Comparative Analysis of CD8⁺ t Cell Priming Efficiencies against a Panel of Leukemia-Associated HLA-A2 Restricted Epitopes Identifies PRAME as a Possible Vaccination Target in AML

Background Therapeutic vaccination with dendritic cells (DCs) is regarded as an option for the treatment of minimal residual disease in acute myeloid leukemia (AML). In this respect, defined epitope sequences derived from Leukemia-Associated Antigens (LAA) are an attractive source of antigen. WT1, PRAME, hTERT, and survivin are all LAA that are expressed in various hematological malignancies and that share a potential for the induction of cellular immune responses. Expression rates in AML of each of these LAA are high, ranging from 60 to 90%. To evaluate the feasibility of using these LAA in DC based vaccine formulations in AML, we investigated the relative Cytotoxic T Lymphocyte (CTL) priming efficiencies in response to a panel of LAA-derived HLA-A2 binding epitopes.

Methods CD8⁺ cells were isolated from peripheral blood of healthy donors (HD) and stimulated with peptide-loaded DCs. DCs were derived from the human AML cell line MUTZ-3 (MUTZ3-DCs).¹ The peptides used were WT1 (126–134), PRAME (100–108), hTERT (540–548), and survivin (96–104). Irradiated CD8⁻ cells from the same donor were also added. CD8⁺ cells were plated at one million per well in 8 to 30 wells. Weekly CD8⁺ cells were restimulated with peptide-loaded MUTZ3-DCs. HLA-tetramer positivity (Tm⁺) for WT1, PRAME, hTERT or survivin epitopes was weekly screened by flow cytometry. Wells were scored positive if more than 0.02% Tm⁺, viable CTLs were present for at least 2 weeks. Functionality of Tm⁺ cells was studied by analysis of IFN-gamma production after stimulation with the JY cell line loaded with relevant LAA peptides or with an irrelevant control peptide (BCR-ABL).

Results Percentages of the induced peptide-specific CTLs were low as shown in the table below. Priming efficiencies were highest for PRAME. Furthermore, PRAME and WT1 specific CTLs produced IFN-gamma in an epitope specific fashion, confirming functionality of the primed T cells.

Conclusions On the whole, the detected LAA-specific CD8⁺ T cell frequencies were low. Most consistently successful priming results were obtained for PRAME, indicating that this peptide may be an attractive candidate for inclusion in AML-specific DC-based vaccine regimens.