Background. The complement fragment C5a is involved in many pathways of autoimmune diseases through activation of its cognate receptors C5aR1 and C5aR2. (Immunobiol 217:1067). Whereas the role of C5aR1 in autoimmune diseases is well appreciated, the contribution of C5aR2 is still enigmatic. Initially considered as a decoy receptor negatively regulating the functions of C5aR1 (Biochemistry 42:9406), C5aR2 was recently shown to exert pro- and anti-inflammatory properties (Trends Biochem Sci Immunol doi.org./10.1016/j.tibs.2020.04.004). Often C5aR1 and C5aR2 are co-expressed. However, using floxed tdTomato-C5aR2-reporter-mice, we found that B cells and NK cells express C5aR2 but not C5aR1. In NK cells C5aR2 regulates IFN-Gamma expression independent of C5aR1 (J Immunol 199:3234). Currently, no data exist regarding the role of C5aR2 in B cells. In preliminary experiments, we observed altered maturation of B cells in C5ar2-/- mice (Fig. A8-1). While B cell numbers did not differ between wildtype and C5ar2-/- mice, IgM and IgD expression was decreased in C5ar2-/- animals, suggesting a pivotal role of C5aR2 in B cell maturation. Further, B cells isolated from the spleen of C5aR2-deficient mice produced higher amounts of IgG2c and IgG2b antibodies after T-independent stimulation with LPS for 24h, whereas the production of other IgG subclasses was comparable with that of wildtype mice.

Objectives. Define the impact of C5aR2 activation for the (i) maturation of B cells; and (ii) regulation of plasma cell induction and autoantibody production.

Work program. First, we will assess the role of C5aR2 for B cell maturation and differentiation under homeostatic conditions in B cells from bone marrow, spleen and lymph nodes of wt and C5ar2-/- mice. Further, we will determine proliferation, maturation/activation, and differentiation into antibody-producing plasma cells or memory cells in vitro in response to T-dependent and T-independent B cell activation. Also, we will define the signaling pathways downstream of C5aR2 in B/plasma cells. Finally, we will determine the importance of C5aR2 activation for IgG autoantibody formation in vivo in using a collagen Type VII immunization-induced model of skin blistering disease (J Immunol 191:2978). For this purpose, we will use C5ar2fl/flCD19Cretg/tg mice, lacking C5aR2 in B cells, backcrossed to the EBA-susceptible B6.SJL-H2s background. We will monitor C5aR2 expression in lymph nodes and measure clinical disease manifestation, IgG autoantibody formation, plasma cell differentiation and germinal center reaction/affinity maturation. Herein, we will define the role of C5aR2 activation as a novel checkpoint for B cell activation and a potential target in autoimmune disease.