Background/Purpose:

Response Gene to Complement (RGC)-32 is an intracellular protein initially discovered in rat oligodendrocytes in response to complement activation. It plays a role in cell growth and promotes cell cycle activation and Akt phosphorylation. RGC-32 is also a downstream target of TGF-β in fibroblasts and renal proximal tubular cells and plays a role in renal fibrogenesis. In the immune system RGC-32 is expressed by both T and B lymphocyte subsets.  Our prior studies have indicated that RGC-32 promotes Th17 differentiation of mouse CD4 T cells and is highly expressed in human IL-17+ CD4 cells. Whether RGC-32 expression in B cells plays a role their activation, differentiation and development of autoimmunity is not known. To address this question we used WT and RGC-32 KO mice to determine whether lack of RGC-32 impairs B cell differentiation and activation and/or alters autoimmune parameters in the Bm12-into-B6 chronic graft versus host disease (cGVHD) model of lupus.

Methods:

Purified B cells from WT or RGC-32 KO mice were cultured with lps, anti-CD40mAb, IL-21 and IL-6, IL-4 or TGFβ and RGC-32 mRNA and protein expression was determined by flow cytometry and RT-PCR. TLR-dependent and T-dependent B cell differentiation to plasma cells (PC) was induced with lps and with CD40mAb plus IL-4, respectively. The number of CD138^hiB220^lo PC was determined 3 days later. Bm12-into-B6 cGVHD was induced by i.p. injection of 100×10⁶Bm12 splenocytes into WT or RGC-32 KO recipients. B cell parameters of cGVHD including host B cell number and activation, anti-dsDNA Ab production, GC B cell number and proliferation, PC number in spleen and bone marrow (BM), expression of transcription factors IRF4 and Blimp1 were assessed at 2 and 4 weeks by flow cytometry, RT-PCR, ELISA and ELISPOT.

Results:

RGC-32 mRNA was expressed at baseline in B cells and was upregulated by lps, anti-CD40mAb, IL-21 and IL-6. RGC-32KO B cells failed to differentiate normally to PC in vitro as demonstrated by a 2 fold reduction in PC numbers generated after lps and anti-CD40mAb+IL-4 stimulation and impaired upregulation of Prdm1 and IRF4 mRNA. In vivo, mRNA expression of RGC-32 was significantly upregulated in spleen cells from cGVHD mice compared to uninjected WT B6 mice.  RGC-32 upregulation was detected in both B220⁺ B cells and B220⁺ PNA⁺ germinal center (GC) cells. Induction of cGVHD in RGC-32KO hosts resulted in an attenuated autoimmune phenotype as demonstrated by: 1) decreased production of anti-dsDNA autoAb. 2) decreased number and proliferation of GC B cells. 3) decreased number of IgG anti-dsDNA secreting PC in BM and 4) decreased IRF4 and Prdm1 mRNA expression.

Conclusion:

These results suggest that expression of RGC-32 in B cells is critical for optimal GC proliferation, PC differentiation and autoantibody production in a murine model of lupus. These data support the idea that RGC-32 blockade has the potential to attenuate autoimmune parameters of cGVHD and possibly reverse abnormalities in the T and B cell pathways that contribute to lupus pathogenesis. These observations provide a compelling rationale for further investigating the therapeutic potential of RGC-32 blockade in murine and human lupus.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/response-gene-to-complement-32-promotes-plasma-cell-differentiation-and-enhances-lupus-like-chronic-graft-versus-host-disease/