The present study was designed to examine whether cyclophosphamide augmented induction of antitumor cells and antitumor resistance in C57BL/6 mice pretreated with mitomycin-C-treated EL4 cells (EL4MMc) plus OK-432, a streptococcal preparation. C57BL/6 mice were pretreated with EL4MMc (107) plus OK-432 (2.5 KE) i.p. twice at 1-week intervals. When the mice received an i.p. injection of cyclophosphamide at 200 mg/kg 2 days before the last treatment, the antitumor activity of their spleen cells and peritoneal exudate cells (PEC) was effectively augmented 7 -8 days after the last treatment. Splenic antitumor activity disappeared 15 days after the last treatment whereas augmented antitumor activity of the PEC was detected even 28 days after the last treatment. This cyclophosphamide effect was dose-dependent and 200 rag/ kg was the most effective among the doses tested. If the EL4MMc plus OK-432 treatment was injected at a s.c. site, it was also effective in combination with cyclophosphamide. The antitumor activity of the PEC from s.c.-pretreated mice, however, was lower than that from i.p.-pretreated mice. Despite the fact that cyclophosphamide effectively augmented induction of antitumor cells in C57BL/6 mice pretreated with EL4MMc plus OK-432, it diminished rather than augmented, under all conditions tested, the ability of the mice to resist a challenge of live EL4 cells. Reduction of antitumor resistance by cyclophosphamide was also observed in an experimental system of a semi-syngeneic host (BDF1) tumor (EL4). These results indicate that augmentation of in vivo induction of certain kinds of antitumor cells does not necessarily result in a beneficial augmentation of the host's ability to resist tumor growth.
have a major role. These eels have been reported to be removed or inactivated following in vitro incubation of a cell population of which they formed a part, following which a cytotoxic activity to spontaneous (non-immunogenic) tumors or tumors of low immunogenicity appears in the population [1,3,5,13,21]. In a similar way, the weakness of in vivo induction of splenic cytotoxic T lymphocytes (CTL) may be due to suppression of the in vivo differentiation of sensitized CTL precursors into effective CTL, and this suppression can be eliminated by the treatment with cyclophosphamide [7,17,22].
A streptococcal preparation, OK-432, augments induction of antitumor resistance when combined with attenuated tumor cells [12,18,23]. This augmented resistance is specific to the tumor used in combination with 23], and is detected in semi-syngeneic [12,18,23] and syngeneic [24] mice.
As Williams et al. [25] stated, an in vitro assay that could evaluate both cytolytic and cytostatic antitumor activities would allow one to detect the effector functions of various classes of lymphocytes and other leukocytes, including macrophages and neutrophils. We therefore used an in vitro assay, based on the growth inhibition of tumor cells in the presence of effector cells, to evaluate in vitro antitumor activities of peritoneal exudate cells (PEC) and spleen cells.
The study presented in this report was designed to examine whether, in C57BL/6 mice pretreated with EL4MMc plus OK-432, cyclophosphamide could augment the induction of antitumor cells and antitumor resistance against a challenge of EL4 thymoma of low immunogenicity.