Natural cytotoxic reactivity of rat lymphocytes against syngeneic Gross virus-induced lymphoma.

Lymphoid cells from many normal W/Fu rats reacted in a 51Cr release cytotoxicity assay against (C58NT)D, a syngeneic Gross leukemia virus-induced tumor. The reactivity was maximal in young rats at 5-8 weeks of age and rapidly declined thereafter. Within reactive rats, the cytotoxicity was widely distributed among the various lymphoid organs. Since immunization of W/Fu rats with (C58NT)D was shown to elicit specific cell-mediated cytotoxic reactivity, studies were done to compare the characteristics of the natural reactivity with those of the immune reactivity. The specificity of both types of reactivity was analyzed in detail by an inhibition assay. The natural and the immune reactivities appeared directed against antigens associated with rat endogenous type-C viruses. The major differences between the natural reactivity and immune reactivity were the nature of the effector cells. Whereas immune reactivity was T-cell dependent, normal reactivity was not affected by treatment with antisera against T cells plus complement. Natural effector cells were not adherent and did not have macrophage properties. The active cells also did not have receptors for Ig or complement. The absence of detectable cell-surface markers on the natural effector cells was seen in studies of natural cytotoxic reactivity of mice, and it is proposed that the natural cytotoxicity in both systems is mediated by a unique subpopulation of lymphoid cells, tentatively designated "N" cells.