Inhibition of rat mammary tumor growth by purified protein A--a potential anti-tumor agent.

Intravenous inoculations of purified Protein A of Staphylococcus aureus Cowan I cause significant (p less than 0.01) regression of di-methyl-benz-anthracene (DMBA)-induced mammary adenocarcinomas in Sprague-Dawley rats. Direct tumor cell counts of treated tumors showed fewer (p less than 0.005) viable tumor cells than did tumors from untreated controls. Plasma immunoglobulin G concentration showed a significant (p less than 0.05) increase compared to that of controls. However, the concentration of immune complexes and percentages of T-rosettes did not change. Peripheral blood mononuclear cells (PBMNC) from treated animals showed increased cytotoxicity (p less than 0.005) compared to that of controls. Plasma of treated animals potentiated PBMNC cytotoxicity (p less than 0.05) and showed increased antibody and complement-mediated cytotoxicity (p less than 0.025). The exact mechanism of protein A-induced potentiation of anti-tumor immune reactivities leading to tumoricidal response is not known. However, our data are suggestive of the involvement of both cellular and humoral immunity of the host in the tumor regressive phenomenon.