Characterization of normal human CD3⁺ CD5^- and γδ T cell receptor positive T lymphocytes

The functional and phenotypic properties of normal human CD3⁺ CD5^- T cells which have a higher frequency of cytotoxic cells than CD3⁺CD5⁺ T lymphocytes have been described. Using three- and four-colour immunofluorescence flow cytometric cell sorting, the CD3⁺CD5^- and CD3⁺CD5⁺ populations were subdivided into αβ or γδ T cell receptor positive cells. The four subsets were examined for the in vitro cytotoxic activity and were also stimulated with mitogens in limiting-dilution assays to measure the frequencies of proliferating and interleukin-2 (IL-2) producing cells. CD3⁺CD5^-αβ, CD3⁺CD5^-γδ⁺ and CD3⁺CD5⁺γδ⁺ cells had lower frequencies of proliferating and IL-2-producing cells than did CD3⁺CD5⁺αβ⁺ cells. However, the cytotoxic activity of the different phenotypes was higher in the CD3⁺CD5^- subsets, especially when these cells were γδ⁺ Expression of γδ or lack of expression of CD5 appeared to be associated with the acquisition of cytolytic potentials. CD8 was expressed on 20% of fresh CD3⁺γδ⁺ cells. Cultured γδ cells retained the expression of γδ, but quickly lost that of CD8 and with time modulated the expression orCD5, The expression of CD5 was found to be higher on sorted CD3⁺CD5⁺γδ^- than on CD3⁺CD5⁺γδ⁺ cells. These observations indicate that γδ is preferentially expressed on CD5-negative or weakly positive T lymphocytes and that CD3⁺CD5^-γδ cells appear to constitute a discrete small subset of mature T lymphocytes which are cytotoxic in nature. However, the exact immunological function of these cells and their place in T cell ontogeny are yet to be elucidated.