Studies on the immunosuppressive properties of a pregnancy-associated alpha-macroglobulin.

A pregnancy-associated serum glycoprotein was shown to have an inhibitory effect on several in vitro methods of immunological assessment. This suppressing activity was evident at physiological concentrations and did not appear to be due to cytotoxicity. Transformation, induced by agents often regarded as preferential stimulators of T lymphocytes (concanavalin A, phytohaemagglutinin, allogeneic cells and tuberculin) was significantly depressed by the alpha-globulin. However, this phenomenon was much less evident when lipopolysaccharide or goat anti-human F(ab')2 serum was employed to selectively stimulate B-cells. The glycoprotein also blocked antigen-induced inhibition of leucocyte migration and caused a significant reduction in the number of lymphocytes binding sheep erythrocytes, in the spontaneous rosette test. It is proposed that the non-specific inhibitory activity of the alpha-macroglobulin depends upon some direct effect exerted on the lymphocyte itself and that it is levelled primarily at the cell-mediated immune response.