Biological functions of human Fc gamma RIIa/Fc gamma RIIc in B cells

The human immunoglobulin receptor IIa (FcRIIa) was transfected into the FcR- mouse B cell line IIA1.6 to study its role in mediating endocytosis of human IgG complexes as well as its possible function in inhibiting the attenuated increased calcium level in B cells after antigen receptor cross-linking. Using FcRIIa mutants with truncated cytoplasmic domains we show that the region within the cytoplasmic region necessary 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 mutant 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 suggests 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 intracellular stores. Using the FcRIIa mutants we could further show that the signal delivered by FcRIIa is strongly dependent on the last 14 amino acids of the cytoplasmic tail. Analyses of the tyrosine phosphorylation of FcRIIa revealed that the calcium release mediated by FcRIIa cross-linking is dependent on tyrosine phosphorylation. In contrast, inhibition of the antigen receptor (sIgG) induced rise in intracellular calcium concentration by FcRIIa-sIgG co-cross-linking was not impaired when 30 amino acids were deleted at the carboxy terminus. FcRIIa wild type was rapidly phosphorylated when co-cross-linked with sIgG, but phosphorylation is not a prerequisite to inhibit calcium influx induced by sIgG cross-linking. These results show that distinct regions within the cytoplasmic tail of FcRIIa are necessary for the various signals transmitted to the cell.