Biphasic expression and cytokine regulation of the complement C3 in heart allograft

Abstract Locally synthesized complement may play a more important role in regulating antigen-specific immune response than circulating complement; however, complement production in transplanted heart remains obscure. In the current study, we investigated local production of the complement C3 in mouse cardiac allografts and examined the relationship between C3 production and cytokine expression. The cardiac grafts of allogeneic (C57BL/6–BALB/c mice) and syngeneic mouse models were subjected to histopathological and immunohistochemical analyses for C3 expression on days 0–6 after operation. mRNA and protein expression of C3, IL-2, IFN-γ in transplanted heart and primary culture cardiomyocytes were analyzed. Our results demonstrated that C3 mRNA exhibited biphasic patterns in transplanted heart. The first expression phase correlated with ischemical reperfusion injury over 2 days post-transplant. The second peak of C3 deposition was found only in allografts on day 5, concurrent with the secretion of IL-2 and IFN-γ accompanied by severe diffuse leukocyte infiltration. Furthermore, in vitro studies showed that IL-2 and IFN-γ enhanced C3 mRNA and protein production in cardiocytes. Together, these observations suggest that both ischemical reperfusion injury and the subsequent acute rejection result in elevated cardiocyte secretion of C3, and the second phase of expression appears to be regulated by cytokines secreted by the infiltrating cells.