A Novel Non-Classical Monocyte-derived Macrophage Subset That Provides A Splenic Replication Niche For Intracellular Salmonella [Bulk RNA-Seq II]

Interactions between intracellular bacteria and mononuclear phagocytes give rise to diverse cellular phenotypes that may determine the outcome of infection. Recent advances in single-cell RNA-seq (scRNA-seq) have identified multiple subsets within the mononuclear population, but implications to their function during infection remain unknown. Here, we applied microscopy, flow cytometry and scRNA-seq to survey the mononuclear niche of intracellular Salmonella Typhimurium (S.Tm) during early systemic infection in mice. We describe eclipse-like growth kinetics in the spleen, with a first phase of bacterial control mediated by tissue-resident red-pulp macrophages. A second phase involved extensive bacterial replication within a newly identified macrophage population characterized by CD9 expression. Using Nr4a1e2-/- mice we established that CD9+ macrophages originated from non-classical monocytes (NCM), and NCM-depleted mice were more resistant to S.Tm infection. Our study defines a novel host-pathogen interface, with macrophage subset-specific interactions that determines early infection dynamics and the infection outcome of the entire organism. Overall design: Splenocytes were isolated from 6 mice 24 hours post infection with Salmonella enterica serovar Typhimurium. RNA was extracted from CD9 Mf and iNOS Mf