Download Mycobacterium leprae interactions with the host cell: recent

Review Article
Indian J Med Res 123, June 2006, pp 748-759
Mycobacterium leprae interactions with the host cell:
recent advances
Lucia P. Barker
The University of Minnesota Medical School, Departments of Anatomy
Microbiology & Pathology. Duluth, Minnesota, USA
Received December 22, 2005
The significance of Hansen disease, or leprosy, is underscored by fact that detection of this
disease has remained stable over the past 10 yr, even though disease prevalence is reduced.
Due to the long incubation time of the organism, health experts predict that leprosy will be
with us for decades to come. Despite the fact that Mycobacterium leprae, the causative agent of
leprosy, cannot be cultured in the laboratory, researchers are using innovative and imaginative
techniques to discern the interactions of M. leprae with host cells both in vitro and in vivo to
identify the host and bacterial factors integral to establishment of disease. The studies described
in this review present a new and evolving picture of the many interactions between M. leprae
and the host. Specific attention will be given to interactions of M. leprae bacilli with host
Schwann cells, macrophages, dendritic cells and endothelial cells. The findings described also
have implications for the prevention and treatment of leprosy.
Key words Interactions - leprosy - macrophages - Mycobacterium leprae - Schwann cells
The global significance of Hansen disease
has remained constant 3. The sustained incidence
of the disease is due, in part, to an inability of health
professionals to reach isolated areas endemic for
the disease. There is also little known about the
route of transmission of leprosy. Because of delays
in diagnosis and treatment, especially in rural areas,
millions of people are permanently disabled by
current or past infections 4-6 . Further, no vaccine
exists for the prevention of leprosy. In addition,
and related to vaccine development, there are huge
gaps in our knowledge of the cell biology associated
with M. leprae infection.
In 1997, the World Health Organization (WHO)
estimated that 2 million people worldwide were
infected with Mycobacterium leprae, the causative
agent of Hansen disease or leprosy 1. Prevalence
of leprosy has diminished in recent years, down
to approximately 460,000 infections globally, in part
because of the success of multiple drug therapy
(MDT) to treat infected patients 2. This has led to
a perception that leprosy is no longer a significant
health problem. The case detection rate, however,
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BARKAR: HOST-PATHOGEN INTERACTION IN LEPROSY
The elimination of Hansen disease
In 1981, the WHO recommended MDT to
address the emergence of drug resistant strains
and to promote compliance and cost effectiveness 7.
In 1991, the World Health Assembly set a year
2000 target for the elimination of leprosy as a public
health problem, and defined elimination as less than
1 case per 10,000 population 8. It should be noted
that different presentations of leprosy, e.g.
paucibacillary (PB) or multibacillary (MB), require
different treatment regimens, and the guidelines
for current MDT are available 9. Since the inception
of the elimination plan, the worldwide prevalence
of leprosy has decreased dramatically 10 . However,
the disease detection rate has remained almost
constant over the past 10 yr, with a high rate
(17%) of infection in children 11 . Because of these
statistics and the potentially long (up to 20 yr)
incubation period of leprosy 12 , there has been
speculation that the priority of elimination should
be reduced in favour of a long-term plan to deal
with the chronic and global burden of leprosy13 .
Though millions have benefited from the
implementation of MDT treatment to pursue the
goal of elimination, lacunae remain in knowledge
regarding the basic biology of the disease. This,
appropriately, is being slowly addressed as scientists
worldwide address the interaction of these
organisms with the host.
Mycobacterium leprae research challenges
The causative agent of leprosy, M. leprae, has
a unique host cell tropism in that it preferentially
infects and grows within Schwann cells surrounding
the axons of nerve cells. This tropism is believed
to contribute to the pathology of leprosy and the
resulting injury to patients with this disease.
Perhaps, the greatest challenge to investigators is
the fact that M. leprae cannot be cultured by normal
laboratory methods. As a result, no genetic systems
for the study of M. leprae currently exist. On a
more positive note, the genome of M. leprae has
been sequenced and is readily available for
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comparative genomics with regard to phenotypic
traits of different mycobacterial pathogens 14 .
Interestingly, when the genome of M. leprae
(3.27Mb) is compared to the Mycobacterium
tuberculosis (the causative agent of tuberculosis
or TB) genome (4.41Mb), the M. leprae bacterium
appears to have undergone “reductive evolution.”
There are high levels of inactivated or pseudogenes,
and the level of gene duplication is approximately
34 per cent. Only about half of the genome contains
functional genes 15. This gene deletion and decay
could, in part, explain the specific niches and host
tropism of the organism as well as the inability of
researchers to culture the organism in the laboratory.
What has been lacking until recent years is a
definition of the molecular basis of M. leprae
association with the host cell, as well as information
relating to the phenotype of the organism in the
intracellular milieu. The tropism of M. leprae for
Schwann cells plus the difficulty in obtaining live
M. leprae doubles the difficulty in studying bacillihost interactions. First, it is difficult to obtain
primary mammalian Schwann cells. The cells can
be harvested from laboratory animals, e.g., the
sciatic nerves of rats, or can be isolated from
humans. These cells can also be immortalized 16 .
Schwannoma cells, in particular the ST 8814
Schwannoma cell line 17 , have also been used to
model the primary cell system 18,19 . A disadvantage
of the ST 8814 cell line is that it is, to be sure, a
model system. In addition, the immortalized ST 8814
cells may be prone to genetic lesions and
rearrangements that would not be seen in primary
cells. Regardless, there are distinct advantages in
using this cell line, including ease of maintenance
and the speed at which relevant data can be
obtained.
Secondly, the inability of researchers to grow
M. leprae in laboratory media adds an additional
level of difficulty to the study of this organism.
Irradiated organisms are available, but are limited
in utility as they are a model, or reflection of, the
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interaction of live organisms with the host. The
bacteria can be grown to relatively high
concentrations in nine-banded armadillo tissue or
in the footpads of nude mice, with the latter system
appearing to provide organisms at significantly higher
viability levels 20. These techniques have provided
highly viable organisms for the M. leprae
researchers, but the organisms can only be utilized
in the short term. In addition, no animal models
exist for the human disease, and neither the mouse
nor the armadillo experience a course of infection
that sufficiently parallels the human clinical features
of leprosy. Regardless of these problems, a small
international cadre of M. leprae researchers has
identified many characteristics of host-pathogen
interactions relevant to leprosy that are, in part,
described herein. It should be noted that the
immunological traits and consequences of M. leprae
infection are also being described at a commendable
rate. Except for specific examples of the influence
of host cell-pathogen interactions on the immunology
of leprosy disease, this review will be limited to
specific host cell-M. leprae interactions.
Interactions of M. leprae with the Schwann cell
Building on histological and clinical studies in
the 1950s that indicated that M. leprae primarily
invade Schwann cells in peripheral nerves 21, it was
further postulated that nerve damage in leprosy,
and the resultant deformities and disabilities related
to the disease, resulted from M. leprae invasion
of human Schwann cells 22-24 . The molecular basis
of this interactions between M. leprae and the
human Schwann cell has been almost completely
unknown until just recently. The late 1990’s saw
an intense level of research on, and interest in,
the molecular basis of Schwann cell-M. leprae
interactions. These studies continue to give more
and more insight into the basic biology of leprosy
and M. leprae interactions with the host cell.
The Schwann cell is part of a Schwann
cell-axon complex or unit which may or may not
be myelinated 25. This Schwann cell-axon unit is
surrounded by a layer of basal lamina 26 . This
phenotype is specific to the Schwann cell, leading
Rambukkana et al to postulate specific interactions
of M. leprae with basal lamina of the Schwann
cell-axon unit as a first step to infection of Schwann
cells 27 . This group in fact, showed that a host
molecule, laminin-2, was the initial target for
M. leprae seeking the Schwann-cell niche. The
laminin-2 molecule consists of α2, β1 and γ1 chains,
and this study further demonstrated that the globular
(G) domain of the laminin α2 chain was the specific
subunit with which M. leprae interacted 27 .
Consistent with this postulated mechanism of entry,
the tissue distribution of the laminin α2 chain is
limited to Schwann cells, striated muscle and the
placenta 28. These tissues correspond to natural sites
of M. leprae infection in the human.
The importance of laminin-2 to M. leprae
invasion is based on work from several studies.
For example, laminin-2 is anchored to Schwann
cells via the laminin receptor 29 , but laminin also
interacts with α -Dystroglycan (α-DG), another
receptor on the Schwann cell-axon unit 30 . A most
exciting extension of the study described above 27
was published by Rambukkana et al in 1998 in
which the specific receptor for M. leprae on
Schwann cells was determined to be the α -DG
receptor 16 . This study further demonstrated that
the G domain of the laminin α2 chain (α 2LG)
was required for binding to, and subsequent infection
of, Schwann cells. It should be noted that other
Schwann cell receptors have been implicated as
possible routes of entry for the uptake of the
M. leprae bacterium into the Schwann cell 31. It is
also interesting to note that if there is a deficiency
in the interaction between laminin-2 and α –DG,
certain types of muscular dystrophy and other
neuropathies can result 32,33 .
M. leprae adhesins
The questions that naturally follow from the
studies described above have to do with which
specific bacterial factors contribute to the observed
BARKAR: HOST-PATHOGEN INTERACTION IN LEPROSY
Schwann cell tropism. One way to narrow down
which bacterial factors are involved in this Schwann
cell-M. leprae interaction is to compare the cell
wall of M. leprae with that of M. tuberculosis
and other mycobacteria and determine those
molecules unique to the species M. leprae. One
such molecule, phenolic glycolipid-1, (PGL-1) is
unique to M. leprae and has been shown to
specifically bind to the laminin α2 chain in vitro 34 .
Specifically, the terminal triglyceride of this molecule
has been shown to bind to laminin-2. There is further
evidence that this bacterial cell wall component
may induce demyelination of nerve cells 35 . There
are two potentially devastating outcomes for the
demyelination observed with infection by M. leprae
and attributed to PGL-1. First, non-myelinated
Schwann cells are more susceptible to invasion
by M. leprae 16,35 . The organism can infect,
demyelinate yet more Schwann cells, and this can
lead to the second effect, axonal damage 36 . This
potential cascade effect of M. leprae invasion,
demyelination, growth within the host cell and
release, leading to more invasion, etc. is a
fascinating theory that could explain, in part, the
early events leading to the devastating progression
of leprosy disease. Finally, it should be noted that
high levels of PGL-1 can be found in the tissue of
leprosy patients 37 .
In addition to the interaction of M. leprae
PGL-1 with host laminin-2, another specific
bacterial adhesin, LBP21, potentiates the interaction
of this bacterium with the Schwann cell. The LBP21
protein, coded for by M. leprae gene ML1683,
also specifically binds laminin-2 on peripheral
nerves 38. This protein was identified by two groups
almost simultaneously. The Shimoji et al study 38
used α2 laminins as a probe to identify ML1683
in cell wall fractions by Western blot, and the protein
was determined by N-terminal amino acid
sequencing. This study showed that when fluorescent
polystyrene beads were coated with recombinant
LBP21, they avidly bound to primary rat Schwann
cells as compared to bovine serum albumin (BSA)coated beads. These experiments indicated a
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specific role for LBP21 as an adhesin in interactions
of M. leprae with Schwann cells.
Using a similar Western blot strategy, Marques
et al isolated a protein that bound to α2 laminin
and analyzed peptide fragments of this protein by
mass spectrometry 39 . The later group designated
this protein Hlp for 21 kDa histone-like protein.
The Hlp protein is identical to the LBP21 protein.
In the Marques study, the gene encoding the Hlp
was cloned and expressed. It was determined that
the addition of exogenous Hlp enhanced the
attachment of mycobacteria to an ST 8814
Schwannoma cell line 39. This group further reported
that cationic proteins, e.g., host-derived histones,
were also able to enhance binding of mycobacteria.
In an interesting experiment, the mycobacterial
species M. smegmatis, which is able to bind ST8814
cells, was compared to an M. smegmatis Hlp mutant
in a Schwannoma cell binding assay 39 . Even though
the mutant did not produce Hlp, there was no
reduction in binding of the mutant to the ST8814
cells as compared to the wild type. These data
indicate that other adhesins, or perhaps even nonspecific host or other bacterial cationic proteins,
may be involved in the binding of M. leprae to
laminin-2. In additional studies this group,
demonstrated that other mycobacterial species are
able to bind to α2-laminins, and that alanine/lysine
rich residues in Hlp and eukaryotic histones may
be involved in laminin binding 40, 41.
Taken together, these studies indicate that there
is more than one adhesin responsible for the initial
attachment of M. leprae to the Schwann cell. It
is clear, however, that α2-laminin is an important
component of the basal lamina with respect to
M. leprae attachment to, and invasion of, Schwann
cells. Though some work remains to be done
regarding the specific host-bacterium interactions
when M. leprae binds to Schwann cells, basic
research thus far indicates that the M. lepraespecific PGL-1 molecules, in addition to the Hlp/
LBP21 putative adhesin, are important bacterial
factors in attachment events. It follows that these
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factors can be potential targets for the treatment
or prevention of Hansen disease.
M. leprae uptake and trafficking in the
Schwann cell
Though much has recently been discovered
about the factors required for binding of M. leprae
to the host cell, the intracellular compartment of
the organism is virtually uncharacterized. In a study
by Alves et al the ST 8814 Schwannoma cell line
was shown to readily phagocytize both viable, nudemouse derived M. leprae and irradiated organisms 19.
This phagocytic event was shown to be dependant
upon host cell kinases, specifically protein tyrosine
kinase, calcium-dependent protein kinase and
phosphatidylinositol 3-kinase in a series of assays
where specific kinase inhibitors were shown to limit
uptake of M. leprae 19 . These results are not
surprising, as phagocytosis is an actin-mediated
process and these kinases have been implicated
in the actin-mediated phagocytosis of other bacteria.
It should be noted that cAMP-dependent protein
kinase inhibitors did not affect phagocytosis of
M. leprae by the ST 8814 cell line. Once M. leprae
is internalized by the Schwann cell, the bacilli must
survive and perpetuate to cause disease. There
have been several recent studies that hint at the
mechanisms M. leprae utilizes to persist in the
host cells and cause, in part, the unique pathology
seen in leprosy.
In general terms, after organisms have been
internalized by host cells, there is a series of events
that normally progress to result in the death of
invading organisms. In normal endocytic processing,
host cells phagocytose particles (including
microorganisms), and the resulting phagosomes are
transported through a series of events along the
phagosomal and endocytic pathway. After exhibiting
early, and then late, endosomal characteristics as
determined by membrane markers, the phagosome
will fuse with lysosomes. These compartments,
designated phagolysosomes, are highly acidified and
contain degradative enzymes from the lysosomal
compartment. This fusion event leads to the
degradation of the phagolysosomal contents 42 .
Much work has been done to characterize the
intracellular compartments of other mycobacterial
species 43 , including our work on the intracellular
phenotype of M. marinum 44. Phagosomes of other
pathogenic mycobacteria, including M. tuberculosis,
M. bovis, and M. avium have been shown to contain
early endosomal, but not late endosomal or lysosomal
markers 45-47 . In other words, they are arrested for
phagosomal development in the early endosomal
state. Most importantly, this enables these
mycobacterial species to survive the normally hostile
intracellular environment by avoiding fusion with
the lysosome.
Using the acidotrophic probe Lysotracker™ to
label lysosomal compartments, we were able to
demonstrate that live M. leprae did not reside in
acidified compartments at early (up to 48 h) time
points in the ST 8814 cell line 19 . Heat killed
organisms tested in parallel were, however,
trafficked to the lysosomal compartments, even
at the earliest (4 h) time point. These data suggest
that trafficking of M. leprae through the Schwann
cell endocytic pathway may be similar to other
mycobacteria studied. It should be noted that similar
results were obtained in the RAW 264.7 macrophage
cell line. Further, the fact that heat killed organisms
were not able to evade the host cell endocytic
pathway in either host cell model (Schwann cell
or macrophage) indicates an active process by which
these organisms perpetuate in the host cell. More
experiments, however, are required to determine
the cellular mechanisms by which phagocytosis of
M. leprae occurs and to fully characterize the
M. leprae-containing phagosome.
Recently elucidated effects of M. leprae
infection of Schwann cells
Several studies indicated that a result of
M. leprae infection of Schwann cells was an
increase in Schwann cell proliferation 34,35 . One
signaling pathway involving extracellular signal-
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regulated kinases 1 and 2 (Erk1, Erk2) has been
shown to play a significant role in cell
proliferation 48,49 . To determine whether this
proliferation was a direct result of bacterial insult,
long-term (30 day) infections of primary Schwann
cells with live M. leprae derived from mouse
footpads were carried out. The investigators then
examined the proliferation of the infected Schwann
cells and determined that higher levels of mitosis
were occurring at 30 days post-infection and that
total numbers of Schwann cells increased as
compared to uninfected controls 50. Affimetrix chips
containing human cell cycle gene substrates were
probed with DNA from M. leprae-infected
Schwann cells. Cyclin D1 and p21, two key G1
phase cell cycle regulators, were found to be
upregulated at day 30 post-infection. As several
signaling pathways could contribute to this G1-phase
cycle induction, a series of elegant inhibition
experiments and in vitro kinase assays were
performed that indicated that the Erk1/2 signaling
cascade was activated by intracellular M. leprae 50 .
One implication of this study is that the organisms
could be increasing non-myelinated Schwann cell
proliferation in order to maintain the niche in which
they reside. This study 50 also elucidated an
interesting alternative signaling mechanism for cell
proliferation. It will be interesting to follow
subsequent experiments that will address specific
bacterial factors that mediate the host cell
proliferation observed and determinations of
whether viable organisms are required for this
effect.
activation in order to modulate the innate immune
response 53 . The regulation of NF-kB can also be
affected by TNF-α. The Pereira study 52 indicates
that M. leprae can modulate the immune response
of the host via specific signaling pathways. These
pathways are, interestingly, also subject to the
influence of the TNF-α inhibitor, thalidomide 54 .
Thalidomide is used for the treatment of some forms
of leprosy in which inflammation and subsequent
tissue damage are a factor in the progression of
the disease 55,56 .
There has been speculation that other effects
of Schwann cell infection by M. leprae can lead
directly to the Schwann cell presentation of
M. leprae antigens to T-cells and result in the
production of immune modulators such as TNFα 51 . A recent publication by Pereira et al 52 that
also investigated the effect of M. leprae infection
on specific signaling molecules indicated that NFkB-dependent transcription repression in ST 8814
Schwannoma cells was a response to infection with
irradiated M. leprae. Other pathogenic
microorganisms have been shown to influence NF-kB
M. leprae has recently been shown to be readily
phagocytosed by RAW 264.7 cells and to evade
phagosome-lysosome fusion, at least up to
48 h 19. In experiments similar to the kinase inhibitor
assays described in this study for the uptake of
M. leprae by Schwann cells, a study by Lima
et al 60 indicated that the uptake of M. leprae by
the monocytic cell line THP-1 was also kinase
dependant. In addition, a study by Charlab et al 61
has shown that human mononuclear cells preexposed
to M. leprae can be stimulated to produce TNF-α
after the addition of PGL-1.
Interactions of macrophages with M. leprae
Krahenbuhl and Adams 57 put forth an excellent
review of the basic biology of macrophage
interactions with M. leprae in 1994. More recently,
however, these investigators, in addition to other
researchers, have elucidated interesting facets of
the macrophage-M. leprae interplay. In a patient
infected with M. leprae, the organisms can be found
in a variety of tissues and cell types, but macrophages
can internalize and/or contain many bacilli, especially
in bacteraemic infections58. Macrophages in specific
tissue and infection sites can play an important
role in the pathogenesis of leprosy for two reasons.
First, whole M. leprae and/or cell wall components
can stimulate macrophages to release cytokines,
including TNF-α, in vitro 59 . This indicates a direct
role of the macrophage in the immune response to
infection. Secondly, macrophages, like dendritic
cells are antigen-presenting cells and will bridge
innate and acquired immunity by evoking T-cell
and B-cell responses.
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Recent studies also define a role for the
activation of macrophages in the control of leprosy
infection. In an imaginative in vitro model system,
which utilizes macrophages isolated from the
granulomas in the footpads of M. leprae-infected
mice, Hagge et al 62 established a co-culture system
of the granuloma macrophages containing viable
M. leprae with either activated or normal effector
macrophages. This study then assayed the viability
of M. leprae that had been recovered from the
respective co-culture systems. It was determined
that the M. leprae recovered from the granuloma
macrophages co-cultured with normal macrophages
had significantly more metabolic activity, as
measured by radiorespirometry 62 . These data
indicated that those organisms in a background of
non-activated macrophages were more viable, i.e.,
activation of the macrophages is an integral part
of the immune response to leprosy infection. Most
interestingly, the normal effector macrophages were
able to acquire M. leprae from the macrophages
of granuloma origin and subsequently augmented
the metabolism of resident M. leprae bacilli. This
leads to intriguing speculation that this, or some
modification of, the co-culture system could lead
to the maintenance of viable M. leprae cultures
over time.
In a recent study, activated and normal mouse
peritoneal macrophages infected with M. leprae
were treated with thalidomide to determine whether
the viability of intracellular organisms could be
affected by this drug 63. Thalidomide did not exhibit
any antimicrobial activity against intracellular
organisms in either normal or activated (with
endotoxin and IFN-α) macrophages. Further, these
investigators suggest that thalidomide does not,
therefore, inhibit the release of M. leprae antigens
that have been previously shown to exhibit an
immunostimulatory effect. The use of thalidomide
to modulate the immune response and treat some
forms of leprosy is, therefore, most likely directly
related to an interaction of the drug with hostspecific modulators such as TNF-α.
Interaction of M. leprae with dendritic cells
Recent work has focused on the interactions
of M. leprae with another antigen presenting cell,
the dendritic cell. Sieling et al 64 demonstrated that
dendritic cells (DCs) were present in tuberculoid
lesions of leprosy patients. Soon after, Yamauchi
et al 65 found that T-cells in a similar patient’s
tuberculoid lesions expressed CD40 ligand, which
is involved in the differentiation and activation of
DCs. The DC is thought to be one of the most
effective antigen presenting cells, as it can stimulate
both CD4+ and CD8+ T cells in the mounting of
an effective protective immune response to
infection 66 . An actual in vitro study of infection
of DCs with footpad-derived M. leprae was
published recently 67 that indicated that monocytederived DCs were able to actively phagocytose
M. leprae. This study further showed that DCs
were able to effectively present M. leprae specific
antigens, including PGL-1 67 . However, in
experiments assaying T-cell activation by DCs,
there was less T-cell stimulation with DCs infected
with M. leprae than with M. bovis BCG or
M. avium. These results indicated a higher level
of resistance to DC mediated T-cell immunity.
Interestingly, a recent report by Hunger et al 68
have determined that Langerhans cells, a subset
of DCs that initiate the immune response in the
skin, are more efficient at M. leprae antigen
presentation than monocyte-derived DCs.
In other studies, Makino and colleagues 70 have
shown that an isolated and purified M. leprae
protein, designated MMP-II (major membrane II)
was highly immunogenic. The MMP-II molecule
stimulated DCs directly 69 and, when used to pulse
monocyte-derived DCs, would result in high levels
of T-cell activation 70. These studies are particularly
noteworthy in that this group has identified a novel,
immunostimulatory M. leprae protein that could
have relevance in the development of vaccines or
treatments against leprosy. Continuing the study
of the ability of M. leprae to persist in and contribute
BARKAR: HOST-PATHOGEN INTERACTION IN LEPROSY
to the immunologic functioning of both macrophages
and DCs will provide additional information as to
the host-pathogen interface that results in disease
pathology in the context of the immune response,
or lack thereof.
M. leprae and endothelial cells – a potential
delivery system
An interesting observation that has been made
since the establishment of histological techniques
for the study of lesions from individuals infected
with M. leprae has been the frequency with which
bacilli are found in endothelial cells lining the blood
vessels and lymphatics. Many studies have
described the presence of M. leprae in the
endothelium of the skin, nervous tissue and nasal
mucosa. These early studies indicate that the
endothelial cell may be a site of M. leprae
persistence and replication 71,72 . In 1999, Scollard
et al 73 used an armadillo model of M. leprae
infection to determine the extent to which the bacilli
could be found in endothelial cells. In this seminal
study, histopathological evidence suggested that the
endothelial cells in the epineurial and perineurial
blood vessels could be a reservoir for actively
replicating M. leprae that would subsequently infect
the Schwann cells in adjacent tissue 73 . The
implications are that some mechanism of M. leprae
attachment to endothelial cells could be required
for the establishment of infection and that M. leprae
can reach the peripheral nerve tissue through the
bloodstream. This hypothesis was counter to the
long-held belief that leprosy infection was a direct
result of injury and subsequent Schwann cell
exposure to viable bacteria from another exogenous
reservoir 74 .
A more detailed study of M. leprae association
with endothelial cells in vitro followed, wherein
monolayers of human umbilical vein endothelial cells
(HUVEC) were infected with M. leprae 75 . Though
the kinetics of uptake of M. leprae by the HUVEC
was much less than those seen with macrophages,
the association appeared to be specific and
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internalization of the bacilli was observed with both
confocal and electron microscopy. Any specific
endothelial cell receptors or bacterial ligands
responsible for this association and uptake remain
to be elucidated. An interesting review of the possible
implications of this work is available 76 , but the
question as to the contribution of endothelial cell
infection to the pathogenesis of leprosy is rather
open-ended. A larger question remains in regard
to whether endothelial cells or some other cell type
can “deliver” viable bacilli from potential exposure
sites (e.g. nasal mucosa) to Schwann cells in the
extremities and whether tissue damage is integral
to the establishment of disease.
The question of how M. leprae can be
disseminated systemically in the leprosy patient is
nicely addressed by Pessolani et al 77 . As the major
site of bacterial excretion is the nasal mucosa, a
fascinating possibility outlined in this report is that
M. leprae in nasal discharges from leprosy patients
can be transmitted to others via an airborne route.
If the bacilli enter the lung, the elucidation of
bacterial adhesins that initiate cell contact and
infection will be very important. Much remains to
be done to determine how, if the organisms are
taken up by the respiratory mucosa, the bacilli reach
the nerves and skin.
Conclusions
It is not difficult to develop an appreciation for
the various host cells with which M. leprae interact
in the pathogenesis of leprosy. The difficulties in
working with M. leprae in the laboratory are not
trivial, but despite this, there has been an explosion
of studies on leprosy host-pathogen interactions
and other related information in the past few years.
This information, now available to researchers and
clinicians, will direct the basic science, prevention,
and treatment of leprosy in coming decades.
Concomitant with studies elucidating the
immunologic consequences of, and effects on,
leprosy infection, are the recent advances in
mycobacterial genomics, eukaryotic cell biology,
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INDIAN J MED RES, JUNE 2006
new microscopic and histological techniques, and
the introduction and/or improvement of in vitro
and in vivo models. These advances, in combination
with some of the findings described herein, give
the leprosy researcher a variety of exciting
investigational roads from which to choose.
information regarding the basic biology of M. leprae
survival and dissemination in the host is the best
hope for the true eradication of leprosy.
Acknowledgment
The author would like to thank Shriyuts O. Brun,
Some of the most intriguing questions that still
remain have to do with the actual reservoirs of
M. leprae in the infected individual. Though the
organism is thought to reside primarily in Schwann
cells and macrophages, many other cell types,
including DCs and endothelial cells, appear to
harbour the organism. Specific Schwann cell
receptors have been elucidated, as have some of
the bacterial factors responsible for colonization
of the Schwann cells. This begs the question as to
whether these bacterial adhesins, e.g. PGL-1 and
LBP21, are acting as specific adhesins in binding
to other cell types.
A further question has to do with the intracellular
survival of M. leprae in Schwann cells,
macrophages, and other host cells. Though there
is some evidence that supports the evasion of normal
endocytic processing by intracellular M. leprae,
it remains to be seen whether the intracellular niche
of this organism is similar to that of other pathogenic
mycobacteria. In addition, it appears that M. leprae
activates proliferation pathways in the Schwann
cell host. This observation is significant for two
reasons. First, the organism may be inducing the
perpetuation of its own intracellular niche. Secondly,
it is intriguing to speculate that specific M. leprae
factors, without infecting organisms, of course, can
be a tool for the induction of nerve regeneration
and myelination in the medical setting.
The elucidation of specific bacterial factors
important for the organism to establish disease may
also lead to candidates for vaccine preparations
or targets for specific antimicrobials. Though the
volume of work on M. leprae interactions with
host cells appears to be growing rapidly, many details
remain to be resolved. The application of new
B. Clarke, J. Regal, L. Repesh and G. Trachte for helpful
suggestions and critical review of this manuscript.
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Reprint requests: Dr Lucia P. Barker, Department of Anatomy, Microbiology and Pathology
The University of Minnesota Medical School, 1035 University Drive Duluth
Minnesota 55811, USA
e-mail: [email protected].