Cross-reactive epitope with Klebsiella pneumoniae nitrogenase in articular tissue of HLA-B27+ patients with ankylosing spondylitis

Molecular mimicry in the pathogenesis of autoimmune rheumatic diseases

Autoimmune rheumatic diseases (ARDs) are a heterogeneous group of conditions characterized by excessive and misdirected immune responses against the body's own musculoskeletal tissues. Their exact aetiology remains unclear, with genetic, demographic, behavioural and environmental factors implicated in disease onset. One prominent hypothesis for the initial breach of immune tolerance (leading to autoimmunity) is molecular mimicry, which describes structural or sequence similarities between human and microbial proteins (mimotopes). This similarity can lead to cross-reactive antibodies and T-cell receptors, resulting in an immune response against autoantigens. Both commensal microbes in the human microbiome and pathogens can trigger molecular mimicry, thereby potentially contributing to the onset of ARDs. In this review, we focus on the role of molecular mimicry in the onset of rheumatoid arthritis and systemic lupus erythematosus. Moreover, implications of molecular mimicry are also briefly discussed for ankylosing spondylitis, systemic sclerosis and myositis.

Association between Klebsiella pneumoniae and ankylosing spondylitis: A systematic review and meta-analysis

AIM

The aim of this study is to evaluate the association between Klebsiella pneumoniae infection and ankylosing spondylitis (AS).

METHOD

Five electronic databases, PubMed, Embase, Medline, Web of Science, and Scopes, were searched until September 29, 2021. Cohort and case-control studies that assessed the association between K. pneumoniae infection and AS were included. Pooled odds ratio (OR) was selected to show the effect size. Subgroup analysis (active or inactive AS) and 2 forms of sensitivity analysis were conducted. All statistical analyses were conducted by using STATA 12.0.

RESULTS

There were 25 case-control studies finally included, including 8 studies concerning presence of K. pneumoniae in feces, and 17 studies concerning serum antibody (immunoglobulin [Ig]G, IgM, IgA) against K. pneumoniae. The results suggested that when compared with healthy people, presence of K. pneumoniae in feces was associated with AS (OR: 5.65; 95% CI: 1.68-19.00). Similarly, when compared with healthy people, higher positive rates of IgA (OR: 6.28; 95% CI: 3.32-11.91) and IgG (OR: 5.22; 95% CI: 1.36-19.99) were observed. Subgroup analyses suggested that association between K. pneumoniae and AS appears stronger in active AS.

CONCLUSION

When compared with healthy people, a significantly higher positive rate of K. pneumoniae in feces, serum IgA and IgG were observed in patients with AS, suggesting that K. pneumoniae probably plays a crucial role in the occurrence of AS. The findings in this study need further prospective investigations for confirmation.

The role of proteomics in defining autoimmunity

INTRODUCTION

Proteomics, i.e. the study of the set of proteins produced in a cell, tissue, organism, or biological entity, has made possible analyses and contextual comparisons of proteomes/proteins and biological functions among the most disparate entities, from viruses to the human being. In this way, proteomic scrutiny of tumor-associated proteins, autoantigens, and pathogen antigens offers the tools for fighting cancer, autoimmunity, and infections.

AREAS COVERED

Comparative proteomics and immunoproteomics, the new scientific disciplines generated by proteomics, are the main themes of the present review that describes how comparative analyses of pathogen and human proteomes led to re-modulate the molecular mimicry concept of the pre-proteomic era. I.e. before proteomics, molecular mimicry - the sharing of peptide sequences between two biological entities - was considered as intrinsically endowed with immunologic properties and was related to cross-reactivity. Proteomics allowed to redefine such an assumption using physicochemical parameters according to which frequency and hydrophobicity preferentially confer an immunologic potential to shared peptide sequences.

EXPERT OPINION

Proteomics is outlining peptide platforms to be used for the diagnostics and management of human diseases. A Molecular Medicine targeted to obtain healing without paying the price for adverse events is on the horizon. The next step is to take up the challenge and operate the paradigm shift that the current proteomic era requires.

Spondyloarthritis, Acute Anterior Uveitis, and Fungi: Updating the Catterall-King Hypothesis

Spondyloarthritis is a common type of arthritis which affects mostly adults. It consists of idiopathic chronic inflammation of the spine, joints, eyes, skin, gut, and prostate. Inflammation is often asymptomatic, especially in the gut and prostate. The HLA-B*27 allele group, which presents intracellular peptides to CD8+ T cells, is by far the strongest risk factor for spondyloarthritis. The precise mechanisms and antigens remain unknown. In 1959, Catterall and King advanced a novel hypothesis explaining the etiology of spondyloarthritis: an as-yet-unrecognized sexually acquired microbe would be causing all spondyloarthritis types, including acute anterior uveitis. Recent studies suggest an unrecognized sexually acquired fungal infection may be involved in prostate cancer and perhaps multiple sclerosis. This warrants reanalyzing the Catterall-King hypothesis based on the current literature. In the last decade, many links between spondyloarthritis and fungal infections have been found. Antibodies against the fungal cell wall component mannan are elevated in spondyloarthritis. Functional polymorphisms in genes regulating the innate immune response against fungi have been associated with spondyloarthritis (CARD9 and IL23R). Psoriasis and inflammatory bowel disease, two common comorbidities of spondyloarthritis, are both strongly associated with fungi. Evidence reviewed here lends credence to the Catterall-King hypothesis and implicates a common fungal etiology in prostate cancer, benign prostatic hyperplasia, multiple sclerosis, psoriasis, inflammatory bowel disease, and spondyloarthritis. However, the evidence available at this time is insufficient to definitely confirm this hypothesis. Future studies investigating the microbiome in relation to these conditions should screen specimens for fungi in addition to bacteria. Future clinical studies of spondyloarthritis should consider antifungals which are effective in psoriasis and multiple sclerosis, such as dimethyl fumarate and nystatin.

Human Leukocyte Antigen-B27: The Genetic Predisposition Leading to Reactive Arthritis during Induction Phase Chemotherapy for Acute Myeloid Leukemia

We report a case of reactive arthritis (ReA) during induction phase chemotherapy of a 15-year-old male patient with acute myeloid leukemia (AML) M4 with inv(16), most probably due to a genetic predisposition of being human leukocyte antigen b27 (HLA-B27) positive. The episode of ReA recurred during consolidation therapy; however, the patient was asymptomatic after the completion of treatment. The link between HLA-B27 and a large family of inflammatory rheumatic diseases is a well-established fact, but interestingly, there is also a molecular link between HLA-B27 and hematological malignancies. This case brings to our notice, the common immunological, molecular, and microbiological link between AML, HLA-B27, and ReA. It also emphasizes the fact that clinicians should have a high index of suspicion of HLA-B27 positivity, if a case of AML develops arthritis during chemotherapy, since early introduction of immunosuppressive medications for arthritis may reduce morbidity and prevent delay in the administration of further chemotherapy cycles.

Raised incidence of ankylosing spondylitis among Inuit populations could be due to high HLA-B27 association and starch consumption

Ankylosing spondylitis (AS) is a chronic inflammatory arthritis mainly affecting the spinal joints. It would appear that the most likely causative agent in the development of AS is an environmental factor in the genetically susceptible, HLA-B27 positive, individuals. Extensive data from several countries support the notion that Klebsiella pneumonia bacteria are the most likely culprit in the causation of AS. These microbes possess antigens which resemble HLA-B27 and spinal collagens. Increased intake of high-starch diet is directly proportional to the gut-associated bacterial load, especially in the large intestine, and among these microbial agents, Klebsiella is considered as one of the main constituting components. Therefore, a low-starch diet intake alongside the currently used medical therapeutic modalities could be beneficial in the management of patients with early AS. It is suggested that a change in the dietary habits from high protein, low-starch marine components to the Westernized high-starch diet among the Inuit peoples of Alaska and Canada could be considered as one of the main contributing factors in the increased prevalence of AS during the last few decades within this genetically unmixed native population.

No human protein is exempt from bacterial motifs, not even one

The hypothesis that mimicry between a self and a microbial peptide antigen is strictly related to autoimmune pathology remains a debated concept in autoimmunity research. Clear evidence for a causal link between molecular mimicry and autoimmunity is still lacking. In recent studies we have demonstrated that viruses and bacteria share amino acid sequences with the human proteome at such a high extent that the molecular mimicry hypothesis becomes questionable as a causal factor in autoimmunity. Expanding upon our analysis, here we detail the bacterial peptide overlapping to the human proteome at the penta-, hexa-, hepta- and octapeptide levels by exact peptide matching analysis and demonstrate that there does not exist a single human protein that does not harbor a bacterial pentapeptide or hexapeptide motif. This finding suggests that molecular mimicry between a self and a microbial peptide antigen cannot be assumed as a basis for autoimmune pathologies. Moreover, the data are discussed in relation to the microbial immune escape phenomenon and the possible vaccine-related autoimmune effects.

The link between ankylosing spondylitis, Crohn's disease, Klebsiella, and starch consumption

Both ankylosing spondylitis (AS) and Crohn's disease (CD) are chronic and potentially disabling interrelated conditions, which have been included under the group of spondyloarthropathies. The results of a large number of studies support the idea that an enteropathic pathogen, Klebsiella pneumoniae, is the most likely triggering factor involved in the initiation and development of these diseases. Increased starch consumptions by genetically susceptible individuals such as those possessing HLA-B27 allelotypes could trigger the disease in both AS and CD by enhancing the growth and perpetuation of the Klebsiella microbes in the bowel. Exposure to increased levels of these microbes will lead to the production of elevated levels of anti-Klebsiella antibodies as well as autoantibodies against cross-reactive self-antigens with resultant pathological lesions in the bowel and joints. Hence, a decrease of starch-containing products in the daily dietary intake could have a beneficial therapeutic effect on the disease especially when used in conjunction with the currently available medical therapies in the treatment of patients with AS and CD.

Autoimmunity in Rheumatic Diseases Is Induced by Microbial Infections via Crossreactivity or Molecular Mimicry

A general consensus supports fundamental roles for both genetic and environmental, mainly microbial, factors in the development of autoimmune diseases. One form of autoimmune rheumatic diseases is confined to a group of nonpyogenic conditions which are usually preceded by or associated with either explicit or occult infections. A previous history of clinical pharyngitis, gastroenteritis/urethritis, or tick-borne skin manifestation can be obtained from patients with rheumatic fever, reactive arthritis, or Lyme disease, respectively, whilst, other rheumatic diseases like rheumatoid arthritis (RA), ankylosing spondylitis (AS), and Crohn's disease (CD) are usually lacking such an association with a noticeable microbial infection. A great amount of data supports the notion that RA is most likely caused by Proteus asymptomatic urinary tract infections, whilst AS and CD are caused by subclinical bowel infections with Klebsiella microbes. Molecular mimicry is the main pathogenetic mechanism that can explain these forms of microbe-disease associations, where the causative microbes can initiate the disease with consequent productions of antibacterial and crossreactive autoantibodies which have a great impact in the propagation and the development of these diseases.

The pathogenesis of ankylosing spondylitis

✓ Ankylosing spondylitis (AS) is a chronic inflammatory disease that can cause significant functional complications by affecting the sacroiliac joints and axial skeleton. Despite a longstanding knowledge about the familial associations of this disease, particularly among patients positive for human leukocyte antigen (HLA)–B27, the fundamental pathogenetic mechanism by which this disease arises in genetically susceptible individuals remains ill defined. Furthermore, the molecular predilection for characteristic articular site involvement remains under ongoing investigation. Current theories about the HLA-B27 association range from the presentation of novel arthritogenic peptides, to abnormal autoimmune stimulation, to anomalous microbial tolerance. The immune effectors of this damage include CD4+, CD8+, and natural killer cells, with marked heterogeneity at different sites. Biomechanical stresses may trigger this disease by exposing the body to previously immune-sequestered autoantigens or by providing a route for bacterial seeding. Environmental triggers such as infection have not been definitively established but may represent a primary pathogenic step in a molecular-mimicry process. In this article, the authors review the current literature on the origin and pathophysiology of AS, focusing on genetic and molecular associations, consequent pathomechanisms, and associated triggers. An improved understanding of the sequence of molecular events that predispose and initiate the onset of this disease will allow for more specific and targeted therapy and better avoidance of the significant side effects of systemic immunomodulation.

Ankylosing spondylitis is linked to Klebsiella--the evidence

Ankylosing spondylitis (AS) is a chronic inflammatory spinal and large-joint arthritic and potentially disabling condition, mainly affecting males of young age groups. Extensive literature based on the results of various genetic, microbiological, molecular and immunological studies carried out by independent research groups suggests that Klebsiella pneumoniae is the main microbial agent being implicated as a triggering and/or perpetuating factor in the etiopathogenesis of AS. Novel diagnostic markers and criteria based on the association with high anti-Klebsiella antibodies could be used in the detection of AS patients during early stages of the disease, and together with the current treatments might help in implementing the use of new therapeutic anti-microbial measures in the management of AS. Prospective longitudinal studies with the use of anti-microbial measures in patients with AS are required to establish the therapeutic benefit of this microbe-disease association.

A possible link between Crohn's disease and ankylosing spondylitis via Klebsiella infections

Crohn's disease (CD) is an immune-mediated gastrointestinal inflammatory disease, which could arise from an interplay between genetic and environmental factors. Klebsiella microbes were suggested to have a vital role in the initiation and perpetuation of the disease through the mechanism of molecular mimicry. This proposition is based on the results of various studies where significantly elevated levels of antibodies against the whole bacteria or preparations from Klebsiella microbes and antibodies to collagen types I, III, IV, and V were detected in patients with CD and patients with ankylosing spondylitis (AS). Molecular similarities were found between Klebsiella nitrogenase and HLA-B27 genetic markers and between Klebsiella pullulanase and collagen fibers types I, III, and IV. Furthermore, significantly positive correlations and cross-reactivity binding activities were observed between anti-Klebsiella and anticollagen antibodies among patients with CD and AS. Early treatment of CD patients with anti-Klebsiella measures is proposed, which may involve the use of antibiotics and low starch diet together with other traditionally used immunomodulatory, immunosuppressive, or biologic agents.

The potential use of antibacterial peptide antibody indices in the diagnosis of rheumatoid arthritis and ankylosing spondylitis

BACKGROUND

Both rheumatoid arthritis (RA) and ankylosing spondylitis (AS) are potentially disabling arthritic disorders for which as yet no highly sensitive and reliable diagnostic laboratory markers are available.

OBJECTIVE

The objective of this study was to evaluate the levels of antibodies against Proteus and Klebsiella antigenic peptides in an endeavor to develop diagnostic indices for the identification of patients with RA and AS, respectively.

METHODS

Sera from 50 patients with RA, 34 patients with AS, and 38 healthy subjects were screened for antibodies against "ESRRAL" and "IRRET" synthetic amino acid peptides obtained from Proteus hemolysin and urease (HU) as well as against "QTDRED" and "DRDE" peptides from Klebsiella nitrogenase and pullulanase (NP) proteins, respectively. Multiplication of the 2 antibodies against each organism produced indices for RA-HU and AS-NP.

RESULTS

Significantly increased levels of anti-HU antibodies (P<0.0001) were observed in patients with RA when compared with patients with AS or with healthy control subjects. Patients with AS were found to have significantly elevated levels of anti-NP (P<0.0001) antibodies when compared with patients with RA or with healthy subjects. Furthermore, all patients with RA were found to have values of anti-HU antibody (RA-HU) index above 95% confidence limit (CL) of the mean of healthy control subjects; meanwhile, all patients with AS were having values of anti-NP antibody (AS-NP) index above the 95% CL of the mean of healthy control subjects (100% sensitivity). However, the specificity of the RA-HU index in RA and the AS-NP index in patients with AS were 92% and 95%, respectively.

CONCLUSION

The use of the RA-HU or AS-NP diagnostic index as a sole marker or in combination with other autoantibody markers could be used in the identification of patients with RA or AS, respectively. Longitudinal investigations starting with patients with early disease will be needed.

Microbial factors in spondyloarthropathies: insights from population studies

Infections and genetics play a role in the development of reactive arthritis. The clinical manifestations and severity of the features depend on the triggering infections and the epidemiologic setting. Reports from hospital-based series show the lowest frequency of reactive arthritis, but often, patients have severe arthritis associated with a high frequency of HLA-B27. At the population level, reactive arthritis occur in 7 to 15% of the infected subjects. The disease is usually mild, affects small joints, can be polyarticular, often rapidly disappears, and has a low association with HLA-B27. There also seems to be a change in the spectrum of triggering infections. Reports of Yersinia arthritis are less common, whereas arthritis in association with Campylobacter or Salmonella infections seems to be increasing. The role of early antimicrobial chemotherapy for the prevention of reactive arthritis needs to be studied.

Molecular mimicry between HLA-DR alleles associated with rheumatoid arthritis and Proteus mirabilis as the Aetiological basis for autoimmunity

Molecular mimicry is one of the pathological mechanisms proposed to explain the association between microorganisms and autoimmune diseases. This review deals with the association between bacteria and rheumatic diseases with a special emphasis on rheumatoid arthritis where upper urinary tract infection by Proteus mirabilis is the possible cause of this severe, arthritic condition. Prospective trials involving anti-Proteus therapy should be carried out.

Therapeutic aspects of spondyloarthropathies -- a review

In the pathogenesis of spondyloarthropathies, infection and gut inflammation are the most important external triggering factors. Early antimicrobial therapy to treat urethritis caused by Chlamydia trachomatis is effective in preventing a recurrent reactive arthritis. When the arthritis appear, a short term conventional antimicrobial therapy is unable to modify its course. In acute chlamydia arthritis, patients benefit from a prolonged (3-month) treatment with tetracycline, while such a treatment has not proved to be effective in enteroarthritis or in chronic forms of reactive arthritis. The role of sulfasalazine in the treatment of patients with spondyloarthropathies is controversial. It might modify the disease course during acute and chronic reactive arthritis, and is working for patients with ankylosing spondylitis, especially patients with peripheral arthritis. Data showing an effect of sulfasalazine in the prevention of chronic spondyloarthropathy or in modification of the long-term prognosis of ankylosing spondylitis are, however, lacking.

Reiter's syndrome among Asian shipboard immigrants: the case of The Golden Venture

OBJECTIVE

To assess the incidence of Reiter's syndrome aboard The Golden Venture, a ship carrying illegal immigrants from China to the United States.

METHODS

After identification of an index case, we conducted telephone interviews with medical staff at immigrant detention centers in Pennsylvania, New York, and Virginia. When a potential case was identified at one facility, we performed a site inspection, reviewing the medical records of all detainees and performing histories and physicals on all those with joint and/or ocular complaints.

RESULTS

We identified two patients, both HLA B27 positive, with Reiter's syndrome. The observed incidence (0.87%) approximated the predicted incidence but may have underestimated the actual incidence. We review the history of shipboard Reiter's syndrome, and discuss the pathogenic roles of HLA B27 and particular infectious agents.

CONCLUSION

Continued transportation of illegal immigrants from China and other parts of the world is likely to result in occasional clusters of Reiter's syndrome. Physicians treating immigrant populations should remain aware of the possibility of reactive arthritis.

Presentation of HLA class I-derived peptides: potential involvement in allorecognition and HLA-B27-associated arthritis

Some 25 years ago, when purified HLA class I allotypes were first being analyzed, of major concern was that the papain used for solubilization might produce a mess of proteolytic fragments that would prove impossible to separate and sequence. Those fears proved unfounded (Parham et al. 1975), and the homogeneity of the preparations was sufficient to allow crystallization and determination of the three-dimensional structure (Bjorkman et al. 1987). Ironically the least ordered region of the electron density map provoked the most interest because it gave a first view of the diverse peptides bound by an MHC molecule. With this image a second chapter of HLA class I biochemistry began, its charge to determine the structures of bound peptides and their influence on the immune system. The extraordinary polymorphism of HLA class I heavy chains now seems quite manageable compared to the vast complexity of the peptides, and our present ignorance as to which ones are important for health and disease. The comparative weakness of most HLA class I associations with disease has made HLA-B27 an especially favored target for investigation, and more is known of the structure and peptide-presenting function of HLA-B27 than for any other HLA-B allotype (López de Castro 1994). Much of this information relates to the native HLA-B27 molecule and has been collected in the belief that disease is a direct consequence of its antigen-presenting function. If one subscribes to the relevance of the transgenic rodent models, this position has almost become untenable. For rats and mice 'non-functional' forms of HLA-B27 are the agents of disease, raising the possibility that B27-associated arthritis is induced by HLA class II presentation of a B27-derived peptide, a variant of the mechanism advanced for the classical HLA class II-associated diseases: type 1 diabetes, multiple sclerosis and rheumatoid arthritis (Gregersen et al. 1987, Roudier et al. 1989, Cucca & Todd 1996, Hall & Bowness 1996). Such speculation invites the obvious question as to whether other diseases associated with HLA class I and chronic inflammation, HLA-C and psoriasis for example (Tiilikainen et al. 1980, Yanagisawa et al. 1995), result from class II presentation of class I peptides.

Molecular mimicry: the geographical distribution of immune responses to Klebsiella in ankylosing spondylitis and its relevance to therapy

The discovery that HLA-B27 is linked to ankylosing spondylitis (AS) and HLA-DR1/DR4 to rheumatoid arthritis (RA) has provided new approaches to the study of the possible causation of these diseases. Several theories have been proposed to explain these associations but only one, namely "molecular mimicry", has provided a specific aetiological agent for each of these diseases. Molecular mimicry between HLA-B27 and two molecules in Klebsiella microbes: nitrogenase and pullulanase D has been reported whilst in Proteus microbes, the haemolysin molecule shows sterochemical similarity to HLA-DR1/DR4. Elevated immune responses to Klebsiella microbes have been demonstrated in AS patients from 10 different countries and this wide geographical distribution suggests that the same aetiological agent is probably acting in producing this condition. Furthermore RA patients show similar immune responses to Proteus microbes. Whether AS or RA are caused by these bacteria can only be resolved by tissue typing all rheumatological patients early, in the course of their disease and then assessing their response to antibiotic chemotherapy in longitudinal studies involving double-blind crossover trials. It is possible that in the future, the course of AS or even RA could be modified by adequate antibiotic chemotherapy or even diets which affect the substrates on which these bacteria grow.

HLA-B27 and bacteria

Most HLA-B27 positive individuals live without noticing the risk factor they carry. Only when certain bacteria come into play, usually in the form of intestinal or genital infection, some of the HLA-B27 positive individuals will develop a reactive disease. Such an association is so well established that it has formed a basis to study the relation of autoimmune reactions to microbial infections. Early epidemiological investigations have been followed by studies on immunological cross-reactivity and recently by studies on presentation of bacterial peptides by the HLA-B27 molecule. This introductory review briefly summarizes the evolution of the research, the goal of which still remains to clarify the etiopathogenetic mechanisms of HLA-B27 associated diseases.

Klebsiella pneumoniae-reactive T cells in blood and synovial fluid of patients with ankylosing spondylitis. Comparison with HLA-B27+ healthy control subjects in a limiting dilution study and determination of the specificity of synovial fluid T cell clones

OBJECTIVE

To study the frequency of Klebsiella pneumoniae-responsive T cells in the peripheral blood (PB) of ankylosing spondylitis (AS) patients compared with that in healthy HLA-B27+ donors, and to examine T lymphocyte clones (TLC) derived from AS patient synovial fluid (SF) for the presence of Klebsiella reactivity.

METHODS

Limiting dilution analysis of PB T cells in 8 patients with active AS and in 8 HLA-B27+ healthy subjects was used to determine the frequency of PB T cells responsive to K pneumoniae and Escherichia coli GroEL. SF T cells from a patient with active AS were cloned, and 125 TLC were characterized in proliferation assays.

RESULTS

There were fewer T cells in the PB of AS patients that reacted with K pneumoniae than in the PB of healthy HLA-B27+ subjects. The frequencies of E coli GroEL-responsive T cells were approximately 5-10 times lower in all subjects tested (healthy donors and AS patients), but without significant differences between the 2 groups. Two CD4+ TLC that recognized K pneumoniae (1 cross-reactive with E coli) as well as 3 TLC that recognized GroEL (2 CD4+, 1 T cell receptor gamma/delta+) were isolated from the SF of a patient with actige AS.

CONCLUSION

Our results indicate that there is a quantitative reduction of K pneumoniae-responsive T cells in the PB of AS patients as compared with healthy controls. This may reflect a defective peripheral T cell defense in the immune response to Klebsiella and may allow bacterial antigens to reach the synovium, where they initiate specific T cell responses.

Molecular mimicry: any role in the pathogenesis of spondyloarthropathies?

Ankylosing spondylitis and reactive arthritis are seronegative spondyloarthropathies, which are strongly associated with HLA-B27. Despite intensive investigation, the basis for this association is not clear. However, in recent years one favored hypothesis to explain this linkage has been that of molecular mimicry, i.e., sharing of linear or conformational epitopes common to microbial antigens and host structures. During the past few years several examples of molecular mimicry between HLA-B27 and microbial antigens have been described. Heat shock proteins, among others, have been considered as target candidates for autoimmune phenomena, because of the high degree of homology between bacterial and mammalian species. Reactive arthritis triggered by Yersinia or Salmonella provides a unique model for studying the pathogenetic mechanisms underlying human inflammatory joint diseases in general, because the arthritogenic microbes are known and well-characterized. We have described two bacterial proteins that share amino acid homology with HLA-B27, namely YadA (Yersinia adhesin) and OmpH, outer surface proteins of Yersinia and Salmonella, respectively. Notably, the area of identity of these amino acid sequences is located in the same place on the HLA-B27 molecule as a hexapeptide identical between Klebsiella nitrogenase and HLA-B27, and a pentapeptide shared by a Shigella flexneri protein and HLA-B27. We have investigated immune responses to a panel of synthetic peptides based on the HLA-B27-homologous portions of pathogen-specific antigens in patients with reactive arthritis and ankylosing spondylitis. One third of the patients have antibodies to the synthetic peptides. However, instead of recognizing the HLA-B27-homologous portion, the antibodies are directed against the flanking sequences of the synthetic peptides. The concept of the role of molecular mimicry between HLA-B27 and microbial antigens in the pathogenesis of spondyloarthropathies is discussed, with a conclusion that no convincing evidence for its significance exists at the present.

Absence of autoantibodies to peptides shared by HLA-B27.5 and Klebsiella pneumoniae nitrogenase in serum samples from HLA-B27 positive patients with ankylosing spondylitis and Reiter's syndrome

Some microorganisms which are pathogenic in humans share amino acid sequences with human proteins (molecular mimicry). It has been suggested that molecular mimicry might be a reason for autoimmunity as a result of immunological cross reactivity. A homologous sequence of six amino acids has been found in both Klebsiella pneumoniae nitrogenase and the HLA-B27.5 molecule. In addition, (auto)antibodies to a synthetic peptide that contained the HLA-B27.5/klebsiella mimicking epitope have been detected in serum samples from HLA-B27 positive patients with ankylosing spondylitis and Reiter's syndrome. Confirmation of these data is important, because ankylosing spondylitis and Reiter's syndrome have so far been assumed to be 'seronegative' rheumatic diseases. It was, however, not possible to confirm the presence of autoantibodies against the mimicking peptide in serum samples from patients with ankylosing spondylitis and Reiter's syndrome. Serum samples from 81 patients with ankylosing spondylitis, 38 patients with Reiter's syndrome, and 81 healthy blood donors were tested against the 'mimicking peptide' in an enzyme linked immunosorbent assay (ELISA). Some of the serum samples from patients showed high but non-specific binding to the mimicking peptide. A highly significant correlation between binding to plastic coated with the mimicking peptide, to plastic coated with an irrelevant peptide, and even to non-coated plastic was observed. The nature of the serum component(s) in these patient serum samples (and some control serum samples) responsible for the high non-specific binding to plastic remains unclear. It was also shown that antibodies to the HLA-B27 peptide (containing the mimicking epitope) induced in rabbits do not cross react with the klebsiella peptide and vice versa.

Chronic otitis media: a new extra-articular manifestation in ankylosing spondylitis?

Following a study reporting a fourfold increase in the occurrence of chronic otitis media in patients with ankylosing spondylitis, this prospective study examines this association with respect to severity, duration of disease, and acute phase in ankylosing spondylitis. Forty two consecutive patients with classical ankylosing spondylitis seen at the rheumatology clinic of a teaching hospital where the features of ankylosing spondylitis were recorded had an otological examination by an otolaryngologist. The occurrence of chronic otitis media (all categories) was 12/42 (29%). The acute phase serum markers (C reactive protein and IgG) were increased in patients with active or inactive chronic otitis media. Extra-articular manifestations were significantly more common in the chronic otitis media group than in those with no history of chronic otitis media. The results of this study suggest that chronic otitis media may be another extra-articular manifestation of ankylosing spondylitis. Alternative explanations, however, include similar aetiological factors for the two conditions or a previously unrecognised increased occurrence of HLA-B27 in patients with chronic otitis media.

HLA and disease

Many human diseases are associated with HLA class I, class II and class III antigens. It appears that the class III antigen disease associations can be explained by a direct defect operating at the level of either the class III gene or its gene product. The mechanism underlying class I and class II antigen disease associations is at present unknown. In this review we have considered thirty diseases which have been ranked according to their relative risk as defined by the frequency of a given HLA antigen in patient and control populations. The chronic inflammatory disorder, ankylosing spondylitis and its association with HLA B27 has been used as a model to study the HLA linked diseases. We have suggested that the disease may be caused by the Gram-negative microorganism Klebsiella which has antigenic similarity to HLA B27. It is proposed that some antibodies made against Klebsiella bind to HLA B27, thereby acting as autoantibodies leading to the pathological sequelae of chronic inflammatory arthritis. This is the crosstolerance hypothesis or molecular mimicry model and it has been compared to the receptor model. It is further suggested that the crosstolerance hypothesis can be utilised as a general theory to explain the association of other diseases with the class I and class II antigens, and offer a possible explanation for the polymorphism of HLA.

Molecular mimickry between HLA B27 and Yersinia, Salmonella, Shigella and Klebsiella within the same region of HLA alpha 1-helix

Two new examples of amino acid homology between HLA B27 and microbes triggering HLA B27-associated diseases are described. An outer membrane protein YadA (Yersinia adhesin, previously called Yop1) of Yersinia enterocolitica and Y. pseudotuberculosis shares a linear tetrapeptide with HLA B27. A cationic outer membrane protein OmpH of Salmonella typhimurium shares homology with five amino acids of HLA B27 in a non-linear fashion. The four amino acids of YadA are also notably included in the hexapeptide identical between Klebsiella pneumoniae nitrogenase and HLA B27, and three of them occur in the pentapeptide shared by a Shigella flexneri protein and HLA B27. Antibodies against synthetic peptides including HLA B27 homologues sequences of YadA and OmpH were observed in one-third of the patients with HLA B27 associated diseases. Antibodies were directed against a flanking sequence next to the amino acid sequences shared by arthritis-triggering microbes and HLA B27. The area of identity in each example of this molecular mimicry (Yersinia, Salmonella, Shigella and Klebsiella) is located in the same place on the HLA B27 molecule: between amino acids 70 to 78 in the variable region of alpha 1-helix. This area of HLA B27 molecule includes sites predicted to be important for binding processed antigens.

Chronic otitis media and ankylosing spondylitis: an HLA association?

A prospective study was undertaken to investigate an apparent correlation between ankylosing spondylitis (AS) and the incidence of otological conditions. 42 consecutive patients attending the ankylosing spondylitis clinic were examined otoscopically and had audiometric assessment. 19% of the patients (8/42) had active or inactive chronic otitis media. The prevalence of chronic otitis media in this group was significantly greater (P less than 0.01) than a stratified sample of the general population from the Medical Research Council Institute of Hearing Research National Study of Hearing, controlled for age, sex and socio-economic group, where the expected incidence was 5%. A hypothetical explanation for the association is given and the need for HLA studies into chronic otitis media is discussed.

Antigenic mimicry and autoimmune diseases

The finding of cross-reactive autoantibodies or sequence homology does not necessarily mean that this molecular mimicry is biologically meaningful or associated with disease pathogenesis. For example, relatives of persons with putative autoimmune insulin-dependent diabetes [123], and elderly humans [124] have a high incidence of autoantibodies which are generally not associated with autoimmune disease. In addition, natural antibodies to cell constituents [125] may be present in normal sera. These antibodies need to be directed against biologically important domains of host cell proteins in order to mediate autoimmune disease [27]. In spite of extensive homology between two sequences, a cross-reactive immune response may not be generated. The dissimilar amino acids should not be radical substitutions or affect the binding properties of the molecule. For instance, antibodies to synthetic peptides with only one substitution in a 19 amino acid sequence may not bind the whole protein [126]. Despite an identical six amino acid sequence shared by HLA-B27 and an EBV protein, no cross-reactive antibodies to EBV peptides were found in HLA-B27 positive patients with AS or RS. Unless the homology and subsequent crossreactive immune response can recognize a host protein intimately involved in disease pathogenesis, autoimmune disease is unlikely to occur.

Stress proteins, autoimmunity, and autoimmune disease

At birth, the immune system is biased toward recognition of microbial antigens in order to protect the host from infection. Recent data suggest that an important initial line of defense in this regard involves autologous stress proteins, especially conserved peptides of hsp60, which are presented to T cells bearing gamma delta receptors by relatively nonpolymorphic class lb molecules. Natural antibodies may represent a parallel B cell mechanism. Through an evolving process of "physiological" autoreactivity and selection by immunodominant stress proteins common to all prokaryotes, B and T cell repertoires expand during life to meet the continuing challenge of infection. Because stress proteins of bacteria are homologous with stress proteins of the host, there exists in genetically susceptible individuals a constant risk of autoimmune disease due to failure of mechanisms for self-nonself discrimination. That stress proteins actually play a role in autoimmune processes is supported by a growing body of evidence which, collectively, suggests that autoreactivity in chronic inflammatory arthritis involves, at least initially, gamma delta cells which recognize epitopes of the stress protein hsp60. Alternate mechanisms for T cell stimulation by stress proteins undoubtedly also exist, e.g., molecular mimicry of the DR beta third hypervariable region susceptibility locus for rheumatoid arthritis by a DnaJ stress protein epitope in gram-negative bacteria. While there still is confusion with respect to the most relevant stress protein epitopes, a central role for stress proteins in the etiology of arthritis appears likely. Furthermore, insight derived from the work thus far in adjuvant-induced arthritis already is stimulating analyses of related phenomena in autoimmune diseases other than those involving joints. Only limited data are available in the area of humoral autoimmunity to stress proteins. Autoantibodies to a number of stress proteins have been identified in SLE and rheumatoid arthritis, but their pathogenetic significance remains to be established. Nevertheless, the capacity of certain stress proteins to bind to multiple proteins in the nucleus and cytoplasm both physiologically and during stress or injury to cells, suggests that stress proteins may be important elements in the "immunogenic particle" concept of the origin of antinuclear and other autoantibodies. In short, this fascinating group of proteins, so mysterious only a few years ago, has impelled truly extraordinary new lines of investigation into the nature of autoimmunity and autoimmune disease.

Antibodies to the peptide from the plasmid-coded Yersinia outer membrane protein (YOP1) in patients with ankylosing spondylitis

Seventy-five Norwegian patients with ankylosing spondylitis (AS) were studied for class-specific antibody response against synthetic peptide, P81, representing the sequence of plasmid-coded outer membrane protein of Yersinia (YOP1) containing four amino acid homology (TDRE) with HLA-B27 sequence. Ten (16.7%), five (8.3%) and seven (11.2%) of 60 male AS patients showed elevated anti-YOP1 P81 antibody of IgA, IgG, and IgM class, respectively, whereas for each isotype only one (4%) of 25 healthy male controls was positive. Differences were not observed between female patients and controls. In all isotypes, antibody-positive patients were more frequently found in patients with active disease. The anti-YOP1 P81 antibody levels of the patients were generally not correlated with the antibody levels against the peptide representing the hypervariable region of HLA-B27 (B27 peptide). However, in one patient the antibody was shown to react with both peptides by cross-inhibition analysis. Overall, it appears that any causal relationship between YOP1 and pathogenesis of AS is not strong. Immunogenicity and cross-reactivity of the YOP1 region encompassing the TDRE sequence particularly at the T cell level require further study.

Autoantibodies to the HLA-B27 sequence cross-react with the hypothetical peptide from the arthritis-associated Shigella plasmid

We previously reported elevated serum antibody levels to a peptide representing the HLA-B27 polymorphic region (B27 peptide) in HLA-B27(+) ankylosing spondylitis (AS) patients. A plasmid (pHS-2) isolated from arthritogenic Shigella flexneri strains had been shown to encode an amino acid sequence homologous to HLA-B27. Rabbit antibody to this sequence (pHS-2 peptide) strongly cross-reacted with B27 peptide and, to a much lesser extent, with Klebsiella nitrogenase peptide. Serum antibody levels to pHS-2 peptide were studied in 160 spondylarthropathy patients. 12 of 115 (10.4%) AS patients, 2 of 45 (4.4%) patients with Reiter's syndrome or reactive arthritis as well as 6 of 147 (4.1%) normal controls were shown to have elevated anti-pHS-2 peptide antibodies. Antibody levels to B27 and pHS-2 peptides were significantly correlated in 134 HLA-B27(+) patients (r = 0.333, P less than 0.001). 13 of 15 affinity-purified anti-B27 peptide antibodies from patients strongly cross-reacted with pHS-2 peptide, whereas only 3 weakly cross-reacted to nitrogenase peptide. Leucine appeared to be a critical residue for this cross-reaction. AS patients' anti-B27 peptide antibodies reacted with HLA-B27 transfected L cells. These results may suggest that pHS-2 peptide more efficiently "mimics" B27 peptide than does nitrogenase peptide. Involvement of pHS-2 in pathogenesis of spondylarthropathy through molecular mimicry mechanisms requires further study.

Epitope mapping of an HLA-B27 monoclonal antibody that also reacts with a 35-kD bacterial outer-membrane protein

Ye-3 is an HLA-B27-specific murine monoclonal antibody recognizing the heat-modifiable protein of Enterobacteriaceae. Here we used recombinant hybrid molecules between the HLA-B7 and HLA-B27 antigens to delineate the epitope recognized by Ye-3. Results of these experiments indicated that the segment of HLA-B*2705 spanning residues 77-81 was critical to the reactive epitope. It is known to be a major serological and functional recognition site of HLA-B*2705 and our data give support for its involvement also in the serological cross-reactivity with bacterial antigens.

Antibody activity in ankylosing spondylitis sera to two sites on HLA B27.1 at the MHC groove region (within sequence 65-85), and to a Klebsiella pneumoniae nitrogenase reductase peptide (within sequence 181-199)

74 overlapping peptides of varying lengths from Klebsiella pneumoniae nitrogenase reductase (residues 181-199) and from the HLA B27.1 molecule (residues 65-85) were synthesized and tested by ELISA against sera from HLA B27+ ankylosing spondylitis (AS) patients, and sera from HLA B27+ and HLA B27- healthy first-degree relatives. Antibody activity in AS sera to Klebsiella peptides of four to eight amino acids was maximal with the peptide NSRQTDR. Activity to HLA B27 peptides was maximal with the peptide KAKAQTDR (named epitope I). These peptides overlap with, but are proximal to the NH2 terminus from QTDRED, which is homologous in HLA B27.1 and K. pneumoniae nitrogenase reductase. A second weaker reactive site was noted in the HLA B27.1 peptides, proximal to the COOH terminus from the homologous sequence, namely peptide REDLRTLL (named epitope II). Little activity was seen against peptides that included the entire homologous sequence. Sera from 50 AS patients showed higher total Ig activity against peptides KAKAQTDR (p less than 0.001) and NSRQTDR (p less than 0.02) than did sera from 22 B27+ and 22 B27- healthy controls. These data indicate that AS patient sera contain antibodies that bind to K. pneumoniae nitrogenase peptides and HLA B27.1 peptides, and that there are at least two epitopes on HLA B27.1 in the alpha 1 domain, at the MHC groove region, that are autoantigenic in AS patients. Epitope I may be a site for crossreactivity between HLA B27 and Klebsiella.

Dysfunction of HLA-B27

Every HLA antigen, as defined by the WHO-HLA Nomenclature, is unique. The major function of these molecules is to present antigen-peptides to the T-cell receptor, thereby contributing to the immunological defence mechanism. This function is regulated for each MHC antigen by its unique structure, with the peptide-binding pockets of the three-dimensional groove of the corresponding molecules playing the critical role. However, HLA-B27 is special by virtue of its disease association(s). Various aspects which might provide an explanation for--or at least a clue to an understanding of the specific role of--B27 in its disease associations are reviewed. Since it appears that there are no published experimental data which would support either of the alternative hypothetical possibilities, the bulk of current theories must therefore be purely speculative. The only lead to a better understanding of the function of B27 in disease associations is the postinfectious reactive arthritis. If it is the B27 molecule itself which is involved, further in vivo work on B27 transgenic animals might help solve this problem with its numerous unknown factors.

The use of enzyme immunoassay (EIA) and radiobinding assay to investigate the cross-reactivity of Klebsiella antigens and HLA B27 in ankylosing spondylitis patients and healthy controls

The binding of rabbit anti-Klebsiella antibodies to tissue-typed lymphocytes obtained from 30 ankylosing spondylitis (AS) patients and 54 healthy subjects has been measured by an enzyme immunoassay method. HLA B27 positive lymphocytes obtained from either AS patients (t = 3.60; p less than 0.001) or healthy subjects (t = 3.77; p less than 0.001) were found to bind Klebsiella antibodies to a significantly greater extent than non-B27 lymphocytes obtained from healthy controls. Absorption studies demonstrated that HLA B27 positive lymphocytes absorbed out significantly more anti-Klebsiella antibodies than HLA B27-negative lymphocytes (t = 6.76; p less than 0.005). These results are compatible with cross-reactivity or molecular mimicry between HLA B27 and epitopes on some Gram-negative bacteria such as Klebsiella.