ROLE FOR A GLYCAN PHOSPHOINOSITOL ANCHOR IN FC-GAMMA RECEPTOR SYNERGY

While many cell types express receptors for the Fc domain of IgG (Fc g amma R), only primate polymer-phonuclear neutrophils (PMN) express an Fc gamma R linked to the membrane via a glycan phosphoinositol (GPI) a nchor. Previous studies have demonstrated that this GPI-linked Fc gamm a R (Fc gamma RIIIB) cooperates with the transmembrane Fc gamma R (Fc gamma RIIA) to mediate many of the functional effects of immune comple x binding. To determine the role of the GPI anchor in Fc gamma recepto r synergy, we have developed a model system in Jurkat T cells, which l ack endogenously expressed Fc gamma receptors. Jurkat T cells were sta bly transfected with cDNA encoding Fc gamma RIIA and/or Fc gamma RIIIB . Cocrosslinking the two receptors produced a synergistic rise in intr acytoplasmic calcium ([Ca2+](i)) to levels not reached by stimulation of either Fc gamma RIIA or Fc gamma RIIIB alone. Synergy was achieved by prolonged entry of extracellular Ca2+. Cocrosslinking Fc gamma RIIA with CD59 or CD48, two other GPI-linked proteins on Jurkat T cells al so led to a synergistic [Ca2+](i) rise, as did crosslinking CD59 with Fc gamma RIIA on PMN, suggesting that interactions between the extrace llular domains of the two Fc gamma receptors are not required for syne rgy. Replacement of the GPI anchor of Fc gamma RIIIB with a transmembr ane anchor abolished synergy. In addition, tyrosine to phenylalanine s ubstitutions in the immunoreceptor tyrosine-based activation motif (IT AM) of the Fc gamma RIIA cytoplasmic tail abolished synergy. While the ITAM of Fc gamma RIIA was required for the increase in [Ca2+](i), tyr osine phosphorylation of crosslinked Fc gamma RIIA was diminished when cocrosslinked with Fc gamma RIIIB. These data demonstrate that Fc gam ma RIIA association with GPI-linked proteins facilitates Fc gamma R si gnal transduction and suggest that this may be a physiologically signi ficant role for the unusual GPI-anchored Fc gamma R of human PMN.