1995;15:272C281. Opsonization allows the efficient elimination of foreign antigens, through recognition of the Fc portion of immunoglobulins by a family of Fc immunoreceptors (FcRs). Receptor engagement in macrophages and neutrophils activates signaling pathways leading to cytoskeletal changes and the phagocytosis of IgG-coated particles, as well as to granule secretion. FcR signaling also stimulates the production of cytotoxic reactive oxygen intermediates and nitric oxide and induces the expression of cytokines, chemokines, and cell surface proteins. In addition to the destruction of pathogens, the ingestion and subsequent presentation of pathogen-derived peptide determinants by macrophages enhances T-cell-mediated immune functions. FcRs belong to the immunoglobulin gene superfamily, and all share a highly homologous extracellular portion, which harbors the Fc binding domain. Three distinct classes of FcRs have been identified. Class I and III receptors form multimeric complexes with disulfide-linked – or -chain dimers, while class II receptors exist as monomers (22). Interestingly, FcRIIB, which harbors a distinct phosphorylation motif, apparently transmits an inhibitory signal after receptor engagement. Signaling from FcRs appears to proceed through a series of interactions similar to those described for antigen receptors in lymphoid cells. Clustering of FcRs induces the activation of a Src family kinase, resulting in the phosphorylation of an ITAM within the receptors signaling subunit. Syk is recruited through TLR4 its SH2 domains to the FcR and subsequently undergoes autophosphorylation and induces the phosphorylation of multiple substrates, including other FcR ITAMs and downstream effectors (17, 32). Several lines of evidence suggest that Syk is a direct mediator of FcR signaling. Upon transfection with human FcRs, Cos-1 cells acquire phagocytic properties which, in the case of the FcRI and FcRIIIA isoforms, are dependent on an ITAM within the chain of the receptor (19, 20, 36). However, reconstitution of the receptor complex results in only marginal Setiptiline phagocytic activity, which can be significantly potentiated by cotransfection with Syk (23). Following FcR engagement in monocytes/macrophages, Syk is associated with the chain, becomes phosphorylated on tyrosine, and is enzymatically activated. Introduction of a protein containing the two Setiptiline SH2 domains of Syk into permeabilized mast cells abolished degranulation and leukotriene production following Fc?RI activation (47). Furthermore, clustering of FcRIII-Syk fusions in Cos-1 cells results in a phagocytic signal, which is dependent on an intact Syk kinase domain (18). Cross-linking of ectopically expressed FcR fusion proteins in Syk-deficient lymphocytes failed to initiate cytoskeletal changes indicative of phagocytosis, while re-expression of Syk restored the response (9). In addition, treatment of monocytes with antisense oligodeoxynucleotides has been reported to abrogate phagocytic activity (35). Targeted disruption of the gene has demonstrated an essential role in murine development Setiptiline (5, 50). Syk-deficient mice show profound bleeding and edema at midgestation, commonly leading to death late during embryogenesis or shortly after birth. Adoptive transfer of Syk-deficient fetal liver into RAG?/? recipients revealed a block of B-cell development at the pre-B-cell stage consistent with the notion that Syk acts downstream of the pre-BCR (5). Setiptiline Syk also plays a unique role in the development of / T cells (34) and acts in early T cells in conjunction with ZAP-70 (4). mutant mice appear to respond normally to thrombin. We have investigated the role of Syk in mediating FcR-dependent and -independent signaling in macrophages and neutrophils by using bone marrow radiation chimeras reconstituted with wild-type or locus (5) were selected for homozygosity at the locus (and 4C for 5 min and resuspended in 4 ml of Hanks balanced salt solution (HBSS; without Ca2+; Gibco, BRL) supplemented with 0.38% sodium citrate. The crude marrow suspension was placed.