A more sensitive immunological monitoring tool for cancer vaccine trials: A direct comparison of HLA-A2 dimer and tetramer molecules in measuring the in vivo immune response to a HER2/neu peptide-based vaccine (E75) in prostate and breast cancer patients

Introduction: Prelimary results of our E75 vaccine clinical trials in breast and prostate cancer suggest that immunological responses correlate with recurrence prevention. The most widely used tool to measure immunologic responses in vaccine trials is the tetramer, despite known difficulties with this assay. Our own experience with the alternative, dimer assay, suggests it is comparable or superior to the tetramer but a direct comparison has never been performed. Methods: From 8 vaccinated breast and prostate cancer patients, we obtained peripheral blood mononuclear cell samples, stained them with E75 and FluM dimers and tetramers, and analyzed them by flow cytometry. Results: Of 8 patients, 6 samples stained positive for E75-dimer (0.1–0.9% CD8 cells) while only 3 samples stained positive for E75-tetramer (0.1–0.2% CD8 cells). Both dimer-negative samples were also negative by tetramer. While all samples stained positive for FluM-dimer (0.6–1.3% CD8 cells), only 5 samples stained positive for FluM-tetramer (0.1–0.2% CD8 cells). When E75- and FluM-stimulated cells were stained with dimer or tetramer, 1.6% and 0.4% of E75-stimulated cells stained positive by E75-dimer and -tetramer, respectively; 1.2% and 0.1% of FluM-stimulated cells stained positive by FluM-dimer and by FluM-tetramer. When co-stained with both, the tetramer stained only high-affinity dimer-positive cells while the dimer stained all tetramer-positive cells. Conclusions: In direct comparison, the HLA-A2:Ig dimer is more sensitive and versatile than the HLA-A2 tetramer in detecting vaccine-specific CD8 cells. The dimer may become the standard for the assessment of peptide-based cancer vaccine trials.