Mitogenic Effect of Cytoplasmic Membranes and a Cytoplasmic Fraction of Staphylococcus aureus L‐Forms on Human Peripheral Blood Lymphocytes

The cytoplasmic membranes and a cytoplasmic fraction of Staphylococcus aureus L‐forms increased the incorporation of [³H] thymidine by human lymphocytes in the presence of fetal bovine serum. Both fractions stimulated cord blood lymphocytes as well as adult peripheral lymphocytes, suggesting the possibility that the observed effect was not due to an antigen‐specific reaction, but to an immunologically nonspecific action. The membrane mitogen(s) was resistant to trypsin, although it was partially solubilized by trypsin treatment. The mitogen (s) could not be extracted with a chloroform‐methanol mixture (2:1, v/v), although the chloroform‐methanol soluble fraction was strongly mitogenic to murine splenocytes. Human serum which was added to the assay system in place of fetal bovine serum definitely suppressed the mitogenic effect of both cytoplasmic membranes and the cytoplasmic fraction, especially the latter.