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New antigenic specificities, not detectable on parental cells and transmissible after the withdrawal of the drug treatment, have been induced in mouse lymphomas. Studies were conducted of proliferative stimulation of syngeneic lymphocytes and the generation of cytotoxic lymphocytes (CL's) in a mixed lymphocyte-tumor cell culture system by 5-(3,3-dimethyl-1-triazeno)imidazole-4-carboxamide (DTIC)-induced antigens in L1210 and EL4 leukemia sublines. The DTIC-induced antigens were observed to stimulate [3H]thymidine uptake by normal and primed syngeneic lymphocytes and to generate specific CL's to DTIC-altered cells. The specificity of the in vitro immune reactivity was demonstrated. Characteristics of lymphocyte triggering, including the optimal ratio of stimulating cells to responding cells, the kinetics of CL activation, and the quantitation of CL activity, were also evaluated. DTIC antigens on leukemic cells can activate syngeneic lymphocytes and can act as target antigens in cell-mediated immunity. The experimental data support the transplantation antigen-like nature of DTIC-induced antigens.

A panel of murine monoclonal antibodies (MAbs) was produced by fusing NS-0 myeloma cells with spleen cells of a BALB/c X DBA/2 F1 mouse hyperimmunized against a highly immunogenic subline of the L1210 leukemia obtained by in vivo treatment of the L1210 parental line with the antitumor drug 5-(3,3'-dimethyl-1-triazeno)imidazole-4-carboxamide (DTIC). Among the 52 MAbs produced 16 (anti-D) were specifically cytotoxic in a complement-dependent cytotoxicity assay for the drug-altered subline and the others (anti-L) also cross-reacted with the L1210 parental leukemia. Six anti-D and three anti-L MAbs were selected for detailed studies of tissue specificity. In quantitative absorption experiments the antigens defined by these antibodies could not be detected on cells from normal mouse tissues (lung, liver, kidney, heart, spleen, and thymus). The reactivities of both anti-D and anti-L MAbs against a panel of L1210/DTIC sublines obtained at different times were assayed. The results showed that the antigenic specificities defined by anti-L MAbs were expressed on almost every L1210/DTIC subline while the anti-D MAbs detected antigenic structures specific for the L1210/DTIC used for the immunization. None of the MAbs tested cross-reacted with the L5178Y lymphoma or with its DTIC-altered sublines. The failure of anti-D MAbs to cross-react with cells from other L1210/DTIC sublines supports the hypothesis that the immunological alterations induced by the DTIC treatment are the consequence of mutagenic activity of the drug. On the other hand the presence of anti-L antigens on the cells of every L1210 subline indicates that the DTIC alteration is not accompanied by a loss of the tumor-associated antigen from the L1210 leukemia.

New antigenic properties of experimental lymphomas have been reported previously following in vivo treatment with antitumor agents. 5-(3,3-Dimethyl-1-triazeno)imidazole-4-carboxamide (DIC) induced new antigenic characteristics on L1210 and L5178Y lymphomas, that were previously investigated in studies in animals compatible with the original untreated parental tumors. Here the L1210/DIC and L5178Y/DIC susceptibility to the cytotoxic effects of allogeneic and xenogeneic lymphocytes and sera obtained from animals sensitized to DBA/2 histocompatibility antigens were studied. The original and the DIC tumors showed the same sensitivity to anti-DBA/2 cellular and humoral cytotoxicity. The immune response electied in allogeneic mice by the original and DIC sublines was evaluated by in vitro cell-mediated and humoral cytotoxic assay. Beyond the immune response to histocompatibility antigens, a specific, anti-DIC-antigen immunoreaction was not found. Inhibition assay of the cell-mediated cytotoxicity and absorption of the humoral cytotoxicity demonstrated that DIC-induced antigens are not reciprocally related in cell-surface concentration to the natural DBA/2 histocompatibility antigens associated with tumor cells of DIC lines. An experiment was conducted in which specific activity against the DIC-treated L5178Y/DIC cells was observed with anti-L5178Y/DIC rabbit immune serum absorbed with the parental L5178Y lymphoma. This finding provides additional support to previous studies indicating that treatment with DIC induced new antigens on the lymphoma cells.

Eight sublines of the radiation-induced lymphoma S-1033 of C57BL/10 (hereafter called B10) origin were established by exposing the cells in vivo to eight antineoplastic agents for a number of transplant generations. The parental and drug-treated sublines were tested for immunogenic properties, i.e., the ability to elicit allograft reactions in the host of origin and in congenic-resistant mice differing for the S-D or K-I-S regions of the H-2 complex. Lymphoma S-1033 and all drug-treated sublines except one were found to be essentially nonimmunogenic for B10 mice. The S-DIC subline, when exposed for 8 to 12 transplant generations to dimethyltriazenoimidazolecarboxamide, became immunogenic for syngeneic B10 mice, as judged from prolongation of survival time. Large i.v. inocula (10(7) cells) of S-1033 and of the drug-treated sublines, with the possible exception of the cyclophosphamide-treated and dimethyltriazenoimideazolecarboxamide-treated lymphomas, were more effectively rejected by K-I-S- than by S-D-incompatible mice. Dilution escape (i.e., tumor rejection after challenge with large inocula, and lethal tumor growth after injection of small inocula of lymphoma cells in allogeneic recipients) occurred in K-I-S-incompatible mice that were inoculated with S-1033 and three drug-treated (5-fluorouracil, cyclophosphamide, and pyrazocarboxamideamino) sublines. No dilution escape occurred with dimethyltriazenoimidazolecarboxamide or bischloroethylnitrosourea sublines. These data favor the hypothesis that various types of immunogenic changes of neoplastic cells may occur in tumor-bearing hosts following treatment with antineoplastic agents in vivo.