REGULATORY SUBSTANCES PRODUCED BY LYMPHOCYTES .3. EVIDENCE THAT LYMPHOTOXIN AND PROLIFERATION INHIBITORY FACTOR ARE IDENTICAL AND DIFFERENT FROM INHIBITOR OF DNA-SYNTHESIS

Abstract

L cell [mouse fibroblast] mutant lines, which were considerably more resistant to rat lymphotoxin (LT) than the original cell line, were obtained by periodic additions of LT, partially purified from sensitized lymph node cell culture supernatants by DEAE-cellulose chromatography and Sephadex gel filtration. Addition of actinomycin D to cultures of these mutant cells abrogated resistance to LT, suggesting that resistance was not due to a loss of LT receptors, but probably to increased activity of a repair mechanism. These mutant lines were more resistant to the proliferation inhibitory effect of LT in low concentration and to that of diluted culture supernatants of lymph node cells stimulated with antigen (ovalbumin) than the original cell line, but they remained as sensitive to inhibitor of DNA synthesis as the original line. The mutant lines also remained fully sensitive to [guinea pig] complement-dependent lysis by antibody and antibody-dependent cellular cytotoxicity, but showed increased resistance to the killing effect of rat lymph node cells sensitized with original L cells in vivo. The partially purified and characterized lymphotoxic substance may be an important mediator of thymus cell-mediated cytotoxicity.