Enumerating Antigen-Specific T-Cell Responses in Peripheral Blood

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Abstract
Detection of the circulating antigen-specific T-cell response to immunization is an important biologic end point in clinical trials of cancer vaccines. Typically employed assays are peptide MHC tetramer, ELISpot, and intracellular cytokine analysis. Although there is no agreement on the definition of a positive response in these assays, many groups have chosen a number of T cells greater than 2 standard deviations above the mean of the negative controls. The authors wished to determine how well this cutoff performed for each of these assays in detecting positive and negative T-cell responses to a model antigen, the immunodominant HLA-A*0201-restricted epitope of cytomegalovirus (CMV) pp65. For each assay, the mean + 2 standard deviations of the response for CMV seronegatives was the point that best separated the two groups. Using this value, each assay had a sensitivity of 87.5% and specificity of 95% to 100% and exhibited a high degree of concordance (kappa 0.76-0.9) with the other two. The authors conclude that currently available immunologic assays perform well in detecting biologically relevant levels of antigen-specific T cells. These assays will better define the quantity and quality of protective immune responses to viral disease and offer insight into the requirements for protective anti-cancer immunity.