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AB0041 B-Cell Proliferation and Plasmablast Generation from Naive and Memory B Cells Are Differentially Regulated by Baff, IL-21, and CD40L and Inhibited by the Systemic Lupus Erythematosus Drug Candidate CC-220

Background Systemic lupus erythematosus (SLE) is a B-cell-driven autoimmune disease that leads to long-term chronic inflammation. The B-cell stimulatory cytokine B-cell activating factor (BAFF) and IL-21 are implicated in the development of SLE, and both are elevated in the serum of SLE patients.1,2 B-cell differentiation signals through CD40 and the B-cell receptor (BCR) mediate B-cell differentiation and antibody production. The Celgene clinical candidate CC-220 is being developed for the treatment of SLE. Objectives Determine the requirements for BAFF, IL-2, IL-21, CD40, and BCR in plasmablast differentiation from naive and memory B cells in T-cell contact-dependent and -independent models of plasmablast differentiation and assess the impact of the SLE clinical drug candidate CC-220. Methods Peripheral blood mononuclear cells (PBMC) were isolated from healthy and SLE donors for flow cytometry analysis of B-cell subtypes. For B-cell differentiation experiments, naive and memory B cells were purified from healthy donors and treated with DMSO or CC-220 (1, 10, and 100 nM) 1 hour before culturing 5 days with combinations of IL-2, IL-21, BAFF, CD40L, and anti-IgM. B cells were assessed for proliferation, activation, and plasmablast generation by flow cytometry. IgM and IgG secretion in the culture supernatants were measured by ELISA, and gene expression was measured by qRT-PCR. Results BAFF combined with IL-2 and IL-21 was required to induce proliferation and plasmablast formation from memory B cells, but did not induce naive B-cell differentiation. Naive B-cell differentiation required CD40 signaling. Anti-IgM combined with IL-21 induced extensive cell death. Treatment of naive B cells with BAFF, IL-2, and IL-21 increased naive B-cell basal survival, but could not counter the cell death observed when IL-21 was present in combination with anti-IgM. In all cases, induced proliferation, plasmablast differentiation, and antibody production were inhibited by CC-220. Conclusions The soluble factors IL-2, IL-21, and BAFF in combination were sufficient to induce memory B-cell differentiation and antibody secretion in the absence of CD40L, but were insufficient to induce differentiation of naive B cells. Plasmablast generation from naive B cells required CD40L. Regardless of the stimulus used to induce B-cell proliferation and differentiation, CC-220 had a dominant inhibition effect. This has implications for the potential use of CC-220 in the treatment of B-cell-mediated pathology in SLE, regardless of whether B-cell proliferation and differentiation are occurring in secondary lymphoid organs in contact with T cells or differentiating at sites of inflammation in the presence of soluble factors such as IL-2, BAFF, and IL-21. References Elbirt D, et al. BLyS levels in sera of patients with systemic lupus erythematosus: clinical and serological correlation. IMAJ 2014;16:491. Wang L, et al. Increased interleukin 21 and follicular helper T-like cells and reduced interleukin 10+ B cells in patients with new-onset systemic lupus erythematosus. J Rheumatol 2014;41:1781. Disclosure of Interest Y. Nakayama Employee of: Celgene Corporation, P. Schafer Employee of: Celgene Corporation, J. Kosek Employee of: Celgene Corporation, G. Ringheim Employee of: Celgene Corporation