Download Contribution of the outer surface proteins of Borrelia

Contribution of the outer surface proteins of
Borrelia burgdorferi s. I. to the pathogenesis of
Lyme disease
O
• V
AKADEMISK AVHANDLING
som för avläggande av doktorsexamen i medicinsk vetenskap vid Umeå
Universitet, offentligen kommer att försvaras på engelska språket i
föreläsningssalen, byggnad 6L, Institutionen för Mikrobiologi, Umeå
Universitet, fredagen den 7 oktober 1994, kl 09.00
av
Maria Jonsson
Umeå 1994
ABSTRACT
Contribution of the outer surface proteins of Borrelia burgdorferi s. L to the
pathogenesis of Lyme disease.
Maria Jonsson, Department of Microbiology, Umeå University, Sweden.
Borrelia burgdorferi s. I is a spirochete which causes the multisystemic disorder Lyme
disease. As the borreliae lack toxin production, the pathogenicity is thought to involve, at
least in part, molecules from the outer surface. Most Lyme disease Borrelia strains
express two major outer surface lipoproteins, OspA (31 kD) and OspB (34 kD), on their
surface. However, some strains lack the expression of OspA and OspB, but express a
smaller 21 to 25 kD OspC protein instead. This thesis focuses on the importance of these
proteins in the pathogenesis of Lyme disease.
Biochemical and immunochemical studies of the OspA and OspB proteins from
strains of various geographic origins show considerable differences in the apparent
molecular weights and in their reactivities to monoclonal antibodies. The cloning and
sequencing of the ospAB opérons from strains of different origins has demonstrated that
the heterogeneity is found also at the DNA level Comparison of the ospAB sequences
allows the classification of the strains into three types, which coincide with the recent
species designations, B. burgdorferi sensu stricto, B. afzelii and B. garinii The genes are
located on a linear plasmid about 50 kb in size, and are cotranscribed as a single message.
The expression of the osp operon in different strains was studied by Western blot
and Northern blot analysis. The ospAB operon of strains expressing varying amounts of
the Osp proteins was cloned and sequenced. The DNA sequence was found to be >99%
identical. The regulation appears to be primarily at the transcriptional level
In patients who have received incomplete treatment, B. burgdorferi have been
isolated several years after the onset of the disease. As mentioned above, the ospAB loci
of different strains show considerable heterogeneity, and it has been speculated that the
spirochetes evade the host’s immune system by antigenic variation of the Osp proteins. In
a mouse model system it was shown that no variation of the osp genes occurs over the
course of an infection, and that other escape mechanisms must be used.
The OspB proteins in particular have been shown to be very heterogeneous in
different isolates. The MAb 84C recognizes a wide variety of B. burgdorferi strains, and
the binding epitope was mapped to a conserved region in the carboxyl terminus of the
OspB protein with putative structural and/or functional importance
It is well known that antibodies can kill bacteria in the presence of complement and
phagocytes. Some antibodies seem to have a bactericidal effect by themselves. H6831 is a
monoclonal antibody recognizing the OspB protein of some B. burgdorferi strains. The
bactericidal action of univalent FAb fragments from H6831 was further characterized, and
the binding epitope was mapped to a very heterogeneous region of the carboxyl end of the
OspB protein
Key words: Borrelia burgdorferi / Lyme disease / Osp proteins / ospAB operon / gene
regulation / pathogenicity
ISBN: 91-7174-938-1
UMEÅ UNIVERSITY MEDICAL DISSERTATIONS
New Series No. 413
From the Department of Microbiology
Umeå University, Umeå, Sweden
ISBN 91-7174-938-1
ISSN 0346-6612
Contribution of the outer surface proteins of
Borrelia burgdorferi s. I. to the pathogenesis of
Lyme disease
by
Maria Jonsson
Department of Microbiology
Umeå University
Umeå 1994
2
Front cover:
Borren Borrelia
Illustrator:
Anna Winberg
Printed in Sweden by
Solfjädern Offset AB
Umeå 1994
3
TABLE OF CONTENTS
Abbreviations
Abstract
Papers in this thesis
Introduction
1. General introduction
1.1 History of Lyme disease
1.2. Taxonomy
1.3. General characteristics
1.4. The vector
1.5 Clinical manifestations
1.6. Therapy
2. DNA content
2.1 The linear chromosome
2.2 Linear plasmids
3. Pathogenicity
3.1. Bacterial virulence factors
3.2. Antigenic variation
3.3 B. burgdorferi s. I. outer membrane proteins
3.4 Regulation of the Osp proteins
3.5 Pathogenicity
3.6 Adhesion of Lyme disease Borrelia to human mammalian cells
3.7 Animal models and vaccination studies
Aims of the present work
Summary and discussion of results
4.
Heterogeneity of the osp operon(Paper I)
4.1 Prologue
4.2 Cloning of the osp operon of ACAI and Ip90
4.3 Genetic organization
4.4 Sequence comparison
5.
Expression of theosp operon (Paper II)
5.1 Why clone the osp operon from B. afzeliiFI ?
5 .2 Characterization of protein expression byWestern blot analysis
5.3 Genetic organization
5.4 Transcription
5
6
7
9
9
9
10
11
12
13
14
14
16
18
19
19
20
22
24
25
27
28
31
33
33
33
33
34
34
37
37
38
38
41
4
6.
8.
9.
5.5 Model for Osp regulation
Molecular characterization of the OspA and OspB proteins during
infection in vivo (Paper III)
6.1 Methodology
6.2 Results of cultivation
6.3 Cloning and sequencing of the ospA and ospB genes
6.4 Immune response
Mapping of anti-OspB antibody epitopes
7.1 Method of selecting mutants
7.2 Cloning and sequencing
7.3 Mapping of the binding epitope for the highly cross-reactive
OspB MAb 84C (Paper IV)
7.4 Characterization of the anti-OspB bactericidal antibody H6831
7.4.1 Borrelicidal activity
7.4.2 Epitope mapping
Binding studies (unpublished results)
8.1 Binding of spirochetes to glycolipids
Discussion of the pathogenicity of Lyme disease borreliae and
the involvement of the Osp proteins
Conclusions
Acknowledgements
References
Papers I-
41
42
43
43
44
44
45
45
45
48
49
49
50
51
51
52
55
56
59
ABBREVIATIONS
ACA
ACAI
B31
CNS
CSF
dnaN
DNA
EM
FI
FAb
gyrA, gyrB
HB19
HUVE
Ip90
kb
kD
LABC
IP
MAb
MBC
MIC
N40
ori
Osp
PAGE
PBS
PCR
RNA
Rpa
rRNA
rrf rrl, rrs
SCID
SDS
s. /.
.S', s.
Vmp
VSG
ZS7
acrodermatitis chronica atrophicans
Borrelia afzelii strain ACAI
Borrelia burgdorferi sensu stricto strain B31
central nervous system
cerebrospinal fluid
gene encoding DNA polymerase III
deoxyribonucleic acid
erythema migrans
Borrelia afzelii strain FI
fragment antigen binding
genes encoding DNA gyrases
Borrelia burgdorferi sensu stricto strain HB19
human umbilical vein epithelium
Borrelia garinii strain Ip90
kilo basepair
kilo Dalton
lymphadenosis benigna cutis
linear plasmid
monoclonal antibody
minimal bactericidal concentration
minimal inhibitory concentration
Borrelia burgdorferi strain N40
origin of replication
outer surface protein
polyacrylamide gel electrophoresis
phosphate buffered saline
polymerase chain reaction
ribonucleic acid
Borrelia burgdorferi sensu stricto strain RPa
ribosomal RNA
genes for the 5S, 23S and 16S ribosomal RNA respectively
severe combined immunodeficiency
sodium dodecyl sulphate
sensu lato
sensu stricto
variable major protein
variable surface glycoprotein
Borrelia burgdorferi strain ZS7
6
ABSTRACT
Contribution of the outer surface proteins of Borrelia burgdorferi s. L to the
pathogenesis of Lyme disease.
Maria Jonsson, Department of Microbiology, Umeå University, Sweden.
Borrelia burgdorferi s. /. is a spirochete which causes the multisystemic disorder Lyme
disease. As the borreliae lack toxin production, the pathogenicity is thought to involve, at
least in part, molecules from the outer surface. Most Lyme disease Borrelia strains express
two major outer surface lipoproteins, OspA (31 kD) and OspB (34 kD), on their surface.
However, some strains lack the expression of OspA and OspB, but express a smaller 21 to
25 kD OspC protein instead. This thesis focuses on the importance of these proteins in the
pathogenesis of Lyme disease.
Biochemical and immunochemical studies of the OspA and OspB proteins from strains
of various geographic origins show considerable differences in the apparent molecular
weights and in their reactivities to monoclonal antibodies. The cloning and sequencing of the
ospAB opérons from strains of different origins has demonstrated that the heterogeneity is
found also at the DNA level. Comparison of the ospAB sequences allows the classification
of the strains into three types, which coincide with the recent species designations,
B. burgdorferi sensu stricto, B. afzelii and B. garinii. The genes are located on a linear
plasmid about 50 kb in size, and are cotranscribed as a single message.
The expression of the osp operon in different strains was studied by Western blot and
Northern blot analysis. The ospAB operon of strains expressing varying amounts of the Osp
proteins was cloned and sequenced. The DNA sequence was found to be >99% identical.
The regulation appears to be primarily at the transcriptional level.
In patients who have received incomplete treatment, B. burgdorferi have been isolated
several years after the onset of the disease. As mentioned above, the ospAB loci of different
strains show considerable heterogeneity, and it has been speculated that the spirochetes
evade the host’s immune system by antigenic variation of the Osp proteins. In a mouse
model system it was shown that no variation of the osp genes occurs over the course of an
infection, and that other escape mechanisms must be used.
The OspB proteins in particular have been shown to be very heterogeneous in different
isolates. The MAb 84C recognizes a wide variety of B. burgdorferi strains, and the binding
epitope was mapped to a conserved region in the carboxyl terminus of the OspB protein with
putative structural and/or functional importance.
It is well known that antibodies can kill bacteria in the presence of complement and
phagocytes. Some antibodies seem to have a bactericidal effect by themselves. H6831 is a
monoclonal antibody recognizing the OspB protein of some B. burgdorferi strains. The
bactericidal action of univalent FAb fragments from H6831 was further characterized, and
the binding epitope was mapped to a very heterogeneous region of the carboxyl end of the
OspB protein.
7
PAPERS IN THIS THESIS
This thesis is based on the following articles and manuscripts, which will be referred to
in the text by Roman numerals.
I.
Jonsson, M., L. Noppa, A. G. Barbour, and S. Bergström. 1991.
Heterogeneity of outer membrane proteins in Borrelia burgdorferi:
Comparison of osp opérons of three isolates of different origins.
Infection and Immunity 60:1845-1853.
II.
Jonsson, M., and S. Bergström. 1994.
Evidence for transcriptional and translational regulation of the expression of the
major outer surface proteins OspA, OspB and OspC in Lyme disease Borrelia.
Manuscript
III. Jonsson, M., T. Elmros, and S. Bergström. 1994.
Genetic stability of the outer surface proteins OspA and OspB of Borrelia
burgdorferi in subcutaneous chambers in mice.
Manuscript.
IV. Shoberg, R. J., M. Jonsson, A. Sadziene, S. Bergström, and D. D. Thomas.
1994.
Identification of a highly cross-reactive outer surface protein B epitope among
diverse geographic isolates of Borrelia spp. causing Lyme disease.
Journal of Clinical Microbiology. 32:489-500.
V.
Sadziene, A., M. Jonsson, S. Bergström, R. K. Bright, R. C. Kennedy, and
A. G. Barbour. 1994.
A bactericidal antibody to Borrelia burgdorferi is directed against a variable
region of the OspB protein.
Infection and Immunity. 62:2037-2045.
9
INTRODUCTION
1.
General background
1.1
History of Lyme disease
The first likely case of Lyme disease to be documented was described by Buchwald in
Germany , who in 1883 reported on a patient with an idiopathic skin atrophy
(Buchwald, 1883). In 1902 similar cases were described, and the disease was named
acrodermatitis chronica atrophicans, ACA (Herxheimer and Hartman, 1902). Erythema
migrans (EM) was described in Sweden in 1909 (Afzelius,1910) and in Austria in 1913
(Lipschutz, 1914). In 1922, a patient in France was reported with neurological
symptoms and an erythema, not recognized as EM (Garin and Bujadoux, 1922). The
Swedish physician Hellerström was the first to report the connection between EM and
neurological symptoms in 1930 (Hellerström, 1951). The first case of EM in the USA
was reported in 1970 (Scrimenti, 1970).
In 1975 there was an outbreakof juvenilearthritis in Lyme,Connecticut,USA.
This is normally a very rare condition, affecting onlyone in100,000 children. Two
mothers of affected children responded to the many cases, mostly children, but also
adults, of this severe condition at the same time, and contacted the Health Department
officials. Steere and his colleagues found that in Lyme, the occurence of juvenile
arthritis was about 100 times the normal incidence (Steere, 1989). Moreover, the cases
were associated with heavily wooded areas, and very few victims were from town
centers. In some discrete regions the frequency was up to 10,000 times higher than
normal. About 25 % of the patients remembered a strange skin rash from one to several
weeks before the outbreak of the arthritis. This was in agreement with the findings of
Afzelius in Europe in 1909, who described an expanding red skin rash, EM, in patients
who had been bitten by the tick Ixodes ricinus. The EM had been successfully treated
with penicillin, indicating a bacterial agent, not a virus
In 1977, nine patients affected by the skin rash remembered a tick bite, and one of
them had even removed and saved the tick. It was identified to be of the species Ixodes
dammini, closely related to the European I. ricinus (Steere, 1989). Finally, in 1981
Burgdorfer found some long, irregularly shaped spirochaete bacteria in the gut of an
Ixodes tick, and Barbour managed to grow them in culture (Barbour, 1984a). Sera from
patients infected with the mysterious disease were tested by Burgdorfer and
colleagues, and they showed a pronounced antibody response to the bacteria
(Burgdorfer et al., 1982). In 1983, the spirochete was also isolated from blood, skin
10
lesions and cerebrospinal fluid of patients (Benach et a i , 1983; Steere et al., 1983).
Three of Kochs four postulates were then fulfilled. The fourth postulate, that
inoculation with the isolated agent should give rise to the same symptoms as was
originally found, cannot, of course, be tested for ethical reasons (Habicht et a i , 1987).
The disease is now named Lyme disease, or Lyme borreliosis, and the agent of the
disease was named Borrelia burgdorferi in honour of Willy Burgdorfer (Johnson et
al., 1984b).
1.2
Taxonomy
The Borrelia genus, named after the french bacteriologist A. Borrel
(Nationalencyklopedin, 1990), is distinct from the genera of Treponema and
Leptospira by differences in guanine plus cytosine content, and the lack of DNA
homology. The DNA of Borrelia have mole % guanine and cytosine contents of 27.3
to 30.5, Leptospira of 35.3 to 39.9, and Treponema pallidum of 52 to 53.7. The DNA
homology between Borrelia and the other two genera is 0 to 2 % (Hyde and Johnson,
1984)
In the past, differentiation of borrelia was based on identification of the specific
vector of transmission of the spirochete. Today, genetic methods are used for the
classification. DNA homology studies have shown that three North American tickborne relapsing fever species, B. hermsii, B. turicatae and B. parkeri, show high
homologies (76-100 %) and should therefore belong to the same species (Hyde and
Johnson, 1986). There are other cases where reclassification will be needed.
B. burgdorferi was described as a new species based on genotypic, i. e. DNADNA hybridization, and phenotypic characteristics such as reactivity of the outer
surface protein A, OspA, to the monoclonal antibody H5332 (Johnson et al., 1984b;
Barbour et al., 1983). This definition was based on few isolates. Today, several
hundred strains from different sources and geographic areas have been isolated and
further characterized. Isolates from Europe have been shown to be especially
heterogeneous, both antigenically and at the DNA level (Wilske et al., 1986, 1988b ).
Based on rRNA gene restriction patterns, protein electrophoresis patterns and
reactivity with murine monoclonal antibodies, Lyme disease Borrelia was divided into
three genomic species; B. burgdorferi sensu stricto, B. garinii and VS461. VS461 has
now been named B. afzelii (Baranton et al., 1992; Marconi and Garon, 1992; Canica et
a i, 1993). In this thesis, I will use the terms B. burgdorferi s. i or Lyme disease
Borrelia to refer to these three species. A fourth genomic species, B. japonica has
11
been isolated from the Japanese tick I. ovatus (Kawabata et al., 1993; Postic et al.,
1993). B. japonica does not appear to be a human pathogen, as no patient isolates of
this group have been found. Furthermore, the ticks from which it has been isolated do
not use humans as a host. Recent reports indicate that the number of Borrelia species
is increasing. At least some of these new groups were isolated from vectors with
enzootic cycles different from the ordinary Lyme disease strains, indicating that they
may not be human pathogens (Assous et al., 1994; Postic et al., 1994).
1.3
General characteristics
Borrelia burgdorferi belongs to the genus of Borrelia, and the order of Spirochaetales.
They are flexuous, thin, gram-negative, chemoheterotrophic, helical-shaped organisms.
A fluid outer cell membrane surrounds the protoplasmic cylinder complex which
consists of the cytoplasm, the inner cell membrane and the peptidoglycan layer.
Isolated outer membranes are made up of 45 to 62 % protein, 23 to 50 % lipid and 3 to
4 % carbohydrate. The outer sheaths of Borrelia species are characterized by a lack of
structural detail (Holt, 1978).
In the tick, the large blood meal is held in the midgut. The digestion of blood
occurs intracellularly in the epithelium of the gut lining (Akov, 1982). As a
consequence, the Borrelia in the tick midgut are not exposed to proteases or acidity,
which makes it unnecessary for them to have a defense against proteases, which would
be required if they were harboured in a hematophageous insect, like a mosquito. Thus,
both the tick borne B. hermsii and B. burgdorferi are susceptible to attack by proteases
like trypsin (Barbour et al., 1984b; Barbour, 1985).
The Borrelia differ morphologically from other procaryotes in that they have an
axial fibril called the endoflagellum. B. burgdorferi have 7 to 20 endoflagella, around
which the cell is coiled (Holt, 1978; Steere, 1989). The flagella are located between
the outer cell membrane and the protoplasmic cylinder, attached to the poles. The
bacterial diameter is about 0.2 to 1 pm and the length of the cell is 20 to 30 pm,
depending on the physiological state of the bacteria. The length increases as cells reach
the stationary phase of growth, but the length also depends on the quality of the growth
medium or the experimental animal in which the bacteria is harboured. (Holt, 1978;
Barbour and Hayes, 1986; Steere, 1989).
12
1.4
The vector
The vectors of Lyme borreliosis are hard shelled ticks of the genus Ixodes. In Europe
the species is I. ricinus, and in Eastern Europe and Asia, the tick species is primarily
I. persulcatus. In the USA, three species of Ixodes ticks are implicated. I. dammini is
found in North-eastern and mid-western USA, I. paciflcus in Western USA, and
I. scapularis in South-eastern USA (Gustafson, 1993). It should be noted that recent
work has shown that I. dammini and I. scapularis are the same species (Oliver et ai,
1993). In this thesis, I will use the name applied in the littérature cited, usually
I. dammini.
The Ixodes ticks go through different life stages. Eggs are usually deposited in the
spring, and it takes several weeks for the larvae to hatch. At this stage they are very
small, barely visible, and have only six legs. They feed once, usually on blood from
small mammals like mice, and then the larvae crawl into the ground and moult to
become the slightly larger, eight-legged nymphs. These also feed once, on mice or
larger mammals like dogs, deers or humans, and then moult into adults. Both sexes
have eight legs, but the female is larger than the male. The females feed once more,
while the adult males do not. The females die after laying their eggs (Steere, 1989).
The frequency of transovarial transmission of spirochetes from an infected female
to her offspring is variable. In one study, the transmission of spirochetes to the progeny
of two I. ricinus female ticks was 60 % and 100 % (Burgdorfer et al., 1983). In
another study, the occurence was <1 % of infection in I. dammini larvae (Magnarelli et
al., 1987). The infection is usually spread by larvae or nymphs becoming infected by
feeding on an infected animal, and then transmitting it to their next host (Magnarelli et
al., 1987).
B. burgdorferi can be found in the midgut, hindgut and the rectal area of the tick,
and eventually in the salivary glands. From the intestinal region the borreliae may be
transmitted by the regurgitation of the gut contents during the feeding act (Burgdorfer,
1984), but usually they are transmitted via the infected saliva. To avoid infection, it is
best to remove the tick from the skin as soon as possible after detection. The best way
to remove an attached tick is to use a pair of forceps, without squeezing the abdomen.
Oil covers the tracheas of the tick and suffocates it. If covered with oil or butter, the
tick will withdraw from the skin, but while doing so, the contractions of the abdomen
will increase the transfer of spirochetes in the midgut into the host. Gasoline or acetone
are also often used to remove ticks. These chemicals will kill the tick, with the result
that it is often caught fastened in the skin (Magnarelli, personal communication).
In Sweden it was previously assumed that Dalälven was the northern boundary of
13
the tick population. Thus, lacking ticks, there were no Borrelia infections in Northern
Sweden. It has now been documented that ticks are more widely spread than
previously thought. We have shown that B. burgdorferi infected ticks and Lyme
disease exist along the coast in the north of Sweden (Bergström et a i , 1992; Jaenson et
al., 1989). Ixodes ricinus has also been found in areas close to the arctic circle (Olsén,
personal communication).
1.5
Clinical manifestations
One of the problems with Lyme borreliosis is that the symptoms of the disease mimic a
lot of other clinical conditions (Stechenberg, 1988). B. burgdorferi may spread locally
in the skin causing a migrating erythematous lesion, erythema migrans (EM) (Åsbrink
et al., 1984a). The period of time between the tick bite and the appearance of EM has
been reported to be 3 to 32 days, with a median of 7 days. The most common EM
lesion is a circular expanding erythema, which may be homogeneous, but usually
shows a central clearing. It is occasionally accompanied by constitutional symptoms
such as fever, headache, malaise and fatigue. Secondary erythema, due to spreading in
the blood, may occur at other locations than the primary lesion, but these are often
smaller. Untreated, the EM usually disappears within weeks to months, however,
relapses are known to occur. B. burgdorferi can also cause a tumor-like bluish-red
swelling or nodule, borrelia lymphocytoma, BL„ usually on the ear lobes or the nippleareola mammae area. BL can appear together with EM or acrodermatitis chronica
atrophicans, ACA, and be accompanied by neurological and arthritic symptoms
(Stechenberg, 1988; Steere, 1989; Karlsson, 1990).
The third characteristic skin lesion is, as mentioned above, ACA. The onset is
usually gradual, and characterized by a bluish-red discoloration and swollen skin,
predominantly located on the extremities. The inflammatory phase may persist for
years or decades, and is gradually replaced by atrophy of the skin and sometimes also
underlying tissues.
B. burgdorferi can spread not only in the skin and underlying tissues, but also
through the lymph or blood to the skin at other sites or to other organs like the heart,
joints and the nervous system (Steere, 1989; Karlsson, 1990). Infection of the heart
leads to fatigue and arrythmia. It usually lasts for approximately a week, and is not
followed by recurrences, although the arrythmia may persist (Karlsson, 1990). Cardiac
manifestations occur in less than 10 % of the patients (Stechenberg, 1988), and so far,
only one fatal case has been reported (Marcus et al., 1985).
14
Arthritis affects a few or several small or large joints. It is often accompanied by
migrating pains in the tendons and muscles, and can last from about a week to months.
Artralgias may recur for several years, interrupted by long, symptom-free periods.
Some patients suffer from chronic arthritis in the larger joints, with erosion of the
cartilage and bone.
Both the peripheral and the central nervous systems (CNS) may be affected in
Lyme disease. The symptoms can be diffuse, including profound fatigue, loss of
appetite, slight headache or change of personality. However, they can also be more
profound, with facial palsy, double vision and loss of hearing. Neuroborreliosis may
resolve itself spontaneously within weeks or months, but a small percent of the cases
become chronic and may last for decades. The fatigue symptoms resemble those
observed in patients with chronic fatigue symptoms, CFS, which are not triggered by
Borrelia infections (Coyle et al., 1994).
1.6
Therapy
In vitro studies show that B. burgdorferi is most susceptible to erythromycin,
ceftriaxone and cefotaxime, although erythromycin is less effective in vivo (PreacMursic et al., 1987). It was also shown that Penicillin G, which is considered an
appropriate antibiotic for the treatment of Lyme disease, has very poor activity both m
vitro and in vivo. In EM, LB and ACA, oral treatment with penicillin, amoxycillin or
doxycycline is usually recommended. In disseminated stages, per oral treatment with
doxycycline and parenteral treatment with Penicillin G, cefotaxime and ceftriaxone has
been succesful (Steere, 1989).
There are cases where failure of the antibiotic treatment has been documented by
positive culture or serological findings. This can be explained by insufficient dosages
of antibiotic, and/or inadequate absorption or penetration to the affected tissues.
Another explanation is deficient bacteriocidal effect of the antibiotic (Karlsson, 1990).
In late, chronic stages it is possible that the symptoms are caused by autoimmune
reactions rather than the presence of spirochetes (Aberer et al., 1989).
2.
DNA content
As mentioned above, the DNA of B. burgdorferi has a low G+C content of 28-30.5
mole % (Hyde and Johnson, 1984; Johnson et al., 1984b; Schmid et al., 1984). The
15
DNA homology to other Borreliae is 30 to 59 % (Hyde and Johnson, 1984; Schmid et
al., 1984). The low G+C content alters the codon usage in the spirochetes. A and T
rich codons, which are regarded as rare codons in E. coli, are preferred codons in
Borrelia (Bergström et al., 1989). Long stretches of poly(dA) is known to cause
natural bending in DNA (Koo et al., 1986). In E. coli, stretches of such structures have
been found upstream of strong promoters, and are thought to be important for the
promoter strength (Bracco et al., 1989). Being very AT rich, stretches of poly(dA) are
frequent in the Borrelia genome. The regulation of gene expression in the spirochetes
may, therefore, be quite different from what is found in other bacteria with higher G+C
contents.
vwvywv
: ..-. =;
Sb
Borrelia burgdorferi s. 1.
/
v
-
-
.....
------------
■
■
■--------
Chromosome 1.000 kb
■ .................o
-----------------o
Linear plasmids 15-50 kb
Circular plasmids 5-30 kb
Phage 1-2 kb
Figure 1. The DNA content of B. burgdorferi s. I.
A schematic drawing of the DNA organization in Borrelia burgdorferi s. I. is shown in
Figure 1. Like most, if not all, bacteria, B. burgdorferi has supercoiled circular
plasmids of 5 to 30 kb (Hyde et al., 1984, 1986; Simpson et al., 1990). However,
B. burgdorferi and all other Borrelia investigated, contain linear plasmids which are
uncommon in eubacteria (Plasterk et al., 1985; Barbour and Garon, 1987; Pemg and
LeFebvre, 1990). Both the size and the number of linear plasmids vary between strains,
but the smallest are around 15 kb, and the largest is about 50 kb. Even more unique
than the linear plasmids is that the chromosome also is linear. To date, Streptomyces
lividans (Lin et al., 1993) and Agrobacterium tumefaciens (Allardet-Servent et al.,
16
1993) are the only other examples of eubacteria having linear chromosomes. Besides
the chromosome and plasmids, phage have been visualized inside the bacteria by
electron microscopy (Hayes et al., 1983 ).
2.1
The linear chromosome
The discovery that the B. burgdorferi chromosome is linear was made simultaneously
in two separate laboratories (Ferdows and Barbour, 1989; Baril et a i , 1989). High
molecular weight Borrelia DNA was subjected to pulse field gel electrophoresis.
Under the running conditions used, circular DNA should not enter the gel. Both groups
found a distinct band about 950-1 000 kb in size, with little material remaining in the
wells. To avoid shearing, DNA was prepared in situ by lysing the spirochetes
embedded in agarose. Other members of the Borrelia genus, including B. hermsii,
B. coriaceae, B. turicatae, B. parkeri and B. anserina, have linear chromosomes of
roughly the same size as B. burgdorferi (Casjens and Huang, 1993; Rosa and Schwann
1992; Kitten and Barbour, 1992 ). Chromosomal DNA from other spirochetes, /. e.
Leptospira interrogans, Spirochaeta aurantia and Treponema denticola, does not
enter the gel, indicating that linear chromosomes are not common to all spirochetes
(Ferdows and Barbour, 1989; Baril et al., 1989).
Figure 2. Map o f the linear chromosome. The chromosome is depicted by the horizontal line. The
numbers above this line indicate the distance in kb from the left end. The regions to which the genes
have been mapped, are indicated by filled triangles. The two putative origins of replication are
indicated above the chromosome (Davidson et a l., 1992; Casjens and Huang, 1993).
The linear nature of the chromosome has also been suggested by the physical and
genetic maps of the chromosome which have been generated (Davidson et al., 1992;
Casjens and Huang, 1993). Figure 2 represents a compilation of the two available
17
maps. If the chromosome were circular, all restriction endonucleases would have to
cleave at one location. The probability of this is quite low. The possibility that the
chromosome contains one extremely fragile locus that is always broken during
preparation cannot be eliminated until the structure of the chromosomal ends has been
determined. The ends do not hybridize to each other. Genes homologous to replication
genes in other eubacteria as well as genes unique to Borrelia have been found near the
telomeric regions (Casjens and Huang, 1993, 1994). Rapid reannealing of the
chromosome after dénaturation suggests that the ends of the telomeres are hairpins
similar to those of the characterized 16 kb linear plasmid, see below (Ferdows and
Barbour, 1989; Barbour 1993).
The rRNA genes, 23S (rrl), 5S (rrf) and 16S (rrs), have been cloned on the basis
of their sequence homology to the E. coli genes, and mapped to the Borrelia
chromosome, see Figure 2. Normally, rRNA genes are encoded by opérons in the
order; promoter, 16S, 23S, 5S. This results in the production of equimolar amounts of
the three rRNA species. The number of rRNA opérons varies from one to ten, in
different bacterial species. In general, fast-growing bacteria have more copies than
slow-growing bacteria (Krawiec and Riley, 1990). B. anserina, B. turicatae and
B. hermsii have a single operon with the usual organization (Schwartz, et al., 1992),
but in B. burgdorferi, rrl and rrf are tandemly duplicated, while the single rrs gene is
located more than two kb upstream of the first rrl gene (Schwartz, et a i , 1992;
Davidson et al., 1992; Fukunaga, et al., 1992).
In bacteria with circular chromosomes, like Escherichia coli, Bacillus subtilis and
Pseudomonas aeruginosa, the replication of the chromosome is initiated and
terminated at specific regions denoted the origin of replication (ori) and the termination
of replication (ter). The initial step in replication is the binding of the protein DnaA to
the DnaA boxes, which consist of at least four characteristic 9 bp repeats, usually
located close to the dnaA gene (Smith, et al., 1991; Holz, et a l , 1992; Ogasawara, et
al., 1990; Ogasawara and Yoshikawa, 1992). Subsequently, DnaB and DnaC bind to
the DnaA/DNA complex, and replication proceeds bidirectionally around the
chromosome until the ter sites are reached. Other genes important for the replication
are dnaN encoding the DNA polymerase III, and gyrA and gyrB encoding DNA
gyrases. In B. burgdorferi s. I these three genes map together with the dnaA gene next
to the rRNA genes in the middle of the chromosome (Old et a l , 1992, 1993a, b;
Casjens and Huang, 1993). In other bacteria, gidA, involved in cell division, is also
situated by the chromosomal origin. In B. burgdorferi gidA has been mapped near the
left end. This suggests that the Borrelia chromosome is either replicated bidirectionally
18
from the centre of the chromosome out to the ends, or from the left end towards the
right end. No cluster of DnaA boxes have been identified in B. burgdorferi to date.
The two putative ori regions are indicated in Figure 2.
Comparison of the genetic maps of the chromosomes from B. burgdorferi s. s.,
B. afzelii and B. garinii show a significant degree of conservation of the position of the
mapped genes, indicating a very stable chromosome (Saint Girons et al., 1994).
2.2
Linear plasmids
The first bacterial linear plasmids were described in Streptomyces rochei in 1979
(Hayakawa et al., 1979). Linear plasmids have since been detected in at least 10 other
species of Streptomyces, as well as in Nocardia opaca, Ascobulus immersus and
Rhodococcus fascians (Kalkus et al., 1990; Francou, 1981; Crespi et al., 1992;
Hinnebusch and Tilly, 1993). All members of the genus Borrelia contain linear
plasmids (Plasterk et al., 1985; Marconi et al., 1993c). They account for most of the
extrachromosomal DNA in B. burgdorferi cells. The B. burgdorferi strain B31
harbours four linear plasmids of 16, 29, 38 and 49 kb, as well as two circular plasmids
of 27-30 kb (Barbour and Garon, 1987, Saint Girons et al., 1994). The most studied of
the plasmids are the linear 16 kb and 49 kb plasmids. The ends of these plasmids are
covalently closed, which can be demonstrated by electron microscopy (Barbour and
Garon, 1987; Barbour, 1988; Hinnebusch et al., 1990). Reannealing of the plasmid
after alkaline dénaturation is very fast, but this feature is destroyed if the plasmid has
been pretreated by SI nuclease. When examining the denatured plasmid, single
stranded circles of about 100 kb were observed. Chemical sequencing of both ends of
the 16 kb plasmid and the left end of the 49 kb plasmid has revealed 19 bp of almost
perfect inverted repeats proceeded by four unpaired nucleotides forming hairpin loops
at the ends (Hinnebusch et al., 1990; Hinnebusch and Barbour, 1991; Barbour 1993;
Hinnebusch and Tilly, 1993). Some animal viruses have the same kind of covalently
closed ends, but, to date similar plasmids have not been observed in any other
prokaryotes (Ploeg et al., 1984; Bergström et al., 1990). The plasmids are structurally
similar to a vaccinia virus, African swine fever virus, which is carried by the
Omithodoros ticks, and also harbours B. duttoni (Hinnebusch and Barbour, 1991;
Hinnebusch and Tilly, 1993).
In B. hermsii, the amount of the different DNA species has been studied under
various growth conditions (Kitten and Barbour, 1992). The copy numbers of the
chromosome and the two linear plasmids containing the silent and expressed forms of
19
one of the vmp genes, vmp7, during in vitro culturing and growth in mice were
determined. Spirochetes grown in vitro had approximately 4 chromosomes per
bacteria.The same value was found for the plasmid harbouring the silent vmp gene,
while the expression plasmid was found in about 6 copies per cell. When grown in
mice, with a generation time of 6-8 h compared to 10-12 h in vitro, the copy numbers
of the replicons changed both absolutely and relative to one another. The number of
chromosomes and the expression plasmids was 15, while the silent storage plasmid
was present at about 7 copies per cell. This difference could be a result of the higher
gene dosage required by the expressed genes. In B. burgdorferi, only the relative copy
numbers of the chromosome and the linear plasmids have been determined. The 16 and
49 kb plasmids have copy numbers of approximately one per chromosome, suggesting
that the replication and partitioning of the chromosome and the plasmids are tightly
coupled (Hinnebusch and Barbour, 1992).
Méthylation of DNA is considered to be an important regulatory mechanism
involved in, for example, protecting the DNA from degradation, DNA replication and
recombination, gene expression and development. Some but not all B. burgdorferi
strains possess an adenine méthylation system. There is no obvious correlation
between strains capable of méthylation and infectivity (Norton Hughes and Johnson,
1990).
B. burgdorferi lose their infectivity after many passages in culture media. Some of
the plasmids, both linear and circular, are also lost after prolonged cultivation, which
has led to the conclusion that at least one of the genes responsible for infectivity should
be plasmid encoded (Schwan et al., 1988; Simpson et al., 1990). Recently, an 18 kD
protein, EppA, apparently only expressed during infection, was cloned from a 9 kb
circular plasmid present only in low-passage, virulent B. burgdorferi strains
(Champion et a i , 1994).
3.
Pathogenicity
3.1
Bacterial virulence factors
An infection is established when the body is colonized by a bacterium capable of
causing disease. Virulence and pathogenicity describe the ability of a bacterium to
cause disease. A virulence factor is a bacterial product or strategy contributing to
virulence or pathogenicity (Salyers and Whitt, 1994). As was described in section 1.5,
20
Borrelia burgdorferi can be isolated from various organs in patients suffering from
Lyme disease (Steere, 1989; Karlsson, 1990). Natural infections are initiated by a tick
bite, where the spirochetes are deposited in the skin (Shih et al., 1992). From this
position, the spirochetes enter the circulatory system and are spread to distant parts of
the body, where they can invade different organs and cause infection. Since they are
able to do this, the spirochetes probably possess several virulence factors.
A very important bacterial virulence factor is adhesion to host cells. This is
required both for the colonization of body surfaces, and for the subsequent invasion of
the host cells. Different types of adhesion are known. Pili are structures extending from
the bacterial surface, which usually bind to carbohydrate moieties of host cell
glycoproteins or glycolipids. As the pili structure is quite fragile, the contact formed
between the bacterium and the host cell is easily broken. Afimbrial adhesins are
bacterial surface proteins that mediate a tighter association between the bacterium and
the host cell. Many bacteria bind to extracellular host proteins that, in turn, attach to
host cells (Isberg, 1991). Bacteria can also adhere to surfaces by forming biofilms
consisting of bacteria held together by a polysaccharide matrix (Salyers and Whitt,
1994).
The invasion of host tissue is accomplished by bacterial invasins. The invasins are
surface proteins that cause actin rearrangements in normally nonphagocytic host cells
that lead to pseudopod formation and engulfment of the bacterium in a phagocytic
vesicle (Isberg, 1991). The bacterium can remain there or escape the phagocytic
vacuole and proliferate in the cell cytoplasm. Through the M cells (macrophage like
cells found in different membranes), the bacteria can also pass through the mucous
membrane to the underlying tissue (Finlay and Falkow, 1989).
Different mechanisms have evolved which allow certain organisms to evade the
host’s immune system. Capsules minimize complement activation and prevent
phagocytosis (Cross, 1990). To avoid recognition by the host’s defence system, some
bacteria cover the bacterial surface with host proteins, others can undergo antigenic
variation (see below) (Finlay and Falkow, 1989).
3.2
Antigenic variation
Many pathogenic organisms change their surface antigens in order to escape the host’s
immune response. Antigenic variation is used by both bacteria and protozoa to promote
the persistence in the host. Neisseria gonorrhoeae has the ability to switch on and off
the expression of pili (Bergström et al., 1986; Swanson et al., 1986). Different strains
21
of gonococci also show extreme antigenic variation in both pili and outer membrane
proteins. The protein subunit of pili, pilin, consists of constant, variable and
hypervariable regions, and genetic rearrangements and recombinations occurring in the
repertoire of pilin genes form the basis of the antigenic variation (Sparling et al., 1986;
Jonsson, 1991).
The causative agent of sleeping sickness, Trypanosoma brucei, is a unicellular
protozoan infecting the bloodstream of mammalian hosts. T. brucei survive by
changing their surface coat, which consists of an abundant glycoprotein, called variant
surface glycoprotein, VSG. The genome contains as many as 1 000 different vsg genes.
Usually only one vsg is expressed at any given time, and about 10 copies of the
protein completely cover the outer membrane of the trypanosome. Only genes located
at special expression sites adjacent to the chromosomal ends are expressed. The rest of
the vsg genes are stored internally on the chromosomes as silent copies. The switch
from the transcription of one vsg to another is accomplished by activating a new gene
in an expression site. The situation is complicated by the fact that there are more than
one potential expression sites, but only one is normally activated at any time
(Donelson, 1989).
The bacterial counterparts of the trypanosomes are the etiology agents of
relapsing fever, of which B. hermsii is the most studied. The variable proteins in
B. hermsii are lipoproteins, called variable major proteins, Vmp’s. The repertoire of
genes is probably not as large as in the trypanosomes, but 25 different serotypes have
been isolated from a single bacteria (Stoenner et a l , 1982). The organization of the
genes is similar, with expressed genes situated at an expression locus near the end of a
28 kb linear plasmid, and silent copies are stored internally on the linear plasmids of
around 30 kb in size (Stoenner et a l , 1982; Plasterk et a l, 1985). Formerly silent vmp
genes replace the actively expressed gene by non-reciprocal recombination between
the linear plasmids (Meier et a l , 1985; Barbour, 1989).The silent and expressed genes
of serotypes vmp7 and vmp21 from the B. hermsii strain HS1 have been cloned and
sequenced. The upstream region of expressed vmp7 and vmp21 are identical, while the
equivalent regions differ when corresponding silent genes are compared (Plasterk et
a l , 1985; Barbour, 1989; Burman et a l , 1990). The silent copies lack promoters, and
can be activated by a recombination event that places them downstream of a promoter
(Barbour et a l , 1991).
There are many indications that antigenic variation occurs in B. burgdorferi as
well. Spirochetes have been isolated from an ACA patient who had had the disease for
more than 10 years (Asbrink and Hovmark, 1985). Some patients with Lyme
22
borreliosis have developed new antibodies a year or more after the initial infection
(Craft et al., 1986). Clonal polymorphism has also been observed in vitro. A European
strain of B. burgdorferi isolated from human cerebrospinal fluid (CSF) contained a 32
kD protein that became more dominant after being passaged in culture for an
unspecified length of time (Wilske et al., 1986). In another study, a B. burgdorferi
strain lost the expression of OspB after only 11-15 passages in BSKII. As the initial
population was not clonal, it cannot be excluded that a strain lacking OspB was
selected from a mixed Borrelia population. Interestingely, Western blot analysis with
two different monoclonal antibodies indicated that changes in certain epitopes of the
OspB protein also occurred (Schwan and Burgdorfer, 1987). Clonal B. burgdorferi
strains grown in vitro have also shown clonal polymorphism of OspB. Subsequent
cloning of different passages from an originally clonal B. burgdorferi, HB19, showed
variations both in the size and expression of the OspB protein. These changes at the
protein level could not be explained by large DNA rearrangements, as no differences
between the distinct clones were detected by Southern or Northern blot analysis
(Bundoc and Barbour, 1989). Rearrangements of the ospA and ospB genes during
growth in vitro creating chimaeric gene fusions were later shown to occur by
homologous recombination, either within or between the two genes. Osp variation was
also shown to arise from nonsense mutations and sequence divergence (Rosa et al.,
1992).
Antigenic variation during B. burgdorferi infection has been contradicted by
recent studies (Barthold, 1993; Jonsson and Bergström, Paper III this thesis). Mice
were immunized intradermally, and then the infection was erradicated by antibiotic
treatment. The treated mice were immune to a challenge by either the initial isolate, or
cultures, as well as biopsies, from animals where the infection was allowed to proceed
for as long as 180 days before antibiotic treatment (Barthold, 1993). The persistence of
viable spirochetes despite a functional immune response shows that the spirochetes
avoid the immune defence in some way. The fact that they were immune to not only
the early, but also the late isolates suggests that surviving the immune response is
achieved by some other mechanism than antigenic variation.
3.3
B. burgdorferi s. I outer membrane proteins
As mentioned above, 45 to 62 % of the outer membrane in Borrelia consists of protein.
More than 100 polypeptide species have been detected by two-dimensional SDSpolyacrylamide gel electrophoresis (SDS-PAGE) (Luff et al., 1989). Labeling with
23
125] or biotin has identified 13 major surface proteins with apparent molecular weights
of 22, 24, 29, 31, 34, 37, 39, 41, 52, 66, 70, 73, and 93 kD. Some of these proteins
have been named Osp proteins. So far, six Osp proteins have been identified. They are
designated OspA, B, C, D, E and F. The common features of the Osp proteins are that
they are all surface exposed, have a hydrophobic leader sequence, and a consensus
cleavage sequence
(L-X-Y-C) recognized by signal peptidase II. They are also all lipoproteins, and can be
labelled by [3H] palmitate (Brandt et al., 1990). The lipid moiety is probably required
to anchor the proteins in the fluid outer membrane. Analysis of the fatty acids in
B. burgdorferi show that the lipoproteins contain palmitate (Belisle et al., 1994). In
E. coli, lipoproteins are synthesized as precursor proteins. The prolipoprotein is
covalently modified with diacylglycerol via a thioether linkage to cysteine and with
fatty acid by an amide bonde. Thereafter, the signal peptide is cleaved off and the
apolipoprotein is further modified at the ot-NH2 group of the glyceride-cysteine residue
with palmitate by N-acyl transferase. The same mechanism is probably used in other
bacteria, like B. burgdorferi s. I. as well (Hayashi and Wu, 1990).
The most studied proteins in B. burgdorferi are the OspA and OspB proteins, of
31 and 34 kD in size respectively. The osp genes from the American type strain B31
were the first to be cloned and sequenced (Howe et al., 1985, 1986; Bergström et al.,
1989). The genes are located in an operon, separated by just a few bases, and
transcribed as one transcriptional unit from the 49 kb linear plasmid. The ospAB
sequences contain signal sequences similar to those for E. coli lipoproteins.
When the protein profiles from different isolates are compared, the Osp proteins
are quite heterogeneous. Both the size and reactivity with various antibodies differ.
The OspB proteins are particularly heterogeneous (Barbour et al., 1984; Lane and
Pascocello, 1989). The OspA proteins are homogeneous in American isolates, but
show a higher degree of polymorphism in European isolates (Barbour et al., 1984,
1985; Barbour and Schrumpf, 1986; Bundoc and Barbour, 1989; Wilske et al., 1985,
1986, 1988b). Most strains express both OspA and OspB proteins, but some isolates
express only one of the proteins. Others express neither OspA nor OspB, but express a
smaller, 21 to 25 kD protein, OspC, instead (Barbour et al., 1985; Wilske et al.,
1988b). In European isolates, more than 90 % of B. burgdorferi isolates express
OspA, and about 40 % express OspC (Wilske et al., 1988b). Like the OspA and OspB
proteins, OspC is heterogeneous and shows immunological and molecular
polymorphism in different isolates (Wilske et al., 1993). The ospC gene was found in
all strains tested in one study despite the fact that it is not expressed in all of these
24
strains (Marconi et al., 1993a). In contrast to the ospA and ospB genes, the ospC gene
is located on a circular plasmid, 26-27 kb in size in B. burgdorferi (Sadziene et al.,
1993a; Marconi et al., 1993a). Amino acid sequencing demonstrated that the Nterminus is blocked. This, together with nucleotide sequence homologies to the
cleavage site of E. coli signal peptidase II also found in OspA and OspB, suggest that
OspC is a lipoprotein (Fuchs et al., 1992).
OspC is found not only in B. burgdorferi. It was first shown that an OspC
specific polyclonal sera recognized a 20 kD Vmp in B. hermsii (Wilske et al., 1993).
Further studies revealed ospC homologues also in B. coriaceae, B. anserina,
B. turicatae and B. parkeri. Interestingly, all these genes are encoded on linear
plasmids, in contrast to the localization in B. burgdorferi (Marconi et al., 1993b).
OspC, together with Vmp’s of 19 to 22 kD, form a genus-wide family of 20 kD
proteins (Carter et al., 1994).
OspD is a 28 kD lipoprotein expressed only in low-passage B31 strains, but
absent in most high-passage, nonvirulent strains tested. The ospD gene is located on a
38 kb linear plasmid, usually lost during prolonged in vitro cultivation (Norris et al.,
1992).
OspE and OspF, 19 and 26 kD in size respectively, are the most recent Osp
proteins to be reported. Like the ospA and ospB genes, their genes are arranged in
tandem as one transcriptional unit, with ospE first, and separated by 27 bp. The whole
operon hybridizes to a 45 kb plasmid in B. burgdorferi N40 (Lam et al., 1994).
Using an antiserum from a patient with cutaneous manifestations, another surface
exposed, 27 kD lipoprotein, P27, has been isolated from the European B. burgdorferi
strain B29 (Reindl et al., 1993). The p27 gene is expressed on the largest, 55 kD,
linear plasmid. In B. burgdorferi B31, the gene for P27 is missing.
3.4
Regulation of the Osp proteins
The expression of OspA and OspB proteins appears to be regulated at the
transcriptional level. A B. burgdorferi isolate named CA-11-90 was cloned by plating,
and then inoculated as single colonies into BSKII liquid medium. One clone, CA-11
2A, did not express detectable amounts of OspA or OspB on Coomassie stained
protein gels. Instead, a major 24-kDa protein could be seen. After 40 in vitro passages,
OspA and OspB were detectable on the stained gels. At the same time, the 24-kDa
protein decreased in amount. Northern blot analysis using a probe covering the whole
ospAB operon showed no detectable ospAB transcript in the clone lacking OspA and
25
OspB, while a strong signal could be seen in the extract from the high passage clone
(Margolis and Rosa, 1993). This suggests that the lack of expression is primarily due
to lack of transcription of the ospAB operon.
B. burgdorferi B31 usually lacks the expression of OspC, but mutant clones of
B31 which had lost a 16 kb linear plasmid, lpl6, started expressing the OspC protein.
This together with the finding that only strains lacking lpl6 homologous DNA
expresses the protein and vice versa, suggests that OspC expression is negatively
regulated by some factor from this plasmid (Hinnebusch and Barbour, 1991; Sadziene
et al., 1993a).
It was also found that the plating efficiency was reduced dramatically when Ipl 6
was lost, suggesting that something on this plasmid is essential for growth on solid
media (Sadziene et al., 1993a). A conflicting result was reported in another study
where OspC expressing clones were obtained from a nonexpressing isolate of
B. burgdorferi strain Pka2 by culture on solid agar (Wilske et al., 1993b). These
findings suggest that:
a) The negative regulation of OspC expression from lpl6 can be turned off without
losing the ability to grow on solid media.
b) Something in the solid media inhibits the negative effects of the lpl6 factor.
c) Some other factors induce the expression of OspC.
d) The regulatory mechanisms are different in the two strains.
Sequence comparison between B31 and an OspC expressing strain, 2591, showed
that a 54 bp fragment just upstream of the -35 and -10 consensus promoter sequences
was absent in strain B31. This fragment may be an enhancing element, promoting the
transcription of the ospC gene (Padula et al., 1993). Further sequencing of the 27 kb
circular plasmid showed that the guaA and guaB genes were located upstream of the
ospC gene, transcribed in the opposite direction. Those genes are involved in the
purine synthesis. Mammalian blood has very low purine levels, while one of the major
waste products in ticks is guanine. Therefore, the guaAB operon is probably only
expressed in mammals. This may also influence the expression of ospC (Rosa and
Margolis, 1994), and thereby the expression of the ospAB operon indirectly (see
discussion in section 5.5).
3.5
Pathogenicity
It now appears that variations in symptoms of Lyme disease are related to the group of
Borrelia that caused the infection. Seven patients with ACA all reacted strongly with a
26
skin isolate, now classified as B. afzelii (Wilske et al., 1988a). In another study, an
OspA serotyping system was developed (Wilske et a i, 1993a). Seven different
serotypes were found. Serotype 1 corresponds to B. burgdorferi s. s., serotype 2 to B.
afzelii, and serotypes 3 to 7 to B. garinii. As in the previous study, serotype 2 was
most prevalent among European skin isolates. In the heterogeneous B. garinii group,
serotypes 4 and 5 were only isolated from patients, while serotype 6 was the most
frequent isolate found in ticks. Assous et al performed a study where the serological
response of Lyme borreliosis patients was analyzed by Western blot using strains of
the three distinct genomic species. They found that about half of the patients suffering
from meningoradiculitis showed preferential reactivity with B. garinii. All patients with
ACA reacted mainly with B. afzelii, and 50 % of the patients with arthritis reacted
preferentially with B. burgdorferi s. s. (Assous et al., 1993). It was only in the arthritis
group that a strong reaction against the OspA and OspB proteins was found. In another
study, Western blot analysis with patient sera on antigens from the three different
genomic groups showed that patients with meningopolyneuritis react with more
antigens of B. garinii than the other two groups. Patients with acrodermatitis or
arthritis had more antibodies to B. afzelii antigens (Dressier et al., 1994).
Persistent infection of SCID mice with B. turicatae, which normally causes
relapsing fever, gave rise to symptoms that resembled human Lyme borreliosis
(Cadavid et al., 1994). In that study, two different serotypes of B. turicatae were
isolated and further characterized. The only discernable difference between the two
variants, was the size of the Vmp proteins expressed. Interestingely, the mice
developed different symptoms depending on what serotype they were infected with,
suggesting that the outer membrane proteins are indeed important for the clinical
manifestations.
The later stages of infection by B. burgdorferi are characterized by the
persistence of the organisms despite a strong anti-borreliocidal immune response. This
suggests that the spirochetes invade tissues where they are protected. Borrelia may
enter and survive inside both human umbilical vein endothelial (HUVE) cells and
human skin fibroblasts (Ma et al., 1991; Klempner et al., 1993). The importance of the
skin in natural B. burgdorferi infections was established in another study. Excision of
the tick attachment site as long as two days after detachment of fully engorged,
infected I. dammini was shown to totally abolish the infection. These results suggest
that the tick-inoculated spirochetes proliferate where they are deposited, doubling
several times during the first two days. The bacteria then remain close to this site for
another week, until they start disseminating in the skin and to other parts of the body
27
(Shih et a i, 1992). In this study, it is also implied that no immune response is elicited
until the dissemination of spirochetes starts. The saliva of ticks contains anti­
inflammatory agents, that may create a protective environment suitable for proliferation
of the spirochetes and the establishment of an infection (Ribeiro et al., 1985). That
ticks are important for the infection is further emphasized in a study showing that mice
infected by the natural route are substantially more infective for ticks than
experimentally inoculated animals (Gem et al., 1993).
3.6
Adhesion of Lyme disease Borrelia to human mammalian cells
As discussed above, spirochetes can adhere to and invade different cell types. Most
binding studies of B. burgdorferi have been performed with HUVE cells. Specific
adherence was shown to be time and temperature dependent. Pretreatment of the
bacteria with heat, immune serum or monoclonal antibodies directed against the outer
surface proteins results in reduced attachment, indicating that the binding involvs
borrelial surface proteins (Thomas and Comstock, 1989). The spirochetes are not only
capable of specific binding to the eukaryotic cells, but also penetration (Comstock and
Thomas, 1989). The passage occurs mainly through the cytoplasm of the host cell, and
may take place by directional transcytosis or parasite-directed endocytosis.
Reorganization of actin and cytoskeletal filaments around the intracellular
B. burgdorferi was observed by transmission electron microscopy (Comstock and
Thomas, 1991). Further studies have shown that OspA is probably involved in the
adhesion, while the OspB as well as the flagellin are important for the invasion.
Binding of the FAb fragments of one specific monoclonal antibody directed against
OspA reduced the binding by 69 %. FAb fragments of other monoclonal antibodies did
not interfere with the binding (Comstock et al., 1993). B. burgdorferi probably has
more than one adhesin, as total inhibition of the binding could not be accomplished.
This is also suggested by the analysis of an Osp-less mutant totally lacking Osp
proteins. This mutant retained some ability to bind HUVE cells but less than the
wildtype (Sadziene et al., 1994).
An Sh2-82 strain containing a mutant OspB protein showed a greatly diminished
capability of penetrating HUVE cells. It also had reduced infectivity in SCID mice,
indicating that this is an important virulence factor (Sadziene et al., 1993b). Analysis
of a spontaneous non-motile flagellin mutant showed a 95 % reduction in invasion,
displaying that motility is an important factor for spirochetal invasion (Sadziene et a i ,
1991).
28
The spirochetes do not only bind to epithelial cells, but also to glial cells and cells
of glial origin (Garcia-Monco et al., 1989), as well as human platelets (Cobum et al.,
1993). They have been shown to specifically bind to galactocerebroside (GarciaMonco et al., 1992).
3.7
Animal models and vaccination studies
Most animal studies of B. burgdorferi have been performed in mice. There are several
reasons for this. Mice constitute the main reservoir for B. burgdorferi in nature, and
persistent infections with Lyme disease symptoms resembling those found in human
patients, can be established. The immune system of mice is very well characterized,
which is helpful in studies of the pathogenesis of Lyme disease. Several inbred mouse
strains with specific déficiences in their immune systems further facilitates these
studies. These immune deficient mice are also very suitable for vaccine studies (Simon
et al., 1991b). Other animals used in experimental B. burgdorferi infections are rats
(Barthold et al., 1988), Syrian hamsters (Johnson et al., 1984a), rabbits (Komblatt et
al., 1984), dogs (Appel et al., 1993), Rhesus monkeys (Philipp et al., 1993) and
Guinea pigs (Sonnesyn et al., 1993).
Most of the vaccination efforts have focused on OspA. Both native OspA from
B. burgdorferi ZS7 and B31, as well as recombinant ZS7 OspA, can generate
hyperimmune sera in immunocompetent mice. Transfer of immune sera or antibodies
specific for OspA to severe combined immunodeficient (SCID) mice, totally protects
from challenge with strain ZS7 ( Schaible et al., 1990; Simon et al., 1991). In another
study, both passive and active immunization with recombinant OspA from
B. burgdorferi N40 conferred protection upon subsequent challenge with N40 in
immunocompetent C3H mice (Fikrig et al., 1990). The active immunization with
purified recombinant N40 OspA also conferred long-lasting protective immunity. All
challenged animals were still immune 150 days after the last boost (Fikrig et al.,
1992a). The lipid moiety is very important for eliciting a strong immune response. In
experiments using both lipidated and de-lipidated OspA, it was shown that the lipid
moiety was needed to activate the immune system, while the antibodies produced were
directed against the peptide moiety (Erdile et al., 1993). 100 to 1000 fold increases in
protective antibody levels were found when the lipidated OspA was fused to the
Mycobacterium bovis strain Bacille Calmette-Guerin (BCG) and expressed as a
membrane associated lipoprotein (Stover et al., 1993). Not only OspA, but also OspB,
OspC and a 39 kD protein, P39, can induce protection by active immunization
29
(Schmitz et al., 1991; Fikrig et al., 1992b; Probert and LeFebvre, 1994; Norton
Hughes et al., 1993). Moreover, borreliae within ticks which engorge on mice
immunized with OspE and OspF get partially destroyed (Nguyen et al., 1994).
To be protective, it is necessary that the vaccination is performed before or at the
time of initiation of infection. Immune sera administered before or at the time of
inoculation can protect against the challenge, but not sera administered as little as 24 h
after the start of infection (Barthold and Bockenstedt, 1993). Active immunization with
OspA after an infection has been established cannot eliminate the infection, but
appears to accelerate the resolution phase of the disease (Fikrig et al., 1993). A recent
study showed that more severe arthritis was induced in hamsters vaccinated with
whole-cell preparations of formalin-inactivated spirochetes than in unvaccinated
animals if the challenge was made before sufficient levels of borreliacidal antibodies
had developed. Even after high levels of specific antibodies could be detected,
challenge with heterogeneous strains induced severe arthritis (Lim et al., 1994).
There are also problems in using only the OspA protein as a vaccine, for example
the heterogeneity displayed in different strains (Marconi et al., 1993a; see also section
3.3). Vaccination with the OspA protein from one strain only confers immunity to
homologous strains. This is a problem especially in Europe. To circumvent this
problem, chimeric proteins containing conserved regions from the OspA of different
genomic groups could be used. Another possibility is to construct recombinant proteins
consisting of parts from several different immunogenic proteins. Trials of this kind
have been initiated (Luft et al., 1994).
So far, the Osp proteins, and in particular the OspA protein, is the best vaccine
candidate. The importance of the Osp proteins in the virulence of B. burgdorferi s. /. is
clear. Therefore, a molecular and structural characterization of the Osp proteins will be
beneficial to our understanding of the pathogenicity of this spirochete. More
knowledge will aid in the development of better diagnostic and therapeutic methods. It
may also be useful in the development of a reliable human vaccine against Lyme
disease.
AIMS OF THE PRESENT WORK
This thesis has examined the outer surface proteins of the Lyme disease Borrelia. The
aims have been to understand the heterogeneity of Osp proteins between different
strains, the regulation of Osp expression and their role in the pathogenesis of this
spirochete.
33
SUMMARY AND DISCUSSION OF RESULTS
4.
Heterogeneity of the osp operon (Paper I)
4.1
Prologue
When I started my work, the osp operon from the North American B. burgdorferi
strain B31 had been cloned and sequenced (Howe et al., 1985; Bergström et al.,
1989). The first part of my work was to continue the examination of the Osp proteins
and their genes in other Borrelia burgdorferi strains. The primary objective was to
look for similarities and differences in distinct isolates, in order to gain insight into the
pathogenesis of this organism. In addition to B31, the strains AC AI and Ip90 were
chosen for this study. ACAI is a Swedish strain isolated from a patient with
acrodermatitis chronicum migrans (Åsbrink et al., 1984b). Ip90, like B31, is a tick
isolate, but which originates from the Khabarovsk territory in the Asian part of Russia,
on the border with China (Kryuchechnikov et al., 1988). These strains were chosen
because the bacteria originated in geographically distinct regions, and were isolated
from different sources. When total protein extracts from the three strains B31, ACAI
and Ip90, were separated by SDS-PAGE, the various strains all had different protein
patterns. Different reactivity of the strains to monoclonal antibodies raised against the
Osp proteins of strain B31 demonstrated that the Osp proteins differ antigenically
between the strains.
4.2
Cloning of the osp operon of ACAI and Ip90
A X phage gene library was constructed by EcoRI digestion of genomic ACAI DNA.
The ACAI osp operon was cloned from this library by probing with labelled
oligonucleotides synthesized from the B31 ospAB DNA sequence. A positive clone
containing the entire ospAB operon was isolated, and subcloned into pUC18.
Two different pUC18 plasmid gene libraries were constructed from genomic Ip90
DNA. From the partial Hindlll digested gene library, a clone containing the entire osp
operon except for the beginning of ospA and the control region was obtained using an
ospA PCR fragment as probe. Clones containing the missing regions were obtained by
screening an EcoRI gene library with an oligonucleotide homologous to the beginning
of ospA.
34
Most of the sequence data was obtained by sequencing deletion libraries made on
the ACAI clone and the shorter of the Ip90 clones. To obtain the complete sequences,
internal oligonucleotides were used as primers.
4.3
Genetic organization
The ospA and ospB genes are organized in one operon, separated by 9 bp. ospA is
located at the 5’ end. They are transcribed as one transcriptional unit. The
transcriptional start site was shown for B31 to be at position +1 in Figure 3, Paper I.
Based on sequence homologies, the same transcriptional start point was assumed for
the two other strains. This has subsequently been confirmed for ACAI (Paper II).
Southern blot analysis showed that the osp operon is located on a 49 kb linear
plasmid, lp49, in B31. In AC AI and Ip90, the operon hybridizes to homologous
plasmids, slightly larger in size (Figure 4, Paper I). The overall plasmid content of the
three strains is very different. In B31, only the linear plasmids lpl 6, lp29 and lp49, are
observed under the conditions used in the pulsed field gel electrophoresis. In both
ACAI and Ip90, several additional plasmids of varying sizes are detected. The only
plasmid that is common to all three strains is the one harbouring the osp operon, even
if the size of it varies slightly between the strains.
4.4
Sequence comparison
The only available sequences at the time were the B31 osp operon, and the ospA
sequences from B. burgdorferi strains N40, isolated from a tick in the USA (Fikrig et
al., 1990), and ZS7, a German tick isolate (Wallich et al., 1989). Comparison of the
ospA and ospB DNA sequences and the deduced amino acid sequences gives the
percentage of identity shown in Table 1 and Table 2. The previously sequenced ospA
genes from B31, ZS7 and N40 are almost identical, there are only single base pair
exchanges between the strains. The osp sequences from ACAI and Ip90 show that the
Osp proteins are indeed heterogeneous, which is in agreement with earlier
immunochemical findings (Wilske et al., 1988b). As can be seen in Figure 3, Paper I,
the ospA genes are very conserved in the 5’ region of the gene. The deduced amino
acid sequences are compared in Figure 5, Paper I. The N-terminal end of OspA is
totally identical in all of the strains studied. This indicates that the beginning of OspA
is functionally and/or structurally very important. This conservation of the N-terminal
region is not found in OspB, where differences occur even in the leader sequence.
35
Table 1. Percentage of the identity of the asp/l/OspA sequences
between the strains
N40
Ip90
ZS7
ACAI
99/99
>99/99
86/19
B31
&5777b
86/ 81
85/11
85/11
ACAI
86/19
86/19
Ip90
99/99
ZS7
-
-
a) nucleotide sequence identity
b) amino acid sequence identity
Table 2. Percentage of the identity of the
B31
ACAI
ACAI
79766h
-
Ip90
79/67
$7/68
a) nucleotide sequence identity
b) amino acid sequence identity
In Figure 3, a cladiogram of the DNA sequences is shown. Phylogenetically, the ospA
and ospB genes first diverge into two branches. Within each branch, the strains can be
placed into three groups. N40 and ZS7 belong to the B31 group. The ACAI and Ip90
strains are slightly more related to each other than to the B31 group. Attempts to
systemize different strains of B. burgdorferi based on other criteria have been made
previously (Postic et a i, 1990; Adam et al., 1991; Rosa et al., 1991), and those results
are quite consistent with the findings presented here. As mentioned in the introduction,
B. burgdorferi has now been split into three genomic species, i. e. B. burgdorferi s. s.,
B. afzelii and B. garinii (Baranton et al., 1992; Canica et al., 1993). ACAI has been
placed into the B. afzelii group, Ip90 into the B. garinii group, and B31, N40 and ZS7
are now classified as B. burgdorferi s. s.
When comparing the OspA and OspB proteins to each other, more than 50 %
similarity is observed (Bergström et a i, 1989). This indicates that the osp operon has
been formed by a duplication of a primitive osp gene. Because of the high similarity,
36
- ZS7-ospA
N40-ospA
41
B31-ospA
157
30
ACAI-ospA
32
Ip90-ospA
23
37
ACAI-ospB
36
Ip90-ospB
42
39
B31-ospB
Figure 3. A phylogenetic tree made from the ospA and ospB sequences of strains B31, AC AI and
Ip90, and the ospA sequences from strains N40 and ZS7. Parsimony analysis was performed using
the PAUP program for the Macintosh, version 3.0s (Swofford, 1990). The numbers indicate the
number of nucleotides diverging between the compared subjects.
37
this is probably quite a recent evolutionary event. The cladiogram in Figure 3 clearly
shows that this duplication must have taken place before the strains started to diverge
into different species.
As all the ospA DNA sequences published at the time came from strains
belonging to B. burgdorferi s. s., it was speculated that ospA was very conserved at
the DNA level although Coomassie stained protein gels and Western blot data
indicated large variations both in size and immunoreactivity (Fikrig et a i , 1990;
Barbour et a i, 1985; Wilske et al., 1986,1988). The results herein show that variation
occurs at the DNA level too. This has implications for the use of the osp genes or
proteins in diagnostic methods as well as in vaccinations. PCR is becoming a very
popular method to use in diagnostics. If the osp genes are to be used as target genes, it
is important to find regions that are highly conserved between the strains. The most
conserved regions are definitely found in the control region and in the beginning of
ospA. Conserved regions suitable for the second primer can also be found further
downstream in both the ospA and ospB genes, although these stretches with homology
are not as long as those previously mentioned.
Using the Osp proteins as vaccines will probably require a mixed vaccine, using
Osp proteins, preferrable OspA, from the different genomic species in order to be
universally protective. In the USA, most strains isolated belong to B. burgdorferi s. s,
and it may therefore be possible to use a simpler form of vaccine than in Europe where
all three genomic species are present.
5.
Expression of the osp operon (Paper II)
5.1
Why clone the osp operon from B. afzelii FI?
One approach to the examination of the regulation of proteins is to compare systems
with different expression patterns. The Osp proteins are differentially expressed in the
two B. afzelii strains ACAI and FI. ACAI expresses both OspA and OspB proteins,
and the ACAI ospAB operon was characterized in Paper I. FI is a Swedish tick isolate,
previously reported to lack the expression of OspA and OspB (Barbour and Schrumpf,
1986). Instead, it expresses the smaller OspC protein in large amounts, as seen on
Coomassie stained protein gels. Studying the ospAB operon from strain FI in more
detail could give information concerning the regulation of expression of Osp proteins in
Lyme disease Borrelia.
38
5.2
Characterization of protein expression by Western blot analysis
To analyse the amount of and structure of the OspA and OspB proteins we have
employed Western blot analysis. This required a panel of antisera directed against the
Osp proteins. We had access to several monoclonal antibodies raised against
B. burgdorferi s. s. antigens. However, these reagents, except for the OspB MAb 84C
(see Paper IV), have a low affinity for the Osp proteins of B. afzelii. Therefore, a
specific rabbit polyclonal anti-OspA sera was produced by immunizing a rabbit with
purified, recombinant OspA protein cloned from the B. afzelii strain ACAI.
Whole cell extracts from strains B31, ACAI and FI were separated on a 12.5 %
SDS-PAGE. After staining with Coomassie brilliant blue, major OspA and OspB
proteins were detected in B31 and ACAI. In FI, a major protein of approximately 25
kD in size is observed. No major OspA and OspB proteins are detected in FI, but
there are some faint bands in the size range of the Osp proteins of ACAI.
In the first Western blot experiments, the anti-OspA MAb 184.1 (Jiang et al.,
1990) and the anti-OspB specific MAb 84C (Comstock et al., 1993) were used. Both
antibodies reacted with the OspA and OspB proteins of B31 and ACAI. Surprisingly,
the 84C antibody reacted with a protein of the size of OspB in strain FI as well. The
OspA antibody gave no signal with strain FI. This indicated that the FI strain
expresses the OspB protein in low amounts but not the OspA protein. Using the rabbit
anti-OspA polyclonal sera, a signal was obtained from the FI strain. Compared to the
OspA signal from ACAI, less OspA protein is found in strain FI, but it varies in
different preparations. These results show that the FI strain is capable of expressing
the ospAB operon in variable amounts.
The differences in the amount in Osp proteins found could be due not only to
differences in expression, but also in protein stability. Therefore, a pulse chase
expreiment was performed that showed that the OspA proteins in both ACAI and FI
are very stable. Therefore, the differences in the protein amount is not due to an
alteration of the turnover rate.
5.3
Genetic organization
The DNA content of strains B31, ACAI and FI is depicted in Figure 4a. Genomic
DNA was separated in a 1.2 % agarose gel by pulse field gel electrophoresis using the
same parameters as described in Paper I. The DNA is visualized by ethidium bromide
staining. The plasmid profiles of the two B. afzelii strains differ although they belong to
the same species. Both strains have the 50 kb linear plasmid harbouring the ospAB
39
operon. DNA homologous to ospA in Fl was identified by Southern blot of the pulse
field gel (Figure 4b).
co O
CD <
co O
CD <
- 29 -
- 16 -
Figure 4. Separation o f genomic DNA from
B. burgdorferi B 31 and B. afzelii strains AC AI
and FI by pulsed field gel electrophoresis.
A. DNA is visualized by ethidium bromide
staining.
B. Southern blot analysis o f the osp opérons
using an oligonucleotide specific for ospA
The ospAB operon from FI was cloned from plasmid libraries as described in Paper II
and sequenced using internal primers. Comparison of the DNA sequence with the osp
operon from ACAI, demonstrated that they are almost 100 % identical. Within the
coding region there are only two base pair substitutions. In the beginning of ospA there
is an A to G transition in position 264 giving a glycine instead of a glutamate. At
position 1419, an A is substituted for a C in strain FI, resulting in a leucine instead of
isoleucine in the end of OspB. As can be seen in Figure 6, Paper II, there is also one
difference in the highly conserved region 5’ of the ospA start. Upstream of the -35
region, ACAI has a stretch of 10 T:s. FI has only 9 T:s, and B31 8 T:s in this area. It is
possible that this region is involved in the regulation of expression of the Osp proteins
in Lyme disease Borrelia. However, this region does not influence the expression of
cloned ospAB genes in E. coli, as can be seen in Figure 10, Paper II.
Figure 5. Schematic model of the regulation of the Osp proteins
40
41
5.4
Transcription
Transcription of the ospAB operon was assessed by Northern blot analysis. In order to
compare different preparations, the amount of ospAB transcript was normalized to the
amount of flagellin transcript. As can be seen in Figure 8, Paper II, the relative amount
of ospAB transcript obtained from different cultures of FI varies dramatically. The
highest level of ospAB mRNA was found in Flb3. This culture had been stored at 4 °C
for some time before the mRNA was prepared. Thus, the transcription of ospAB may
be induced by low temperatures, or by the spirochetes entering the stationary phase. It
is also possible that the ospAB transcript is particularly stable, and is thereby enriched
in the stationary phase. Hardly any ospC mRNA could be detected in this FI culture.
The relative amounts of ospAB transcript together with the amount of OspA,
OspB and OspC proteins detected in ACAI and different cultures of FI are shown in
Figure 4, Paper II Interestingly, the correlation is not perfect between the amount of
transcript and protein observed. Relatively high levels of ospAB transcript are observed
in Flb2, but hardly any OspA and OspB protein was detected. More OspA protein is
found in the other two FI cultures, where the level of ospAB transcript is lower. The
presence of OspA in the culture F iel, which lacks detectable amounts of transcript,
could be explained by the stability of the OspA protein. Nothing is known about the
rate of protein synthesis in Borrelia. It is possible that there is a delay between
transcription and translation, and this results in the low levels of OspA compared to
transcript seen in preparations from culture Flb2. Another possibility could be that the
expression is not only regulated at the level of transcription, but also at the translational
level.
5.5
Model for Osp regulation
A schematic representation of the regulation of Osp expression is shown in Figure 5.
The Lyme disease Borrelia must be adaptable to differences in their surroundings, as
they move between ticks and mammals in their life cycle. Mammalian blood is very
low in purines, while guanine is one of the primary waste products in ticks. Upstream
of the ospC. gene on the 26 kb circular plasmid, the guaA and guaB genes have been
found. These genes, involved in purine biosynthesis, are expressed when the bacteria is
present in mammals but not in ticks (Rosa and Margolis, 1994). OspC cannot be
detected on the surface of spirochetes in unfed ticks, but is expressed by spirochetes in
engorged ticks. Temperature also appears to influence OspC expression (Schwan,
1994). It is possible that factors in the ingested warm mammalian blood signal the
42
activation of expression from the 26 kb plasmid. The presence of a 16 kb linear
plasmid has been reported to correlate with OspC expression (Hinnebusch and
Barbour, 1991). This would suggest that a factor is synthesized from this plasmid that
may function as a negative regulator of OspC, and possibly also GuaA and GuaB,
expression. High levels of guanine is probably sensed by the guaAB operon, either
directly or via some sensor molecule. The ospC and guaAB genes are transcribed in
opposite directions from separate promoters, but it is possible that activation of the
guaAB promoter also influences ospC transcriptional activity by conformational
changes that facilitate binding to the ospC promoter.
In contrast to OspC, OspA is present on the surface of spirochetes in unfed ticks
(Schwan, 1994), but is probably not expressed in the mammalian stage as antibodies
against OspA are usually absent in sera from Lyme disease patients (Wilske et al.,
1986, 1988a). In Paper II, the inverse relationship between expressed OspA and OspB
contra OspC exists both at the protein and RNA level, see Figures 5 and 9. Therefore,
it is possible that OspAB expression is somehow coupled to the transcription of the
26 kb plasmid. When the 26 kb plasmid is inactive, expression of the ospAB operon is
fully induced. Activation of the 26 kb plasmid may produce some kind of repressor for
the ospAB operon. This factor could be either a protein or an RNA molecule with anti­
sense RNA properties, synthesized from this circular plasmid.
6.
Molecular characterization of the OspA and OspB proteins during
infection in vivo (Paper III)
It is well documented that B. burgdorferi can cause chronic infections in both humans
and other mammals. To be able to survive inside the host, the spirochetes must avoid
the immune system in some way. As the closely related B. hermsii use antigenic
variation of the vmp genes to achieve this (Stoenner et al., 1982), it has been
speculated that B. burgdorferi may use some kind of variation of the Osp proteins to
survive. Both the ospA and ospB genes are composed of small, tandem repeats, as can
be seen in Figure 3, Paper I. PCR sequencing of the osp genes from B. burgdorferi
Sh2-82 has shown that antigenic variation occurs during growth in vitro. The
mechanism for this variation has been proposed to be recombination between the
repeats, either intra- or intergenetically. This is possible due to the high homology
between the ospA and ospB genes. The absolute copy number of the plasmid
harbouring the osp operon has not been determined, but it is probable that more than
43
one copy exists per cell. This would allow for recombination between the plasmids
(Rosa et al., 1992). To determine whether this is the mechanism used in vivo, we
decided to study the ospA and ospB genes in B. burgdorferi at the molecular level
during an ongoing infection.
6.1
Methodology
To perform these studies, we needed an animal model where infection could be
established, and sampling once a week could be done in a convenient way. We chose
to use mice as they are easy to handle and can be infected by B. burgdorferi. The
strain B. burgdorferi Sh2-82 was chosen since it had previously been shown to be
infective for mice (Schwan et al., 1988) and in vitro data suggested that antigenic
variation could occur in this strain (Rosa et al., 1992) To be able to take sequential
comparative samples over several weeks, we decided to use a tissue cage model
previously successfully used to study streptococcal infections (Holm et al., 1978; Knöll
et al., 1985). Small, cylindrical net cages were placed in subcutaneous pouches on the
backs of anaesthesized mice. Infection, and then sampling was performed as described
in Paper V. Addition of NaCl or PBS to the cages to replace the volume taken out at
sampling, prolonged the time before the cages became dried out. This also made it
easier to take out sufficient fluid at each sampling. The presence of spirochetes was
demonstrated by the cultivation of chamber fluid.
6.2
Results of cultivation
In mice numbers 1 to 18, an Sh2-82 culture, cloned by limiting dilution and passed
more than 10 times in vitro, was used, while a low-passage, non-clonal culture of Sh282 was used in mice numbers 19 to 36. The DNA pattern of the inoculated strains was
found to be identical by PCR amplification, indicating that they were monocultures, at
least with respect to the osp genes. It is a well known fact that B. burgdorferi lose their
infectivity during prolonged cultivation in medium. This is probably because factors
required for infectivity, but not for in vitro growth, are lost during the cultivation. As
can be seen in Table I, Paper V, the low passage strain was much more infectious than
the strain passaged for a longer period of time in vitro. In the first group, only the
highest dosage of spirochetes used, 106, could induce an infection, and only in the
C3H/Tif mice (No 16 and 18). When the low-passage strain was used, 100 fold less
spirochetes were needed to establish an infection, and it was possible to cultivate
44
spirochetes not only from C3H/Tif, but also from the BalbC/cJ mice. Furthermore, in
this group, it was easier to establish a long-lasting infection in the C3H/Tif mice.
The growth pattern found in Table 1, Paper V, resembles the variation expected
from an organism surviving by antigenic variation: Positive culture indicating iarge”
amounts of spirochetes in the circulation, followed by negative cultures resulting from
the removal of spirochetes by the immune system. Positive cultures later on indicate
that a new serotype, not recognized by the immune system, has arised, and so on.
6.3
Cloning and sequencing of the ospA and ospB genes
The results of the cultivation experiment fit with the antigenic variation found in vitro
by Rosa and coworkers (Rosa et al., 1992). To verify that this was responsible for the
fluctuations, we PCR amplified the ospA and ospB genes from the obtained cultures.
No difference in the size of amplified DNA fragments from early or late cultures
compared to the original strains could be detected. To study if recombination within
the genes, giving rise to new, antigenically different Osp proteins, had taken place, the
fragments were cloned into pET7-vectors and sequenced. Surprisingly, no DNA
rearrangements at all could be detected, implying that antigenic variation of the Osp
proteins is not a mechanism used to survive inside an immunocompetent host.
6.4
Immune response
To examine whether the Borrelia infections induced any immune response in the mice,
and if there was any difference in mice positive or negative in culture, we performed
ELISA with the mouse sera on whole cell lysates of Sh2-82. Recombinant B31 OspA
was also used as antigen to test the immune response against the Osp proteins. The osp
opérons of B31 and Sh2-82 are almost identical, and polyclonal antibodies to OspA
should react with both strains. Figure 1 Paper III show the mean values of the ELISA
results obtained. As can be seen, the most pronounced responses were found in mice
where infection had been initiated as indicated by a positive culture. This shows that
persistence of the infection is not because these mice were unable to mount an immune
response. Similarly, the lack of detectable infection in mice No 1-15 and No 17 is not
because these mice responded with a stronger immune defence. The lack of infection in
these mice is probably because not enough of infectious bacteria were injected. That a
relevant immune response was raised in the infected mice was further established by
the observation that several of the C3H/Tif mice had swollen joints 10 weeks after the
45
onset of infection. Only two mice, numbers 9 and 18, raised an immune response
towards OspA that was above the cut off value 0.2. These mice were both injected
with 106 spirochetes. In natural infections, immune responses to Osp proteins are rarely
found (Wilske et al., 1986, 1988a). But when higher amounts of spirochetes are
injected, fast responses to OspA and OspB are seen (Gem et a i , 1993). The data in
Paper V indicate that 106 spirochetes is a borderline dose to elicit an OspA response.
7.
Mapping of anti-OspB antibody epitopes
7.1
Method of selecting mutants
To date, there are no genetic methods available for creating mutants in Borrelia. It is
not possible to introduce new DNA into the spirochetes since no vector system and no
method of transformation has been developed. The situation is further complicated by
the fact that the modified BSKII medium used for cultivation is very complex and
contains, among other things, rabbit semm and can therefore not be considered
defined. This makes it very difficult to screen for mutants in the ordinary way, by
plating on selective plates.
An alternative method for the generation of mutants has been developed by
Sadziene et al (1992a). Antibody resistant mutants are isolated under selective
pressure from anti Borrelia antibodies in complete BSKII medium. After incubation at
34°C, surviving borrelia are cloned by plating with low melting point agarose on solid
BSKII plates. By comparing the DNA sequences from such mutants with the wild type
sequence, it is possible to get an idea of the binding epitopes of the antibodies used. In
combination with functional studies of the mutants, it is possible to characterize the
studied proteins.
7.2
Cloning and sequencing
The ospB genes from wild type and antibody resistant mutants were amplified by PCR
and cloned into TA-vectors as described in Paper III and IV. Internal, as well as
plasmid-encoded, primers were used for sequencing. Normally, sequencing was
performed from several clones isolated from separate PCR rounds, to ensure that base
pair exchanges were true mutations, and not artefacts due to the PCR procedure.
Occasionally, the sequences of the variable regions were confirmed by sequencing of
asymmetrical PCR products. The results of the sequencing can be seen in Table 3 and
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Figure 6. The DNA sequence of the ospB gene of B . burgdorferi B31 is shown in capital letters below the amino acid sequence in italics.
The ribosomal binding site (RBS) is underlined, and stop codons are indicated by three stars The mutations in the different mutants are indicated
in bold.
46
47
Table 3. Description of the antibodies and genotypes
Selecting
Mutant
base-pair
antibody
exchange
B31-2
H6831
1738 A-»G
B31-3,4
H6831
1570 G-»A
1802 C-»A
H6831
B31-7
1738 A—»G
H614
1799 C-»T
B31-8
B31-9
3A3
1153 T-»C
B31-10
3D3
1331 C-»T
1769 A-»G
B31-11
84C
B31-12
84C
1730 C-»T
HB191*
1697 G-»A
Polyclonal
amino acid
exchange
253 Lys—»Glu
197 V algile
274 Ala—»Asp
253 Lys—»Glu
273 Thr-»Ile
58 Ser-»Pro
117 Ala-» Val
263 Asp-»Gly
250 Ala-» Val
239 Trp—»Stop
* The H B 191 sequence was compared to HB19 wild type sequence, not to the
B31 sequence
Figure 6. As can be seen in Figure 6, most of the mutations have occurred in the Cterminal end of the protein. The only exceptions are mutants B31-9 and B31-10, which
have mutations in amino acids 58 and 117 respectively. They are selected for by the
monoclonal antibodies 3A3 and 3D3. Mutants derived from selection with Mab 84C
are further described in Paper IV, and Mab H6831 mutants are described in Paper V.
Analysis of the predicted secondary structures of the mutants revealed that the
differences in amino acids 126 and 128, where B31 has an alanine and an aspartate,
while UBI9 has a threonine and an asparagine probably confers a slight difference in
the secondary structure on the C-terminal side of this area, around amino acid 150.
Because of the truncation of the OspB protein in mutant HB191, the secondary
structure in the C-terminal end differs from the wild type HB19 OspB sequence.
Among the B31 OspB mutants, it is only in mutants B31-3 and B31-4, together
with mutant 84C’, that the amino acid exchanges appears to affect the secondary
structure. Therefore, all the other mutants apparently have altered amino acids directly
involved in the binding epitopes of the respective antibodies. B31-3 and B31-4 are the
only mutants where more than one nucleotide has been changed. Therefore, it is likely
that the epitopes of the selecting antibodies are very precise, and very sensitive to even
slight changes in the primary structure. Mutant B31-8, generated by selection with
Mab H614, is mutated next to one of the amino acids that is changed in the mutants
48
B31-3 and B31-4. The other amino acid which is changed in this double mutant, is
situated next to amino acid 198, which is changed in strains Rpa and HB19 compared
to the wild type B31 OspB sequence. The mutations in mutants 84C’ and B31-2 and
B31-7 are separated by only two amino acids. Thus, the OspB protein contains regions
that are apparently more easily mutated than other parts. It is probable that these areas
are exposed on the surface of the bacteria, as they are targets for several different
antibodies. It is possible that mutations in other parts of the OspB protein also occurs,
but that these mutants are not allowed to grow.
7.3
Mapping of the binding epitope of highly cross-reactive OspB MAb 84C
(Paper IV)
As was described in Paper I, the OspB protein is particularily heterogeneous among
different isolates. In Paper III, an OspB monoclonal antibody, MAb 84C, is described,
which, despite the diversity, recognizes a wide variety of B. burgdorferi isolates. Of 38
strains tested by Western blot analysis, 32 reacted with MAb 84C. Strains reactive to
this monoclonal antibody belong to all three genomic species, /. e. B. burgdorferi s. s.,
B. garinii, and B. afzelii. The isolates came from North America, Europe, Russia and
Japan, and were isolated from ticks, humans and other mammals. This shows that the
epitope of MAb 84C is very conserved, and indicates that it may be functionally
important, although binding of MAb 84C does not interfere with Borrelia attachment
or the invasion of HU VE cells. MAb 84C is specific for Lyme disease borrelias, and
does not react with other Borrelia species , like B. hermsii.
The epitope for MAb 84C is probably surface exposed, as it causes agglutination
when mixed with spirochetes. It was also possible to isolate escape variants, which
further implies that the recognition site is exposed on the outside of viable bacteria.
Deletion mutagenesis of recombinant B31 OspB mapped the MAb 84C epitope to
the carboxy-terminal end of the protein, between amino acids 230 and 273. To further
map the binding site, MAb 84C escape variants were selected as described above.
Two different mutants, 84C and 84C’, were isolated. Mutant 84C’ still reacts with
MAb 84C in Western blot analysis, while mutant 84C had obviously lost the ability to
interact with the antibody. When sequenced, both mutants had a single base pair
exchange. Aspartate-263 was substituted for a glycine in mutant 84C, and alanin-250
was exchanged for a valine in 84C\ As can be seen in Figure 6, Paper III, the 250
Ala->Val substitution confers a predicted disruption of the secondary structure, which
probably hides the MAb 84C epitope from exposure on the surface, allowing the 84C’
49
mutant to survive. Amino acid 263 is situated within the area defined by the deletion
studies. When the amino acid sequences between positions 230 and 273 from B31,
ACAI, Ip90 and the two mutants were aligned, amino acids 259 to 269 are totally
conserved, except in the non-binding 84C mutant. Therefore, it is suggested that the
MAb 84C epitope is situated within this area, and that amino acid 263 is particularily
important. The epitope is probably linear, as neither dénaturation nor fixation
apparently affect the binding. Furthermore, the wild type predicted secondary structure
is not changed by the amino acid substitution in the 84C mutant.
7.4
Characterization of the anti-OspB bactericidal antibody H6831
(Paper V)
7.4.1 Borrelicidal activity
It is well known that specific antibodies in the presence of complement or phagocytes
can kill bacteria. Antibodies with inherent inhibitory or bactericidal activity are less
common. In an previous study (Sadziene et al., 1992b), Fab fragments from the OspB
MAb H6831 were shown to inhibit growth of B. burgdorferi in vitro, in the absence of
complement. In Paper IV, this activity of H6831 was further characterized.
To establish whether H6831 simply inhibits growth,or if it has true bactericidal
activity as well, a time-kill study was performed. The MAb H4825, specific for
serotype C of B. hermsii was used as a negative control in all experiments. This
antibody has previously been shown to inhibit the growth of B. hermsii HS1 at a 10
fold lower concentration than H6831 is effective on strain B31. MIC, the minimal
inhibitory concentration, was 0.2 and 2.0 pg/ml, respectively (Sadziene et al., 1992b).
In this time-kill study, B31 was mixed with 20 jug/ml of either H6831 or H4825, and
samples were plated at different time intervals to measure the concentration of live
spirochetes. As can be seen in Figure 1A, Paper IV, there is a rapid decline in viable
B31 when exposed to H6831, but no effect at all could be seen when B31 was
incubated with the B. hermsii antibody H4825. In Figure 2, Paper IV, electron
photomicrographs depict the fate of the spirochetes. The bactericidal antibodies
apparently disrupt the target cells, and numerous blebs are formed.
The MBC is the minimal bactericidal concentration needed for killing 99.9 % of
the inoculum. To determine the MBC of H6831, bacteria were incubated with different
concentrations of antibody for 2 h, and then plated as described above. Figure IB,
50
Paper IV, shows that the MBC of H6831 for B. burgdorferi B31 is 10 pg/ml, /'. e. 5
times higher than the MIC.
7.4.2 Epitope mapping
The extraordinary effect of H6831 on the viability of the spirochetes suggests that its
epitope on the OspB protein may be important. In an previous study (Barbour and
Schrumpf, 1986), it was shown that H6831 bound only to some B. burgdorferi s. s.
strains, and not at all to strains from the other genomic species. In that study, H6831
was shown to bind to B31 and RPa, a human skin isolate from New York, but not to
the North American blood isolate HB19. In a first attempt to delimit the binding
epitope, ospB sequences from binding and nonbinding strains were compared. The
ospB gene of HB19 was cloned from a phage library and sequenced. The RPa ospB
was PCR amplified, cloned and sequenced as described above. These sequences were
also compared to the non-binding sequences of strains N40 and Sh2-82. All sequences
were >99 % identical, but strains binding to H6831 had a lysine in position 253, while
non-binding strains had a threonine in this position.
To further map the epitope, four H6831 B31 OspB mutants, which had all lost the
ability to bind the antibody, were isolated, and the ospB genes were cloned and
sequenced. The results can be seen in Table 4 and Figure 6. Mutants B31-3 and B31-4
had two missense mutations, resulting in amino acid exchanges in positions 197 and
274. These mutations may interfere with the secondary structure of the epitope. The
other two mutants, B31-2 and B31-7, had substituted the lysine at position 253 with a
glutamate.
To define the role of the linear epitope around amino acid 253, a synthetic 21-mer
peptide covering this area was synthesized. Sera raised from this peptide could block
the binding of H6831, showing the importance of this area for the binding of H6831.
The mechanism of the bactericidal activity of the antibodies is not known. By
electron microscopy, the effect of H6831 on B31 cells resembles the action of
penicillin. Penicillin causes lysis of actively growing cells binding to special penicillin
binding proteins in the cell wall (Boyd and Hoerl, 1986). Binding of H6831 probably
does not interfere with the cell wall in the same way as the antibiotic, but it is possible
that binding of H6831 may interfere with the anchoring of OspB in the membrane.
Detachment of OspB may introduce holes that cause cell lysis.
51
8.
Binding studies (unpublished results)
8.1
Binding of spirochetes to glycolipids
B. burgdorferi s. /., like many other microorganisms, binds specifically to certain cell
types. Bacterial surface lectins recognizing glycolipid and glycoprotein receptors
present on the surface of host cells are known determinants of this cell-specificity
(Strömberg et al., 1992). To investigate the involvement of carbohydrate receptors also
in B. burgdorferi adherence, reference glycolipids separated on thin-layer plates were
tested for their ability to mediate binding of 35S-metabolically-labelled B. afzelii strain
ACAI (Strömberg et al., 1990).
1
2
3
4
5
6
7
8
9
2
3
4
5
6
7
8
9
1
■ ■
• •
1
À,
jjp
ilir illf:
im
ÆÈL
.'
A
J fc l
[
!
B
Figure 7. Binding of radioactively labelled B. afzelii to glycolipids separated on thin-layer
chromatograms. Nine lanes are shown after spray detection with anisaldehyde (A) and after
autoradiography following overlay with labeled B. afzelii. Lanes: 1 through 6, non-acidic glycolipids
(20-40 ug) isolated from human erythrocytes (lane 1), monkey intestine (lane 2), dog intestine (lane
3), guinea pig intestine (lane 4), mouse faeces (lane 5) and rat faeces (lane 6); lanes 7 to 8, acidic
glycolipids (gangliosides) isolated from calf brain (lane 7) and human erythrocytes (lane 8); lane 9,
non-acidic glycolipids from a japanese sea urchin.
The results are shown in Figure 7 and Table 4. Selective binding to some but not all
glycolipids was observed (see Table 4). Lactosylceramide (number 3 in Table 4) was
the smallest receptor-active glycolipid. Certain terminal substitutions, such as Gala 1-3
(number 4) and GalNAcßl-4 (number 8) were tolerated, while others seemed to block
receptor activity.
52
Table 4. Binding of Borrelia afzelii AC AI to purified and structurally
characterized glycolipids.
Number
Glycolipid
1
GalßCer
2
GlcßCer
3
4
5
6
7
8
9
GalßMGlcßCer
Binding
+
Galal -3Galß 1-4GlcßCer
Galal -4Galß 1-4GlcßCer
GalNacß 1-3Gala 1-4Galß 1-4GlcßCer
+
NeuAca2-3Galß 1-4GlcßCer
+
Galß 1-3GalNAcß 1-4Galß 1-4Glcß 1-Cer
Galß 1-3GalNac(NeuAca2-3) ß 1-4Galß 1-4GlcßCer
-
-
Lectin-carbohydrate interactions mediate the specificity of host-parasite interactions as
well as cell-to-cell interactions. The specific interaction between lactosylceramide and
related glycolipids by B. afzelii could be implicated in the association of Borrelia with
brain tissues or in other adherence-related events associated with this organism. The
delineation of a recognition site related to lactose is in line with the previous report of
lactosylceramide being a receptor molecule (Garcia-Monco et al., 1992). A large
number of additional bacteria have previously been reported to interact with
lactosylceramide, which is closely associated with the membrane. In this respect, it
should be noted that the HIV virus have been reported to interact specifically with
another membrane associated glycolipid, galactosylceramide (Strömberg et al., 1992).
It may be speculated that lactosylceramide and galactosylceramide may be involved in
direct interactions with the eucaryotic cellular membrane.
9.
Discussion of the pathogenicity of Lyme disease borreliae and the
involvement of the Osp proteins
The data in Paper III and mouse immunization experiments indicate that the Lyme
disease Borrelia do not survive inside the host by antigenic variation of the OspA and
OspB proteins as was earlier believed (Barthold and Bockenstedt, 1993; Barthold,
1993). Instead, it appears that the spirochetes invade parts of the body that are not
accessible to the immune response. Bacteria can for example invade and survive
53
antibiotic treatments inside human skin fibroblasts (Klempner et al., 1993). From these
immunological cryptic pools, the bacteria can reenter the blood, and from there access
more distant places in the body.
Immunization with OspA, OspB or OspC proteins confers protection against
infection in mice (Probert and LeFebvre, 1994). Another study has shown that only
patient sera containing anti-Osp antibodies could confer resistance to infection in mice
(Fikrig et a i , 1994). Those data all indicate that the Osp proteins are important for the
initiation of the infection. The blocking of OspA with antibodies reduces the binding
while blocking of OspB seems to interfere with the invasion of HUVE cells (Comstock
and Thomas, 1989). Perhaps OspA mediates the binding to lactosylceramide in the
eukaryotic cell membrane (this thesis), and OspB the entry of spirochetes into the cells
where they are protected from the immune defence system. The presence of anti-OspA
and/or anti-OspB antibodies would inhibit these processes, and the spirochetes could
therefore be erradicated.
Not all anti-OspB antibodies protect from invasion. Binding to the highly
conserved 84C epitope in the C-terminal end of the protein does not affect the entry
into HUVE cells (Comstock et a i , 1993; Shoberg et al., 1994). Thus, this region of the
protein is probably not directly interacting with other molecules involved in the
invasion. The high degree of conservation of this region between different strains
indicates that it must be essential for a functional OspB protein, and maybe involved in
the tertiary structure.
In patient sera, antibodies towards OspA and OspB are rarely found, particularily
in the beginning of the infection (Wilske et a i, 1986, 1988a). Recent studies show that
B. burgdorferi transmitted to mice by ticks induce an antibody response to a 39 kD
protein, but not to OspA (Golde et al., 1994). This, together with data presented in
Paper II, indicates that the expression of OspA and OspB can be modulated, and may
be turned off upon entry into the mammalian host. The stability of the OspA, and
probably also the OspB, protein mean that these highly immunogenic proteins not
directly vanish from the surface of the bacteria. They are probably present long enough
to mediate the invasion process. The anti-inflammatory components of the tick saliva
may inhibit the immune response towards the OspA and OspB proteins at this stage of
infection. After the initial entry, the OspA and OspB proteins may no longer be needed
for the subsequent infection, and this is perhaps the reason why antibodies against
these proteins are not found in the sera of Lyme disease patients.
Another explanation is that the primary function of the OspA and OspB proteins
are not within the mammalian host but in the tick, possibly for the transport from the
54
midgut and entry into the salivary glands. The reason that anti-Osp antibodies appear
to be protective is then that not all OspA and OspB proteins have disappeared from the
surface of the bacteria upon entering the mammalian host, although the production of
the OspA and OspB proteins may terminate when the ticks are filled with blood.
55
CONCLUSIONS
• The ospA and ospB genes are located on linear plasmids about 50 kb in size, and are
transcribed as one transcriptional unit.
• The ospA genes of the B. burgdorferi strains B31, ACAI and Ip90, are 85 %
identical to each other, and the ospB genes show approximately 80 % identity.
Nucleotide sequence comparisons of the ospAB genes allow the classification of the
strains into three groups that coincide with the recent species designations of
B. burgdorferi sensu stricto, B. afzelii and B. garinii.
• The expression of the Osp proteins is regulated primarily at the transcriptional level,
but may also be adjusted at the translational level.
• The expression of the OspA and OspB proteins is probably shut off in favour of the
OspC protein upon, or soon after, entry into the mammalian host.
• No antigenic variation of the OspA and OspB proteins occurs during infection. The
spirochetes probably evade the immune system by migrating to places within the
body not reached by the immune system.
• The monoclonal antibody 84C recognizes a highly conserved region of the carboxyl
end of the OspB protein. The high degree of conservation indicates that this part of
the protein is important for the structure and/or function of the protein.
• The monoclonal antibody H6831 binds to a variable region of the OspB protein.
FAb fragments of the MAb H6831 have borrelicidal activity that mimics the action
of ß-lactam antibiotics.
56
ACKNOWLEDGEMENTS
This is the part of the thesis that I have worried most about, as this is the only section
that people read. But please, don’t have any expectations!
This work had not been possible to do, if I had not been surrounded by
wonderfully helpful and talented people, both at the Department of Microbiology,
where most of the work has been performed, and ‘outside’ (there actually exists a
world on the other side of the door!).
First of all, I am grateful to my supervisor, Sven “Bärka” Bergstrom, for
persuading me to enter the interesting field of Borrelia. There have been times when
I’ve kind of regretted this, but I don’t ‘seem to’ do that any more. In the end, I think
I’ve realized how much You have taught me.
During this 6 year period (is it really that long?!) I’ve had the pleasure to work
with several present and former members of the Borrelia team; Nisse, Laila, Björn,
Jonas, Anna-Karin, Sara, Jonas II, Annika and Ingela, in the order you showed up.
I am especially grateful to Laila, who, besides being a very good friend, has helped me
with a lot of different things during the years. Thanks also to Ingela for taking over the
lab-work in the end. A special thanks also to Jonas, for struggling through one of the
premature versions of my thesis and trying to convert them into some sense. Björn has
also helped in trying to improve my medical knowledge, and Nisse has saved the life of
some computers.
The binding studies presented in these thesis were performed in colaboration with
Niklas Strömberg at the Department of Cariology. I especially want to thank You for
your helpful cooperation to finish that section in time for deadline.
Many people have helped to make Micro a pleasant place to work at.
Unfortunately, 1 don’t have the space to acknowledge you all by name. But Berit must
be mentioned, as being a wonderful person and a close friend that you can always rely
on.
A big thanks to the technical personnel; P-A, who can fix anything, Marita for
being able to hold track of all administration, Gerd and Doris at the media, and the
‘wash-ladies’ Beth, Ulla, Maj-Britt and Karin. I will probably not realize how much
You do before I’ve left this place.
And thanks Amanda for converting my thesis (except for this part) and the two
manuscripts into something that hopefully will make sense, and are written in a
readable international language. I’m really impressed with your linguistic talents and
your speed!
57
The time at Micro has not only been spent by the lab bench, but also performing
different sport activities. The first thing happening after I had entered the front door,
was Tord coming asking me what 4km-time’ I had, which I, at that time, had no idea
of. Thanks to P-A and Micke for being good coaches, even if You always have a
tendency to highly overestimate my abilities!
I also want to mention Åsa, Micke and Anna, who all have been close friends
during all this time, and never hesitated to listen to my complaints. Anna is not only a
good friend, but she also took care of my problems with the front cover despite a very
short notice.
Not many students have had the privilege of having such a supporting and
understanding family as I have. My parents Bjarne and Inga-Britt, my sister Lena,
and my grand parents Rinaldo and Ingrid, have always shown a great interest in what
I’ve been doing, supported me and understood that studies take time and are not
always performed during normal working hours.
Last, but definitely not least, I want to acknowledge Göran, for all Your love,
understanding, encouragement and a million other things like drawing the computer
illustrations in these thesis. During this last period, it has been wonderful living
together with someone who knows what it is all about. Thanks a lot!
This work was financially supported from the Swedish Medical Research
Council, the Swedish Society for Medical Research, the Swedish Research Council
for Engineering Science, the National Institute of Health, USA, the Medical
Faculty, the Kempe Foundation and the Lenander Foundation.
59
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