Cytochalasin B (CB), administered i.p. to C57B1/6 mice in a single dose as a suspension in carboxymethylcellulose 2%/Tween 20 1%, inhibits in a dose-dependent and time-dependent manner the ability of spleen cells to respond to allogeneic P815 mastocytoma tumor cells in vitro. Spleen cells from CB-treated animals sensitized to X-irradiated P815 cells in 4-day cultures at a 50:1 responder:stimulator ratio and tested for specific cytotoxicity against 51Cr-labelled P815 target cells showed strong inhibition 3 h after CB treatment at a dose of 50 mg/kg. A dose of 25 mg/kg showed measureable but not statistically significant inhibition at 3 h, whereas 10 mg/kg produced only slight inhibition, and 5 mg/kg and 2 mg/kg were noninhibitory. None of the doses produced significant suppression 19 h or 72 h after CB treatment. Addition to the sensitization cultures of human recombinant interleukin-2 (rhIL-2) at 350 BRMP units/ml completely restored tumor lytic capacity. C57B1/1 mice treated with CB 50 mg/kg, i.p. and challenged i.p. with 3 x 10(7) allogeneic P815 mastocytoma cells showed a brief, time-dependent, statistically significant abrogation of allogeneic responsiveness consistent with transient reversible immunosuppression within 3-12 h following CB treatment. No such inhibition of host allogeneic responsiveness in vivo was observed when CB was administered 24 h prior to, simultaneously with, or 1, 2, or 4 days after tumor challenge. Thus CB at the highest tolerated i.p. dose in vivo causes only a transient inhibition of anti-allo-responsiveness measured in culture, and rhIL-2 used in vitro restores lytic capacity. The anti-allo effect of CB is also seen to be transient directly in vivo since allogeneic tumor outgrowth is permitted for only a brief period following administration of CB. These results indicate that the use of CB in vivo in anti-tumor chemotherapy protocols will not be complicated by profound or prolonged immunosuppressive effects.

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