Origins of γδ T cell effector subsets: a riddle wrapped in an enigma.

αβ and γδ T cells are thought to arise from a common precursor in the thymus but play distinct roles in pathogen resistance. Although conventional αβ T cells exit the thymus in a naive state and acquire effector function in the periphery, the effector fate of many γδ T cells is specified in the thymus and exhibits limited plasticity thereafter. This review describes the current models that have been proposed to explain the acquisition of effector fate by γδ T cells, as well as the apparent linkage to Vγ gene usage. The two predominant models are the predetermination model, which suggests that effector fate is determined prior to TCR expression, perhaps in association with the developmental timing of Vγ rearrangement, and the TCR-dependence model, which proposes that the nature of the TCR signal, particularly its intensity or duration, plays an important role in influencing effector fate.