Our findings suggest that Mast cells (MCs) can have different cytokine profiles in the skin and that the skin microbiome has a strong influence on shaping these different profiles. We have demonstrated that bacterial products at the skin surface induce an anti-inflammatory phenotype in human MCs that contributes to maintaining skin homeostasis and helps avoid unnecessary inflammation. In this proposal, we aim to demonstrate that MCs, conditioned by the skin microbiome products, can shape skin responses to allergens in contexts that are relevant to the human immune system. Using both in vivo and in vitro assays, we intend to define the relative contribution of MC-bacterial interactions in directing dendritic and T cell trafficking in response to allergens. In our preliminary studies, we employ components from two representative bacterial strains: the pathogen S. aureus community acquired methicillin resistant USA 300 (USA 300) and the commensal S. epidermidis and show that they are both capable of shifting MC inflammatory phenotypes. We will use these bacterial components to explore the possibility of manipulating MCs during allergic reactions with the end goal of developing new therapeutic strategies for the treatment of allergic inflammation and related skin diseases. To characterize of the effects of MC-bacterial interactions in the skin, we propose the followings: 1. To test whether preconditioning of MCs with bacterial products results in the modulation of skin allergic reactions in vivo. We hypothesize that exposure of MCs to commensal bacterial small molecules will induce anti-inflammatory MC cytokine profiles, resulting in changes to the skin immunological environment and the reduction of the allergic response in the skin. Using two different mouse models of allergic reaction we will investigate the effects of MC preconditioning on allergic reactions in th skin. We will also verify the effects of LTA and bacterial products on MC IL-10 expression in human skin using laser capture micro dissection (LCM) in an ex-vivo system. Data generated by the in vivo allergic reaction studies in this aim will clarify the importance of the cytokine profies generated by MCs and how they can modulate allergic reactions in the skin. 2. To investigate the mechanism of how preconditioning of MCs with bacterial products modifies dendritic cell cytokine profiles to modulate allergic reactions in vitro. We hypothesize that MCs preconditioned with commensal byproducts or LTA alone, will prevent or decrease dendritic cell (DC) differentiation and T cell maturation in vitro and consequently dampen the allergic response. In the second part of this proposal, we will focus our interest on APC cytokines, which are responsible for T cell polarization. Data generated by this in vitro study will give us important information regarding the mechanism of how the release of cytokines by preconditioned MCs can skew the allergic response.