New anti-tumor vaccine and immunotherapeutic strategies based on optimal stimulation of CD4+ T Helper cells.

Vaccination with a poorly or not immunogenic tumor fails to protect the host from a subsequent challenge with the same tumor. The mechanisms underlying the failure of these tumors to sensitize therapeutic T cells are not clearly understood, but the inability of host T cells to recognize tumor has been implicated. Previous works from our laboratory have shown that the highly aggressive BALB/c mammary adenocarcinoma TS/A, can be rejected if genetically modified to express MHC class II genes upon transfection of the MHC class II transactivator CIITA. MHC class II molecules are required for presenting antigenic peptides to T helper cell and thus activate the cascade of events leading to immune effector functions such as antibody production and cytolytic T cell activity. The purpose of my thesis was to investigate whether the above approach could be generalized and thus extended to tumors of distinct histological origin. Moreover, it was assessed whether anti-tumor lymphocytes generated by this approach could be used as an immunotherapeutic tool for established cancers. Beside the previously described TS/A-CIITA breast carcinoma cells, stable CIITA-transfectants of colon adenocarcinoma C51, renal adenocarcinoma RENCA, and sarcoma WEHI-164, were generated. Tumor cells transfectants were injected in vivo and their grow kinetics and recipient’s immune response were analyzed. Tumor rejection and/or retardation of growth was found for CIITA-transfected C51 and RENCA adenocarcinomas, as well as WEHI-164 sarcoma. As for TS/A, this tumor rejection correlated mostly with the stability and the amount of CIITA-induced MHC class II expression. Interestingly, mice rejecting CIITA-transfected tumors acquired specific immunological memory, as demonstrated by resistance to challenge with untransfected parental tumors. Adoptive cell transfer experiments demonstrated that tumor immunity correlates with CD4+ and CD8+ T lymphocytes. In this thesis, it is also shown that CIITA-induced MHC class II expression on tumor cells is able to generate antitumor therapeutic T cells. Indeed, T cells from TS/A-vaccinated mice were used in an adoptive cell immunotherapy (ACT) model of established tumors. The results showed the cure at early stages and a significantly prolonged survival at later stages of tumor progression. Importantly, CD4+ T cells were clearly superior to CD8+ T cells in anti-tumor protective function. Interestingly, the protective phenotype was associated to both a Th1 and Th2 polarization of the immune effectors. Of great importance, the T cells obtained from vaccinated mice were therapeutic also without other in vitro activation passages, like treatment with anti-CD3 and IL-2, differently to some other approaches of vaccination and ACT. Furthermore, ACT induced proliferation of tumor-specific immune splenocytes in receiving mice, leading to rejection of subsequent tumor challenge or to protection from metastasis in animals having established tumors. These data support our original idea that CIITA-expressing tumor cells can act as antigen-presenting cells (APCs) for their own tumor-associated antigens to CD4+ T cells, to induce specific and potent anti-tumor responses. These results establish the general application of our tumor vaccine model. This approach let us to envisage additional applications of this strategy for producing better lymphocyte effectors for adoptive anti-tumor cell immunotherapy, particularly in those cases of tumors non-responsive to existing therapies.