IDENTIFICATION OF SIGNALING MOTIFS WITHIN HUMAN FC-GAMMA-RIIA AND FC-GAMMA-RIIB ISOFORMS

To assess the functional capacity of the heterogeneous Fc gamma RII (C D32) family and to identify critical regions for functioning, we gener ated a panel of B-cell transfectants. The Fc gamma R-negative B-cell l ine IIA1.6 was transfected with wildtype or mutant human Fc gamma RIIa and IIb molecules. Solely Fc gamma RIIa-expressing IIA1.6 cells were capable of phagocytosing opsonized Staphylococcus aureus bacteria, and crosslinking of Fc gamma RIIa triggered a rapid induction of tyrosine phosphorylation after 20 seconds. Analysis of Fc gamma RIIa mutants i dentified the immunoreceptor tyrosine-based activation motif (ITAM; pr eviously described as ARH-1 motif) within the IIa cytoplasmic tail to be critical for B-cell activation, In contrast, Fc gamma RIIb isoforms triggered tyrosine phosphorylation on cross-linking with much slower kinetics (> 3 minutes) than Fc gamma RIIa. Furthermore, solely Fc gamm a RIIb molecules proved capable of downregulating [Ca2+](i) and interl eukin-2 production on co-cross-linking with slgG in IIA1.6. The Fc gam ma RIIb-mediated functions were absent in Fc gamma RIIb mutants in whi ch the tyrosine or leucine within the YSLL motif in a conserved 13-aa region (now known as immunoreceptor tyrosine-based inhibitor motif [IT IM]) were changed into phenylalanines. In conclusion, these data show the presence of functionally critical motifs within Fc gamma RII cytop lasmic tails. Fc gamma RIIa contains an ITAM involved in B-cell activa tory functions, whereas the downregulatory activity of Fc gamma RIIb i soforms is linked to an ITIM. (C) 1995 by The American Society of Hema tology.