Lineage diversion of T cell receptor transgenic thymocytes revealed by lineage fate mapping

Background. The binding of the T cell receptor (TCR) to major histocompatibility complex (MHC) molecules in the thymus determines fates of TCRαβ lymphocytes that subsequently home to secondary lymphoid tissue. TCR transgenic models have been used to study thymic selection and lineage commitment, Most TCR transgenic mice express the rearranged TCRαβ prematurely at the double negative stage and abnormal TCRαβ populations of T cells that are not easily detected in non-transgenic mice have been found in secondary lymphoid tissue of TCR transgenic mice. Methodology and Principal Findings. To determine developmental pathways of TCR-transgenic thymocytes, we used Cre-LoxP-mediated fate mapping and show here that premature expression of a transgenic TCRαβ diverts some developing thymocytes to a developmental pathway which resembles that of gamma delta cells. We found that most peripheral T cells with the HY-TCR in male mice have bypassed the RORyt- positive CD4′ 8′ (double positive, DP) stage to accumulate either as CD4 ^-8^- (double negative, DN) or as CD8α ⁺ T cells in lymph nodes or gut epithelium. Likewise, DN TCRαβ cells in lymphoid tissue of female mice were not derived from DP thymocytes. Conclusion. The results further support the hypothesis that the premature expression of the TCRαβ can divert DN thymocytes into gamma delta lineage cells.