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To evaluate the potential role of human placental endothelial cells in the transport of IgG from maternal to fetal circulation, we studied Fcγ receptor (FcγR) expression by immunohistology and immunoblotting. Several pan-FcγRII Abs that label the placental endothelium displayed a distribution pattern that correlated well with transport functions, being intense in the terminal villus and nil in the cord. In contrast, the MHC class 1-like IgG transporter, FcRn, and the classical FcγRIIa were not expressed in transport-related endothelium of the placenta. Our inference, that FcγRIIb was the likely receptor, we confirmed by analyzing purified placental villi, enriched in endothelium, by immunoblotting with a new Ab specific for the cytoplasmic tail of FcγRIIb. These experiments showed that the FcγRII expressed in villus endothelium was the b2 isoform whose cytoplasmic tail is known to include a phosphotyrosyl-based motif that inhibits a variety of immune responses. We suggest that this receptor is perfectly positioned to transport IgG although as well it may scavenge immune complexes.

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Fc␥R clustering in monocytes initiates a cascade of signaling events that culminate in biological responses such as phagocytosis, production of inflammatory cytokines, and generation of reactive oxygen species. We have identified and determined the function of the adapter protein linker of activation of T cell (LAT) in Fc␥R-mediated signaling and function. Clustering of Fc␥Rs on the human monocytic cell line, THP-1, induces phosphorylation of a major 36-kDa protein which immunoreacts with anti-LAT antisera. Our data indicate that although both the 36-kDa and 38-kDa isoforms of LAT are expressed in THP-1 and U937 human monocytic cells, Fc␥R clustering induces phosphorylation of the 36-kDa isoform only. Co-immunoprecipitation experiments revealed a constitutive association of p36 LAT with both Fc␥RI and Fc␥RIIa immunoprecipitates, and an activation-induced association of LAT with PLC␥1, Grb2, and the p85 subunit of phosphatidylinositol 3-kinase. Transient transfection experiments in COS-7 cells indicated that overexpression of a wild type but not a dominant-negative LAT, that is incapable of binding to p85, enhances phagocytosis by Fc␥RI. Furthermore, bone marrow-derived macrophages from LAT-deficient mice displayed reduced phagocytic efficiency in comparison to the macrophages from wild-type mice. Thus, we conclude that p36 LAT serves to enhance Fc␥R-induced signal transduction in myeloid cells. Clustering of the Fc␥ receptors (Fc␥R)1 on monocytes/macrophages initiates a series of intracellular biochemical events that are necessary for induction of the various biological outcomes, such as phagocytosis, production of inflammatory cyto- The biochemical pathways initiated by Fc␥Rs leading to phagocytosis is highly analogous to that of other immunoreceptors. Thus, Fc␥R aggregation by IgG-coated particulate antigen induces the activity of Src kinases, which phosphorylate a conserved receptor-associated amino acid motif known as the immunoreceptor tyrosine-based activation motif (ITAM) (2-4). ITAMs of the Fc␥Rs are found in the receptor-associated ␥-subunit except in the case of Fc␥RIIa (4, 5), which uniquely expresses the ITAM within its cytoplasmic tail (5, 6). ITAM phosphorylation initiates ITAM recruitment of a variety of enzymes that propagate the antigenic signal, and lead to and are essential for phagocytosis (reviewed in Refs. 4 and 7). However, while reports over the past several years have elucidated some of the Fc␥R-triggered signaling pathways leading to phagocytosis, the proximal events induced by Fc␥R clustering are not fully understood.Studies from our laboratory (8) and others (9, 10) have demonstrated that the tyrosine kinase Syk is directly recruited to the phosphorylated ITAM of the receptor. Syk recruitment is followed by the recruitment of other SH2 domain-containing enzymes such as PLC␥1, the Grb2-Sos complex and the p85-p110 complex of PI 3-kinase (11, 12). Translocation of PLC␥1 to the membrane brings it in contact with its lipid substrate phosphatidylinositol 4,5-bisphosphate, thus generating s...