Alloreactive gamma delta thymocytes utilize distinct costimulatory signals from peripheral T cells

Interactions between CD28/CTLA-4 on T cells and CD80 (B7.1) and CD86 (B7.2) counter receptors provide crucial costimulatory signals for TCR-alpha beta+ lymphocytes. To test the role of CD28 in thymic development and activation of TCR-gamma delta+ T cells, we introduced the alloreactive V gamma 2V alpha 11.3 TCR into CD28-deficient mice (CD28-/-). We show that positive and negative selection of gamma delta Tg thymocytes proceeded normally in the absence of CD28. Although mature Tg gamma delta+ thymocytes required a second costimulatory signal for proliferation, gamma delta+ thymocytes from CD28-/- and CD28+/- littermates responded equally well to the alloantigen Tlab. Alloreactivity of CD28-/- and CD28+/- Tg gamma delta+ thymocytes could not be blocked with mAbs against CD80 and CD86 ligands. Thus gamma delta thymocytes utilize a costimulatory system during development and alloresponses that is independent of CD28/CD80 and CD28/CD86 interactions. By contrast to V gamma 2V alpha 11.3+ thymocytes, alloreactivity of V gamma 2V alpha 11.3+ lymph node T cells depended on CD28 costimulation and was severely impaired in CD28-/- mice. These data provide functional evidence that maturation and selection of gamma delta cells is independent of CD28. These results also indicate that distinct costimulatory pathways are operational in mature thymocytes and peripheral T cells.