Protein A from Staphylococcus aureus (SpA) is known to bind specifically the Fc portion of IgG from several species. In the present paper, SpA was demonstrated to be a potent mitogen for T and non-T lymphocyte populations from both human adult periperal blood and cord blood. SpA was equivalent in mitogenic activity for T lymphocytes as compared to phytohemagglutinin (PHA), and was a much stronger mitogen than concanavalin A.

Experiments separating non-T lymphocytes into B lymphocytes (Fc-) and L lymphocytes (Fc+) by rosetting with IgG-coated sheep erythrocytes demonstrated that SpA can stimulate the proliferation of B lymphocytes but not L lymphocytes.

The addition of mitomycin-treated T lymphocytes to the culture of non-T lymphocytes greatly enhanced the non-T lymphocyte response to SpA, and vice versa. In contrast, enhanced response with either mitomycin-treated monocyte and T lymphocyte combinations or mitomycin-treated monocyte and non-T lymphocyte combinations was not observed. It is suggested that T and non-T lymphocyte synergism is needed for the maximal response to SpA.

Discontinuous bovine serum albumin density gradient fractionation of T lymphocytes demonstrated a population of low density T lymphocytes which responded to SpA better than to PHA, and a population of high density T lymphocytes which responded to PHA better than to SpA. These results indicate that some of SpA-responsive T lymphocytes in human peripheral blood appear to be distinct from a population of T lymphocytes responding to PHA.

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