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

Abstract

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 anti-C5 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.

Carboxypeptidase B2 and carboxypeptidase N in the crosstalk between coagulation, thrombosis, inflammation, and innate immunity

“…The CPB2 generated will then be able to inactivate C3a and C5a. This hypothesis is in agreement with the data from the HUS model, in which deficiency of CPB2 has a greater effect than deficiency of CPN . In humans, the importance of CPB2 in regulating C5a is shown by the association of the long‐acting CPB2 form with a reduced risk of developing severe rheumatoid arthritis .…”

“…No phenotype has been observed in these animals without a challenge, suggesting that there is no requirement for any plasma basic carboxypeptidase during reproduction and development. When challenged with CVF, 50% of the Cpb2 À/À /Cpn À/ À mice died, in comparison with complete survival of Cpb2 À/À , Cpn À/À and WT mice [52]. Thirty minutes after administration of CVF, the clinical health score showed that WT mice were unaffected, that there was little effect on Cpb2 À/À mice, and that Cpn À/À mice were almost as badly affected as Cpb2 À/À /Cpn À/À mice.…”

“…When challenged with CVF, 50% of the Cpb2 − / − / Cpn − / − mice died, in comparison with complete survival of Cpb2 − / − , Cpn − / − and WT mice . Thirty minutes after administration of CVF, the clinical health score showed that WT mice were unaffected, that there was little effect on Cpb2 − / − mice, and that Cpn − / − mice were almost as badly affected as Cpb2 − / − / Cpn − / − mice.…”

“…Properties of Cpb2 À/À /Cpn À/À mice possessing neither plasma basic carboxypeptidase Mice that are deficient in both CPB2 and CPN (Cpb2 À/À / Cpn À/À mice) are viable and are born to compound heterozygous animals (Cpb2 +/À /Cpn +/À ) in the expected Mendelian ratio [52]. Both genders of Cpb2 À/À /Cpn À/À mice are fertile, and the females carry the pups to term, giving birth to normal-sized litters.…”

Basic weapons to degrade C3a and C5a

“…The work of Morser et al . described in this issue of JTH is a major step forwards in that direction.…”