CD28-mediated costimulation is necessary for the activation of T cell receptor-γδ⁺ T lymphocytes

The role of costimulation in the activation of TCR-γδ cells in normal mice and mice transgenic (tg) for a TCR-γδ receptor was investigated. Activation of TCR-γδ cells required two signals. One signal was mediated by TCR occupancy, whereas a second signal was provided by accessory cells. The importance of the CD28/B7 interaction in the delivery of the second signal was demonstrated in multiple ways. First, addition of a soluble fusion protein homolog of CD28, CTLA41g, significantly inhibited the activation of G8 tg splenic TCR-γδ lymphocytes and intestinal epithelial TCR-γδ lymphocytes by Ag-bearing lymphocytes during primary stimulation. Similarly, both proliferation and IFN-γ production were inhibited by addition of CTLA41g to secondary antigenic stimulation of G8 tg TCR-γδ cells. Second, an Ag-bearing thymoma, EL-4, was only able to stimulate expanded G8 tg TCR- γδ cells when the thymoma expressed B7. This stimulation was blocked by both CTLA41g and anti-B7 antibody. Third, antibodies to CD28 were able to mimic the costimulatory affect of APC. TCR-γδ cells cultured with either Ag-bearing fixed stimulator cells or submitogenic concentrations of immobilized anti-pan TCR-γδ mAb proliferated only in the presence of anti- CD28 mAb. Finally, G8 tg cells produced IL-2 only in the presence of APC costimulation or anti-CD28 antibodies, and the addition of exogenous rIL-2 overcame the need for costimulation. Thus, autocrine IL-2 production is one of the major consequences of TCR-γδ cell costimulation. Together these data demonstrate that costimulation is necessary for the activation of TCR-γδ cells and can occur through CD28 interaction.