CD4⁺CD8^dim T cells represent a minor subset of the total CD3⁺ T cell population in peripheral blood. Although transient and persistent expansions of these cells havebeen reported in both healthy and diseased individuals, the functional properties of the CD4⁺CD8^dim population are largely unknown. In this study, we examined antigen‐specific cytokine and proliferative responses of the CD4⁺CD8^dim subset. In whole blood cultures stimulated with the viral antigens HCMV and HIV‐1, a significant fraction of the CD4⁺CD8^dim subset exhibited cytokine expression and proliferation in response to antigen activation. Typically, the CD4⁺CD8^dim population contained two‐ to eightfold higher frequencies of antigen‐specific cytokine producing cells than the CD4⁺CD8^‐ population. Phenotypic analysis of the cytokine expressing CD4⁺CD8^dim population indicated that these cells are memory T cells, with a high frequency of this population expressing the cytotoxic markers CD56 and perforin. Furthermore, the CD4⁺CD8^dim cytokine responses to CMV were shown to be MHC class II dependent. Significantly, purified CD4⁺CD8^dim T cells were found to possess higher CMV‐specific cytotoxic activity than purified CD4⁺CD8^– T cells in a standard ⁵¹Cr‐release CTL assay. Thus, CD4⁺CD8^dim T cells appear to be MHC class II dependent, are capable of cytolytic effector activity, and are highly enriched within the CD4⁺ cell populations specific for HCMV and HIV‐1.

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