Effect of released lymphocyte proteins on human lymphocytes in vitro

The effect on lymphocytes and Hep2 target cells of proteins obtained from the medium of human lymphocytes incubated for short times was studied. The supernatants were precipitated and fractionated with ammonium sulfate in three steps. Two protein fractions were found to inhibit protein synthesis in target cells and to increase in vitro release of chromium from prelabelled lymphocytes. The proteins studied may be related to lymphotoxin produced by mitogen-activated lymphocytes. It is assumed that in vitro interactions for short times between different types of lymphocytes may cause the release of proteins. The same proteins play probably a part in the in vivo regulation of the metabolic activity of lymphocytes.