Extracellular vesicles mediate immune escape and cancer progression of MHC class II constitutively expressing melanoma cells.

Melanoma is the deadliest form of skin cancer and the most common cancer whose incidence rate is increasing worldwide. Indeed, the metastatic progression of melanoma is associated to the expression of Major Histocompatibility Complex (MHC) class II molecules. In particular, the signalling activated by the engagement of MHC class II molecules, leads to the increase of the expression and activation of signalling proteins, kinases and adhesion receptors. Melanoma cells secrete in their microenvironment extracellular vesicles (EVs), that regulating the immune cell functions and modifying the tumour microenvironment, play a main role in the metastatic progression of melanoma. Indeed, the EVs for their nanoscale size, can circulate in advancing tumour front and in distant tissues interacting with different cell types as mediators of metastasis. Therefore, the aim of our work was to understand the role of EVs in the metastatic progression of melanoma as a consequences of MHC class II mediated signalling. Indeed, we showed in EVs secreted by MHC class II constitutive expressing melanoma cell lines, the increased expression of HLA-DR, CAMs adhesion receptors, PD-L1 and STAT3 signalling proteins mediated by the MHC class II activated signalling. Furthermore, through co-culture experiments of EVs and PBMCs, we showed though flow cytometry analysis that MHC class II mediated signalling enhances the cytotoxic effects of EVs on human PBMCs. Finally, using a transwell migration assay we showed the autocrine and paracrine function of EVs that, in response to class II mediated signalling, induce the increased migration of fibroblast and melanoma cells. Indeed, our results suggest that MHC class II mediated signalling plays a new role to promote melanoma progression enhancing, through the extracellular vesicles secreted, the metastatic dissemination of melanoma cells as well as inhibiting the immune response in tumour microenvironment. In particular, our results showed a complex interaction between tumour cells and fibroblasts involving EVs secreted by tumour cells that, as important mediators of cell communication, establish a paracrine and autocrine dynamic signalling circuit that plays a crucial role in the regulation of tumour cell and fibroblast migration and, therefore, cancer progression. In-depth experiments will be needed in the future to explore the underlying mechanisms.