Modulation of MHC class-I molecules on melanoma cells after photodynamic treatment

Full text: Endogenous antigenic peptides are presented in the context of MHC class-I molecules on the cell surface for recognition by CD8+ T lymphocytes. Down-regulation of MHC molecules is a frequently observed strategy of tumor cells to escape immune attack. E.g., B16 melanoma is characterized by extremely low MHC-I surface expression and high tumorigenicity in syngeneic mice. Generally, the efficiency of photodynamic therapy is low for melanotic tumors. On the other hand, PDT has been shown capable of inducing anti-tumoral immunity. Therefore, we investigated the effect of PDT treatment in vitro on the MHC class-I surface expression of surviving B16 cells. When sensitized with 50 ng/mL hypericin and then irradiated the viability of the cells gradually decreased with increasing light dose. However, with 4 J/cm2 50 % of cells were still viable after 24 hours. Analysis by flow cytometry revealed that a subpopulation of these cells had significantly elevated the surface density of MHC class-I molecules (fluorescence intensity approx. 5-fold over that of untreated cells). These findings suggest that repetitive PDT might facilitate CTL-mediated cytolysis of tumor cells and might, therefore, synergize with immunotherapeutic approaches for at least some tumors. (author)