Staphylococcus aureus produces a protein identified as protein A (SpA) with a molecular weight of 41,000 (ref. 1) which binds to the Fc region of many types of mammalian IgG2, activates complement3, blocks opsonins4 and is both chemotactic5 and mitogenic 6,7. SpA has been used for various immunological purposes including removal of immune complexes8, arming effector cells with antibody9, distinguishing between antibody-dependent cell-mediated cytotoxicity (ADCC) and natural killing (NK)10, and exerting anti-tumour activity11. We have now demonstrated that SpA induces the production of γ interferon (IFN-γ) in nonadherent, T lymphocyte-depleted (E-), Fc receptor-bearing (FcR+) human peripheral blood lymphocytes. Our data also suggest that IFN-γ is a potent stimulator of both NK and ADCC activity.
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