Defective costimulatory function is a striking feature of antigen-presenting cells in an HLA-B27-transgenic rat model of spondylarthropathy

TGFβ signaling pathway is altered by HLA-B27 expression, resulting in pathogenic consequences relevant for spondyloarthritis

BACKGROUND

Association of HLA-B27 with spondyloarthritis (SpA) has been known for 50 years, but still remains unexplained. We recently showed that HLA-B27 expressed in wing imaginal disc from HLA-B27/human-β2 microglobulin (hβ2m) transgenic Drosophila deregulated bone morphogenetic protein (BMP) pathway by interacting physically with type I BMP receptor (BMPR1) Saxophone (Sax), leading to crossveinless phenotype.

METHODS

Genetic interaction was studied between activin/transforming growth factor β (TGFβ) pathway and HLA-B27/hβ2m in transgenic Drosophila wings. The HLA-B27-bound peptidome was characterized in wing imaginal discs. In mesenteric lymph node (mLN) T cells from HLA-B27/hβ2m rat (B27 rat), physical interaction between HLA-B27 and activin receptor-like kinase-2 (ALK2), ALK3 and ALK5 BMPR1s, phosphorylation of small mothers against decapentaplegic (SMADs) and proteins of the non-canonical BMP/TGFβ pathways induced by its ligands, and the transcript level of target genes of the TGFβ pathway, were evaluated.

RESULTS

In HLA-B27/hβ2m transgenic Drosophila, inappropriate signalling through the activin/TGFβ pathway, involving Baboon (Babo), the type I activin/TGFβ receptor, contributed to the crossveinless phenotype, in addition to deregulated BMP pathway. We identified peptides bound to HLA-B27 with the canonical binding motif in HLA-B27/hβ2m transgenic Drosophila wing imaginal disc. We demonstrated specific physical interaction, between HLA-B27/hβ2m and mammalian orthologs of Sax and Babo, i.e. ALK2 and ALK5 (i.e. TGFβ receptor I), in the mLN cells from B27 rat. The magnitude of phosphorylation of SMAD2/3 in response to TGFβ1 was increased in T cells from B27 rats, showing evidence for deregulated TGFβ pathway. Accordingly, expression of several target genes of the pathway was increased in T cells from B27 rats, in basal conditions and/or after TGFβ exposure, including Foxp3, Rorc, Runx1 and Maf. Interestingly, Tgfb1 expression was reduced in naive T cells from B27 rats, even premorbid, an observation consistent with a pro-inflammatory pattern.

CONCLUSIONS

This study shows that HLA-B27 alters the TGFβ pathways in Drosophila and B27 rat. Given the importance of this pathway in CD4 + T cells differentiation and regulation, its disturbance could contribute to the abnormal expansion of pro-inflammatory T helper 17 cells and altered regulatory T cell phenotype observed in B27 rats.

The Experimental Animal Models in Psoriasis Research: A Comprehensive Review

Psoriasis is an autoimmune, chronic, inflammatory skin condition mediated by T cells. It differs from other inflammatory conditions by causing significant alterations in epidermal cell proliferation and differentiation that are both complicated and prominent. The lack of an appropriate animal model has significantly hindered studies into the pathogenic mechanisms of psoriasis since animals other than humans typically do not exhibit the complex phenotypic features of human psoriasis. A variety of methods, including spontaneous mutations, drug-induced mutations, genetically engineered animals, xenotransplantation models, and immunological reconstitution approaches, have all been employed to study specific characteristics in the pathogenesis of psoriasis. Although some of these approaches have been used for more than 50 years and far more models have been introduced recently, they have surprisingly not yet undergone detailed validation. Despite their limitations, these models have shown a connection between keratinocyte hyperplasia, vascular hyperplasia, and a cell-mediated immune response in the skin. The xenotransplantation of diseased or unaffected human skin onto immune-compromised recipients has also significantly aided psoriasis research. This technique has been used in a variety of ways to investigate the function of T lymphocytes and other cells, including preclinical therapeutic studies. The design of pertinent in vivo and in vitro psoriasis models is currently of utmost concern and a crucial step toward its cure. This article outlines the general approach in the development of psoriasis-related animal models, aspects of some specific models, along with their strengths and limitations.

Paradoxical Effects of Endoplasmic Reticulum Aminopeptidase 1 Deficiency on HLA-B27 and Its Role as an Epistatic Modifier in Experimental Spondyloarthritis

OBJECTIVE

We undertook this study to examine the functional basis for epistasis between endoplasmic reticulum aminopeptidase 1 (ERAP1) and HLA-B27 in experimental spondyloarthritis (SpA).

METHODS

ERAP1-knockout rats were created using genome editing and bred with HLA-B27/human β2 -microglobulin-transgenic (HLA-B27-Tg) rats and HLA-B7-Tg rats. The effects of ERAP1 deficiency on HLA allotypes were determined using immunoprecipitation and immunoblotting, flow cytometry, allogeneic T cell proliferation assays, and gene expression analyses. Animals were examined for clinical features of disease, and tissue was assessed by histology.

RESULTS

ERAP1 deficiency increased the ratio of folded to unfolded (β2 m-free) HLA-B27 heavy chains, while having the opposite effect on HLA-B7. Furthermore, in rats with ERAP1 deficiency, HLA-B27 misfolding was reduced, while free HLA-B27 heavy chain dimers on the cell surface and monomers were increased. The effects of ERAP1 deficiency persisted during up-regulation of HLA-B27 and led to a reduction in endoplasmic reticulum stress. ERAP1 deficiency reduced the prevalence of arthritis in HLA-B27-Tg rats by two-thirds without reducing gastrointestinal inflammation. Dendritic cell abnormalities attributed to the presence of HLA-B27, including reduced allogeneic T cell stimulation and loss of CD103-positive/major histocompatibility complex class II-positive cells, were not rescued by ERAP1 deficiency, while excess Il23a up-regulation was mitigated.

CONCLUSION

ERAP1 deficiency reduced HLA-B27 misfolding and improved folding while having opposing effects on HLA-B7. The finding that HLA-B27-Tg rats had partial protection against SpA in this study is consistent with genetic evidence that loss-of-function and/or reduced expression of ERAP1 reduces the risk of ankylosing spondylitis. Functional studies support the concept that the effects of ERAP1 on HLA-B27 and SpA may be a consequence of how peptides affect the biology of this allotype rather than their role as antigenic determinants.

Interleukin 27 is a novel cytokine with anti-inflammatory effects against spondyloarthritis through the suppression of Th17 responses

Introduction

Spondylarthritis (SpA) development in HLA-B27/human β2-microglobulin transgenic rat (B27-rat) is correlated with altered conventional dendritic cell (cDC) function that promotes an inflammatory pattern of CD4+T cells, including a biased expansion of pro-inflammatory Th₁₇ population and imbalance of regulatory T cells cytokine profile. Transcriptomic analysis revealed that cDCs from B27-rats under express IL-27, an anti-inflammatory cytokine which induces the differentiation of IL-10⁺ regulatory T cells and inhibits Th₁₇ cells.

Methods

Here, we first investigated whether in vitro addition of exogenous IL-27 could reverse the inflammatory pattern observed in CD4⁺ T cells. Next, we performed preclinical assay using IL-27 to investigate whether in vivo treatment could prevent SpA development in B27-rats.

Results

in vitro addition of IL-27 to cocultures of cDCs and CD4⁺ T cell subsets from B27-rats reduced IL-17 and enhanced IL-10 production by T cells. Likewise, IL-27 inhibited the production of IL-17 by CD4⁺ T cells from SpA patients. Interestingly, in vivo treatment with recombinant IL-27 starting before SpA onset, inhibited SpA development in B27-rats through the suppression of IL-17/TNF producing CD4⁺ T cells.

Discussion

Overall, our results reveal a potent inhibitory effect of IL-27 and highlight this cytokine as a promising new therapeutic target in SpA, especially for SpA patients non responders to currently approved biotherapies.

Intestinal and enthesis innate immunity in early axial spondyloarthropathy

Axial SpA (axSpA), encompassing AS, is a multifactorial disease that localizes to sites of high spinal biomechanical stress. Much has been written on T cells and adaptive immunity in axSpA, which is understandable given the very strong HLA-B27 disease association. Extra-axial disease characteristically involves the anterior uveal tract, aortic root, lung apex and terminal ileum. Under recent classification, axSpA is classified as an intermediate between autoimmunity and autoinflammatory disease, with the latter term being synonymous with innate immune dysregulation. The purpose of this review is to evaluate the ‘danger signals’ from both the exogenous intestinal microbiotal adjuvants or pathogen-associated molecular patterns that access the circulation and endogenously derived damaged self-tissue or damage-associated molecular patterns derived from entheses and other sites of high biomechanical stress or damage that may serve as key drivers of axSpA onset, evolution, disease flares and eventual outcomes.

Lessons on SpA pathogenesis from animal models

Understanding the complex mechanisms underlying a disorder such as spondyloarthritis (SpA) may benefit from studying animal models. Several suitable models have been developed, in particular to investigate the role of genetic factors predisposing to SpA, including HLA-B27, ERAP1, and genes related to the interleukin (IL)-23/IL-17 axis. One of the best examples of such research is the HLA-B27 transgenic rat model that fostered the emergence of original theories regarding HLA-B27 pathogenicity, including dysregulation of innate immunity, contribution of the adaptive immune system to chronic inflammation, and influence of the microbiota on disease development. Very recently, a new model of HLA-B27 transgenic Drosophila helped to expand further some of those theories in an unexpected direction involving the TGFβ/BMP family of mediators. On the other hand, several spontaneous, inducible, and/or genetically modified mouse models-including SKG mouse, TNF^ΔARE mouse and IL-23-inducible mouse model of SpA-have highlighted the importance of TNFα and IL-23/IL-17 axis in the development of SpA manifestations. Altogether, those animal models afford not only to study disease mechanism but also to investigate putative therapeutic targets.

Advances in nanomedicine for the treatment of ankylosing spondylitis

Ankylosing spondylitis (AS) is a complex disease characterized by inflammation and ankylosis primarily at the cartilage–bone interface. The disease is more common in young males and risk factors include both genetic and environmental. While the pathogenesis of AS is not completely understood, it is thought to be an immune-mediated disease involving inflammatory cellular infiltrates, and human leukocyte antigen-B27. Currently, there is no specific diagnostic technique available for this disease; therefore conventional diagnostic approaches such as clinical symptoms, laboratory tests and imaging techniques are used. There are various review papers that have been published on conventional treatment approaches, and in this review work, we focus on the more promising nanomedicine-based treatment modalities to move this field forward.

Detection of novel diagnostic antibodies in ankylosing spondylitis: An overview

Ankylosing spondylitis (AS) is a debilitating, chronic, rheumatic disease characterized by inflammation and new bone formation resulting in fusion of the spine and sacroiliac joints. Since early treatment is impeded by a delayed diagnosis, it is highly important to find new biomarkers that improve early diagnosis and may also contribute to a better assessment of disease activity, prognosis and therapy response in AS. Because of the absence of rheumatoid factor, AS was long assumed to have a seronegative character and antibodies are thus not considered a hallmark of the disease. However, emerging evidence suggests plasma cells and autoantibodies to be involved in the disease course. In this review, the role of B cells and antibodies in AS is discussed. Furthermore, an overview is provided of antibodies identified in AS up till now, and their diagnostic potential. Many of these antibody responses were based on small study populations and further validation is lacking. Moreover, most were identified by a hypothesis-driven approach and thus limited to antibodies against targets that are already known to be involved in AS pathogenesis. Hence, we propose an unbiased approach to identify novel diagnostic antibodies. The already successfully applied techniques cDNA phage display and serological antigen selection will be used to identify antibodies against both known and new antigen targets in AS plasma. These newly identified antibodies will enhance early diagnosis of AS and provide more insight into the underlying disease pathology, resulting in a more effective treatment strategy and eventually an improved disease outcome.

Ankylosing spondylitis patients display altered dendritic cell and T cell populations that implicate pathogenic roles for the IL-23 cytokine axis and intestinal inflammation

Objective. AS is a systemic inflammatory disease of the SpA family. Polymorphisms at loci including HLA-B27, IL-23R and ERAP-1 directly implicate immune mechanisms in AS pathogenesis. Previously, in an SpA model, we identified HLA-B27–mediated effects on dendritic cells that promoted disease-associated Th17 cells. Here we extend these studies to AS patients using deep immunophenotyping of candidate pathogenic cell populations. The aim of our study was to functionally characterize the immune populations mediating AS pathology.

Methods. Using 11-parameter flow cytometry, we characterized the phenotype and functions of lymphocyte and myeloid cells from peripheral blood, and the synovial phenotype of AS patients and age-matched healthy controls.

Results. Significantly fewer circulating CD1c-expressing dendritic cells were observed in AS patients, offset by an increase in CD14⁻ CD16⁺ mononuclear cells. Ex vivo functional analysis revealed that this latter population induced CCR6 expression and promoted secretion of IL-1β and IL-6 when co-cultured with naive CD4⁺ T cells. Additionally, systemic inflammation in AS patients significantly correlated with increased proportions of activated CCR9⁺ CD4⁺ T cells.

Conclusion. CD14⁻ CD16⁺ mononuclear cells may contribute to AS by promoting Th17 responses, and antigen-presenting cells of mucosal origin are likely to contribute to systemic inflammation in AS.

ERAP1 Gene Expression Is Influenced by Nonsynonymous Polymorphisms Associated With Predisposition to Spondyloarthritis

OBJECTIVE

Several polymorphisms in ERAP1 are strongly associated with susceptibility to spondyloarthritis (SpA). The combination of rs17482078, rs10050860, and rs30187 results in the construction of 3 major haplotypes that are associated with SpA (the "protective" haplotype T/T/C, the "neutral" haplotype C/C/C, and the "susceptibility" haplotype C/C/T). The aim of the present study was to determine whether such haplotypes might affect endoplasmic reticulum aminopeptidase 1 (ERAP-1) messenger RNA (mRNA) expression, protein level, and/or enzymatic activity in antigen-presenting cells, a type of cell that is potentially relevant to disease pathogenesis.

METHODS

Monocyte-derived dendritic cells (DCs) were generated in 2 cohorts (a discovery cohort and a replication cohort) comprising a total of 23 SpA patients and 44 healthy controls. Lymphoblastoid B cell lines were established from individuals who were homozygous for the risk, the neutral, or the protective ERAP1 haplotype, respectively. In those samples, we investigated the relationship between ERAP1 haplotypes and mRNA expression level. We also used Western blot analysis to measure the relative protein expression of ERAP-1 and a fluorogenic assay to measure its enzymatic activity.

RESULTS

In monocyte-derived DCs, there was a strong association between ERAP1 haplotypes and the ERAP-1 mRNA expression level, with higher levels in subjects harboring the susceptibility haplotype (P = 0.001 and P = 5.6 × 10(-7) in the discovery and replication cohorts, respectively). In lymphoblastoid B cell lines, we observed a significant correlation between haplotype risk score and ERAP1 transcript or protein level (P = 0.003, ρ = 0.92 for both). Enzymatic activity followed a similar trend both in monocyte-derived DCs and in lymphoblastoid B cell lines.

CONCLUSION

These data provide strong evidence that SpA-associated ERAP1 polymorphisms affect the level of gene expression in antigen-presenting cells. How increased production/activity of ERAP-1 may influence susceptibility to SpA remains to be determined.

Increased production of interleukin-17 over interleukin-10 by treg cells implicates inducible costimulator molecule in experimental spondyloarthritis

OBJECTIVE

HLA-B27/human β2 -microglobulin (hβ2 m)-transgenic (B27-transgenic) rats develop an inflammatory disorder resembling spondyloarthritis, with accumulation of proinflammatory Th17 cells. Because Treg cells and Th17 cells have opposing effects in inflammatory disorders, we sought to determine whether biased expansion of Th17 cells could result from altered Treg cell frequency and/or function in B27-transgenic rats.

METHODS

We characterized the phenotype and function of Treg cells from B27-transgenic rats in comparison with those from control rats, by examining their expression of cell surface markers, suppressive activity, cytokine production, and differentiation pattern.

RESULTS

In B27-transgenic rats, the preferential accumulation of CD4+ Teff cells over Treg cells was not associated with a defect in Treg cell differentiation or suppressive activity. The expression of Treg cell markers was similar between B27-transgenic and control rats, with the exception of the inducible costimulator (ICOS) molecule, which was overexpressed in B27-transgenic rats. High levels of ICOS are considered to be a hallmark of Treg cells with heightened suppressive activity and interleukin-10 (IL-10) expression. Paradoxically, the production of IL-10 by Treg cells was reduced in B27-transgenic rats, whereas the production of IL-17 was enhanced. Moreover, the addition of anti-ICOS monoclonal antibodies during Treg cell differentiation in the presence of dendritic cells from B27-transgenic rats reversed this cytokine profile, restoring the balance between IL-10 and IL-17 in Treg cells from B27-transgenic rats.

CONCLUSION

We observed dysregulated production of IL-10 and IL-17 by Treg cells from B27-transgenic rats, which may contribute to disease development. Moreover, our data highlight a key role for ICOS signaling in the generation of imbalanced production of IL-10 and IL-17 by Treg cells in this experimental model of spondyloarthritis.

Monocyte-derived dendritic cells from HLA-B27+ axial spondyloarthritis (SpA) patients display altered functional capacity and deregulated gene expression

Introduction

This study aimed to compare the functional capacity and gene expression profile of monocyte-derived dendritic cells (MD-DCs) in HLA-B27⁺ axial spondyloarthritis (SpA) patients and healthy controls.

Methods

MD-DCs were differentiated with interleukin 4 (IL-4) and granulocyte-macrophage colony-stimulating factor (GM-CSF) for seven days, starting from purified CD14⁺ monocytes and stimulated with lipopolysaccharide (LPS) for six and twenty four hours. Their capacity to stimulate allogeneic CD4⁺ T cells from unrelated healthy donor was tested. Transcriptomic study was performed with Affymetrix HuGene 1.0 ST microarrays. Gene expression levels were compared between patients and controls using a multivariate design under a linear model (LIMMA). Real-time quantitative PCR (qRT-PCR) was performed for validation of the most striking gene expression differences.

Results

The stimulatory capacity of allogeneic CD4⁺ T cells by MD-DCs from SpA patients was decreased. Transcriptomic analysis revealed 81 genes differentially expressed in MD-DCs between SpA patients and controls (P <0.01 and fold-change <0.66 or >1.5). Four selected genes were validated by qRT-PCR: ADAMTS15, CITED2, F13A1 and SELL. Expression levels of ADAMTS15 and CITED2, encoding a metallopeptidase and a transcription factor, respectively, were inversely correlated with each other (R = 0.75, P = 0.0003). Furthermore, in silico analysis identified several genes of the Wnt signaling pathway having expression co-regulated with CITED2.

Conclusion

This study revealed altered function and gene expression pattern in MD-DCs from HLA-B27⁺ axial SpA. Co-expression study showed an inverse correlation between ADAMTS15 and CITED2. Moreover, the Wnt signaling pathway appeared as deregulated in SpA MD-DCs, a finding which may be connected to Th17-driven inflammatory responses.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-014-0417-0) contains supplementary material, which is available to authorized users.

Reverse interferon signature is characteristic of antigen-presenting cells in human and rat spondyloarthritis

OBJECTIVE

In HLA-B27-transgenic rats, the development of a disorder that mimics spondyloarthritis (SpA) is highly correlated with dendritic cell (DC) dysfunction. The present study was undertaken to analyze the underlying mechanisms of this via transcriptome analysis.

METHODS

Transcriptome analysis of ex vivo-purified splenic CD103+CD4+ DCs from B27-transgenic rats and control rats was performed. Transcriptional changes in selected genes were confirmed by quantitative reverse transcriptase-polymerase chain reaction. A meta-analysis of our rat data and published data on gene expression in macrophages from ankylosing spondylitis (AS) patients was further performed.

RESULTS

Interferon (IFN) signaling was the most significantly affected pathway in DCs from B27-transgenic rats; the majority of genes connected to IFN were underexpressed in B27-transgenic rats as compared to controls. This pattern was already present at disease onset, persisted over time, and was conserved in 2 disease-prone B27-transgenic rat lines. In DCs from B27-transgenic rats, we further found an up-regulation of suppressor of cytokine signaling 3 (which may account for reverse IFN signaling) and a down-regulation of interleukin-27 (a cytokine that opposes Th17 differentiation and promotes Treg cells). The meta-analysis of data on conventional DCs from rats and data on monocyte-derived macrophages from humans revealed 7 IFN-regulated genes that were negatively regulated in both human and rat SpA (i.e., IRF1, STAT1, CXCL9, CXCL10, IFIT3, DDX60, and EPSTI1).

CONCLUSION

Our results suggest that expression of HLA-B27 leads to a defect in IFNγ signaling in antigen-presenting cells in both B27-transgenic rats and SpA patients, which may result in Th17 expansion and Treg cell alteration (as shown in B27-transgenic rats) and contribute to disease pathogenesis.

Resveratrol as a natural anti-tumor necrosis factor-α molecule: implications to dendritic cells and their crosstalk with mesenchymal stromal cells

Dendritic cells (DC) are promising targets for inducing tolerance in inflammatory conditions. Thus, this study aims to investigate the effects of the natural anti-inflammatory molecule resveratrol on human DC at phenotypic and functional levels, including their capacity to recruit mesenchymal stem/stromal cells (MSC). Primary human monocyte-derived DC and bone marrow MSC were used. DC immunophenotyping revealed that small doses of resveratrol (10 µM) reduce cell activation in response to tumor necrosis factor (TNF)-α, significantly decreasing surface expression of CD83 and CD86. Functionally, IL-12/IL-23 secretion induced by TNF-α was significantly reduced by resveratrol, while IL-10 levels increased. Resveratrol also inhibited T cell proliferation, in response to TNF-α-stimulated DC. The underlying mechanism was investigated by Western blot and imaging flow cytometry (ImageStream^X), and likely involves impairment of nuclear translocation of the p65 NF-κB subunit. Importantly, results obtained demonstrate that DC are able to recruit MSC through extracellular matrix components, and that TNF-α impairs DC-mediated recruitment. Matrix metalloproteinases (MMP) produced by both cell populations were visualized by gelatin zymography. Finally, time-lapse microscopy analysis revealed a significant decrease on DC and MSC motility in co-cultures, indicating cell interaction, and TNF-α further decreased MSC motility, while resveratrol recovered it. Thus, the current study points out the potential of resveratrol as a natural anti-TNF-α drug, capable of modulating DC phenotype and function, as well as DC-mediated MSC recruitment.

The biochemistry and immunology of non-canonical forms of HLA-B27

HLA-B27 (B27) is strongly associated with the spondyloarthritides. B27 is expressed at the cell surface of antigen presenting cells (APC) both as canonical β2m-associated and non-canonical β2m-free heavy chain (FHC) forms which include B27 dimers (termed B272). B27 FHC forms arise in an endosomal compartment from recycling β2m-associated B27. Formation of cell surface FHC dimers is critically dependent on an unpaired reactive cysteine 67 in the α1 helix of the class I heavy chain. HLA-B27 also form redox-inducible β2m-associated dimers on exosomes and apoptosing cells. By contrast with cell surface expressed cysteine 67-dependent heavy chain dimers these dimers are dependent on a cytoplasmic cysteine 325 for their formation. HLA-B27 binds to immunoregulatory receptors including members of the Killer cell Immunoglobulin-like (KIR) and Leukocyte Immunoglobulin-like receptor family. B27 FHC bind to different but overlapping sets of these immunoreceptors compared to classical β2m-associated HLA-B27. B27 FHC bind more strongly to KIR3DL2 and LILRB2 immune receptor than other β2m-associated HLA-class I ligands. Genetic studies have implicated genes which control production of the important proinflammatory cytokine IL-17 in the pathogenesis of spondyloarthritis. Cell surface HLA-B27 FHC binding to these immune receptors or acting through other mechanisms could impact on the pathogenesis of spondyloarthritis by promoting immune cell production of IL-17. Here we review the literature on these non-canonical forms of HLA-B27 and the immune receptors they bind to and discuss the possible relevance of these interactions to the pathogenesis of spondyloarthropathy.

The link between HLA-B27 and SpA--new ideas on an old problem

The strong association of the HLA-B27 with AS was first discovered independently by groups in London and California in 1972 and has recently been confirmed beyond reasonable doubt by fine mapping in the latest and most sophisticated genome-wide association study (GWAS) published this July. Yet, despite nearly four decades of extensive research, the exact role that HLA-B27 plays in pathogenesis remains unknown. However, we believe that recent developments in three fields have allowed us to view this conundrum in a new light and to propose coherent theories of disease pathogenesis. These areas are as follows: (i) GWASs, (ii) studies of B27 biology and (iii) lessons from biologic therapies. In this review we will discuss these recent advances before discussing the current models of AS pathogenesis under investigation.

Phenotype of resting and activated monocyte-derived dendritic cells grown from peripheral blood of patients with ankylosing spondylitis

Decreased levels of class II major histocompatibility complex (MHC) expression and impaired formation of immunological synapse by dendritic cells (DCs) of HLA-B27 transgenic rats have been recently demonstrated. The resulting dysfunction of DCs may be implicated in the pathogenesis of the HLA-B27-related disease in transgenic animals. The phenotype of DCs in patients with ankylosing spondylitis (AS) has not been evaluated. Monocyte-derived DCs (MDDCs) were grown from patients with active AS and age-matched healthy volunteers. Surface expression of HLA-DR, co-stimulation molecules CD80, CD86 and CD40, as well as CD83 was assessed by flow cytometry and compared between the groups under 3 conditions: in resting state, after stimulation by lipopolysaccharide (LPS) and after stimulation by LPS in the presence of etanercept, a soluble receptor of tumor necrosis factor α. Lower baseline expression of class II MHC molecules (HLA-DR) was observed by MDDCs grown from AS patients, as compared to healthy subjects. Post-stimulated levels of HLA-DR were comparable in both groups, suggesting greater up-regulation of class II MHC molecules by MDDCs from AS in response to LPS. No difference between groups in the levels of expression of co-stimulation molecules and CD83 was observed. Lower basic expression of class II MHC by the MDDCs grown from patients with AS may be associated with impaired regulation of their activity. Functional studies on DCs from patients with AS are needed to evaluate the integrity of their antigen-presenting function.

Proinflammatory Th17 cells are expanded and induced by dendritic cells in spondylarthritis-prone HLA-B27-transgenic rats

OBJECTIVE

HLA-B27/human β2-microglobulin-transgenic (B27-transgenic) rats, a model of spondylarthritis (SpA), develop spontaneous colitis and arthritis under conventional conditions. CD4+ T cells are pivotal in the development of inflammation in B27-transgenic rats. This study was undertaken to characterize the phenotype of CD4+ T cells in this model and to determine whether dendritic cells (DCs) induce proinflammatory T cells.

METHODS

The phenotype of CD4+ T cells from rat lymph nodes (LNs) draining the sites of inflammation was analyzed by flow cytometry. Immunostaining was used to detect interleukin-17 (IL-17)-producing cells in the rat joints. DCs from B27-transgenic or control rats (transgenic for HLA-B7 or nontransgenic) were cocultured with control CD4+ T cells and stimulated with anti-T cell receptor α/β.

RESULTS

IL-17A- and tumor necrosis factor α (TNFα)-producing CD4+ T cells were expanded in mesenteric and popliteal LNs from B27-transgenic rats. The accumulation of Th17 cells correlated with disease development, in contrast to Th1 or Treg cells. IL-17-positive mononuclear cells were detected in the arthritic joints of B27-transgenic rats but not in the joints of control rats. Finally, in vitro cocultures demonstrated that Th17 cells were preferentially induced and expanded by DCs from B27-transgenic rats, by a process that may involve defective engagement of costimulatory molecules.

CONCLUSION

Our findings indicate that expanded CD4+ T cells in B27-transgenic rats exhibit a proinflammatory Th17 phenotype characterized by IL-17A and TNFα production. Furthermore, this population is preferentially induced by DCs from B27-transgenic rats. These data point toward an induction of Th17 cells as a possible pathogenic mechanism in this model of SpA. However, their pathogenic role still needs to be shown.

Interactions between gut inflammation and arthritis/spondylitis

PURPOSE OF REVIEW

The spectrum of spondyloarthritis is characterized by the intriguing co-occurrence of gut and joint inflammation, although no obvious anatomical link exists.

RECENT FINDINGS

Data from animal models identify stromal cells as important players in pathogenesis, although signalling through TNFRI appeared to be sufficient for development of combined gut and joint inflammation. Interleukin-23 receptor was identified as a susceptibility locus for ankylosing spondylitis.

SUMMARY

Human genome studies combined with animal model research provide us with new evidence in the fascinating field of the gut-joint axis. However, how these newly identified genetic associations can influence the immunological environment remains to be elucidated.

From HLA-B27 to spondyloarthritis: a journey through the ER

Almost four decades of research into the role of human leukocyte antigen-B27 (HLA-B27) in susceptibility to spondyloarthritis has yet to yield a convincing answer. New results from an HLA-B27 transgenic rat model now demonstrate quite convincingly that CD8(+) T cells are not required for the inflammatory phenotype. Discoveries that the HLA-B27 heavy chain has a tendency to misfold during the assembly of class I complexes in the endoplasmic reticulum (ER) and to form aberrant disulfide-linked dimers after transport to the cell surface have forced the generation of new ideas about its role in disease pathogenesis. In transgenic rats, HLA-B27 misfolding generates ER stress and leads to activation of the unfolded protein response, which dramatically enhances the production of interleukin-23 (IL-23) in response to pattern recognition receptor agonists. These findings have led to the discovery of striking T-helper 17 cell activation and expansion in this animal model, consistent with results emerging from humans with spondyloarthritis and the discovery of IL23R as an additional susceptibility gene for ankylosing spondylitis. Together, these results suggest a novel link between HLA-B27 and the T-helper 17 axis through the consequences of protein misfolding and open new avenues of investigation as well as identifying new targets for therapeutic intervention in this group of diseases.

Dendritic cells from spondylarthritis-prone HLA-B27-transgenic rats display altered cytoskeletal dynamics, class II major histocompatibility complex expression, and viability

OBJECTIVE

Spondylarthritis (SpA) is characterized by spinal and peripheral joint inflammation, frequently combined with extraarticular manifestations. Despite the well-established association of SpA with the class I major histocompatibility complex (MHC) allele HLA-B27, there are still different, parallel hypotheses on the relationship between HLA-B27 and disease mechanisms. The present study was undertaken to investigate several characteristics of mature dendritic cells (DCs), which are believed to be essential for triggering disease in a model of SpA in HLA-B27-transgenic rats.

METHODS

We combined different whole-proteome approaches (2-dimensional polyacrylamide gel electrophoresis and iTRAQ) to define the most aberrant molecular processes occurring in spleen DCs. Videomicroscopy and flow cytometry were used to confirm both cytoskeletal and class II MHC expression deficiencies.

RESULTS

Our proteome studies provided evidence of up-regulation of proteins involved in class I MHC loading, and unfolded protein response, along with a striking down-regulation of several cytoskeleton-reorganizing proteins. The latter result was corroborated by findings of deficient motility, altered morphology, and decreased immunologic synapse formation. Furthermore, class II MHC surface expression was reduced in DCs from B27-transgenic rats, and this could be linked to differences in class II MHC-induced apoptotic sensitivity. Finally, we found reduced viability of the CD103+CD4- DC subpopulation, which likely exerts tolerogenic function.

CONCLUSION

Taken together, our findings have different important implications regarding the physiology of B27-transgenic rat DCs, which have a putative role in spontaneous disease in these rats. In particular, the reduced motility and viability of putatively tolerogenic CD4+ DCs could play an important role in initiating the inflammatory process, resulting in SpA.

Increased production of soluble CTLA-4 in patients with spondylarthropathies correlates with disease activity

INTRODUCTION

Spondylarthropathies (SpA) are characterized by abnormal immune responses including T cell activation. Cytotoxic T lymphocyte associated molecule-4 (CTLA-4) is involved in down-regulating immune responses. A soluble form of CTLA-4 (sCTLA-4), resulting from an alternative splicing, has been identified and was found increased in several autoimmune diseases. Here, we evaluated circulating levels of sCTLA-4 as a marker of immune dysregulation in SpA. Intracellular CTLA-4 and levels of CTLA-4 transcript expression in peripheral blood lymphocytes (PBL) were also studied.

METHODS

Sera from 165 patients with SpA were evaluated for sCTLA-4 measurements. Results were compared with those from 71 patients with rheumatoid arthritis (RA) and 88 healthy subjects. In 32 patients with SpA, 22 patients with RA and 15 healthy controls, we analyzed the intracellular CTLA-4 expression in CD4+ T cells, CD8+ T cells, activated (HLA-DR+Foxp3-) CD4+ T cells, CD4+ regulatory (CD25+Foxp3+) T cells and in CD3 negative cells by flow cytometry. Expression of the full length (coding for membrane CTLA-4) and spliced form (coding for sCTLA-4) of CTLA-4 transcripts in PBL were analyzed by quantitative real-time polymerase chain reaction (QRT-PCR).

RESULTS

High levels of sCTLA-4 were found in the SpA group compared to the RA group and healthy controls (P < 0.0001). Soluble CTLA-4 serum levels strongly correlated with clinical index of disease activity BASDAI (r = 0.42, P < 0.0001) and C-reactive protein (CRP) levels (r = 0.17, P = 0.037). In contrast to RA patients, SpA patients did not exhibit changes in intracellular CTLA-4 expression in the different PBL subsets tested. Finally, the SpA group showed a preferential expression of the spliced CTLA-4 mRNA (P = 0.0014) in PBL.

CONCLUSIONS

SpA patients exhibit high levels of circulating sCTLA-4 that may result from an alternative splicing of CTLA-4 transcripts. This may influence immune activation and regulation in SpA.

Progress in spondylarthritis. Immunopathogenesis of spondyloarthritis: which cells drive disease?

Spondyloarthritides, or SpA, form a cluster of chronic inflammatory diseases with the axial skeleton as the most typical disease localisation, although extra-articular manifestations such as intestinal inflammation may frequently occur during the course of the disease. This review summarises recent progress in our understanding of the immunopathogenesis of SpA with special emphasis on the cellular constituents considered to be responsible for the initiation and/or perpetuation of inflammation. There are several arguments favouring a role for haematopoietic cells in the pathophysiology of spondyloarthritis, including HLA-B27-associated dendritic cell disturbances, HLA-B27 misfolding properties and T helper 17 cells. In addition, recent studies have pointed toward a pivotal role for stromal cells. A major challenge, however, remains to determine how recently identified genetic associations such as interleukin-23 receptor polymorphisms may influence cellular targets in spondyloarthritis.

The HLA-B27 transgenic rat, a model of spondyloarthritis, has decreased bone mineral density and increased RANKL to osteoprotegerin mRNA ratio

OBJECTIVE

Bone metabolism in spondyloarthritis (SpA) is not well elucidated. We investigated alterations in bone in the HLA-B27 transgenic rat, a model of SpA.

METHODS

Femur, tibia, and lumbar vertebral bodies of disease-prone HLA-B27 transgenic, healthy HLA-B7 transgenic, and nontransgenic control rats were used for bone histomorphometric and dual energy x-ray absorptiometry (DEXA) analysis. Serum levels of type I collagen C-telopeptides (CTX), N-terminal propeptide of type I procollagen (P1NP), and osteocalcin, as well as receptor activator of nuclear factor-kappa B ligand (RANKL) and osteoprotegerin (OPG), were measured. RNA was isolated from the bone tissue of the femura to analyze gene expression of RANKL, OPG, and osteocalcin.

RESULTS

Histomorphometric analysis indicated a significant decrease in bone volume as well as trabecular number and thickness in the HLA-B27 rats. Trabecular separation was increased. Numbers of osteoblasts, osteoclasts, and osteoid volume were not altered significantly. The decrease in bone mineral density was confirmed using DEXA. Levels of RANKL mRNA were significantly increased in the bone tissue of HLA-B27 transgenic rats, resulting in an increased RANKL to OPG ratio. Osteocalcin mRNA expression was also significantly elevated in bone of HLA-B27 rats. Serum levels of CTX, RANKL, OPG, P1NP, and osteocalcin did not differ significantly.

CONCLUSION

Our data indicate that, similarly to SpA in humans, HLA-B27 transgenic rats show a reduced bone mass, and suggest an involvement of the RANKL/OPG system in the mechanism of bone loss in this disease. This model may be adequate to study osteoporosis in SpA.

Correlation between dendritic cell functional defect and spondylarthritis phenotypes in HLA-B27/HUMAN beta2-microglobulin-transgenic rat lines

OBJECTIVE

To examine the functional capacity of dendritic cells (DCs) from a panel of HLA-B27/human beta2-microglobulin (Hubeta2m)-transgenic rat lines and crosses with varying susceptibilities to spondylarthritis (SpA)-like disease.

METHODS

Mature splenic DCs were isolated from HLA-B27-transgenic, HLA-B7-transgenic, and/or Hubeta2m-transgenic rats and tested for support of allogeneic proliferation, compared with nontransgenic controls (all male rats on Lewis background). Graded numbers of DCs were cultured with allogeneic lymph node CD4+ T cells (dark agouti background). Proliferation was assayed by incorporation of tritiated deoxythymidine after 2-4 days of culture.

RESULTS

Allogeneic proliferation stimulated by DCs from the healthy HLA-B27/Hubeta2m-transgenic line 21-3 and from the healthy Hubeta2m-transgenic line 283-2 was weakly decreased (21-3) or close to normal (283-2) as compared with that observed with control nontransgenic Lewis rat DCs. In contrast, the ability of DCs from (21-3 x 283-2)F1 rats, which develop a dramatic SpA phenotype, to stimulate allogeneic proliferation was markedly defective. When DC-induced allogeneic proliferation was compared among different transgenic lines and crosses with distinct levels of susceptibility to SpA-like disease, stimulatory capacity was inversely correlated with disease susceptibility.

CONCLUSION

In HLA-B27/Hubeta2m-transgenic rats, a defective functional capacity of DCs correlates with susceptibility to SpA. Since it was previously demonstrated that defective DC function is not a consequence of disease, it could well be a principal factor in the spontaneous development of SpA in these lines.

Animal models of spondyloarthritis

Animal models are available for the study of several different aspects of spondyloarthritis. The models include naturally occurring spontaneous disorders in primates and rodents, spontaneous disorders in transgenic or gene-deleted rodents and induced disorders in rodents. Areas of investigation to which these models contribute include the role HLA-B27, processes of spinal and peripheral joint inflammation and calcification, immune responses to candidate antigens and the role of TNF.

The role of B27 heavy chain dimer immune receptor interactions in spondyloarthritis

HLA-B27 (B27) is strongly associated with spondyloarthopathy. The classical role of B27 is to present peptides from intracellular pathogens as a heterotrimeric complex with beta2 microglobulin for recognition by the T-cell receptor (TCR) of CD8 T-cells. In addition to heterotrimers, B27 can also be expressed as cell surface beta2-microglobulin (beta2m)-free homodimers (B27(2)). In addition to the TCR, MHC class I molecules bind to immunoregulatory receptors including members of the killer immunoglobulin-like receptor (KIR) and leukocyte immunoglobulin-like receptor (LILR) families. Rodents express the paired immunoglobulin receptor (PIR) family which are related to LILR. B27(2) but not beta2m-associated B27 binds to KIR3DL2 and rodent PIR. NK and T-cells expressing the immune receptor KIR3DL2, which interacts with B27(2), are expanded in B27 AS patients. Ligation of immune receptors by B27(2) promotes the survival of KIR-expressing leukocytes and modulates immune cytokine production. Upregulation ofB272 in spondyloarthritis and differential interaction of beta2m-associated HLA-B27 and B27(2) with immune receptors could be involved in the pathogenesis of B27-associated spondyloarthritis (AS).

HLA-B27 and host-pathogen interaction

HLA-B27 is a risk factor closely associated to spondyloarthropathies (SpA). One form of SpA is reactive arthritis (ReA), which develops as a complication after certain bacterial infections (e.g., Salmonellae, Yersiniae, Shigellae, Campylobacteriae and Chlamydiae). The development of infection-triggered complication is a complex train of events between the triggering bacteria and the host. Since most of the patients suffering from ReA are HLA-B27 positive, it has been proposed that HLA-B27 may modulate the interaction between ReA-triggering bacteria and host cell. Besides antigen presenting function, HLA-B27 displays other unusual properties that might be of importance in the development of ReA. These properties (homodimer formation and misfolding of HLA-B27 heavy chain in the endoplasmic reticulum (ER)) may trigger ER-stress signaling pathways in host cell, which in turn may modulate cell signaling in favor of ReA-triggering bacteria. Here we summarize the observations of HLA-B27 modulating the interaction between ReA-triggering bacteria and host cell and discuss potential mechanisms behind the interaction.

Dendritic cells modulated by innate immunity improve collagen-induced arthritis and induce regulatory T cells in vivo

Dendritic cells (DCs) mediate interactions between innate and specific immunity and may induce regulatory mechanisms. We investigated the effects of modulated DCs in mice with collagen-induced arthritis (CIA) and tested the responses of cells to induced naturally occurring regulatory T cells. DCs were stimulated or not with DNA or lipopolysaccharide (LPS) for 24 hr. DC maturation was assayed, and then modulated DCs were intraperitoneally injected on day 14 into DBA/1 mice to treat CIA. In addition to arthritis scores and type 2 collagen (CII) response, the induction of CD4(+) CD25(+) T cells was analysed by flow cytometry in peripheral blood and the expression of Foxp3, transforming growth factor (TGF)-beta, interleukin (IL)-10 and cytotoxic T-lymphocyte antigen (CTLA)-4 was quantified. Finally, the expression of indoleamine-2,3-dioxygenase (IDO) was assayed in DCs. In comparison with LPS-stimulated DCs, plasmid-stimulated DCs expressed lower levels of major histocompatibility complex (MHC) class II, CD40, CD80 and CD86 molecules and secreted less IL-12p70, interferon (IFN)-gamma, IL-10 and TNF-alpha, displaying a semi-mature phenotype. Compared with non-stimulated DCs, stimulated DCs improved arthritis scores when injected after immunization, without modifying the T helper type 1 (Th1)/Th2 balance of the immune response against collagen. Stimulated DCs induced markers for regulatory T cells (Foxp3, TGF-beta1 and CTLA-4) in vivo. Only LPS-stimulated DCs expressed IDO, which may explain their better therapeutic efficacy. Regulatory mechanisms were induced using DCs modulated by innate immunity stimulators. Innate immunity mechanisms do not require the presence of the disease-causing antigen, even in T- and B-cell specific diseases. Our results have implications for the treatment of rheumatoid arthritis, an autoimmune disease whose triggering antigen has not been identified, and substantially clarify the role of regulatory T cells in CIA.

TNFalpha blockade prevents the development of inflammatory bowel disease in HLA-B27 transgenic rats

Rats transgenic for HLA-B27 and human β2microglobulin (B27TR) develop a multi-systemic disease resembling inflammatory bowel disease (IBD) and spondyloarthritis. TNFα has a crucial role in chronic inflammation. Our objective was to evaluate the effect of anti-TNFα treatment on spontaneous IBD in B27TR. Nine-week-old B27TR received monoclonal anti-TNFα or an isotypic IgG2a,k up to age of 18 weeks. A second group was monitored up to 18 weeks and then randomly assigned to anti-TNFα or IgG2 a,k treatment. Each rat was monitored for clinical IBD manifestations. After sacrifice, the colon was examined for pathological changes. TNFα receptors (TNF-R1, TNF-R2), Fas/Fas-L expression and apoptosis were evaluated. IgG2a,k-treated and untreated B27TR presented signs of IBD at 11 weeks, whereas in anti-TNFα-treated B27TR no IBD signs were detected. In the late treatment, IBD signs improved after 1 week. Histopathological analysis of IgG2a,k-treated B27TR colon showed inflammatory signs that were widely prevented by early anti-TNFα treatment. Late treatment did not significantly reduce inflammation. TNF-R1 was weakly expressed in intestinal epithelial cells of IgG2a,k-treated B27TR, while it was comparable to controls in anti-TNFα-treated animals. TNF-R2 immunopositivity was strongly evident in IgG2a,k-treated B27TR, whereas was absent in anti-TNFα-treated rats. RT-PCR confirmed these results. IgG2a,k-treated B27TR showed, at 18 weeks, few Fas-positive cells and an increase of Fas-L-positive cells. At 27 weeks, Fas-/Fas-L-positive cell number was significantly low. Anti-TNFα treatment increased Fas-L expression, whereas Fas increased only with the early treatment. TNFα blockade is effective in preventing inflammation in early phase of IBD, maintaining the homeostatic balance of apoptosis.

Alteration of antigen-independent immunologic synapse formation between dendritic cells from HLA-B27-transgenic rats and CD4+ T cells: selective impairment of costimulatory molecule engagement by mature HLA-B27

OBJECTIVE

To investigate the molecular mechanism responsible for the reduced capacity of dendritic cells (DCs) from HLA-B27-transgenic rats to form conjugates with naive T cells.

METHODS

We monitored interactions between DCs derived from HLA-B27-transgenic, HLA-B7-transgenic control, and nontransgenic rats and naive CD4+ T cells. Chemoattraction was studied in Transwell assays, and the formation of an immunologic synapse was examined by videomicroscopy and electron microscopy. Involvement of specific molecules in the defective interaction was examined in antibody-blocking assays.

RESULTS

T cells migrated normally toward B27 DCs, but upon contact, the frequency of T cells undergoing a Ca2+ response was decreased, indicating impaired immunologic synapse formation. The immunologic synapse formed between B27 DCs and T cells appeared to be normal, as assessed by electron microscopy and by the Ca2+ response. Blocking lymphocyte function-associated antigen 1 on T cells or blocking activated leukocyte cell adhesion molecules on DCs inhibited an equivalent proportion of conjugates from forming between B27 or control DCs and T cells, whereas blocking CD86 on DCs and blocking CD28, CD2, or CD4 on T cells inhibited a greater number of conjugates from forming with control DCs, indicating specific involvement of costimulatory molecules in the reduced formation of conjugates with B27 DCs. Mature B27 molecules on the DC surface were responsible for this decreased formation of conjugates.

CONCLUSION

In the HLA-B27-transgenic rat model of spondylarthropathy, mature B27 molecules expressed by DCs impair the formation of an antigen-independent immunologic synapse with naive CD4+ T cells by interfering with the engagement of costimulatory molecules. This phenomenon could potentially affect the production and/or maintenance of regulatory T cells and contribute to the expansion of pathogenic CD4+ T cells.

Two HLA–B27 alleles differently associated with spondylarthritis, B*2709 and B*2705, display similar intracellular trafficking and oligomer formation

OBJECTIVE

To examine whether and to what extent the intracellular trafficking features of HLA-B*2705, which is associated with the development of spondylarthritis (SpA), differ from those of HLA-B*2709 and HLA-B*0702, which are not associated with SpA.

METHODS

HeLa cells were transfected with complementary DNA encoding for HLA-B proteins fused to Renilla luciferase or yellow fluorescent protein. The subcellular distribution of properly folded and unfolded/misfolded HLA-B proteins was examined by flow cytometry and confocal microscopy of cells labeled with ME1 and HC-10 antibodies, respectively. HLA-B/HLA-B interactions were monitored in endoplasmic reticulum (ER)- and plasma membrane-enriched subcellular fractions, by bioluminescence resonance energy transfer (BRET).

RESULTS

All 3 HLA-B alleles displayed a similar distribution pattern (properly folded heavy chain at the cell surface, unfolded/misfolded proteins only in the cytoplasm). By means of BRET, we provided evidence that both HLA-B*2705 and HLA-B*2709 formed more oligomers in the ER and the plasma membrane than did HLA-B*0702. The propensity of HLA-B*2705 to form oligomers in the ER was partly attributable to residue Cys(67) of the molecule. For all 3 alleles, increased expression of HLA-B proteins was associated with intracytoplasmic accumulation of unfolded/misfolded proteins and intracellular vesicles, probably corresponding to expanded ER-Golgi intermediate compartments, in which these proteins accumulated together with the stress sensor BiP.

CONCLUSION

Our results suggest that the difference in disease susceptibility conferred by HLA-B*2705 and HLA-B*2709 cannot be explained by their different propensity to form dimers or misfolded proteins, thus presumably implicating other, still unknown factors.

Luminal bacterial antigen-specific CD4+ T-cell responses in HLA-B27 transgenic rats with chronic colitis are mediated by both major histocompatibility class II and HLA-B27 molecules

Rats transgenic (TG) for the human major histocompatibility complex (MHC) class I HLA-B27 and beta2-microglobulin genes develop chronic colitis under specific pathogen-free (SPF) but not sterile (germ-free, GF) conditions. We investigated the role of antigen-presenting molecules involved in generating immune responses by CD4+ mesenteric lymph node (MLN) cells from colitic HLA-B27 TG rats to commensal enteric micro-organisms. All TG MLN cells expressed HLA-B27. A higher level of MHC class II was expressed on cells from TG rats, both SPF and GF, compared to non-TG littermates. In contrast, rat MHC class I expression was lower on TG than non-TG cells. Both TG and non-TG antigen presenting cells (APC) pulsed with caecal bacterial antigens induced a marked interferon-gamma (IFN-gamma) response in TG CD4+ T lymphocytes but failed to stimulate non-TG cells. Blocking MHC class II on both TG and non-TG APC dramatically inhibited their ability to induce TG CD4+ T cells to produce IFN-gamma. Blocking HLA-B27 on TG APC similarly inhibited IFN-gamma responses. When the antibodies against MHC class II and HLA-B27 were combined, no APC-dependent IFN-gamma response was detected. These data implicate both native rat MHC class II and TG HLA-B27 in CD4+ MLN T-cell IFN-gamma responses to commensal enteric microflora in this colitis model.

Adoptive transfer of nontransgenic mesenteric lymph node cells induces colitis in athymic HLA-B27 transgenic nude rats

HLA-B27 transgenic (TG) rats develop spontaneous colitis when colonized with intestinal bacteria, whereas athymic nude (rnu/rnu) HLA-B27 TG rats remain disease free. The present study was designed to determine whether or not HLA-B27 expression on T cells is required for development of colitis after transfer of mesenteric lymph node (MLN) cells into rnu/rnu HLA-B27 recipients. Athymic nontransgenic (non-TG) and HLA-B27 TG recipients received MLN cells from either TG or non-TG rnu/+ heterozygous donor rats that contain T cells. HLA-B27 TG rnu/rnu recipients receiving either non-TG or TG MLN cells developed severe colitis and had higher caecal MPO and IL-1beta levels, and their MLN cells produced more IFN-gamma and less IL-10 after in vitro stimulation with caecal bacterial lysate compared to rnu/rnu non-TG recipients that remained disease free after receiving either TG or non-TG cells. Interestingly, proliferating donor TG T cells were detectable one week after adoptive transfer into rnu/rnu TG recipients but not after transfer into non-TG recipients. T cells from either non-TG or TG donors induce colitis in rnu/rnu TG but not in non-TG rats, suggesting that activation of effector T cells by other cell types that express HLA-B27 is pivotal for the pathogenesis of colitis in this model.

HLA-B27 misfolding in transgenic rats is associated with activation of the unfolded protein response

The mechanism by which the MHC class I allele, HLA-B27, contributes to spondyloarthritis pathogenesis is unknown. In contrast to other alleles that have been examined, HLA-B27 has a tendency to form high m.w. disulfide-linked H chain complexes in the endoplasmic reticulum (ER), bind the ER chaperone BiP/Grp78, and undergo ER-associated degradation. These aberrant characteristics have provided biochemical evidence that HLA-B27 is prone to misfold. Recently, similar biochemical characteristics of HLA-B27 were reported in cells from HLA-B27/human beta2-microglobulin transgenic (HLA-B27 transgenic) rats, an animal model of spondyloarthritis, and correlated with disease susceptibility. In this study, we demonstrate that the unfolded protein response (UPR) is activated in macrophages derived from the bone marrow of HLA-B27 transgenic rats with inflammatory disease. Microarray analysis of these cells also reveals an IFN response signature. In contrast, macrophages derived from premorbid rats do not exhibit a strong UPR or evidence of IFN exposure. Activation of macrophages from premorbid HLA-B27 transgenic rats with IFN-gamma increases HLA-B27 expression and leads to UPR induction, while no UPR is seen in cells from nondisease-prone HLA-B7 transgenic or wild-type (nontransgenic) animals. This is the first demonstration, to our knowledge, that HLA-B27 misfolding is associated with ER stress that results in activation of the UPR. These observations link HLA-B27 expression with biological effects that are independent of immunological recognition, but nevertheless may play an important role in the pathogenesis of inflammatory diseases associated with this MHC class I allele.

Dysregulated luminal bacterial antigen-specific T-cell responses and antigen-presenting cell function in HLA-B27 transgenic rats with chronic colitis

HLA-B27/beta2 microglobulin transgenic (TG) rats spontaneously develop T-cell-mediated colitis when colonized with normal commensal bacteria, but remain disease-free under germ-free conditions. We investigated regulation of in vitro T-cell responses to enteric bacterial components. Bacterial lysates prepared from the caecal contents of specific pathogen-free (SPF) rats stimulated interferon-gamma (IFN-gamma) production by TG but not non-TG mesenteric lymph node (MLN) cells. In contrast, essentially equivalent amounts of interleukin-10 (IL-10) were produced by TG and non-TG cells. However, when cells from MLNs of non-TG rats were cocultured with TG MLN cells, no suppression of IFN-gamma production was noted. Both non-TG and TG antigen-presenting cells (APC) pulsed with caecal bacterial lysate were able to induce IFN-gamma production by TG CD4+ cells, although non-TG APC were more efficient than TG APC. Interestingly, the addition of exogenous IL-10 inhibited non-TG APC but not TG APC stimulation of IFN-gamma production by cocultured TG CD4+ lymphocytes. Conversely, in the presence of exogenous IFN-gamma, production of IL-10 was significantly lower in the supernatants of TG compared to non-TG APC cultures. We conclude that commensal luminal bacterial components induce exaggerated in vitro IFN-gamma responses in HLA-B27 TG T cells, which may in turn inhibit the production of regulatory molecules, such as IL-10. Alterations in the production of IFN-gamma, and in responses to this cytokine, as well as possible resistance of TG cells to suppressive regulation could together contribute to the development of chronic colitis in TG rats.

Animal models of HLA-B27-associated diseases: new outcomes

The HLA-B27 molecule is strongly associated with the spondyloarthropathies, a group of chronic inflammatory diseases, affecting the skeleton, the bowel and the skin. This association has been largely studied, but mechanisms of pathology remain unclear. The HLA-B27 transgenic rats develop a spontaneous disease that strikingly resembles human spondyloarthropathies, dependent of bacterial flora and implicating the immune system. The presence of CD4+ T cells is required, and antigen presenting cells (APC) expressing high levels of HLA-B27 likely play an important role. Indeed, APC are defective in naive T lymphocytes stimulation. This default appears to implicate the APC/T cells contact, and may result in a loss of tolerance toward microbial flora. Two models of skeletal inflammation linked to HLA-B27 have been developed in mice. The ANKENT mice develop a spontaneous ossifying enthesitis affecting ankle and tarsal joints, with increased frequency in the presence of an HLA-B27 transgene. The HLA-B27 transgenic mice lacking endogenous beta2 microglobulin develop arthritis of hind-paws. In this model, homodimers of B27 heavy chains could be implicated in the pathogenesis by presenting exogenous peptides to CD4+ T cells.

Pathogenesis of ankylosing spondylitis and reactive arthritis

PURPOSE OF REVIEW

The hallmark of ankylosing spondylitis is acute and chronic spinal inflammation initiating in the sacroiliac joints, often coupled with enthesitis, presenting as chronic inflammation at the sites of ligamentous and tendinous insertions into bone. Peripheral joint synovitis can be a prominent feature as well. Reactive arthritis is a sterile synovitis arising after an extra-articular infection of enteric or urogenital tracts. HLA-B27 has been known for about the past 30 years to be associated with ankylosing spondylitis and reactive arthritis, but the pathogenesis of ankylosing spondylitis and reactive arthritis is still not well defined. Although the clinical manifestations of ankylosing spondylitis and reactive arthritis may differ, this update discusses the two diseases together and focuses on recent evidence in both.

RECENT FINDINGS

With respect to HLA-B27 several recent studies address arthritogenic peptides, molecular mimicry, and aberrant forms of B27. Several candidate genes in addition to B27 have been implicated in recent genetic studies. With respect to bacterial infection, recent findings in bacterial antigenicity, host response through interactions of antigen-presenting cells, T cells, and cytokines are providing new understanding of host-pathogen interactions and the pathogenesis of arthritis. Endogenous host factors such as proteoglycans may play a role as autoantigens and contribute to chronic inflammation on that basis.

SUMMARY

Recent advances provide additional new insights into distinct pathogenetic mechanisms in AS and ReA that arise from a complex interplay between genetic factors including HLA-B27 and environmental factors.

MHC-I recognition by receptors on myelomonocytic cells: New tricks for old dogs?

Receptors on cytotoxic T lymphocytes and natural killer cells play well-established roles in the immunological response and share a common ligand in the form of MHC-I. We discuss how a variety of MHC-I receptors are also expressed on myelomonocytic cells such as macrophages and dendritic cells. Since myelomonocytic MHC-I receptors recognise a broad range of alleles and MHC-I structures, we propose that their task is to discern expression levels and folding forms of MHC. We describe a model in which these recognition events would regulate bidirectional cross talk between cells of innate and adaptive immune systems to organise an ongoing combined immune response. We discuss how such a model is supported by recent literature and might function in a variety of contexts, including immunoregulation during pregnancy. Our model also offers an alternative explanation of immune dysregulation rather than autoimmunity during HLA-B27-associated spondyloarthropathies and addresses a number of conundrums in this field.