Missing HLA class I expression on Daudi cells unveils cytotoxic and proliferative responses of human γδ T lymphocytes

The major subset of human blood γδ T lymphocytes expresses the variable-region genes Vγ9 and Vδ2. These cells recognize non-peptidic phosphoantigens that are present in some microbial extracts, as well as the β2-microglobulin-deficient Burkittʹs lymphoma Daudi. Most cytotoxic human Vγ9/Vδ2 T cells express inhibitory natural killer cell receptors for HLA class I that downmodulate the responses of the γδ T lymphocytes against HLA class I expressing cells. In this study we show that transfection of the human β2-microglobulin cDNA into Daudi cells markedly inhibits the cytotoxic and proliferative responses of human Vγ9/Vδ2 T cells. This provides direct evidence that the “innate” specificity of human Vγ9/Vδ2 T-lymphocytes for Daudi cells is uncovered by the loss of β2m by Daudi. However, Daudi cells that express HLA class I in association with mouse β2m at the cell surface are recognized by human Vγ9/Vδ2 T cells close to the same degree as the parental HLA class I deficient Daudi cell line. Thus, proper conformation of the HLA class I molecules is required for binding to natural killer cell receptors. Cloning of the HLA class I A, B, and C molecules of Daudi cells and transfer of the individual HLA class I molecules of Daudi cells into the HLA class I deficient recipient cell lines .221 and C1R demonstrate that for some human γδ T-cell clones cytolysis can be entirely inhibited by single HLA class I alleles while for other clones single HLA class I alleles only partially inhibit cytotoxicity. Thus, most human Vγ9/Vδ2 T cells represent a population of killer cells that evolved like NK cells to destroy target cells that have lost expression of individual HLA class I molecules but with a specificity that is determined by the Vγ9/Vδ2 TCR.