Induction of a CD4+ T cell response against MAGE-A3 in mRNA electroporated dendritic cells: introducing T cell help in cancer immunotherapy

Despite the fact that major progress has been made in the treatment of cancer patients, many cancers remain incurable with conventional therapies such as chemotherapy, radiotherapy or surgery. In addition, many patients who initially showed clinical improvement eventually relapse. Their tumors often acquire a multi-drug resistant phenotype, and thus they do not longer benefit from classical therapies. Therefore, these patients could benefit from alternative strategies such as the administration of anti-tumor vaccines.Dendritic cells (DCs) are the most potent antigen presenting cells of the immune system because they have the unique ability to stimulate naive T lymphocytes. Therefore, these cells are ideal candidates to act as vectors for immunotherapy of cancer. It has been shown that DCs presenting epitopes derived from tumor associated antigens (TAAs) have the capacity to induce antigen specific anti-tumor responses. Among the broad range of strategies to load DCs with TAA derived epitopes, we chose the electroporation of DCs with in vitro produced mRNA. This method has already proven its efficacy since mRNA loaded DCs have been shown to be efficient stimulators of the immune system both in vitro and in vivo. It is also a very safe tool for clinical trials, since mRNA is not immunogenic, has a relatively short half-life and lacks the potential to integrate into the host genome.After immunization, the DCs will present the TAA to the different cells of the immune system, which should ultimately result in the generation of a cellular immune response against tumor cells expressing this TAA. Since CD8+ cytotoxic T lymphocytes (CTLs) are important effector cells during tumor rejection, most attention has been given to these cells. However, it has become clear over the years that CD4+ T helper cells play an essential role in initiating and maintaining both primary and memory CD8+ T cell responses. Consequently, the development of new strategies to induce T helper responses is very important for the development of optimal anti-cancer vaccines. In this project, we mainly focused on the induction of a CD4+ T cell response against MAGE-A3 by mRNA electroporated dendritic cells.The increased interest in introducing T cell help in anti-tumor immunotherapy has rekindled the search for tumor antigens recognized by CD4+ T cells, which are now being identified with increasing frequency. The identification of defined tumor antigen derived, HLA class II restricted epitopes has made it possible to target these peptides to the HLA class II processing compartments. We describe the targeting of defined MAGE-A3 derived epitopes to the class II compartments by virtue of a recombinant invariant chain (Ii). We engineered genetic constructs where CLIP has been replaced by sequences encoding HLA-DR13 or -DP4 restricted MAGE-A3 epitopes. Monocyte derived DCs were electroporated with in vitro transcribed mRNA encoding the recombinant Ii constructs. These electroporated DCs could efficiently stimulate MAGE-A3 specific T cell clones and were able to stimulate unprimed CD4+ T cells specific to the HLA-DR13 or -DP4 restricted MAGE-A3 epitope in vitro. In accordance with other groups, we describe that the presentation of the MHC class II epitopes using CLIP replacement is superior to peptide pulsing.A disadvantage of the 'CLIP replacement' strategy is that only defined epitopes can be used, and these will only be presented in the context of HLA class II molecules. In order to obtain presentation of TAA derived epitopes in the context of both HLA class I and class II molecules, we linked the TAA encoding information to a HLA class II pathway sortin