Introduction␥␦ T cells are key players in the immune surveillance of cellular distress, thanks to their ability to recognize conserved determinants up-regulated after inflammation, infection, or cell transformation. 1,2 Although ␥␦ T-cell receptors (TCRs) contribute to detection of danger-associated determinants, ligands for these receptors have been identified in a few cases only. 3 Thus, the antigenic specificity of ␥␦ T cells and their fine activation modalities in response to cell stress remain largely unknown.One of the best studied ␥␦ T-cell subsets in humans expresses V␥9V␦2 TCR and predominates in blood, composing several percent of the whole peripheral lymphoid pool in most adults. V␥9V␦2 T cells are activated by nonpeptidic phosphorylated isoprenoid pathway metabolites, 4-6 hereafter referred to as phosphoagonists (PAg). Natural V␥9V␦2-stimulating PAg include isopentenyl pyrophosphate (IPP), 7 a metabolite of the mevalonate pathway found in mammalian cells and the desoxyxylulose phosphate pathway shared by many microorganisms, and hydroxy-methyl-butyl-pyrophosphate, 8 an intermediate of the latter pathway. PAg detection by ␥␦ T cells underlies their broad reactivity toward infected and transformed cells. Indeed, tumor cell recognition by V␥9V␦2 T cells is linked to enhanced production of the weak agonist IPP, resulting from increased cell metabolism and cholesterol biosynthesis. Accordingly, pharmacologic inhibitors of the mevalonate pathway that up-regulate (eg, aminobisphosphonates, NBP) or down-regulate (eg, statins) IPP production, respectively, increase or decrease antitumor V␥9V␦2 T-cell responses. 9,10 Moreover, because of the high V␥9V␦2 T cell-stimulating activity of the microbial agonist hydroxy-methyl-butyl-pyrophosphate, V␥9V␦2 T-cell responses are elicited by infected cells producing even traces of this PAg. 8 Although PAg-induced activation is restricted to V␥9V␦2 T cells and can be conferred by V␥9V␦2 TCR gene transfer, 11,12 attempts to detect cognate interactions between PAg and V␥9V␦2 TCR have failed so far. 13 So how V␥9V␦2 T cells sense PAg remains an enigma. PAg rapidly induce Ca 2ϩ signaling and activation of V␥9V␦2 T-cell clones, but this requires cell-to-cell contact, suggesting the implication of additional target cell surface receptors in this phenomenon. 11,14 PAg elicit V␥9V␦2 T-cell responses against basically all human cells, irrespective of their tissue origin, but do not induce recognition of any murine target cells. Therefore, the putative target cell receptors involved in PAg-mediated T-cell activation are expected to be broadly expressed by human, but not murine, cells.Activation of antigen-stimulated T cells is tuned by interactions involving T cell-derived CD28-related receptors and target cellderived B7-related counter-receptors, 15 which family includes members, such as Skint and butyrophilin (BTN) receptors. The mandatory role played by Skint-1 in the intrathymic positive There is an Inside Blood commentary on this article in this issue.The online version of this...

The cellular and molecular events that drive early innate lympoid cell (ILC) development remain poorly understood. We show that transcription factor TCF-1 is required for the efficient generation of all known adult ILC subsets and their precursors. Using novel reporter mice, we identified a new subset of early ILC progenitors (EILP) that expressed high amounts of TCF-1. EILP lacked efficient T and B lymphocyte potential, but efficiently gave rise to NK cells and all known adult helper-ILC lineages, indicating that they are the earliest identified ILC-committed progenitors. Our results suggest that upregulation of TCF-1 expression denotes the earliest stage of ILC fate specification. The discovery of EILP provides a basis to decipher additional signals that specify the ILC fate.

Background: Phosphoisoprenoid stimulation of V␥9V␦2 T cells can be modulated by anti-BTNA3 antibodies. Results: Agonist and antagonist antibodies associate differently with BTN3A structurally and biophysically. Conclusion: Differential binding of antibodies to BTN3A modulates its oligomerization on the cell surface. Significance: Defining how ␥␦ T cells recognize antigen is critical for understanding their functions in the immune response.

Summary T follicular helper (TFH) and Th1 cells generated after viral infections are critical for the control of infection and the development of immunological memory. However, the mechanisms that govern the differentiation and maintenance of these two distinct lineages during viral infection remain unclear. Here, we found that viral-specific TFH and Th1 cells showed reciprocal expression of the transcriptions factors TCF1 and Blimp1 starting early after infection, even before the differential expression of the canonical TFH marker CXCR5. Furthermore, TCF1 was intrinsically required for the TFH-cell response to viral infection; in the absence of TCF1, the TFH-cell response was severely compromised and the remaining TCF1 deficient TFH cells failed to maintain TFH-associated transcriptional and metabolic signatures, which were distinct from those in Th1 cells. Mechanistically, TCF1 functioned through forming negative feedback loops with IL-2 and Blimp1. Our findings demonstrate an essential role of TCF1 in TFH-cell responses to viral infection.

T cell development is orchestrated by transcription factors that regulate the expression of genes initially buried within inaccessible chromatin, but the transcription factors that establish the regulatory landscape of the T cell lineage remain unknown. Profiling chromatin accessibility at eight stages of T cell development revealed the selective enrichment of TCF-1 at genomic regions that became accessible at the earliest stages of development. TCF-1 was further required for the accessibility of these regulatory elements and at the single-cell level, it dictated a coordinate opening of chromatin in T cells. TCF-1 expression in fibroblasts generated de novo chromatin accessibility even at chromatin regions with repressive marks, inducing the expression of T cell-restricted genes. These results indicate that a mechanism by which TCF-1 controls T cell fate is through its widespread ability to target silent chromatin and establish the epigenetic identity of T cells.

Harly et al. show that early innate lymphoid progenitors (EILPs) are a developmental intermediate between all-lymphoid progenitors (ALPs) and ILC precursors (ILCps) and identify requirements for their generation and further differentiation.

Innate lymphoid cells (ILCs) play important functions in immunity and tissue homeostasis, but their development is poorly understood. Through the use of single-cell approaches, we examined the transcriptional and functional heterogeneity of ILC progenitors and studied the precursor–product relationships that linked the subsets identified. This analysis identified two successive stages of ILC development within TCF-1 + early innate lymphoid progenitors (EILPs), which we named ‘specified EILPs’ and ‘committed EILPs’. Specified EILPs generated dendritic cells, whereas this potential was greatly decreased in committed EILP. TCF-1 was dispensable for the generation of specified EILPs, but required for the generation of committed EILPs. TCF-1 used a pre-existing regulatory landscape established in upstream lymphoid precursors to bind chromatin in EILPs. Our results provide insight into the mechanisms by which TCF-1 promotes developmental progression of ILC precursors, while constraining their dendritic cell lineage potential and enforcing commitment to ILC fate.

Notch signaling induces gene expression of the T cell lineage and discourages alternative fate outcomes. Hematopoietic deficiency in the Notch target Hes1 results in severe T cell lineage defects; however, the underlying mechanism is unknown. We found here that Hes1 constrained myeloid gene-expression programs in T cell progenitor cells, as deletion of the myeloid regulator C/EBPa restored the development of T cells from Hes1-deficient progenitor cells. Repression of Cebpa by Hes1 required its DNA-binding and Groucho-recruitment domains. Hes1-deficient multipotent progenitor cells showed a developmental bias toward myeloid and dendritic cells after Notch signaling, whereas Hes1-deficient lymphoid progenitor cells required additional cytokine signaling for diversion into the myeloid lineage. Our findings establish the importance of constraining developmental programs of the myeloid lineage early in T cell development.