Synergistic interaction of a cytokine produced by embryonic fibroblasts and a lymphokine contained in Con A-stimulated spleen cell culture for murine macrophage differentiation: a model experiment using cell line cells, M1

The modulating activity of the culture supernatant of Con A-stimulated murine spleen cells for macrophages was investigated by using M-1 cells, which could differentiate into macrophage-like cells (referred to as M+1 hereafter), cocultured in a conditioned medium (CM) containing a differentiation factor (DF) obtained from the secondary culture of murine embryonic fibroblasts. DF induced Ia antigens on M-1 cells at a high rate in parallel with the appearance of Fc-receptor (FcR)-dependent phagocytic activity for erythrocytes coated with an antibody (EA). In contrast, Con A-sup alone had no modulating effect on M-1 cells. However, the Con A-sup stimulates synergistically M-1 cells with DF. Thus, coculture of M-1 cells with Con A-sup and DF generates M++1 cells which possess higher phagocytic activity than M+1 cells. These cells also exhibited stronger accessory cell activity than M+1 cells when tested for their promoting effect on IL-2 production by Sephadex G-10-passed spleen cells. The accessory cell activity of M++1 cells was further enhanced by culture with lymphocytes in the presence of indomethacin while that of M+1 cells did not change. These findings suggest that M-1 cells probably acquire potentiating, as well as inhibitory activity at the same time when cultured with DF and Con A-sup. The functional maturation caused by Con A-sup seemed to be associated with the expression of a receptor for a lymphokine, termed phagocytosis-augmenting factor (PAF) which is present in the Con A-sup. Such a receptor appeared to be common to macrophage lineage, since PAF in Con A-sup was absorbed out with splenic adherent cells and peritoneal exudate cells (PEC) in addition to M+1 cells, but not with nonadherent splenic lymphocytes or lymphoid cell line cells, such as EL-4 and L-1210. This fact suggests that PAF is different from interferon-gamma (IFN) which is known to modulate the function of lymphocytes. Inability of PAF to bind Cibacron Blue-Sepharose, unlike IFN, supports this notion. The molecular weight of PAF is approximately 2-3 X 10(4). Thus, the present studies suggest the requirement of at least two signals for the full maturation of macrophages, a cytokine represented by DF and a lymphokine, by PAF.