Crosstalk between Sentinel and Helper Macrophages Permits Neutrophil Migration into Infected Uroepithelium

The phagocytes of the innate immune system, macrophages and neutrophils, contribute to antibacterial defense, but their functional specialization and cooperation is unclear. Here, we report that three distinct phagocyte subsets play highly coordinated roles in bacterial urinary tract infection. Ly6C− macrophages acted as tissue-resident sentinels that attracted circulating neutrophils and Ly6C+ macrophages. Such Ly6C+ macrophages played a previously undescribed helper role: once recruited to the site of infection, they produced the cytokine TNF, which caused Ly6C− macrophages to secrete CXCL2. This chemokine activated matrix metalloproteinase-9 in neutrophils, allowing their entry into the uroepithelium to combat the bacteria. In summary, the sentinel macrophages elicit the powerful antibacterial functions of neutrophils only after confirmation by the helper macrophages, reminiscent of the licensing role of helper T cells in antiviral adaptive immunity. These findings identify helper macrophages and TNF as critical regulators in innate immunity against bacterial infections in epithelia. [Display omitted] •Neutrophils, Ly6C− and Ly6C+ macrophages perform distinct antibacterial functions•Ly6C+ macrophages act as innate immune helper cells in antibacterial defense•TNF is the innate helper factor; it permits neutrophil migration into epithelium•Distinct CXCR2 ligands mediate endothelial and epithelial neutrophil migration In urinary tract infection, neutrophils are recruited from the blood to the infected organ by tissue-resident sentinel macrophages but are only directed to the frontline, the uroepithelium, after receiving a confirmation signal from recruited helper macrophages