Spontaneous release of Fc gamma receptors from normal human peripheral blood lymphocytes in vitro.

Certain normal human peripheral blood lymphocyte (PBL) membrane receptors for the Fc region of IgG (i.e. Fc gamma receptors) are known to be released spontaneously when incubated at 37 degrees in serum-free medium into the culture supernatant. However, the mechanisms involved in spontaneous Fc gamma receptor release are not clear. In this study we have shown that Fc gamma receptors appear to be released in two stages. Stage 1 occurs within the first hour of incubation, and is probably mediated by limited proteolysis at the cell surface. Stage 2 occurs between 2 hr and 4 hr, and requires active synthesis of Fc gamma receptors. The kinetics of Fc gamma receptor release from activated PBL was found to be slightly different from normal, but no appreciable increase in Fc gamma receptor 'activity' was found in the culture supernatants when compared with supernatants from normal 'resting' cells. It is doubtful whether these mechanisms operate in vivo since spontaneous release of Fc gamma receptors was completely inhibited in the presence of serum. The serum factors that prevent receptor release were found to be confined to two distinct fractions corresponding to those that contain the major serum protease enzyme inhibitors-alpha 2-macroglobulin and alpha 1-trypsin inhibitor.