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Osteoclasts, the only bone-resorbing cells, are central to the pathogenesis of osteoporosis, yet their development and regulation are incompletely understood. Multiple receptors of the immune system use a common signaling paradigm whereby phosphorylated immunoreceptor tyrosine-based activation motifs (ITAMs) within receptor-associated adapter proteins recruit the Syk tyrosine kinase. Here we demonstrate that a similar mechanism is required for development of functional osteoclasts. Mice lacking two ITAM-bearing adapters, DAP12 and the Fc receptor [gamma]-chain (FcR[gamma]), are severely osteopetrotic. [DAP12.sup-/-][FcR[gamma].sup.-/-] bone marrow cells fail to differentiate into multinucleated osteoclasts or resorb bone in vitro and show impaired phosphorylation of the Syk tyrosine kinase, [syk.sup.-/-] progenitors are similarly defective in osteoclast development and bone resorption. Intact SH2-domains of Syk, introduced by retroviral transduction, are required for functional reconstitution of [syk.sup.-/-] osteoclasts, whereas intact ITAM-domains on DAP12 are required for reconstitution of [DAP12.sup.-/-][FcR[gamma].sup.-/-] cells. These data indicate that recruitment of Syk to phosphorylated ITAMs is critical for osteodastogenesis. Although DAP12 appears to be primarily responsible for osteoclast differentiation in cultures directly stimulated with macrophage-colony stimulating factor and receptor activator of NF-[kappa]B ligand cytokines, DAP12 and FcR[gamma] have overlapping roles in supporting osteoclast development in osteoblast-osteoclast cocultures, which mirrors their overlapping functions in vivo. These results provide new insight into the biology of osteoclasts and suggest novel therapeutic targets in diseases of bony remodeling.

We demonstrate that binding of different IgE molecules (IgEs) to their receptor, Fc[epsilon]RI, induces a spectrum of activation events in the absence of a specific antigen and provide evidence that such activation reflects aggregation of Fc[epsilon]RI. Highly cytokinergic IgEs can efficiently induce production of cytokines and render mast cells resistant to apoptosis in an autocrine fashion, whereas poorly cytokinergic IgEs induce these effects inefficiently. Highly cytokinergic IgEs seem to induce more extensive Fc[epsilon]RI aggregation than do poorly cytokinergic IgEs, which leads to stronger mast cell activation and survival effects. These effects of both types of IgEs require Syk tyrosine kinase and can be inhibited by Fc[epsilon]RI disaggregation with monovalent hapten. In hybridoma-transplanted mice, mucosal mast cell numbers correlate with serum IgE levels. Therefore, survival effects of IgE could contribute to the pathogenesis of allergic disease.