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MEDIATION OF IMMUNE RESPONSES TO TUMOR ANTIGENS IN VITRO BY IMMUNE RNA *

It was shown that normal nonimmune C3H mouse spleen cells became specifically cytotoxic to chemically-induced syngeneic C3H tumor cells by incubation with xenogeneic I-RNA extracted from the lymphoid organs of specifically immunized guinea pigs. This response was specific for the tumor used to immunize the I-RNA donor. In a totally syngeneic system, we showed that syngeneic I-RNA extracted from the spleens of tumor-bearing rats mediated cytotoxic immune reactions which were directed specifically against the tumor-associated antigens of syngeneic rat tumor target cells. Active antitumor I-RNA synthesis in the lymphoid organs of I-RNA donor animals reached a maximum between days 14 and 21, depending on the route of administration and the nature of the immunizing tumor. Active I-RNA preparations were insensitive to treatment with deoxyribonuclease or pronase, but were inactivated by ribonuclease treatment; thereby indicating that the active moiety was one or more species of RNA. The active fractions of the I-RNA preparations had sedimentation values in sucrose density gradients of 12-16S, and comprised only a small fraction of the total RNA present in the lymphoid cells. Active antitumor I-RNA appeared to be localized in the cytoplasm of sensitized lymphoid cells, rather than in the nucleus. Lymphocytes from normal human donors as well as from cancer patients, when incubated with xenogeneic or allogeneic I-RNA, became specifically cytotoxic for human tumor cells in vitro. Crossreactivity among tumors of the same histologic type was observed, but not crossreactivity with tumors of other histologic types. Xenogeneic I-RNA extracted from the lymphoid organs of donor animals immunized either iwth tumor cells or normal tissues, following incubation with normal allogeneic lymphocytes, mediated cytotoxic immune reactions which were directed both against tumor-associated antigens and normal transplantation antigens. However, when autologous lymphocytes were used as effector cells, only immune reactions directed against tumor-associated antigens were observed. Allogeneic I-RNA extracted from peripheral blood lymphocytes of human cancer patients mediated specific cytotoxic immune reactions that were directed against common tumor-associated antigens shared by human tumors of similar histologic type. I-RNA's directed against "self" normal cell surface antigens appear to be recognized as self by lymphocytes, and immune responses against these self antigens are not elicited. On the other hand, I-RNA's directed against "nonself" tumor-associated antigens induce lymphocytes to effect specific antitumor immune responses. Our data are consistent with the hypothesis that I-RNA is an information-containing ribonucleic acid molecule capable of mediating immune reactions in vitro which are specific for the tumor-associated antigens of the tumor used to immunize the I-RNA donor.