Cross-tissue organization of myeloid cells in scleroderma and related fibrotic diseases

Purpose of review Scleroderma and other fibrotic diseases have been investigated using single-cell RNA sequencing (scRNA-Seq), which has demonstrated enrichment in myeloid cell populations in multiple tissues. However, scRNA-Seq studies are inconsistent in their nomenclature of myeloid cell types, including dendritic cells, monocytes, and macrophages. Using cell type-defining gene signatures, I propose a unified nomenclature through analysis of myeloid cell enrichment across fibrotic tissues. Recent findings scRNA-Seq of human blood and skin identified a new subset of dendritic cells called DC3. DC3 express similar inflammatory genes to monocytes, including FCN1 , IL1B, VCAN, S100A8, S100A9 , and S100A12 . DC3 can be distinguished from monocytes by expression of EREG and Fc receptor genes such as FCER1A and FCGR2B . scRNA-Seq analyses of scleroderma skin and lung, idiopathic pulmonary fibrosis (IPF), COVID-19 lung fibrosis, myelofibrosis, and liver, kidney, and cardiac fibrosis all showed enrichment in myeloid cell types. Although they were called different names, studies of scleroderma skin and lung as well as liver cirrhosis datasets demonstrated enrichment in DC3. By contrast, lung, heart, and kidney fibrosis were enriched in SPP1 macrophages. High numbers of DC3 in the skin was associated with worse SSc skin and lung fibrosis severity. Summary scRNA-Seq of multiple diseases showed enrichment of DC3 in fibrotic skin, lung, and liver, whereas SPP1 macrophages occurred in fibrotic lung, heart, and kidney. Because DC3 and SPP1 macrophages showed organ-specific enrichment, understanding their signaling mechanisms across tissues will be important for future investigation.