Low activation threshold as a mechanism for ligand-independent signaling in pre-T cells

Pre-TCR complexes are thought to signal in a ligand-independent manner because they are constitutively targeted to lipid rafts. We report that ligand-independent signaling is not a unique capability of the pre-TCR complex. Indeed, the TCR subunit restores development of pT-deficient thymocytes to the CD4CD8 stage even in the absence of conventional MHC class I and class II ligands. Moreover, we found that pre-TCR and TCR complexes exhibit no appreciable difference in their association with lipid rafts, suggesting that ligand-independence is a function of the CD4CD8 (DN) thymocytes in which pre-TCR signaling occurs. In agreement, we found that only CD44CD25 DN thymocytes (DN3) enabled activation of extracellular signal-regulated kinases by the pre-TCR complex. DN thymocytes also exhibited a lower signaling threshold relative to CD4CD8 thymocytes, which was associated with both the markedly elevated lipid raft content of their plasma membranes and more robust capacitative Ca2 entry. Taken together these data suggest that cell-autonomous, ligand-independent signaling is primarily a property of the thymocytes in which pre-TCR signaling occurs. The Journal of Immunology, 2003, 170: 2853–2861. D evelopment of immature CD4CD8 double negative(DN)3 thymocytes to the CD4CD8 double positive(DP) stage is linked to productive rearrangement of the TCR locus by signals transduced through a developmental fore-runner of the TCR termed the pre-TCR complex. Although it is clear that the transition from the DN to DP stage requires pre-TCR signaling, the way in which those pre-TCR signals are initiate