Spontaneous inflammatory disease in HLA-B27 transgenic mice does not require transporter of antigenic peptides

Spondyloarthritides: Theories and beyond

Spondyloarthritides (SpA) are a family of interrelated rheumatic disorders with a typical disease onset ranging from childhood to middle age. If left untreated, they lead to a severe decrease in patients' quality of life. A succesfull treatment strategy starts with an accurate diagnosis which is achieved through careful analysis of medical symptoms. Classification criterias are used to this process and are updated on a regular basis. Although there is a lack of definite knowledge on the disease etiology of SpA, several studies have paved the way for understanding plausible risk factors and developing treatment strategies. The significant increase of HLA-B27 positivity in SpA patients makes it a strong candidate as a predisposing factor and several theories have been proposed to explain HLA-B27 driven disease progression. However, the presence of HLA-B27 negative patients underlines the presence of additional risk factors. The current treatment options for SpAs are Non-Steroidal Anti-Inflammatory Drugs (NSAIDs), TNF inhibitors (TNFis), Disease-Modifying Anti-Rheumatic Drugs (DMARDs) and physiotherapy yet there are ongoing clinical trials. Anti IL17 drugs and targeted synthetic DMARDs such as JAK inhibitors are also emerging as treatment alternatives. This review discusses the current diagnosis criteria, treatment options and gives an overview of the previous findings and theories to clarify the possible contributors to SpA pathogenesis with a focus on Ankylosing Spondylitis (AS) and enthesitis-related arthritis (ERA).

Salmonella as an Inducer of Autoimmunity

A clear etiological link has been established between infection with several gram-negative enteric pathogens, including Salmonella spp., and the incidence of reactive arthritis (ReA), an autoimmune disease that largely affects the joints. ReA is sometimes referred to as Reiter's syndrome, particularly when accompanied by uveitis and urethritis. This review reviews the evidence etiologically linking Salmonella infection with autoimmune disease and addresses the roles that bacterial and host elements play in controlling disease outcome. ReA is an autoimmune disease that largely consists of painful joint inflammation but also can include inflammation of the eye, gastrointestinal tract, and skin. ReA is a member of a broad spectrum of chronic inflammatory disorders termed the seronegative spondyloarthropathies (SNSpAs) that includes ankylosing spondylitis (AS), psoriatic arthritis, and enteropathic arthritis. Salmonella species, as well as other enteric pathogens associated with postgastroenteritis ReA, are facultative intracellular gram-negative bacteria. Many studies have analyzed the association of the HLA class I molecule, HLA-B27, with SNSpAs. Whereas B27 has been shown to be present in 90 to 95% of cases of AS, the association of the B27 haplotype with other SNSpAs is more tenuous. The clear association between ReA and infection with Salmonella or other gram-negative enteric pathogens has led to the suggestion that the adaptive immune response to infection has an autoimmune component. In addition to various Salmonella species, other gram-negative enteric pathogens have been linked to the development of ReA. Given their close relationship to Salmonella, this review considers the involvement of Shigella species in ReA.

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.

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.

HLA-B27 Heavy Chain Homodimers Are Expressed in HLA-B27 Transgenic Rodent Models of Spondyloarthritis and Are Ligands for Paired Ig-Like Receptors

HLA-B27 transgenic rats and strains of HLA-B27-transgenic beta(2)-microglobulin (beta(2)m)-deficient mice develop a multisystem inflammatory disease affecting the joints, skin, and bowel with strong similarity to human spondyloarthritis. We show that HLA-B27 transgenic mice and rats express HC10-reactive, beta(2)m-free HLA-B27 homodimers (B27(2)) and multimers, both intracellularly and at the cell surface of leukocytes, including rat dendritic cells. Fluorescent-labeled tetrameric complexes of HLA-B27 homodimers (B27(2) tetramers) bind to populations of lymphocytes, monocytes, and dendritic cells. The murine (and probably rat) paired Ig-like receptors (PIRs) are ligands for B27(2). Thus, B27(2) tetramers stain RBL cells transfected with murine activating PIR-A4 and inhibitory PIR-B receptors. Murine PIR-A and -B can be immunoprecipitated from the RAW264.7 macrophage cell line, and murine PIR-A can be immunoprecipitated from the J774.A1 line using B27(2). B27(2) tetramer staining corresponds to the distribution of PIR expression on lymphoid and myeloid cells and on murine macrophage cell lines. B27(2) can induce TNF-alpha release from the J774.A1 macrophage cell line. The binding of B27(2) to PIR is inhibited by HC10, an mAb that ameliorates arthritis in HLA-B27(+) beta(2)m(-/-) mice. The expression and PIR recognition of B27(2) could explain the pathogenesis of rodent spondyloarthritis.

CD8 alpha beta T cells are not essential to the pathogenesis of arthritis or colitis in HLA-B27 transgenic rats

The class I MHC allele HLA-B27 is highly associated with the human spondyloarthropathies, but the basis for this association remains poorly understood. Transgenic rats with high expression of HLA-B27 develop a multisystem inflammatory disease that includes arthritis and colitis. To investigate whether CD8alphabeta T cells are needed in this disease, we depleted these cells in B27 transgenic rats before the onset of disease by adult thymectomy plus short-term anti-CD8alpha mAb treatment. This treatment induced profound, sustained depletion of CD8alphabeta T cells, but failed to suppress either colitis or arthritis. To address the role of CD8alpha(+)beta(-) cells, we studied four additional groups of B27 transgenic rats treated with: 1) continuous anti-CD8alpha mAb, 2) continuous isotype-matched control mAb, 3) the thymectomy/pulse anti-CD8alpha regimen, or 4) no treatment. Arthritis occurred in approximately 40% of each group, but was most significantly reduced in severity in the anti-CD8alpha-treated group. In addition to CD8alphabeta T cells, two sizeable CD8alpha(+)beta(-) non-T cell populations were also reduced by the anti-CD8alpha treatment: 1) NK cells, and 2) a CD4(+)CD8(+)CD11b/c(+)CD161a(+)CD172a(+) monocyte population that became expanded in diseased B27 transgenic rats. These data indicate that HLA-B27-retricted CD8(+) T cells are unlikely to serve as effector cells in the transgenic rat model of HLA-B27-associated disease, in opposition to a commonly invoked hypothesis concerning the role of B27 in the spondyloarthropathies. The data also suggest that one or more populations of CD8alpha(+)beta(-) non-T cells may play a role in the arthritis that occurs in these rats.

Mice lacking endogenous major histocompatibility complex class II develop arthritis resembling psoriatic arthritis at an advanced age

OBJECTIVE

To describe and characterize a novel inflammatory toe disease with severe bone destruction that developed spontaneously in "humanized" (HLA transgenic) mice lacking their own major histocompatibility complex (MHC).

METHODS

We studied 5 different HLA transgenic mouse lines (HLA-DR2.Ab(0), DR3.Ab(0), DR4.Ab(0), DQ6.Ab(0), and DQ8.Ab(0)) in similar genetic background for an extended period of time (>14 months). Clinical, radiologic, and histologic abnormalities were monitored, and the MHC-related major immunologic parameters in affected and resistant mice were compared.

RESULTS

Animals of 4 transgenic lines (HLA-DR2.Ab(0), DR4.Ab(0), DQ6.Ab(0), and DQ8.Ab(0)) developed severe toe inflammation accompanied by progressive bone resorption, hyperkeratosis, alopecia, loss of nails, and shortening and thickening of the distal phalanges. HLA-DR3.Ab(0) transgenic mice were resistant to inflammation. The disease manifested only at advanced ages (6 months or older) and affected 70-100% of the mice, with a female preponderance. The clinical signs and the radiographic and histopathologic features of the affected toes were not similar to those of any disease previously described in mice but did resemble those described for human psoriatic arthritis (PsA). Mice from the 4 susceptible lines expressed lower levels of the HLA transgene and exhibited significantly fewer CD4+ cells in the peripheral blood and reduced natural killer cell activity compared with mice from the resistant HLA-DR3.Ab(0) line.

CONCLUSION

This novel, spontaneously developing PsA-like toe disease in MHC-manipulated mice seems to be related to the absence of endogenous MHC class II. Replacement with HLA transgene expression that is insufficient (or no replacement at all) may result in imbalanced MHC class I and class II functions and lead to development of the disease.

HLA-B27 and pathogenesis of spondyloarthropathies

Although the influence of HLA-B27 on the development of spondyloarthropathies is undisputed, its role in pathogenesis remains unclear. New ideas have focused on abnormal characteristics of HLA-B27 resulting from aberrant folding, disulfide bond formation, or both, rather than a predilection for selecting arthritogenic peptides. This reflects, in part, unanswered questions about whether immunologic recognition of HLA-B27 is required for disease. Recent studies suggest that CD4+ T cells, immunomodulatory killer cell Ig receptors, and Ig-like transcript receptors may recognize aberrant forms of HLA-B27. Other reports suggest that HLA-B27 expression can alter cytokine production from monocytes and T cells-effects that appear unrelated to antigen presentation. Novel bioinformatics approaches have led to the identification of HLA-B27-restricted pathogen-derived peptides and may prove useful in determining whether HLA-B27 presents arthritogenic peptides. Elucidating the role of HLA-B27 in the pathogenesis of these conditions will require an integration of information from animal models, genome-wide screens for susceptibility alleles, and translational studies using human samples.

The recognition of HLA-B27 by human CD4(+) T lymphocytes

HLA-B27 transgenic animal models suggest a role for CD4(+) T lymphocytes in the pathogenesis of the spondyloarthropathies, and murine studies have raised the possibility that unusual forms of B27 may be involved in disease. We demonstrate that CD4(+) T cells capable of recognizing B27 can be isolated from humans by coculture with the MHC class II-negative cell line T2 transfected with B27. These CD4(+) T cells recognize a panel of B27-transfected cell lines that are defective in Ag-processing pathways, but not the nontransfected parental cell lines, in a CD4-dependent fashion. Inhibition of responses by the MHC class I-specific mAb w6/32 and the B27 binding mAb ME1 implicates the recognition of a form of B27 recognized by both of these Abs. We suggest that B27-reactive CD4(+) T cells may be pathogenic in spondyloarthropathies, particularly if factors such as infection influence expression of abnormal forms of B27.