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A tumor-expressed inhibitor of the early but not late complement lytic pathway enhances tumor growth in a rat model of human breast cancer.

Membrane-bound complement inhibitors protect host cells from inadvertent complement attack, and complement inhibitors are often up-regulated on tumors, possibly representing a selective adaptation by tumors to escape elimination by a host antitumor immune response. Relevant in vivo studies using rodent models of human cancer have been hampered by the fact that human complement inhibitors are not effective against rodent complement. Using nude rats and MCF7 cells expressing different rat complement inhibitors, a model of human breast cancer was established to investigate the role of complement and complement inhibitors in tumor progression. Expression of rat CD59, an inhibitor of the terminal cytolytic membrane attack complex of complement, had no effect on the incidence or growth rate of MCF7 tumors. In contrast, expression of rat Crry, an inhibitor of complement activation, dramatically enhanced the tumorigenicity of MCF7 cells. The expression of rat Crry on MCF7 inhibited the in vivo deposition of complement C3 fragments that serve as opsonins for receptors on phagocytes and natural killer cells. These data provide direct in vivo evidence that an inhibitor of complement activation can facilitate tumor growth by modulating C3 deposition. These data indicate an important role for complement opsonization in promoting cell-mediated antitumor immune function, a conclusion further supported by the demonstration that expression of rat Crry, but not rat CD59, on MCF7 cells inhibits rat cell-mediated cytotoxicity in vitro. Rat complement activation on MCF7 tumors was mediated by tumor-reactive antibodies present in the serum of naïve nude rats, but there was also an IgM response to MCF7 tumors, a situation with similarities to some human cancers. These data support a hypothesis that blocking complement inhibitor function on tumor cells will not only enhance monoclonal antibody-mediated immunotherapy but may also be effective at enhancing a normally ineffective humoral immune response in the absence of administered antitumor antibody.