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The physiological basis and mechanistic requirements for a large number of functional immunoreceptor tyrosine-based activation motifs (ITAMs; high ITAM multiplicity) in the complex of the T cell antigen receptor (TCR) and the invariant signaling protein CD3 remain obscure. Here we found that whereas a low multiplicity of TCR-CD3 ITAMs was sufficient to engage canonical TCR-induced signaling events that led to cytokine secretion, a high multiplicity of TCR-CD3 ITAMs was required for TCR-driven proliferation. This was dependent on the formation of compact immunological synapses, interaction of the adaptor Vav1 with phosphorylated CD3 ITAMs to mediate the recruitment and activation of the oncogenic transcription factor Notch1 and, ultimately, proliferation induced by the cell-cycle regulator c-Myc. Analogous mechanistic events were also needed to drive proliferation in response to weak peptide agonists. Thus, the TCR-driven pathways that initiate cytokine secretion and proliferation are separable and are coordinated by the multiplicity of phosphorylated ITAMs in TCR-CD3. [ABSTRACT FROM AUTHOR]

Summary: V(D)J recombination is essential for generating a diverse array of B and T cell receptors that can recognize and combat foreign antigens. As with any recombination event, tight control is essential to prevent the occurrence of genetic anomalies that drive cellular transformation. One important aspect of regulation is directed targeting of the RAG recombinase. Indeed, RAG accumulates at the 3′ end of individual antigen receptor loci poised for rearrangement; however, it is not known whether focal binding is involved in regulating cleavage, and what mechanisms lead to enrichment of RAG in this region. Here, we show that monoallelic looping out of the 3′ end of the T cell receptor α (Tcra) locus, coupled with transcription and increased chromatin/nuclear accessibility, is linked to focal RAG binding and ATM-mediated regulation of monoallelic cleavage on looped-out 3′ regions. Our data identify higher-order loop formation as a key determinant of directed RAG targeting and the maintenance of genome stability. [ABSTRACT FROM AUTHOR]

It is well established that the pre-T cell receptor for antigen (TCR) is responsible for efficient expansion and differentiation of thymocytes with productive TCRβ rearrangements. However, Ptcra- as well as Tcra-targeting experiments have suggested that the early expression of Tera in CD4-CD8- cells can partially rescue the development of αβ CD4+CD8+ cells in Ptera-deficient mice. In this study, we show that the TCR Eδ but not Eα enhancer function is required for the cell surface expression of αβTCR on immature CD4-CDS- T cell precursors, which play a crucial role in promoting αβ T cell development in the absence of pre-TCR. Thus, βTCR expression by CD4-CD8- thymoeytes not only represents a transgenic artifact but occurs under physiological conditions. [ABSTRACT FROM AUTHOR]

Commitment of hematopoietic progenitors to the T cell lineage requires the integration of multiple signaling pathways. Evidence has suggested involvement of hedgehog (Hh) signaling in T cell differentiation through its signal transducer smoothened (Smo). However, the precise function of the Hh pathway remains controversial, mainly because T cell–specific in vivo genetic models have not been used. Using pre–T cell–specific, mature T cell–specific and poly(I)·poly(C)-inducible deletions of Smo and antagonists of Smo signaling, we report here that Hh is an essential positive regulator of T cell progenitor differentiation. Furthermore, we localize Hh function to a stage preceding pre–T cell receptor signaling, connect Smo signaling to the activity of the Gli1 and Gli2 transcription factors and demonstrate that Hh affects regulators of thymocyte survival and proliferation. [ABSTRACT FROM AUTHOR]

Pre-T-cell receptor (pre-TCR) functions and the study of early thymocyte development continue to fascinate immunologists more than 10 years after the first description and cloning of the receptor. Although multiple reports have addressed several aspects of pre-TCR signaling and function, its ability to regulate diverse functions, including proliferation, survival, and allelic exclusion of the TCR-β locus, remains an open question. What fascinates us is its central role in the fine balance between physiological differentiation and thymocyte transformation that leads to T-cell leukemia and lymphomas. In this review, we integrate pre-TCR signaling pathways and study their effects on the regulation of T-cell progenitor cell-cycle entry and cell survival. We also connect aberrant pre-TCR signaling to deregulated proliferation and apoptotic balances and thymocyte transformation. [ABSTRACT FROM AUTHOR]

Intrathymic T cell development represents one of the best studied paradigms of mammalian development. Lymphoid committed precursors enter the thymus and the Notch1 receptor plays an essential role in committing them to the T cell lineages. The pre-T cell receptor (TCR), as an autonomous cell signaling receptor, commits cells to the αβ lineage while its rival, the ϒδTCR, is involved in generating the ϒδ lineage of T cells. Positive and negative selection of immature αβTCR-expressing cells are essential mechanisms for generating mature T cells, committing them to the CD4 and CD8 lineages and avoiding autoimmunity. Additional lineages of αβT cells, such as the natural killer T cell lineage and the CD25[sup+] regulatory T cell lineage, are formed when the αβTCR encounters specific ligands in suitable microenvironments. Thus, positive selection and receptor-instructed lineage commitment represent a hallmark of the thymus. Ectopically expressed organ-specific antigens contribute to thymic self-nonself discrimination, which represents an essential feature for the evolutionary fitness of mammalian species. [ABSTRACT FROM AUTHOR]

A transgenic reporter mouse strain, which expressed the human CD25 (hCD25) surface marker as a reporter under the control of the pre-T cell receptor a(pTa) promoter, was used to identify lymphoid precursors that expressed pTa intracellularly. The hCD25 reporter marked intra- and extrathymic precursors of lymphocytes but not myeloid cells. The earliest intrathymic precursors were CD4loCD8-CD25-CD44+c-Kit+ cells that expressed elevated levels of Notch-1 mRNA. Clonogenic assays showed that the extrathymic precursors were common lymphoid progenitors (CLPs) that included CD19-, B220+, Thy1+ and CD4+ cells. Thus, the pTa reporter can be used to trace lymphopoiesis between CLPs and a? T cells. The slower extinction of the hCD25 reporter compared to pTa enabled us to define points at which pTa- lineages branched off. [ABSTRACT FROM AUTHOR]

Signals that emanate from the pre-T cell receptor (pre-TCR) regulate multiple processes required for the development of the a? T cell lineage. In contrast to the ?d TCR, the pre-TCR localizes cell-autonomously to membrane rafts, where it appears to signal in a constitutive and ligand-independent manner. We addressed here the role played by structural features specific to the cytoplasmic domain of the pre-TCRa chain (pTa). More specifically, we examined a COOH-terminal proline-rich sequence that might play a role in signal transduction and a juxtamembrane cysteine residue that could be a target for palmitoylation, thus allowing spontaneous raft localization. Expression of pTa mutants in transgenic mice, retrovirally transduced T cell precursors and cell lines showed that the pTa cytoplasmic tail, in particular the proline-rich domain, plays a crucial role in pre-TCR signaling and T cell development. In contrast, the pTa juxtamembrane cysteine appeared to be dispensable for pre-TCR function. [ABSTRACT FROM AUTHOR]

Pre-T cell antigen receptor (pre-TCR) signaling plays a crucial role in the development of immature T cells. Although certain aspects of proximal pre-TCR signaling have been studied, the intermediate signal transducers and the distal transcription modulators have been poorly characterized. We report here a correlation between pre-TCR signaling and a biphasic rise in the cytosolic Ca2+ concentration. In addition, we show that constitutive pre-TCR signaling is associated with an increased rate of Ca2+ influx through store-operated plasma membrane Ca2+ channels. We show also that the biphasic nature of the observed pre-TCR?induced rise in cytosolic Ca2+ differentially modulates the activities of the transcription factors NF-κB and NFAT in developing T cells. [ABSTRACT FROM AUTHOR]

Summary: The analysis of T-cell receptor (TCR) β selection, TCRβ allelic exclusion and TCRβ rearrangement in γδ T cells from normal and pre-TCR-deficient mice has shown that the pre-TCR has a crucial role in T-lymphocyte development. • The pre-TCR is by far the most effective receptor that generates large numbers of CD4[sup+]8[sup+] T cells with productive TCRβ rearrangements. • In the absence of the pre-TCR, TCRβ rearrangement proceeds in developing cells irrespective of whether they already contain a productive TCRβ gene. • The pre-TCR directs developing T cells to the αβ lineage because γδ T cells from pTα[sup-/-] mice proceed much further in TCRβ rearrangement than γδ T cells from wild-type mice. It is argued that the pre-TCR commits developing T cells to the αβ lineage by an instructive mechanism, which has largely replaced an evolutionarily more ancient mechanism that involves stochastic αβ lineage commitment. [ABSTRACT FROM AUTHOR]

An aberrant MHC-I antigen presentation pathway has been closely linked to low tumor immunogenicity and immunotherapy resistance. Genetic alteration or reduced expression of MHC-I positive regulators has been observed in certain cancers, but is considered a challenge for the development of targeted therapies. MHC-I modulation by cancer-associated inhibitory mechanisms is becoming an emerging field of study, calling for a search for specific modulators and their underlying biology and function in MHC-I modulation and tumor immunity. A membrane-associated MHC-I inhibitory axis was recently documented, mediating MHC-I ubiquitination and subsequent lysosomal degradation in leukemia and solid cancers. We posit that the future exploration of MHC-I inhibitory pathways and their potential application represents an exciting direction for cancer immunotherapy. Cancer cells efficiently hijack the MHC-I antigen presentation (AP) pathway to escape from immunosurveillance and cause resistance to immunotherapy. Exploring specific mechanisms that actively inhibit MHC-I expression and/or AP in tumors can improve our understanding of immune regulation in cancers and provide potential targets for next-generation cancer immunotherapy. The MHC-I antigen presentation (AP) pathway is key to shaping mammalian CD8+ T cell immunity, with its aberrant expression closely linked to low tumor immunogenicity and immunotherapy resistance. While significant attention has been given to genetic mutations and downregulation of positive regulators that are essential for MHC-I AP, there is a growing interest in understanding how tumors actively evade MHC-I expression and/or AP through the induction of MHC-I inhibitory pathways. This emerging field of study may offer more viable therapeutic targets for future cancer immunotherapy. Here, we explore potential mechanisms by which cancer cells evade MHC-I AP and function and propose therapeutic strategies that might target these MHC-I inhibitors to restore impaired T cell immunity within the tumor microenvironment (TME). [ABSTRACT FROM AUTHOR]

TLX1 and TLX3 are two closely-related homeobox transcriptional repressors frequently misexpressed and translocated in T cell acute lymphoblastic leukemia (T-ALL). In this issue of Cancer Cell, Dadi et al. provide new insights into how these factors are recruited by ETS-1 to the TCRα enhancer and actively repress differentiation. [Copyright &y& Elsevier]

Highly coordinated changes in gene expression underlie T cell activation and exhaustion. However, the mechanisms by which such programs are regulated and how these may be targeted for therapeutic benefit remain poorly understood. Here, we comprehensively profile the genomic occupancy of mSWI/SNF chromatin remodeling complexes throughout acute and chronic T cell stimulation, finding that stepwise changes in localization over transcription factor binding sites direct site-specific chromatin accessibility and gene activation leading to distinct phenotypes. Notably, perturbation of mSWI/SNF complexes using genetic and clinically relevant chemical strategies enhances the persistence of T cells with attenuated exhaustion hallmarks and increased memory features in vitro and in vivo. Finally, pharmacologic mSWI/SNF inhibition improves CAR-T expansion and results in improved anti-tumor control in vivo. These findings reveal the central role of mSWI/SNF complexes in the coordination of T cell activation and exhaustion and nominate small-molecule-based strategies for the improvement of current immunotherapy protocols. [Display omitted] • mSWI/SNF targeting and activity is specific to T cell activation and exhaustion states • Genetic and chemical disruption of cBAF complexes enhances T cell persistence • cBAF complex activities facilitate T cell exhaustion and prevent memory features • mSWI/SNF pharmacologic disruption improves CAR-T expansion and anti-tumor control mSWI/SNF ATP-dependent chromatin remodeling complexes direct chromatin accessibility and gene expression during T cell activation and exhaustion. Targeting mSWI/SNF complexes with clinically relevant small-molecule inhibitors and degraders attenuates T cell exhaustion and increases T cell persistence and anti-tumor activity in cell culture systems and in vivo. [ABSTRACT FROM AUTHOR]