Single-cell transcriptomics identifies the differentiation trajectory from inflammatory monocytes to pro-resolving macrophages in a mouse skin allergy model

Both monocytes and macrophages are heterogeneous populations. It was traditionally understood that Ly6C hi classical (inflammatory) monocytes differentiate into pro-inflammatory Ly6C hi macrophages. Accumulating evidence has suggested that Ly6C hi classical monocytes can also differentiate into Ly6C lo pro-resolving macrophages under certain conditions, while their differentiation trajectory remains to be fully elucidated. The present study with scRNA-seq and flow cytometric analyses reveals that Ly6C hi PD-L2 lo classical monocytes recruited to the allergic skin lesion sequentially differentiate into Ly6C lo PD-L2 hi pro-resolving macrophages, via intermediate Ly6C hi PD-L2 hi macrophages but not Ly6C lo non-classical monocytes, in an IL-4 receptor-dependent manner. Along the differentiation, classical monocyte-derived macrophages display anti-inflammatory signatures followed by metabolic rewiring concordant with their ability to phagocytose apoptotic neutrophils and allergens, therefore contributing to the resolution of inflammation. The failure in the generation of these pro-resolving macrophages drives the IL-1α-mediated cycle of inflammation with abscess-like accumulation of necrotic neutrophils. Thus, we clarify the stepwise differentiation trajectory from Ly6C hi classical monocytes toward Ly6C lo pro-resolving macrophages that restrain neutrophilic aggravation of skin allergic inflammation.