Dynamics of the Immune Response to M RSA Subcutaneous Skin Infection

Abstract Staphylococcus aureus is a leading cause of skin and soft tissue infections (SSTIs). While studies have examined how the immune system responds to S. aureus SSTIs, most are limited in scope to a single time point or outcome. To set a foundation for future studies, we conducted a S. aureus skin infection experiment using female C57BL/6J mice and USA300 S. aureus to examine the host-pathogen interface up to 15 days post-infection (d.p.i.). We measured lesion size, bacterial titers, local cytokine and chemokine levels, phenotyped responding leukocytes, and collected skin for histopathologic analysis and Gram staining. Lesions were largest at 1 d.p.i. with peak necrotic tissue area at 3 d.p.i. and were largely resolved by 15 d.p.i. IL-6, IL-4, CCL2, and IL-1β were highest during early infection, while IL-17A/F peaked 7 days post-infection. Neutrophils were abundant in the skin throughout the infection and were the primary immune cell present until 9 d.p.i. Histopathologic analysis demonstrated swift and extensive keratinocyte death and robust and persistent neutrophil infiltration. Gram staining revealed subdermal S. aureus colonization and, later, limited migration into upper skin layers. Most leukocyte subsets decreased within 8h after infection and returned during wound resolution coincident with declining bacterial titers. These (re)emerging leukocytes included CD4+ T cells, innate lymphoid cells, inflammatory monocytes, macrophages, and dendritic cells. Collectively, these data illustrate a dynamic immune response to S. aureus skin infections and provide a foundation for further studies characterizing the host-pathogen interface. This research was supported by funding from the National Institute of Allergy and Infectious Diseases (NIAID) award R01AI121073 as well as pilot project grants from K-INBRE (P20GM130418) and University of Kansas Chemical Biology of Infectious Disease COBRE grant P20GM113117 to JLB. We acknowledge the Kansas Intellectual and Developmental Disabilities Research Center Histology Core which is sponsored in part by the NIH/NICHD (U54 HD090216) and the Flow Cytometry Core Laboratory, which is sponsored, in part, by the NIH/NIGMS COBRE grant P30 GM103326 and the NIH/NCI Cancer Center grant P30 CA168524.