Interleukin-36γ–producing macrophages drive IL-17–mediated fibrosis

Single-cell RNAseq defines macrophage phenotypes in fibrotic biomaterial environments (see the related Focus by Perciani and MacParland). Sequencing in the matrix Biological scaffolds that mimic tissue microenvironments can be used to model wound healing. Using two distinct biomaterial environments, one that promotes regeneration and another that promotes fibrosis in conjunction with single-cell RNA sequencing, Sommerfeld et al. have characterized macrophages involved in both fibrosis and regeneration. They have defined populations of macrophages that are involved in both processes, and they have identified a subset of CD9+ interleukin-36γ (IL-36γ)–producing macrophages that participate in IL-17–driven fibrosis. By evaluating wound healing in IL-17–deficient mice, they report IL-17 to be essential for the generation of CD9+ IL-36γ–producing macrophages during fibrosis. Further studies are needed to understand the functional relationship between IL-17– and IL-36γ–producing cells in fibrosis and in other settings. Biomaterials induce an immune response and mobilization of macrophages, yet identification and phenotypic characterization of functional macrophage subsets in vivo remain limited. We performed single-cell RNA sequencing analysis on macrophages sorted from either a biologic matrix [urinary bladder matrix (UBM)] or synthetic biomaterial [polycaprolactone (PCL)]. Implantation of UBM promotes tissue repair through generation of a tissue environment characterized by a T helper 2 (TH2)/interleukin (IL)–4 immune profile, whereas PCL induces a standard foreign body response characterized by TH17/IL-17 and fibrosis. Unbiased clustering and pseudotime analysis revealed distinct macrophage subsets responsible for antigen presentation, chemoattraction, and phagocytosis, as well as a small population with expression profiles of both dendritic cells and skeletal muscle after UBM implantation. In the PCL tissue environment, we identified a CD9hi+IL-36γ+ macrophage subset that expressed TH17-associated molecules. These macrophages were virtually absent in mice lacking the IL-17 receptor, suggesting that they might be involved in IL-17–dependent immune and autoimmune responses. Identification and comparison of the unique phenotypical and functional macrophage subsets in mouse and human tissue samples suggest broad relevance of the new classification. These distinct macrophage subsets demonstrate previously unrecognized myeloid phenotypes involved in different tissue responses and provide targets for potential therapeutic modulation in tissue repair and pathology.