By using mAb and flow cytometry, a constitutive expression of the p75 IL-2R was revealed in human peripheral blood CD8+ T cells and TCR delta-1+ T cells as well as in CD16+ NK cells. Anti-p75 IL-2R mAb almost completely inhibited the induction of cytolytic activity in these T cells by brief exposure to IL-2, as estimated by anti-TCR/CD3 mAb-targeted cytotoxicity. While anti-p55 IL-2R mAb alone inhibited the response only modestly, maximal inhibition was achieved by combining both anti-p55 and anti-p75 IL-2R mAbs. These results indicate that the p75 IL-2R constitutively expressed on peripheral blood CD8+ T cells and TCR delta-1+ T cells is predominantly responsible for the direct activation of these cells by IL-2.

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