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Keywords:

  • CD8+ T cells;
  • immune monitoring;
  • melanoma;
  • peptide-based vaccine;
  • tetramer

Abstract

ECOG 1696 was a Phase II multi-center trial testing vaccination with melanoma peptides, gp100, MART-1 and tyrosinase delivered alone, with GM-CSF, IFN-α2b or both cytokines to HLA-A2+ patients with metastatic melanoma. Here, the frequency of circulating CD8+tetramer+ (tet+) T cells and maturation stages of responding T cells were serially monitored and compared with baseline values in a subset of patients (n = 37) from this trial. Multiparameter flow cytometry was used to measure the frequency of CD8+ T cells specific for gp100, MART-1, tyrosinase and influenza (FLU) peptides. Expression of CD45RA/CCR7 on CD8+tet+ T cells and CD25, CD27, CD28 on all circulating T cells was determined. Vaccine-induced changes in the CD8+tet+ T cell frequency and phenotype were compared with results of IFN-γ ELISPOT assays and with clinical responses. The frequency of CD8+tet+ T cells in the circulation was increased for the melanoma peptides (p < 0.03–0.0001) but not for FLU (p < 0.9). Only gp100- and MART-1-specific T cells differentiated to CD45RA+CCR7- effector/memory T cells. In contrast to the IFN-γ ELISPOT frequency, previously correlated with overall survival (Kirkwood et al., Clin Cancer Res 2009;15:1443-51), neither the frequency nor differentiation stage of CD8+tet+ T cells correlated with clinical responses. Delivery of GM-CSF and/or IFN-α2b had no effects on the frequency or differentiation of CD8+tet+, CD8+ or CD4+ T cells. Phenotypic analyses of CD8+tet+ T cells did not correlate with clinical responses to the vaccine, indicating that functional assessments of peptide-specific T cells are preferable for monitoring of anti-tumor vaccines.