BIOLOGICAL FUNCTIONS OF HUMAN FC-GAMMA-RIIA FC-GAMMA-RIIC IN B-CELLS/

The human immunoglobulin receptor IIa (FcRIIa) was transfected into th e FcR(-) mouse B cell line IIA1.6 to study its role in mediating endoc ytosis of human IgG complexes as well as its possible function in inhi biting the attenuated increased calcium level in B cells after antigen receptor cross-linking. Using FcRIIa mutants with truncated cytoplasm ic domains we show that the region within the cytoplasmic region neces sary for endocytosis differs from that necessary for the abrogation of the elevated calcium level in B cells induced after antigen receptor cross-linking. Deletion of 14 amino acids at the carboxy terminus led to a slow internalization of FcRIIa bound human IgG, whereas the mutan t lacking 30 amino acids completely failed to mediate IgG uptake. The mutant lacking 14 amino acids at the carboxy terminus is not tyrosine phosphorylated in the course of receptor-mediated IgG uptake. This sug gests that phosphorylation of FcRIIa is not necessary to mediate this function. FcRIIa cross linking leads to a rapid transient rise in the intracellular calcium concentration due to calcium release from intrac ellular stores. Using the FcRIIa mutants we could further show that th e signal delivered by FcRIIa is strongly dependent on the last 14 amin o acids of the cytoplasmic tail. Analyses of the tyrosine phosphorylat ion of FcRIIa revealed that the calcium release mediated by FcRIIa cro ss-linking is dependent on tyrosine phosphorylation. In contrast, inhi bition of the antigen receptor (sIgG) induced rise in intracellular ca lcium concentration by FcRIIa sIgG co-cross-linking was not impaired w hen 30 amino acids were deleted at the carboxy terminus. FcRIIa wild t ype was rapidly phosphorylated when co-cross-linked with sIgG, but pho sphorylation is not a prerequisite to inhibit calcium influx induced b y sIgG cross-linking. These results show that distinct regions within the cytoplasmic tail of FcRIIa are necessary for the various signals t ransmitted to the cell.