Early induction of immune resistance against leukemia in mice after lethal irradiation followed by syngeneic bone marrow transplantation and injection of syngeneic leukocytes

BALB/cxDBA/2Wf F₁(CD2F₁) mice were lethally irradiated and reconstituted with syngeneic bone marrow cells untreated or treated with 75 µg/ml of mafosfamide. One day after bone marrow transplantation some groups of mice were injected with syngeneic splenocytes, peripheral blood leukocytes, or thymocytes. Seven days after marrow grafting the anti-L1210 leukemia immunization of mice, consisting of four i.p. injections of 106 L1210-Maf cells (L1210 cells treated in vitro with mafosfamide for Inhibition of their growth in vivo), was started. Strong resistance against leukemia could be obtained only in mice receiving splenocytes or peripheral blood leukocytes, not in mice injected with thymocytes or in those not receiving any cells. In vitro elimination of various subpopulations from among the splen-ocytes before their injection into the mice made it possible to deduce which are necessary for early induction of antitumor resistance after bone marrow transplantation in mice. These cells are: Thy 1.2^‑, Ig^‑, AsGM 1^‑, Mac 1⁺, l-A^d+/^‑, are adherent and nonsusceptible to carrageenan toxicity.