Interleukin-21 and Foxp3 are crucial regulators of autoimmunity development in the K/BxN rheumatoid arthritis model

Abstract

Rheumatoid arthritis is a joint-specific autoimmune inflammatory disease of unknown etiology. The K/BxN mouse is a model of rheumatoid arthritis that is mainly due to inflammatory responses mediated by autoantibodies against a self antigen glucose-6-phosphate isomerase. We found that homeostatic proliferation of autoreactive CD4+ T cells was required for disease initiation in the K/BxN mice, and the disease activity correlated with the level of interleukin (IL)-21, that was mainly produced by homeostatically proliferating CD4+CD25– T cells. To determine whether IL-21 signals play a role in the pathogenesis of autoimmune arthritis, we generated IL-21R-deficient K/BxN mice (IL-21R-/- K/BxN) and found that these mice were completely refractory to the development of spontaneous arthritis. They contained fewer CD4+ T cells with a reduced proportion of homeostatically proliferating cells, fewer follicular helper T cells, and, surprisingly, more IL-17-producing helper T (Th17) cells than their control counterparts. They also failed to develop autoantigen-specific IgG1 antibody-secreting cells. IL-21 induced expression of receptor activator of NF-κB ligand (RANKL), a regulator of osteoclastogenesis, and few RANKL-expressing infiltrates were found in the synovia of IL-21R-/- K/BxN mice. This result demonstrates that IL-21 forms a positive feedback autocrine loop linking homeostatically activated CD4+ cells to humoral autoimmunity and that it is indispensable for the development of autoimmune arthritis. Foxp3+ regulatory T cells (Tregs) are crucial for maintaining peripheral tolerance. Although the arthritis development observed in the K/BxN model have been shown to be dependent on the functions of autoreactive T and B cells, less is known about the in vivo roles of Tregs in the pathology of K/BxN mice. We found that K/BxN mice contained a higher percentage of Tregs than their BxN littermates and these Tregs were anergic and efficiently suppressed the proliferation of naï ve CD4+ T cells and cytokine production by effector CD4+ T cells in vitro. To determine the in vivo roles of Tregs in the pathogenesis of K/BxN mice, we crossed K/BxN mice with scurfy mice, which bear a loss-of-function mutation on Foxp3 gene, leading to failure of Treg development. These Treg-deficient scurfy K/BxN (K/BxNsf) mice exhibited significant acceleration of both the onset and progression of arthritis, and this coincided with increases in extrafollicular helper T cells, follicular hepler T cells, and germinal centers. Surprisingly, the K/BxNsf mice exhibited an abnormal accumulation of mature plasma cells in the spleen and a corresponding loss of bone marrow plasma cells. These effects were due to unresponsiveness of plasma cells to the bone marrow homing chemokine CXCL12, despite normal expression of the chemokine receptor CXCR4. Importantly, the K/BxNsf splenic plasma cells were long-lived and less susceptible to the cytotoxic action of cyclophosphamide. They also expressed less FcγRIIb and were less apoptotic in response to autoantigen-autoantibody immune complexes. This suggests Tregs contrtol plasma cell susceptibility to cell death induced by engagement of FcγRIIb with immune complexes. Direct cytotoxic effects of Tregs also contribute to the death of plasma cells. Thus, our results revealed that Tregs suppress the emergence of long-lived splenic plasma cells by affecting plasma cell-autonomous mechanisms as well as T cell help, thereby avoiding persistence of humoral autoimmunity. These studies highlight that IL-21 signals are essential for the initiation of autoimmune responses, which can be tuned by Foxp3+ Tregs, in the K/BxN arthritis model. Therefore, relative activities of these two factors may shape the outcome of immune responses between autoimmunity and tolerance.