Melphalan-induced enhancement of tumor cell immunostimulatory capacity as a mechanism for the appearance of potent antitumor immunity in the spleen of mice bearing a large metastatic MOPC-315 tumor

Summary Exposure of MOPC-315 cells from the primary tumor nodule to a low concentration (0.5 nmol/ml) of melphalan (L-phenylalanine mustard; L-PAM) rendered the tumor cells capable of bringing about the generation of a potent primary antitumor cytotoxic response. Accordingly, the level of antitumor cytotoxicity generated by normal spleen cells immunized in vitro with L-PAM-treated tumor cells was at least five-fold greater than the level generated in response to untreated tumor cells. The marked superiority of L-PAM-treated tumor cells over untreated tumor cells in bringing about the generation of antitumor cytotoxicity was evident over a wide range of responder to stimulator cell ratios. The higher level of antitumor cytotoxicity exhibited by normal spleen cells immunized with L-PAM-treated tumor cells as compared with untreated tumor cells was not merely the result of direct drug-mediated tumoricidal activity, thereby reducing the number of tumor cells present which can act as cold target cell inhibitors during the ⁵¹Cr release assay. This is apparent from the observation that the level of antitumor cytotoxicity generated in response to a given percentage of stimulator tumor cells pretreated with 0.5 nmol L-PAM/ml, a drug concentration associated with retention of 60% tumor cell proliferative capacity, is substantially greater than that generated in response to less than half that percentage of untreated stimulator tumor cells. Moreover, stimulator tumor cells exposed to a fully antiproliferative concentration of L-PAM brought about the generation of a higher level of antitumor cytotoxicity than stimulator tumor cells exposed to mitomycin C at a concentration which inhibited the proliferation of the tumor cells to the same extent as the L-PAM. A low concentration of L-PAM which was effective in rendering isolated tumor cells from the primary tumor nodule capable of bringing about the generation of antitumor cytotoxicity was also effective in inducing the appearance of potent antitumor immune potential in tumor bearer splenic cells containing metastatic tumor cells. Thus, metastatic tumor cells within the spleen of tumor-bearing mice may have the potential to act as in situ stimulators for the generation of potent antitumor immunity which brings about the eradication of a large, metastatic tumorigenic load remaining after clearance of the drug from the circulation.Supported by United States Public Health Service Research Grants CA-30088 and CA-35761 from the National Cancer InstituteIn partial fulfillment of the requirements of the Graduate College for the Doctor of Philosophy degree for RCBSBE was visiting Fulbright Professor from the Department of Human Microbiology, Sackler School of Medicine, Tel Aviv University, Israel, and was supported by the Council for International Exchange of Scholars