The involvement of complement activation products in promoting tumor growth has not yet been recognized. Here we show that generation of complement C5a in the tumor microenvironment enhanced tumor growth by suppressing the anti-tumor CD8+ T cell-mediated response. This suppression was associated with the recruitment of myeloid-derived suppressor cells (MDSCs) into tumors and augmentation of their T cell-directed suppressive capabilities. Amplification of MDSC suppressive capacity by C5a occurred through regulation of the production of reactive oxygen and nitrogen species. Pharmacological blockade of C5a receptor significantly impaired tumor growth to a degree comparable to the effect produced by the anti-cancer drug Taxol. Thus, this study demonstrates a therapeutic role for complement inhibition in the treatment of cancer.

Our understanding of the biology of the complement system has undergone a drastic metamorphosis since its original discovery. This system, which was traditionally primarily described as a "complement" to humoral immunity, is now perceived as a central constituent of innate immunity, defending the host against pathogens, coordinating various events during inflammation, and bridging innate and adaptive immune responses. Complement is an assembly of proteins found in the blood and body fluids and on cell surfaces. Soluble complement components form the proteolytic cascade, whose activation leads to the generation of complement effectors that target various cells involved in the immune response. Membrane-bound receptors and regulators transmit signals from complement effectors to target cells and limit complement activation to the surfaces of pathogens and damaged or activated host cells. The multiple interconnections among complement proteins, immune cells, and mediators provide an excellent mechanism to protect the organism against infections and support the repair of damaged tissues. However, disturbances in this "defense machinery" contribute to the pathogenesis of various diseases. The role of complement in various inflammatory disorders is multifaceted; for example, the activation of complement can significantly contribute to inflammation-mediated tissue damage, whereas inherited or acquired complement deficiencies highly favor the development of autoimmunity.

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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/TNFα induction and lack of C3 and C5a receptor stimulation attenuated nuclear factor–κB/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.

The impact of complement on cancer metastasis has not been well studied. In this report, we demonstrate in a preclinical mouse model of breast cancer that the complement anaphylatoxin C5a receptor (C5aR)