POSTN+ fibroblast‑secreted small extracellular vesicles drive macrophage M2 polarization through BMP4/BMPR2/Smad signaling

Carcinoma‑associated fibroblasts (CAFs) exhibit notably heterogeneity and are closely implicated in immune checkpoint blockade (ICB) resistance in head and neck squamous cell carcinoma (HNSCC). However, the specific subtypes and mechanisms involved remain to be elucidated. By analyzing two single‑cell RNA sequencing datasets (GSE103322 and GSE139324) and The Cancer Genome Atlas (TCGA)‑HNSCC dataset, two distinct fibroblasts subtypes were identified: Perostin (POSTN)^‑ and POSTN⁺ fibroblasts. A comparison with reported markers revealed that extracellular matrix‑related markers were highly expressed in POSTN⁺ fibroblasts. In addition, fibroblast activation protein and POSTN expression were positively associated with macrophage infiltration and predicted ICB resistance in TCGA‑HNSCC dataset. Immunogold labeling confirmed the enrichment of POSTN on the membrane surface of CAF‑derived small extracellular vesicles (sEVs) and it was indicated that these POSTN⁺ sEVs may promote THP‑1‑derived macrophages polarization toward the M2 phenotype. Additionally, sEVs derived from CAFs with POSTN knockdown reduced bone morphogenetic protein (BMP) 4 expression in macrophages, thereby inhibiting M2 polarization through the BMP receptor 2/Smad pathway. Collectively, these findings revealed that a POSTN⁺ fibroblasts fosters an immunosuppressive microenvironment via sEV‑mediated macrophage polarization, nominating POSTN as a potential therapeutic target to overcome ICB resistance in HNSCC.