In vivo regulation of infected and bystander monocyte phenotype by the Mycobacterium marinum ESX-1 type VII secretion system

Pathogenic mycobacteria require the conserved ESX-1 type VII secretion system to cause disease. Using a murine Mycobacterium marinum infection model we recently demonstrated an antagonistic interplay between neutrophils and monocytes, where monocytes play a host-protective role by restricting ESX-1-dependent intragranuloma accumulation of neutrophils. Here, we demonstrate that the protective function of monocytes is maintained in RAG-deficient mice, indicating that their ability to restrict neutrophil-dependent inflammation is independent of adaptive immunity. Further, we performed single cell RNA sequencing of infected and bystander Ly6C+MHCII+ monocytes, isolated from the infected tissue, to explore how M. marinum and ESX-1 alter the monocyte phenotype in vivo. Our findings indicate that M. marinum intrinsically impacts on the transcription profile of infected monocytes in an ESX-1-independent fashion. Infection alters cytokine expression, induces glycolytic metabolism, hypoxia-mediated signaling, nitric oxide synthesis, tissue remodeling, and suppresses responsiveness to IFNg. In contrast to infected cells, the phenotype of bystander monocytes is extrinsically regulated in an ESX-1-dependent manner, including a reduced responsiveness to IFNg. These findings suggest that mycobacterial infection has pleiotropic effects on monocyte phenotype, with potential implications in bacterial growth restriction and granuloma formation.