Reactive arthritis and HLA-B27
Mechanisms of disease.
A common feature of reactive arthritis , ankylosing spondylitis and rheumatoid arthritis is that all three conditions are caused by chronic inflammation which is probably stimulated by microbial infection and which has a very strong association with particular human genes. In rheumatoid arthritis, more than 90% of patients possess the genes encoding either HLA-DR1 or some subtypes of HLA-DR4 class II histocompatability antigens [lay reader: particular human tissue types], whilst the frequency of this marker in the general population is roughly 35%. In ankylosing spondylitis, 96% of patients are positive for HLA-B27 class I histocompatability antigen, which occurs in only about 8% of the normal population [Ebringer & WIlson, 2000]. A majority of patients with the clinical presentation of reactive arthritis are HLA-B27 positive and it has been proposed that only such patients should be considered as meeting a new strict Arthritis Presentation of the disease [Braun et al., 2000]. It is evident that any satisfactory explanation of the pathogenesis of reactive arthritis must take account of the diseases strong association with the HLA-B27 gene and with a limited number of micro organisms including chlamydiae [see: Arthritis and Chlamydiae]. It is also necessary to explain why antibiotics, with the possible exception of chlamydial-induced disease, have relatively little effect on fully developed reactive arthritis [Sieper et al., 1999; Toivanen & Toivanen, 1999].
In reactive arthritis the prime target cell for Chlamydia in the joint is probably the synovial fibroblast in which intracellular persistence of viable chlamydiae is the probable trigger for the initiation and maintenance of the inflammatory process [Hanada et al., 2003; Kuipers et al., 2001; Rodel et al., 1998; Vilareal et al., 2002]. The infected synovial fibroblasts produce Iinterleukin-6, tissue growth factor-beta, and granulocyte and macrophage colony stimulating factor, GMCSF. In the presence of gamma interferon derived from the host cellular immune response they also produce the pro-inflammatory tumor necrosis factor alpha. Thus chlamydial-induced cytokine release from synovial fibroblasts might contribute to alterations in the synovial membrane that promote the development of joint inflammation [Rodel et al., 1998]. This was confirmed by Hanada et al., 2003 who reported that infection of human fibroblast-like synovial fibroblasts with C. trachomatis was characterised by persistent infection and that interleukin 6 production was dependent on chlamydial viability. Furthermore, synovial fibroblasts produce interferon-beta in response to chlamydial infection via the activation of interferon regulatory factor-1 and interferon-stimulated gene factor 3-gamma [Rodel et al., 1999; Rodel et al., 2002]. This process is in turn enhanced by tumour necrosis factor-alpha. Upregulation of interferon regulatory factor-1 also leads to the Induction of the tryptophan-catabolizing enzyme indoleamine 2,3-dioxygenase which, by depleting intracellular pools of the essential amino acid tryptophan, is thought to be an important factor in the induction of chronic chlamydial infection [see: Persistence And IFNg]. This might explain why, in reactive arthritis associated with both C. trachomatis [Gerard et al., 1998] or, less commonly, C. pneumoniae [Gerard et al., 2000; Schumacher et al., 1999] metabolically active chlamydiae can be detected as shown by the turn-over of chlamydial nucleic acid yet, as a defining characteristic of reactive arthritis [Braun et al., 1999], the organism can seldom be grown from samples of infected synovium. Chlamydial-induced up-regulation of HLA class 1 antigen expression in synovial fibroblasts appears to be primarily caused by the induction of IFN-beta [Rodel et al., 2002].
Interestingly, expression of the omp1 gene encoding the chlamydial major outer membrane protein which is itself an important target for the host immune response, is strongly down-regulated in chlamydiae persistently infecting the joint, whereas expression of of chlamydial heat shock protein 60 (hsp60) was unimpaired [Gerard et al., 1998]. It has long been speculated [with little hard evidence] that, as chlamydial heat shock proteins share some common antigenic structures with human heat shock proteins, they might trigger the body to mount a cellular immune response that damages its own components [see: Heat Shock Proteins]. However, when the precise target of the human cell mediated immune response to chlamydial hsp60 was identified, it was directed against a region on the chlamydial protein that is unique to C. trachomatis and C. pneumoniae and quite distinct to the comparable region on human hsp60. Thus although immune responses to chlamydial hsp60 may be immunodominant in reactive arthritis and other chronic chlamydial infections, cross-reactive recognition of self hsp60 was not implicated in the pathogenesis of disease [Deane et al., 1997; Gaston, 2000].
Support for chlamydiae as a persisting but non cultivatable trigger of arthritis comes from experimental observations in the Lewis rat. When synovial fibroblasts from the knee of Lewis rats are infected with C. trachomatis then re-inoculated back into the rat knee joint, intense arthritis ensues. The early phase of this arthritis was characterized by an intense local acute inflammatory response involving polymorphs and accelerated cartilage injury; plus the dissemination of chlamydiae to the liver and spleen with viable chlamydiae in the joints. Subsequently a chronic inflammatory response developed, characterised by mononuclear cells and both injury and repair of cartilage. There was a marked antibody response and viable chlamydiae were absent. This model therefore demonstrates that intense synovial inflammation can be induced by chlamydiae, persisting beyond the culture-positive phase. It also indicates the potential of the synoviocyte as a host cell for C. trachomatis capable of acting as a reservoir of chlamydial antigen sufficient to perpetuate joint injury [Inman & Chiu, 1998].
In reactive arthritis a bacteria-specific T cell response to the triggering microbe is detected in the joint [Thiel et al., 2000]. In reactive arthritis the potentially protective T-helper 1 response is depressed in favour of a T-helper 2 response as a result of antigen-specific secretion of the regulatory chemical messenger, interleukin-10 [Thiel et al., 2000]. Other data [Kotake et al., 1999; Yin et al., 1997] also indicate that a T-helper 2 cytokine pattern predominates in the joints of patients with reactive arthritis. Since T-helper 1 response cytokines, notably gamma interferon, are necessary for the elimination of the facultative or obligate intracellular bacteria associated with reactive arthritis, it was postulated that cytokines generated by the T-helper 2 response, conversely, contributed to bacterial persistence in the joint. The T-helper responses are regulated by the balance of the cytokines interleukin 10 and interleukin 12. Thus the balance of these cytokines is seen as crucial for regulation of the cytokine pattern in the joints of patients with reactive arthritis [Kotake et al., 1999; Yin et al., 1997]. yle="line-height: 150%">Role of HLA-B27
The key to understanding the pathogenicity of reactive arthritis in relationship to chlamydiae lies in understanding the role of the HLA-B27 tissue type antigen, which most patients with reactive arthritis or ankylosing spondylitis, unlike the normal population, characteristically possess. HLA-B27 is a so-called class I histocompatability antigen, which means it is involved in the presentation of foreign ( i.e. chlamydial) antigen to the cell mediated immune system. It has long been postulated that HLA-B27 presents so-called arthritogenic microbial peptides [lay reader: protein fragments] to T lymphocytes of the host cellular immune system. It has been postulated that these peptides might, by molecular mimicry, trigger a damaging T-lymphocyte response against the bodies own tissues. Indeed, studies of cloned T lymphocytes from cases of reactive arthritis indicate a surprisingly high degree of conservation in the T cell response independent of which micro-organism triggered the response [Dulphy et al., 1999]. The role of HLA-B27 is probably complex, modulating the cell mediated immune response against chlamydiae and directly affecting chlamydial replication.
As far as the cellular immune response is concerned, animal studies consistently indicate that T-helper 1 type cellular immune responses to chlamydiae are involved in acquired protective immune responses to chlamydiae [and other intracellular bacteria] whereas T-helper 2 responses are not. Interestingly, in chlamydial reactive arthritis, lower levels of synovial fluid interferon gamma, the main T-helper 1 response effector, are found in HLA-B27 positive individuals than in HLA-B27 negatives [Bas et al., 2003]. This requires verification in other settings.
The interaction of chlamydial antigen with the main antigen processing cell, the dendritic cell, is likely to be crucial in determining the type of cellular immune response which is mounted [see Antigen Processing]. Matyszak et al., 2002 studied the uptake and processing of C. trachomatis serovar L2 by human dendritic cells. Entry of C. trachomatis into dendritic cells was mediated via chlamydial attachment to heparan sulfates containing glycosamino glycans and did not involve micropinocytosis. Infection of the dendritic cells led to their activation and the production of IL-12 [which supports T helper 1 cellular immune responses]and TNF-alpha but not IL-10. The chlamydiae were confined to distinct vacuoles and did not develop into characteristic inclusions. Although there was no obvious co-localization between chlamydia-containing vacuoles and MHC loading compartments, infected dendritic cells nevertheless efficiently presented chlamydial antigens to CD4+ T helper cells and also supported the expansion of C. trachomatis specific CD8+ T cells. However thee role of these latter cells in the pathogenesis of chlamydial reactive arthritis is uncertain [Matyszak et al., 2002].
Information is beginning to emerge on the chlamydial antigens which, following processing, stimulate the cell mediated immune response. C. trachomatis-specific CD4+ T cell clones were isolated from a patient with chlamydia-induced reactive arthritis. T cell immunoblotting indicated that for these clones the main stimulatory molecule was the chlamydial 60 Kilodalton cysteine rich outer envelope protein, omp2. The most stimulatory portion of this molecule for presentation via HLA-DRB1*0401 was a nonamer peptide with high predicted binding affinity for DRB1*0401. The sequence of the epitope is conserved in all C. trachomatis strains but not in C. pneumoniae. Investigation of patients with acute urethritis and additional patients with sexually acquired reactive arthritis showed that OMP2-reactive T cells were readily detectable in peripheral blood and synovial fluid. Thus OMP2 is a target antigen of the T cell-mediated immune response to CT infection [Goodall et al., 2001].
Using a different approach, Kuon et al., 2001 searched the C. trachomatis proteome for HLA-B27 binding peptides that were stimulatory for CD8+ T cells, both in a model of HLA-B27 transgenic mice and in patients. This was done by combining two biomathematical computer programs, the first of which predicts HLA-B27 peptide binding epitopes, and the second the probability of HLA-B27 peptide generation during antigen processing. Selected peptides were further tested by Ag-specific flow cytometry leading to the identification of eight murine-derived peptides which were recognized by cytotoxic T cells. The same strategy was used to identify B27-restricted chlamydial peptides in three patients with reactive arthritis. Eleven peptides were found to be stimulatory for patient-derived CD8+ T cells, of which eight overlapped with those found in mice.
One question is whether HLA-B27 contributes to disease pathogenesis by functioning as a restriction element for the presentation of an arthritis inducing (arthritogenic) peptide, or whether B27 itself serves as a self-antigen which could be targeted by the cellular immune system. Popov et al., 2002 explored this question using HLA-B27 transgenic rats expressing human B27 antigen. Unlike their normal (wild type) counterparts, HLA-B27-transgenic rats are tolerant of B27 immunization using either B27(+) splenocytes [as source of T and B lymphocytes] or B27-expressing plasmid DNA. Thus they do not develop a cytotoxic T lymphocyte response to the artificial self [B27] antigen. However, if splenocytes from such immunized animals are exposed to Chlamydia in vitro, cytotoxic T cells are generated that kill cells expressing HLA-B27. This was not observed if cells were transfected with other HLA antigens, such as B7, B14, B40, or B44. Nor was it observed if rats were immunised with non-transgenic spleen cells or with a DNA construct expressing a different antigen, HLA-A2. The specific site on the "self" HLA-B27 that was recognized by the cytotoxic T cells involved the Lys(70) amino acid residue in the alpha(1) domain of the HLA-B27 MHC class I molecule. Thus, normal tolerance to B27 can be subverted by C. trachomatis, implying that there is a dynamic relationship between the chlamydiae, HLA-B27 and the cellular immune system which may have important implications for understanding the HLA-B27-related spondyloarthropathies that are triggered by chlamydiae and other intracellular bacteria [Popov et al., 2002].
Besides its role as a histocompatability antigen involved in antigen presentation to T lymphocytes, there is evidence that HLA-B27 can directly influence the invasion and/or replication of chlamydiae [Kuipers et al., 2001]. Human Hela cells, or HeLa cells expressing various parts of the HLA-B27 gene or control genes were infected with C. trachomatis. [Researchers: Hela cells were transfected with either HLA-B27 cDNA or controls consisting of either: HLA-A1 cDNA; cDNA from an HLA-B27 mutant without the cytoplasmic tail, or B27Q10 cDNA consisting of HLA-B27 Exon 1-4 linked to Exon 5 of murine Q10]. Chlamydial invasion was determined 4 hours post inoculation while chlamydial replication was determined 2 days and 4 days post inoculation. Chlamydial entry into HeLa cells was not affected by HLA-B27 gene expression. However, chlamydial replication and formation of inclusion bodies was suppressed by HLA-B27 expression and in particular by the cytoplasmic tail. This ability of the cytoplasmic tail of HLA-B27 to alter the replication of intracellular bacteria associated with reactive arthritis [C. trachomatis and Salmonella] suggests that the expression of this gene, as well as affecting antigen presentation, may particularly favour persistent infection in the joint [Kuipers et al., 2001]. However, Young et al., 2001 using a different [less satisfactory] approach found no direct influence of HLA-B27 on chlamydial replication. This is an important issue requiring resolution.
[MEW] Updated August 2003
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