New approaches of cancer vaccination and cancer immunotherapy based on the optimal stimulation of tumor-specific, MHC class II-restricted CD4+ T helper cells

Recently we have shown that a number of murine tumors arising from mice of the Balb/c (MHC H-2d) strain, induced to express MHC class II molecule after transfection with CIITA (MHC class II transactivator) can be rejected effectively, generating tumor specific T helper cell (TH) triggering, immunological memory and protection even against parental tumor. The aims of this investigation were: a) ??? to assess whether the same results can be extended to tumors of different MHC background (H-2b); b) ??? to investigate, using dendritic cell knock-out C57Bl6 mice, whether CIITA transfected tumor cells can act as ???surrogate APC??? for their tumor antigens in vivo. To this end, LLC (Lewis lung carcinoma) cells were stably transfected with CIITA and selected for expression of MHC class II molecules. Parental tumor cells and CIITA-transfected cells (LLC-CIITA) were injected subcutaneously into C57/BL6 mice and tumor take and growth kinetics were assessed. Mice injected with LLC-CIITA cells fully rejected the tumor or remained tumor-free for longer time than mice injected with parental tumor cells. The growth kinetics and the size of CIITA-expressing tumors were significantly reduced compared to the parental tumor. Adoptive cell transfer of purified CD4+ TH cells from mice injected with LLC-CIITA into naïve mice demonstrated that these cells were able to protect from LLC parental tumor growth. Taken together these results strongly suggest that, similarly to H-2d strain, also H-2b tumors can be rejected if expressing CIITA-mediated MHC class II molecules, confirming the general applicability of our tumor vaccination model. To achieve the second goal we performed in vivo experiments in a novel transgenic mouse model designated CD11.c DOG, in which it is possible to induce a conditional depletion of dendritic cells (DC) by diphtheria toxin (DT) treatment. These mice express the diphtheria toxin receptor (DTR) under the control of the promoter of the CD11c molecule, expressed preferentially in DC. Once injected with DT, dendritic cells are eliminated for the period of treatment up to 12 days. DT-treated CD11c.DOG mice where injected with LLC-CIITA tumor cells and tumor growth was followed during time. We found that LLC-CIITA cells can be recognized and rejected better than parental tumor even in CD11c-DOG mice. These results suggest that CIITA-tumor cells may act in vivo as surrogate APCs for their own tumor antigens and, most importantly,they can prime virgin CD4+ TH cells to trigger a protective adaptive immune response against the tumor.