Competition between foetal tissue and macrophage-dependent natural tumour resistance.

Prolonged interaction in vitro between C. parvum-induced adherent predominantly phagocytic rat peritoneal cells and syngeneic or xenogeneic tumour targets consistently produces marked cytotoxicity. In the presence of irradiated foetal liver cells, expression of cytotoxicity is blocked in a dose-dependent manner. The ability of liver cells to compete with tumour targets is rapidly lost after birth. Irradiated liver cells from adult donors showed no such competition with tumour cells. The in vivo growth in ascites form of rat fibrosarcoma cells of low immunogenicity is significantly enhanced by irradiated foetal liver cells administered locally shortly before or on the day of tumour-cell challenge. The findings may provide an indication as to the nature of the structures recognized as non-self by mononuclear phagocytes.