Collagen promotes PD-L1 overexpression in fibroblasts through binding to CD44 and activating YAP1 signaling during keloid formation

Abstract Keloid is a skin collagen disease secondary to skin injury, characterized by excessive collagen deposition and a markedly high rate of recurrence. To date, there is still a lack of effective prevention and treatment methods for keloid. Thus, it is urgent to explore the main pathological mechanism of keloid formation, which requires to identify the biological mechanism of deposited collagen regulating the formation of fibroblast-mediated local immunosuppressive microenvironment. Collagen accumulation, CD4 + T, CD8 + T, CD86 + Macrophages, FOXP3 + Treg cells infiltration, CD44 expression, and α-smooth muscle actin (α-SMA), platelet-derived growth factor receptor (PDGFR) and programmed cell death 1 ligand 1 (PD-L1) levels in keloid and normal skin tissue samples were determined by tissue immunofluorescence assay. The effects of collagen I on the expression of PD-L1, YAP1 and phosphorylated YAP1 (p-YAP1) in primary keloid fibroblasts were examined by reverse transcription-quantitative PCR and western blot assays. The co-location of collagen I and CD44 or PD-L1 and α-SMA was determined by cell immunofluorescence assay. Collagen accumulated in keloid pathological tissues, and fibroblasts proliferated abnormally in keloid pathological tissues. The infiltration of CD4 + T, CD86 + Macrophages and FOXP3 + Treg cells in keloid tissues was significantly upregulated, while the infiltration of CD8 + T cells in keloid tissues was significantly reduced. PD-L1 was significantly overexpressed in the fibroblasts of keloid tissues. Further mechanistic exploration found that deposited collagen (induced by inflammation) could bind to CD44 on the surface of fibroblasts, activate the YAP1 signaling pathway and lead to PD-L1 overexpression to form a local immunosuppressive microenvironment, resulting in the inability to remove the deposited collagen and the continuous proliferation of fibroblasts, ultimately forming keloid. The present study revealed that collagen deposition induced an immunosuppressive microenvironment and promoted the formation of keloid, thus providing potential new targets and theoretical basis for the prevention and treatment of keloid.