(209) - IL-6 Blockage by Anti-Interleukin-6 Receptor Antibody Suppresses Plasma Cells during Development of Alloantibody Responses

Abstract

We previously reported that IL-6 blockage by anti-IL6R antibody (mMR16-1) resulted in attenuation of donor-specific antibody (DSA) responses in a mouse model of allo-sensitization. However, the mechanism(s) responsible for antibody suppression is poorly understood. We recently conducted series of studies by examining the peripheral T and B cells to explore the DSA suppression mechanism. C57BL/6 mice were sensitized with skin allografts from a HLA.A2 transgenic mouse, and treated with intraperitoneal injections of mMR16-1 or control antibody. Alloantibody responses were monitored weekly by measurement of serum anti-HLA.A2 antibodies. Cellular kinetics of B subsets, including B1a (CD5+ B220+), B1b (CD11b+CD5-B220+) and follicular B2 (CD5-CD11b-B220+) cells were analyzed in FACS. IgG producing plasma cells were quantitated in an ELISpot assay. mMR16-1 significantly reduced DSA IgM (treated vs. control, MFI: 12.7+2.3 vs. 22.9+3.2 at Day 14, p=0.00022), and IgG (MFI: 99.41+35.31 vs. 300.6+53.04 at Day 28, p=6.621E-05) responses. FACS analysis of the splenic B cells showed that there was a transient reduction of B1a cells at the early responses (p<0.01 vs. control) in the treatment group while no significant difference was found in the B2 cell population between the treatment and control mice. IgG ELISpot assay demonstrated a significant reduction in IgG generating plasma cells in the spleens of the treatment mice (p<0.01 vs. control). Immune fluorescent microscopy showed that the IgG-secreting plasma cells were CD138+ localized in the marginal zones of the splenic lymphoid nodes. The data indicate that anti-IL6R antibody may affect the final stage of B cell maturation into plasma cells, or suppression of antibody forming plasma cells during the development of de novo alloantibody responses. Thus, one of the mechanisms by which mMR16-1 attenuates alloantibody responses is consistent with a blockage of a described IL-6 activity as a B/plasma cell growth factor.