Maturation of thymocytes with a monoclonal TCR under control of Trac promoter elements in the absence of β-selection.

Thymocyte maturation is a tightly controlled and sequential process of T cell receptor (TCR) gene rearrangement that generates a broad repertoire of T cells with minimal self-reactivity. We previously generated TCR exchange (TRex) mice by targeting a mesothelin-specific "1045" TCR to the Trac locus in murine zygotes. While 1045 T cells from TRex mice display physiological development and function, some T cells coexpress endogenous TCRβ chains, suggesting that β-selection is required for 1045 T cell development. Here, we evaluate thymocyte maturation in the setting of compromised β-selection by deleting endogenous Tcrb or Rag2 in TRex mice. T cells readily form in TRex mice lacking Tcrb, though thymocytes mature through developmental trajectories that appear dependent on interleukin-7 and γδTCR. In contrast, mature T cells fail to form in the absence of Rag2. Maturation of αβ thymocytes bypassing β-selection is reduced by 100-fold, in part because γδTCR+ precursors are biased to form conventional γδ T cells. Nevertheless, in TRex mice, these unconventional β-selection-independent trajectories yield a mature αβ T cell population with uniform TCR expression and pronounced function.