The resistance of in vivo selected tumor cells of malignant phenotype to the cytolytic action of activated peritoneal macrophages in vitro]

Using radioisotope cytolytic 3H-thymidine release assay the sensitivity of low-malignant spontaneously in vitro transformed hamster embryo cells (STHE strain) and its in vivo selected malignant variants (STHE-LM-4, STHE-LM-8, STHE-75/18) as well as Rous sarcoma virus (RSV) (Schmidt-Ruppin strain) tumorigenic transformants (HET-SR-1, TU-SR) were tested to cytolysis by peritoneal Syrian hamster macrophages (MP) activated in vitro with levan, LPS, MDP and PMA. It has been shown that the only parental cells STHE strain (non-selected in vivo) were sensitive to cytolysis by activated MP. All in vivo selected malignant variants of STHE cell sublines as well as tumorigenic RSV-SR-transformants were resistant to cytolysis by activated MP. Thioglycollate-elicited but not activated MP did not destroy any tumour target cells.