Mechanisms of Disease: the immunopathogenesis of spondyloarthropathies

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Mechanisms of Disease: the immunopathogenesis of spondyloarthropathiesHill Gaston

INTRODUCTIONThe spondyloarthropathies (SpAs) are the second-commonest cause of inflammatory arthritis in humans, after rheumatoid arthritis. In adults, there are five conditions within this family of diseases: ankylosing spondylitis (AS), psoriatic arthritis (PsA), arthritis associated with inflammatory bowel disease, reactive arthritis and undifferentiated spondyloarthropathy (Table 1). Although members of the family vary substantially in their clinical features—for instance, reactive arthritis is often a relatively mild and self-limiting condition, whereas AS is progressive and lifelong—there are good reasons for regarding them as related conditions with common underlying pathologic mechanisms. As with members of any family, these diseases share certain pathologic features (e.g. skin, gut and eye involvement), whilst maintaining their own unique features (e.g. triggering by specific infections in reactive arthritis).

For some time, the pathogenesis of SpA has been discussed in terms of interactions between genes, including but not limited to HLA-B27;1,2 with bacteria, whether pathogenic or commensal,3 and with innate and acquired immune responses.4,5 This article focuses on the influence of HLA-B27 (and other poten-tial susceptibility genes) on the pathogenesis of these diseases, and provides an insight into the validity of the gene–bacteria–immune response paradigm, and how interactions between these factors might result in disease.

NEW THOUGHTS ON HLA-B27HLA-B27 is a class I MHC molecule consisting of a heavy chain, β2-microglobulin (β2M), and a short peptide, which can be derived from intracellular pathogens, and is bound within a groove formed by the heavy chain. These peptides are presented to CD8+ T cells in order to initiate immune responses. In this respect HLA-B27 resembles all other class I HLA alleles, and has been maintained at a significant frequency in most human populations, indicating

Research into the pathogenesis of the spondyloarthropathies has examined the role of HLA-B27 and other genes in susceptibility to these diseases. Novel characteristics of HLA-B27 have been discovered, which have allowed hypotheses for an influence of HLA-B27 on disease to be developed that do not reflect its ability to present arthritogenic peptides to CD8+ T cells. Although a role for CD8+ T cells has not been excluded, they are not required in the HLA-B27 transgenic rat model, and do not dominate at sites of disease in humans. Studies have also focused on the consequences of the (rather inefficient) intracellular folding of the HLA-B27 heavy chain, the ability of cells to deal with intracellular infection, and their expression of unusual forms of HLA-B27 on cell surfaces (including free heavy chains and dimers). Unusual surface forms of HLA-B27 interact with a different set of receptors from those that recognize conventional class I MHC molecules and thus can be implicated in driving inflammatory responses. Additional candidate susceptibility genes are being identified, either using gene-targeting technology in mice, or genomic screening approaches in humans. In several cases, as with HLA-B27, the evidence suggests that these genes influence the response of the host to bacteria, including pathogens and commensal organisms of the skin and gastrointestinal tract. The concept that spondyloarthropathies are the result of interactions between susceptibility genes, bacteria and the immune system remains a useful model for the pathogenesis of these diseases.

KEYWORDS bacteria, genes, HLA-B27, immune responses, spondyloarthropathies

H Gaston is a Professor of Rheumatology at the University of Cambridge, Cambridge, UK.

CorrespondenceAddenbrooke’s Hospital, University of Cambridge, Cambridge CB2 2QQ, [email protected]

Received 25 October 2005 Accepted 10 April 2006

www.nature.com/clinicalpracticedoi:10.1038/ncprheum0219

REVIEW CRITERIAPublished articles for inclusion in this review were identified from the author’s extensive records of papers on spondyloarthropathies dating from 1970 to the present. PubMed and MEDLINE searches of the medical literature under the terms “spondyloarthropathy” and “ankylosing spondylitis” were performed to update and cross-check these ongoing records.

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its utility in defense against pathogens, partic-ularly viruses. Despite the involvement of HLA antigens in the response to infection, the diseases that are associated with particular HLA alleles have almost all turned out to be known autoimmune dis orders (e.g. type 1 diabetes, autoimmune thyroid disease) or inflamma-tory conditions that might be autoimmune. The most outstanding example of the latter, and the strongest association between HLA and a disease, is between HLA-B27 and AS, first discovered over 30 years ago. More than 90% of AS patients are positive for HLA-B27, and there are significant asso ciations between HLA-B27 and other members of the SpA family of diseases.

It had been assumed that the asso ciation between HLA-B27 and SpA must reflect some aspect of its principal function, that of presenting antigenic peptides to CD8+ T cells; however, several other properties of HLA-B27 have been considered as possible explana-tions for its strong association with SpA. These include the relative inefficiency of folding of the HLA-B27 heavy chain following synthesis in the endoplasmic reticulum (ER); its relative independence from tapasin in achieving surface expression; its ability to be expressed as a free heavy chain (i.e. without bound β2M) which, through disulphide bond formation, leads to surface expression of heavy-chain dimers; and the presence of a peptide-binding groove that allows peptides to take up more than one conformation (Figure 1).

HLA-B27 and the unfolded-protein responseCells have sensitive mechanisms for detecting the accumulation of misfolded proteins in the ER, and initiate a complex response to this by altering gene transcription.6 The HLA-B27 heavy chain has been shown to fold inefficiently compared with other class I MHC alleles, a property related to the B pocket of its peptide-binding groove.7 The main question is to what extent HLA-B27 misfolding elicits the unfolded-protein response (UPR) in vivo, which can result in activation of the transcription factor nuclear factor κB (NFκB) and downstream effects on gene transcription.

Macrophages isolated from the bone marrow or gut of HLA-B27 transgenic rats show evidence of formation of HLA-B27 dimers in the ER and of activation of the UPR, measured by altera-tion in the splicing of the gene encoding X-box binding protein, and upregulated expression of ER chaperone proteins such as the 70 kDa heat shock protein family member immuno-globulin-heavy-chain-binding protein (BiP) and C/EBP-homologous protein (CHOP);8 similar data for BiP have been reported in the adherent fraction of SpA cells.9 In addition, the pattern of gene transcription in these cells indicated that they had responded to interferon (IFN). IFNs could be induced by the UPR but, alternatively, IFNs generated during immune responses could upregulate HLA-B27 expression and exacer bate the UPR. HLA-B27 transgenic rats that develop colitis and arthritis, however, express a high number of copies of the HLA-B27

Table 1 Diseases comprising the spondyloarthropathies.

Disease Joints affected Skin involvement

Gut involvement Eye involvement % HLA-B27+

Ankylosing spondylitis SpineSacroiliac jointsHips, shoulders

No Subclinical in a proportion of patients

Iritis 95

Psoriatic arthritis Peripheral joints and/orSpineSacroiliac joints

Psoriasis Subclinical in a proportion of patients

Conjunctivitis 15–50 (higher with spinal involvement)

Arthritis associated with inflammatory bowel disease

Peripheral joints and/orSpineSacroiliac joints

Erythema nodosum

Yes, by definition Iritis 20–50 (higher with spinal involvement)

Reactive arthritis Peripheral joints (especially lower limb)Sacroiliac jointsSpine (late)

Psoriasis-like rashErythema nodosum

Yes, when disease is triggered by enteric pathogens

Conjunctivitis 20–80 (higher with more-severe and chronic disease)

Undifferentiated spondyloarthritis

Peripheral jointsSacroiliac joints

No Yes Iritis ~50



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transgene (50–90 copies), so these findings might not be relevant to human SpA. Transfection of HLA-B27 into human cells does not initiate a UPR10 (also J Goodall and H Gaston, unpub-lished data), but it is possible that under addi-tional stresses, such as the presence of cytokines (particularly IFNγ) or perhaps infection, a UPR could be initiated. This process, in turn, could lead to activation of NFκB through inhibition of synthesis of its cytosolic inhibitor, IκBα. Although faster and more prolonged activa-tion of NFκB has been described in a myeloid cell line (U937) transfected with HLA-B27,10 other experiments using gene arrays and the same cell line have, however, not confirmed this finding.11

The relative tapasin independence of HLA-B27The ability of HLA-B27 to reach the cell surface in cells that do not express tapasin is unusual.12 Tapasin is a component of the class I MHC antigen-processing pathway that facilitates asso-ciation between the class I molecules and the transporter associated with antigen processing, which delivers antigenic peptides to be bound by the class I MHC molecule.13,14 The polymorphic amino acids in the class I HLA heavy chain that influence tapasin dependence have been deter-mined, and are those at positions 114, 116 and 152; interestingly, these vary between HLA-B27 subtypes.15,16 Since some HLA-B27 subtypes are clearly associated with susceptibility to SpA (B*2705 and B*2704) and some are not (B*2706 and B*2709),17,18 it has been possible to ask whether tapasin dependence correlates with disease susceptibility. There was no such correla tion: the disease-associated allele B*2705, and the non-disease-associated alleles B*2706 and B*2709 showed similar cell-surface expres-sion in the absence of tapasin, whereas another disease-associated allele, B*2704, showed impaired expression.16,19

Although these studies are inevitably arti ficial, as patients with SpA are not deficient in tapasin, the results might be relevant to SpA patho-genesis, because infectious agents can inhibit tapasin function. This inhibition has mainly been demonstrated for viruses,20,21 but might occur with intracellular bacteria as well. Indeed, it has been suggested that the relative tapasin indepen-dence of HLA-B27 might confer an advantage, possibly accounting for its maintenance at a high frequency is some human populations.14

B*2706, an allele which is not associated with disease, exhibits a number of properties that are different from the disease-associated subtypes. These include lack of an association with the peptide-loading complex, expres-sion of an increased proportion of free heavy chains at the cell surface, and a faster rate of maturation through the secretory pathway. The fact that these properties are not shared by the other non-disease-associated allele B*2704, however, brings into question whether they are truly protective.16

Expression of HLA-B27 free heavy chains, disulphide-linked dimers and multimersThe relative independence of HLA-B27 from tapasin leads to HLA-B27 molecules that are loaded with suboptimal antigenic peptides.13,14,22 These peptides are likely to dissociate from the HLA-B27 molecule at the cell surface, and in the absence of peptide, β2M no longer binds. The result is a supply of free HLA-B27 heavy chains that are available to form disulphide-linked multimers, especially dimers,23 which are expressed at the cell surface.24–26 The contribution of these dimers and multimers to the pathogenesis of SpA is a subject of ongoing research; their effect on SpA pathogenesis will depend on their ability to engage receptors on cells of the immune system and generate inflamma tory responses. HLA-B27 dimers can bind to various receptors on macrophages, natural killer cells and T cells. In rodents, the paired immunoglobulin-like receptors bind HLA-B27 multimers,25 whereas in humans the killer cell immunoglobulin-like receptors (KIRs, expressed by some T cells as well as natural killer cells) and the (now renamed) leukocyte immuno-globulin-like subfamily B receptors (LILBRs) are involved. Previously termed immuno globulin-like transcripts, these receptors resemble KIRs in that they likewise interact with Class I HLA antigens, but LILBRs are also expressed by macrophages and dendritic cells. Importantly, HLA-B27 dimers bind to a different set of recep-tors from those that bind conventional peptide-containing HLA-B27 molecules. For example, conventional HLA-B27 binds KIR3DL1, LILRB1, and LILRB2, whereas dimers interact with KIR3DL2 but fail to interact with LILRB1.24,27,28 Dimers, or free heavy chains, might also be capable of recognition by CD4+ T cells;29,30 this recognition is an important consideration, since CD4+ T cells (rather than the CD8+ cells



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that naturally interact with conventional HLA-B27 molecules) seem to be critical in HLA-B27 transgenic rodent models of SpA.

Experiments in transgenic rats seem to support the importance of free heavy chains and dimer formation. Disease was ameliorated in rats that expressed a minigene encoding a

high-affinity HLA-B27 binding peptide.31 This finding was originally interpreted as evidence for recognition of an arthritogenic peptide that would normally be bound by HLA-B27, but was displaced by the transgenic peptide; however, in view of the noninvolvement of CD8+ T cells in the rat model, the effect might instead be caused by stabilization of HLA-B27 molecules by a plentiful supply of high-affinity peptide. Such stabilization would decrease surface expression of HLA-B27 molecules containing low-affinity peptides that dissociate to free heavy chains and dimerize. Against this interpretation, a recent paper showed that HLA-B27 transgenic rats that overexpressed β2M developed more-severe arthritis, despite decreased expression of free HLA-B27 heavy chains on the cell surface.32

The atypical peptide-binding groove of HLA-B27Unusual aspects of the ability of HLA-B27 to present peptides have been described. A crystallo-graphic study of HLA-B*2705 showed that, uniquely amongst HLA class I alleles, peptides could take up alternative configurations within the peptide-binding groove, and that these could be distinguished by T cells.33 This property was not shared by the non-SpA-associated allele B*2709. The peptide in question was derived from a self protein, vasoactive intestinal peptide receptor 1,34 which was previously suggested to be an autoantigen in AS. Further evidence that this autoreactivity is critical to AS pathogenesis has, however, yet to be reported.

NON-HLA-B27 GENES IMPLICATED IN DISEASE SUSCEPTIBILITYCARD15—a bacterial sensorInvestigation into the genes conferring suscep-tibility to Crohn’s disease met with conspic-uous success when mutations in the CARD15 (caspase recruitment domain family member 15, also known as NOD2) gene were identified in a significant proportion of patients (~30%), with heterozygotes having a twofold to fourfold, and homozygotes a 15-fold to 40-fold increased susceptibility to Crohn’s disease.35,36 This finding led to a search for associations with other forms of SpA, and although no association with AS has been found, one has been reported with PsA: an odds ratio of ~3 for possession of one or more of the Crohn’s disease-associated CARD15 mutants was seen in a Newfoundland population.37 This finding might not apply to all populations,

Antigenicpeptide

Freeheavychain

Class I MHCheavy chain

Heavychain

dimers

T cellNK cell

APC

Loss of peptideand β2M

αβTCR β2M

A

B

C

T cell APC

APC

Unfoldedprotein response

Endoplasmicreticulum

NFκBactivation

MisfoldedHLA-B27

heavy chains

Figure 1 Characteristics of HLA-B27. (A) The conventional interaction between the T cell receptor and an HLA-B27 heterotrimer (i.e. B27 heavy chain, β2 microglobulin and antigenic peptide). (B) Generation of additional forms of HLA-B27, such as free heavy chains and dimers on the cell surface, and their interactions with T cells, natural killer cells and antigen-presenting cells. (C) Formation of a misfolded form of HLA-B27 within the endoplasmic reticulum of an antigen-presenting cell and the elicitation of stress and proinflammatory responses. Abbreviations: β2M, β2 microglobulin; APC, antigen-presenting cell; MHC, major histocompatibility complex; NFκB, nuclear factor κB; NK, natural killer; TCR, T-cell receptor.



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but the same is true of the asso ciation with Crohn’s disease.

What is the function of CARD15? This question remains somewhat controversial, with different phenotypes being described in different CARD15 gene-targeted mice. There is considerable evidence, however, that in macrophages, dendritic cells and neutrophils, CARD15 can act as an intracellular sensor for bacterial products, particularly the muramyl dipeptide moiety present in bacterial cell wall peptidoglycan.38 Interestingly, mutations that are associated with Crohn’s disease are located in the peptide domain implicated in this bacte-rial recognition. Following binding of the bacterial ligand, CARD15 has been shown by one laboratory to downregulate production of interleukin (IL)-12 in response to peptidoglycan, which is mediated by Toll-like receptor 2.39 This process fits well with the pathogenesis of Crohn’s disease, in which a failure to regu-late responses to bacterial products in the gut could explain the inflammatory lesions which develop. Another laboratory, using different gene-targeted mice, however, did not find evidence for this mechanism.40 It is unclear, therefore, whether signaling through Toll-like receptor 2 has a critical role in the patho-genesis of Crohn’s disease, or whether the CARD15 mutations associated with Crohn’s disease affect the response of these patients to a combination of muramyl dipeptide moiety and peptidoglycan.41 An alternative hypoth-esis is that processing of prointerleukin-1β is enhanced in CARD15 mutants; evidence for this hypothesis has been found in mice, but the same does not seem to be true in human Crohn’s disease. Whatever the mechanism, the finding that a gene encoding a protein that func-tions in the recognition of bacteria is associated with two elements of SpA reinforces the basic gene–bacteria–immune response hypothesis outlined above.

Jun transcription factors—a new model of psoriatic arthritisThe concept that SpA is the result of inter-actions between susceptibility genes, bacteria and the immune system has been illustrated recently by elegant studies in gene-targeted mice. These studies have resulted in the develop-ment of a novel murine model of psoriasis and psoriatic arthritis; although this animal model is by no means the only one that is relevant

to psoriatic arthritis,42 it has several features that are worth considering in some detail.43 The model is complex, and is illustrated in Box 1. The genes that were targeted are two components of the activator protein (AP)-1 tran scription factor, which has effects on many cellular processes, including proliferation, differentiation, and cytokine production. The genes in question, c-Jun and Junb, are believed to be mutually antagonistic, and were of interest in the context of psoriasis since the Junb gene is present in one of the psoriasis susceptibility loci, PSORS6. Simple knockout of these genes is embryonically lethal, so a conditional model was used in which the genes are knocked out at a time determined by the investigator, and only in keratinocytes. This powerful technology is now widely used, and depends on the ability of the P1 bacteriophage DNA recombinase (Cre) to excise genes that are engineered to contain certain recognition sites (see Box 1). The timing of expression, and specificity of Cre is deter-mined by use of a Cre–steroid receptor fusion protein that is only activated when tamoxifen is administered.

Strikingly, mice in which deletion of both c-Jun and JunB in keratinocytes was induced by treatment with tamoxifen developed psori-atic skin lesions 8–10 days later and, impor-tantly, a destructive inflammatory arthritis, which is not a common feature of most animal models of psoriasis. The skin lesions showed typical histology, with T cell, neutrophil and macrophage infiltration (Figure 2). There was upregula tion of proinflammatory cytokines, including IL-1, IFNγ and tumor necrosis factor (TNF), and chemokines, including macro-phage inflammatory protein 2 (MIP-2 or CXC-chemokine ligand [CXCL] 2, which is the mouse equivalent of IL-8), MIP-1α (CC-chemokine ligand [CCL] 3), MIP-1β (CCL4), interferon-inducible protein 10 (IP-10 or CXCL10) and monocyte chemotactic protein 1 (MCP-1 or CCL2). Transforming growth factor β2, which inhibits keratinocyte proliferation, was down-regulated. Interestingly, studies of mouse skin at earlier time points (after tamoxifen treat-ment but before lesion development) showed upregula tion of two members of the S100 family of proteins, S100A8 and S100A9, which bind the receptor for advanced glycation end-products (RAGE)44 and are known to have chemotactic actions. These S100 proteins are encoded in the PSORS4 susceptibility locus.



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By use of this model it was possible to address the importance of other factors, in addition to c-Jun and Junb, in the development of skin disease and arthritis. Perhaps unexpectedly, the requirements for the two diseases differed. When c-Jun and Junb were deleted in mice lacking T and B cells (recombinase activating gene [RAG] 2 knockout mice, which lack an enzyme that is essential for recombination of T-cell and B-cell receptor genes), skin disease

was still seen, and although it was less severe, the typical changes in cytokine and chemokine expression were still present. Arthritis in these mice, however, was mild and no bone destruc-tion occurred. This result can be compared with that obtained in another transgenic mouse model of psoriasis (but not psoriatic arthritis), which involves an altered transcription factor in keratinocytes. In this model, keratinocytes are engineered to express a constitutively active

Box 1 A new model of psoriasis and psoriatic arthritis.

Mouse 1Both copies of the normal c-Jun genes are replaced, by standard gene-targeting techniques, with c-Jun genes with a loxP sequence at both ends of the gene (a ‘floxed’ gene) which is recognized by a recombinase, Cre, found in the P1 bacteriophage. c-Jun function in this mouse is entirely normal.

Mouse 2Both copies of the normal Junb genes are replaced by floxed c-Junb genes, as for c-Jun in mouse 1. Again Junb function in this mouse is entirely normal.

Mouse 3Obtained by crossing mouse 1 and 2; these mice have floxed c-Jun and Junb genes, but both function normally.

Mouse 4Expresses Cre recombinase transgenically, but only in keratinocytes (i.e. the Cre gene is under control of the keratinocyte-specific keratin 5 gene promoter) and only when the mice are treated with tamoxifen, because the Cre gene is fused to an estrogen-receptor gene. Without tamoxifen treatment no Cre is expressed.

Mouse 5Obtained by crossing mouse 3 and 4 and selecting those which have inherited both the floxed genes and the transgenic Cre recombinase. These mice express c-Jun and JunB normally until treated with tamoxifen, when the Cre gene is induced. The Cre recombinase gene then brings about recombination between the two lox sites in each gene, effectively ‘chopping out’ both the c-Jun and Junb genes. This only happens in keratinocytes (the only cells that express Cre). In this way c-Jun and JunB expression is normal during mouse development and is then switched off in a tissue-specific manner.

Abbreviation: ER, estrogen receptor.

loxP loxP

loxP loxP

loxP loxP

×

×

Mouse 1 Mouse 2

c-Jun

c-Jun

Junb

Junb

Mouse 3

c-Jun

Junb

Mouse 4 Mouse 3

Keratinocyte-specific promoter

Treat with tamoxifen to activate Cre–ER recombinase

Cre ER c-Jun

Junb

loss of c-Jun

loss of Junb

Mouse 5

c-Jun

Junb



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transcription factor, signal transducer and activator of inflammation (STAT) 3.45 These mice develop psoriasis, albeit only in the pres-ence of T lymphocytes; together, these results suggest that crosstalk between keratinocytes and T lymphocytes is required for the establishment of psoriatic lesions (the STAT-3 model), or for their amplification (the c-Jun/Junb model). In humans, this crosstalk might be reflected by the effectiveness of alefacept, which targets the CD2 molecules expressed by T cells, in the treatment of psoriasis, and possibly psoriatic arthritis.46

Likewise, when c-Jun and Junb were knocked out in mice lacking one of the TNF receptors, TNFR1, skin disease, cytokine and chemokine dysregulation were still present, but no joint disease was observed. This finding is somewhat surprising, since skin disease in human psori-asis is highly responsive to anti-TNF therapy, which has a somewhat less dramatic (though still impressive) effect on joint disease.47 These findings might be explained by differences in the timing of TNF blockade, as the roles for this cytokine in disease initiation and after the develop ment of disease might differ.

Lastly, and of considerable interest, treat-ment with the antibacterial agent cipro floxacin delayed the onset of skin disease and also prevented arthritis, although the last findings are preliminary and require confirmation. The finding that arthritis was prevented is reminis-cent of results in HLA-B27 transgenic rodents, where arthritis did not occur in germ-free animals. In the transgenic rat, population of the gut with particular bacteria allowed disease to develop.48,49

What does the c-Jun/Junb gene-knockout model teach us about the pathogenesis of psori-atic arthritis, one component of the SpA spec-trum? First, it suggests that arthritis is secondary to skin disease, rather than arthritis and skin disease being two parallel manifesta tions of the same process. The discordance between skin disease and arthritis in humans has encour-aged the latter idea, including the ‘psoriatic arthritis sine psoriasis’, which is encountered particularly in patients with a strong family history of psoriasis. The role of bacteria needs to be considered, however, since they seem to be critical for the development of arthritis in the mouse model, and might also be relevant for skin disease, since onset of skin lesions was delayed by broad-spectrum anti biotics. Second, the genes of interest were deleted in all

keratino cytes, but psoriatic lesions only occurred at discrete sites, raising the possibility that a local factor, such as skin bacterial flora, is involved in disease pathogenesis. Patients with minimal amounts of skin psoriasis, or even a subclinical impairment or alteration in their ability to deal with skin bacteria might, there-fore, still be at risk of arthritis. Since arthritis in this model required T or B lymphocytes, the easiest way to tie these observations together would be to argue that the arthritis relates to immune responses to bacteria that occur within joints as a consequence of skin inflammation and a breakdown in the normal host relation-ship with commensal skin bacteria. Bacteria are already clearly implicated in one form of psoriasis, guttate psoriasis, which arises after streptococcal infection, but in genetically susceptible hosts commensal bacteria might fulfill this role.

How relevant are the findings in this mouse model to human psoriasis and psoriatic arthro-pathy? Clearly, neither the c-Jun nor Junb genes are deleted in human patients, although the investigators who developed the mouse model did report altered expression of c-Jun (increased) and JunB (decreased) in human lesional skin. The effect of manipulating AP1 component gene expression in the mouse, and perhaps of more subtle alterations in humans, might be to increase the expression of the chemotactic proteins S100A8 and S100A9, perhaps resulting in enhanced or uncontrolled recruitment of cells that mediate innate and acquired immune responses. Responses to bacteria could then amplify the inflamma-tion, leading to trafficking of bacteria or their constituents (most likely via phagocytes) to joints. Immune responses in the joint, both innate and acquired, could then initiate the destructive arthropathy.

Bone morphogenetic proteinsThe role of these factors in enthesitis has been examined using a nontransgenic mouse model: male DBA/1 mice that develop spontaneous enthesitis and ankylosis of the hind paws.50 The relationship between this model and SpA is not wholly clear. The study showed that several bone morphogenetic protein (BMP) family members were expressed in the enthesis lesions, and there was evidence of activation of their downstream signaling pathways, which involve phosphoryla tion of SMAD proteins and their



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translocation to the nucleus. More importantly, the study showed inhibition of arthritis by gene therapy using Nog, whose product noggin is a BMP antagonist. Although this study clearly showed the involvement of BMP signaling in enthesitis and ankylosis, it did not establish BMP signaling as a probable primary defect in SpA, rather than as a mechanism whereby the SpA phenotype in expressed.

CONCLUSIONSThe research investigating the link between susceptibility genes, bacteria and the immune system in SpA pathogenesis, much of which has been performed in experimental models, has been discussed. To what extent are the conclusions

obtained from these models backed up (or refuted) by investigations of the diseases them-selves? It has been suggested that the principal lesion in SpA involves the enthesis,51 with syno-vitis possibly being secondary to local enthesitis. Unfortunately, the enthesis is notoriously diffi-cult to investigate directly, and biopsies are hard to justify. A recent study, however, suggests that the entheses in SpA are infiltrated by CD3+CD8+ T cells, mainly in the adjacent bone marrow, together with edema.52 These findings reflect the ‘bone edema’ that is a feature of active lesions in SpA when visualized by MRI. Synovitis in SpA differs to some extent from that seen in RA, with less hypertrophy of the synovial membrane and a substantial increase in vascularity (particularly in psoriatic arthritis), though predominance of CD8+ T cells has not been reported.53–55 The demonstration that TNF was highly expressed in sacroiliac joints led to a highly successful trial of anti-TNF therapy in SpA; this study also reported infiltration of sacroiliac joints by both CD4+ and CD8+ T cells (CD4+ cells comprising the majority).56,57

Findings from both entheses and synovia in SpA are, therefore, consistent with a T-cell-driven inflammatory process, perhaps occurring in response to bacterial antigens. Indeed, anti-TNF therapy has been shown to decrease recruit-ment of T cells and macrophages to the site of disease.58 If this hypothesis is correct, genes could influence the likelihood of bacterial prod-ucts reaching the site of disease, for example by compromising relationships with gut commensal bacteria (via CARD15) or skin bacteria (via c-Jun and JunB). HLA-B27 could also act at this level, predisposing patients to colitis as it does in HLA-B27 transgenic rats, or might alter the immune response to bacterial antigens. These effects need not occur solely, or even predomi-nantly, by means of presenting bacterial peptides to HLA-B27-restricted T cells. HLA-B27 could also be involved in the modulation of these responses through the engagement of receptors such as KIRs or LILRs with unusual dimeric or multimeric forms of HLA-B27. Other properties of HLA-B27 might alter host responses to intra-cellular infection (perhaps in a similar way to CARD15), and alter the cytokines produced by infected antigen presenting cells. These cytokines might be directly inflammatory, for example TNF, IL-1 and IL-6, or might alter the nature of the ensuing T-cell response, (as IL-12 and IL-23 are known to do).

Figure 2 Abnormalities seen in gene-targeted mice after tamoxifen-induced deletion of c-Jun and Junb genes, as compared with untreated controls. Psoriaform lesions are seen on wrists, (B) ears (D) and tail (F), compared with unaffected controls (A,C,E). Histology of affected toes shows joint infiltration, bone destruction and periostitis (G,H). Reprinted with permission from Macmillan Publishers Ltd: Nature,43 © (2005).

A

C

E

B

D

F

G H



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KEY POINTS■ HLA-B27 proteins possess several

unusual properties that might contribute to susceptibility to spondyloarthropathies (SpAs)

■ Several non-HLA-B27 genes also influence susceptibility to SpAs

■ The influence of susceptibility genes is often connected with interactions with pathogenic or commensal bacteria

■ A better understanding of the pathogenesis of SpA is urgently required; despite notable advances in treatment no curative therapy is currently available

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Competing interestsThe author declared he has no competing interests.

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Mechanisms of Disease: the immunopathogenesis of spondyloarthropathies