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Positive selection of CD4/CD8 double-positive (DP) thymocytes generates helper and cytotoxic T lineages whose CD4 or CD8 co-receptor expression matches their αβ T cell receptor (TCRαβ) specificity MHCII or MHCI, respectively. Notch1 signaling is critical for early T cell development and was suggested to regulate CD4/CD8 lineage determination during DP thymocyte selection. However, early studies expressing activated Notch1 in pre-DP progenitors did not resolve this question. Here we showed that CD4-Cre-mediated activation of canonical Rbpj-dependent Notch1 signalling potently skews DP selection to the CD8 lineage. We used a tamoxifen-inducible 'time-stamp' model coupled with phenotypic staging to show that that activated Notch1 decreases generation of Gata3+ CD4+CD8lo selection intermediates during the earliest stages of DP selection, thereby preventing ThPOK induction and emergence of CD4 lineage cells. Finally, activated Notch1 efficiently re-directed DP thymocytes expressing MHCII-specific TCRαβ into the CD8 lineage. These studies show that activated Notch1 acts early during DP selection to skew selection of CD4-fated, MHCII-specific DP precursors to the CD8 lineage.Competing Interest StatementThe authors have declared no competing interest.