C3AR1 and peritonitis: First, we induced MSU-elicited peritonitis in mice deficient in both C3a and C5a receptors (C3ar1-/-C5ar1-/-): significantly fewer neutrophils and monocytes were present in peritoneal lavage fluid from C3ar1-/-C5ar1-/- mice compared with WT controls (Figure 4A), indicating that anaphylatoxin-mediated signaling regulates leukocyte infiltration following MSU administration in vivo.