Carboxypeptidase B2 deficiency reveals opposite effects of complement C3a and C5a in a murine polymicrobial sepsis model

Abstract

Summary Background and Objectives Carboxypeptidase B2 (CPB2) is a basic carboxypeptidase with fibrin and complement C3a and C5a as physiological substrates. We hypothesized that in polymicrobial sepsis, CPB2-deficient mice would have sustained C5a activity, leading to disease exacerbation. Methods Polymicrobial sepsis was induced by cecal ligation and puncture (CLP). Results Contrary to our hypothesis, Cpb2−/− mice had significantly improved survival, with reduced lung edema, less liver and kidney damage, and less disseminated intravascular coagulation. Hepatic proCPB2 was induced by CLP leading to increase in proCPB2 levels. Thrombomodulin present on mesothelium supported thrombin activation of proCPB2. Both WT and Cpb2−/− animals treated with a C5a receptor antagonist had improved survival, demonstrating that C5a was detrimental in this model. Treatment with a fibrinolysis inhibitor, tranexamic acid, caused a decrease in survival in both genotypes, however, the Cpb2−/− animals still retained their survival advantage. Administration of a C3a receptor antagonist exacerbated the disease in both WT and Cpb2−/− mice, and eliminated the survival advantage of Cpb2−/− mice. C5a receptor is expressed in both peritoneal macrophages and neutrophils; in contrast, C3a receptor expression is restricted to peritoneal macrophages, and C3a induced signaling in macrophages but not neutrophils. Conclusions Thus while C5a exacerbates the peritonitis, resulting in a deleterious generalized inflammatory state, C3a activation of peritoneal macrophages may limit the initial infection following CLP, thereby playing a diametrically opposing protective role in this polymicrobial sepsis model.

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

“…A role for CPB2 in protection against sepsis is consistent with data showing that CPB2 levels in the cerebrospinal fluid correlate with the severity of disease and the levels of complement activators such as C3a, iC3b, and C5b‐9 . In a polymicrobial sepsis model, Cpb2 − / − mice were protected and the relevant physiological substrate causing the phenotype was shown to be C3a, although both C5a and fibrin also played a role in the disease . This is consistent with the hypothesis that C3a plays an essential role in stimulating peritoneal macrophages to combat the infection, whereas C5a and fibrin exacerbate the outcome.…”

The immunopathology of sepsis and potential therapeutic targets

“…Compstatin, a synthetic C3 convertase inhibitor, not only inhibited complement activation during E. coli sepsis in baboons, but also attenuated other inflammatory responses, coagu lation activation and multiple organ failure 13 . The receptors for C3a and C5a may have differential roles in sepsis, as indicated by a recent study in which treatment with a C5a receptor antagonist improved survival during polymicrobial sepsis, whereas the administration of a C3a receptor antagonist reduced survival 14 .…”

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

“…Studies using the Cpb2 ‐deficient ( Cpb2 −/− ) mouse confirm that the observed in vitro inactivation of complement C3a and C5a by CPB2 also occurs in vivo . Cpb2 −/− mice developed more severe alveolitis than WT mice upon tracheal instillation of C5a .…”