Systemic lupus erythematosus is a complex autoimmune disease characterized by dysregulated interactions between autoreactive T and B lymphocytes and the development of anti-nuclear Abs. The recently described pleiotropic cytokine IL-21 has been shown to regulate B cell differentiation and function. IL-21 is produced by activated T lymphocytes and its interactions with IL-21R are required for isotype switching and differentiation of B cells into Ab-secreting cells. In this report, we studied the impact of blocking IL-21 on disease in the lupus-prone MRL-Faslpr mouse model. Mice treated for 10 wk with IL-21R.Fc fusion protein had reduced proteinuria, fewer IgG glomerular deposits, no glomerular basement membrane thickening, reduced levels of circulating dsDNA autoantibodies and total sera IgG1 and IgG2a, and reduced skin lesions and lymphadenopathy, compared with control mice. Also, treatment with IL-21R.Fc resulted in a reduced number of splenic T lymphocytes and altered splenic B lymphocyte ex vivo function. Our data show for the first time that IL-21 has a pathogenic role in the MRL-Faslpr lupus model by impacting B cell function and regulating the production of pathogenic autoantibodies. From a clinical standpoint, these results suggest that blocking IL-21 in systemic lupus erythematosus patients may represent a promising novel therapeutic approach.

MRL/MpJ-Faslpr/lpr/J (MRLlpr) mice develop lupus-like disease manifestations in an IL-21–dependent manner. IL-21 is a pleio-tropic cytokine that can influence the activation, differentiation, and expansion of B and T cell effector subsets. Notably, auto-reactive CD4+ T and B cells spontaneously accumulate in MRLlpr mice and mediate disease pathogenesis. We sought to identify the particular lymphocyte effector subsets regulated by IL-21 in the context of systemic autoimmunity and, thus, generated MRLlpr mice deficient in IL-21R (MRLlpr.IL-21R−/−). Lymphadenopathy and splenomegaly, which are characteristic traits of the MRLlpr model were significantly reduced in the absence of IL-21R, suggesting that immune activation was likewise decreased. Indeed, spontaneous germinal center formation and plasma cell accumulation were absent in IL-21R–deficient MRLlpr mice. Correspondingly, we observed a significant reduction in autoantibody titers. Activated CD4+ CD44+ CD62Llo T cells also failed to accumulate, and CD4+ Th cell differentiation was impaired, as evidenced by a significant reduction in CD4+ T cells that produced the pronephritogenic cytokine IFN-γ. T extrafollicular helper cells are a recently described subset of activated CD4+ T cells that function as the primary inducers of autoantibody production in MRLlpr mice. Importantly, we demonstrated that T extrafollicular helper cells are dependent on IL-21R for their generation. Together, our data highlighted the novel observation that IL-21 is a critical regulator of multiple pathogenic B and T cell effector subsets in MRLlpr mice.

CSF-1, the major regulator of macrophage (Mφ) development, has three biologically active isoforms: a membrane-spanning, cell surface glycoprotein, a secreted glycoprotein, and a secreted proteoglycan. We hypothesized that there are shared and unique roles of individual CSF-1 isoforms during renal inflammation. To test this, we evaluated transgenic mice only expressing the cell surface or precursors of the secreted CSF-1 isoforms for Mφ accumulation, activation, and Mφ-mediated tubular epithelial cell (TEC) apoptosis during unilateral ureteral obstruction. The only difference between secreted proteoglycan and secreted glycoprotein CSF-1 isoforms is the presence (proteoglycan) or absence (glycoprotein) of an 18-kDa chondroitin sulfate glycosaminoglycan. We report that 1) cell surface CSF-1 isoform is sufficient to restore Mφ accumulation, activation, and TEC apoptosis to wild-type levels and is substantially more effective than the secreted CSF-1 isoforms; 2) the chondroitin sulfate glycosaminoglycan facilitates Mφ accumulation, activation, and TEC apoptosis; 3) increasing the level of secreted proteoglycan CSF-1 in serum amplifies renal inflammation; and 4) cell-cell contact is required for Mφ to up-regulate CSF-1-dependent expression of IFN-γ. Taken together, we have identified central roles for the cell surface CSF-1 and the chondroitin sulfate chain on secreted proteoglycan CSF-1 during renal inflammation.

Objectives. To characterize the in vitro binding and effector function properties of CD20-directed small modular immunopharmaceutical (SMIP) 2LM20-4, and to compare its in vivo B-cell depletion activity with the mutated 2LM20-4 P331S [no in vitro complement-dependent cytotoxicity (CDC)] and rituximab in cynomolgus monkeys.Methods. Direct binding is examined in flow cytometry, confocal microscopy, scatchard and lipid raft assays. Effector function assays include CDC and Fc-mediated cellular toxicity. In the 6-month-long in vivo B-cell depletion study, single i.v. dosages of 1 or 10 mg/kg of anti-CD20 proteins were administered to monkeys and B-cell counts were monitored in peripheral blood, bone marrow and lymph nodes.Results. 2LM20-4 has lower saturation binding to human primary B cells and recruits fewer CD20 molecules into lipid rafts compared with rituximab; however, it induces higher in vitro CDC. In competitive binding, 2LM20-4 only partially displaces rituximab, suggesting that it binds to a fraction of CD20 molecules within certain locations of the plasma membrane as compared with rituximab. In monkeys, 2LM20-4 had more sustained B-cell depletion activity than rituximab in peripheral blood and had significantly more profound and sustained activity than 2LM20-4 P331S and rituximab in the lymph nodes.Conclusions. SMIP 2LM20-4, which binds to a fraction of CD20 molecules as compared with rituximab, has more potent in vitro CDC, and more potent and sustained B-cell depletion activity in cynomolgus monkeys. Our work has considerable clinical relevance since it provides novel insights related to the emerging B-cell depletion therapies in autoimmune diseases.

IL-21 is a pleiotropic cytokine that can influence the activation, differentiation and expansion of B and T cells. Lupus prone MRL/MpJ-Faslpr/lpr/J (MRLlpr) mice have been shown to develop renal pathology and autoantibodies in an IL-21 dependent manner. We generated MRLlpr mice deficient in IL-21 receptor (MRLlpr.IL-21R-/-) to assess the impact of loss of IL-21R signaling on the systemic B and T cell lymphoaccumulation and activation that occurs in MRLlpr mice. Consistent with therapeutic blockade of IL-21, MRLlpr.IL21R-/- mice developed reduced proteinuria, renal pathology and serum IgG autoantibody levels. Lymphoaccumulation was significantly reduced in MRLlpr.IL-21R-/- mice, as evidenced by decreased lymphadenopathy and splenomegaly. Importantly, spontaneous germinal center formation and plasma cell accumulation were absent in IL-21R deficient MRLlpr mice. In addition, activated CD4+ CD44+ CD62Llo T cells failed to accumulate. We also demonstrate that T extrafollicular helper cells, which are a subset of activated CD4+ T cells that are reported to be the primary inducers of antibody production in MRLlpr mice, require IL-21R for their generation. T helper cell differentiation was also found to be impacted by loss of IL-21R, as IFN-γ producing CD4+ T cells were significantly reduced in MRLlpr.IL21R-/- mice. Together, our data highlight that IL-21 promotes multiple pathogenic B and T cell effector responses in MRLlpr mice.

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