The effect of human monocytes and macrophages on lymphocyte proliferation.

Human monocytes suppressed both the phytohaemagglutin (PHA) or antigen-induced lymphocyte proliferative response, when the monocyte: lymphocyte ratio was increased or when the monocytes were stimulated with zymosan or endotoxin. The effect of monocytes on autologous lymphocyte proliferation was compared with that of macrophages obtained by culturing monocytes in vitro for 7 days. The lymphocyte proliferative responses were increased in the presence of macrophages, however, neither increasing their number nor stimulation by zymosan or endotoxin altered the autologous lymphocyte proliferative response to PHA or purified protein derivative (PPD). The PGE2 concentration in the medium of both the cultured monocytes or macrophages activated by zymosan and endotoxin rose markedly without a corresponding suppressor effect on lymphocyte proliferation in the presence of macrophages in the culture. Thus it seems that while PGE2 is a useful marker of mononuclear phagocyte activation, other molecular species are of importance in determining the lymphocyte proliferative response to mitogens and antigens.