Notch proteins influence cell-fate decisions in many developing systems. Several gain-of-function studies have suggested a critical role for Notch 1 signaling in CD4-CD8 lineage commitment, maturation and survival in the thymus. However, we show here that tissue-specific inactivation of the gene encoding Notch 1 in immature (CD25+CD44-)T cell precursors does not affect subsequent thymocyte development. Neither steady-state numbers nor the rate of production of CD4+ and CD8+ mature thymocytes is perturbed in the absence of Notch 1. In addition, Notch 1-deficient thymocytes are normally sensitive to spontaneous or glucocorticoid-induced apoptosis. In contrast to earlier reports, these data formally exclude an essential role for Notch 1 in CD4-CD8 lineage commitment, maturation or survival.

The extraordinary sensitivity of CD8+ T cells to recognize antigen impinges to a large extent on the coreceptor CD8. While several studies have shown that the CD8β chain endows CD8 with efficient coreceptor function, the molecular basis for this is enigmatic. Here we report that cell-associated CD8αβ, but not CD8αα or soluble CD8αβ, substantially increases the avidity of T cell receptor (TCR)-ligand binding. To elucidate how the cytoplasmic and transmembrane portions of CD8β endow CD8 with efficient coreceptor function, we examined T1.4 T cell hybridomas transfected with various CD8β constructs. T1.4 hybridomas recognize a photoreactive Plasmodium berghei circumsporozoite (PbCS) peptide derivative (PbCS (4-azidobezoic acid [ABA])) in the context of H-2Kd, and permit assessment of TCR-ligand binding by TCR photoaffinity labeling. We find that the cytoplasmic portion of CD8β, mainly due to its palmitoylation, mediates partitioning of CD8 in lipid rafts, where it efficiently associates with p56lck. In addition, the cytoplasmic portion of CD8β mediates constitutive association of CD8 with TCR/CD3. The resulting TCR-CD8 adducts exhibit high affinity for major histocompatibility complex (MHC)-peptide. Importantly, because CD8αβ partitions in rafts, its interaction with TCR/CD3 promotes raft association of TCR/CD3. Engagement of these TCR/CD3-CD8/lck adducts by multimeric MHC-peptide induces activation of p56lck in rafts, which in turn phosphorylates CD3 and initiates T cell activation.

Considerable progress has been made in characterizing four key sets of interactions controlling antigen responsiveness in T cells, involving the following: the T cell antigen receptor, its coreceptors CD4 and CD8, the costimulatory receptors CD28 and CTLA-4, and the accessory molecule CD2. Complementary work has defined the general biophysical properties of interactions between cell surface molecules. Among the major conclusions are that these interactions are structurally heterogeneous, often reflecting clear-cut functional constraints, and that, although they all interact relatively weakly, hierarchical differences in the stabilities of the signaling complexes formed by these molecules may influence the sequence of steps leading to T cell activation. Here we review these developments and highlight the major challenges remaining as the field moves toward formulating quantitative models of T cell recognition.