FC-GAMMA-R-MEDIATED ENDOCYTOSIS AND EXPRESSION OF CELL-SURFACE FC-GAMMA-RIIB1 AND FC-GAMMA-RIIB2 BY MOUSE BONE-MARROW CULTURE-DERIVED PROGENITOR MAST-CELLS

Mouse IL-3-dependent, bone marrow culture-derived mast cells (BMMC) bind IgG immune complexes through Fc receptors for IgG (FcgammaR) but express minimal FcgammaRIII on their surfaces. BMMC do not degranulate appreciably when their FcgammaR are perturbed with the rat anti-mouse FcgammaRII/III mAb 2.4G2 and F(ab')2 mouse anti-rat IgG (MAR). In contrast, after their FcgammaR were cross-linked with mAb 2.4G2 and (NaI)I-125-labeled MAR at 37-degrees-C, BMMC rapidly internalized the complex. To identify the FcgammaR species expressed on the surface of BMMC and therefore implicated in the endocytic response, two rabbit antipeptide antisera were raised, one against a sequence common to the cytoplasmic regions of FcgammaRIIb1 and FcgammaRIIb2 and the other to a unique cytoplasmic region of FcgammaRIIb1. When FcgammaR were immunoprecipitated with mAb 2.4G2 from detergent extracts of BMMC, digested with N-glycosidase F, subjected to SDS-PAGE, and immunoblotted with the FcgammaRIIb1- and FcgammaRIIb1/b2-specific antibodies, BMMC were found to express FcgammaRIIb1 and FcgammaRIIb2. Selective immunoprecipitation of plasma membrane-localized FcgammaRIIb1 and FcgammaRIIb2 from [H-3]leucine-labeled BMMC showed that their ratio at the cell surface was similar to their initial biosynthetic ratio. Thus, in contrast to mature serosal mast cells that degranulate on binding of IgG complexes, immature mast cells, of which BMMC are a prototype, may have a role in the clearance of complexes without concomitant release of proinflammatory mediators.