Extracellular vesicles from melanoma cells and their potential role in immune escape and cancer progression as a consequence of the MHC class II signalling.

Melanoma, is one of the most aggressive cancers worldwide, its incidence rate increases rapidly in western population and unfortunately it is responsible of about 20% overall cancer mortality as well as of approximately 80% skin cancer related mortality. Indeed, the aggressive metastatic trend of melanoma is associated to the increase of the cellular dissemination and is a consequence of the tumour cells detachment from primary tumour as well as to the interaction between tumour cells and cellular blood components. Melanoma cells also secrete in their microenvironment microvesicles and exosomes, that for their nanoscale size, can circulate and interact with immune cells, tumour cells and with different cell types localized in advancing tumour front and in distant tissues. Indeed, the extracellular vesicles secreted by melanoma cells play a main role in the metastatic progression of melanoma regulating the immune cell functions and modifying the tumour microenvironment. Furthermore, the metastatic progression of melanoma is associated to the constitutive expression of Major Histocompatibility Complex (MHC) class II molecules that are constitutively expressed in almost 50% of metastatic lesions and therefore are a marker of poor prognosis in melanoma. Therefore, the aim of our work was to understand the consequences on the extracellular vesicles secreted by melanoma cells, of the MHC class II mediated signalling. Indeed, the MHC class II molecules are signalling receptors whose engagement leads to the activation of several signalling pathways. In particular, we showed in the extracellular vesicles secreted by two class II constitutive expressing melanoma cell lines, that the MHC class II mediated signalling increases the expression of HLA-DR?, ICAM and MCAM adhesion receptors, FasL and STAT3 signalling proteins and also increases the proteolytic activity of some members of the matrix metalloproteinases (MMPs) family. Interestingly, the tumour microvesicles and exosomes secreted by the MHC class II stimulated melanoma cells, through the expression of these proteins and the activation of the proteolytic activity of MMP family members, could have immune suppressive function inducing the apoptosis of activated T cells and the inhibition of dendritic cells differentiation and could be implicated in tumour cells invasion of the surrounding tissues and also in the host-tumour communication. The increased amount of these receptors and signalling proteins in the extracellular vesicles as well as of the MMP gelatinases activity, would act in concert to enhance the breakdown and the remodelling of the extracellular matrix, thus enhancing the tumour cell invasion of microenvironment and the metastatic progression of melanoma cells. Therefore, our results suggest that the 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.