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Although the complement system has been implicated in liver regeneration after toxic injury and partial hepatectomy, the mechanism or mechanisms through which it participates in these processes remains ill-defined. In this study, we demonstrate that complement activation products (C3a, C3b/iC3b) are generated in the serum of experimental mice after CCl(4) injection and that complement activation is required for normal liver regeneration. Decomplementation by cobra venom factor resulted in impaired entry of hepatocytes into S phase of the cell cycle. In addition, livers from C3-deficient (C3(-/-)) mice showed similarly impaired proliferation of hepatocytes, along with delayed kinetics of both hepatocyte hyperplasia and removal of injured liver parenchyma. Restoration of hepatocyte proliferative capabilities of C3(-/-) mice through C3a reconstitution, as well as the impaired regeneration of C3a receptor-deficient mice, demonstrated that C3a promotes liver cell proliferation via the C3a receptor. These findings, together with data showing two waves of complement activation, indicate that C3 activation is a pivotal mechanism for liver regeneration after CCl(4) injury, which fulfills multiple roles; C3a generated early after toxin injection is relevant during the priming of hepatocytes, whereas C3 activation at later times after CCl(4) treatment contributes to the clearance of injured tissue.

Complement has been implicated in liver repair after toxic injury. Here, we demonstrate that complement components are essential for liver regeneration, and mediate their effect by interacting with key signaling networks that promote hepatocyte proliferation. C3- or C5-deficient mice exhibited high mortality, parenchymal damage, and impaired liver regeneration after partial hepatectomy. Mice with dual C3 and C5 deficiency had a more exacerbated phenotype that was reversed by combined C3a and C5a reconstitution. Interception of C5a receptor signaling resulted in suppression of IL-6/TNFalpha induction and lack of C3 and C5a receptor stimulation attenuated nuclear factor-kappaB/STAT-3 activation after hepatectomy. These data indicate that C3a and C5a, two potent inflammatory mediators of the innate immune response, contribute essentially to the early priming stages of hepatocyte regeneration.