The structural basis for autonomous dimerization of the pre-T-cell antigen receptor

The preTCR-dependent DN3 to DP transition requires Notch signaling, is improved by CXCL12 signaling and is inhibited by IL-7 signaling

The requirement for Notch signaling during T-cell development has been extensively studied. Nevertheless, the developmental stage at which it is required and whether additional signaling pathways are needed are still poorly understood. By using a stromal-cell-free culture system, we show that sorted double-negative 3 (DN3) thymocytes only require a Delta-like-4-induced Notch signal to differentiate into double-positive (DP) cells. This differentiation process is preTCR-α dependent. DN3 cells undergo 4-5 proliferation cycles, and the addition of the chemokine CXCL12 improves proliferation. IL-7 blocks the differentiation of DN3 cells to DP cells but not the Notch-induced proliferation of cultured DN3 cells. The impaired differentiation correlates with an inhibition of Rag-2 up-regulation. Overall, the in vitro stromal-cell-free culture system presented here also provides a powerful and unique tool for studying the mechanisms involved in the positive and negative selection of T cells.

PreTCR and TCRγδ signal initiation in thymocyte progenitors does not require domains implicated in receptor oligomerization

Whether thymocytes adopt an αβ or a γδ T cell fate in the thymus is determined at the β selection checkpoint by the relatively weak or strong signals that are delivered by either the pre-T cell receptor (preTCR) or the γδ TCR, respectively. Signal initiation at the β selection checkpoint is thought to be independent of ligand engagement of these receptors. Some reports have suggested that receptor oligomerization, which is thought to be mediated by either the immunoglobulin (Ig)-like domain of the preTCRα (pTα) chain or the variable domain of TCRδ, is a unifying mechanism that initiates signaling in early CD4(-)CD8(-) double-negative (DN) thymocyte progenitors. Here, we demonstrate that the extracellular regions of pTα and TCRδ that are implicated in mediating receptor oligomerization were not required for signal initiation from the preTCR or TCRγδ. Indeed, a truncated TCRγδ that lacked all of its extracellular Ig-like domains still formed a signaling-competent TCR that drove cells through the β selection checkpoint. These observations suggest that signal initiation in DN thymocytes is simply a consequence of the surface-pairing of TCR chains, with signal strength being a function of the abundances of surface TCRs. Thus, processes that regulate the surface abundances of TCR complexes in DN cells, such as oligomerization-induced endocytosis, would be predicted to have a major influence in determining whether cells adopt an αβ versus γδ T cell fate.

Pre-TCR signaling and CD8 gene bivalent chromatin resolution during thymocyte development

The CD8 gene is silent in CD4(-)CD8(-) double-negative thymocytes, expressed in CD4(+)CD8(+) double-positive cells, and silenced in cells committing to the CD4(+) single-positive (SP) lineage, remaining active in the CD8(+) SP lineage. In this study, we show that the chromatin of the CD8 locus is remodeled in C57BL/6 and B6/J Rag1(-/-) MOM double-negative thymocytes as indicated by DNaseI hypersensitivity and widespread bivalent chromatin marks. Pre-TCR signaling coincides with chromatin bivalency resolution into monovalent activating modifications in double-positive and CD8 SP cells. Shortly after commitment to CD4 SP cell lineage, monovalent repressive characteristics and chromatin inaccessibility are established. Differential binding of Ikaros, NuRD, and heterochromatin protein 1α on the locus during these processes may participate in the complex regulation of CD8.

A conserved hydrophobic patch on Vβ domains revealed by TCRβ chain crystal structures: Implications for pre-TCR dimerization

The αβ T cell receptor (TCR) is a multimeric complex whose β chain plays a crucial role in thymocyte development as well as antigen recognition by mature T lymphocytes. We report here crystal structures of individual β subunits, termed N15β (Vβ5.2Dβ2Jβ2.6Cβ2) and N30β (Vβ13Dβ1Jβ1.1Cβ2), derived from two αβ TCRs specific for the immunodominant vesicular stomatitis virus octapeptide (VSV-8) bound to the murine H-2K^b MHC class I molecule. The crystal packing of the N15β structure reveals a homodimer formed through two Vβ domains. The Vβ/Vβ module is topologically very similar to the Vα/Vβ module in the N15αβ heterodimer. By contrast, in the N30β structure, the Vβ domain’s external hydrophobic CFG face is covered by the neighboring molecule’s Cβ domain. In conjunction with systematic investigation of previously published TCR single-subunit structures, we identified several conserved residues forming a concave hydrophobic patch at the center of the CFG outer face of the Vβ and other V-type Ig-like domains. This hydrophobic patch is shielded from solvent exposure in the crystal packing, implying that it is unlikely to be thermodynamically stable if exposed on the thymocyte surface. Accordingly, we propose a dimeric pre-TCR model distinct from those suggested previously by others and discuss its functional and structural implications.

Insight into the basis of autonomous immunoreceptor activation

Expression of the pre-T cell receptor (pTCR) by immature thymocytes is crucial for T cell development. The pTCR comprises an invariant pre-Tα chain that pairs with a newly rearranged TCRβ chain and CD3 signaling components. Despite its similarity to the mature αβTCR, which binds to specific peptide-loaded major histocompatibility molecules, the pTCR functions in a ligand-independent manner. Precisely how pTCR functions autonomously has remained a source of intense debate. Recently, the structure of the extracellular domain of the pTCR has been determined, providing insight into the mechanism of pTCR autonomous signaling. In this review, we reflect on the current understanding of pTCR function and draw comparisons to the mechanisms employed by the mature αβTCR and the related pre-B cell receptor.

Signaling in thymic selection

T cell receptor signaling allows the developing thymocyte to undergo positive or negative selection, which is required for the formation of a useful mature T cell repertoire. Recent developments include the finding that much of the Lck kinase (required to initiate T cell signaling) is already in an active configuration before signaling. The analog strength of antigen binding to the T cell receptor binding may be translated into a digital signal by the amount of time the TCR is paired with a co-receptor carrying Lck. Downstream, the cellular localization of MAP kinase signaling is determined by the strength of the signal and in turn predicts positive or negative selection. A novel protein, Themis, is important in crossing the positive selection developmental checkpoint, but its mode of action is still uncertain. Commitment to the CD4 or CD8 lineage is influenced by the amount of ZAP-70 signaling and also by closely regulated responsiveness to intrathymic cytokines such as IL7.

The translational repressor 4E-BP called to order by eIF4E: new structural insights by SAXS

eIF4E binding protein (4E-BP) inhibits translation of capped mRNA by binding to the initiation factor eIF4E and is known to be mostly or completely unstructured in both free and bound states. Using small angle X-ray scattering (SAXS), we report here the analysis of 4E-BP structure in solution, which reveals that while 4E-BP is intrinsically disordered in the free state, it undergoes a dramatic compaction in the bound state. Our results demonstrate that 4E-BP and eIF4E form a ‘fuzzy complex’, challenging current visions of eIF4E/4E-BP complex regulation.

Germ-line elimination of electric charge on pre-T-cell receptor (TCR) impairs autonomous signaling for beta-selection and TCR repertoire formation

The pre-T-cell receptor (TCR) is crucial for the early T-cell development, but the ligand for pre-TCR remains unidentified. We recently proposed a model that pre-TCR complexes oligomerize spontaneously through interactions of the pre-TCRα chain. To investigate the mechanism underlying this ligand-independent signaling in vivo, we established knock-in mice that express a pre-TCRα mutant lacking charged amino acids (D(22)R(24)R(102)R(117) to A(22)A(24)A(102)A(117); 4A). CD4(+)CD8(+) thymocyte number was significantly reduced in invariant pre-TCRα (pTα(4A/4A)) mice, whereas CD4(-)CD8(-) thymocytes were unaffected. The percentages of double-negative 3 (DN3) cells and γδ T cells were increased in the pTα(4A/4A) thymus, indicating that β-selection is impaired in pTα(4A/4A) mice. Pre-TCR-mediated tyrosine phosphorylation and clonal expansion into double-positive thymocytes were also defective in the knock-in mice. Pre-TCR was expressed at higher levels on pTα(4A/4A) cell surfaces than on those of the wild type, suggesting that the charged residues in pTα are critical for autonomous engagement and subsequent internalization of pre-TCR. Pre-TCR-mediated allelic exclusion of the TCRβ gene was also inhibited in pTα(4A/4A) mice, and thereby, dual TCRβs were expressed on pTα(4A/4A) T cells. Furthermore, the TCRβ chain variable region (Vβ) repertoire of mature T cells was significantly altered in pTα(4A/4A) mice. These results suggest that charged residues of pTα are critical for β-selection, allelic exclusion, and TCRβ repertoire formation.