Immunotargetting of the Wilms’ Tumor WT1 Antigen for Dendritic Cell and B-Cell-Based Vaccination of Leukemia.

The Wilms’ tumor antigen (WT1) is overexpressed in almost all leukemias and in several solid tumors. Overexpression of WT1 blocks the normal differentiation and enhances proliferation of hematopoietic progenitor cells. WT1 is also used in the detection of minimal residual disease. Using WT1-specific MHC class I tetramers, we were able to detect ex vivo low numbers of WT1-specific CD8+ T cells in the peripheral blood or bone marrow of leukemia patients, but not of healthy donors. In one particular donor we could detect up to 24% WT1 tetramer positive cells at the time of diagnosis. WT1 tetramer positive cells were present in all types of leukemia, except for CLL, and also in patients with MDS. Because WT1 plays an important role in leukemogenesis, it could serve as an antigenic target for dendritic cell-based immunotherapy. We used the mRNA electroporation strategy that allows presentation of multiple WT1 epitopes by MHC class I molecules, irrespective of the HLA haplotype. Monocyte-derived DC (Mo-DC) were electroporated with in vitro transcribed WT1 mRNA. RT-PCR and Western blot analysis showed that WT1 RNA and protein, respectively, was present for up to 5 days in WT1-electroporated DC, but not in mock- or EGFP mRNA-electroporated Mo-DC. Importantly, using a CD8+ T cell clone that secretes IFN-gamma upon recognizing the HLA-A2 immunodominant WT1_126–134 epitope, we showed that WT1 mRNA-electroporated Mo-DC processed the WT1 protein via the MHC class I pathway and presented the WT1 epitope to the T cells in an HLA- and antigen-specific manner. Since Mo-DCs are a non-expandable source of antigen-presenting cells, we also used proliferating CD40-activated B (CD40-B) cells as inducers for WT1-specific T cell immunity. CD40-B cells were expanded to high numbers from a limited amount of peripheral blood and subsequently electroporated with WT1 mRNA. In T cell clone activation experiments, WT1 mRNA-electroporated CD40-B cells were as efficient as Mo-DC in presenting the WT1 epitope in a MHC class I-restricted manner. Based on these results, we are currently focusing on the in vitro (re)activation of autologous WT1-specific cytotoxic T cells of leukemia patients using WT1-loaded autologous Mo-DC or CD40-B cells and on the immunological parameters to break immune tolerance against the WT1 tumor self antigen.