The in vitro discrimination of rat immune and natural cytotoxicity by serum choice.

Natural in vitro cytotoxicity of non-immune lymphoid cells against a variety of transformed and normal iso- and allogeneic target cells has been described in a number of species (for review see Herberman, Djeu, Kay, Ortaldo, Riccardi, Bonnard, Holden, Fagnani, Santoni & Puccetti, 1979). This background cytotoxicity is a hindrance to the estimation of immune lymphocyte cytolysis (Herberman & Oldham, 1975). Other workers have demonstrated that the choice of serum in which effector (EC) or target cells (TC) are prepared or tested may greatly influence the degree of killing (Irie, Irie & Morton, 1974; Zielske & Golub, 1976; Akira & Takasugi, 1977; Ortaldo, Bonnard & Herberman, 1977). It was found that high levels of rat normal effector cell (NEC) cytotoxicity against normal rat fibroblasts observed in the presence of foetal calf serum (FCS) were markedly reduced by the inclusion of normal rat serum (NRS). This effect on NEC activity was consistently noted in eleven batches of FCS and eight batches of NRS, whereas, normal human serum (NHuS) gave variable results. Decreased TC lysis was proportional to the concentration of NRS, was independent of heat inactivation and was primarily due to an effect on the EC population. The effects of FCS and NRS were not affected by prior absorption on TC or EC nor was NEC activity related to a proliferative response of EC during the 18-48 h microcytotoxicity assay employed in these studies. Since the inclusion of NRS did not markedly diminish the levels of immune effector cell cytotoxicity we have been able consistently to demonstrate that the use of 10% NRS in long-term rat cytotoxicity assays provides a means of dissociating the measurement of immune and normal lymphoid cell cytotoxicity.