Role of Different T Cell Receptors in the Development of Pre–T Cells

The development of pre–T cells with productive TCR-β rearrangements can be mediated by each the pre–T cell receptor (pre-TCR), the TCR-αβ as well as the TCR-γδ, albeit by distinct mechanisms. Although the TCR-γδ affects CD4⁻8⁻ precursor cells irrespective of their rearrangement status by TCR-β mechanisms not involving TCR-β selection, both the preTCR and the TCR-αβ select only cells with productive TCR-β genes for expansion and maturation. The TCR-αβ appears to be much less effective than the pre-TCR because of the paucity of TCR-α proteins in TCR-β–positive precursors since an early expressed transgenic TCR-αβ can largely substitute for the pre-TCR. Thus, the TCR-αβ can assume a role not only in the rescue from programmed cell death of CD4⁺8⁺ but also of CD4⁻8⁻ thymocytes. In evolution this double function of the TCR-αβ may have been responsible for the maturation of αβ T cells before the advent of the pre–TCR-α chain.