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Tumor growth inhibition by protein A and non-protein A containing Staphylococcus aureus in a mouse mammary carcinoma model.

Reinfusion of tumor-bearer plasma after absorption with killed Staphylococcus aureus strain Cowan I may be followed by inhibition or even acute necrosis of animal and human tumors. The effect has been attributed to protein A produced in large amounts by this staphylococcus. We have examined the effect upon the growth of a transplanted GR mouse mammary tumor of intraperitoneal inoculation of three strains of S. aureus characterized by being either protein A-rich or protein A-free. A significant tumor growth inhibition was found with all three strains of S. aureus. Serum levels of IgG1, IgG2 and IgM were found to be substantially increased. Crossed immunoelectrophoresis revealed increased numbers and titres of precipitins against staphylococcal antigens. It is concluded that staphylococcal moieties other than protein A may be involved in the tumor growth inhibition. The possibility and role of complement activation by protein A-like molecules through the alternative F(ab)2 reactivity is discussed.