The HLA-DR mediated signalling in melanoma cells

Background: The metastatic progression of melanoma, one of the most widespread cancers in the Western population, is associated to the expression of Major Histocompatibility Complex (MHC) class II molecules. Indeed, the MHC class II molecules are signalling receptors whose engagement leads to the activation of several signalling pathways and are constitutively expressed in almost 50% of melanoma. Melanoma cells secrete in their microenvironment extracellular vesicles, that circulating in advancing tumour front could interact with immune cells and different cell types as mediators of metastasis. Material and Methods: The HLA-DR signalling was studied in class II constitutive expressing melanoma cell lines through western blot experiments of cell extracts, lipid raft compartments and extracellular vesicles. The HLA-DR signalling mediated migration, extracellular matrix proteins adhesion and cell-cell adhesion was also analysed in melanoma cells. Dynamic Light Scattering (DLS) analysis was performed to measure the size distribution of extracellular vesicles and through flow cytometry analysis we studied the apoptotic PBMCs treated with extracellular vesicles secreted by melanoma cells. Results: HLA-DR mediated signalling increases the expression and the lipid raft localisation of HLA-DR?, PD-L1, Integrin and CAM adhesion receptors, FAK, AKT and STAT3 signalling proteins and their activation. The HLA-DR mediated signalling increases in extracellular vesicles the expression of HLA-DR, adhesion receptors, PDL1 and STAT3. Furthermore, through co-culture experiments of PBMCs and extracellular vesicles, we showed that HLA-DR mediated signalling enhance the cytotoxic effects of extracellular vesicles on PBMCs. Conclusion: The results showed suggest a new model in which HLA-DR stimulation activates a signalling in melanoma cells that provides a platform useful to frustrate an effective anti-tumour response and to increase melanoma migration and metastatic dissemination regulating cell adhesion, motility and immune escape. Furthermore, our results suggest that HLA-DR mediated signalling promotes melanoma progression enhancing, through the extracellular vesicles secreted, the inhibition of immune response.