PUB - Publikationen an der Universität Bielefeld

Human B cells express four immunoglobulin G receptors, Fc gamma RIIa, Fc gamma RIIb1, Fc gamma RIIb2, and Fc gamma RIIc. Coligation of either Fc gamma RII isoform with the B-cell antigen receptor (BCR) results in the abrogation of B-cell activation, but only the Fc gamma RIIa/e and Fc gamma RIIb1 isoforms become phosphorylated. To identify the Fc gamma RII-phosphorylating protein tyrosine kinase (PTK), we used the combination of an in vitro and an in vivo approach. In an in vitro assay using recombinant cytoplasmic tails of the different Fc gamma RII isoforms as well as tyrosine exchange mutants, we show that each of the BCR-associated PTKs (Lyn, Blk, Fyn, and Syk) shows different phosphorylation patterns with regard to the different Fc gamma R isoforms and point mutants. While each PTK phosphorylated Fc gamma RIIa/c, Fc gamma RIIb1 was phosphorylated by Lyn and Blk whereas Fc gamma RIIb2 became phosphorylated only by Blk Mutants lacking both tyrosine residues of the immune receptor tyrosine-based activation motif (ITAM) of Fc gamma RIIa/c were not phosphorylated by Blk and Fyn, while Lyn-mediated phosphorylation was dependent on the presence of the C-terminal tyrosine of the ITAM. Results obtained in assays using an Fc gamma R(-) B-cell line transfected with wild-type or mutated Fc gamma RIIa demonstrated that exchange of the C-terminal tyrosine of the ITAN of Fc gamma RIIa/c was sufficient to abolish Fc gamma RIIa/c phosphorylation in B cells. Additionally, we could show that Lyn and Fyn bind to Fc gamma RIIa/c, with the ITAM being necessary for association. Comparison of the phosphorylation pattern of each PTK observed in vitro with the phosphorylation pattern observed in vivo suggests that Lyn is the most likely candidate for Fc gamma RIIa/c and Fc gamma RIIb1 phosphorylation in vivo.