Immunoregulatory role of in vitro differentiated macrophages on human natural killer (NK)-cell activity

Immunoregulatory effects of human macrophages on natural killer (NK) activity were studied. Monocytes were isolated by adherence to plastic, after leukapheresis of normal blood donors, and cultured for 1 to 14 days. In vitro-differentiated (5–7 days) human macrophages consistently and significantly ( P < 0.01) augmented NK activity of fresh autologous or allogeneic PBMNC. During culture, these macrophages also developed increased antitumor cytostatic activity. The optimal time for both the expression of cytostatic activity and up-regulation of NK activity was 5–7 days in culture. In contrast, 12- to 14-day macrophages significantly suppressed NK activity and had less cytostatic activity. Macrophages in culture demonstrated shifts in Leu-M3 +HLA-DR + phenotype from the mean of 60% ± 11 (SD) in fresh monocytes to 90% ± 5 between Days 5 and 7 in culture and then down to 10% ± 5 in 14-day cultures. The activity of NK (CD56 +CD3 −) cells, purified by Percoll gradient centrifugation and flow cytometry, was up-regulated directly by in vitro-differentiated macrophages at low macrophage to NK cell ratios, and this up-regulation was not dependent on T lymphocytes or other accessory cells. The modulation of NK activity by differentiated macrophages was not MHC-restricted and depended on the viability and cellular integrity of macrophages. Sonicated macrophages could no longer up-regulate NK activity. This study shows that antitumor effects mediated by human in vitro differentiated LeuM3 +HLA-DR + macrophages may simultaneously involve more than one mechanism, namely direct cytostasis of tumor cells and activation of NK cells.