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Tumor immunity to a transplantable diethylnitrosamine-induced hepatoma in inbred guinea pigs has been found to be immunologically specific and cell mediated. We have investigated this cellular immunity using a quantitative, reproducible, and simple assay based on the ability of leucocytes to inhibit the incorporation of tritiated thymidine (TdR(3)H) by tumor cells. Tumor cell suspensions were obtained from the ascites form or tissue culture monolayers of the hepatoma. Cells from the peritoneal exudate of immunized guinea pigs inhibited tritiated thymidine uptake by tumor target cells to a significantly greater degree than cells from the peritoneal exudate of unimmunized animals. Immune lymph node, peripheral blood, and spleen leucocytes were not inhibitory. The assay was sufficiently sensitive to detect relatively weak tumor immunity. The in vitro inhibition was correlated directly with the presence of delayed hypersensitivity and/or inhibition of tumor cell growth in local passive transfer studies. Irradiation of peritoneal exudate cells (3000 R) blocked their inhibitory effects on tumor cells. Fractionation of the peritoneal exudate cells by centrifugation in zones of bovine serum albumin of different density also revealed the lymphocytes to be responsible for the specific inhibitory effects whereas macrophages inhibited in an immunologically nonspecific fashion.

SPECIFIC INHIBITION OF TUMOR CELL DNA SYNTHESIS In Vitro BY LYMPHOCYTES FROM PERITONEAL EXUDATE OF IMMUNIZED SYNGENEIC GUINEA PIGS