Signalling of the major histocompatibility complex (MHC) class II molecules in melanoma cells.

Background: Melanoma is the cancer with the higher incidence in western populations and is notoriously resistant to all current cancer therapy. Indeed, the reason of the limited success of immunotherapeutic approaches could be the ability of melanoma cells to escape immune response and alter the function and survival of immune cells. Interestingly, almost 50% of metastatic melanoma, in contrast to skin melanocytes, expresses constitutively the major histocompatibility complex (MHC) class II which is associated to disease progression and is linked to a poor prognosis. The MHC class II molecules during T cell/ professional antigen-presenting cells (APCs) interactions are localised, as signalling receptors, to membrane lipid rafts which are thought to be sites where transmembrane signalling complexes assemble. In the aim to understand the molecular mechanisms used by melanomas to frustrate an effective anti-tumour response, we studied in MHC class II constitutive expressing melanoma cell lines, the membrane localization of the class II molecules as well as the MHC class II signalling. Material and methods: The class II constitutive expressing melanoma cells (A375 and HT144 cell lines) were stimulated with a specific anti-HLA-DR mAb (L243) that mimics the TCR interaction with the class II molecules or left unstimulated. The lipid rafts of stimulated and unstimulated melanoma cells were isolated by discontinuous sucrose gradient and analysed by western blot. Exosomes secreted by stimulated and unstimulated melanoma cells were purified and analysed by western blot. Results: Within the hypothesis that MHC class II constitutive expressing melanoma cells might mimic an APC, we stimulated the HT144 melanoma cells with L243 monoclonal antibody and we isolated the lipid rafts. In these membrane domains of stimulated HT144 cells we observed an increased localisation of HLA-DR molecules as reported for A375 cells. Therefore, we compared the expression level of some adhesion receptors as well as the exosomes secreted by stimulated and unstimulated A375 and HT144 melanoma cells. Conclusions: Therefore, our results underline the role played in melanoma cells by the MHC class II dependent signalling on motility and exosomes functionality. The study of the signalling activated by class II molecules could help to elucidate how affect the metastatic dissemination ability of melanoma cells as well as the role of exosomes on the microenvironment of tumour sites.