We examined the relationship of I-A expression by normal murine macrophages to their immunoregulatory role on natural killer cell activity. Macrophages were isolated on the basis of plastic adherence; characterized on the basis of conventional markers such as phagocytic ability, cytoplasmic non-specific esterase activity, surface MAC-1 and F4/80 antigen expression; and then used for functional studies relative to their expression of surface I-A. Two functional macrophage subsets were identified: NK-stimulatory and NK-suppressive subsets. The former function was associated with splenic macrophages, which were predominantly I-A+ as identified with a radioautographic immunolabeling technique; the latter function with peritoneal macrophages which were predominantly I-A-. Loss of macrophage I-A expression in vitro was delayed in the presence of indomethacin and enhanced in the presence of PGE2, indicating that PGE2 down-regulates I-A expression on macrophages. The NK stimulatory function of I-A+ macrophages was attributable to a soluble mediator, identified as IFN-gamma, since the stimulatory ability was abrogated with an anti-IFN-gamma antibody. I-A expression appears to be important for the stimulatory function, since some interference with this function was noted in the presence of anti-I-A antibody. The NK-suppressor function of I-A- macrophages was attributable to the soluble mediator PGE2, since this function was abrogated with indomethacin or anti-PGE2 antibody. These results are relevant to the understanding of normal in vivo immunoregulation by macrophages.

During the subcutaneous growth of a highly metastatic mammary adenocarcinoma line, C3-L4 in C3H/HeJ mice, there was a rapid decline in macrophage I-A expression in vivo. The incidence of I-A+ macrophage subset in the spleen declined from 90% to 10% or less within 5 days of tumor transplantation, with a parallel decline in their absolute number. I-A expression in these cells remained suppressed for a long time until tumors became necrotic and ulcerated. In spite of a low incidence (15-20%) of I-A+ macrophages in the normal peritoneal space, tumor transplantation caused a long-lasting decline in their incidence to one-fourth of the original level. Tumor-associated macrophages were predominantly I-A- throughout the tumor life span. Thus I-A- macrophages dominated in all anatomical compartments in tumor-bearing mice. Macrophage I-A expression was substantially restored (spleen) or stimulated (peritoneal space and tumor) in tumor-transplanted mice subjected to chronic indomethacin therapy in the drinking water, indicating a reversal of prostaglandin-mediated down-regulation of macrophage I-A expression in situ. Concomitantly, this therapy caused regression of the primary tumors and prevented lung metastasis. These results are relevant to the understanding of tumor-host interactions and its exploitation for immunotherapy.