Activation of complement by spontaneous leukemic cells of AKR mice

Complement activation via the alternative pathway by tumor cells was tested by rosette formation of human erythrocytes (HuE). Test cells were treated with C4‐deficient guinea‐pig serum (C4DS) in the presence of Mg ++ and absence of Ca ++ , following which 10–30% of lymphoma cells and 10–17% of thymoma cells of spontaneous leukemia in AKR mice formed rosettes. The incidence of rosette formation did not change significantly after transplantation of these cells to other non‐leukemic AKR mice or after in vitro cultivation. Following treatment with fresh C4DS, however, normal AKR mouse thymocytes as well as lymph‐node cells contained only 3–4% HuE‐positive cells. These findings suggest that the ability of non‐specific activation of complement (NAC) may be increased when normal AKR mouse cells become transformed into leukemic cells and/or that the leukemic cells arise from thymus cells which possess NAC ability. This interpretation was further confirmed by the finding that a higher proportion of leukemic cells than of normal thymocytes were lysed by mouse complement in Mg ++ ‐EGTA. The fact that NAC‐positive AKR thymoma cells were lysed effectively by homologous C3H/He plasma agrees with earlier findings that in leukemic AKR mice supplemented with C5 a temporary regression of leukemia occurs.