Active and passive re-expression of Fcγ receptors on human lymphocytes

Modulation of receptors for IgG (FcγR) on human lymphocytes was induced by the interaction with erythrocyte-IgG antibody (EA_G) complexes followed by incubation at 37°C. The re-expression of FcγR could be achieved by two independent processes, (a) Active synthesis, susceptible to inhibition by puromycin or cycloheximide was shown to peak 4 to 6 h after removal of EA_G complexes; it required addition of at least 2% fetal calf serum. (b) Insertion of soluble FcγR into the membrane of modulated lymphocytes was shown to occur within 20 min of contact between cells and FcγR-containing supernatants; it was not altered by protein synthesis inhibitors. FcγR-like material, spontaneously released by unstimulated peripheral blood lymphocytes or by polymorphonuclear cells, was taken up by modulated lymphocytes. This soluble material was fully absorbed on polymerized human IgG; it was non- dialyzable, thermolabile (56 °C, 30 min) and partially destroyed by freezing and thawing; it was recovered as two broad peaks from chromatography on polyacrylamide gel; it was shown to bind to both T and non-T FcγR-bearing lymphocytes capable of forming EA_G rosettes before modulation; and it could be inserted into allogeneic lymphocytes. These results demonstrate that the FcγR structure bears two active sites, one binding to the Fcγ and the other to the surface of EA_G rosette-forming cells. Rapid release of soluble FcγR from the cells and their possible insertion into the membrane may have important implications with respect to the biological functions associated with these receptors.