Staphylococcus aureus skin infections have emerged as a major public health threat in the U.S. as a result the dramatic increase in incidence and the ongoing emergence of methicillin-resistant S. aureus (MRSA) and multi-drug resistant strains. This application will focus on investigating mechanisms of neutrophil recruitment and host defense against S. aureus skin infections and using the information gained from understanding these pathways to develop innovative therapeutic strategies. Based on our published findings and preliminary data, we hypothesize that a major pathway by which the innate immune system combats S. aureus skin infections involves pathogen recognition by Toll-like receptors (TLRs), secretion of IL-1 followed by induction of IL-23/IL-17, which subsequently triggers neutrophil recruitment to the skin. We further hypothesize that different components of this pathway can serve as therapeutic targets against S. aureus skin infections. To test these hypotheses, an in vivo mouse cutaneous S. aureus skin infection model in combination with in vivo imaging techniques to track both the neutrophil recruitment and bacterial clearance in real-time will be used. In addition, innovative human skin culture systems, including cultures of specific innate immune cell populations derived directly from human skin, organotypic keratinocytes, and full thickness human skin explants in combination with human neutrophil chemotaxis assays will be used to determine the predominant cytokines and chemokines that promote neutrophil recruitment in the context of S. aureus infection. Lastly, novel therapeutic strategies will be developed that target human resident skin innate immune cell populations to enhance production of those mediators found to be important in triggering neutrophil recruitment. This application will provide important new insights into mechanisms of neutrophil recruitment and host defense S. aureus skin infections. Furthermore, this study has significant translational potential, since the development of innovative therapeutic strategies to engage the host's neutrophilic response will provide the groundwork for future prevention and treatment interventions against S. aureus skin infection. We believe that this application is timely and relevant, since the need for new and effective treatment modalities are desperately needed to help combat these infections, which are becoming increasingly resistant to antibiotic therapy. PUBLIC HEALTH RELEVANCE. Staphylococcus aureus skin infections represent a major public health threat in the U.S. as a result of the dramatic increased incidence of these infections and the widespread emergence of virulent and antibiotic- resistant strains. The ultimate objective of this application is to uncover novel insights into mechanisms of host defense against S. aureus skin infection and to use these insights to develop innovative immune-based therapeutic strategies that will provide the groundwork for future prevention and treatment interventions against S. aureus skin infections. We believe that this area of research is highly significant, since the need for new and effective treatment modalities are desperately needed to help combat these infections, which are becoming increasingly resistant to antibiotic therapy.