Background. The receptors C5aR1 and C5aR2, belonging to the complement system, can contribute to autoimmune diseases when activated by the ligand C5a. The extent to which C5aR2 contributes to this is not yet fully understood. C5aR2 is thought to have both pro- and anti-inflammatory effects. Using reporter mice, it has been shown that B2 cells express C5aR2 in a steady state, but not C5aR1. Recently, we found that C5aR2-deficient B-2 cells showed altered maturation and reduced activity and differentiation compared to wild-type cells. Since both receptors and B-2 cells influence the onset of autoimmune diseases, two in vivo models were ultilized to investigate the influence of C5aR2-deficiency on Epidermolysis bullosa acquisita (EBA) progression. This autoimmune skin disease occurs due to IgG autoantibodies being formed against type VII collagen (COL7). The effector phase of the disease was mimicked by passive transfer of anti-COL7 IgG autoantibodies. Active immunization with von Willebrand factor A-like domain 2 of COL7 mimicked both the afferent and effector phases of the disease. While C5aR2-deficient mice were only partly protected in passive EBA, they were almost completely protected from disease development in the active model. In addition, serum and flow cytometric analyses revealed attenuated expression of inflammatory markers and reduced activation and differentiation of B-2 cells. Based on these observations, it is tempting to speculate that C5aR2 signaling on B-2 cells has a pro-inflammatory effect and promotes the onset of autoimmune diseases. 

Objectives. 1. Create B-2 cell-specific C5aR2 knockout. mice 2. Investigate the phenotype during active EBA in cell-specific knockout mice. 3. Examine expression of C5aR2 during active EBA using tdTomato-C5aR2 reporter mice. 

Work program. Since former investigations used global C5aR2 knockouts, it is also possible that other cell types are responsible for the observed phenotype. In order to investigate the specific influence of C5aR2 expressed by B-2 cells on the pathogenesis of EBA, a targeted knockout of C5aR2 in this cell population will be established. For this purpose, the Cre/loxP system will be used to generate mice in which C5aR2 is switched off exclusively in B-2 cells. In further experiments, the course of EBA will be induced in this model system and the resulting phenotype will be compared with both wild-type and global knockout mice. In addition, serological analyses will be performed to determine the concentration of pro-inflammatory markers in serum and to investigate a possible correlation between the absence of C5aR2 in B-2 cells and the severity of the disease. Additionally, using the C5aR2 reporter mice, we will examine the expression of C5aR2 during active EBA