T cell-innate immune crosstalk regulates Staphylococcus aureus craniotomy infection

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A craniotomy is performed to access the brain for procedures that include tumor resection, localization and resection of epileptogenic foci, and aneurysm clipping, where the bone flap is replaced intraoperatively. Despite prophylaxis, infectious complications after craniotomy range from 1-3%, with approximately half caused by Staphylococcus aureus (S. aureus), which forms a biofilm on the bone flap that is recalcitrant to antibiotics. We have developed a mouse model of S. aureus craniotomy infection that shares important ultrastructural, MRI, and immune attributes with human disease, which can be exploited to identify mechanisms for infection persistence. Our preliminary results suggest that T cells maintain S. aureus in a biofilm state to minimize the shedding of planktonic bacteria into the brain. Ultimately, this helps protect the CNS parenchyma but does not clear the biofilm. This is supported by the fact that bacterial burden was unchecked in Rag1 KO mice and in WT animals following CD4+ T cell depletion, reflecting increased planktonic bacteria in the brain and galea. Furthermore, T cell loss coincided with an attenuated activation signature in microglia, macrophages, and granulocytes, with significant reductions in several IFN-ɣ-regulated genes, including CXCL10, indicative of T cell-innate immune crosstalk. We aim to understand this regulation in addition to T cell-biofilm crosstalk to devise novel strategies to promote biofilm eradication. This possibility is feasible given our innovative bacterial scRNA-seq data, where activated CD4+ T cells induced the expression of several S. aureus virulence genes, including protein A (spa) that binds the Fc portion of antibody to inhibit opsonophagocytosis. CXCL10 has been reported to induce Spa shedding from the bacterial membrane, which raises the intriguing possibility that CXCL10 induction by T cells promotes Spa release to block S. aureus phagocytosis, which is supported by our preliminary data where craniotomy infection was significantly reduced with a S. aureus spa mutant. This proposal will examine the hypothesis that CD4+ T cells regulate antimicrobial responses in the brain and galea by targeting microglia/macrophages vs. PMNs/G-MDSCs, respectively, to prevent bacterial outgrowth and limit invasion into the brain. In response, S. aureus alters its transcriptiome to augment virulence factor expression to promote biofilm persistence. This represents a host-pathogen triad that dictates infection outcome and the molecular mechanisms responsible for bacterial persistence in the brain and galea will be examined in the following Specific Aims: 1) Identify the critical CD4+ Th subset and antigen specificity in controlling S. aureus outgrowth during craniotomy infection; 2) Determine key mechanisms of T cell-innate immune crosstalk that dictate biofilm growth and CNS invasion; and 3) Identify S. aureus biofilm genes that are critical for subverting T cell effector function during craniotomy infection. An improved understanding of how T cells shape the innate immune landscape and S. aureus virulence during craniotomy infection may be leveraged to enhance antimicrobial activity and biofilm clearance to reduce patient morbidity.