Th1-mediated immune programming is protective during craniotomy infection

A craniotomy is a neurosurgical procedure performed to access the intracranial space. In 3-5% of cases, infection can develop, most commonly caused by Staphylococcus aureus biofilm formation on the surface of the skull. Medical management of this infection is difficult, as biofilm properties confer immune- and antimicrobial recalcitrance to the infection and necessitate additional surgical procedures. Still, treatment failure rates can be appreciably high. These factors, in conjunction with rapidly expanding rates of antibacterial resistance, necessitate the development of alternative treatment strategies targeting and reversing the dysfunctional immune response that develops during biofilm infection. Our recent work has identified Th1 and Th17 CD4+ T cells as potent regulators of innate immune antimicrobial capacity during craniotomy infection, where enhanced signatures of IFN-γ production and signaling were observed at the site of infection. Here, we follow up on these findings to determine the specific role of IFN-γ in programing the immune response to biofilm infection using global and cell-type-restricted IFN-γR deficient mice. We show exaggerated bacterial outgrowth in the absence of IFN-γR, despite few changes in immune cell recruitment relative to WT mice. Single-cell transcriptomic analysis identified candidate explanations for this phenotype as alterations in cell death pathways, innate immune cell activation, MHC-II expression, and CD4+ T cell skewing in IFN-γR KO mice. While PMN pyroptosis in vitro and macrophage/microglia MHC-II levels in vivo displayed sensitivity to IFN-γ signaling, granulocyte- and macrophage/microglia-specific conditional IFN-γR KO experiments isolated these factors from observed increases in bacterial burden. Instead, we uncovered evidence of a decreased Th1/Th17 ratio which was corroborated by elevated IL-17 levels during IFN-γR deficiency and correlated with dysfunctional cellular communication with the innate immune system. Further, Th17 cells were less effective than Th1 cells in stimulating macrophage and microglia bacterial killing. In sum, this work identifies Th1 cells as uniquely protective during craniotomy infection, where they likely enhance macrophage and microglial antibacterial abilities in an IFN-γ-independent manner. Therefore, augmentation of CD4+ T cell differentiation towards a Th1 bias may represent a novel therapeutic strategy for treating craniotomy infection in the future.