Segregation of type 1 cytokine production in human peripheral blood lymphocytes: Phenotypic differences between IFN-γ and IL-2-producing cells in the CD8+ T cell subset

T cell clones are classified as type 0, 1 or 2 depending on the lymphokines they produce. However, it has remained unclear whether single cells of a given type produce one or several cytokine species. Flow cytometric analysis of peripheral blood lymphocytes (PBL) obtained from 20 healthy donors for the production of the type 1 cytokines IFN-γ and IL-2 revealed very few cells that co-expressed both cytokines independently of the mitogenic stimulus used for PBL activation. Similarly, kinetic studies of cytokine synthesis indicated a low percentage of IFN-γ/IL-2 double-positive T cells at all time points. Reverse transcription-PCR analysis of sorted IL-2- and IFN-γ-positive T cells showed the presence of IL-2- or IFN-γ-specific mRNA only in those cells expressing the corresponding cytokine. This segregation of the two type 1 cytokines was lost in long-term cultured T cells and in T cell clones. A high percentage of cells expressing only IL-2 or IFN-γ was observed even when the production of these cytokines was evaluated on CD4+ and CD8+ subsets. Moreover, in some healthy individuals, IFN-γ and IL-2 production by CD8+ T cells was related to CD8+ expression levels and cell size, i.  e. IL-2-expressing cells were generally smaller with more intense CD8+ staining as compared with IFN-γ-producing T cells. These data indicate that activated T lymphocytes are strongly committed in vivo to produce IFN-γ or IL-2 and emphasizes the independent regulation of the two cytokine genes.