A machine learning analysis enables demonstration that fibroblasts mediate neutrophil recruitment during type 17 skin inflammation

Neutrophil-associated skin diseases such as bacterial infection, psoriasis, and other dermatoses involve type 17 inflammation. Advancements in treatment may come from a more complete understanding of the cell types that participate in the pathogenesis of disease. To address this, we first performed an unbiased investigation of cell-cell communication during type 17 skin inflammation using a machine learning approach with single-cell RNA sequencing (scRNA-Seq) data from several different human skin diseases and mouse models. This network analysis revealed that most communication to myeloid cells in acne, psoriasis, and S. aureus infection comes from dermal fibroblasts, not keratinocytes. Further analysis showed that subsets of fibroblasts were induced during inflammation to highly express chemokines and other host defense genes and had an activation state consistent with response to IL-17A and TNFa. These immune acting fibroblasts were visualized in situ and modeled with primary dermal fibroblasts and 3T3-L1 fibroblasts. Following treatment with IL-17A and TNFa, cultured fibroblasts showed activation of NF-kB signaling and high expression of AMPs and several neutrophil chemokines, validating our scRNA-Seq analysis. Conditioned media from fibroblasts that were activated by IL-17A and TNFa had robust chemotactic activity for neutrophils that was dependent on CXCR2 and CXCR4. To determine the in vivo significance of this response by fibroblasts, we deleted Il17ra specifically in fibroblasts (PdgfraΔIl17ra). PdgfraΔIl17ramice had impaired defense against S. aureus skin infection and reduced skin neutrophil recruitment and expression of CXCR2 and CXCR4 ligands after topical imiquimod, intradermal injection of IL-17A and TNFa, or wounding. These findings show for the first time that dermal fibroblasts are critical for the type 17 inflammatory response in skin. Thus, dermal fibroblast subsets may represent a novel therapeutic target for treatment of infection and inflammatory skin diseases.