Download Licentiate thesis from the Department of Immunology

Licentiate thesis from the Department of Immunology
Wenner-Gren Institute, Stockholm University, Sweden
Does IgA play a role in protection against
pulmonary tuberculosis?
Anna Tjärnlund
Stockholm 2005
PUBLICATIONS
This licentiate thesis is based on the following papers, which are referred to by their Roman
numerals in the text:
I.
Rodríguez A*., A. Tjärnlund*, J. Ivanyi, M. Singh, I. García, A. Williams, P. D.
Marsh, M. Troye-Blomberg, and C. Fernández. 2005. Role of IgA in the defense
against respiratory infections. IgA deficient mice exhibited increased susceptibility
to intransal infection with Mycobacterium bovis BCG. Vaccine 23: 2565-2572.
II.
Tjärnlund A*., A. Rodríguez*, P-J. Cardona, E. Guirado, J. Ivanyi, M. Singh, M.
Troye-Blomberg, and C. Fernández. 2005. Polymeric Ig receptor knockout mice
are more susceptible to mycobacteria infection in the respiratory tract.
Submitted to Vaccine
* These authors contributed equally
CONTENTS
ABBREVIATIONS
SUMMARY
INTRODUCTION
1
TUBERCULOSIS
1
MYCOBACTERIAL INFECTIONS
Immune evasion
IMMUNE RESPONSE TO MYCOBACTERIAL INFECTION
1
2
3
Innate immune responses
3
Adaptive immune responses
4
Granuloma formation
5
Cellular requirements for protective immunity against TB
5
+
5
+
CD8 T cells
6
γδ T cells
7
Macrophages
8
Dendritic cells
9
B cells
10
CD4 T cells
MUCOSAL IMMUNITY IN THE RESPIRATORY TRACT
11
Specific immune responses
11
Secretory IgA
13
Functions of IgA
14
Fcα receptors
14
IgA deficiency
15
TB: THE DISEASE
17
Diagnosis
17
Tuberculin skin test
18
Microscopy
18
Cultivation
18
Molecular methods
19
Treatment
19
Current vaccine
19
New vaccine candidates
20
ANIMAL MODELS IN TB
21
PRESENT STUDY
23
AIM
23
MATERIALS AND METHODS
24
Antigen and adjuvant
24
Immunizations
24
Intranasal infection with BCG and quantification of bacterial load
24
M. tuberculosis aerosol infection and quantification of bacterial load
25
Antibody production
25
Cytokine-producing cells
25
Cytokine production
26
Quantification of mRNA expression
26
Histology and morphometry
26
Statistical analysis
27
RESULTS AND DISCUSSION
28
Paper I
28
Paper II
29
CONCLUDING REMARKS
33
ACKNOWLEDGEMENTS
34
REFERENCES
35
APPENDIX: Paper I-II
ABBREVIATIONS
APC
Antigen-presenting cell
BAL
Broncho-alveolar lavage
BCG
Mycobacterium bovis Bacillus Calmette-Guérin
CFU
Colony forming unit
CR
Complement receptor
CT
Cholera toxin
DC-SIGN
DC-specific intercellular adhesion molecule-3 grabbing nonintegrin
FcR
Fc-receptor
HIV
Human immunodeficiency virus
HPRT
Hypoxanthine guanine phosphoribosyl transferase
Ig
Immunoglobulin
i.n.
Intranasal
iNOS
Inducible nitric oxide synthase
IFN-γ
Interferon-gamma
IL
Interleukin
MALT
Mucosal-associated lymphoid tissue
MDR
Multidrug resistant
MHC
Major histocompatibility complex
MR
Mannose receptor
MyD88
Myeloid differentiation factor 88
NALT
Nasal-associated lymphoid tissue
NO
Nitric oxide
pIgA
Polymeric immunoglobulin A
pIgR
Polymeric immunoglobulin receptor
PPD
Purified protein derivative
RANTES
Regulated-upon-activation normal T-cell sequence
RMI
Reactive-nitrogen intermediate
SC
Secretory component
sIgA
Secretory immunoglobulin A
sIgAD
Selective IgA deficiency
TAP
Transporters associated with antigen processing
TB
Tuberculosis
TCR
T-cell receptor
Th
Helper-T cell
TLR
Toll-like receptor
TNF-α
Tumor necrosis factor-alpha
TST
Tuberculin skin test
SUMMARY
More than a century after the identification of the tubercle bacillus and the first attempts at
vaccination, tuberculosis (TB) still remains one of the world’s most serious infectious
diseases. TB is typically a disease of the lung, which serves both as port of entry and as the
major site of disease manifestation. The currently used vaccine, Mycobacterium bovis bacillus
Calmette-Guérin (BCG), is administered parentally and induces a systemic immune response.
However, it fails to protect against pulmonary TB, thereby raising the question whether
vaccination targeting the mucosal immunity in the lungs could be favourable.
The respiratory mucosal surfaces represent the first line of defence against a multitude of
pathogens. Secretory IgA (sIgA) in mucosal secretions has an important function by blocking
entrance of pathogenic organisms and preventing infections. Yet, another role for IgA in
protection against intracellular pathogens has lately been appreciated, when sIgA was
demonstrated to neutralize viruses intracellulary. We aimed to investigate the relevance of
sIgA in protection against mycobacterial infections using mice deficient for IgA and the
polymeric Ig receptor. Mice were immunized intranasally with a mycobacterial antigen which
elicited, in wild-type mice, a strong IgA response in mucosal secretions in the respiratory
tract. Gene-targeted mice failed to induce the same response and more importantly, were more
susceptible to mycobacterial infections in the respiratory tract, as demonstrated by higher
bacterial loads in the lungs than wild-type mice. Analysis of immune responses after infection
revealed reduced production of proinflammatory, and protective, factors such as IFN-γ and
TNF-α in the lungs of deficient mice, which was in concordance with the higher bacterial
burden seen in the lungs of these mice. The mechanisms explaining the defective
proinflammatory responses in the lungs of deficient mice are not clear but might involve
impaired signalling through Fcα receptors, or homologous receptors, which could lead to
inadequate activation of pulmonary macrophages. This could subsequently result in
suboptimal induction and production of cytokines and chemokines important for attraction
and migration of cells to sites of infection in the lungs.
Our results demonstrate a role for IgA in protection against mycobacterial infection in the
respiratory tract by blocking the entrance of the mycobacterium into the lungs, and/or by
modulating the locally induced proinflammatory immune responses.
INTRODUCTION
TUBERCULOSIS
Tuberculosis (TB) remains one of the leading infectious diseases and causes high mortality in
humans, resulting in more than 2 million deaths annually (WHO, 2001). The increasing global
health burden of TB is due both to the synergistic pathogenesis of coinfection with the human
immunodeficiency virus (HIV), as well as to continued dissemination of multidrug-resistant
(MDR) Mycobacterium tuberculosis strains (Coker, 2004; Toossi, 2003). Despite this
alarming health challenge, the capacity for treating and preventing TB remains limited, and a
uniformly effective vaccine is lacking.
The M. tuberculosis complex, the cause of TB, is comprised of M. tuberculosis, M. bovis, M.
africanum, M. microtti and M. canetti, and although all members can cause TB, M.
tuberculosis is the most prevalent. The natural reservoir of M. tuberculosis and M. canetti is
limited to humans and that of M. microtti is mainly limited to small rodents (Kremer et al.,
1998). In contrast, the host range of M. bovis is very broad and these species can cause
disease among a wide range of wild and domestic animals, as well as in humans (Ayele et al.,
2004). M. africanum has been isolated from humans and diverse animal species (Thorel,
1980; Alfredsen and Saxegaard, 1992). All members in the complex are slow-growing
organisms with generation times ranging from 12 to 24 hours depending on environmental
and microbial variables.
MYCOBACTERIAL INFECTIONS
The causative agent of infectious TB is M. tuberculosis, a rod-shaped obligate aerobic
bacillus, which is shielded by a unique wax-rich cell wall, composed of long-chain fatty acids,
glycolipids and other components (reviewed in Kaufmann, 2001). This robust cell wall of the
bacteria contributes to intracellular survival in host phagocytes.
TB can manifest itself at any tissue site, but the lungs represent both the main port of entry
and the most important site of disease manifestation. Droplets containing bacilli are expelled
from individuals with active pulmonary TB, and subsequently inhaled into the respiratory
1
tract (Riley et al., 1995) and phagocytosed by alveolar macrophages. These cells are
considered to be the main cellular host for mycobacteria, and their major role is rapid killing
of the invading organism, through the release of toxic reactive oxygen and nitrogen
intermediates, or killing by lysosomal enzymes following fusion with the bacterial
phagosome. The receptors that have been implicated in the uptake of mycobacteria include
mannose receptors (MRs) that recognize mannose residues on mycobacteria (Schlesinger,
1993; Schlesinger et al., 1996), Fc receptors (FcRs) binding opsonised cells, complement
receptor (CR) 1, CR3, and CR4, after opsonisation with complement factor C3 (Aderem and
Underhill, 1999; Hirsch et al., 1994; Schlesinger et al., 1990), surfactant receptors (Downing
et al., 1995), and scavenger receptors (Zimmerli et al., 1996).
Once M. tuberculosis has entered the lungs, one of four potential fates might occur
(Dannenberg, 1994):
1. The initial host response can be effective in killing and eliminating the bacilli. These
individuals will not develop TB at any time point in the future.
2. The bacilli can grow and multiply immediately after infection, thereby causing clinical
disease known as primary TB.
3. The bacilli may become dormant and never cause disease, resulting in a latent
infection that is manifested only as positive tuberculin skin test (TST).
4. The dormant bacilli can eventually begin to grow with resultant clinical disease known
as reactivation TB.
Immune evasion
Despite the preferred target cell, whose function is the elimination of microbes; M.
tuberculosis can remain viable after phagocytosis through different strategies evolved to
evade host immune responses. The use of certain CRs may be advantageous for the
bacterium, since engagement of these receptors does not induce the release of cytotoxic
reactive oxygen intermediates (Wright and Silverstein, 1983). Moreover, it is well known that
mycobacteria can evade the normal phagosome-lysosome fusion pathway resulting in
2
persistence of the bacteria within the host cell (Armstrong and Hart, 1975). In this way the
bacteria not only remain viable, but bacterial antigens are prevented from presentation to T
cells. Mycobacteria appear to increase retention of a macrophage protein, termed the
tryptophan-aspartate containing coat (TACO) protein, on the surface of the mycobacterial
phagosome, and thereby preventing phagosome-lysosome fusion (Ferrari et al., 1999). In
addition, expression of major histocompatibility complex (MHC) II molecules is decreased in
M. tuberculosis-infected macrophages (Noss et al., 2000). Another immune evasion
mechanism is the secretion of proteins such as superoxide dismutase and catalases by M.
tuberculosis, which are antagonistic to reactive oxygen intermediates (Andersen et al., 1991),
and inhibition of macrophage apoptosis (Fratazzi et al., 1999). Macrophages infected with M.
tuberculosis produce inhibitory cytokines, such as transforming growth factor-β1 and
interleukin (IL)-10, which reduce macrophage activation, thereby leading to decreased
clearance of bacteria (Barnes et al., 1992; Toossi et al., 1995).
IMMUNE RESPONSE TO MYCOBACTERIAL INFECTION
Innate immune responses
Besides phagocytosis, recognition of M. tuberculosis or mycobacterial products is crucial for
an effective host response. Phagocytic cells play an important role in the initiation and
direction of the adaptive immunity by presentation of mycobacterial antigens and expression
of costimulatory molecules and cytokines. Central to immune defence against microbial
pathogens are pattern recognition receptors such as the Toll-like receptors (TLRs). Emerging
evidence suggest that TLRs play an important role in the activation of immune cells by
pathogens, including M. tuberculosis. TLR2, TLR4, and more recently, TLR1/TLR6 that
heterodimerise with TLR2, have been implicated in the recognition of mycobacterial antigens
(Bulut et al., 2001; Hajjar et al., 2001). Predominantly, a role for TLR2 in immune
recognition of M. tuberculosis has been demonstrated. Mycobacterial products have been
demonstrated to induce secretion of tumor necrosis factor-α (TNF-α) and nitric oxide (NO)
by macrophages via interaction with TLRs, as well as inducing apoptosis in the host cell
(Aliprantis et al., 1999; Brightbill et al., 1999).
Infection studies using TLR gene-disrupted mice have, however, provided conflicting data,
depending on the experimental settings, for instance the dose of bacteria used. TLR2-/- mice
3
have been demonstrated to be more susceptible to M. tuberculosis infection than wild-type
mice, while others have reported a redundant role for TLR2 in this context (Sugawara et al.,
2003; Reiling, et al., 2002; Drennan et al., 2004). In addition, outcomes from infection studies
with TLR4 deficient mice show disparity (Kamath et al., 2003; Shim et al., 2003; Reiling et
al., 2002). Signal transduction by most TLRs, with the exception of TLR3, requires the
adapter molecule myeloid differentiation factor 88 (MyD88) (Medzhitov et al., 1998; Adachi
et al., 1998; Kawai et al., 1999). MyD88 is an intracellular adaptor protein in the IL-1
receptor/IL-1 receptor associated kinases (IRAK) pathway that links TLR recognition with
activation of IRAK and TNF receptor associated factor (TRAF), translocation of NF-κB, and
gene transcription (Akira et al., 2003). Mice deficient in MyD88 fail to generate
proinflammatory responses when stimulated through TLRs (Adachi et al., 1998; Kawai et al.,
1999), and demonstrate high susceptibility to several infectious agents, including
mycobacteria (Muraille et al., 2003; Mun et al., 2003; Feng et al., 2003; Fremond et al.,
2004).
Along with triggering cytokine secretion, TLR activation triggers the maturation of
monocyte-derived dendritic cells (DCs), resulting in higher levels of T-cell costimulatory
molecules, such as CD80 and CD86, along with antigen-presentation molecules such as MHC
II (Hertz et al., 2001; Michelsen et al., 2001; Tsuji et al., 2000).
In this way TLRs contribute to the innate immune system by the induction of antimicrobial
effector molecules, upon ligation. In addition, recognition of mycobacterial products by TLRs
induces secretion of cytokines and upregulation of immunostimulatory molecules, and
subsequently modulation of the adaptive immune response.
Adaptive immune responses
M. tuberculosis is a classic example of a pathogen for which the protective immune response
relies on cell mediated immunity. The initial interactions in the lungs is with alveolar
macrophages, but after this first encounter DCs and monocyte-derived macrophages, recruited
to the site of infection, also take part in the phagocytic process (Henderson et al., 1997;
Thurnher et al., 1997). Infected DCs mature and migrate to draining lymph nodes to prime
naïve T cells via processed antigens. Inflammation in the lungs provides the signals that direct
the effector T lymphocytes back to the site of infection where granulomas are formed. The
4
anatomic affinity of these cells appears to be mainly determined by site-specific integrins,
“homing receptors”, on their surface and complementary mucosal tissue-specific receptors,
“addressins”, on vascular endothelial cells (Kunkel and Butcher, 2003). In addition,
chemokines produced in the local microenvironment promote chemotaxis toward mucosal
tissues and regulate integrin expression on mucosal lymphocytes, thereby controlling cell
migration (Champbell et al., 2003).
Granuloma formation
Granuloma formation is the hallmark of M. tuberculosis infection. Granulomas are formed in
response to chronic local antigenic stimulation, and can be observed in different infectious
diseases, including schistosomiasis, leprosy, and leishmaniasis (Reyes-Flores, 1986; Modlin
and Rea, 1988; Palma and Saravia, 1997; Rumbley and Phillips, 1999; Boros, 1999).
Structure and composition of granulomas vary depending on the organism. A tuberculous
granuloma is observed concomitant with a highly activated cell-mediated immune response,
which generally mediates control of mycobacterial numbers in the lungs. The granuloma is
composed of many different cells, including macrophages, CD4+-, and CD8+-T cells, and B
cells (Gonzales-Juarrero et al., 2001). These cells control the infection by providing a local
environment for the cells to interact, leading to an effective immune response where cytokine
production, macrophage activation and CD8+ T cell-effector functions lead to killing of the
mycobacteria. Granulomas also provide a way of containing the bacilli by walling off and
preventing spread of infection. Altogether these actions lead to inhibition of growth, or death
of M. tuberculosis. However, the resulting pathology can cause additional problems for the
host.
Cellular requirements for protective immunity against TB
CD4+ T cells
CD4+ T cells are generally considered to be among the most important players in the
protective immune response against M. tuberculosis. CD4+ T cells recognize peptide antigens
from mycobacteria degraded in the phagolysosomal compartments and complexed with MHC
class II molecules (Davis and Björkman, 1988). Murine studies with antibody depletion of
CD4+ T cells (Muller et al., 1987), adoptive transfer (Orme and Collins, 1984), or the use of
5
gene-deficient mice (Caruso et al., 1999), have demonstrated that the CD4+ T cell subset is
required for control of infection. The primary effector function of CD4+ T cells is the
production of cytokines; first and foremost interferon-γ (IFN-γ), which is crucial for induction
of microbicidal activities by macrophages, but also TNF-α. Production of these cytokines by
CD4+ T cells are important for the control of TB (Flynn et al., 1993; Flynn et al., 1995), and
studies using IFN-γ gene depleted mice demonstrate that these mice are highly susceptible to
virulent M. tuberculosis, with defective macrophage activation and uncontrolled bacilli
growth (Cooper et al., 1993). Humans defective in genes for IFN-γ or the IFN-γ receptor are
prone to serious mycobacterial infections, including M. tuberculosis (Ottenhof et al., 1998).
TNF-α plays a key role in the granuloma formation (Kindler et al., 1989; Senaldi et al.,
1996), induces macrophage activation, and has immunoregulatory properties (Orme et al.,
1999; Tsenova et al., 1999). In mice, TNF-α is also important for containment of latent
infection in granulomas (Mohan et al., 2001).
Although IFN-γ production by CD4+ T cells is a very important effector function, these cells
probably have other roles in controlling M. tuberculosis infection. In MHC class II-/- or CD4-/mice, levels of IFN-γ were severely diminished early in infection, but returned to wild-type
levels later on (Tascon et al., 1998; Caruso, et al., 1999). Nevertheless, the gene-deficient
mice were not rescued by this later IFN-γ production and succumbed to the infection. Both
CD4+ T cell clones and mycobacterial-antigen-expanded CD4+ T cells have been shown to
exhibit cytolytic effector function against mycobacterial-antigen pulsed- or mycobacteriainfected-macrophages (Orme et al., 1992).
CD8+ T cells
A role for CD8+ T cells in mycobacterial infection has also been described despite the
residence of the bacteria within phagosomes (reviewed in Smith and Dockrell, 2000).
Mycobacterial antigen-specific CD8+ T cells are restricted by either MHC class I or CD1
molecules. CD1 molecules are nonpolymorphic molecules that present lipids or glycolipids to
T cells, as opposed to peptide epitopes presented by MHC molecules (reviewed in Porcelli
and Modlin, 1999). Mice genetically disrupted in the genes for β2-microglobulin or
transporter of antigen processing (TAP), and therefore deficient in MHC class I and nonclassical MHC class Ib molecules and CD8+ T cells, displayed increased susceptibility to M.
6
tuberculosis infection compared to wild type mice (Flynn et al., 1992; Behar et al., 1999;
Sousa et al., 1999).
The mechanism by which mycobacterial proteins gain access to the MHC class I molecules is
not clear. Mycobacteria-induced pores or breaks in the phagosomal membrane have been
proposed as mechanisms (Myrvik et al., 1984; Mazzaccaro et al., 1996; Teitelbaum et al.,
1999). The bacteria could use this as a way of gaining access to cytosolic nutrients and
introducing toxic molecules into the cytoplasm. Additionally, mycobacterial antigens would
be allowed to enter the cytoplasm of infected cells and the MHC class I pathway.
Two principal effector functions for CD8+ T cells have been suggested: lysis of infected cells
and production of cytokines, explicitly IFN-γ, although the relative contributions of these
functions are not established. CD1- and MHC class I-restricted CD8+ T cells, specific for
mycobacterial antigens, have been shown to induce lysis of infected human DCs and
macrophages, resulting in reduced intracellular bacterial numbers (Stenger et al., 1997; Cho et
al., 2000). This was dependent on perforin, which was required for pore formation (Stenger et
al., 1997), while granulysin was responsible for killing the intracellular bacteria (Stenger, et
al., 1998). Moreover, antigen-specific CD8+ T cells can target and kill infected macrophages,
and also induce growth inhibition of M. tuberculosis, through apoptotic mechanisms (Oddo et
al., 1998). The role of IFN-γ in mycobacterial infections, on the other hand, is considered to
be activation of macrophages. CD8+ T cells from lungs of infected mice have been shown to
be primed for IFN-γ production although this production appeared to be limited in the lungs
(Serbina and Flynn, 1999).
γδ T cells
T cells that express the γδ T-cell receptor (TCR) also participate in the immune response
against M. tuberculosis (Kaufmann, 1996), and are believed to play a role in the early immune
response. Data from animal studies suggest that γδ T cells play a significant role in the host
response to TB in mice (Izzo and North, 1992). In addition, studies using γδ TCR knockout
mice indicate that γδ T cells may be involved in regulation of granuloma formation, which is
critical for control of mycobacteria (D´Souza et al., 1997). Dieli et al., have demonstrated an
early accumulation of γδ T cells in the lungs of BCG-infected mice, reaching a peak 3 weeks
7
before αβ T cells, as well as regulatory role for γδ T cells in the induction of CD8+ T cells in
the lungs (Dieli et al., 2003). These results indicate that γδ T cells might be important for
control of mycobacterial infection in the period between innate and adaptive immunity.
Human γδ T cells, predominantly the Vγ9/Vδ2 TCR subset, have also been demonstrated to
respond to M. tuberculosis antigens (Kabelitz et al., 1991), and monocytes infected with live
M. tuberculosis were particularly effective in expanding this subset of γδ T cells (Havlir et al.,
1991).
The effector functions of γδ T cells in the immune response to M. tuberculosis appear to be
both cytokine secretion and cytotoxicity. Studies with M. tuberculosis antigen-activated γδ T
cell clones or primary cells demonstrated IFN-γ production, but also TNF-α production in
response to phosphate antigens (reviewed in Boom, 1999). Likewise, cytotoxicity mediated
by γδ T cells has been confirmed and was dependent upon activation through the TCR (Munk
et al., 1990; Dieli et al., 2003). M. tuberculosis reactive γδ T cells from the peripheral blood
of TST positive subjects were cytotoxic for monocytes pulsed with mycobacterial antigens.
Macrophages
Macrophages are regarded as the phagocytic cell that initially ingest M. tuberculosis, and
provide an important cellular niche during infection. Upon infection, macrophages have been
shown to secrete proinflammatory cytokines such as TNF-α, IL-1, and IL-6, suggesting a role
for these cytokines in the recruitment of cells to the site of infection (Giacomini et al., 2001).
Furthermore, the secretion of TNF-α may aid in the activation of macrophages to produce
reactive oxygen and nitrogen intermediates, and help granuloma formation (Roach et al.,
2002; Flynn et al., 1995). The significance of these toxic nitrogen oxides in host defence
against M. tuberculosis has been well documented, both in vitro and in vivo, particularly in
the murine system (MacMicking et al., 1997; Shiloh and Nathan, 2000). In the mouse,
reactive nitrogen intermediates (RMI) play a protective role in both the acute and chronic
persistent infection (MacMicking et al., 1997; Flynn et al., 1998). More important,
accumulating evidence supports a role for these reactive molecules in host defence in human
TB (Nicholson et al., 1996; Wang et al., 1998), although this still remains controversial.
8
The mechanisms by which NO and other RMI may affect antimicrobial activity could be
through modification of bacterial DNA, proteins and lipids (reviewed in Knowledge et al.,
2001). NO can also deaminate, as well as directly damage bacterial DNA. RMI has also the
potential to disrupt signalling pathways, and NO has been demonstrated to induce apoptosis.
Dendritic cells
While DCs do not display effective anti-microbial activity upon mycobacterial encounters,
their secretion of cytokines and expression of co-stimulatory molecules help in modulating
the adaptive immune response, supporting a helper T cell (Th) 1 biased T-cell response. It was
recently shown that DCs, but not macrophages, infected with M. tuberculosis were capable of
driving Th 1 polarization of naïve CD4+ T cells (Hickman et al., 2002). Activation of human
monocyte-derived DCs with the 19-kDa mycobacterial lipoprotein results in the preferential
secretion of IL-12, a key player in host defence against M. tuberculosis (Thoma-Uszynski et
al., 2000). An additional property of DCs contributing to their effectiveness in initiating
immune responses is their ability to migrate from peripheral tissues to secondary lymphoid
tissues after acquiring antigens. Naïve T cells are thereby activated via antigen presentingand costimulatory-molecules in the presence of polarizing cytokines such as IL-12.
DCs and macrophages appear to have different roles during infection with mycobacteria.
Macrophages but not DCs, for instance, have the ability to kill intracellular M. tuberculosis
(Bodnar et al., 2001). The different intracellular behaviour of M. tuberculosis in macrophages
and DCs may reflect differences in the receptors involved in bacterial uptake in the two cell
types. DCs have lectin-surface receptors such as the recently identified DC-specific
intercellular adhesion molecule-3 grabbing nonintegrin (DC-SIGN) (Geijtenbeek et al., 2000),
that are not expressed on most macrophages and that facilitate antigen uptake (Engering et al.,
2002). Whereas CR3 and MR are also expressed by human DCs, they seem to be neglected by
the tubercle bacillus (Tailleux et al., 2003). Ligation of DC-SIGN with the M. tuberculosisderived lipoarabinomanan induces IL-10 secretion by DCs, and thereby suppresses their
functions (Geijtenbeek et al., 2003).
9
B cells
While an essential role for T cells in the control of M. tuberculosis is well established, the role
of B cells is less well understood. Infection studies with B cell-deficient mice have revealed
conflicting data. It has been reported that B-cell-deficient mice had an increase in viable
bacilli compared to the wild-type mice (Vordermeier et al., 1996). Other studies demonstrated
that B cells were recruited to the lungs of mice infected with M. tuberculosis and contributed
to granuloma formation, yet mice that lack B cells are able to control the growth of bacteria in
the lungs (Johnson et al., 1997; Bosio et al., 2000). The absence of B cells resulted in reduced
recruitment of neutrophils, macrophages, and CD8+ T to the lungs, suggesting that B cells
may influence the cellular composition at this site. An additional role for B cells as antigenpresenting cells (APCs) has also been suggested (Vordermeier et al., 1996).
10
MUCOSAL IMMUNITY IN THE RESPIRATORY TRACT
Mucosal surfaces lining the respiratory-, gastrointestinal-, and urogenital tracts are the major
sites of entry for pathogens. These mucosal surfaces thereby provide the first line of defence
against entrance of various bacteria and viruses. Protection of mucosal membranes against
colonization, possible entry and invasion by microbes is provided by a combination of nonspecific and specific mechanisms. Production of mucos is part of the non-specific
mechanisms which acts as a physical barrier by containing substances such as lysozyme,
lactoferrin, collectin and defensins. Ciliary action can force microbes out of the respiratory
tract. The movement of microbes by the ciliae decreases the time available for adherence by
pathogens to the epithelium. Tight junctions between neighbouring epithelial cells lining the
mucosal membranes also act as a physical barrier against penetration.
Specific immune responses
Mucosal surfaces contain specialized mucosa-associated lymphoid tissues (MALT) necessary
for antigen sampling and induction of mucosal immune responses. The immune system in the
upper and lower respiratory tract can be divided into three parts (Davis, 2000):
1. an epithelial compartment at the surface of the epithelium and the underlying
connective tissue that contains immunocompetent cells
2. the MALT, subdivided according to anatomical location: the nose-associated
lymphoid tissue (NALT), larynx-associated lymphoid tissue (LALT), and the
bronchus-associated lymphoid tissue (BALT)
3. lymph nodes draining the respiratory system
Mucosal antigen-specific immune responses are elicited in the MALT, where foreign material
from epithelial surfaces can be sampled and transported by microfold (M) cells, and
subsequently taken up by underlying DCs and macrophages (reviewed in Kiyono and
Fukuyama, 2004). The follicles of the MALT contain all immunocompetent cells, i. e. T cells,
B cells and APCs that are required for initiation of an immune response. The MALT, as well
as local and regional draining lymph nodes, thereby constitutes the inductive site for
11
generation of mucosal immunity. The common mucosal immune system connects these
inductive sites with effector sites where antigen-specific lymphocytes perform their effector
functions after extravasation from peripheral blood, directed by the local profile of vascular
adhesion molecules and chemokines (Fig. 1).
Dendritic cells in the lymphoid tissue capture antigens, process and present them to
lymphocytes in the context of MHC molecules. After antigen-induced priming, proliferation
and partial differentiation in the MALT, lymphoid memory and effector cells migrate to
regional lymph nodes where further differentiation can take place (Brandtzaeg et al., 1999).
The lymphocytes thereafter pass into the peripheral blood circulation whereby extravasation
at mucosal effector sites occurs. These primed cells express adhesion molecules or “homing
receptors” specific for corresponding determinants on endothelial cells in mucosal and
exocrine glandular tissues (Butcher and Picker, 1996).
Figure 1. The common mucosal immune system. (Modified from Nature Reviews Immunology 2004).
12
Secretory IgA
IgA is the predominant immunoglobulin (Ig) isotype induced at mucosal sites (Brandtzaeg,
1989), where it is believed to mediate defense mechanisms (Lamm, 1997; Mazanec et al.,
1993). IgA monomers are polymerized through the J chain, which is added just before
secretion of IgA by plasma cells (Johansen et al, 2000). IgA-producing plasma cells migrate
to the basolateral surface of mucosal epithelial cells, where secreted IgA is transported to the
luminal side by the polymeric Ig receptor (pIgR), expressed at the basolateral side of
epithelial cells lining the mucosal surfaces (Mostov, 1994) (Fig. 2). Translocation of IgA
involves enzymatical cleavage of the pIgR whereby the extracellular part of the molecule, the
secretory component (SC), in complex with polymeric IgA (pIgA) forms the secretory IgA
(sIgA) that is released into the luminal secretions (Norderhaug et al., 1999). pIgR-mediated
transcytosis does not require the presence of a ligand, resulting in a continuous release of the
SC into external secretions. In human exocrine fluids, 30 to 60% of SC is normally in a free
form (Brandtzaeg, 1973). The constitutive expression of pIgR in epithelial cells can be further
upregulated by certain cytokines, such as IFN-γ (Sollid et al., 1987; Loman et al., 1997;
Youngman et al., 1994), and TNF-α (Kvale et al., 1988).
Figure 2. Transport of mucosal IgA. (A) IgA translocation through the epithelium. (B) Structure of sIgA.
13
Functions of IgA
IgA is thought to be the most important Ig for lung defence by shielding the mucosal surfaces
from penetration by microorganisms and foreign antigens, as well as neutralizing bacterial
products such as enzymes and toxins (Lamm, 1997; Mazanec et al., 1993). Other mechanisms
include its ability to agglutinate microbes and interfere with bacterial motility by interacting
with their flagella. Immune complexes of IgA and encountered antigens can be transported
across epithelial cells from basal to apical surfaces in vitro (Kaetzel et al., 1991), and in vivo
(Robinson et al., 2001). Foreign substances that have breached the mucosal surface could
thereby be eliminated from the body by IgA-mediated transport back through the epithelium.
Additionally, IgA appears to be able to interact with viral antigens during transcytosis and
interfere with viral synthesis and/or assembly, thereby neutralizing viruses intracellularly
(Mazanec et al., 1992). In vitro studies have demonstrated evidence for such intraepithelial
cell neutralization against Sendai and measles viruses (Fujioka et al., 1998; Yan et al., 2002),
influenza virus (Mazanec et al., 1995), rotavirus (Burns et al., 1996; Feng et al., 2002), and
recently against HIV (Huang et al., 2005).
Whereas the role of sIgA is established in mucosal immunology, the function of serum IgA
antibodies is mostly unknown. Serum IgA is considered to be a “discrete housekeeper”
because IgA-immune complexes can be removed by the phagocytic system with little or no
resulting inflammation.
Fcα Recptors
IgA has traditionally been viewed as a non-inflammatory antibody. The inability of sIgA to
fix complement efficiently or to act as an opsonin is an advantage in secretions, where
induction of an inflammatory reaction would likely affect the integrity of the mucosal surface
(Kerr, 1990). However, characterization of FcRs for IgA (FcαRs) has challenged the
paradigm of IgA as a non-inflammatory or even anti-inflammatory immunoglobulin. The
human FcαR (FcαRI, CD89) is expressed on eosinophils (Monteiro, et al., 1990), neutrophils
(Honorio-Franca et al., 2001), monocytes (Patry et al., 1996), macrophages subsets, Kupffer
cells, and DCs (Geissmann et al., 2001). Whereas interaction of IgA with the pIgR is noninflammatory, CD89 mediates a broad spectrum of pro-inflammatory and immunomodulatory
14
effects (Morton et al., 1996). A second line of defence at the interface of mucosal and
systemic immunity, provided by FcαR-serum IgA interactions on Kupffer cells has been
proposed (van Egmond et al., 2000). Under physiological conditions, sIgA inhibits invasion
of pathogens into the mucosal surface, as a first line of defence, without activating
inflammatory responses. Under pathological conditions the pathogens can invade the portal
circulation due to disruption of the mucosal barrier, where they subsequently will be exposed
to serum IgA. Concomitantly produced inflammatory cytokines induce FcαRI expression on
Kupffer cells, and FcαRI-positive Kupffer cells can thereby phagocytose the pathogens that
have entered the circulation.
Despite extensive studies on FcαRs in man, there is scarce knowledge about the structure and
function of FcαRs in mice. A CD89 homologue in rats was recently identified (Maruoka et
al., 2004), but no mouse homologue has yet been found. A common Fcα/µR was newly
characterized on human and mouse B cells and macrophages, and on different tissues like
liver, spleen, and intestine (Shibuya et al., 2000). Using the human FcαR probe, transcripts of
two cDNAs, PIR-A and PIR-B, isolated from a mouse splenic library, were detected
(Kubagawa et al., 1997). The PIR-A and PIR-B genes were fund to be expressed on B cells
and cells from the myeloid lineage.
IgA deficiency
Selective IgA deficiency (SIgAD), using 0.05 g/l as the upper limit for diagnosis in adults, is
the most common form of primary immunodeficiency in the western world and affects
approximately 1/600 individuals (reviewed in Hammarström et al., 2000). The variability in
the prevalence in different ethnic groups is striking, 1/18000 in Japanese and 1/4000 in
Chinese, suggesting a genetic implication for the disorder. Although sIgA has a clear
biological role, SIgAD is a heterogeneous condition with symptoms ranging from none to
recurrent respiratory or gastrointestinal diseases, atopy, asthma, inflammatory or autoimmune
disorders such as systemic lupus erythematosus, rheumatoid arthritis, and pernicious anaemia
(Burks and Steele, 1986; Burrows and Cooper, 1997; Cunningham-Rundles, 2001), although
most individuals remain without symptoms. IgA deficiency can be associated with IgG
subclass deficiency (Oxelius et al., 1981), making it, in some cases, difficult to distinguish
between the cause and the symptoms. The fact that most IgA-deficient individuals are healthy
15
may be partly explained by a compensatory increase in IgM-bearing B cells and increased
secretory IgM in mucosal fluids (Brandtzaeg and Nilssen, 1995; Norhagen et al., 1989).
However, secretory IgM does not completely replace sIgA functionally, particularly not in the
upper respiratory tract (Brandtzaeg et al., 1987).
16
TB: THE DISEASE
TB is primarily a pulmonary infectious disease. Following infection with M. tuberculosis
there is an early transient influx of granulocytes, but the hallmark of mycobacterial infections
is the development of granulomatous lesions (Rook and Bloom, 1994). Although granuloma
formation provides a mean of containing infection, granulomas may displace and destroy
adjacent tissues. Initially well-formed granulomas may gradually develop central caseation
which may lead to extensive fibrosis or cavity formation in the lungs.
TB can involve any organ system in the body. While pulmonary TB is the most common
clinical manifestation, extrapulmonary TB is also an important clinical problem. The bacilli
can spread from the initial site of infection in the lung through the lymphatics or blood to
other parts of the body an cause extrapulmonary TB of the pleura, lymphatics, bone, genitorurinary system, meninges, peritoneum, or skin. Before the HIV pandemic, and in studies
involving immunocompetent adults, it was observed that extrapulmonary TB constituted
about 10-20% of all cases with TB (Fanning, 1999; Weir and Thornton, 1985). In HIVpositive patients, extrapulmonary TB accounts for more than 50% of all cases of TB (Theuer
et al., 1990). The most common extrapulmonary sites in HIV-positive individuals are the
lymph nodes.
Disseminated, or miliary, TB refers to involvement of two or more organs simultaneously,
and can occur during primary infection or after reactivation of a latent infection, as well as
after reinfection.
Diagnosis
Early confirmation of the diagnosis of TB is a challenging problem. The established methods
have limitations of speed, sensitivity and specificity. In general, it is more difficult to
diagnose extrapulmonary TB than pulmonary TB, since this often requires invasive
procedures to obtain diagnostic specimens for histological or bacteriological confirmation.
17
Tuberculin skin test
The TST is currently the only widely used method for identifying infection with M.
tuberculosis in persons who do not have TB. This test is based on the fact that infection with
M. tuberculosis produces a delayed-type hypersensitivity reaction to certain mycobacterial
components in the extracts of culture filtrates called “tuberculins”. The test (Mantoux method)
is administered by intradermal injection of tuberculin purified protein derivative (PPD), which
produces a wheal of the skin. The visible induration (in mm) is measured 48 and 72 hours
after injection. There are however concerns regarding this test. Several factors may contribute
to false-negative results such as age, poor nutrition and general health, overwhelming acute
illness, or immunosuppression, such as medications or HIV infection (American Thoracic
Society, 2000). In addition, false-positive results can occur in individuals who have been
infected with other mycobacteria, including vaccination with BCG. Because of its low
sensitivity, TST can not be used to rule out the possibility of active TB.
Microscopy
The detection of acid-fast bacilli in stained smears examined microscopically is the first
bacteriologic evidence of the presence of mycobacteria in clinical specimens. Acid-fast
staining procedure depends on the ability of mycobacteria to retain dye when treated with
mineral acid or an acid-alcohol solution. Smear examination is rapid, inexpensive, technically
simple, and highly specific for acid-fast bacilli, such as M. tuberculosis. Additionally, it gives
a quantitative estimation of the number of bacilli being excreted. Identification of smear
positive patients is of major importance because only smear positive pulmonary TB patients
are regarded as highly infectious to others. However, smear microscopy can not discriminate
between M. tuberculosis and other mycobacteria, and in addition, lacks sensitivity since 5000
to 10000 bacteria/ml is needed for a positive result (American Thoracic Society, 2000).
Cultivation
Mycobacterial culture is the ultimate proof of mycobacterial infection and is often used as the
reference method due to its high sensitivity and specificity (Schirm et al., 1995; Walker,
2001). As few as 10 bacteria/ml of material is necessary for detection using this method
(American Thoracic Society, 2000). Also, the cultivation of the etiological agent has been
18
essential for species identification, drug susceptibility testing and monitoring the response to
therapy. Nevertheless, the slow growth rate of M. tuberculosis and most other mycobacterial
pathogens complicates the use of cultivation as diagnostic technique.
Molecular methods
The use of nucleic acid amplification for the diagnosis of TB is rapidly evolving. These
technologies allow for the amplification of specific target sequences of nucleic acids that can
be detected through the use of a nucleic acid probe, and both RNA and DNA amplification
systems are commercially available (Cohen et al., 1998).
Treatment
Treatment against TB requires a combination of different drugs. Isoniazid, rifampicin,
pyrazinamide and streptomycin constitute the first line of TB drugs to be used. A combination
of three drugs is required to avoid the development of MDR TB, and must be taken for a long
period of time, usually 6 months or more.
The WHO declared TB as a global emergency in 1993. One reason for this extraordinary
declaration was an epidemic of MDR TB in the USA in the late 1980s and early 1990s,
predominantly in HIV infected prisoners and homeless (Centers for Disease Control and
Prevention, 1991). The management of MDR TB is a challenging problem, given that
treatment is less effective, more toxic and much more expensive compared to treatment of
patients with drug susceptible TB. In the last few years the fluoroquinolone group of drugs
has been added to the chemotherapy to TB, and has been used as a part of regimens to treat
patients with MDR TB.
Current vaccine
The current vaccine against TB, the attenuated M. bovis bacillus Calmette-Guérin (BCG), was
developed by the French scientists Calmette and Guérin in the first decade of the 20th
century. It has been used for more than 70 years and has been given to more people than any
other vaccine. Although it can prevent disseminated and meningeal TB in young childen, its
efficacy against the most prevalent disease form, pulmonary TB in adults, has been strongly
19
questioned. Data concerning the protective efficacy of BCG in adults range from 0% in South
India to 80% in the UK (Fine, 1995). The reason for this variation in efficacy might depend
on several factors, including variation in BCG strains, vaccination dose, vaccination
protocols, or inappropriate handling of vaccine (Hess and Kaufmann, 1999). Additionally,
studies from animal models suggest that prior exposure to live environmental mycobacteria
primes the host immune system against mycobacterial antigens shared with BCG, and recall
of this immune response upon vaccination results in accelerated clearance of BCG and
therefore decreased protection against TB (Kamala et al., 1996; Brandt et al., 2002).
Moreover, the hypothesis that the protection by BCG vaccination wanes over time has been
brought up (Sterne et al., 1998).
Another factor underlying the failure of BCG could be the route of vaccination. BCG is
currently administered intradermally which might not be optimal for inducing protective
immunity in the respiratory tract. Vaccination at the mucosal site has been believed to be
superior to vaccination at other sites for eliciting protective immune responses against
mucosal infectious diseases (Davis, 2001). Falero-Diaz and colleagues reported that intranasal
(i.n.) vaccination with BCG conferred potent protection against airway M. tuberculosis
challenge (Falero-Diaz et al., 2000). Another study demonstrated that a single i.n. BCG
vaccination is superior to the subcutaneous route for protection against pulmonary TB in mice
(Chen et al., 2004). In addition, i.n. vaccination offers desirable vaccination advantages, such
as ease, feasibility, and the ability to trigger to trigger both mucosal and systemic immune
activation (Davis, 2001).
New vaccine candidates
Over the past several years there has been an intensive effort to develop a new vaccine against
TB, and the TB vaccines under development can be divided into two categories: preexposure
or postexposure vaccines. Preexposure vaccines prevent infection and subsequent disease and
should be given to uninfected persons. Postexposure vaccines aim to prevent or reduce
progression to disease, and would be given to persons already infected with M. tuberculosis.
To date, a multitude of vaccine candidates have been tested in animal models. The increasing
knowledge of the tubercle proteins and development of techniques to help identify the most
immunogenic antigens, have generated numerous subunit vaccine candidates. Another area in
which there has been substantial interest and progress is the DNA vaccines, and several
20
mycobacterial antigens have been targeted in this manner (Huygen, 1998). Whole bacterial
vaccines have the advantage of a built-in adjuvanticity, as well as containing both protein- and
non-protein antigens. This strategy includes live attenuated bacteria, as well as engineering
and overexpression of distinct antigens to improve the immunogenicity.
ANIMAL MODELS IN TB
Experimental animal models of TB are central to vaccine development. As for many
infectious diseases, there is no ideal model for TB. The most common models of M.
tuberculosis infection are the mouse and the guinea pig. In neither of these species does the
disease perfectly match that seen in human, however many aspects of immunity are the same.
Nevertheless, the importance of being careful when extrapolating results from animal
experiments to human TB must be emphasised.
The mouse is the most widely used species and provides many advantages (reviewed in
Kaufmann, 2003). The mouse genome has been completely sequenced, and besides mice
being relatively low in cost, there is a wealth of information on their immune system and the
techniques and reagents for mechanistic studies are abundant. Additionally, the availability of
genetically targeted mice makes it possible for in vivo studies of the relevance of particular
cells and molecules. A large number of gene knockout and knock-in mice, both constitutive
and conditional have been generated. However, the mouse is relatively resistant to M.
tuberculosis, and does not develop the severe pathology seen in some human patients.
The guinea pig is generally considered even more susceptible to M. tuberculosis than humans
and therefore provides a very sensitive model for testing the efficacy of novel vaccine
candidates. Moreover, granulomatous lesions in guinea pigs are very similar to those in
human TB patients. Finally, the group 1 CD1 molecules, responsible for presentation of
mycobacterial glycolipids to CD1-restricted T cells, are present in humans and guinea pigs
but absent in mice (Ulrichs and Kaufmann, 2002; Schaible and Kaufmann, 2000).
The non-human primate model is considered the closest match for human disease in terms of
pathology. The immune response in this species is very similar to those in humans and most
reagents for human cells and molecules can be applied to primate studies. Experiments in
21
non-human primates should be limited to critical experiments, and for final validations
directly preceding clinical trials of vaccines and therapeutic agents in humans.
22
PRESENT STUDY
AIM
The upper respiratory tract is the port of entrance for the mycobacterium, and the first
encounter between the host and the pathogen is taking place in the lung. This raises the
hypothesis that protective immunity against mycobacterial infections should include a local
respiratory mucosal immunity in the lungs, in addition to a cell-mediated immune response.
We therefore aimed to investigate the importance of IgA, the major Ig isotype present at
mucosal sites, in the respiratory mucosal immunity against infection with intracellular
mycobacteria.
The specific objectives were:
ƒ
To evaluate the relevance of IgA in the respiratory tract in protection against
mycobacterial infection using IgA-deficient mice (paper I).
ƒ
To study the role of actively secreted IgA in protection against mycobacterial infection
in the respiratory tract using pIgR-deficient mice (paper II).
23
MATERIALS AND METHODS
Antigen and adjuvant
The endotoxin-free recombinant PstS-1 protein was used as antigen for immunizations. This
38 kDa lipoprotein is a putative phosphate-transport receptor, and a known B-, and T-cell
stimulant that has been suggested as a potential immunodiagnostic reagent (Wilkinson et al.,
1997). As adjuvant, for targeting the mucosal immune system, cholera toxin (CT) was used.
CT is a strong mucosal adjuvant that markedly increases antigen presentation by DCs,
macrophages and B cells (reviewed in Holmgren, 2005). It acts by up-regulation of MHC-,
and costimulatory molecules as well as chemokine receptors on APCs, and by increasing the
permeability of the epithelium leading to enhanced uptake of co-administered antigen.
Immunizations
IgA-deficient mice and wild-type non-targeted littermate (IgA+/+) mice (Harriman et al.,
1999), provided by Dr. I Mbawuike (Baylor College of Medicine, Houston, USA), were used
in Paper I. pIgR-deficient mice (Shimada et al., 1999), obtained from Dr. M. Nanno (Yakult
Central Institute for Microbiological Research, Tokyo, Japan), and wild-type C57BL/6 mice
were used in Paper II. All mice were kept and bred in the Animal House in the Arrhenius
Laboratories at Stockholm University, Sweden.
For investigating the induced mucosal immune responses, groups of mice were immunized
i.n. using the PstS-1 protein (10 µg/dose) formulated with CT (1 µg/dose). Each group of
mice received three immunizations separated by three-week intervals. I.n. immunizations
were performed on mice anaesthetized with isofluorane and the antigen (30 µl) was applied to
the external nares, 15 µl in each nostril, using a micropipette.
Intranasal infection with BCG and quantification of bacterial load
Mice were inoculated with 106 colony-forming units (CFU) of live BCG i.n., in 30 µl PBS,
under anesthesia with isofluorane, two weeks after the last immunization with PstS-1. At
weeks 1 or 4 after infection, mice were sacrificed and the numbers of viable bacteria in the
24
lungs and broncho-alveolar lavage (BAL) were determined. Serial dilutions of the lung
homogenates and BAL samples were plated on Middlebrook 7H11 agar plates. The numbers
of CFU were determined after 2-4 weeks of incubation at 37 °C.
M. tuberculosis aerosol infection and quantification of bacterial load
All animals were kept under controlled conditions in a BL High Security Facility. Mice were
placed in the exposure chamber of an airborne infection apparatus. The nebulizer
compartment was filled with 7 ml of the bacillar suspension at a previously calculated
concentration to provide an approximate uptake of 20 viable bacilli within the lungs. The
numbers of viable bacteria in the left lung homogenates on weeks 3 and 8 after infection were
followed by plating serial dilutions on Middlebrook 7H11 agar plates and counting bacterial
colonies after 21 days incubation at 37 ºC.
Antibody production
Total Ig and specific antibody levels in serum, BAL and saliva were analyzed by ELISA.
ELISA plates were coated with either the PstS-1 protein (2 µg/ml), or unlabeled anti-mouse Ig
(1 µg/ml). Samples were applied to the wells in serial dilutions starting from 1:200 for serum,
1:4 for saliva, and 1:50 for BAL. Secondary antibodies, alkaline-phosphatase labelled goat
anti-mouse Ig isotypes, were thereafter added. Finally, a colourless substrate for the enzyme,
p-nitrophenyl phosphate, was added which will be cleaved and generated into a coloured
reaction product. Optical density was read in a multiscan plate reader at 405 nm. Background
level was defined as OD obtained using PBS-Tween 0.05% (vol/vol) instead of samples. ODdilution curves were plotted (after background value correction) and titres were defined as the
inverse log10 of the lowest dilution giving OD of 0.1 for BAL and serum, and an OD of 0.2
for saliva samples.
Cytokine-producing cells
The numbers of IFN-γ-, and TNF-α-producing cells in the lungs were determined by
ELISPOT. PVDF plates were coated with anti-IFN-γ antibodies (15 µg/ml), or anti-TNF-α
antibodies (5 µg/ml). Cell suspensions obtained from the lungs of individual mice were added
25
to the wells and incubated in the presence of the PstS-1 protein or PPD (10 µg/ml), to induce
cytokine production. After 12 h (TNF-α), or 36 h (IFN-γ) of incubation at 37°C in a
humidified 5% CO2 incubator, biotinylated anti-IFN-γ antibodies (2 µg/ml), or anti-TNF-α
antibodies (0.5 µg/ml) were added to the wells. Streptavidin conjugated to alkaline
phosphatase at a 1:1000 dilution was thereafter added. Individual cytokine-producing cells
were visualized by addition of the BCIP/NBT substrate solution and enumerated in a
computerized ELISPOT counter.
Cytokine production
Cell suspensions obtained from lungs of individual mice, 4 weeks after i.n. infection with
BCG, were cultured (2x105 cells/well) with BCG (1:2 ratio) as stimulation. Supernatants were
collected after 48 h and production of IFN-γ and TNF-α was assayed by ELISA.
Quantification of mRNA expression
Levels of IFN-γ-, regulated upon activation normal T-cell sequence (RANTES)-, inducible
nitric oxide synthase (iNOS)-, and TNF-α-mRNA expression in the lungs of M. tuberculosisinfected mice were analyzed by real-time PCR. In short, total RNA from the middle right lobe
of the lungs of aerosol-infected mice was extracted with a commercial phenol-chloroform
method, RNAzol, and reverse transcribed using a Superscript RT kit to obtain cDNA. The
quantitative analysis was performed using a LightCycler™ System. Hypoxanthine guanine
phosphoribosyl transferase (HPRT) mRNA expression was analyzed for every target sample
to normalize for efficiency in cDNA synthesis and RNA loading. A ratio based on the HPRT
mRNA expression was obtained for each sample.
Histology and morphometry
Histopathological analysis of the lungs of M. tuberculosis-infected mice were performed.
Briefly, two right lung lobes from each mouse were fixed in buffered formalin and
subsequently embedded in paraffin. Every sample was stained with hematoxilin-eosin. For
histometry, 5 µm thick sections from each specimen were stained with hematoxilin-eosin and
photographed at 6x using a Stereoscopic Zoom SMZ800 microscope and a Coolpix 990
26
digital camera. Sections of 8 lung lobes were studied in each case. A sequence of appropriate
software programs was used to determine the area of each single lesion and the total tissue
area on photomicrographs.
Statistical analysis
Experimental groups were compared by ANOVA followed by Tukey posttest. Differences in
bacterial loads between wild-type and gene-targeted mice were compared by Mann-Whitney
U-test. The level of significance was set at p<0.05.
27
RESULTS AND DISCUSSION
Paper I
In paper I we investigated the role of IgA in the protection against i.n. challenge with BCG
using IgA-deficient mice. The mucosal immune system provides the first line of defence
against entrance of a multitude of ingested and inhaled microorganisms, and the most
characteristic component of mucosal immunity is sIgA. Induction of IgA responses in the
respiratory tract could provide a protective function against diseases caused by respiratory
infections, among them TB. We therefore wanted to investigate the relevance of IgA in the
respiratory mucosal immunity against mycobacterial infection, after i.n. immunization with
the mycobacterial antigen PstS-1 formulated with the mucosal adjuvant CT. It is currently
well established that protective immunity against the intracellular pathogen M. tuberculosis
requires a cell-mediated immune response (Schaible et al., 1999; Orme et al., 1993). The role
of B cells and antibodies in protection against mycobacterial infections has been less
extensively studied, and the few studies performed, using B-cell deficient mice, reveal
contradictory results with reports of increased viable bacterial counts (Vordermeier et al.,
1996), reduced granuloma size but similar bacterial growth (Boise et al., 2000), as well as no
effect at all (Turner et al., 2001). The role of antibodies in protection against intracellular
bacterial infections has been re-evaluated during the last years, and reports demonstrating
antibody-mediated protection in this context have been published (Edelson and Unanue, 2001;
Winslow et al., 2000; Hellwig et al., 2001; Glatman-Freedman, 2003). In concordance with
this, our results demonstrated that IgA-deficient mice, immunized with PstS-1 in combination
with CT, were more susceptible to BCG infection compared to immunized wild-type mice, as
shown by higher bacterial load in the lungs and BAL. This result suggests a protective role for
IgA in protection against infection with mycobacteria in the respiratory tract.
To address the mechanisms underlying the increased susceptibility to BCG infection
displayed by the IgA-deficient mice, we analyzed the induced local immune responses, after
i.n. immunization with PstS-1 formulated with CT, in IgA-knockout and wild-type mice.
Whereas no IgA was detected in either saliva or BAL from IgA-/- mice, they displayed higher
levels of both antigen-specific and total IgM compared to IgA+/+ mice. Moreover, IgAdeficient mice displayed an impaired T cell response, as revealed by significantly reduced
TNF-α- and IFN-γ-production in the lungs, when compared to wild-type mice.
28
Previous studies have demonstrated impaired T cell responses in both IgA-deficient and Bcell-deficient mice (Arulanandam et al., 2001; Zhang et al., 2002; Vordermeier et al., 1996),
but the mechanistic details behind have not been clarified. One possibility is involvement of
signalling through Fcα receptors. The human FcαR (CD89) has been well characterized
whereas no murine homologue has yet been identified. Transcripts of two cDNAs, PIR-A and
PIR-B, have been isolated from a mouse splenic library using the human FcαR probe, and
their expression was restricted to B cells and myeloid cells, including granulocytes,
macrophages, and mast cells (Kubagawa et al., 1997). A common Fcα/µR, both human and
murine, has been identified and is expressed on B cells and macrophages (Shibuya et al.,
2000; Sakamoto et al., 2001), and moreover, activated mouse T cells express FcαRs (Sandor
et al., 1992). In vitro studies have demonstrated that monomeric and polymeric IgA
stimulated TNF-α production and apoptosis in mouse macrophage cell lines (Reljic et al.,
2004). It is therefore possible that absence of IgA could lead to reduced or inadequate
activation of macrophages at mucosal sites.
All together, our results suggest a role for IgA in the protection against mycobacterial
infection in the respiratory tract, by blocking the entrance of the bacilli into the lungs and/or
by modulating the locally induced proinflammatory immune responses.
Paper II
In addition to investigating a possible role of IgA in protection against mycobacterial
infection in the respiratory tract, we also addressed the role of actively secreted antibodies,
specifically sIgA. For this purpose we used pIgR-deficient mice. Although B-cell deficient
mice have previously been used in infection studies with mycobacteria (Vordermeier et al.,
1996; Boise et al., 2000; Turner et al., 2001), mice lacking actively secreted antibodies have
never been used. Generation of pIgR-knockout mice present a unique opportunity to explore
the relative contribution of locally secretory antibodies versus systemic immunity in the
protection against mycobacteria, as has been done previously in virus-infection studies
(Asashi et al., 2002).
29
Initially, the immunological status regarding antibodies, as well as the induced immune
responses upon i.n. immunization with PstS-1 in combination with CT in the pIgR-/- mice was
assessed. Interestingly, although no antigen-specific IgA was detected in saliva from the
knockout mice, IgA levels in BAL were comparable between the two groups. This finding
indicates differences between the upper and lower respiratory tract concerning the
mechanisms of IgA transport to mucosal secretions. The possibility of transport mechanisms
other than through the pIgR can not be ruled out, nor can leakage through the epithelial
membrane, which has been previously reported (Johansen et al., 1999), and their respective
contributions may be different in the upper and lower respiratory tract. Our finding indicates
that transport of IgA in the upper respiratory tract is to a larger extent pIgR mediated than in
the lower respiratory tract, where contributions from other transport mechanisms or leakage
may have a greater impact.
Protection studies against i.n. infection with BCG revealed that pIgR-/- mice were more
susceptible to mycobacterial infection than wild-type mice, with significantly higher bacterial
burden in the lungs. Although these mice had similar levels of IgA antibodies in the BAL
after i.n. immunization as wild-type mice, their higher bacterial load in the lungs after BCG
infection could indicate that the SC plays a role for the protective capacity of sIgA. The SC
has been associated with several functions, such as stabilization and protection of
dimeric/polymeric IgA and, through its carbohydrate residues, ensuring appropriate
localization of the complex to the mucos (Phalipon et al., 2002). Moreover, BCG infected
pIgR-/- mice displayed a considerable reduction in TNF-α and IFN-γ production by lung
mononuclear cells. Since these cytokines are central for protective immunity against
mycobacterial infections (Flynn et al., 1995; Cooper et al., 1993), this impaired
mycobacterium-induced proinflammatory response most likely contributed to the higher
bacterial load in the lungs of these mice.
We next addressed the role of sIgA in protection against natural infection with virulent M.
tuberculosis. Viable bacterial counts in the lungs clearly demonstrated that pIgR-knockout
mice were more susceptible than wild-type mice at the early phase of infection. At the late
phase of infection no differences between the two groups could be seen. The higher
susceptibility to M. tuberculosis infection was associated with significantly reduced
expression of important protective factors in the lungs. Expression of the proinflammatory
cytokines IFN-γ and TNF-α were substantially reduced, which corroborate with the findings
30
from the BCG-infected pIgR-deficient mice. Moreover, expression of iNOS was also reduced.
Production of reactive nitrogen intermediates is an important defence mechanism against
microbial pathogens exerted by macrophages. A reduced level of iNOS in the lungs of
infected pIgR-/- mice is therefore in line with the higher bacterial load in the lungs of these
mice. Moreover, reduced levels of RANTES expression was also seen in the lungs of infected
knockout mice. This C-C chemokine is involved in attracting monocytes and lymphocytes to
sites of infection, as well as promoting Th1 type of responses (Taub et al., 1996; Dairaghi et
al., 1998; Chensue et al., 1999). These findings indicate that pIgR-/- mice display lower
expression of factors important for protective immunity against TB and proper granuloma
formation. Indeed, histological analysis of granulomas in the lungs demonstrated a decreased
infiltration of macrophages and lymphoytes, but increased numbers of neutrophils in the
pIgR-knockout mice compared to wild-type mice. In addition, a higher degree of necrosis and
karyorrhexis was evident in the granulomas in the knockout mice. This qualitative
histopathological difference was detected at the early phase of infection, whereas no major
differences in granuloma composition where seen during the late phase. In this context it is
interesting to note that human SC has been reported to bind and inactivate IL-8 (Marshall et
al., 2001), a chemokine important for the attraction of neutrophils. Although no rodent IL-8
homologue has been identified, a murine IL-8 receptor homologue exists (Cerretti et al.,
1993), and mice deficient for this receptor display impaired neutrophil responses and reduced
leukocyte migration (Lee et al., 1995; Becker et al., 2000; Hang et al., 2000). It is therefore
tempting to speculate that the ligand for this receptor homologue could in a similar fashion
bind to SC as human IL-8, whereby mucosal secretions from pIgR-/- mice would contain
higher levels of IL-8 compared to wild-type mice, resulting in increased chemotaxis of
neutrophils in the knockout mice.
These findings suggest that mycobacteria-infected pIgR-/- mice display a delayed immune
response, due to reduced expression of RANTES and consequently impaired attraction of
macrophages and lymphocytes to the site of infection, as well as lower proinflammatory
response. As a consequence, pIgR-deficient mice exhibit higher bacterial growth in the lungs,
which in turn triggers an increased granulomatous infiltration in order to control the bacilli. At
the later phase of infection pIgR-/- mice have managed to control the bacterial growth and
display comparable viable bacterial counts in the lungs as wild-type mice.
31
Collectively, our results imply a role for sIgA in modulation of mycobacteria-induced
proinflammatory immune responses and subsequently for protection against mycobacterial
infections.
32
CONCLUDING REMARKS
The importance of B cells and antibodies in protection against the intracellular pathogen M.
tuberculosis remains controversial, although a protective capacity of antibodies has lately
been reconsidered. Mucosal surfaces in the respiratory tract are constantly challenged by
massive amounts of inhaled microorganisms and antigens, and in many cases the main
protective effector function against mucosal infections is the production of pathogen-specific
local sIgA responses, achieved solely via activation of the mucosal immune system.
Based on the fact that TB is transmitted via the respiratory tract, and IgA in mucosal
secretions is the most characteristic component of mucosal immunity, we investigated the
relevance of IgA in protection against mycobacterial infection in the respiratory tract. Our
results demonstrate that IgA antibodies play a role in the protection against mycobacterial
infection in the respiratory tract, by blocking the entrance of the bacillus into the lungs and/or
by modulating the locally induced proinflammatory immune responses. The mechanisms
explaining the defective proinflammatory responses in the lungs of IgA- and pIgR-deficient
mice might involve impaired signalling through the FcαR, or a homologous receptor, which
may lead to inadequate activation of pulmonary macrophages. Insufficient activation of
macrophages could consequently result in suboptimal production of factors important for
attraction or migration of immune cells to the lungs.
In a long perspective, it is important to consider the advantages of efficiently targeting
vaccine antigens to appropriate components within the systemic and mucosal immune
compartments. As TB is primarily an infection of the respiratory tract, a successful vaccine
against TB should stimulate appropriate immunological responses to counteract the tubercle
bacillus in the lungs.
33
ACKNOWLEDGEMENTS
I wish to express my warm gratitude to all who have contributed to these studies and in
particular my supervisor Carmen Fernández and co-supervisor Marita Troye-Blomberg.
Many thanks to my co-author and dear friend Ariane Rodríguez Munoz for all the good and
bad times we have shared, and for our fruitful collaboration.
I sincerely thank everyone that has helped me in the lab and especially Margareta “Maggan”
Hagstedt, Ann Sjölund, John Arko-Mensah, Esther Julian, Karin Lindroth and Monika
Hansson.
Nora Bachmayer, Yvonne Sundström, and Lisa Israelsson, thanks for your friendship, support
and all the fun we have together!
Thanks to everyone at the Department of Immunology for creating a nice and friendly
working atmosphere.
Finally I would like to express my deepest and warmest gratitude to my family and all my
friends. Mamma, pappa, Karin and mormor, thanks for all your love, support and patience
with me during the last 30 years. Simon, thanks for all the fun and good football games.
This work was financially supported by the European Commission specific RTD program
QLK2-CT-1999-00367 and Hjärt- och Lungfonden.
34
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