Immunosuppression by human plasma alpha 1-acid glycoprotein: importance of the carbohydrate moiety

Human plasma alpha 1-acid glycoprotein (orosomucoid) and derivatives of this protein produced by sequential enzymatic cleavage of the glycosidic residue (sialic acid, galactose, N-acetylglucosamine, mannose) were tested for the ability suppress a number of immune functions of mouse spleen cells in vitro at physiological concentrations. alpha 1-Acid glycoprotein and especially the agalacto/asialo derivative suppressed (i) the mitogenic responses to concanavalin A, lipopolysaccharide, and alloantigens, (ii) antibody responses to sheep erythrocytes, and (iii) induction of cell-mediated lympholysis against allogeneic target cells. The native protein and its derivative did not inhibit the proliferation of EL-4 lymphoma cells and did not suppress the lysis of YAC-1 lymphoma cells by natural killer cells. The enhanced potency of the agalacto/asialo derivative indicates that the nature of the carbohydrate moiety exposed on the protein determines the effectiveness of the glycoprotein and its deglycosylated derivatives may function to regulate immune responses in various physiological and pathological conditions.