Notch signaling is required for proliferation but not for differentiation at a well-defined β-selection checkpoint during human T-cell development

Notch signaling is absolutely required for β-selection during mouse T-cell development, both for differentiation and proliferation. In this report, we investigated whether Notch has an equally important role during human T-cell development. We show that human CD34+ thymocytes can differentiate into CD4+CD8β+ double positive (DP) thymocytes in the absence of Notch signaling. While these DP cells phenotypically resemble human β-selected cells, they lack a T-cell receptor (TCR)–β chain. Therefore, we characterized the β-selection checkpoint in human T-cell development, using CD28 as a differential marker at the immature single positive CD4+CD3−CD8α− stage. Through intracellular TCR-β staining and gene expression analysis, we show that CD4+CD3−CD8α−CD28+ thymocytes have passed the β-selection checkpoint, in contrast to CD4+CD3−CD8α−CD28− cells. These CD4+CD3−CD8α−CD28+ thymocytes can efficiently differentiate into CD3+TCRαβ+ human T cells in the absence of Notch signaling. Importantly, preselection CD4+CD3−CD8α−CD28− thymocytes can also differentiate into CD3+TCRαβ+ human T cells without Notch activation when provided with a rearranged TCR-β chain. Proliferation of human thymocytes, however, is clearly Notch-dependent. Thus, we have characterized the β-selection checkpoint during human T-cell development and show that human thymocytes require Notch signaling for proliferation but not for differentiation at this stage of development.