Prostaglandin E2 production by Mac-2+ macrophages: tumor-induced population shift

Tumor growth induced a shift in the phenotype of macrophages (M luminal diameter) responsible for factor-mediated suppression of allogeneic mixed lymphocyte reactions (MLR), and the suppression by tumor-bearing host (TBH) Mac-2+ M luminal diameter was in part due to production of prostaglandin E2 (PGE2). Thioglycollate-elicited peritoneal M luminal diameter from normal and TBH BALB/c mice were modulated with anti-Mac-1, -2, or -3 monoclonal antibodies (mAb) or depleted with mAb plus complement and cultured in the presence or absence of indomethacin. Culture supernatants derived from mAb plus complement-depleted M luminal diameter were added to the MLR at time of initiation and showed that the suppressor phenotype shifted from Mac-3+ in the normal host to Mac-2+ in the TBH. Mac-1+ M luminal diameter also appeared to be involved in suppression by normal host, but not TBH, M luminal diameter. Loss of MLR suppression (increase in MLR reactivity) correlated with an increase in protein content of the culture supernatants. In an effort to explain both this relationship and the mechanism of MLR suppression, PGE2 levels of culture supernatants were determined by radioimmunoassay. Mac-1+ M luminal diameter were involved in the regulation of PGE2 production in normal hosts, as both activation and depletion caused an increase in PGE2 production. Depletion caused a more dramatic increase in PGE2 production than did activation, suggesting that Mac-1+ M luminal diameter had a dampening effect on PGE2 production. In contrast, no Mac-1+ M luminal diameter-mediated regulatory function occurred in the TBH. Mac-3+ M luminal diameter were involved in the regulation of PGE2 production in both normal and TBH. Mac-2+ M luminal diameter were the primary producers of PGE2 in the TBH, but not in the normal host, as their depletion in the TBH caused a significant loss of PGE2 production. Thus, immunosuppression in the TBH was at least partly due to the inability of Mac-1+ and/or Mac-3+ M luminal diameter to control production of PGE2 by Mac-2+ M luminal diameter.