Binding of prostate-specific membrane antigen to dendritic cells: a critical step in vaccine preparation

Background aims Dendritic cell (DC)-based vaccines hold promise as a safe therapy for prostate cancer (PCa), and prostate-specific membrane antigen (PSMA) fulfils the requirements for a tumor-associated antigen (TAA) to be clinically effective. We evaluated the actual binding of selected HLA-A2-restricted PSMA peptides to HLA class I molecules on ex vivo-generated mature (m) DC. Methods mDC were generated from peripheral monocytes of HLA-A2 normal donors. The PSMA peptides PSMA(711) (ALFDIESKV), PSMA(27) (VLAGGFFLL) and PSMA(663) (MMNDQLMFL) were selected based on computer-assisted prediction programs, documented CTL epitope activity or previous use in clinical trials. The model cell line T2 and the clinical grade (CD83(+) CCR7(+)) mDC were pulsed with fluorescein (FL)-conjugated peptides and an anti-HLA-A2 monoclonal antibody (MAb) and analyzed. Results Flow cytometry analysis showed best binding efficiency to be by PSMA(27.) Confocal microscopy confirmed coincident fluorescence emission of HLA-A2 MAb and FL-PSMA(27). Virtual co-localization of PSMA(27) and HLA class I molecules was supported further by fluorescence resonance energy transfer (FRET) analysis. The clinical relevance of our findings has to be validated in vivo. Conclusions The present report is the first to score selected PSMA peptides based on their detectable binding to mDC. It identifies PSMA(27) as the choice candidate among other PSMA peptides and it should be included in developing DC vaccine protocols for HLA-A2 PCa patients.