During B lymphopoiesis, Igk recombination requires pre-B cell receptor (pre-BCR) expression and escape from interleukin 7 receptor (IL-7R) signaling. By activating the transcription factor STAT5, IL-7R signaling maintains proliferation and represses Igk germline transcription by unknown mechanisms. We demonstrate that STAT5 tetramer bound the Igk intronic enhancer (Eκi), leading to recruitment of the histone methyltransferase Ezh2. Ezh2 marked H3K27me3 throughout Jκ to Cκ. In the absence of Ezh2, IL-7 failed to repress Igk germline transcription. H3K27me3 modifications were lost after termination of IL-7R–STAT5 signaling and E2A bound Eκi, resulting in acquisition of H3K4me1 and H4Ac. Genome-wide analyses revealed a STAT5 tetrameric binding motif associated with transcriptional repression. These data demonstrate how IL-7R signaling represses Igk germline transcription and provide a general model for STAT5-mediated epigenetic transcriptional repression.

Depletion of B cells in rheumatoid arthritis is therapeutically efficacious. Yet, the mechanism by which B cells participate in the inflammatory process is unclear. We previously demonstrated that Ag-specific B cells have two important functions in the development of arthritis in a murine model of rheumatoid arthritis, proteoglycan (PG)-induced arthritis (PGIA). PG-specific B cells function as autoantibody-producing cells and as APCs that activate PG-specific T cells. Moreover, the costimulatory molecule CD86 is up-regulated on PG-specific B cells in response to stimulation with PG. To address the requirement for CD80/CD86 expression on B cells in the development of PGIA, we generated mixed bone marrow chimeras in which CD80/CD86 is specifically deleted on B cells and not on other APC populations. Chimeras with a specific deficiency in CD80/CD86 expression on B cells are resistant to the induction of PGIA. The concentration of PG-specific autoantibody is similar in mice sufficient or deficient for CD80/86-expressing B cells, which indicates that resistance to PGIA is not due to the suppression of PG-specific autoantibody production. CD80/86-deficient B cells failed to effectively activate PG-specific autoreactive T cells as indicated by the failure of T cells from PG-immunized CD80/86-deficient B cell chimeras to transfer arthritis into SCID mice. In vitro secondary recall responses to PG are also dependent on CD80/86-expressing B cells. These results demonstrate that a CD80/86:CD28 costimulatory interaction between B cells and T cells is required for autoreactive T cell activation and the induction of arthritis but not for B cell autoantibody production.

IL-17 is the hallmark cytokine for the newly identified subset of T helper cells, Th17. Th17 cells are important instigators of inflammation in several models of autoimmune disease; in particular, collagen induced arthritis (CIA) and experimental autoimmune encephalomyelitis (EAE), which were previously characterized as Th1-mediated diseases. Although high levels of IFN-γ are secreted in CIA and EAE, disease is exacerbated in IFN-γ or IFN-γ receptor deficient mice due to the ability of IFN-γ to suppress IL-17 secretion. However, in proteoglycan-induced arthritis (PGIA), severe arthritis is dependent on the production of IFN-γ. We were therefore interested in determining the role of IL-17 in PGIA. We assessed the progression of arthritis in IL-17-deficient (IL-17−/−) mice and found the onset and severity of arthritis equivalent in wildtype (WT) and IL-17−/− mice. Despite evidence that IL-17 is involved in neutrophil recruitment, synovial fluid from arthritic joints showed a comparable proportion of Gr1+ neutrophils in WT and IL-17−/− mice. IL-17 is also implicated in bone destruction in autoimmune arthritis, however histological analysis of the arthritic joints from WT and IL-17−/− mice revealed a similar extent of joint cellularity, cartilage destruction and bone erosion despite significantly reduced RANKL expression. There were only subtle differences between WT and IL-17−/− in pro-inflammatory cytokine expression, T cell proliferation and autoanibody production. These data demonstrate that IL-17 is not absolutely required for autoimmune arthritis and production of other proinflammatory mediators are sufficient to compensate for the loss of IL-17 in PGIA.

B cells have been implicated in the pathogenesis of rheumatoid arthritis (RA) since the discovery of RA as an autoimmune disease. There is renewed interest in B cells in RA based on the clinical efficacy of B cell depletion therapy in RA patients. Although, reduced titers of rheumatoid factor and anti-cyclic citrullinated peptide Abs are recorded, the mechanisms that convey clinical improvement are incompletely understood. In the proteoglycan-induced arthritis (PGIA) mouse model of RA, we reported that Ag-specific B cells have two important functions in the development of arthritis. PG-specific B cells are required as autoantibody-producing cells as well as Ag-specific APCs. Herein we report on the effects of anti-CD20 mAb B cell depletion therapy in PGIA. Mice were sensitized to PG and treated with anti-CD20 Ab at a time when PG-specific autoantibodies and T cell activation were evident but before acute arthritis. In mice treated with anti-CD20 mAb, development of arthritis was significantly reduced in comparison to control mAb-treated mice. B cell depletion reduced the PG-specific autoantibody response. Furthermore, there was a significant reduction in the PG-specific CD4+ T cell recall response as well as significantly fewer PG-specific CD4+ T cells producing IFN-γ and IL-17, but not IL-4. The reduction in PG-specific T cells was confirmed by the inability of CD4+ T cells from B cell-depleted mice to adoptively transfer disease into SCID mice. Overall, B cell depletion during PGIA significantly reduced disease and inhibited both autoreactive B cell and T cell function.

The contribution of the proinflammatory cytokines IFN-γ and IL-17 to the pathogenesis of experimental arthritis is controversial. In proteoglycan (PG)-induced arthritis (PGIA), severe arthritis is dependent on the production of IFN-γ, whereas IL-17 is dispensable. In collagen-induced arthritis and Ag-induced arthritis, although high levels of IFN-γ are secreted, disease is exacerbated in IFN-γ or IFN-γ receptor-deficient mice due to the ability of IFN-γ to suppress IL-17 expression. In the current study, we investigated the effect of IFN-γ on the IL-17 response and its consequences in PGIA. In PG-immunized IFN-γ−/− mice, despite reduction in arthritis, the PG-specific CD4+ T cell IL-17 response was significantly increased. Elevated IL-17 contributed to development of arthritis, as disease in IFN-γ/IL-17−/− was significantly reduced in comparison with either IFN-γ−/− or IL-17−/− mice. A contribution of IFN-γ and IL-17 to the development of arthritis was also identified in T-bet−/− mice. PG-specific CD4+ T cells from T-bet−/− mice produced reduced IFN-γ and elevated concentrations of IL-17. Both IFN-γ and IL-17 contribute to arthritis, as T-bet−/− mice lacking IL-17 (T-bet/IL-17−/−) were resistant, whereas wild-type, T-bet−/−, and IL-17−/− mice were susceptible to PGIA. T cell proliferation and autoantibody production did not correlate with development of disease; however, expression of cytokines and chemokines in joint tissues demonstrate that IFN-γ and IL-17 cooperatively contribute to inflammation. These results demonstrate that both IFN-γ and IL-17 have the potential to induce PGIA, but it is the strength of the IFN-γ response that regulates the contribution of each of these Th effector cytokines to disease.