A heat-stable soluble inhibitor of T cell proliferation was demonstrated in splenic and peritoneal macrophage (Mφ) culture supernatants. Concentrated supernatants were prostaglandin E₂ (PGE₂)-free and yet inhibited proliferation in the mixed lymphocyte reaction (MLR) and mitogen assays. The high mw inhibitory factor was apparently >67 kd, as shown by S-200 Sephacryl chromatography and gel electrophoresis. DEAE-Cellulose chromatography suggested that the pl of the inhibitory factor was < 7.7. lsoelectric focusing revealed that the Mφ-mediated inhibitory activity differed in charge, with a pl of 6.5-7.6 for normal hosts and 4.0-6.0 for tumor bearing hosts (TBH). Normal and TBH Mφ supernatants showed different hydroxylapatite fractionation, with the latter being resistant to proteolytic enzymes but sensitive to neuraminidase. Lectins such as wheat germ agglutinin, concanavalin A, Ricin communi: and Bandeirea simplicifolia were not useful in affinity purification of the high mw inhibitory monokine. Sugar-BSA conjugates suggested that inhibitory activity was vested in a terminal ßl,4 linked galactose. The inhibitory activity was apparently hydrophobic and heat·stable, but heat-stability was lost if supernatants were boiled at an acidic pH. The high mw monokine inhibited the proliferation of P388D₁ and A4A cells, but enhanced the proliferation of BW 5147.3 cells. Time course addition to the MLR revealed that PGE; may be required for inhibitory activity to be manifested early (0 and 24 hr) but not if the high molecular weight (mw) inhibitor was added late (>48 hr). Indomethacin blocked activity of the inhibitory factor early in the MLR using normal host T cells and augmented the proliferation of TBH T cells in the MLR. Both normal and TBH Mo supernatants suppressed the generation of interleukin 2 but with a dose- and time·dependent difference. Cell cycle analysis of mitogen- stimulated cells revealed that TBH M<p supernatants enhanced cell cycle progression at 42 hr, but that both normal and TBH Mφ supernatants suppressed the number of cells in S phase at 66 hr. To determine the phenotype of Mφ that produced the high mw inhibitor, Mφ were modulated with anti-Mac-1, -2, or -3 monoclonal antibodies (mAb) or depleted with mAb plus complement and cultured with or without indomethacin. P388D₁; cells, used as a control, revealed that the anti-Mac mAb were reacting specifically and not via Fc receptors. Culture supernatants added to the MLR showed that the Mφ suppressor phenotype shifted from Mac-3⁺ in the normal host to Mac-2⁺ in the TBH. Production of PGE₂ was susceptible to indomethacin, but in general the relative production of PGE₂ by the Mφ subpopulations remained the same. Depletion of Mac-1⁺ Mφ caused a higher increase in PGE₂; production than did activation, suggesting that Mac-1⁺ Mcp down-regulated PGE; production. In contrast, no Mac-1* Mrp-mediated regulatory function occurred in the TBH. Mac-2⁺ Mφ were the primary producers of PGE; in the TBH, but not in the normal host. Thus, immunosuppression in the TBH was at least partly due to the inability of Mac-1⁺ and/or Mac-3⁺ Mφ to control production of PGE; by Mac-2⁺ Mφ, which then acts in conjunction with the high mw inhibitor to stop T cell proliferation. / Ph. D.