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Human leukocyte antigen (HLA)-E is a nonclassical major histocompatibility complex (MHC) class I molecule of limited sequence variability that is expressed by most tissues albeit at low levels. HLA-E has been first described as the ligand of CD94/NKG2 receptors expressed mainly by natural killer (NK) cells, thus confining its role to the regulation of NK-cell function. However, recent evidences obtained by our and other groups indicate that HLA-E complexed with peptides can interact with αβ T-cell receptor (TCR) expressed on CD8+ T cells. Although, HLA-E displays a selective preference for nonameric peptides, derived from the leader sequence of various HLA class I alleles, several reports indicate that it can present also “noncanonical” peptides derived from both stress-related and pathogen-associated proteins. Because HLA-E displays binding specificity for innate CD94/NKG2 receptors, as well as all the features of an antigen-presenting molecule, its role in both natural and acquired immune responses has recently been re-evaluated.

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 Vbeta 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 Vbeta16, Vbeta9 and Vbeta3 TCR, respectively. Various clones isolated under limiting dilution conditions from Vbeta16(+) cells of donor 1 displayed identical TCR Vbeta and Valpha 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 beta2-microglobulin (beta2m) deficiency. However, the latter targets became susceptible to lysis upon beta2m transfection. Using monoclonal antibodies specific for the relevant TCR Vbeta or beta2m we provide evidence suggesting that target cell lysis by NK-CTL is mediated by the TCR itself upon recognition of beta2m-associated proteins. The cellular distribution of the potential beta2m-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 alpha/beta 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.