Function but not phenotype of melanoma peptide-specific CD8+ T cells correlate with survival in a multiepitope peptide vaccine trial (ECOG 1696).

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. [ABSTRACT FROM AUTHOR]