Fibroblast-Derived Small Extracellular Vesicles Promote M2 Macrophage Polarization and PD-L1 Upregulation in Mycosis Fungoides

Introduction: Cutaneous T cell lymphoma (CTCL), most commonly known as mycosis fungoides (MF), is characterized by an increasingly immunosuppressive tumor microenvironment (TME) as the disease progresses. Cancer-associated fibroblasts (CAFs) are key stromal components that support a permissive niche, in part through the secretion of small extracellular vesicles (sEVs), predominantly exosomes, that mediate intercellular communication. We investigated the immunomodulatory role of exosome-enriched sEVs derived from MF fibroblasts (MF-Fs) compared to normal fibroblasts (N-Fs). Materials and Methods: Primary MF-Fs from early-stage MF biopsies and N-Fs from healthy skin were cultured in vitro. sEVs enriched with exosomes were isolated by ultracentrifugation and characterized by flow cytometry (CD81), electron microscopy, Nanosight analysis, and protein quantification, and their uptake by normal peripheral blood mononuclear cells (nPBMCs) was confirmed using PKH26-labeled sEVs. nPBMCs, monocytes, CD4+ and CD8+ T cells from healthy donors were exposed to MF-F or N-F sEVs. Cell viability was assessed using MTT and trypan blue exclusion assays. Mass cytometry (CyTOF) profiled immune subsets and regulatory proteins for preliminary observation. Monocyte polarization was evaluated by flow cytometry for M1 (CD80, CD86) and M2 (CD163, CD206) markers and PD-L1 expression; M1/M2-associated cytokines and sEV-microRNAs were quantified by qRT-PCR. Results: Both MF-F and N-F sEVs were internalized by nPBMCs and reduced their viability, with a more pronounced effect observed for MF-F sEVs. In nPBMCs, MF-F sEVs also increased the frequency of M2-like macrophages, decreased M1 polarization, and enhanced PD-L1 expression. In primary monocytes, MF-F- compared with N-F-derived sEVs upregulated M2-associated cytokines (IL-10, TGF-β), increased PD-L1 expression, and generated M2-like cells that suppressed CD4+ and CD8+ T cell viability. Conclusions: MF-F sEVs promote an immunosuppressive TME and represent potential therapeutic or biomarker targets in MF.