Signaling through FcγRIII is required for optimal T helper type (Th)2 responses and Th2-mediated airway inflammation

Although inhibitory Fcγ receptors have been demonstrated to promote mucosal tolerance, the role of activating Fcγ receptors in modulating T helper type (Th)2-dependent inflammatory responses characteristic of asthma and allergies remains unclear. Here, we demonstrate that signaling via activating Fcγ receptors in conjunction with Toll-like receptor 4 stimulation modulated cytokine production from bone marrow–derived dendritic cells (DCs) and augmented their ability to promote Th2 responses. Ligation of the low affinity receptor FcγRIII was specifically required for the enhanced Th2 responses, as FcγRIII^−/− DCs failed to augment Th2-mediated airway inflammation in vivo or induce Th2 differentiation in vitro. Further, FcγRIII^−/− mice had impaired Th2 cytokine production and exhibited reduced airway inflammation, whereas no defect was found in FcγRI^−/− mice. The augmentation of Th2 immunity was regulated by interleukin 10 production from the DCs but was distinct and independent of the well-established role of FcγRIII in augmenting antigen presentation. Thus, our studies reveal a novel and specific role for FcγRIII signaling in the regulation of Th cell responses and suggest that in addition to immunoglobulin (Ig)E, antigen-specific IgG also contributes to the pathogenesis of Th2-mediated diseases such as asthma and allergies.