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FUNCTION AND REGULATION OF A MURINE MACROPHAGE-SPECIFIC IgG Fc RECEPTOR, FcyRa BY RICHARD L. WEINSHANK, ANDREW D. LUSTER,

Receptors for the Fc portion of IgG (FcyR)' provide a link between humoral and cellular immune response by targeting antibody/antigen complexes to effector cells. Crosslinking of these receptors on macrophages results in a wide array of cellular responses, which include phagocytosis, secretion of reactive oxygen intermediates, and lysosomal hydrolases, andultimately mediates antibody-dependent cellular cytoxicity (ADCC) (1) . Studies on mouse macrophages have defined three distinct FcyRs: a low-affinity, trypsin-resistant receptor that binds IgGl/IgG2b (2, 3), a high-affinity trypsin-sensitive receptor for monomeric IgG2a (4), and a receptor for IgG3 (5). Characterization of the IgGl/IgG2b FcyR has been facilitated by the mAb 2.4G2, which blocks ligand binding to this receptor (6) . In addition to its expression on macrophages, this FcyR is expressed on lymphocytes, where it displays broader ligand specificities, binding IgG1, IgG2b, as well as IgG2a (7). Its role on lymphocytes is not well defined but it is presumed to be involved in immune regulation (8). This functional heterogeneity of the IgG2b/IgG1 FcyR has been addressed at the molecular level by the characterization of two genes that encode this receptor, referred to as FcyRa and FcyRR (9-11) . These genes encode predicted transmembrane proteins with nearly identical extracellular domains, while differing completely in the transmembrane and cytoplasmic domains. Alternative splicing of the (3 gene results in two transcripts ((31 and 02) that differ in the cytoplasmic domain . The FcyRa transcript is expressed only in macrophages, while the FcyR(3 transcript is found in both macrophages and lymphocytes. Thus, the functional diversity displayed by this receptor on different cell types may result from the structural heterogeneity encoded in these two genes and their alternative transcripts . To begin to address the role of this structural heterogeneity in mediating different FcyR functions, we have studied the contribution of FcyRa and -Q gene expression for two phenotypes, ligand specificity and macrophage activation . Through the transfection of these genes into FcyR negative cells, the contribution of different structural domains was assessed in determining ligand specificity. Further, we have iso-