HLA-B27-derived peptides as autoantigens for T lymphocytes in ankylosing spondylitis

Illuminating the impact of γδ T cells in man and mice in spondylarthritides

Spondylarthritides (SpA) are a group of autoinflammatory diseases affecting the spine, peripheral joints, and entheses, including axial spondyloarthritis (axSpA) and psoriatic arthritis. AxSpA has a multifactorial etiology that involves genetic predispositions, such as HLA-B27 and IL-23R. Although HLA-B27 is strongly associated with axSpA, its role remains unclear. GWAS studies have demonstrated that genetic polymorphisms related to the IL-23 pathway occur throughout the spectrum of SpA, including but not limited to axSpA and PsA. IL-23 promotes the production of IL-17, which drives inflammation and tissue damage. This pathway contributes not only to peripheral enthesitis but also to spinal inflammation. γδ T cells in axSpA express IL-23R and RORγt, crucial for their activation, although specific pathogenic cells and factors remain elusive. Despite drug efficacy in PsA, IL-23R inhibition is ineffective in axSpA. Murine models provide valuable insights into the intricate cellular and molecular interactions that contribute to the development and progression of SpA. Those models are useful tools to elucidate the dynamics of γδ T cell involvement, offering insights into disease mechanisms and potential therapeutic targets. This review aims to illuminate the complex interplay between IL-23 and γδ T cells in SpA pathogenesis, emphasizing their roles in chronic inflammation, tissue damage, and disease heterogeneity.

IRE1α Implications in Endoplasmic Reticulum Stress-Mediated Development and Pathogenesis of Autoimmune Diseases

Inositol-requiring transmembrane kinase/endoribonuclease 1α (IRE1α) is the most prominent and evolutionarily conserved endoplasmic reticulum (ER) membrane protein. This transduces the signal of misfolded protein accumulation in the ER, named as ER stress, to the nucleus as “unfolded protein response (UPR).” The ER stress-mediated IRE1α signaling pathway arbitrates the yin and yang of cell life. IRE1α has been implicated in several physiological as well as pathological conditions, including immune disorders. Autoimmune diseases are caused by abnormal immune responses that develop due to genetic mutations and several environmental factors, including infections and chemicals. These factors dysregulate the cell immune reactions, such as cytokine secretion, antigen presentation, and autoantigen generation. However, the mechanisms involved, in which these factors induce the onset of autoimmune diseases, are remaining unknown. Considering that these environmental factors also induce the UPR, which is expected to have significant role in secretory cells and immune cells. The role of the major UPR molecule, IRE1α, in causing immune responses is well identified, but its role in inducing autoimmunity and the pathogenesis of autoimmune diseases has not been clearly elucidated. Hence, a better understanding of the role of IRE1α and its regulatory mechanisms in causing autoimmune diseases could help to identify and develop the appropriate therapeutic strategies. In this review, we mainly center the discussion on the molecular mechanisms of IRE1α in the pathophysiology of autoimmune diseases.

CRALBP is a highly prevalent autoantigen for human autoimmune uveitis

Cellular retinaldehyde binding protein (CRALBP) is an autoantigen in spontaneous equine recurrent uveitis. In order to test whether CRALBP contributes to human autoimmune uveitis, the specificity of antibodies from human uveitis patient's sera was first evaluated in two-dimensional (2D) Western blot analysis. Subsequent identification of the immunoreactive proteins by mass spectrometry resulted in the identification of CRALBP as a putative autoantigen. Additionally, sera from human uveitis and control patients were by Western blot using purified human recombinant CRALBP. Anti-CRALBP autoantibodies occur more frequently (P<.01) in human uveitis patients than in normal controls. Thirty out of 56 tested uveitis patient's sera contained autoantibodies reactive against CRALBP, compared to only four out of 23 normal control subjects. The presence of CRALBP autoantibodies in 54% of tested uveitis patients supports CRALBP as a possible autoantigen in human autoimmune uveitis.

Th1/Th17 cytokine profiles in patients with reactive arthritis/undifferentiated spondyloarthropathy

OBJECTIVE

Data on synovial fluid (SF) cytokine concentrations in patients with reactive arthritis (ReA) or undifferentiated spondyloarthropathy (uSpA) are limited and contradictory. We measured levels of several proinflammatory and immunoregulatory cytokines in SF and sera from patients with ReA/uSpA.

METHODS

Interleukin 17 (IL-17), IL-6, interferon-g (IFN-g), and IL-12p40, and immunoregulatory cytokines IL-10 and transforming growth factor-beta (TGF-beta) were assayed using ELISA in SF specimens from 51 patients with ReA/uSpA (ReA 21, uSpA 30), 40 patients with rheumatoid arthritis (RA), and 11 patients with osteoarthritis (OA). IL-17, IL-6, IFN-g, and IL-10 levels were also measured in paired sera samples from patients with ReA/uSpA.

RESULTS

SF concentrations of IL-17, IL-6, TGF-beta, and IFN-g were significantly higher in patients with ReA/uSpA as compared to RA patients (for IL-17 median 46 pg/ml, range < 7.8-220 vs median < 7.8 pg/ml, range < 7.8-136, p < 0.05; for TGF-beta median 4.2 ng/ml, range 1.32-12 vs median 3.01 ng/ml, range 0.6-9.6, p < 0.01; for IL-6 median 58 ng/ml, range 2-540 vs median 34.5 ng/ml, range < 0.009-220, p < 0.05; for IFN-g median 290 pg/ml, range < 9.4-1600 vs median 100 pg/ml, range < 9.4-490, p < 0.05). SF levels of IL-10 were comparable but the ratio of IFN-g/IL-10 was significantly higher in ReA/uSpA patients than RA patients (median 3.18, range 0.06-200 for ReA/uSpA vs median 1.0, range 0.03-26.9 for RA; p < 0.05). IL-17, IL-6, IL-10, and IFN-g SF levels were significantly higher than paired serum levels in ReA/uSpA patients (p < 0.01 for IL-17, p < 0.0001 for IL-6, p < 0.0001 for IL-10, and p < 0.001 for IFN-g).

CONCLUSION

Increased IL-17, IL-6, TGF-beta, and IFN-g concentrations in ReA/uSpA than in RA suggest that Th1 and Th17 cells could be the major agents in inflammation in ReA/uSpA.

A novel and major association of HLA-C in Graves' disease that eclipses the classical HLA-DRB1 effect

Association of the major histocompatibility complex (MHC) class II-encoded HLA-DRB1-DQA1-DQB1 haplotype with Graves' disease (GD) has been known for several years. Recent evidence from other autoimmune diseases has suggested that the HLA class I encoded HLA-B/-C molecules could be conferring HLA-DRB1-DQA1-DQB1 independent effects on disease. The aim of this study was to determine the effect of HLA-B and HLA-C in GD in a white ethnic group of 806 patients with GD and 487 control subjects from the UK. Of the five loci (HLA-B, -C, -DRB1, -DQA1, -DQB1), HLA-C demonstrated the strongest association (P = 1.20 x 10(-20)) with HLA-C*07 predisposing [OR = 1.63, 95% CI (1.23-2.17)] and both HLA-C*03 [OR = 0.54, 95% CI (0.38-0.77)], HLA-C*16 [OR = 0.36, 95% CI (0.21-0.61)] protective. The other loci were then tested for HLA-C-independent associations. HLA-B was found to be associated independently of HLA-C (P = 1.54 x 10(-6)) with the other three loci, HLA-DRB1, HLA-DQB1 and HLA-DQA1, also improving the model but with less confidence (P > 10(-5)). This study has for the first time provided evidence of a primary association of HLA-C, and to a lesser extent HLA-B, with GD. Class II loci could still have effects on GD, but they appear smaller than the HLA-C association. A full investigation of the MHC region, including all class I and II loci is now required. Our results point to a primary role for class I-mediated responses in GD, a condition classically assumed to be a straightforward HLA-class II-restricted autoantibody response to the thyroid stimulating hormone receptor.

Spondyloarthropathy: disease at the crossroads of immunity

Up until recently, the prevailing paradigm relating to spondyloarthropathy (SpA) pathogenesis was that they were human leukocyte antigen (HLA)-associated, T-cell-driven autoimmune diseases. This view is now being questioned. Careful studies of well-characterised cohorts of patients with SpA, including detailed analysis of involved tissue, together with clinical trials of targeted treatments, in particular anti-tumour necrosis factor (TNF) therapies, have contributed enormously to both interest in and understanding of disease pathogenesis. In this chapter, our current knowledge and understanding of the relative contributions of the components of the innate and adaptive arms of the immune response to SpA pathogenesis is reviewed. It is clear that both arms of the immune response are involved and inter-dependent in SpA. With continued emphasis on discovery research, including detailed analysis of novel therapeutic interventions, significant additional breakthroughs in SpA are likely to be forthcoming.

Multiple autoantigen mimotopes of infectious agents induce autoimmune arthritis and uveitis in lewis rats

We found multimolecular antigenic mimicry of arthritogenic autoantigens and peptides from several other "self" or foreign antigens sharing amino acid sequence homologies. Many of these new mimotopes induced arthritis and/or uveitis upon immunization in Lewis rats, indicating a role for multiple antigens in the initiation of a certain autoimmune disease.

Immunostimulatory and immunomodulatory peptides derived from the alpha1 domain of HLA-B27 in experimental autoimmune diseases in Lewis rats

Peptides derived from amino acid sequence 60-80 of HLA-B27 (B27PA, aa 60-72 and B2702PA, aa 60-80) mimic cytokeratin and are able to induce in vitro proliferation of human peripheral blood lymphocytes as well as arthritis in Lewis rats. Here we show that the pathogenic epitope recognized by autoaggressive rat T cells is located at the N-terminus of the sequence, between aa 60 and 72. A C-terminally elongated 25mer peptide (B2702.60-84) showed increased pathogenicity, indicating either a second arthritogenic epitope or an immunomodulatory region within this peptide. B2702.60-84 has been described to inhibit murine and human CD8 + cytotoxic T cells (CTL) and was even successfully used for the treatment of allograft rejection. In addition to pathogenicity we have investigated the immunomodulatory effect of peptide B2702.60-84 in our rat model of experimental autoimmune uveitis (EAU), induced with retinal S-Antigen peptide PDSAg. We found that disease exacerbated following coimmunization of PDSAg with B2702.60-84. In vitro, the B27-peptide enhanced the proliferation of CD4+ T cell lines specific for retinal autoantigen peptides during coincubation of B2702.60-84 with the respective antigen. Oral tolerance induction, an effective mechanism to prevent uveitis in Lewis rats, is abrogated by cofeeding peptide B2702.60-84 with the tolerogen PDSAg. In rat EAU, naturally occurring regulatory T cells and orally induced gamma deltaTCR+ suppressor cells are CD8+ which might be impeded by peptide B2702.60-84. As a consequence of their abrogated suppressive capacity disease was exacerbated. We propose a similar role of HLA-B27 in man: disturbing the mechanisms down-regulating self-responses might lead to autoimmune diseases. This could explain the high association of HLA-B27 with a variety of autoimmune diseases targeting different organs or tissues.

Identification of Novel Human Aggrecan T Cell Epitopes in HLA-B27 Transgenic Mice Associated with Spondyloarthropathy

The pathology of ankylosing spondylitis, reactive arthritis, and other spondyloarthropathies (SpA) is closely associated with the human leukocyte class I Ag HLA-B27. A characteristic finding in SpA is inflammation of cartilage structures of the joint, in particular at the site of ligament/tendon and bone junction (enthesitis). In this study, we investigated the role of CD8+ T cells in response to the cartilage proteoglycan aggrecan as a potential candidate autoantigen in BALB/c-B27 transgenic mice. We identified four new HLA-B27-restricted nonamer peptides, one of them (no. 67) with a particularly strong T cell immunogenicity. Peptide no. 67 immunization was capable of stimulating HLA-B27-restricted, CD8+ T cells in BALB/c-B27 transgenic animals, but not in wild-type BALB/c mice. The peptide was specifically recognized on P815-B27 transfectants by HLA-B27-restricted CTLs, which were also detectable by HLA tetramer staining ex vivo as well as in situ. Most importantly, analysis of the joints from peptide no. 67-immunized mice induced typical histological signs of SpA. Our data indicate that HLA-B27-restricted epitopes derived from human aggrecan are involved in the induction of inflammation (tenosynovitis), underlining the importance of HLA-B27 in the pathogenesis of SpA.

HLA-B27-restricted T cells from patients with ankylosing spondylitis recognize peptides from B*2705 that are similar to bacteria-derived peptides

Ankylosing spondylitis (AS) is an inflammatory systemic disease affecting the spine, sacroiliacal and peripheral joints. Although the aetiology of AS remains unknown, the strong association with the HLA-B27 allele might reflect directly a detrimental effect of the HLA-B27 molecule itself, resulting from its potential capability to present 'arthritogenic' peptides to CD8+ T cells. Because some forms of SpA are triggered by enterobacterial infection, such arthritogenic peptides might originate from autologous and/or bacterial proteins triggering cross-reactive CD8+ T cell clones. Intriguingly, two peptides from the second extracellular domain of HLA-B*2705 share sequence homologies with several enterobacterial antigens, exhibit the HLA-B27-binding-motif, and are presented by HLA-B*2705 itself. The objective of this study was to examine the clonal T cell reactivity against these peptides in patients with AS. To this end, we screened peripheral blood lymphocytes (PBL) of 26 patients with AS and 24 healthy donors for TNF-alpha-producing cells using ELISPOT assays. PBL and synovial fluid-derived lymphocytes (SFL) of peptide-responsive patients were then stimulated and cultured with the relevant peptide and control peptides in vitro. Antigen-specific T cell lines (TCL) were identified by standard chromium release assays. Clonal analysis was performed subsequently applying TCRB-CDR3 spectratyping. Among eight peptides tested, only the HLA-B27 168-176 peptide LRRYLENGK was recognized by PBL from B27+ AS patients but not from B27+ healthy controls (P=0.001). LRRYLENGK-specific T cell clones used preferentially the TCRBV5S1 and the BV14 segment. These results suggest that an HLA-B27-derived peptide with homology to bacterial peptides may play a role in AS.

Uveitis in horses induced by interphotoreceptor retinoid-binding protein is similar to the spontaneous disease

Equine recurrent uveitis (ERU) is an inflammatory eye disease with high similarity to uveitis in man. It is the only spontaneous animal model for uveitis and the most frequent eye disease in horses affecting up to 10% of the population. To further investigate the pathophysiology of ERU we now report the establishment of an inducible uveitis model in horses. An ERU-like disease was elicited in seven out of seven horses by injection of interphotoreceptor retinoid-binding protein (IRBP) in complete Freund's adjuvant. Control horses did not develop uveitis. The disease model is characterized by a highly reproducible disease course and recurrent episodes with an identical time course elicited in all horses by repeated IRBP injections. The histology revealed the formation of lymphoid follicle-like structures in the eyes and an intraocular infiltration dominated by CD3(+) lymphocytes, morphological patterns typical for the spontaneous disease. Antigen-specific T cell proliferation of PBL was monitored prior to clinical uveitis and during disease episodes. An initial T cell response to IRBP-derived peptides was followed by epitope spreading to S-antigen-derived peptides in response to subsequent immunizations. Thus, horse experimental uveitis represents a valuable disease model for comparative studies with the spontaneous disease and the investigation of immunomodulatory therapeutic approaches after onset of the disease.

Immune linkages between inflammatory bowel disease and spondyloarthropathies

Gut involvement is a prominent feature of spondyloarthropathy (SpA). Analysis of immune alterations of the gut in SpA have shown two distinct aspects. On the one hand, gut inflammation in SpA seems closely related with gut inflammation seen in Crohn disease. On the other hand, gut inflammation in SpA is associated with peripheral joint inflammation. Recent studies have provided new insights into this gut-synovium axis. First, there is little new evidence to support the concept of viable microbial pathogens recirculating to the joint. In contrast, it seems likely that both bacterial antigens and reactive T cell clones home to the joint, and that adhesion molecules such as the beta7 integrins and VAP1 play an important role in this process. Second, there is increasing evidence that the different disease localizations in SpA are characterized by alterations of the innate immune system, which contribute to a breakdown of the immune tolerance and the creation of an inflammation-prone environment. Mediators of the innate immune system, such as scavenger receptors, interleukin-10 (IL-10), and tumor necrosis factor alpha (TNF-alpha), may therefore be interesting targets for therapeutic intervention, as illustrated by the effect of TNF-alpha blockade in SpA.

Induction of arthritis and uveitis in Lewis rats by antigenic mimicry of peptides from HLA-B27 and cytokeratin

Ankylosing spondylitis (AS) is highly associated with HLA-B27. We have previously shown that peripheral blood lymphocytes from AS patients respond to stimulation with a peptide from the sequence of HLA-B27. Here we report on molecular mimicry of peptides from HLA-B27 and cytokeratin, the latter being specifically expressed in synovial membranes and eyes, the main targets of the autoaggressive immune response in AS patients. Immunization of rats with these peptides induced an inflammatory response in joints, spine and eyes, resembling the symptoms in AS. Furthermore, both HLA-B27- and cytokeratin-derived peptides, are effective oral tolerogens: feeding these peptides ameliorated arthritis and uveitis induced with the cytokeratin peptide. Our model might elucidate the role of peptides from the sequence of HLA-B27 as an antigen of the immune response in AS, introducing a new aspect of antigenic mimicry between HLA-B27 and tissue-specific antigens. We propose this as a mechanism directing a systemic autoimmune response to specific target organs by antigenic mimicry of T cell epitopes.

Gonadal and adrenal sex steroids in ankylosing spondylitis

A role for sex steroids in the pathogenesis of AS is suggested by the male predominance, the peak age of onset in young adults, the increased number of first manifestations and flares after pregnancy, and the fact that sex steroids may modulate immune functions. There is a theoretic possibility that (normal levels of) androgens are indeed relevant in the male sex skew of AS. It has been reported that men with AS have higher than normal androgen levels; however, the evidence that serum testosterone levels are elevated in patients with AS is not robust. Elevated DHEAS and 17 alpha-hydroxyprogesterone levels have been reported in male AS patients; these may be secondary to inflammation and stress but may theoretically also be causally related to AS. These elevations might result from a partial late onset 11 beta- or 21-hydroxylase deficiency. Current data on sex steroid hormones provide no straightforward explanation for the male predominance in AS. It is fair to say that present data in patients with long-standing AS are too limited to suggest a role for androgens in the perpetuation of the disease, but a role in the initiation and the early stages of AS cannot be excluded. Such information can only be obtained from prospective studies. Cross-sectional studies cannot clearly distinguish causal relation from secondary disease effects, because blood sampling to test these hypotheses only takes place many years after the onset of disease. The impact of sex steroids on these features of AS is still unresolved. There is as yet no rationale for the use of medication that modifies sex steroid hormones in the management of AS. Alternative explanations for the higher male prevalence of AS may be found in the different chromosomal configuration and body composition of men and women.

T-cell studies in the spondyloarthropathies

A growing body of evidence suggests that T lymphocytes play an important role in initiating and maintaining the inflammatory process characteristic of the human leukocyte antigen (HLA)-B27-associated spondyloarthropathies. T cells seem to be involved in the primary defense reaction against arthritis-triggering gram-negative bacteria at the site of extra-articular infection, in determining the systemic cytokine pattern, in the recirculation process between gut mucosa and the joint, and in mediating secondary autoimmune joint inflammation. The factors involved in disease chronicity (namely in ankylosing spondylitis and psoriatic arthritis) are still unknown. Autoreactive T cells may contribute to this process by recognition of cross-reactive self-epitopes (ie, molecular mimicry between bacterial and self-antigens). Autoreactive T cells may as well be inappropriately upregulated by bacterial superantigens, or by local inflammatory reactions leading to the uncovering of former cryptic self-epitopes. In this paper, we review recent studies on peripheral blood and synovial T cells in patients with reactive arthritis, enteropathic spondyloarthropathy, psoriatic arthritis, and ankylosing spondylitis.

Anti-MHC autoimmunity in Behçet's disease: T cell responses to an HLA-B-derived peptide cross-reactive with retinal-S antigen in patients with uveitis

Immune response to retinal autoantigens plays a central role in the pathogenesis of uveitis. A synthetic peptide (B27PD) from a common sequence of various HLA-B molecules associated with uveitis, such as HLA-B27 and 51, which shares amino acid homologies with a retinal-S antigen (S-Ag)-derived peptide (PDSAg), was shown to be immunogenic in human and experimental uveitis in the rat. In this study we investigated T cell responses to B27PD and PDSAg in patients with Behçet's disease and posterior uveitis (BD-posterior uveitis; n = 33) in comparison with non-Behçet anterior uveitis (AU, n = 14), Behçet's patients without uveitis (BD, n = 15) and healthy controls (HC, n = 32) in a 6-day proliferation assay. Patients with BD and posterior uveitis had significantly higher responses (stimulation index (SI) 2.8 +/- 1.3) than those with AU (SI 1.5 +/- 0.4), BD without uveitis (SI 1.1 +/- 0.4) and HC (SI 1.1 +/- 0.6) for B27PD (P < 0.0001). Responses to PDSAg were also higher in BD with posterior uveitis patients (SI 3.3 +/- 1.6) than AU (SI 1.5 +/- 0.4), BD without uveitis (SI 1.2 +/- 0.3) and HC (SI 1.1 +/- 0.6) (P < 0. 0001). A significant correlation between the responses to PDSAg and B27PD (r = 0.56, P < 0.001) was observed. Elevated levels of IL-2 and tumour necrosis factor-alpha were also observed in culture supernatants obtained from peripheral blood mononuclear cells after stimulation with the peptides, but no correlation was found between the proliferative responses and cytokine levels. These results suggest that cellular immunity to cross-reactive HLA-B and S-Ag-derived peptides might play a role in the pathogenesis of posterior uveitis in BD.

Misfolding of HLA-B27 as a Result of Its B Pocket Suggests a Novel Mechanism for Its Role in Susceptibility to Spondyloarthropathies

The MHC class I protein HLA-B27 is strongly associated with susceptibility to spondyloarthropathies and can cause arthritis when expressed in rats and mice, implying a direct role in disease pathogenesis. A prominent hypothesis to explain this role suggests that the unique peptide binding specificity of HLA-B27 confers an ability to present arthritogenic peptides. The B pocket, a region of the peptide binding groove that is an important determinant of allele-specific peptide binding, is thought to be critical for arthritogenicity. However, this hypothesis remains unproven. We show that in addition to its role in peptide selection, the B pocket causes a portion of the pool of assembling HLA-B27 heavy chains in the endoplasmic reticulum to misfold, resulting in their degradation in the cytosol. The misfolding phenotype is corrected by replacing the HLA-B27 B pocket with one from HLA-A2. Our results suggest an alternative to the arthritogenic peptide hypothesis. Misfolding and its consequences, rather than allele-specific peptide presentation, may underlie the strong link between the HLA-B27 B pocket and susceptibility to spondyloarthropathies.

HLA-B27 associated spondyloarthropathy, an autoimmune disease based on crossreactivity between bacteria and HLA-B27?

Most autoimmune diseases are associated with certain HLA types. Therefore, spondyloarthropathies (SpA) strongly associated with HLA-B27, are also often classified as autoimmune diseases. This study questions whether SpA indeed fulfils the criteria of an autoimmune disease. The Medline database was searched for all reports between 1966 and April 1998 on the presence of autoimmune reactivity in SpA patients. This search yielded 45 articles on this subject. Only eight articles study T cell reactivity. Twelve reports were found on the assessment of antibodies crossreacting between bacteria and HLA-B27. In the 45 studies demonstrating autoimmune reactions in SpA patients proper controls matched for HLA-B27, sex and age were nearly always lacking. Therefore, it is concluded that the frequency of increased autoreactivity in sera from patients and controls is not significantly different, and that this lack of autoreactivity does not justify classification of SpA as an autoimmune disease. As crossreactive antibodies against bacteria and HLA-B27 were equally present in sera from patients and controls, the pathogenetic significance of molecular mimicry between various bacteria and HLA-B27 is questionable. Furthermore, the regions of the B27 molecule that are supposed to be crossreactive with bacteria, differ in one or more amino acids among the distinct B27 subtypes. Although these differences strongly influence the binding of antibodies to the B27 molecule, there was no relation between the degree of crossreactivity of certain subtypes and the association of these subtypes with SpA. In conclusion, there is no evident proof that SpA is an autoimmune disease attributable to crossreactivity between bacteria and HLA-B27.

A molecular insight on the association of HLA-B27 with spondyloarthropathies

This article focuses on molecular studies concerning HLA-B27 and their relevance for its pathogenetic role in spondyloarthropathy. The peptide binding and T-cell antigen presenting properties of HLA-B27 are discussed, mainly in connection with differential subtype association to ankylosing spondylitis. Molecular studies in transgenic rodents are also considered, with an emphasis on their relevance to the various pathogenetic mechanisms. Recent studies on the putative role of HLA-B27 in bacteria-host interactions are also discussed, as they suggest another level of implication of HLA-B27 in disease whose molecular basis is obscure.

Enthesitis in spondyloarthropathy

Inflammation at the insertions of ligaments, tendons, or joint capsules to bone, which is termed enthesitis, is a characteristic feature of spondyloarthropathy. Because of the relative inaccessibility of the enthesis, the inflammatory, microbiologic, and immunologic events at that site have been poorly defined. Recent magnetic resonance imaging studies have drawn attention to the ubiquitous nature of enthesitis in spondyloarthropathies, especially adjacent to synovial joints. This may have implications for the mechanisms of synovitis in spondyloarthropathies. Magnetic resonance imaging studies also suggest that enthesitis lesions may be extensive, which could explain the diffuse nature of bone changes seen in some patients with spondyloarthropathies. The importance of enthesitis as a skeletal phenomenon in spondyloarthropathies has gained further support from transgenic models in which either tumor necrosis factor-alpha or bone morphogenetic protein-6 overexpression result in entheseal-associated polyarthropathy.

Androgens and ankylosing spondylitis: a role in the pathogenesis?

The frequency and severity of ankylosing spondylitis (AS) show a male preponderance, and androgenic steroids have been implicated in its etiology. Some reports have indicated that serum androgen levels are slightly elevated relative to estrogen levels in patients with AS as compared to controls. In more recent studies, however, serum testosterone, 17 beta-estradiol, and androstenedione levels did not significantly differ between AS patients and controls. Moreover, testosterone levels measured directly in serum can be spuriously elevated, especially in patients using phenylbutazone. Elevated serum levels of the adrenal steroids 17 alpha-hydroxyprogesterone and dehydroepiandrosterone (DHEA) sulfate have been found in patients with AS. These elevations might be explained by partial 11 beta- or 21-hydroxylase deficiencies, but may also be secondary to an enhanced stress response. In vitro studies as well as studies in animals and humans indicate that DHEA enhanced, and 17 beta-estradiol and progesterone inhibit, the cell-mediated immune response, which may play a role in the pathogenesis of AS. Oral estrogen therapy in female patients and human chorionic gonadotrophin injections in male patients with AS, increased the 17 beta-estradiol/testosterone ratio and resulted in a moderate clinical improvement. In conclusion, serum testosterone levels are not elevated in patients with AS. Therefore testosterone probably has no role in the perpetuation of long-standing AS and provides no basis for antiandrogenic treatment. Cross-sectional case-control studies, however, cannot clearly distinguish etiological factors from secondary disease effects, especially when blood sampling occurs many years after the onset of AS. Consequently, the role of sex steroids in the pathogenesis is still insufficiently elucidated.

The role of HLA-B27 polymorphism and molecular mimicry in spondylarthropathy

Ankylosing spondylitis (AS), reactive arthritis (ReA) and other related spondyloarthropathies (SpAs) are characterized by a strong association with the major histocompatibility complex allele HLA-B27. Experimental evidence from humans and transgenic rodents suggests that HLA-B27 is itself involved in the pathogenesis of SpA. Population and peptide-specificity analysis of HLA-B27 suggest it has a pathogenic function related to antigen presentation. Putative roles for infectious agents have been proposed in ReA and suggested in AS. However, the mechanism by which HLA-B27 and bacteria interact to induce arthritis is not clear. Molecular mimicry between bacterial epitopes that cross-react with self-B27 peptides is the most persuasive explanation for the pathogenesis of SpA. The experimental studies reviewed here have greatly increased our knowledge of the structure, function and disease association of HLA-B27.

Pathogenesis of human leukocyte antigen B27-positive arthritis. Information from clinical materials

In the spondyloarthropathies human leukocyte antigen (HLA) B27 confers a strong genetic predisposition to the development and to the chronicity of disease after extra-articular infection with certain gram-negative bacteria. The close relationships between infection, HLA-B27, other genetic factors, and the host immune system, however, still are unexplained. HLA-B27-positive arthritis continues to be an area of intensive investigation in basic and clinical research. New animal models with HLA-B27 transgenic mice and rats, as well as recent developments in understanding the processes involved in signal transduction, cytokine production, and human T-lymphocyte activation, contribute to the development of new pathogenic models of the spondyloarthropathies. This article summarizes the current concepts of the cause and pathogenesis of the spondyloarthropathies resulting from studies of clinical materials. The host-microbial interplay in human disease, namely in bacteria-induced reactive arthritis, may eludicate principle disease mechanisms in acute disease and in the development of chronic autoimmune arthritis or ankylosing spondylitis.

Animal models and in vitro models for the study of aetiopathogenesis of spondyloarthropathies

Among several animal models, HLA-B27 transgenic rodents proved useful for investigating the interplay between genetic factors and the bacterial environment in the aetiopathogenesis of the spondyloarthropathies (SpA). HLA-B27 transgenic rats spontaneously develop a multisystemic inflammatory disease resembling human SpA. This disease is dependent on the presence of a normal bacterial flora and implicates the immune system. The presence of both T cells and antigen-presenting cells expressing high levels of HLA-B27 seems of critical importance in its pathogenesis. HLA-B27 transgenic mice also develop arthritis, under the influence of the bacterial flora. In both types of model, CD8+ T cells seem not to be necessary, arguing against the 'arthritogenic peptide' hypothesis. In vitro models have been used to study the immune response against bacterial agents and the role of HLA-B27 in human SpA. It appears that an impaired immune response against bacteria could be involved in the triggering of human SpA. HLA-B27 could be implicated at the level of interaction between host cells and bacteria in the driving of a specific immune response against bacterial antigens or as a target of an autoimmune response.

Infectious agents as triggers of reactive arthritis

Reactive arthritis was originally defined as a sterile joint inflammation after infection elsewhere in the body, but this view has been challenged in the past decade since different antigens and DNA and RNA of various triggering microbes have been shown to exist at the sites of inflammation in the joints. It has been suggested that microbial antigens, or intact pathogens, are important for the pathogenesis of reactive arthritis, at least in the early phase of the disease, but the exact mechanism of how the pathogens contribute to the development of this usually self-limiting polyarthritis has not been discovered. This article reviews the theories on the role of infectious agents as triggers of reactive arthritis.

The pathogenetic role of HLA-B27 in chronic arthritis

Population and peptide specificity analyses and studies in transgenic rodents support a role of HLA-B27 as an antigen-presenting molecule in spondyloarthropathy. The interplay between HLA-B27 and arthritogenic bacteria on infected cells suggests that HLA-B27 might also influence disease by other mechanisms. Recent genetic advances promise the identification of additional susceptibility genes.