Ontogeny of 'macrophage' function. IV. Newborn mouse macrophages strongly suppress tumour cell growth and readily acquire cytolytic activity in comparison with adult macrophages.

Peritoneal macrophages (PM) were prepared as adherent peritoneal exudate cells from newborn and adult mice injected i.p. with thioglycollate medium 4 days previously. In comparison with adult PM, newborn PM exerted a high suppressive effect on the in vitro growth of syngeneic and allogeneic tumour cells, though they did not manifest cytolytic activity. The high suppressive activity of newborn PM was maintained until about 2 weeks of age, and then declined rapidly until about 3 weeks of age toward the level of adult PM. The treatment with a high dose of LPS enhanced the suppressive effect of both newborn and adult PM, while a low dose of LPS was effective only for newborn PM. Necessary minimum dose of LPS to make PM significantly cytolytic was lower for newborn PM than for adult PM. Addition of a low concentration of LK to the culture for activating adult PM with LPS resulted in the augmentation of cytolytic activity and the reduction of necessary minimum dose of LPS. Activation of newborn PM by LPS was not affected by the addition of such a low concentration of LK. On the other hand, newborn PM were rapidly activated by LPS as compared with adult PM. LK accelerated the activation of adult PM by LPS. The activity of newborn PM to bind to tumour cells was higher than that of adult PM. These results seem to indicate that newborn PM are activated to some extent inherently or by some intrinsic agents.