Abstract
Cancer immunity is mediated through the effective priming and activation of tumour-specific class I MHC molecule-restricted CD8+ cytotoxic T lymphocytes (CTLs). DEC-205+ dendritic cells (DCs) can cross-present the epitope(s) of captured tumour antigens associated with class I MHC molecules alongside co-stimulatory molecules to prime and activate tumour-specific CD8+ CTLs. Immunosuppressive tolerogenic DCs with reduced co-stimulatory molecules may be a cause of impaired CTL induction. Hepa1-6-1 cells were established from the mouse hepatoma cell line Hepa1-6; these cells grow continuously after subcutaneous implantation into syngeneic C57BL/6 (B6) mice and do not prime CD8+ CTLs. In this study, we show that the growth of ongoing tumours was suppressed by activated CD8+ CTLs with tumour-specific cytotoxicity through the administration of the glycolipid α-galactosylceramide (α-GalCer), which is a compound known to stimulate invariant natural killer T (iNKT) cells and selectively activate DEC-205+ DCs. Moreover, we demonstrated that sequential repetitive intraperitoneal inoculation with α-GalCer every 48 hr appeared to convert tolerogenic DEC-205+ DCs into immunogenic DCs with a higher expression of co-stimulatory molecules and a stronger cross-presentation capacity, which primed CTL precursors and induced tumour-specific CD8+ CTLs within the tumour environment without activating iNKT cells. These findings provide a new basis for cancer immunotherapy to convert tolerogenic DEC-205+ DCs within tumours into immunogenic DCs through the sequential administration of an immuno-potent lipid/glycolipid, and then activated immunogenic DCs with sufficient expression of co-stimulatory molecules prime and activate tumour-specific CD8+ CTLs within the tumour to control tumour growth.