Four macrophage (Mφ) surface antigens (Ia, Mac-1, -2, and -3) were examined for their association with Mφ regulatory functions. Observations of antigen expression on Mφ derived from normal or tumor-bearing hosts (TBH) showed that changes occurred in the antigen-defined phenotypes of Mφ which evolve during tumor growth. These changes in antigen expression were correlated with notable changes in Mφ immunoregulatory functions. Experiments using only normal host-derived Mφ showed that in the presence of complement (C), monoclonal antibodies (mAb) directed against Mφ could lyse targeted Mφ and that enrichment of the remaining cells provided populations of Mφ that were altered in their regulatory functions. Analysis of mAb-treated Mφ in the absence of C, suggested that the alterations observed in the presence of C were not due to ligand-receptor activation of peritoneal Mφ and that antibodies alone were not altering Mφ viability. When anti-Mac- I, -2, and -3 antibodies, were used to modify accessory cell activity of whole spleen cell (WSC) or splenic adherent cell (SAC) preparations from normal or TBII, differential susceptibilities of the Mφ were noted. Ligand-receptor activation of WSC by anti-Mac-I was observed in normal but not TBH WSC. With C, anti-Mac-I and -3 each reduced normal and TBH WSC proliferation. To evaluate the possible role of different types of SAC in T cell lectin responsiveness, adherent cells were collected and depleted by antibody plus C treatment and added back to normal T cells. Removal of Mac-1⁺ normal host SAC stimulated the supportive accessory function of the remaining SAC. Enhancing accessory cell function diminished after removal of normal host Mac-2⁺ or TBH Mac-1⁺ SAC. In summary, SAC from normal host demonstrated an accessory cell function corresponding to a Mac-1⁻ phenotype, which was either replaced or obscured by the predominance of a Mac-1⁺ phenotype in TBH. Variable Ia antigen expression by Mφ was examined during tumor growth. Tumor growth induced progressive loss of Ia antigen expression on Mφ. TBH splenic Mφ supported Concanavalin A-induced proliferation of syngeneic T cells (Ia antigen-independent) but did not support syngeneic T cell proliferation in the mixed lymphocyte reaction (MLR) (Ia antigen-dependent). Irrespective of tissue source, normal and TBH Mφ differed in their MLR stimulatory capabilities. In general, splenic Mφ preparations were better stimulators of allogeneic T cell blastogenesis in the MLR than thioglycollate-elicited peritoneal Mφ. Expression of Ia antigens by normal but not TBH Mφ were diminished by 24-hr in vitro plating of the peritoneal Mφ. Indomethacin treatment showed Prostaglandin E₂ was not a direct in vitro factor in Ia antigen-mediated reduction of splenic Mφ MLR stimulatory activity. Taken together, this data suggested a loss of Mφ Ia antigen expression, resulting in a decrease in Ia antigen-mediated functional activities during tumor growth. To continue the assessment of Mφ phenotypes and to determine if alterations in Mφ function during tumor growth included changes in the secretion of soluble regulators of T cell activities, anti-Mac-1, -2, and -3 mAb were used to modulate monokine-mediated regulation of T lymphocyte proliferation. The mAb anti-Mac-1, -2, and -3 (plus C) exhibited differential depletion of normal and TBH Mφ. There was a distinct increase in the number of peroxidase-positive Mφ during tumor growth. Peroxidase-positive TBH Mφ were susceptible to C-mediated lysis by anti-Mac-1 and -3 but not anti-Mac-2, whereas no direct relationship was observed among normal host-derived Mφ. Immunofluorescence of mAb-binding showed a decrease in Mac-2⁺ cells in TBH Mφ populations that was accompanied by an increase in Mac-3 expression. Anti-Mac-2 treatment significantly reduced the ability of TBH Mφ to produce a soluble suppressor(s) but did not alter normal host Mφ-derived suppressor production. In contrast, anti-Mac-1 and -3 treatment of normal host Mφ significantly reduced suppressor production but had diminished effects on TBH Mφ. Anti-Ia plus C treatment of splenic or peritoneal Mφ derived from normal or TBH showed that selection of Ia Mφ increased the secretion of PGE and also increased the T cell suppressor activity in Mφ culture supernatants. Collectively, these data suggest that tumor-induced aberrations in immunoregulation can be attributed to differences in Mφ subpopulations which were discriminated by their surface membrane components. / Ph. D.