Additional human β2-microglobulin curbs HLA–B27 misfolding and promotes arthritis and spondylitis without colitis in male HLA–B27–transgenic rats

The endoplasmic reticulum stress response in immunity and autoimmunity

Many exogenous sources of stress can lead to cell death. In recent years, endogenous cellular sources of stress have also been identified, including the stress that arises from the accumulation of unfolded proteins within a cell's endoplasmic reticulum (ER). To counterbalance this type of ER stress, higher eukaryotic cells possess a three-pronged signal-transduction pathway termed the unfolded-protein response (UPR). This Review focuses on the role of the UPR in the mammalian immune system and how manipulation of this complex signalling pathway may be of therapeutic benefit in human disease.

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.

HLA-B27 misfolding and spondyloarthropathies

HLA-B27 plays a central role in the pathogenesis of many spondyloarthropathies and in particular ankylosing spondylitis. The observation that the HLA-B27 heavy chain has a tendency to misfold has raised the possibility that associated diseases may belong in a rapidly expanding category of protein misfolding disorders. The synthesis of the HLA-B27 heavy chain, assembly with beta(2)m and the loading of peptide cargo, occurs in the endoplasmic reticulum (ER) before transport to the cell surface. The evidence indicates that misfolding occurs in the ER prior to beta(2)m association and peptide optimization and is manifested in the formation of aberrant inter- and intra-chain disulfide bonds and accumulation of heavy chain bound to the chaperone BiP. Enhanced accumulation of misfolded heavy chains during the induction of class I expression by cytokines, can cause ER stress resulting in activation of the unfolded protein response (UPR). Effects of UPR activation on cytokine production are beginning to emerge and may provide important missing links between HLA-B27 misfolding and spondyloarthritis. In this chapter we will review what has been learned about HLA-B27 misfolding in human cells and in the transgenic rat model of spondyloarthritis-like disease, considering it in the context of other protein misfolding disorders. These studies provide a framework to support much needed translational work assessing HLA-B27 misfolding and UPR activation in patient-derived material, its consequences for disease pathogenesis and ultimately how and where to focus intervention strategies.

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 misfolding and spondyloarthropathies

HLA-B27 plays a central role in the pathogenesis of many spondyloarthropathies and in particular ankylosing spondylitis. The observation that the HLA-B27 heavy chain has a tendency to misfold has raised the possibility that associated diseases may belong in a rapidly expanding category of protein misfolding disorders. The synthesis of the HLA-B27 heavy chain, assembly with beta2m and the loading of peptide cargo, occurs in the endoplasmic reticulum (ER) before transport to the cell surface. The evidence indicates that misfolding occurs in the ER prior to b2m association and peptide optimization and is manifested in the formation of aberrant inter- and intra-chain disulfide bonds and accumulation of heavy chain bound to the chaperone BiP. Enhanced accumulation ofmisfolded heavy chains during the induction of class I expression by cytokines, can cause ER stress resulting in activation of the unfolded protein response (UPR). Effects of UPR activation on cytokine production are beginning to emerge and may provide important missinglinks between HLA-B27 misfolding and spondyloarthritis. In this chapter we will review what has been learned about HLA-B27 misfolding in human cells and in the transgenic rat model of spondyloarthritis-like disease, considering it in the context of other protein misfolding disorders. These studies provide a framework to support much needed translational work assessing HLA-B27 misfolding and UPR activation in patient-derived material, its consequences for disease pathogenesis and ultimately how and where to focus intervention strategies.

Pathophysiology of ankylosing spondylitis: what's new?

Ankylosing spondylitis is a chronic inflammatory joint disease that predominantly affects the sacroiliac joints and spine. Its pathophysiology remains one of the most vexing enigmas of rheumatology. However, new insights have been provided by the recent identification of susceptibility genes other than HLA-B27; evidence of a pivotal role for several proinflammatory cytokines including interleukins 23 and 17; and the recognition that inflammation and structural progression proceed separately from each other.

Joint expedition: linking gut inflammation to arthritis

The tight connection between intestinal inflammation and arthritis in spondyloarthritis (SpA) has been studied extensively. Subclinical gut inflammation, which can be considered as a model for early Crohn's disease, was shown to be strongly associated with joint inflammation. Several early mucosal abnormalities were uncovered even in the absence of histological signs of inflammation, providing clues into the pathogenesis of SpA. Nevertheless, many questions remain unanswered. In this review, we highlight recent progress on this intimate relationship between gut and joint inflammation. Emerging evidence exists favoring a role for genes beyond human leukocyte antigen B27 in the genetic predisposition of SpA and intestinal inflammation. Furthermore, the role of these predisposing genes in modulating host-pathogen interaction at mucosal surfaces and the subsequent link between gut and joint inflammation are of utmost importance in understanding the pathogenesis of SpA.

Peptides: the cornerstone of HLA-B27 biology and pathogenetic role in spondyloarthritis

The association of human leukocyte antigen (HLA)-B27 to ankylosing spondylitis is one of the strongest between a major histocompatibility complex molecule and a disease. Yet, the basis for this association remains unknown. Several hypotheses, each based on a particular feature of HLA-B27, guide much of the current research on the pathogenesis of this disease, but none has yet satisfactorily explained its mechanism and the differential association of B27 subtypes to it. In this review, the pathogenetic role of HLA-B27 will be analyzed from a global perspective of its biology, emphasizing the interdependency of multiple molecular features and the likely influence of disease-modifying gene products. From this perspective, peptide binding emerges as the cornerstone of all other biological properties.

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.

The intimate relationship between gut and joint in spondyloarthropathies

PURPOSE OF REVIEW

The aim of this article is to highlight recent progress in the combined joint and gut disease in spondyloarthropathies.

RECENT FINDINGS

A set of genes has been identified that are differentially expressed in the colon of spondyloarthropathy and Crohn's disease patients. Reduction of human leukocyte antigen B27 (HLA B27) misfolding by additional beta2-microglobulin in HLA B27 transgenic rats unexpectedly increased disease severity, with more similarities to spondyloarthropathies. By contrast, colitis disappeared.

SUMMARY

Human genomic studies combined with animal model research provide new clues concerning the common pathogenesis of spondyloarthropathy and Crohn's disease, further substantiating the unique relationship between gut and joint inflammation.

Identification of previously unrecognized predisposing factors for ankylosing spondylitis from analysis of HLA-B27 extended haplotypes in Sardinia

OBJECTIVE

To define the contribution of HLA genes other than HLA-B27 in conferring susceptibility to ankylosing spondylitis (AS), through analysis of HLA-B27 haplotypes in Sardinian subjects.

METHODS

Ninety-eight patients with AS, 133 HLA-B27-positive controls (of whom 33 were positive for HLA-B*2709), and 190 randomly selected controls were genotyped for microsatellites and single-nucleotide polymorphisms (SNPs) spanning the HLA region.

RESULTS

Haplotypes carrying either the B*2705 or the B*2709 allele were found to share a conserved region downstream of the HLA-B gene and a functional polymorphism in the HLA-E gene (R128G), while differing in all other markers. Notably, the presence of an A at SNP rs1264457, encoding for Arg-128, was significantly increased in the cohort of patients (P = 6 x 10(-6), corrected P = 3 x 10(-5)) but not in B*2705- or B*2709-positive controls. Comparing the alleles co-occurring at each HLA marker, we identified a region differentiating patients with AS and B*2705-matched controls. In particular, there was a markedly increased prevalence of heterozygosity at rs1264457 among B27-positive controls (74%, versus 47% in patients and 54% in random controls), suggesting a protective role of G128 in AS. Moreover, other markers around the HLA-B gene were also differentially represented.

CONCLUSION

These results demonstrate a significant difference in the frequency of some HLA markers between AS patients and B*2705-positive controls, which could be attributed to the opposite chromosome. In particular, the differential distribution of a functional polymorphism in the HLA-E gene suggests a possible role of natural killer function in AS pathogenesis.

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.

The pathogenetic role of HLA-B27 and its subtypes

The strong association between HLA-B27 and ankylosing spondylitis has been known for more than 33 years, but the enigma of the pathogenetic role of the gene and its product has not yet been solved. Ongoing studies have produced evidence supporting different theories to explain this association, and structural and functional studies of HLA-B27 allele products at molecular level have provided information of broad and multidisciplinary value and disclosed new avenues leading to autoimmunity and immune disregulation.

HLA-B27 and the pathogenesis of spondyloarthropathies

The association of HLA-B27 with ankylosing spondylitis, a chronic inflammatory disease of the axial skeleton, and other spondyloarthropathies, is among the strongest of an MHC antigen and any disease. Yet, the basis for this association remains unknown. In this review the main current hypotheses concerning the pathogenetic role of HLA-B27 will be discussed. They focus on three molecular properties of the molecule: (1) its peptide-presenting specificity, (2) its slow folding and tendency to misfold, and (3) its capacity to form covalent heavy chain homodimers amenable to recognition by leukocyte receptors. On the basis of the peptide specificity spondyloarthropathies would be triggered through T-cell autoimmunity against a self-ligand of HLA-B27 elicited by a cross-reactive foreign antigen. HLA-B27 misfolding would trigger disease through activation of inflammatory pathways following induction of endoplasmic reticulum stress, thus independently of antigen presentation. Recognition of heavy chain homodimers by leukocyte receptors might be involved in disease through immunomodulation of both innate and adaptive responses to arthritogenic pathogens. None of these hypotheses can yet satisfactorily account for the pathogenesis of spondyloarthritides. It is proposed that the pathogenetic role of HLA-B27 will eventually be explained through a global understanding of its biology, in which the various features of this molecule are envisaged as inter-dependent in their contribution to disease.

Humanized mouse models for organ-specific autoimmune diseases

Murine models for human autoimmune diseases are an essential tool for studying pathogenesis and for identifying new therapeutic targets. Mice are not the natural disease host, and conventional models have proved to be poor predictors of efficacy and safety in recent trials aiming to translate drug and biologic treatments to humans. Evidently, further steps towards recapitulating human diseases are urgently needed, for example using transgenic predisposing human HLA allele(s) plus T-cell receptor(s) implicated in a representative patient's autoimmune disease. The latest development - humanizing most of the immune system by transplanting human hematopoietic stem cells into severely immunodeficient mice - should lead to even better modeling.

Mechanisms of Disease: the immunopathogenesis of spondyloarthropathies

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.

HLA–B27 up-regulation causes accumulation of misfolded heavy chains and correlates with the magnitude of the unfolded protein response in transgenic rats: Implications for the pathogenesis of spondylarthritis-like disease

OBJECTIVE

HLA-B27 is implicated in the pathogenesis of spondylarthritis (SpA), yet the molecular mechanisms are incompletely defined. HLA-B27 misfolding has been associated with endoplasmic reticulum stress and activation of the unfolded protein response (UPR) in macrophages from HLA-B27/human beta(2)-microglobulin-transgenic (B27-transgenic) rats. This study was performed to assess the mechanisms that drive activation of the HLA-B27-induced UPR and to determine whether splenocytes respond in a similar manner.

METHODS

Splenocytes were isolated and bone marrow macrophages were derived from B27-transgenic and wild-type rats. Cells were treated for up to 24 hours with cytokines that induce class I major histocompatibility complex expression. HLA-B27 expression and misfolding were assessed by real-time reverse transcription-polymerase chain reaction, flow cytometry, and immunoblotting. Activation of the UPR was measured by quantifying UPR target gene expression and X-box binding protein 1 messenger RNA (mRNA) splicing.

RESULTS

HLA-B27 mRNA up-regulation was accompanied by a dramatic increase in the accumulation of misfolded heavy chains and preceded robust activation of the UPR in macrophages. When macrophages were treated with various cytokines, the magnitude of the UPR correlated strongly with the degree of HLA-B27 up-regulation. In contrast, B27-transgenic splenocytes exhibited only low-level differences in the expression of UPR target genes after exposure to interferon-gamma or concanavalin A, which resulted in minimal HLA-B27 up-regulation.

CONCLUSION

These results suggest that HLA-B27-associated activation of the UPR in macrophages is attributable to the accumulation of misfolded heavy chains, and that certain cell types may be more susceptible to the effects of HLA-B27 misfolding. Strategies that eliminate HLA-B27 up-regulation and/or the accumulation of misfolded heavy chains may be useful in evaluating the role of these events in the pathogenesis of SpA.