A novel T cell-based vaccine capable of stimulating long-term functional CTL memory against B16 melanoma via CD40L signaling

The ultimate goal of antitumor vaccines is to develop memory CD8+ cytotoxic T lymphocytes (CTLs), which are critical mediators of antitumor immunity. We previously demonstrated that the ovalbumin (OVA)-specific CD4+ T cell-based (OVA-TEXO) vaccine generated using OVA-pulsed dendritic cell (DCOVA)-released exosomes (EXOOVA) stimulate CTL responses via IL-2 and costimulatory CD80 signaling. To assess the potential involvement of other costimulatory pathways and to define the key constituent of costimulation for memory CTL development, we first immunized wild-type (WT) C57BL/6 and gene-knockout mice with WT CD4+ OVA-TEXO cells or OVA-TEXO cells with various molecular deficiencies. We then assessed OVA-specific primary and recall CTL responses using PE-H-2Kb/OVA257–264 tetramer and FITC-anti-CD8 antibody staining by flow cytometry. We also examined antitumor immunity against the OVA-expressing B16 melanoma cell line BL6-10OVA. We demonstrated that CD4+ OVA-TEXO cells stimulated more efficient CTL responses compared to DCOVA. By assessing primary and recall CTL responses in mice immunized with OVA-TEXO or with OVA-TEXO lacking the costimulatory molecules CD40L, 4-1BBL or OX40L, we demonstrated that these costimulatory signals are dispensable for CTL priming by OVA-TEXO. Interestingly, CD40L, but not 4-1BBL or OX40L, plays a crucial role in the development of functional memory CTLs against BL6-10OVA tumors. Overall, this work suggests that a novel CD4+ T cell-based vaccine that is capable of stimulating long-term functional CTL memory via CD40L signaling may represent a novel, efficient approach to antitumor vaccination.