Identification of a novel arthritis-associated osteoclast precursor macrophage regulated by FoxM1.

Macrophages comprise a variety of subsets with diverse biological activity, contributing to tissue homeostasis and a broad spectrum of pathogenesis. In response to environmental cues, they follow distinct developmental pathways such as differentiation into osteoclasts. In rheumatoid arthritis (RA), deep infiltration of the inflamed synovium eventually leads to irreversible destruction of the joints caused by pathological osteoclasts formed on periosteal surface. Although bone marrow macrophages have been extensively examined in the field, the precise entity of osteoclast precursors (OP) in pannus has yet to be identified, so as the trajectory of them. Here we show that OPs in pannus are exclusively derived from bone marrow through circulation, not from locally resident origins, and identify CX3CR1hiLy6CintF4/80+I-A/I-E+ macrophages, which we term ‘arthritis-associated osteoclastogenic macrophages (AtoM)’, as a population of OPs in inflamed synovium, comprising a distinct subset from conventional OPs in bone marrow represented by CX3CR1+Ly6ChiF4/80+I-A/I-E- fraction. Transcriptional profiling showed that AtoM differentially expresses genes related to osteoclastogenesis and mitochondrial biosynthesis, and is regulated by a characteristic transcription factor, FoxM1. (i) We performed an unbiased global transcriptomic comparison of R1 cells in the blood, R2’ and R3’ cells in the inflamed synovium in triplicates by RNA-sequencing analysis. (ii) CX3CR1hiLy6Cint subset in situ cells were sorted and targeted single-cell RNA-seq analysis was conducted (8682 single cells).