Low Activation Threshold As a Mechanism for Ligand-Independent Signaling in Pre-T Cells

Abstract

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 TCRα subunit restores development of pTα-deficient thymocytes to the CD4+CD8+ stage even in the absence of conventional MHC class I and class II ligands. Moreover, we found that pre-TCR and αβTCR complexes exhibit no appreciable difference in their association with lipid rafts, suggesting that ligand-independence is a function of the CD4−CD8− (DN) thymocytes in which pre-TCR signaling occurs. In agreement, we found that only CD44−CD25+ 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 CD4+CD8+ 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.

MHC‐restricted T cell receptor signaling is required for αβ TCR replacement of the pre T cell receptor

“…Thus, and in contrast to previously published data [15,17], in preTCR-deficient animals, a differentiation and survival signal emanating from a prematurely expressed HY-TCR appears to be most efficient in the presence of its positively selecting MHC molecules, facilitating optimal transition from DNIII to DNIV.…”

“…We performed our analysis in a set-up generated through breeding together the respective alleles, in contrast to the data published so far which had revealed no such requirement. One group had addressed the question of the relevance of TCR-MHC interaction for b-selection through ab TCR by retroviral gene transfer of a pigeon cytochrome C-specific TCRa chain into pTa-deficient fetal thymocytes [17]. Upon organ culture in MHC class I/II-deficient or -proficient thymic lobes, significant progression to the DP stage was observed in both environments.…”

“…It seems plausible that either mere expression of the ab TCR at DNII/III or its expression and interaction with its restricting MHC element could be required to imitate the preTCR signal. Two studies indicated that the restricting MHC elements were not required [15,17], but due to the nature of the used model systems -viral TCR expression, bone marrow chimeras, and adoptive transfer -the authors could not rigorously exclude a contribution of MHCmediated signals in the experimental outcome.…”

Function of the Src-family kinases, Lck and Fyn, in T-cell development and activation

“…The pre-TCR requires plasma membrane expression for signaling (O'Shea et al, 1997). DN3 cell membranes are reported to be highly enriched for lipid rafts suggesting an equilibrium shift from elemental to coalescent rafts as a potential mechanism for spontaneous Lck clustering and activation once the ITAMcontaining pre-TCR is expressed (Saint-Ruf et al, 2000;Dykstra et al, 2003;Haks et al, 2003). One study reported constitutive recruitment of Lck and phosphorylation of ZAP-70 in cells expressing a pre-TCR (SaintRuf et al, 2000).…”

Changes in the Role of the CD45 Protein Tyrosine Phosphatase in Regulating Lck Tyrosine Phosphorylation during Thymic Development

“…It is thought that pre-TCR signaling in DN cells occurs autonomously in the absence of an extracellular ligand (44). This is in part facilitated by localization in lipid raft domains and by the tendency of pre-TCR␣-chains to dimerize, and perhaps by an inherent difference in the sensitivity of DN cells to respond to low potency signals (33,45,46). Our current data indicate that in addition to the pre-TCR, Lck is also relatively enriched within lipid raft membrane microdomains in DN compared with Lck in total thymocytes.…”