Gamma interferon expression in CD8(+) T cells is a marker for circulating cytotoxic T lymphocytes that recognize an HLA A2-restricted epitope of human cytomegalovirus phosphoprotein pp65

Antigen-specific CD8(+) T cells with cytotoxic activity are often critical in immune responses to infectious pathogens. To determine whether gamma int erferon (IFN-gamma) expression is a surrogate marker for cytotoxic T lympho cytes (CTL), human cytomegalovirus-specific CTL responses were correlated w ith CD8(+). T-cell IFN-gamma expression determined by cytokine Bow cytometr y, A strong positive correlation was observed between specific lysis of pep tide-pulsed targets in a Cr-51 release assay and frequencies of peptide-act ivated CD8(+) T cells expressing IFN-gamma at 6 h (r(2) = 0.72) or 7 days ( r(2) = 0.91). Enumeration of responding cells expressing perforin, another marker associated with CTL, did not improve this correlation. These results demonstrate that IFN-gamma expression can be a functional surrogate for id entification of CTL precursor cells.