Melphalan-induced appearance of potent antitumor immune reactivity in tumor bearer lymphocytes co-expressing the Lyt 2 and the L3T4 antigens.

We have previously shown that Sephadex G-10-adherent spleen cells from mice bearing a large MOPC-315 tumor can suppress the in vitro generation of a primary anti-MOPC-315 cytotoxic response. Here we show that following low dose melphalan (L-phenylalanine mustard; L-PAM) therapy of such tumor bearing mice their Sephadex G-10-adherent spleen cells no longer suppressed but actually brought about the generation of enhanced antitumor cytotoxicity when added to the immunization culture of normal spleen cells and MOPC-315 tumor cells. This immunopotentiating activity of the Sephadex G-10-adherent spleen cells from L-PAM treated MOPC-315 tumor bearers was attributed to T-cells which co-express the Lyt 2 and the L3T4 antigens based on results of experiments employing negative selection. Specifically, depletion of Lyt 2+ cells or of L3T4+ cells abolished the ability of the Sephadex G-10-adherent splenic cell population from L-PAM treated MOPC-315 tumor bearers to bring about the generation of enhanced antitumor cytotoxicity when added to the immunization culture of normal spleen cells. Moreover, the immunopotentiating activity was not restored when a population of Sephadex G-10-adherent spleen cells depleted of Lyt 2+ cells was admixed with a population of Sephadex G-10-adherent spleen cells depleted of L3T4+ cells. In light of the unusual phenotype of the immunopotentiating cells in the spleens of L-PAM treated MOPC-315 tumor bearing mice (i.e. Lyt 2+ L3T4+), and since the vast majority of thymocytes in normal adult BALB/c mice co-express the Lyt 2 and the L3T4 antigens, we evaluated the effect of low dose L-PAM therapy on the antitumor immune reactivity of thymocytes from MOPC-315 tumor bearing mice. A low dose of L-PAM was found to render thymocytes from MOPC-315 tumor bearers, but not from normal mice, capable of bringing about the generation of enhanced lytic activity when added to the immunization culture of normal spleen cells and MOPC-315 tumor cells. At the same time, the thymocytes from L-PAM treated MOPC-315 tumor bearers were unable to develop an antitumor cytotoxic response of their own when immunized in vitro in the absence of normal spleen cells. The possibility that the Lyt 2+ L3T4+ immunopotentiating cells in the spleens of L-PAM treated MOPC-315 tumor bearers represent immature cells that have been induced by the chemotherapy to migrate from the thymus into the spleen is discussed.