Carboxypeptidase B2 and N play different roles in regulation of activated complements C3a and C5a in mice

Background: There are two basic carboxypeptidases in plasma. Carboxypeptidase B2 (CPB2) is activated from a circulating zymogen, proCPB2, and carboxypeptidase N (CPN) is constitutively active with both inactivating complement C3a and C5a. Aims: To test the roles of CPB2 and CPN in complement-driven mouse models of cobra venom factor (CVF) challenge and hemolytic-uremic syndrome (HUS). Methods: Cpb2(-/-), Cpn(-/-) and wild-type (WT) mice were compared in an HUS model induced by Shiga toxin and lipopolysaccharide administration and following CVF administration. Results: HUS was exacerbated in Cpb2(-/-) mice more than in Cpn(-/-) mice, compared with WT mice. Cpb2(-/-) mice developed the HUS clinical triad of microangiopathic hemolytic anemia, uremia and thrombocytopenia. Treatment with antiC5 antibody improved survival of both Cpb2(-/-) and Cpn(-/-) mice. In contrast, when challenged acutely with CVF, the reverse phenotype was observed. Cpn(-/-) mice had markedly worse disease than Cpb2(-/-) mice, whereas the WT mice were resistant. Conclusions: CPN and CPB2 play overlapping but non-redundant roles in regulating complement activation in vivo. The constitutively active CPN is key for inactivation of systemic C5a, whereas CPB2 functions as an on-demand supplementary anaphylatoxin inhibitor in inactivating excessive C5a formed locally.