Neurosurgery to relieve life-threatening edema (decompressive craniectomy) or gain temporary access to the brain for tumor resection (craniotomy) requires removal of a portion of the skull (i.e. bone flap). The infection incidence after craniotomy/craniectomy ranges from 0.8-12%, with a significant number caused by methicillin- resistant S. aureus (MRSA), which forms a biofilm on both surfaces of the bone flap. Biofilms are bacterial communities encased in a self-produced matrix that are recalcitrant to antibiotics due to their metabolic dormancy. Our laboratory has developed a mouse model of S. aureus craniotomy-associated biofilm infection that shares important ultrastructural and MRI attributes with human disease, which can be exploited to identify mechanisms for infection persistence. We have identified a unique immune compartmentalization in the S. aureus craniotomy model, namely preferential neutrophil (PMN) recruitment and chemokine expression in the subcutaneous galea, whereas monocytes are more prominent in the brain. Myeloid-derived suppressor cells (MDSCs), an immature granulocyte population with anti-inflammatory properties, are present in both compartments, but most abundant in the galea. Our preliminary studies have identified a role for IL-10 in promoting S. aureus survival in both the galea and brain, suggesting that IL-10 may be critical for programming glia and infiltrating leukocytes towards an anti-inflammatory state to promote biofilm persistence. Although IL- 10 is expressed in both the galea and brain, the cytokine is poised to inhibit the antibacterial activity of cell types that are enriched at either site, namely PMNs and MDSCs (galea) vs. microglia, astrocytes, and monocytes (brain). In terms of molecular mechanisms, our preliminary data demonstrate that S. aureus- derived lactate induces IL-10 production in MDSCs. Since lactate is a histone deacetylase inhibitor (HDACi) and the IL-10 promoter is regulated by histone acetylation, this supports the hypothesis that S. aureus biofilm- derived lactate is an exogenous HDACi that promotes IL-10 expression and inhibits antimicrobial responses in the galea and brain by targeting MDSCs/PMNs vs. microglia/astrocytes/monocytes, respectively, to promote infection persistence. The crosstalk between IL-10 and galeal vs. brain populations and the molecular mechanisms responsible for this compartmentalized specificity will be tested in the following Specific Aims. 1) Identify the role of MDSC-derived IL-10 on PMN antimicrobial activity in the galea during S. aureus craniotomy- associated biofilm infection; 2) Determine whether IL-10 promotes S. aureus persistence in the brain during craniotomy-associated biofilm infections by polarizing glia and monocytes towards an anti-inflammatory state; and 3) Establish that S. aureus biofilm-derived lactate enhances IL-10 production by inhibiting HDAC activity. An improved understanding of the immune compartmentalization during craniotomy biofilm ...