Increased tumorigenicity and resistance to antibody lysis of human colon tumor cells xenografted in congenitally athymic mice.

are data to suggest it is immunological. Prehn and Outzen (14) showed that syngeneic tumors of high immunogenicity grew better in irradiated adult or untreated newborn athymic mice than in control adult athymic mice. These authors suggested growth of immunogenic tumors in nude mice may be due to an overloading of a weak immune system. No evidence of immu nological memory to the tumor antigen could be demonstrated in these mice. Campanile et a!. (3) recently demonstrated a strong transplantation resistance in nude mice to allogeneic lymphomas, In contrast to the studies by Prehn and Outzen (14), this refractoriness to tumor growth was radioresistant. The suggestions that athymic mice exert some immunologi cal or nonimmunological restrictions on a variety of cell types are supported by recent observations in our laboratory that human colon tumor cell lines grown in nude mice were more tumorigenic on subsequent in vivo passages (18). It is possible that, even in the case of tumors that grow readily in nude mice, some selective pressures are exerted by the host which may result in alterations in the phenotype and/or genotype of the cell population. The results of this study confirm our earlier observations that human colon tumor cells grown in nude mice are more tumorigenic on the second nude mouse passage and show that the nude mouse passage cells are more resistant to lysis by antibody and complement than the original cell lines.