Differential MHC class II presentation of a pathogenic autoantigen during health and disease

Autoreactive thymic B cells are efficient antigen-presenting cells of cognate self-antigens for T cell negative selection

The thymus contains a population of B cells that colocalize with dendritic cells and medullary thymic epithelial cells in the thymic medulla. The development and functional significance of these cells are largely unknown. Using recombination-activating gene 2 GFP reporter mice along with parabiosis experiments, we demonstrate that the vast majority of thymic B cells develop from progenitors within the thymus. Thymic B cells express unique phenotypic markers compared with peripheral B cells; particularly they express high levels of MHC class II, suggesting that they are poised to present self-antigens efficiently. Using Ig knock-in and T-cell receptor transgenic mice specific for the self-antigen glucose-6-phosphate isomerase, we show that autoreactive thymic B cells serve as efficient antigen-presenting cells for T cell negative selection even when they are present at low frequencies. Furthermore, the endogenous thymic B-cell repertoire also functions in this capacity. These results suggest that developing thymic B cells could efficiently capture a broad array of autoantigens through their B-cell receptors, presenting peptides derived from those autoantigens to developing thymocytes and eliminating cognate T cells.

Autoreactive T and B cells induce the development of bronchus-associated lymphoid tissue in the lung

Rheumatoid arthritis-related interstitial lung disease (RA-ILD) is associated with significant morbidity and mortality. Studies in humans have found that the incidence of bronchus-associated lymphoid tissue (BALT) correlates with the severity of lung injury. However, the mechanisms underlying the development of BALT during systemic autoimmunity remain unknown. We have determined whether systemic autoimmunity in a murine model of autoimmune arthritis can promote the development of BALT by generating a novel murine model derived from K/BxN mice. Transgenic mice with the KRN T-cell receptor specific for the autoantigen, glucose-6-phosphate isomerase (GPI), were crossed with GPI-specific immunoglobulin heavy and light chain knock-in mice, producing mice with a majority of T and B cells specific for the same autoantigen. We found that 67% of these mice demonstrated lymphocytic infiltration in the lungs, localized to either the perivascular or peribronchial regions. Fifty percent of the mice with lymphocytic infiltration manifested lymphoid-like lesions resembling BALT, with distinct T and B cell follicles. The lungs from mice with lymphoid infiltrates had increased numbers of cytokine-producing T cells, including IL-17A(+) T cells and increased major histocompatibility complex Class II expression on B cells. Interestingly, challenge with bleomycin failed to elicit a significant fibrotic response, compared with wild-type control mice. Our data suggest that systemic autoreactivity promotes ectopic lymphoid tissue development in the lung through the cooperation of autoreactive T and B cells. However, these BALT-like lesions may not be sufficient to promote fibrotic lung disease at steady state or after inflammatory challenge.

B cells as effectors and regulators of sex-biased arthritis

B cells have been implicated both with pathogenic as well as protective capabilities in induction and regulation of autoimmune diseases. Rheumatoid arthritis (RA) is an autoimmune disease that occurs more often in women than men. A significant role of B cells as antibody producing and antigen-presenting cells has been demonstrated in RA. Predisposition to RA is associated with the presence of certain HLA class II alleles that share sequences with DRB1*0401. To determine the role of HLA genes and B cells in vivo, we have generated transgenic mice carrying HLA genes, DRB1*0401 and DQ8, known to be associated with susceptibility to RA. Humanized mice can be induced to develop arthritis that mimics human disease in clinical, histopathological and sex bias. Effect of hormones on immune cells and their function has been described in humans and mice and has been suggested to be the major reason for female bias of autoimmune diseases. An immune response to an antigen requires presentation by HLA molecules thus suggesting a critical role of MHC in combination with sex hormones in susceptibility to develop rheumatoid arthritis. Based on our observations, we hypothesize that modulation of B cells by estrogen, presentation of modified antigens by DR4 and production of antigen-specific B cell modulating cytokines leads to autoreactivity in females. These data suggest that considering patient's sex may be crucial in selecting the optimal treatment strategy. Humanized mice expressing RA susceptible and resistant haplotype provide a means to investigate mechanism sex-bias of arthritis and future strategies for therapy.

Sensors of the innate immune system: their link to rheumatic diseases

Evidence strongly suggests that excessive or protracted signaling, or both, by cell-surface or intracellular innate immune receptors is central to the pathogenesis of most autoimmune and autoinflammatory rheumatic diseases. The initiation of aberrant innate and adaptive immune responses in autoimmune diseases can be triggered by microbes and, at times, by endogenous molecules--particularly nucleic acids and related immune complexes--under sterile conditions. By contrast, most autoinflammatory syndromes are generally dependent on germline or de novo gene mutations that cause or facilitate inflammasome assembly. The consequent production of proinflammatory cytokines, principally interferon-alpha/beta and tumor necrosis factor in autoimmune diseases, and interleukin-1beta in autoinflammatory diseases, leads to the creation of autoamplification feedback loops and chronicity of these syndromes. These findings have resulted in a critical reappraisal of pathogenetic mechanisms, and provide a basis for the development of novel diagnostic and therapeutic modalities for these diseases.

Steady-state dendritic cells continuously inactivate T cells that escape thymic negative selection

Induction of peripheral tolerance by steady-state peripheral dendritic cells (DCs) presenting self antigen may be important in preventing autoimmune diseases mediated by self-reactive T cells that escape thymic deletion. However, the relative contribution of thymic and peripheral tolerance to the inactivation of self-specific repertoires is yet to be clearly defined. Here we tested the relative contribution of thymic and peripheral tolerance induction, using mice (11c.OVA) in which ovalbumin (OVA) expression is genetically targeted to DCs in conjunction with mice (Vbeta5 TCR transgenic), where a polyclonal repertoire of OVA-specific T cells of diverse affinity is present. The expression of OVA in thymic DC reduced the frequency of OVA(257-264)-specific mature CD8 single-positive thymocytes although some functional OVA-specific CD8(+) T cells escaped negative selection and were detectable in the periphery. After adult thymectomy, OVA(257-264)-reactive T cells declined in the periphery indicating that the repertoire of OVA(257-264)-specific T cells that escaped negative selection and egressed to the periphery, were susceptible to inactivation by steady-state peripheral DC. Thus, in the face of inefficient negative selection, peripheral tolerance induction to cognate antigen by resting DC is a crucial requirement for the inactivation of a self-specific repertoire.

Increased number and function of FoxP3 regulatory T cells during experimental arthritis

OBJECTIVE

CD4+CD25+FoxP3+ regulatory T (Treg) cells are critical regulators of autoimmunity. Yet the number of Treg cells is paradoxically increased in rheumatoid arthritis (RA) patients, and Treg cells show variable activity in human studies. The objective of this study was to characterize the expansion and function of Treg cells during the initiation and progression of experimental arthritis.

METHODS

To unequivocally identify Treg cells, we crossed FoxP3gfp mice with K/BxN mice to generate arthritic mice in which Treg cells express green fluorescence protein. We examined the expansion and function of Treg cells and effector T (Teff) cells during different stages of arthritis, using flow cytometry and cell proliferation analyses.

RESULTS

In K/BxN mice, thymic selection of KRN T cells resulted in an enrichment of forkhead box P3 (FoxP3)-positive Treg cells. Treg cell numbers increased during arthritis, with significant increases in spleens and draining lymph nodes, indicating selective tropism to sites of disease. In contrast to the in vitro unresponsiveness of Treg cells when cultured alone, substantial proportions of Treg cells proliferated in both nonarthritic and arthritic mice. However, they also underwent greater apoptosis, thereby maintaining equilibrium with Teff cells. Similarly, enhanced Treg cell-suppressive activity during arthritis was offset by greater resistance by their Teff cell counterparts and antigen-presenting cells.

CONCLUSION

In this well-established model of RA, the interplay of Teff cells and Treg cells in K/BxN mice recapitulated many features of the human disease. We demonstrated an ordered expansion of Treg cells during arthritis and dynamic changes in Treg cell and Teff cell functions. By elucidating factors that govern Treg cell and Teff cell development in K/BxN(gfp) mice, we will gain insight into the pathophysiology of and develop novel therapeutics for human RA.

B cells are important as antigen presenting cells for induction of MHC-restricted arthritis in transgenic mice

Rheumatoid arthritis and its animal model, collagen-induced arthritis, are known as a T and B cell dependent disease. To analyze the role of B cells in arthritis, we generated B cell deficient (microMT) mice carrying HLA-DQ8 as transgene, Abetao.DQ8.micromt mice. HLA-DQ8 transgenic mice (Abetao.DQ8) are susceptible to collagen induced arthritis, an animal model for inflammatory arthritis. Deletion of IgM gene led to the absence of B cells while T cells were comparable to Abetao.DQ8 mice. Arthritis and autoantibodies was completely abrogated in B cell deficient DQ8 mice. T cell response and proinflammatory cytokine production in response to type II collagen and its derived peptides in vitro was significantly decreased despite an increased number of Mac-1 positive cells in DQ8.micromt mice compared to DQ8 mice suggesting B cells could be important for antigen presentation as well. In vitro substitution of B cells from wild type mice restored the response in DQ8.micromt mice. B cells could also present CII-derived peptides to antigen-specific DQ8-restricted hybridomas reinforcing the role of B cells in presentation of antigens to T cells. The data suggest that B cells can be involved in pathogenesis of arthritis by producing autoantibodies and antigen presentation.