Evidence that soluble products released by PHA-stimulated human lymphoid cells activate immunosuppressive monocytes

Soluble mediators, lymphokines, released by stimulated lymphoid cells can modify immunological responses in several ways. In this investigation we have examined whether the supernatants of phytohaemagglutinin (PHA)-activated human lymphocytes (active SUPs) contain factors that can suppress proliferative responses of lymphocytes in vitro. The results have shown that crude preparations of peripheral lymphoid cells incubated for 24 h in active SUPs can suppress the responses of cocultured autologous lymphocytes to PPD tuberculin in vitro. Their suppressive activity was not abolished by mitomycin treatment. Some reduction of phytomitogen responses was also noted. Maximal suppressive activity was obtained within 24 h of incubation in active SUPs and it could not be induced in cell preparations depleted of monocytes-macrophages. Similar results were obtained by treating lymphoid cells with lipopolysaccharide from Escherichia coli, which is a known activator of monocytes. These results thus show that lymphokines released by stimulated lymphoid cells can activate monocytes-macrophages in such a way that they become immunosuppressive.