Molecular and cellular analysis of human T lymphocytes expressing γδ T-cell receptor

A minor subset of T lymphocytes expresses a CD3-associated TCR composed of γ and δ chains. The majority of TCR γ/δ⁺ cells lack surface CD4 and CD8 antigen and do not react with WT31 mAb. These negative criteria were utilized in early studies to identify TCR γ/δ⁺ cells. More recently, mAb to TCR γ/δ, selected in different laboratories, have permitted the direct identification of TCR γ/δ⁺ cells and their subsets. TCR γ/δ molecules were found to be heterogeneous in size and charge mobility. Two major forms of TCR γ/δ could be identified that are characterized by the presence or absence of an inter-chain disulphide bond. Biochemical analysis originally suggested that a precise correlation existed between reactivity with BB3 or δTCS1/A13 mAb and expression of a disulphide (Cγ1-encoded) or non-disulphide linked (Cγ2-encoded) form of TCR γ/δ. However, more recent studies have indicated that these mAb react with the molecular product of Vδ2 or Vδ1, respectively. mAb directed to one or another form of TCR γ/δ activate the functional program of the cell, leading to intracellular Ca⁺⁺ mobilization, lymphokine production and triggering of the lytic machinery. Analysis of the target molecules for TCR γ/δ-mediated recognition revealed that at least some TCR γ/δ⁺ cells are capable of specific responses to (allo)antigen and that polymorphic determinants of class I molecules can be recognized (as shown by the specific lysis of P815 cells transfected with HLA-24 allele). Unlike TCR α/β⁺ cells, TCR γ/δ⁺ cells are homogeneously composed of cytolytic precursors, as shown by the analysis of a large panel of clones in both lectin-dependent and redirected killing assays. In spite of their LGL morphology, freshly isolated TCR γ/δ⁺ cells do not lyse NK-sensitive targets but do so after exposure to rIL-2. A modest cytolytic activity, however, could be induced also in fresh cells by anti-TCR/CD3 mAb in a redirected killing assay. Analysis of the distribution of the subsets expressing different TCR γ/δ types showed that BB3⁺ cells are prevalent in the peripheral blood and virtually absent in the thymus; in contrast, A13⁺ (δTCS1⁺) cells represent the majority of TCR γ/δ⁺ thymocytes. Electron microscopic analysis of fresh TCR γ/δ⁺ cells showed an extended cytoplasm containing numerous electron-dense granules identifiable as primary lysosomes. Upon stimulation with IL-2, TCR γ/δ⁺ cells, similar to other LAK cells, display an increase in their cytoplasmic granules together with a redistribution of cytoskeletal structures. In addition, the bizarre cell shape, the nuclear deformations and the emission of uropodia and filopodia with adhesion plaques support the concept of an active mobility of TCR γ/δ⁺ cells.