1. Lymphocyte Biology Section, Laboratory of Immunology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892 , USA
2. Department of Immunology, Duke University Medical Centre, Durham, North Carolina 27710 , USA
3. Cancer Immunology Program, Cardinal Bernardin Cancer Centre, Loyola University Medical Centre, Maywood , Illinois 60153, USA

Correspondence to: Ronald N. Germain¹ Correspondence and requests for materials should be addressed to R.N.G. (e-mail: Email: Ronald_Germain@nih.gov).

Signals elicited by binding of the T-cell antigen receptor and the CD4/CD8 co-receptor to major histocompatibility complex (MHC) molecules control the generation of CD4⁺ (helper) or CD8⁺ (cytotoxic) T cells from thymic precursors that initially express both co-receptor proteins¹. These precursors have unique, clonally distributed T-cell receptors with unpredictable specificity for the self-MHC molecules involved in this differentiation process². However, the mature T cells that emerge express only the CD4 (MHC class II-binding) or CD8 (MHC class I-binding) co-receptor that complements the MHC class-specificity of the T-cell receptor. How this matching of co-receptor-defined lineage and T-cell-receptor specificity is achieved remains unknown^{1, 3, 4}, as does whether signalling by the T-cell receptors, co-receptors and/or general cell-fate regulators such as Notch-1 (refs 5, 6) contributes to initial lineage choice, to subsequent differentiation processes or to both. Here we show that the CD4 versus CD8 lineage fate of immature thymocytes is controlled by the co-receptor-influenced duration of initial T-cell receptor-dependent signalling. Notch-1 does not appear to be essential for this fate determination, but it is selectively required for CD8 ⁺ T-cell maturation after commitment directed by T-cell receptors. This indicates that the signals constraining CD4 versus CD8 lineage decisions are distinct from those that support subsequent differentiation events such as silencing of co-receptor loci.