Promotion of murine antitumour activity by prothymosin α treatment: I. Induction of tumoricidal peritoneal cells producing high levels of tumour necrosis factor α

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Abstract
The effect of prothymosin α (ProTα) on the survival of DBA/2 mice inoculated with syngeneic tumour cells was studied. DBA/2 mice inoculated intraperitoneally (i.p.) with 2×105 syngeneic leukaemic L1210 cells developed ascites within 8–12 days and died 10–14 days later. Treatment with ProTα consistently inhibited the development of ascites in 20% of the treated animals and prolonged the survival of 40%–60% of the animals up to 70 days. The most effective treatment schedule of ProTα was 300 ng/mouse given i.p. at 2-day intervals for 3 weeks followed by a rest period of 7 days, prior to tumour cell inoculation. Peritoneal exudate (PE) cells collected from mice treated with the optimal dose of ProTα produced, in the absence of exogenous stimulus, six- to eightfold higher levels of tumour necrosis factor α (TNFα) than PE cells from control mice. Furthermore these cells exhibited cytotoxic activity against several tumour cell lines including the syngeneic L1210, the TNF-insensitive P815 mastocytoma, the human MOLT-4 lymphoblastic leukaemia, as well as the murine TNF-sensitive L929 fibroblast cell line. Kinetic studies revealed that both production of TNFα and tumoricidal activity peaked 7 days after the last injection of ProTα and were maintained at high levels over a period of 1 month. Injections with 150 ng ProTα slightly improved the survival of mice whereas higher (500 ng and 1000 ng) doses of ProTα and a wide range of thymosin α1 doses remained without any effect. PE cells collected from these mice produced extremely low levels of TNFα and exhibited negligible tumoricidal activity. Our data demonstrate that ProTα has a protective effect in vivo against the growth of adoptively transfered tumour cells and suggest that this effect is, at least in part, mediated by ProTα-activated PE cells. These cells were demonstrated to produce high levels of TNFα in vitro and to exhibit activity against both TNF-sensitive and TNF-resistant cell lines.