Staphylococcus aureus protein A induces the production of interferon-alpha in human lymphocytes and interferon-alpha/beta in mouse spleen cells.

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Abstract
Human and mouse lymphocytes produced IFN-alpha and IFN-alpha/beta, respectively, when incubated with highly purified Staphylococcus aureus protein A (SPA). Maximum IFN induction occurred when lymphocytes were incubated with SPA at 250 to 500 micrograms/ml for 24 hr. The SPA induced-IFN were identified by neutralization of antiviral activity with specific antisera. A commercial preparation of SPA was found to induce both HuIFN-alpha and HuIFN-gamma, with the latter comprising over 90% of the antiviral activity. The SPA preparation that induced IFN-gamma was shown by SDS-polyacrylamide gel electrophoresis and an enzyme-linked immunosorbent assay to be contaminated with 5% S. aureus enterotoxin A (SEA), a potent T lymphocyte mitogen and IFN-gamma inducer. Preincubation of the commercial SPA with a specific antiserum to SEA blocked the induction of IFN-gamma and had no effect on IFN-alpha production. SEA is a very potent IFN-gamma inducer, effective at concentrations of 1 ng/ml or less. Thus, very minor contamination of SPA by SEA or other IFN-gamma inducers could be responsible for previous reports that SPA induces IFN-gamma in lymphocytes or that SPA behaves as a T cell mitogen.