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Infections in the immunopathogenesis of chronic inflammatory bowel disease
seminars in I M M U N OL OG Y, Vol 10, 1998: pp 69]78
Infections in the immunopathogenesis of chronic inflammatory bowel disease Michael Merger and Ken CroitoruU
In chronic inflammatory bowel disease, self-destructive, exaggerated inflammation seems to occur in the absence of a well defined pathogen. However, epidemiological data strongly suggests that development of disease does not depend on endogenous factors alone. In this review, we summarize how a possible role for microbial factors can be reconciled with the current understanding of etiology and pathogenesis of IBD. The data presented does not support that IBD is an infectious disease nor that it is a selfantigen-specific autoimmune disease, however, recent findings increasingly suggest that tissue damage might be caused by a non-specific autoaggressive inflammation which is driven by common, ubiquitous microbial agents derived from the bacterial flora in the intestinal lumen.
and immunosuppressive agents. Development of strictures, fistulae and abscesses often requires surgical intervention. In this review, we summarize the current understanding of the immunopathogenesis of IBD with a particular focus on the possible role of infectious agents.
Etiology and pathogenesis of IBD: convergence of genetic predisposition, immune dysregulation and environmental factors The cause of IBD is unknown. It has been suggested that this chronic inflammatory disease occurs in genetically predisposed individuals, possibly on the basis of an intrinsic immune defect.1,2 In addition, it is believed that as yet unidentified environmental factors play an important role. A somewhat similar concept, induction of self-aggressivity by an exogenous agent in genetically predisposed individuals, has been proposed to explain the epidemiology of autoimmune diseases.3 ] 5 In Crohn’s disease and ulcerative colitis, tissue damage seems to result from an inappropriate and self-destructive inflammation. On the other hand, there is a striking similarity between IBD and a number of infectious intestinal diseases. The primary trigger of inflammation in IBD is unknown. Chronic inflammation in IBD may not be initiated by a single step but may require the additive effects of several factors.6 It is possible that in IBD, the initial event merely enhances the ‘inclination’ of a patient to develop IBD later in life, while clinical disease requires some non-specific promoting factor. Due to these difficulties the initial trigger of IBD has not been identified yet.
Key words: IBD r immunology r autoimmunity r infection Q1998 Academic Press Ltd
CHRONIC INFLAMMATORY BOWEL disease ŽIBD. encompasses two apparently distinct disease entities, Crohn’s disease and ulcerative colitis. Crohn’s disease is characterized by a transmural, granulomatous inflammation of discontinuously affected intestinal segments. Ulcerative colitis, on the other hand, involves mostly the mucosal, but not the deeper layers of the intestinal wall, is restricted to the large intestine and spreads continuously from distal to proximal segments. Both diseases can follow an unpredictable, often chronic relapsing course. Their treatment is symptomatic and based on non-specific anti-inflammatory drugs such as 5-aminosalicylic acid compounds, corticosteroids
From the Intestinal Diseases Research Program, Department of Medicine, McMaster University, Hamilton, Ontario, Canada and U McMaster University Health Sciences Center, Division of Gastroenterology, 1200 Main Street West, Room 4W8, Hamilton, Ontario, Canada L8N 3Z5 Q1998 Academic Press Ltd 1044-5323r 98r 010069q 10 $25.00r 0r si970106
Genetic aspects of IBD Epidemiological data supports the notion that development of IBD requires an inherited predisposition. 69
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For example, first degree relatives of patients with IBD are much more likely to develop disease than second degree relatives.7 Linkage analysis has begun to identify genetic loci associated with IBD. As with autoimmune diseases, there may be a link between IBD and certain HLA alleles.7 HLA molecules may contribute directly to the pathogenesis of immunologically mediated diseases due to their role in shaping the immune repertoire and to presentation of antigen peptides. It is also possible that the association with HLA alleles reflects their close proximity to the truly responsible genes. The gene for the cytokine TNF, which may play an important role in both autoimmune diseases and IBD is closely adjacent to the HLA complex. IBD patients are at an increased risk of developing autoimmune disorders such as autoimmune hemolytic anemia, raising the possibility that there may be a common genetic disposition for these diseases. In addition, some common extraintestinal manifestations of IBD Žaxial arthritis, biliary hepatic inflammation. may reflect association with idiopathic ankylosing spondylitis and sclerosing cholangitis which are believed to be autoimmune diseases.8 However, similar to idiopathic sclerosing cholangitis and ankylosing spondylitis, sclerosing cholangitis and axial arthritis as extraintestinal manifestations of IBD are associated with HLA B8rDR3 and HLA B27, respectively, whereas IBD itself is not.8,9 Genes located outside the MHC complex may predispose to both autoimmune diseases and IBD. For example, an inherited leakiness of the mucosa barrier could allow penetration of gut luminal antigens and pro-inflammatory molecules triggering the subsequent development of IBD, chronic arthritis or other ‘autoimmune’ diseases, depending on which additional disease-specific genes contribute to the predisposition.9 Intestinal permeability is increased in both patients with Crohn’s disease and a significant percentage of their unaffected relatives and interestingly also in patients with idiopathic axial arthritis.9,10 The disease-specific predisposition to develop IBD could be represented by an inherited abnormality of the local immune regulation in the intestinal mucosa.1 Indirect evidence, summarized in the following section, seems to support this concept.
activation of these cells leads to an increased production of cytokines and other proinflammatory mediators.1 Mucosal T cells in IBD tissue display enhanced responsiveness to IL-2 and an increased cytotoxic ability. Normally, CD8 T cells predominate in the epithelium, probably functioning as suppressor cells, however, epithelial cells in IBD preferentially activate CD4 T cells promoting helper cell activity.11 A shift in the balance between pro and anti-inflammatory mechanisms in IBD is also indicated by the increased production of IgG in the lamina propria, in contrast to the normal predominance of IgA.1 IgA prevents antigens from inducing an inflammatory response Ž‘antigen exclusion’., e.g. due to its lack of complement activation, whereas IgG promotes inflammation. In the normal mucosa, the immune system is constantly exposed to environmental antigens and proinflammatory bacterial products. In the normal intestine, antigens are taken up and processed by specialized structures of the mucosa ŽM cells in the epithelium, specialized lymphoid tissue such as Peyer’s Patches.. The observed expression of activation markers and a constitutive production of small amounts of cytokines by T cells and macrophages12 may reflect this constant activation of immune mechanisms in the normal mucosa.13 Usually, a specific immune response in the mucosa leads to tolerance towards the inducing antigen which does not remain restricted to the gut but also pertains to systemic rechallenge with the antigen. Tolerance may depend on antigen processing by the dendritic cells closely associated with M cells and thus also on an intact mucosal barrier.14 Quantitative uptake of bacterial antigens does not occur through the normal epithelium which, in contrast to M cells, is covered by mucus preventing bacterial adhesion. However, trace amounts of proinflammatory macromolecules penetrating through the epithelium, such as bacterial lipopolysaccharides ŽLPS., may contribute to the ‘physiological’ inflammation in the mucosa. Therefore, prevention of tissue damage depends on the tight regulation of a complex balance between proand anti-inflammatory mechanisms. The normal downregulation of the mucosal immune response is for example reflected by the absence of IL-2 receptor expression by mucosal T cells and CD14 expression Ža high-affinity receptor for LPS commonly expressed by myeloid cells. by macrophages despite the constitutive activation of these cells.15,16 The inflammation in IBD may occur due to the failure to contain this
The mucosal immune system in IBD Tissue destruction in IBD is apparently due to an inappropriate chronic inflammation mediated by specific and non-specific immune effector cells, in particular T cells and macrophages. Recruitment and 70
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pro-inflammatory cytokines IL-12 and IFN-g may also be of benefit is suggested by studies in which experimental colitis, induced by the haptenizing agent TNBS,25 was prevented by anti-IL12 or antiIFN-antibodies. The colitis induced by TNBS is of particular interest, since it could be demonstrated that oral feeding of colon-derived antigens prevented the development of inflammation in this animal model of IBD by inducing oral tolerance towards these self-antigens. 26 This suggests that a similar strategy, currently being examined in patients with multiple sclerosis and rheumatoid arthritis,27 may prove feasible in the treatment of human IBD as well.
‘physiologic’ inflammation in response to nonspecific and ubiquitous pro-inflammatory agents. Indeed, the inflammation in IBD seems to be rather non-specific. Yet, that does not exclude the possibility that it is a specific microorganism which causes this disinhibition of mucosal immune mechanisms. Some mice with an intrinsic dysregulation of the immune system, e.g. due to the lack of certain cytokines, develop spontaneous intestinal inflammation.17 IL-2-deficient mice develop colitis accompanied by an increased production of Th1 cytokines apparently driven by IL-12.18 In TCR ab mutant mice, an increased production of IL-1 has been suggested to contribute to the development of colitis.19 The ability of IL-10 to reduce pro-inflammatory cytokines such as IFN- and TNF-a may explain the enterocolitis in IL-10-deficient mice.20 SCID mice reconstituted with the CD45RB high subset of CD4q T cells develop a granulomatous enterocolitis reminiscent of Crohn’s disease due to the production of Th1 cytokines.21 In this model, the importance of a balance between spontaneously produced pro- and anti-inflammatory mediators is strikingly demonstrated by simultaneous transfer of the CD45RB low CD4q T cells. In the presence of these regulatory cells, which apparently produce TGF-b , CD45RB high CD4q T cells are prevented from causing inflammation.22 Although a complete absence of a cytokine has not been found in patients with IBD, an imbalance between pro- and anti-inflammatory mediators, such as an enhanced production of cytokines and an increased IL1rIL1 receptor antagonist ŽIL-1RA . ratio, has been observed.1 Crohn’s disease and ulcerative colitis seem to differ in the profile of cytokines produced in the mucosa. The manner in which this parallels the development of colitis Žsimilar to ulcerative colitis. in some animal models and enterocolitis Žsimilar to Crohn’s disease. in others remains to be determined. It is currently being investigated whether genetically determined differences in the expression of cytokines, due to allelic variants of the gene promotor regions Žmicrosatellites., could explain the propensity to develop IBD. Whereas no association between IL2 microsatellite alleles and IBD could be found, there seems to be an association of Crohn’s disease with TNF microsatellite alleles.23 In recent clinical trials, treatment with human recombinant IL-10 and or a humanized antibody blocking TNF were shown to reduce inflammation in patients with IBD, possibly by re-establishing the cytokine balance.24 The possibility that blockade of the
Environmental agents The epidemiology of IBD cannot be easily reconciled with the hypothesis that endogenous factors alone cause the disease. The initiating event or promotion of inflammation or both could be related to some extent to exogenous factors. For example, an altered regulation of the immune response could be the result of an acute infection. The intriguing possibility that a subtle alteration of the immune regulation may persist for a long time after the acute infection has been overcome is supported by recent studies showing that the incidence of atopy is less in individuals who experienced certain infections during early childhood.28 Also, pathogens or microbial material may persist in tissues, chronically activating the immune system. The intestinal lumen provides a large reservoir of microbial agents. Usually these commensal bacteria are non-pathogenic, and apparently they are normally not recognized as foreign by the immune system Žsimilar to self antigens or ‘extended self’..29 Nonetheless, cell wall constituents and products released from even the normal flora can have strong pro-inflammatory activities. Changes in the mucosal barrier, allowing translocation of bacteria and cell wall-derived pro-inflammatory macromolecules into the mucosa, might therefore result in inflammation despite absence of aggressive pathogenicity factors in these bacteria. Furthermore, usually harmless microbes could gain virulence, e.g. by acquiring pathogenicity factors from other bacteria via plasmids. A plethora of other, non-microbial exogenous factors which can not be discussed in detail in this review, e.g. smoking, food antigens, drugs have all been implicated in the pathogenesis of IBD. Future research will have to clarify the relative importance of these influences. 71
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Initiation and promotion of inflammation in IBD
tionship between known microorganisms and disease can elude the attention of researchers despite considerable efforts to unravel the causes of the disease. Similarly, it still remains a possibility that an as yet unidentified microorganism which is hard to grow with conventional culture methods is the cause of IBD. Epidemiological data supporting transmissibility of IBD is at best controversial. Reports about occasional family clustering of IBD are contrasted by the apparent absence of an increased risk for spouses of patients or medical personnel treating IBD patients to develop IBD themselves.2,7 Acute manifestations of IBD can be hard to distinguish from intestinal infections. The intestinal pathology in Crohn’s disease is almost identical to that in infections with Yersinia enterocolitica or Mycobacterium tuberculosis. Infections with Campylobacter, Shigella, Amoeba, or cytomegalovirus ŽCMV. can be easily mistaken for an acute presentation of ulcerative colitis.30 To some degree this may reflect the limited repertoire of defense mechanisms of the intestine. The development of granulomas, a typical histological feature of Crohn’s disease, may indicate persistence of a microbe or microbial material in the tissue. In reactive arthritis, persistence of microbial factors has been proposed to explain the protracted development of joint inflammation; similarly, inflammation in Crohn’s disease could result from the protracted activation of the immune system in these granulomas. Paramyxovirus Žmeasles. like virus particles have been described in the vascular endothelia and granulomas of Crohn’s disease tissues. In addition, two epidemiological studies from Sweden showed that presentation of Crohn’s disease before the age of 30 was associated with birth in areas of recent measles epidemics31 and that measles infection of the mother during pregnancy was associated with an increased risk of the child developing Crohn’s disease.32 A possible role for viruses in the pathogenesis of ulcerative colitis is further suggested by the anecdotal finding of intranuclear and cytoplasmic inclusion bodies in colonic tissue. The Epstein]Barr virus can be isolated from 75% of patients with ulcerative colitis and anti-CMV antibody titres are frequently increased in these patients.30 Interestingly, slowly replicating EBV seems to be able to cause activation and proliferation of infected T cells. However, a high percentage of the healthy population is latently infected with these herpesviruses. The bacterium Mycobacterium paratuberculosis causes
A number of scenarios can be considered for the role of microbial factors in the immuno-pathology of IBD. An unusually aggressive microbe might cause chronic inflammation due to its ability to penetrate into the mucosa, to induce the release of pro-inflammatory mediators and to persist in tissues. Usually organisms causing acute intestinal infections such as shigella, salmonella and campylobacter are quickly eliminated. Some microbes, such as Entameba histolytica or Mycobacterium tuberculosis, can persist intracellularly particularly in macrophages, after resolution of the acute inflammation. For example, the occurrence of reactive arthritis several months following resolution of intestinal inflammation has been attributed to distribution of macrophages into the joints which contain live intracellular microorganisms or microbial material. An aggressive microbe may induce a breach of the mucosal barrier which allows penetration of the bacteria of the normal flora. Similarly, an acute infection might exacerbate an inherited but previously subclinical permeability defect. An intrinsic defect of the immune system, inapparent as long as nothing else fails, may become obvious during the course of an infection and result for example, in the inability to resolve inflammation once the pathogen has been eliminated. Microbial factors could alter the immune response qualitatively, e.g. by inducing a shift in the balance between Th1 and Th2 cytokines. Inappropriate inflammation may also be the result of a breakdown of the normal tolerance towards gut flora bacteria. Microbial factors might even induce the emergence of self-reactivity in the intestine, in analogy to certain autoimmune diseases. These possibilities are not necessarily mutually exclusive and it is possible that a heterogeneous disease such as IBD may involve a variety of combinations of such hypothetical mechanisms.
IBD as an infectious disease Since Crohn’s disease and ulcerative colitis were described for the first time decades ago the hypothesis that IBD might be caused by an infectious organism has received much attention. Although decades of continuous efforts to isolate or at least detect a microorganism in IBD tissue have failed, the delayed recognition of Helicobacter pylori as the cause of peptic ulcer disease demonstrates very clearly that a rela72
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a granulomatous intestinal disease in ruminants which is strikingly similar to human Crohn’s disease. Mycobacteria can persist in tissues, cause granulomatous disease and are difficult to culture. In addition, mycobacterial products have immuno- modulatory properties Že.g. BCG, Freunds adjuvant.. As a potential source of infection, M. paratuberculosis has been demonstrated in commercially distributed milk and drinking water. Furthermore, intestinal infection with the related M. tuberculosis can appear very similar to Crohn’s disease although their response to treatment is quite different. Isolation of microbial organisms and identification of microbial material have occasionally been successful. However, highly sensitive and specific methods such as nested PCR have not been able to consistently isolate or identify a microorganism such as measles virus or M. paratuberculosis and yielded mostly negative results.30 Thus, evidence supporting a role for an infectious microbe in IBD remains controversial.
fraction of patients with ulcerative colitis indicates that they do not play a central role in the pathogenesis of the disease. Since similar pANCA can be found in patients with sclerosing cholangitis without intestinal disease and even in some healthy individuals, production of these auto-antibodies obviously does not cause disease, although, as an epiphenomenon, they may be indicative of the more profound immune disturbance in ulcerative colitis. Thus, the significance of pANCA lies in their value as disease markers for ulcerative colitis. In summary, direct evidence for a role of autoimmune mechanisms in IBD is scarce and controversial. Under normal conditions, the immune system does not turn against itself, although potentially autoreactive cells are probably generated during thymic maturation. Self-reactive T cells that have escaped clonal deletion in the thymus either express a TCR with a very low affinity to immunodominant self-antigens or are naive cells which can not penetrate through the vascular endothelium into the tissue and therefore are not primed to cause damage Ž‘immune ignorance’.. Cells which do reach the tissue can be deleted peripherally due to constitutive production of the Fas ligand molecule in the tissue or by a TNF-dependent mechanism.36 Activation of autoreactive cells does not normally occur since these cells usually lack the IL-2-dependent mechanisms essential for the amplification of T cell responses Ž‘anergy’.. Furthermore, most peripheral tissues lack organized lymphoid tissue necessary to induce a strong inflammatory response if autoreactive were activated locally.37 The microenvironment also seems to determine the result of activation of autoreactive cells: whereas development of a Th1 type response seems to initiate some autoimmune diseases, Th2 cells are protective Žthe reverse may be the case in other diseases..38 Thus, the normal balance between Th1 and Th2 cytokines seems to deflect autoreactive cells from their antigen by turning them into cells with ‘suppressor’ activity 39 subduing autoreactive responses rather than causing damage Ž ‘immune deviation’..38,40 In the intestine, some of these mechanisms may not be present. The normal intestinal mucosa contains secondary lymphoid tissue. Fas mediated apoptosis does not appear to reduce intestinal damage upon generalized T cell activation. The constitutive Fas L expression and resistance to Fas mediated cell death typical for ‘immunologically privileged’ peripheral tissues is contrasted by constitutive expression of high levels of Fas and absence of FasL production by
Is IBD an autoimmune disease? In IBD, tissue damage is caused by a self-destructive, exaggerated inflammation, and this inflammation appears to be maintained in the absence of a specific pathogen. Self-contained and auto-destructive inflammation could be regarded as the hallmark of autoimmunity in the broadest sense. Recent studies have even found evidence for self-antigen-specific immune mechanisms, suggesting the possibility that IBD may indeed be an autoimmune disease. Some antibodies generated in IBD recognize self-antigens, e.g. colonocytes, mucin and goblet cells.33 ] 35 Autoantibody-dependent cytotoxicity against colonic epithelial cells has been demonstrated in vitro, and in ulcerative colitis, activated complement factors have been described to co-localize with anti-colon antibodies on the luminal surface of the epithelium.35 Anti-mucin and anti-goblet cell antibodies could cause alteration of the protective mucin layer which might lead to increased adherence of luminal bacteria. Antibodies against neutrophils ŽpANCA. are very common in ulcerative colitis. However, identification of the putative autoantigens revealed that pANCA in ulcerative colitis and those found in some autoimmune vasculitides probably recognize different antigens. Although pANCA may bind, e.g. to bactericidalrpermeability increasing protein, a molecule that prevents inflammation in response to bacterial LPS Žsee below., the absence of pANCA in a significant 73
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epithelial cells. Thus, Fas mediated mechanisms may cause tissue damage rather than prevent it in the intestine.41,42 In IBD, it is conceivable that the observed imbalance between pro- and anti-inflammatory mediators could reverse anergy of autoaggressive T cells 43 and might also interfere with ‘immune deviation’, pushing autoreactive cells towards a more aggressive phenotype. Furthermore, the normal induction of ‘oral tolerance’ to exogenous antigens seems to depend on the balance between IL12 and TGF-b . Abrogation of tolerance, for example due to IL-12 overproduction, enhances the chances that an exogenous antigen sharing immunogenic determinants with self-antigens is no longer ignored but induces autoimmunity Ž‘molecular mimicry’.. Whereas unprimed T cells usually do not emigrate into peripheral tissues, altered expression of endothelial adhesion molecules increasingly allows the recruitment of naive T cells into the mucosa. Some of these T cells may recognize self-antigens which normally remain sequestered in the mucosa, were not present in the thymus during maturation and thus failed to induce tolerance. The enhanced expression of MHC class II molecules by epithelial cells and activation of B cells in IBD probably leads to increased antigen presentation by ‘non-professional’ antigen-presenting cells ŽAPC.. These cells may process antigen differently than professional APC and therefore present different peptides which were not immunodominant previously Žpresentation of ‘cryptic’ antigens..4 Furthermore, the association of IBD with known autoimmune diseases and the frequent occurrence of ‘extraintestinal manifestations’ which share all the essential characteristics of their idiopathic counterparts and thus probably represent autoimmune diseases as well, can be regarded as indirect evidence for the occurrence of autoimmune mechanisms in the context of IBD. In summary, autoimmunity and IBD seem to occur together, although reason for their causal relationship is still unclear. At the moment, the notion that IBD itself may be an autoimmune disease remains speculation.
Nonetheless, at least some autoimmune diseases seem to be triggered by infectious agents which are no longer present when disease becomes manifest clinically.44 IBD may be initiated by a similar mechanism, particularly if autoimmune mechanisms turn out to be involved in the pathogenesis. It is possible that an infection itself Že.g. by a virus which latently infects host cells without killing them. remains undetected but manifests as an ‘autoimmune’ disease if coating of the cell membrane with microbial peptides leads to recognition of the host cell as foreign by the immune system. Microbial peptides could haptenize self proteins on host cells, which are then recognized by the immune system as foreign Ž‘altered self hypothesis’., a possibility which is demonstrated by the induction of colitis with the haptenizing agent TNBS in a rat model of IBD. Microbial peptides which share antigenic determinants with self proteins could induce a cross-reactive immune response in the course of an infection Ž‘molecular mimicry’.. The recruitment of T cells with an increasing diversity of antigenic specificities can lead to recognition of other epitopes on the same microbial protein, decreasing the threshold for cross-reactivity with host proteins Ž‘epitope spreading’.. Cross-reactivity between autoantibodies in IBD with microbial antigens has indeed been suggested, but could not be proved to date. The homology between mycobacterial heat shock protein, HSP65 and human HSP60 is particularly intriguing, since mycobacteria have been implicated as causative agents in Crohn’s disease. Whereas some groups have found increased anti-HSP60 autoantibodies, anti-HSP-65 antibodies and HSP65 reactive T cells in the peripheral blood,45 ] 47 as well as increased HSP60 expression in IBD,48 our study found neither increased intestinal expression of HSP60 nor HSP60 reactive T cells in Crohn’s disease.49 The changes in the cytokine profile observed in IBD which might alter the functional properties of activated autoreactive cells 39,50 could be induced by an infectious agent. In addition, infections can be accompanied by increased expression of MHC class II molecules on intestinal epithelial cells 51 and can also induce the expression of costimulatory molecules such as B7.2, both of which have been observed in IBD as well.52,53 Since absence of costimulatory molecules may be crucial for the maintenance of self-tolerance by promoting anergy,54 prolonged alteration of antigen presentation by an infection could be related to the autoimmune phenomena observed in IBD Žsee above.. Lastly, bacterial derived superantigens could activate large subsets of T cells includ-
Is IBD an ‘infection-induced autoimmune disease’? Exogenous factors seem to be necessary to trigger IBD. Whereas no specific infectious microorganisms could be isolated from IBD tissues, the same is true for inflamed tissues in autoimmune diseases. 74
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ing self-reactive clones. Increased frequencies of certain T cell subsets, e.g. TCR Vb 8 q , has been described in Crohn’s disease and altered functional activity of these subsets suggest a possible role for superantigens in IBD.55,56 Alternatively, if regulatory T cells become non-specifically deleted through interactions with superantigens, deficient suppression of self-reactive clones could lead to autoimmune reactivity. The relationship between IBD, intestinal infections, reactive extraintestinal inflammation and autoimmune diseases such as idiopathic ankylosing spondylitis provides compelling indirect evidence for the existence of a connection between IBD, infectious diseases and autoimmune diseases. Reactive arthritis is triggered by mucosal infections for example with Salmonella, Shigella, or Yersinia. Reactive arthritis can occasionally become chronic and then resembles idiopathic ankylosing spondylitis and axial arthritis in IBD. The genetic association of all three manifestations with the same HLA B27 allele seems to suggest a common pathogenetic mechanism. The role of HLA B27 in ankylosing spondylitis may be related to the ability of the molecule to facilitate invasion of microorganisms, its role in shaping the T cell repertoire, presentation of an ‘arthritogenic’ self-peptide, or even cross-reactivity between epitopes of HLA B27 itself and microbial peptides.57,58 Interestingly, some rat strains transgenic for HLA B27 seem to spontaneously develop arthritis, however, in the absence of gut flora in animals raised in a germfree environment, inflammation does not occur.59 In contrast to human IBD, which is not linked to HLA B27, these animals also develop gastrointestinal inflammation which is equally dependent on the presence of gut flora. However, although this data fits into the concept that IBD may be an autoimmune disease related to microbial factors, the relationship between joint inflammation and the intestinal mucosa may be more simple. For example, increased intestinal permeability has been found in patients with reactive arthritis and ankylosing spondylarthritis.9 In an experimental animal model reactive arthritis is the result of small intestinal bacterial overgrowth which seems to lead to increased uptake of the macromolecule peptidoglycan-polysaccharides ŽPG-PS . produced by anaerobic bacteria.59 The same macromolecule, injected into the intestinal wall, also leads to a granulomatous intestinal inflammation which resembles Crohn’s disease. The chronic relapsing inflammation in this model is T cell mediated, but depends on persistence
of PG-PS in the tissues and protracted re-release by the liver, not an autoimmune process. In summary, despite indirect evidence for a possible relation to autoimmune diseases, autoimmune mechanisms do not seem to play an important role in the pathogenesis of IBD.
Microbes, macromolecules and immune dysregulation In the absence of a specific aggressive pathogen and the lack of evidence for an autoimmune mechanism, the only remaining alternative seems to be that nonspecific environmental factors are the cause of exaggerated inflammation in IBD.30 This may be the least attractive hypothesis, since identification of a specific abnormality as a target for causative therapy might then be impossible. There is indeed increasing evidence that in particular the normal intestinal flora contributes significantly to the development of inflammation, although this does not necessarily lead to dismal conclusion that a propensity to develop IBD is simply the price we have to pay for the complex mechanisms allowing uptake of a large variety of nutrients and protection against similarly varied pathogens at the same time. IBD occurs with a predilection for sites with a high luminal content of bacteria, e.g. the terminal ileum and the colon. In Crohn’s disease, reduction of luminal bacteria content by decontamination, bowel rest, lavage or total parenteral nutrition has beneficial effects on the activity of inflammation. Furthermore, there is evidence that in Crohn’s disease the composition of the luminal flora is altered. Indeed, the beneficial effect of antibiotics such as metronidazole in some patients with Crohn’s disease has been attributed to the longterm reduction of Bacteroides spp in the lumen. Interestingly, Bacteroides seems to be responsible for the intestinal inflammation occurring in HLA-B27 transgenic rats and anaerobes may also be the source of PG-PS in the above mentioned model of experimental small intestinal bacterial overgrowth.60 Similarly, the presence of normal intestinal flora is also a prerequisite for the development of intestinal disease in the animal models of immune dysregulation as mentioned above. In ulcerative colitis, increased bacterial adherence, invasiveness and even production of toxins by normal bacterial flora has been demonstrated.61,62 Bacterial adhesion to epithelial cells in vitro induces secretion of proinflammatory mediators, such as IL-1, IL8 and IL-6, which could for example, 75
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be responsible for the massive transmigration of neutrophils through the epithelium into the colonic crypts Ž‘crypt abscesses’. typical for ulcerative colitis.63 ] 65 Usually, the endogenous flora does not induce immune responses. The composition of the intestinal flora is a rather stable individual trait and these bacteria seem to be regarded as ‘extended self’ by the immune system. However, in IBD, recent studies have shown a breakdown of this tolerance which correlates with activity of inflammation.29 However, regardless of tolerance of the host, some bacterial antigens, such as lipopolysaccharides ŽLPS., the major constituent of the cell wall of Gram negative bacteria, are potent pro- inflammatory molecules. Whereas mononuclear cells of the normal mucosa are not reactive to LPS, freshly recruited tissue macrophages in IBD express the high affinity LPS receptor, CD14.16 Activation of these cells via CD14 could lead to release of pro-inflammatory mediators TNF, IL1, IL6 and IL8, procoagulant tissue factor and complement components. Interestingly, in the IL-2 knockout mouse, expression of CD14 by epithelial cells precedes the onset of inflammation.66 The possible role of proinflammatory bacterial macromolecules in IBD has recently been summarized in a comprehensive review.60 In short, some evidence suggests that cell wall constituents, such as LPS and PG-PS, as well as formylated oligopeptides, such as FMLP, can be detected in the serum of IBD patients, which also seem to have increased titres of antibodies against these molecules. All these molecules, alone or synergistically, strongly induce the release of pro-inflammatory mediators through specific ŽFMLP, LPS. and nonspecific receptors ŽLPS, PG-PS . by macrophages and neutrophils. FMLP and PG-PS can undergo enterohepatic recirculation and thus may be related to the biliary hepatic inflammation associated with IBD.
how ubiquitous, non-specific environmental factors induce severe inflammation only in some individuals, but leave the majority of us unscathed. Furthermore, there may even be the possibility that the unraveling of these pathogenetic mechanisms could also lead to a better understanding of reactive arthritis, idiopathic ankylosing spondylitis and autoimmune diseases.
Acknowledgements K.C. is an Ontario Ministry of Health Career Scientist. M.M. is supported by a grant from the Deutsche Forschungsgemeinschaft ŽMe 1432r1]1..
References 1. Elson CO, McCabe RP Ž1995. The immunology of inflammatory bowel disease, in Inflammatory Bowel Disease ŽKirsner JB, Shorter RG, eds., pp 203]251. Williams and Wilkins, Baltimore, Philadelphia, Hong Kong, London, Munch, Sydney, Tokyo 2. Yang H, Rotter JI Ž1993. Genetics of inflammatory bowel disease, in Inflammatory Bowel Disease. From Bench to Bedside ŽTargan SR, Shanahan F, eds., pp 32]64 3. Theofilopoulos AN Ž1995. The basis of autoimmunity: Part II. Genetic predisposition. Immunol Today 16:150]159 4. Theofilopoulos AN 1995. The basis of autoimmunity: Part I. Mechanisms of aberrant self-recognition. Immunol Today 16:90]98 5. Hafler DA, Flavell R Ž1996. Autoimmunity. How to know thy self weditorialx. Curr Opin Immunol 8:805]807 6. Shanahan F, Targan SR Ž1993. Genetics of inflammatory bowel disease, in Inflammatory Bowel Disease. From Bench to Bedside. ŽTargan SR, Shanahan F, eds., pp 78]88 7. Calkins BM, Mendeloff AI Ž1995. The epidemiology of idiopathic inflammatory bowel disease, in Inflammatory Bowel Disease ŽKirsner JB, Shorter RG, eds., pp 31]68. Williams and Wilkins, Baltimore, Philadelphia, Hong Kong, London, Munch, Sydney, Tokyo 8. Retsky JE, Kraft SC Ž1995. The extraintestinal manifestations of inflammatory bowel disease, in Inflammatory Bowel Disease ŽKirsner JB, Shorter RG, eds., pp 474]491. Williams and Wilkins, Baltimore, Philadelphia, Hong Kong, London, Munch, Sydney, Tokyo 9. Veys EM, Mielants H, de Vos M, Cuvelier C Ž1996. Spondylarthropathies: from gut to target organs. Baillere’s Clinical Rheumatology 10Ž1.:123]146 10. Hollander D Ž1993. Permeability in Crohn’s disease: altered barrier functions in healthy relatives? weditorial; commentx. Gastroenterology 104:1848]1851 11. Mayer L, Eisenhardt D Ž1990. Lack of induction of suppressor T cells by intestinal epithelial cells from patients with inflammatory bowel disease. J Clin Invest 86:1255]1260 12. Smith PD, Janoff EN, Mosteller-Barnum M, Merger M, Orenstein JM, Kearney JF, Graham MF Ž1997. Isolation and purification of CD14-negative mucosal macrophages from normal human small intestine. J Immunol Methods 202:1]11 13. Mowat AM, Viney JL Ž1997. The anatomical basis of intestinal immunity. Immunol Rev 156:145]166 14. Morise K, Yamaguchi T, Kuroiwa A, Kanayama K, Matsuura
Conclusion Despite many recent advances in our understanding of the mucosal immune system, the role of environmental factors in the pathogenesis of IBD remains a mystery. Clinical observations and experimental evidence increasingly suggest that the nature of the responsible agents may be rather non-specific. However, in the future, the observation that the normal bacterial flora induces intestinal inflammation only in animals with an intrinsic immune dysregulation of the immune system may lead us to an explanation of 76
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18. 19. 20. 21.
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27. 28. 29.
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T, Shinoda M, Yamamoto H, Horiuchi Y, Furusawa A, Iwase H et al Ž1994. Expression of adhesion molecules and HLA-DR by macrophages and dendritic cells in aphthoid lesions of Crohn’s disease: an immunocytochemical study. J Gastroenterol 29:257]264 Rugtveit J, Haraldsen G, Hogasen AK, Bakka A, Brandtzaeg P, Scott H Ž1995. Respiratory burst of intestinal macrophages in inflammatory bowel disease is mainly caused by CD14 q L1q monocyte derived cells. Gut 37:367]373 Rugtveit J, Nilsen EM, Bakka A, Carlsen H, Brandtzaeg P, Scott H Ž1997. Cytokine profiles differ in newly recruited and resident subsets of mucosal macrophages from inflammatory bowel disease. Gastroenterology 112:1493]1505 Fedorak RN Ž1995. Naturally occurring and experimental models of inflammatory bowel disease, in Inflammatory Bowel Disease ŽKirsner JB, Shorter RG, eds., pp 71]95. Williams and Wilkins, Baltimore, Philadelphia, Hong Kong, London, Munch, Sydney, Tokyo Ehrhardt RO, Ludviksson BR, Gray B, Neurath M, Strober W Ž1997. Induction and prevention of colonic inflammation in IL-2 deficient mice. J Immunol 158:566]573 Mizoguchi E, Mizoguchi A, Bhan AK Ž1997. Role of the cytokines in the early stages of inflammatory bowel disease in TCR a-mutant mice wAbstractx. Gastroenterology 112:A1044 Kuhn R, Lohler J, Rennick D, Rajewsky K, Muller W Ž1993. Interleukin-10-deficient mice develop chronic enterocolitis. Cell 75:263]274 Powrie F, Leach MW, Mauze S, Menon S, Caddle LB, Coffman RL Ž1994. Inhibition of Th1 responses prevents inflammatory bowel disease in scid mice reconstituted with CD45RBhi CD4 q T cells. Immunity 1:553]562 Powrie F, Carlino J, Leach MW, Mauze S, Coffman RI Ž1996. A critical role for transforming growth factor-b but not interleukin 4 in the suppression of T helper type 1-mediated colitis by CD45RBlow CD4 q T cells. J Exp Med 183:2669]2674 Plevy SE, Targan SR, Yang H, Fernandez D, Rotter JI, Toyoda H Ž1996. Tumor necrosis factor microsatellites define a Crohn’s disease-associated haplotype on chromosome 6. Gastroenterology 110:1053]1060 van Dullemen HM, Van Deventer SJ, Hommes DW, Bijl HA, Jansen J Tytgat GN, Woody J Ž1995. Treatment of Crohn’s disease with anti-tumor necrosis factor chimeric monoclonal antibody ŽcA2.. Gastroenterology 109:129]135 Strober W, Kelsall B, Fuss I, Marth T, Ludviksson B, Ehrhardt R, Neurath M Ž1997. Reciprocal IFN-gamma and TGF-beta responses regulate the occurrence of mucosal inflammation. Immunol Today 18:61]64 Neurath MF, Fuss I, Kelsall BL, Presky DH, Waegell W, Strober W Ž1996. Experimental granulomatous colitis in mice is abrogated by induction of TGF-beta-mediated oral tolerance. J Exp Med 183:2605]2616 Weiner HL Ž1997. Oral tolerance for the treatment of autoimmune diseases. Annu Rev Med 48:341]351 Shirakawa T, Enomoto T, Shimazu S, Hopkin JM Ž1997. The inverse association between tuberculin responses and atopic disorder. Science 275:77]79 Duchmann R, Kaiser I, Hermann E, Mayet W, Ewe K, Meyer zum Buschenfelde KH Ž1995. Tolerance exists towards resident intestinal flora but is broken in active inflammatory bowel disease ŽIBD. wsee commentsx. Clin Exp Immunol 102:448]455 Sartor RB Ž1995. Microbial factors in the pathogenesis of Chron’s disease, ulcerative colitis and experimental intestinal inflammation, in Inflammatory Bowel Disease ŽKirsner JB, Shorter RG, eds., pp 96]124. Williams and Wilkins, Baltimore, Philadelphia, Hong Kong, London, Munch, Sydney, Tokyo Ekbom A, Wakefield AJ, Zack M, Adami HO Ž1994. Perinatal
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measles infection and subsequent Crohn’s disease wsee commentsx. Lancet 344:508]510 Ekbom A, Daszak P, Kraaz W, Wakefield AJ Ž1996. Crohn’s disease after in- utero measles virus exposure. Lancet 348:515]517 Folwaczny C, Noehl N, Tschoep K, Endres SP, Heldwein W, Loeschke K, Fricke H Ž1997. Goblet cell autoantibodies in patients with inflammatory bowel disease and their first degree relatives. Gastroenterology 113:101]106 Biancone L, Mandal A, Yang H, Dasgupta T, Paoluzi AO, Marcheggiano A, Paoluzi P, Pallone F, Das KM Ž1995. Production of immunoglobulin G and G1 antibodies to cytoskeletal protein by lamina propria cells in ulcerative colitis. Gastroenterology 109:3]12 Brandtzaeg P Ž1995. Autoimmunity and ulcerative colitis: can two enigmas make sense together? Gastroenterology 109:307]312 Mountz JD, Edwards 3rd CK, Cheng J, Yang P, Wang Z, Liu C, Su X, Bluethmann H, Zhou T Ž1996. Autoimmunity due to defective Nur77, Fas, and TNF-RI apoptosis. Adv Exp Med Biol 406:241]262 Zinkernagel RM, Ehl S, Aichele P, Oehen S, Kundig T, Hengartner H Ž1997. Antigen localisation regulates immune responses in a dose- and time-dependent fashion: a geographical view of immune reactivity. Immunol Rev 156:199]209 Liblau RS, Singer SM, McDevitt HO Ž1995. Th1 and Th2 CD4 q T cells in the pathogenesis of organ-specific autoimmune diseases. Immunol Today 16:34]38 Kroemer G, Hirsch F, Gonzalez-Garcia A, Martinez C Ž1996. Differential involvement of Th1 and Th2 cytokines in autoimmune diseases. Autoimmunity 24:25]33 Rocken M, Shevach EM Ž1996. Immune deviation } the third dimension of nondeletional T cell tolerance. Immunol Rev 149:175]194 Strater J, Wellisch I, Riedl S, Walczak H, Koretz K, Tandara A, Krammer PH, Moller P Ž1997. CD95 ŽAPO-1rFas .-mediated apoptosis in colon epithelial cells: a possible role in ulcerative colitis. Gastroenterology 113:160]167 Sakai T, Kimura Y, Inagaki-Ohara K, Kusugami K, Lynch DH, Yoshikai Y Ž1997. Fas-mediated cytotoxicity by intestinal intraepithelial lymphocytes during acute graft-versus-host disease in mice. Gastroenterology 113:168]174 Croitoru K, Bienenstock J, Ernst PB Ž1994. Phenotypic and functional assessment of intraepithelial lymphocytes bearing a ‘forbidden’ alpha beta TCR. Int Immunol 6:1467]1473 Aichele P, Bachmann MF, Hengartner H, Zinkernagel RM Ž1996. Immunopathology or organ-specific autoimmunity as a consequence of virus infection. Immunol Rev 152: 21]45 Stevens TR, Winrow VR, Blake DR, Rampton DS Ž1992. Circulating antibodies to heat-shock protein 60 in Crohn’s disease and ulcerative colitis. Clin Exp Immunol 90:271]274 Elsaghier A, Prantera C, Bothamley G, Wilkins E, Jindal S, Ivanyi J Ž1992. Disease association of antibodies to human and mycobacterial hsp70 and hsp60 stress proteins. Clin Exp Immunol 89:305]309 Szewczuk MR, Depew WT Ž1992. Evidence for T lymphocyte reactivity to the 65 kilodalton heat shock protein of mycobacterium in active Crohn’s disease. Clin Invest Med 15:494]505 Peetermans WE, GR D’Haens JL Ceuppens, P. Rutgeerts, and K. Geboes Ž1995. Mucosal expression by B7-positive cells of the 60-kilodalton heat-shock protein in inflammatory bowel disease. Gastroenterology 108:75]82 Baca-Estrada ME, Gupta RS, Stead RH, Croitoru K Ž1994. Intestinal expression and cellular immune responses to human heat-shock protein 60 in Crohn’s disease. Dig Dis Sci 39:498]506 Nicholson LB, Kuchroo VK Ž1996. Manipulation of the
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Th1rTh2 balance in autoimmune disease. Curr Opin Immunol 8:837]842 McDonald GB, Jewell DP Ž1987. Class II antigen ŽHLA-DR . expression by intestinal epithelial cells in inflammatory diseases of colon. J Clin Pathol 40:312]317 Noguchi M, Hiwatashi N, Liu Z, Toyota T Ž1995. Enhanced interferon-gamma production and B7-2 expression in isolated intestinal mononuclear cells from patients with Crohn’s disease. J Gastroenterol 30 Suppl 8:52]55 MacDermott RP Ž1996. Alterations of the mucosal immune system in inflammatory bowel disease. J Gastroenterol 31:907]916 Lenschow DJ, Walunas TL, Bluestone JA Ž1996. CD28rB7 system of T cell costimulation. Annu Rev Immunol 14:233]258 Baca-Estrada ME, Wong DK, Croitoru K Ž1995. Cytotoxic activity of V beta 8 q T cells in Crohn’s disease: the role of bacterial superantigens. Clin Exp Immunol 99:398]403 Shalon L, Gulwani-Akolkar B, Fisher SE, Akolkar PN, Panja A, Mayer L, Silver J Ž1994. Evidence for an altered T-cell receptor repertoire in Crohn’s disease. Autoimmunity 17:301]307 Sieper J, Kingsley GH, Marker-Hermann E Ž1996. Aetiological agents and immune mechanisms in enterogenic reactive arthritis. Baillieres Clin Rheumatol 10:105]121 Baum H, Davies H, Peakman M Ž1996. Molecular mimicry in the MHC: hidden clues to autoimmunity? Immunol Today 17:64]70 Rath HC, Herfarth HH, Ikeda JS, Grenther WB, Hamm Jr TE, Balish E, Taurog JD, Hammer RE, Wilson KH, Sartor RB Ž1996. Normal luminal bacteria, especially Bacteroides species, mediate chronic colitis, gastritis, and arthritis in
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HLA-B27rhuman beta2 microglobulin transgenic rats. J Clin Invest 98:945]953 Elson CO, Sartor RB, Tennyson GS, Riddell RH Ž1995. Experimental models of inflammatory bowel disease. Gastroenterology 109:1344]1367 Lobo AJ, Sagar PM, Rothwell J, Quirke P, Godwin P, Johnston D, Axon AT Ž1993. Carriage of adhesive Escherichia coli after restorative proctocolectomy and pouch anal anastomosis: relation with functional outcome and inflammation. Gut 34:1379]1383 von Wulffen H, Russmann H, Karch H, Meyer T, Bitzan M, Kohrt TC, Aleksic S Ž1989. Verocytotoxin-producing Escherichia coli O2:H5 isolated from patients with ulcerative colitis wletterx. Lancet 1:1449]1450 Jung HC, Eckmann L, Yang SK, Panja A, Fierer J, MorzyckaWroblewska E, Kagnoff MF Ž1995. A distinct array of proinflammatory cytokines is expressed in human colon epithelial cells in response to bacterial invasion. J Clin Invest 95:55]65 Eckmann L, Kagnoff MF, Fierer J Ž1993. Epithelial cells secrete the chemokine interleukin-8 in response to bacterial entry. Infect Immunol 61:4569]4574 Rasmussen SJ, Eckmann L, Quayle AJ, Shen L, Zhang YX, Anderson DJ, Fierer J, Stephens RS, Kagnoff MF Ž1997. Secretion of proinflammatory cytokines by epithelial cells in response to Chlamydia infection suggests a central role for epithelial cells in chlamydial pathogenesis. J Clin Invest 99:77]87 Meijssen MAC, Devaney K, Bhan AK, Podolsky DK Ž1996. Altered cytokine and CD14 expression by intestinal epithelial cells of interleukin-2 deficient mice. Gastroenterology 110:A966
Infections in the immunopathogenesis of chronic inflammatory bowel disease Michael Merger and Ken CroitoruU
In chronic inflammatory bowel disease, self-destructive, exaggerated inflammation seems to occur in the absence of a well defined pathogen. However, epidemiological data strongly suggests that development of disease does not depend on endogenous factors alone. In this review, we summarize how a possible role for microbial factors can be reconciled with the current understanding of etiology and pathogenesis of IBD. The data presented does not support that IBD is an infectious disease nor that it is a selfantigen-specific autoimmune disease, however, recent findings increasingly suggest that tissue damage might be caused by a non-specific autoaggressive inflammation which is driven by common, ubiquitous microbial agents derived from the bacterial flora in the intestinal lumen.
and immunosuppressive agents. Development of strictures, fistulae and abscesses often requires surgical intervention. In this review, we summarize the current understanding of the immunopathogenesis of IBD with a particular focus on the possible role of infectious agents.
Etiology and pathogenesis of IBD: convergence of genetic predisposition, immune dysregulation and environmental factors The cause of IBD is unknown. It has been suggested that this chronic inflammatory disease occurs in genetically predisposed individuals, possibly on the basis of an intrinsic immune defect.1,2 In addition, it is believed that as yet unidentified environmental factors play an important role. A somewhat similar concept, induction of self-aggressivity by an exogenous agent in genetically predisposed individuals, has been proposed to explain the epidemiology of autoimmune diseases.3 ] 5 In Crohn’s disease and ulcerative colitis, tissue damage seems to result from an inappropriate and self-destructive inflammation. On the other hand, there is a striking similarity between IBD and a number of infectious intestinal diseases. The primary trigger of inflammation in IBD is unknown. Chronic inflammation in IBD may not be initiated by a single step but may require the additive effects of several factors.6 It is possible that in IBD, the initial event merely enhances the ‘inclination’ of a patient to develop IBD later in life, while clinical disease requires some non-specific promoting factor. Due to these difficulties the initial trigger of IBD has not been identified yet.
Key words: IBD r immunology r autoimmunity r infection Q1998 Academic Press Ltd
CHRONIC INFLAMMATORY BOWEL disease ŽIBD. encompasses two apparently distinct disease entities, Crohn’s disease and ulcerative colitis. Crohn’s disease is characterized by a transmural, granulomatous inflammation of discontinuously affected intestinal segments. Ulcerative colitis, on the other hand, involves mostly the mucosal, but not the deeper layers of the intestinal wall, is restricted to the large intestine and spreads continuously from distal to proximal segments. Both diseases can follow an unpredictable, often chronic relapsing course. Their treatment is symptomatic and based on non-specific anti-inflammatory drugs such as 5-aminosalicylic acid compounds, corticosteroids
From the Intestinal Diseases Research Program, Department of Medicine, McMaster University, Hamilton, Ontario, Canada and U McMaster University Health Sciences Center, Division of Gastroenterology, 1200 Main Street West, Room 4W8, Hamilton, Ontario, Canada L8N 3Z5 Q1998 Academic Press Ltd 1044-5323r 98r 010069q 10 $25.00r 0r si970106
Genetic aspects of IBD Epidemiological data supports the notion that development of IBD requires an inherited predisposition. 69
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For example, first degree relatives of patients with IBD are much more likely to develop disease than second degree relatives.7 Linkage analysis has begun to identify genetic loci associated with IBD. As with autoimmune diseases, there may be a link between IBD and certain HLA alleles.7 HLA molecules may contribute directly to the pathogenesis of immunologically mediated diseases due to their role in shaping the immune repertoire and to presentation of antigen peptides. It is also possible that the association with HLA alleles reflects their close proximity to the truly responsible genes. The gene for the cytokine TNF, which may play an important role in both autoimmune diseases and IBD is closely adjacent to the HLA complex. IBD patients are at an increased risk of developing autoimmune disorders such as autoimmune hemolytic anemia, raising the possibility that there may be a common genetic disposition for these diseases. In addition, some common extraintestinal manifestations of IBD Žaxial arthritis, biliary hepatic inflammation. may reflect association with idiopathic ankylosing spondylitis and sclerosing cholangitis which are believed to be autoimmune diseases.8 However, similar to idiopathic sclerosing cholangitis and ankylosing spondylitis, sclerosing cholangitis and axial arthritis as extraintestinal manifestations of IBD are associated with HLA B8rDR3 and HLA B27, respectively, whereas IBD itself is not.8,9 Genes located outside the MHC complex may predispose to both autoimmune diseases and IBD. For example, an inherited leakiness of the mucosa barrier could allow penetration of gut luminal antigens and pro-inflammatory molecules triggering the subsequent development of IBD, chronic arthritis or other ‘autoimmune’ diseases, depending on which additional disease-specific genes contribute to the predisposition.9 Intestinal permeability is increased in both patients with Crohn’s disease and a significant percentage of their unaffected relatives and interestingly also in patients with idiopathic axial arthritis.9,10 The disease-specific predisposition to develop IBD could be represented by an inherited abnormality of the local immune regulation in the intestinal mucosa.1 Indirect evidence, summarized in the following section, seems to support this concept.
activation of these cells leads to an increased production of cytokines and other proinflammatory mediators.1 Mucosal T cells in IBD tissue display enhanced responsiveness to IL-2 and an increased cytotoxic ability. Normally, CD8 T cells predominate in the epithelium, probably functioning as suppressor cells, however, epithelial cells in IBD preferentially activate CD4 T cells promoting helper cell activity.11 A shift in the balance between pro and anti-inflammatory mechanisms in IBD is also indicated by the increased production of IgG in the lamina propria, in contrast to the normal predominance of IgA.1 IgA prevents antigens from inducing an inflammatory response Ž‘antigen exclusion’., e.g. due to its lack of complement activation, whereas IgG promotes inflammation. In the normal mucosa, the immune system is constantly exposed to environmental antigens and proinflammatory bacterial products. In the normal intestine, antigens are taken up and processed by specialized structures of the mucosa ŽM cells in the epithelium, specialized lymphoid tissue such as Peyer’s Patches.. The observed expression of activation markers and a constitutive production of small amounts of cytokines by T cells and macrophages12 may reflect this constant activation of immune mechanisms in the normal mucosa.13 Usually, a specific immune response in the mucosa leads to tolerance towards the inducing antigen which does not remain restricted to the gut but also pertains to systemic rechallenge with the antigen. Tolerance may depend on antigen processing by the dendritic cells closely associated with M cells and thus also on an intact mucosal barrier.14 Quantitative uptake of bacterial antigens does not occur through the normal epithelium which, in contrast to M cells, is covered by mucus preventing bacterial adhesion. However, trace amounts of proinflammatory macromolecules penetrating through the epithelium, such as bacterial lipopolysaccharides ŽLPS., may contribute to the ‘physiological’ inflammation in the mucosa. Therefore, prevention of tissue damage depends on the tight regulation of a complex balance between proand anti-inflammatory mechanisms. The normal downregulation of the mucosal immune response is for example reflected by the absence of IL-2 receptor expression by mucosal T cells and CD14 expression Ža high-affinity receptor for LPS commonly expressed by myeloid cells. by macrophages despite the constitutive activation of these cells.15,16 The inflammation in IBD may occur due to the failure to contain this
The mucosal immune system in IBD Tissue destruction in IBD is apparently due to an inappropriate chronic inflammation mediated by specific and non-specific immune effector cells, in particular T cells and macrophages. Recruitment and 70
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pro-inflammatory cytokines IL-12 and IFN-g may also be of benefit is suggested by studies in which experimental colitis, induced by the haptenizing agent TNBS,25 was prevented by anti-IL12 or antiIFN-antibodies. The colitis induced by TNBS is of particular interest, since it could be demonstrated that oral feeding of colon-derived antigens prevented the development of inflammation in this animal model of IBD by inducing oral tolerance towards these self-antigens. 26 This suggests that a similar strategy, currently being examined in patients with multiple sclerosis and rheumatoid arthritis,27 may prove feasible in the treatment of human IBD as well.
‘physiologic’ inflammation in response to nonspecific and ubiquitous pro-inflammatory agents. Indeed, the inflammation in IBD seems to be rather non-specific. Yet, that does not exclude the possibility that it is a specific microorganism which causes this disinhibition of mucosal immune mechanisms. Some mice with an intrinsic dysregulation of the immune system, e.g. due to the lack of certain cytokines, develop spontaneous intestinal inflammation.17 IL-2-deficient mice develop colitis accompanied by an increased production of Th1 cytokines apparently driven by IL-12.18 In TCR ab mutant mice, an increased production of IL-1 has been suggested to contribute to the development of colitis.19 The ability of IL-10 to reduce pro-inflammatory cytokines such as IFN- and TNF-a may explain the enterocolitis in IL-10-deficient mice.20 SCID mice reconstituted with the CD45RB high subset of CD4q T cells develop a granulomatous enterocolitis reminiscent of Crohn’s disease due to the production of Th1 cytokines.21 In this model, the importance of a balance between spontaneously produced pro- and anti-inflammatory mediators is strikingly demonstrated by simultaneous transfer of the CD45RB low CD4q T cells. In the presence of these regulatory cells, which apparently produce TGF-b , CD45RB high CD4q T cells are prevented from causing inflammation.22 Although a complete absence of a cytokine has not been found in patients with IBD, an imbalance between pro- and anti-inflammatory mediators, such as an enhanced production of cytokines and an increased IL1rIL1 receptor antagonist ŽIL-1RA . ratio, has been observed.1 Crohn’s disease and ulcerative colitis seem to differ in the profile of cytokines produced in the mucosa. The manner in which this parallels the development of colitis Žsimilar to ulcerative colitis. in some animal models and enterocolitis Žsimilar to Crohn’s disease. in others remains to be determined. It is currently being investigated whether genetically determined differences in the expression of cytokines, due to allelic variants of the gene promotor regions Žmicrosatellites., could explain the propensity to develop IBD. Whereas no association between IL2 microsatellite alleles and IBD could be found, there seems to be an association of Crohn’s disease with TNF microsatellite alleles.23 In recent clinical trials, treatment with human recombinant IL-10 and or a humanized antibody blocking TNF were shown to reduce inflammation in patients with IBD, possibly by re-establishing the cytokine balance.24 The possibility that blockade of the
Environmental agents The epidemiology of IBD cannot be easily reconciled with the hypothesis that endogenous factors alone cause the disease. The initiating event or promotion of inflammation or both could be related to some extent to exogenous factors. For example, an altered regulation of the immune response could be the result of an acute infection. The intriguing possibility that a subtle alteration of the immune regulation may persist for a long time after the acute infection has been overcome is supported by recent studies showing that the incidence of atopy is less in individuals who experienced certain infections during early childhood.28 Also, pathogens or microbial material may persist in tissues, chronically activating the immune system. The intestinal lumen provides a large reservoir of microbial agents. Usually these commensal bacteria are non-pathogenic, and apparently they are normally not recognized as foreign by the immune system Žsimilar to self antigens or ‘extended self’..29 Nonetheless, cell wall constituents and products released from even the normal flora can have strong pro-inflammatory activities. Changes in the mucosal barrier, allowing translocation of bacteria and cell wall-derived pro-inflammatory macromolecules into the mucosa, might therefore result in inflammation despite absence of aggressive pathogenicity factors in these bacteria. Furthermore, usually harmless microbes could gain virulence, e.g. by acquiring pathogenicity factors from other bacteria via plasmids. A plethora of other, non-microbial exogenous factors which can not be discussed in detail in this review, e.g. smoking, food antigens, drugs have all been implicated in the pathogenesis of IBD. Future research will have to clarify the relative importance of these influences. 71
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Initiation and promotion of inflammation in IBD
tionship between known microorganisms and disease can elude the attention of researchers despite considerable efforts to unravel the causes of the disease. Similarly, it still remains a possibility that an as yet unidentified microorganism which is hard to grow with conventional culture methods is the cause of IBD. Epidemiological data supporting transmissibility of IBD is at best controversial. Reports about occasional family clustering of IBD are contrasted by the apparent absence of an increased risk for spouses of patients or medical personnel treating IBD patients to develop IBD themselves.2,7 Acute manifestations of IBD can be hard to distinguish from intestinal infections. The intestinal pathology in Crohn’s disease is almost identical to that in infections with Yersinia enterocolitica or Mycobacterium tuberculosis. Infections with Campylobacter, Shigella, Amoeba, or cytomegalovirus ŽCMV. can be easily mistaken for an acute presentation of ulcerative colitis.30 To some degree this may reflect the limited repertoire of defense mechanisms of the intestine. The development of granulomas, a typical histological feature of Crohn’s disease, may indicate persistence of a microbe or microbial material in the tissue. In reactive arthritis, persistence of microbial factors has been proposed to explain the protracted development of joint inflammation; similarly, inflammation in Crohn’s disease could result from the protracted activation of the immune system in these granulomas. Paramyxovirus Žmeasles. like virus particles have been described in the vascular endothelia and granulomas of Crohn’s disease tissues. In addition, two epidemiological studies from Sweden showed that presentation of Crohn’s disease before the age of 30 was associated with birth in areas of recent measles epidemics31 and that measles infection of the mother during pregnancy was associated with an increased risk of the child developing Crohn’s disease.32 A possible role for viruses in the pathogenesis of ulcerative colitis is further suggested by the anecdotal finding of intranuclear and cytoplasmic inclusion bodies in colonic tissue. The Epstein]Barr virus can be isolated from 75% of patients with ulcerative colitis and anti-CMV antibody titres are frequently increased in these patients.30 Interestingly, slowly replicating EBV seems to be able to cause activation and proliferation of infected T cells. However, a high percentage of the healthy population is latently infected with these herpesviruses. The bacterium Mycobacterium paratuberculosis causes
A number of scenarios can be considered for the role of microbial factors in the immuno-pathology of IBD. An unusually aggressive microbe might cause chronic inflammation due to its ability to penetrate into the mucosa, to induce the release of pro-inflammatory mediators and to persist in tissues. Usually organisms causing acute intestinal infections such as shigella, salmonella and campylobacter are quickly eliminated. Some microbes, such as Entameba histolytica or Mycobacterium tuberculosis, can persist intracellularly particularly in macrophages, after resolution of the acute inflammation. For example, the occurrence of reactive arthritis several months following resolution of intestinal inflammation has been attributed to distribution of macrophages into the joints which contain live intracellular microorganisms or microbial material. An aggressive microbe may induce a breach of the mucosal barrier which allows penetration of the bacteria of the normal flora. Similarly, an acute infection might exacerbate an inherited but previously subclinical permeability defect. An intrinsic defect of the immune system, inapparent as long as nothing else fails, may become obvious during the course of an infection and result for example, in the inability to resolve inflammation once the pathogen has been eliminated. Microbial factors could alter the immune response qualitatively, e.g. by inducing a shift in the balance between Th1 and Th2 cytokines. Inappropriate inflammation may also be the result of a breakdown of the normal tolerance towards gut flora bacteria. Microbial factors might even induce the emergence of self-reactivity in the intestine, in analogy to certain autoimmune diseases. These possibilities are not necessarily mutually exclusive and it is possible that a heterogeneous disease such as IBD may involve a variety of combinations of such hypothetical mechanisms.
IBD as an infectious disease Since Crohn’s disease and ulcerative colitis were described for the first time decades ago the hypothesis that IBD might be caused by an infectious organism has received much attention. Although decades of continuous efforts to isolate or at least detect a microorganism in IBD tissue have failed, the delayed recognition of Helicobacter pylori as the cause of peptic ulcer disease demonstrates very clearly that a rela72
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a granulomatous intestinal disease in ruminants which is strikingly similar to human Crohn’s disease. Mycobacteria can persist in tissues, cause granulomatous disease and are difficult to culture. In addition, mycobacterial products have immuno- modulatory properties Že.g. BCG, Freunds adjuvant.. As a potential source of infection, M. paratuberculosis has been demonstrated in commercially distributed milk and drinking water. Furthermore, intestinal infection with the related M. tuberculosis can appear very similar to Crohn’s disease although their response to treatment is quite different. Isolation of microbial organisms and identification of microbial material have occasionally been successful. However, highly sensitive and specific methods such as nested PCR have not been able to consistently isolate or identify a microorganism such as measles virus or M. paratuberculosis and yielded mostly negative results.30 Thus, evidence supporting a role for an infectious microbe in IBD remains controversial.
fraction of patients with ulcerative colitis indicates that they do not play a central role in the pathogenesis of the disease. Since similar pANCA can be found in patients with sclerosing cholangitis without intestinal disease and even in some healthy individuals, production of these auto-antibodies obviously does not cause disease, although, as an epiphenomenon, they may be indicative of the more profound immune disturbance in ulcerative colitis. Thus, the significance of pANCA lies in their value as disease markers for ulcerative colitis. In summary, direct evidence for a role of autoimmune mechanisms in IBD is scarce and controversial. Under normal conditions, the immune system does not turn against itself, although potentially autoreactive cells are probably generated during thymic maturation. Self-reactive T cells that have escaped clonal deletion in the thymus either express a TCR with a very low affinity to immunodominant self-antigens or are naive cells which can not penetrate through the vascular endothelium into the tissue and therefore are not primed to cause damage Ž‘immune ignorance’.. Cells which do reach the tissue can be deleted peripherally due to constitutive production of the Fas ligand molecule in the tissue or by a TNF-dependent mechanism.36 Activation of autoreactive cells does not normally occur since these cells usually lack the IL-2-dependent mechanisms essential for the amplification of T cell responses Ž‘anergy’.. Furthermore, most peripheral tissues lack organized lymphoid tissue necessary to induce a strong inflammatory response if autoreactive were activated locally.37 The microenvironment also seems to determine the result of activation of autoreactive cells: whereas development of a Th1 type response seems to initiate some autoimmune diseases, Th2 cells are protective Žthe reverse may be the case in other diseases..38 Thus, the normal balance between Th1 and Th2 cytokines seems to deflect autoreactive cells from their antigen by turning them into cells with ‘suppressor’ activity 39 subduing autoreactive responses rather than causing damage Ž ‘immune deviation’..38,40 In the intestine, some of these mechanisms may not be present. The normal intestinal mucosa contains secondary lymphoid tissue. Fas mediated apoptosis does not appear to reduce intestinal damage upon generalized T cell activation. The constitutive Fas L expression and resistance to Fas mediated cell death typical for ‘immunologically privileged’ peripheral tissues is contrasted by constitutive expression of high levels of Fas and absence of FasL production by
Is IBD an autoimmune disease? In IBD, tissue damage is caused by a self-destructive, exaggerated inflammation, and this inflammation appears to be maintained in the absence of a specific pathogen. Self-contained and auto-destructive inflammation could be regarded as the hallmark of autoimmunity in the broadest sense. Recent studies have even found evidence for self-antigen-specific immune mechanisms, suggesting the possibility that IBD may indeed be an autoimmune disease. Some antibodies generated in IBD recognize self-antigens, e.g. colonocytes, mucin and goblet cells.33 ] 35 Autoantibody-dependent cytotoxicity against colonic epithelial cells has been demonstrated in vitro, and in ulcerative colitis, activated complement factors have been described to co-localize with anti-colon antibodies on the luminal surface of the epithelium.35 Anti-mucin and anti-goblet cell antibodies could cause alteration of the protective mucin layer which might lead to increased adherence of luminal bacteria. Antibodies against neutrophils ŽpANCA. are very common in ulcerative colitis. However, identification of the putative autoantigens revealed that pANCA in ulcerative colitis and those found in some autoimmune vasculitides probably recognize different antigens. Although pANCA may bind, e.g. to bactericidalrpermeability increasing protein, a molecule that prevents inflammation in response to bacterial LPS Žsee below., the absence of pANCA in a significant 73
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epithelial cells. Thus, Fas mediated mechanisms may cause tissue damage rather than prevent it in the intestine.41,42 In IBD, it is conceivable that the observed imbalance between pro- and anti-inflammatory mediators could reverse anergy of autoaggressive T cells 43 and might also interfere with ‘immune deviation’, pushing autoreactive cells towards a more aggressive phenotype. Furthermore, the normal induction of ‘oral tolerance’ to exogenous antigens seems to depend on the balance between IL12 and TGF-b . Abrogation of tolerance, for example due to IL-12 overproduction, enhances the chances that an exogenous antigen sharing immunogenic determinants with self-antigens is no longer ignored but induces autoimmunity Ž‘molecular mimicry’.. Whereas unprimed T cells usually do not emigrate into peripheral tissues, altered expression of endothelial adhesion molecules increasingly allows the recruitment of naive T cells into the mucosa. Some of these T cells may recognize self-antigens which normally remain sequestered in the mucosa, were not present in the thymus during maturation and thus failed to induce tolerance. The enhanced expression of MHC class II molecules by epithelial cells and activation of B cells in IBD probably leads to increased antigen presentation by ‘non-professional’ antigen-presenting cells ŽAPC.. These cells may process antigen differently than professional APC and therefore present different peptides which were not immunodominant previously Žpresentation of ‘cryptic’ antigens..4 Furthermore, the association of IBD with known autoimmune diseases and the frequent occurrence of ‘extraintestinal manifestations’ which share all the essential characteristics of their idiopathic counterparts and thus probably represent autoimmune diseases as well, can be regarded as indirect evidence for the occurrence of autoimmune mechanisms in the context of IBD. In summary, autoimmunity and IBD seem to occur together, although reason for their causal relationship is still unclear. At the moment, the notion that IBD itself may be an autoimmune disease remains speculation.
Nonetheless, at least some autoimmune diseases seem to be triggered by infectious agents which are no longer present when disease becomes manifest clinically.44 IBD may be initiated by a similar mechanism, particularly if autoimmune mechanisms turn out to be involved in the pathogenesis. It is possible that an infection itself Že.g. by a virus which latently infects host cells without killing them. remains undetected but manifests as an ‘autoimmune’ disease if coating of the cell membrane with microbial peptides leads to recognition of the host cell as foreign by the immune system. Microbial peptides could haptenize self proteins on host cells, which are then recognized by the immune system as foreign Ž‘altered self hypothesis’., a possibility which is demonstrated by the induction of colitis with the haptenizing agent TNBS in a rat model of IBD. Microbial peptides which share antigenic determinants with self proteins could induce a cross-reactive immune response in the course of an infection Ž‘molecular mimicry’.. The recruitment of T cells with an increasing diversity of antigenic specificities can lead to recognition of other epitopes on the same microbial protein, decreasing the threshold for cross-reactivity with host proteins Ž‘epitope spreading’.. Cross-reactivity between autoantibodies in IBD with microbial antigens has indeed been suggested, but could not be proved to date. The homology between mycobacterial heat shock protein, HSP65 and human HSP60 is particularly intriguing, since mycobacteria have been implicated as causative agents in Crohn’s disease. Whereas some groups have found increased anti-HSP60 autoantibodies, anti-HSP-65 antibodies and HSP65 reactive T cells in the peripheral blood,45 ] 47 as well as increased HSP60 expression in IBD,48 our study found neither increased intestinal expression of HSP60 nor HSP60 reactive T cells in Crohn’s disease.49 The changes in the cytokine profile observed in IBD which might alter the functional properties of activated autoreactive cells 39,50 could be induced by an infectious agent. In addition, infections can be accompanied by increased expression of MHC class II molecules on intestinal epithelial cells 51 and can also induce the expression of costimulatory molecules such as B7.2, both of which have been observed in IBD as well.52,53 Since absence of costimulatory molecules may be crucial for the maintenance of self-tolerance by promoting anergy,54 prolonged alteration of antigen presentation by an infection could be related to the autoimmune phenomena observed in IBD Žsee above.. Lastly, bacterial derived superantigens could activate large subsets of T cells includ-
Is IBD an ‘infection-induced autoimmune disease’? Exogenous factors seem to be necessary to trigger IBD. Whereas no specific infectious microorganisms could be isolated from IBD tissues, the same is true for inflamed tissues in autoimmune diseases. 74
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ing self-reactive clones. Increased frequencies of certain T cell subsets, e.g. TCR Vb 8 q , has been described in Crohn’s disease and altered functional activity of these subsets suggest a possible role for superantigens in IBD.55,56 Alternatively, if regulatory T cells become non-specifically deleted through interactions with superantigens, deficient suppression of self-reactive clones could lead to autoimmune reactivity. The relationship between IBD, intestinal infections, reactive extraintestinal inflammation and autoimmune diseases such as idiopathic ankylosing spondylitis provides compelling indirect evidence for the existence of a connection between IBD, infectious diseases and autoimmune diseases. Reactive arthritis is triggered by mucosal infections for example with Salmonella, Shigella, or Yersinia. Reactive arthritis can occasionally become chronic and then resembles idiopathic ankylosing spondylitis and axial arthritis in IBD. The genetic association of all three manifestations with the same HLA B27 allele seems to suggest a common pathogenetic mechanism. The role of HLA B27 in ankylosing spondylitis may be related to the ability of the molecule to facilitate invasion of microorganisms, its role in shaping the T cell repertoire, presentation of an ‘arthritogenic’ self-peptide, or even cross-reactivity between epitopes of HLA B27 itself and microbial peptides.57,58 Interestingly, some rat strains transgenic for HLA B27 seem to spontaneously develop arthritis, however, in the absence of gut flora in animals raised in a germfree environment, inflammation does not occur.59 In contrast to human IBD, which is not linked to HLA B27, these animals also develop gastrointestinal inflammation which is equally dependent on the presence of gut flora. However, although this data fits into the concept that IBD may be an autoimmune disease related to microbial factors, the relationship between joint inflammation and the intestinal mucosa may be more simple. For example, increased intestinal permeability has been found in patients with reactive arthritis and ankylosing spondylarthritis.9 In an experimental animal model reactive arthritis is the result of small intestinal bacterial overgrowth which seems to lead to increased uptake of the macromolecule peptidoglycan-polysaccharides ŽPG-PS . produced by anaerobic bacteria.59 The same macromolecule, injected into the intestinal wall, also leads to a granulomatous intestinal inflammation which resembles Crohn’s disease. The chronic relapsing inflammation in this model is T cell mediated, but depends on persistence
of PG-PS in the tissues and protracted re-release by the liver, not an autoimmune process. In summary, despite indirect evidence for a possible relation to autoimmune diseases, autoimmune mechanisms do not seem to play an important role in the pathogenesis of IBD.
Microbes, macromolecules and immune dysregulation In the absence of a specific aggressive pathogen and the lack of evidence for an autoimmune mechanism, the only remaining alternative seems to be that nonspecific environmental factors are the cause of exaggerated inflammation in IBD.30 This may be the least attractive hypothesis, since identification of a specific abnormality as a target for causative therapy might then be impossible. There is indeed increasing evidence that in particular the normal intestinal flora contributes significantly to the development of inflammation, although this does not necessarily lead to dismal conclusion that a propensity to develop IBD is simply the price we have to pay for the complex mechanisms allowing uptake of a large variety of nutrients and protection against similarly varied pathogens at the same time. IBD occurs with a predilection for sites with a high luminal content of bacteria, e.g. the terminal ileum and the colon. In Crohn’s disease, reduction of luminal bacteria content by decontamination, bowel rest, lavage or total parenteral nutrition has beneficial effects on the activity of inflammation. Furthermore, there is evidence that in Crohn’s disease the composition of the luminal flora is altered. Indeed, the beneficial effect of antibiotics such as metronidazole in some patients with Crohn’s disease has been attributed to the longterm reduction of Bacteroides spp in the lumen. Interestingly, Bacteroides seems to be responsible for the intestinal inflammation occurring in HLA-B27 transgenic rats and anaerobes may also be the source of PG-PS in the above mentioned model of experimental small intestinal bacterial overgrowth.60 Similarly, the presence of normal intestinal flora is also a prerequisite for the development of intestinal disease in the animal models of immune dysregulation as mentioned above. In ulcerative colitis, increased bacterial adherence, invasiveness and even production of toxins by normal bacterial flora has been demonstrated.61,62 Bacterial adhesion to epithelial cells in vitro induces secretion of proinflammatory mediators, such as IL-1, IL8 and IL-6, which could for example, 75
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be responsible for the massive transmigration of neutrophils through the epithelium into the colonic crypts Ž‘crypt abscesses’. typical for ulcerative colitis.63 ] 65 Usually, the endogenous flora does not induce immune responses. The composition of the intestinal flora is a rather stable individual trait and these bacteria seem to be regarded as ‘extended self’ by the immune system. However, in IBD, recent studies have shown a breakdown of this tolerance which correlates with activity of inflammation.29 However, regardless of tolerance of the host, some bacterial antigens, such as lipopolysaccharides ŽLPS., the major constituent of the cell wall of Gram negative bacteria, are potent pro- inflammatory molecules. Whereas mononuclear cells of the normal mucosa are not reactive to LPS, freshly recruited tissue macrophages in IBD express the high affinity LPS receptor, CD14.16 Activation of these cells via CD14 could lead to release of pro-inflammatory mediators TNF, IL1, IL6 and IL8, procoagulant tissue factor and complement components. Interestingly, in the IL-2 knockout mouse, expression of CD14 by epithelial cells precedes the onset of inflammation.66 The possible role of proinflammatory bacterial macromolecules in IBD has recently been summarized in a comprehensive review.60 In short, some evidence suggests that cell wall constituents, such as LPS and PG-PS, as well as formylated oligopeptides, such as FMLP, can be detected in the serum of IBD patients, which also seem to have increased titres of antibodies against these molecules. All these molecules, alone or synergistically, strongly induce the release of pro-inflammatory mediators through specific ŽFMLP, LPS. and nonspecific receptors ŽLPS, PG-PS . by macrophages and neutrophils. FMLP and PG-PS can undergo enterohepatic recirculation and thus may be related to the biliary hepatic inflammation associated with IBD.
how ubiquitous, non-specific environmental factors induce severe inflammation only in some individuals, but leave the majority of us unscathed. Furthermore, there may even be the possibility that the unraveling of these pathogenetic mechanisms could also lead to a better understanding of reactive arthritis, idiopathic ankylosing spondylitis and autoimmune diseases.
Acknowledgements K.C. is an Ontario Ministry of Health Career Scientist. M.M. is supported by a grant from the Deutsche Forschungsgemeinschaft ŽMe 1432r1]1..
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