Heterogeneity and Functional Divergence of Ly6C hi Monocytes in Acute Inflammation Identifies a Requirement for Metabolic Reprogramming

Acute inflammation is rapid and dynamic process involving the recruitment and activation of multiple cell types in a co-ordinated and precise manner. Using cell tracking, linage tracing and single cell transcriptomics we investigated the origin and transcriptional reprogramming of monocyte and macrophages in acute inflammation. Monocyte trafficking and adoptive transfer experiments revealed that monocytes undergo rapid phenotypic change as they exit the blood and give rise to monocyte-derived macrophages that persist during the resolution of inflammation. Single cell transcriptomics revealed significant heterogeneity within the surface marker defined CD11b+ Ly6G-Ly6Chi monocyte population within the blood and at the site of inflammation. Lineage trajectory analysis revealed that Ly6Chi monocytes in the blood are pre-programmed into a defined differentiation pathway prior to inflammatory stimulus.  We show that two major transcriptional reprogramming events occur during the initial 6 h of Ly6Chi monocyte mobilisation, one in the blood priming monocytes for migration and a second at the site of inflammation. Pathway analysis revealed an important role for oxidative phosphorylation (OxPhos) during both these reprogramming events in a subset of M2-like cells. Experimentally we demonstrate that OxPhos is essential for monocyte chemotaxis and monocyte to macrophage differentiation. Critically OxPhos is needed for M(IL-4) polarisation in both murine and human primary monocytes and macrophages . Our results reveal metabolic reprogramming towards OxPhos in the blood identifies Ly6Chi monocytes that differentiate to become M2-like monocyte-derived macrophages.