Mast cells and basophils, which are activated by immunoglobulin E (IgE) and allergen, play a prominent role in anaphylaxis. However, they express at least three types of IgE receptor, including the high affinity IgE receptor (Fc epsilon RI). The relative contribution of these IgE receptors, and possibly other receptors such as Fc epsilon RII/CD23 and Mac-2, to the genesis of in vivo anaphylaxis is still unclear. To address this question, we have generated Fc epsilon RI-deficient mice. These mice appear normal and express a normal number of mast cells, but they are resistant to cutaneous and systemic anaphylaxis. These data demonstrate that Fc epsilon RI is necessary for the initiation of IgE-dependent anaphylactic reactions. Therefore, interfering with its function should be an effective means of treating allergy, regardless of the allergen specificity.

Lipopolysaccharide activates plasma-membrane signaling and endosomal signaling by Toll-like receptor 4 (TLR4) through the TIRAP-MyD88 and TRAM-TRIF adaptor complexes, respectively, but it is unclear how the signaling switch between these cell compartments is coordinated. In dendritic cells, we found that the p110δ isoform of phosphatidylinositol-3-OH kinase (PI(3)K) induced internalization of TLR4 and dissociation of TIRAP from the plasma membrane, followed by calpain-mediated degradation of TIRAP. Accordingly, inactivation of p110δ prolonged TIRAP-mediated signaling from the plasma membrane, which augmented proinflammatory cytokine production while decreasing TRAM-dependent endosomal signaling that generated antiinflammatory cytokines (interleukin 10 and interferon-β). In line with that altered signaling output, p110δ-deficient mice showed enhanced endotoxin-induced death. Thus, by controlling the 'topology' of TLR4 signaling complexes, p110δ balances overall homeostasis in the TLR4 pathway.

A role for the Fc receptor beta chain (FcRbeta) in the pathogenesis of allergy has been suggested by genetic studies. FcRbeta is a subunit common to the high-affinity IgE (FcepsilonRI) and low-affinity IgG (FcgammaRIII) receptors, both of which contribute to the initiation of allergic reactions. Current in vitro data suggest that FcRbeta can function as either a positive or negative regulator, leaving a mechanistic explanation for its association with the development of atopy unclear. To address this controversy, we have generated novel mouse models relevant to human Fc receptor function. Analysis of FcepsilonRI- and FcgammaRIII-dependent responses in these mice provides unequivocal genetic evidence that FcRbeta functions as an amplifier of early and late mast cell responses and, remarkably, in vivo anaphylactic responses.