The analysis of the natural killer-like activity of human cytolytic T lymphocytes revealed HLA-E as a novel target for TCR α/β-mediated recognition

Cytolytic T lymphocytes (CTL) are known to recognize antigen peptides in association with major histocompatibility complex (MHC) class I molecules expressed on target cells. However, a fraction of human CD8⁺ CTL has been shown to lyse certain natural killer (NK)-susceptible target cells via still undefined mechanism(s). These CD8⁺ T cells, hereafter referred to as NK-CTL, are frequently composed of cells expressing one single TCR Vβ expansion (different in different individuals), display a memory phenotype and express HLA class I-specific inhibitory NK receptors. Here we show that cell populations or clones of NK-CTL isolated from three healthy donors homogeneously expressed Vβ16, Vβ9 and Vβ3 TCR, respectively. Various clones isolated under limiting dilution conditions from Vβ16⁺ cells of donor 1 displayed identical TCR Vβ and Vα rearrangements, thus suggesting a substantial monoclonality of the NK-CTL subset analyzed. NK-CTL lysed a number of NK-susceptible tumor target cells with the exception of those characterized by β2-microglobulin (β2m) deficiency. However, the latter targets became susceptible to lysis upon β2m transfection. Using monoclonal antibodies specific for the relevant TCR Vβ or β2m we provide evidence suggesting that target cell lysis by NK-CTL is mediated by the TCR itself upon recognition of β2m-associated proteins. The cellular distribution of the potential β2m-associated proteins in susceptible target cells suggested, as a likely candidate for TCR-mediated recognition, the non-classical HLA-E molecule. The use, as target cells, of the murine TAP2-deficient RMA-S cells, either untransfected or transfected with HLA-E, and loaded with an appropriate HLA-E-binding peptide, provided the direct demonstration that HLA-E represents a ligand recognized by the TCR expressed by NK-CTL. This is the first evidence that human TCR α/β can recognize HLA-E molecules, thus revealing a novel type of TCR-mediated recognition, which may offer new insight in immune responses in both normal and disease conditions.