Regulation of T cell fate by Notch.

The transmembrane receptor Notch participates in diverse cell fate decisions throughout embryonic development. Notch receptors and their ligands are expressed in the mammalian thymus, raising the possibility that Notch could regulate T cell fate decisions. Expression of a constitutively activated form of Notch in developing thymocytes causes thymocytes normally destined for the CD4 lineage to adopt the CD8 lineage instead. This suggests that Notch activity normally acts to direct CD4+CD8+ precursors to the CD8 lineage. The choice between CD4 and CD8 T cell fates is also controlled by MHC recognition during positive selection, implying that recognition of class I or II MHC might regulate Notch signaling. Possible models for the regulation of Notch by MHC recognition during CD4 versus CD8 lineage determination are discussed.