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Protein A from Staphylococcus aureus is known to stimulate human lymphocytes. Using ³H-thymidine incorporation, virus plaque assay and induction of cytotoxic T lymphocytes (CTL), this study showed that soluble or insoluble protein A stimulated different lymphocyte subpopulations. Soluble protein A is highly mitogenic to T lymphocytes. In both ³H-thymidine incorporation and virus plaque assay, its maximum stimulation was as high as the stimulation by the nonspecific mitogens phytohemagglutinin and concanavalin A, and a higher CTL response than that induced by phytohemagglutinin or concanavalin A was induced. Mitogenic activity to B lymphocytes was negligible. S. aureus (Cowan I strain) is itself considered to be an insoluble form of protein A and is 3–4 times more mitogenic to B lymphocytes than pokeweed mitogen without any increase in virus plaque-forming cells. No mitogenicity was noted to T lymphocytes. Sepharose CL-4B-protein A, also known as insoluble protein A, stimulated both T and B lymphocytes effectively, but its mitogenicity to T lymphocytes was considered to be due to the soluble protein A released from the Sepharose CL-4B beads.

Killer cells with natural killer (NK) cell-like target selectivity were generated when C3H or C57BL/6 mouse spleen cells were cultured in vitro. Compared with NK cells, the in vitro-generated killer cells with NK-like reactivity were slightly adherent to nylon wool fibers, a little more sensitive to anti-Thyl.2 antibody plus complement and almost entirely resistant to cytotoxic treatment with anti-asialo GM₁ antiserum which is selectively cytotoxic to NK cells. The culture of NK-depleted spleen cells yielded a lower degree of NK-like cytotoxicity, thus it seems likely that NK cells changed to NK-like killer cells in vitro. Competitive inhibition studies suggested that the in vitro-generated NK-like cells were different from alloantigen-reactive killer T cells. The above results, therefore, might suggest that the in vitro-induced NK-like cells do not belong to any known subset of killer cells.

SUMMARY With strictly selected controls natural killer cell activity was evaluated in 10 untreated patients with systemic lupus erythematosus. Natural killer levels of the patients were significantly lower than those of the age-and sex-matched normal controls. Natural killer levels, however, did not correlate with disease activity.

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