Distinct developmental pathways generate functionally distinct populations of natural killer cells.

Natural killer (NK) cells function by eliminating virus-infected or tumor cells. Here we identified an NK-lineage-biased progenitor population, referred to as early NK progenitors (ENKPs), which developed into NK cells independently of common precursors for innate lymphoid cells (ILCPs). ENKP-derived NK cells (ENKP_NK cells) and ILCP-derived NK cells (ILCP_NK cells) were transcriptionally different. We devised combinations of surface markers that identified highly enriched ENKP_NK and ILCP_NK cell populations in wild-type mice. Furthermore, Ly49H+ NK cells that responded to mouse cytomegalovirus infection primarily developed from ENKPs, whereas ILCP_NK cells were better IFNγ producers after infection with Salmonella and herpes simplex virus. Human CD56dim and CD56bright NK cells were transcriptionally similar to ENKP_NK cells and ILCP_NK cells, respectively. Our findings establish the existence of two pathways of NK cell development that generate functionally distinct NK cell subsets in mice and further suggest these pathways may be conserved in humans.

Intrathymic dendritic cell-biased precursors promote human T cell lineage specification through IRF8-driven transmembrane TNF.

The cross-talk between thymocytes and thymic stromal cells is fundamental for T cell development. In humans, intrathymic development of dendritic cells (DCs) is evident but its physiological significance is unknown. Here we showed that DC-biased precursors depended on the expression of the transcription factor IRF8 to express the membrane-bound precursor form of the cytokine TNF (tmTNF) to promote differentiation of thymus seeding hematopoietic progenitors into T-lineage specified precursors through activation of the TNF receptor (TNFR)-2 instead of TNFR1. In vitro recapitulation of TNFR2 signaling by providing low-density tmTNF or a selective TNFR2 agonist enhanced the generation of human T cell precursors. Our study shows that, in addition to mediating thymocyte selection and maturation, DCs function as hematopoietic stromal support for the early stages of human T cell development and provide proof of concept that selective targeting of TNFR2 can enhance the in vitro generation of T cell precursors for clinical application.

Mucosal-associated invariant T cells restrict reactive oxidative damage and preserve meningeal barrier integrity and cognitive function.

Increasing evidence indicates close interaction between immune cells and the brain, revising the traditional view of the immune privilege of the brain. However, the specific mechanisms by which immune cells promote normal neural function are not entirely understood. Mucosal-associated invariant T cells (MAIT cells) are a unique type of innate-like T cell with molecular and functional properties that remain to be better characterized. In the present study, we report that MAIT cells are present in the meninges and express high levels of antioxidant molecules. MAIT cell deficiency in mice results in the accumulation of reactive oxidative species in the meninges, leading to reduced expression of junctional protein and meningeal barrier leakage. The presence of MAIT cells restricts neuroinflammation in the brain and preserves learning and memory. Together, our work reveals a new functional role for MAIT cells in the meninges and suggests that meningeal immune cells can help maintain normal neural function by preserving meningeal barrier homeostasis and integrity.

Transcription factor Tox2 is required for metabolic adaptation and tissue residency of ILC3 in the gut.

Group 3 innate lymphoid cells (ILC3) are the major subset of gut-resident ILC with essential roles in infections and tissue repair, but how they adapt to the gut environment to maintain tissue residency is unclear. We report that Tox2 is critical for gut ILC3 maintenance and function. Gut ILC3 highly expressed Tox2, and depletion of Tox2 markedly decreased ILC3 in gut but not at central sites, resulting in defective control of Citrobacter rodentium infection. Single-cell transcriptional profiling revealed decreased expression of Hexokinase-2 in Tox2-deficient gut ILC3. Consistent with the requirement for hexokinases in glycolysis, Tox2-/- ILC3 displayed decreased ability to utilize glycolysis for protein translation. Ectopic expression of Hexokinase-2 rescued Tox2-/- gut ILC3 defects. Hypoxia and interleukin (IL)-17A each induced Tox2 expression in ILC3, suggesting a mechanism by which ILC3 adjusts to fluctuating environments by programming glycolytic metabolism. Our results reveal the requirement for Tox2 to support the metabolic adaptation of ILC3 within the gastrointestinal tract.

Distinct and temporary-restricted epigenetic mechanisms regulate human αß and γδ T cell development.

The development of TCRαß and TCRγδ T cells comprises a step-wise process in which regulatory events control differentiation and lineage outcome. To clarify these mechanisms, we employed RNA-sequencing, ATAC-sequencing and ChIPmentation on well-defined thymocyte subsets that represent the continuum of human T cell development. The chromatin accessibility dynamics show clear stage specificity and reveal that human T cell-lineage commitment is marked by GATA3- and BCL11B-dependent closing of PU.1 sites. A temporary increase in H3K27me3 without open chromatin modifications is unique for ß-selection, whereas emerging γδ T cells, which originate from common precursors of ß-selected cells, show large chromatin accessibility changes due to strong T cell receptor (TCR) signaling. Furthermore, we unravel distinct chromatin landscapes between CD4+ and CD8+ αß-lineage cells that support their effector functions and reveal gene-specific mechanisms that define mature T cells. This resource provides a framework for studying gene regulatory mechanisms that drive normal and malignant human T cell development.

Bone marrow plasma cells require P2RX4 to sense extracellular ATP.

Plasma cells produce large quantities of antibodies and so play essential roles in immune protection1. Plasma cells, including a long-lived subset, reside in the bone marrow where they depend on poorly defined microenvironment-linked survival signals1. We show that bone marrow plasma cells use the ligand-gated purinergic ion channel P2RX4 to sense extracellular ATP released by bone marrow osteoblasts through the gap-junction protein pannexin 3 (PANX3). Mutation of Panx3 or P2rx4 each caused decreased serum antibodies and selective loss of bone marrow plasma cells. Compared to their wild-type counterparts, PANX3-null osteoblasts secreted less extracellular ATP and failed to support plasma cells in vitro. The P2RX4-specific inhibitor 5-BDBD abrogated the impact of extracellular ATP on bone marrow plasma cells in vitro, depleted bone marrow plasma cells in vivo and reduced pre-induced antigen-specific serum antibody titre with little posttreatment rebound. P2RX4 blockade also reduced autoantibody titre and kidney disease in two mouse models of humoral autoimmunity. P2RX4 promotes plasma cell survival by regulating endoplasmic reticulum homeostasis, as short-term P2RX4 blockade caused accumulation of endoplasmic reticulum stress-associated regulatory proteins including ATF4 and B-lineage mutation of the pro-apoptotic ATF4 target Chop prevented bone marrow plasma cell demise on P2RX4 inhibition. Thus, generating mature protective and pathogenic plasma cells requires P2RX4 signalling controlled by PANX3-regulated extracellular ATP release from bone marrow niche cells.

Integrated scRNA-Seq Identifies Human Postnatal Thymus Seeding Progenitors and Regulatory Dynamics of Differentiating Immature Thymocytes.

During postnatal life, thymopoiesis depends on the continuous colonization of the thymus by bone-marrow-derived hematopoietic progenitors that migrate through the bloodstream. The current understanding of the nature of thymic immigrants is largely based on data from pre-clinical models. Here, we employed single-cell RNA sequencing (scRNA-seq) to examine the immature postnatal thymocyte population in humans. Integration of bone marrow and peripheral blood precursor datasets identified two putative thymus seeding progenitors that varied in expression of CD7; CD10; and the homing receptors CCR7, CCR9, and ITGB7. Whereas both precursors supported T cell development, only one contributed to intrathymic dendritic cell (DC) differentiation, predominantly of plasmacytoid dendritic cells. Trajectory inference delineated the transcriptional dynamics underlying early human T lineage development, enabling prediction of transcription factor (TF) modules that drive stage-specific steps of human T cell development. This comprehensive dataset defines the expression signature of immature human thymocytes and provides a resource for the further study of human thymopoiesis.

Distinct Notch1 and BCL11B requirements mediate human γδ/αß T cell development.

γδ and αß T cells have unique roles in immunity and both originate in the thymus from T-lineage committed precursors through distinct but unclear mechanisms. Here, we show that Notch1 activation is more stringently required for human γδ development compared to αß-lineage differentiation and performed paired mRNA and miRNA profiling across 11 discrete developmental stages of human T cell development in an effort to identify the potential Notch1 downstream mechanism. Our data suggest that the miR-17-92 cluster is a Notch1 target in immature thymocytes and that miR-17 can restrict BCL11B expression in these Notch-dependent T cell precursors. We show that enforced miR-17 expression promotes human γδ T cell development and, consistently, that BCL11B is absolutely required for αß but less for γδ T cell development. This study suggests that human γδ T cell development is mediated by a stage-specific Notch-driven negative feedback loop through which miR-17 temporally restricts BCL11B expression and provides functional insights into the developmental role of the disease-associated genes BCL11B and the miR-17-92 cluster in a human context.

HES1 and HES4 have non-redundant roles downstream of Notch during early human T-cell development.

In both mouse and human, Notch1 activation is the main initial driver to induce T-cell development in hematopoietic progenitor cells. The initiation of this developmental process coincides with Notch1-dependent repression of differentiation towards other hematopoietic lineages. Although well described in mice, the role of the individual Notch1 target genes during these hematopoietic developmental choices is still unclear in human, particularly for HES4 since no orthologous gene is present in the mouse. Here, we investigated the functional capacity of the Notch1 target genes HES1 and HES4 to modulate human Notch1-dependent hematopoietic lineage decisions and their requirement during early T-cell development. We show that both genes are upregulated in a Notch-dependent manner during early T-cell development and that HES1 acts as a repressor of differentiation by maintaining a quiescent stem cell signature in CD34+ hematopoietic progenitor cells. While HES4 can also inhibit natural killer and myeloid cell development like HES1, it acts differently on the T- versus B-cell lineage choice. Surprisingly, HES4 is not capable of repressing B-cell development, the most sensitive hematopoietic lineage with respect to Notch-mediated repression. In contrast to HES1, HES4 promotes initiation of early T-cell development, but ectopic expression of HES4, or HES1 and HES4 combined, is not sufficient to induce T-lineage differentiation. Importantly, knockdown of HES1 or HES4 significantly reduces human T-cell development. Overall, we show that the Notch1 target genes HES1 and HES4 have non-redundant roles during early human T-cell development which may relate to differences in mediating Notch-dependent human hematopoietic lineage decisions.

Identification and Purification of Human T Cell Precursors.

During their development, human T cells undergo similar genomic changes and pass through the same developmental checkpoints as developing thymocytes in the mouse. The difference between both species, however, is that some of these developmental stages are characterized by different phenotypic markers, and as a result, evidence emerges that the molecular regulation of human T cell development subtly differs from the mouse (Taghon et al., Curr Top Microbiol Immunol 360:75-97, 2021; Haddad et al., Immunity 24:217-230, 2006; Hao et al., Blood 111:1318-1326, 2008; Taghon and Rothenberg, Semin Immunopathol 30:383-398, 2008). In this chapter, we describe in detail how the different stages of human T cell development can be characterized and isolated using specific surface markers.

Expression of the transcription factor Klf6 by thymic epithelial cells is required for thymus development.

Thymic epithelial cells (TEC) control T cell development and play essential roles in establishing self-tolerance. By using Foxn1-Cre-driven ablation of Klf6 gene in TEC, we identified Klf6 as a critical factor in TEC development. Klf6 deficiency resulted in a hypoplastic thymus-evident from fetal stages into adulthood-in which a dramatic increase in the frequency of apoptotic TEC was observed. Among cortical TEC (cTEC), a previously unreported cTEC population expressing the transcription factor Sox10 was relatively expanded. Within medullary TEC (mTEC), mTEC I and Tuft-like mTEC IV were disproportionately decreased. Klf6 deficiency altered chromatin accessibility and affected TEC chromatin configuration. Consistent with these defects, naïve conventional T cells and invariant natural killer T cells were reduced in the spleen. Late stages of T cell receptor-dependent selection of thymocytes were affected, and mice exhibited autoimmunity. Thus, Klf6 has a prosurvival role and affects the development of specific TEC subsets contributing to thymic function.

Single-cell profiling identifies a novel human polyclonal unconventional T cell lineage.

In the human thymus, a CD10+ PD-1+ TCRαß+ differentiation pathway diverges from the conventional single positive T cell lineages at the early double-positive stage. Here, we identify the progeny of this unconventional lineage in antigen-inexperienced blood. These unconventional T cells (UTCs) in thymus and blood share a transcriptomic profile, characterized by hallmark transcription factors (i.e., ZNF683 and IKZF2), and a polyclonal TCR repertoire with autoreactive features, exhibiting a bias toward early TCRα chain rearrangements. Single-cell RNA sequencing confirms a common developmental trajectory between the thymic and blood UTCs and clearly delineates this unconventional lineage in blood. Besides MME+ recent thymic emigrants, effector-like clusters are identified in this heterogeneous lineage. Expression of Helios and KIR and a decreased CD8ß expression are characteristics of this lineage. This UTC lineage could be identified in adult blood and intestinal tissues. In summary, our data provide a comprehensive characterization of the polyclonal unconventional lineage in antigen-inexperienced blood and identify the adult progeny.

T cell development made EZ.

In this issue of Cell Stem Cell, Jing et al. inhibit EZH1 expression in a system that supports mature T cell development from iPSCs in vitro. The authors efficiently generate T cells that are highly functional against tumors.

Transcriptional dynamics and epigenetic regulation of E and ID protein encoding genes during human T cell development.

T cells are generated from hematopoietic stem cells through a highly organized developmental process, in which stage-specific molecular events drive maturation towards αß and γδ T cells. Although many of the mechanisms that control αß- and γδ-lineage differentiation are shared between human and mouse, important differences have also been observed. Here, we studied the regulatory dynamics of the E and ID protein encoding genes during pediatric human T cell development by evaluating changes in chromatin accessibility, histone modifications and bulk and single cell gene expression. We profiled patterns of ID/E protein activity and identified up- and downstream regulators and targets, respectively. In addition, we compared transcription of E and ID protein encoding genes in human versus mouse to predict both shared and unique activities in these species, and in prenatal versus pediatric human T cell differentiation to identify regulatory changes during development. This analysis showed a putative involvement of TCF3/E2A in the development of γδ T cells. In contrast, in αß T cell precursors a pivotal pre-TCR-driven population with high ID gene expression and low predicted E protein activity was identified. Finally, in prenatal but not postnatal thymocytes, high HEB/TCF12 levels were found to counteract high ID levels to sustain thymic development. In summary, we uncovered novel insights in the regulation of E and ID proteins on a cross-species and cross-developmental level.

Conventional and Computational Flow Cytometry Analyses Reveal Sustained Human Intrathymic T Cell Development From Birth Until Puberty.

The thymus is the organ where subsets of mature T cells are generated which subsequently egress to function as central mediators in the immune system. While continuously generating T cells even into adulthood, the thymus does undergo involution during life. This is characterized by an initial rapid decrease in thymic cellularity during early life and by a second age-dependent decline in adulthood. The thymic cellularity of neonates remains low during the first month after birth and the tissue reaches a maximum in cellularity at 6 months of age. In order to study the effect that this first phase of thymic involution has on thymic immune subset frequencies, we performed multi-color flow cytometry on thymic samples collected from birth to 14 years of age. In consideration of the inherent limitations posed by conventional flow cytometry analysis, we established a novel computational analysis pipeline that is adapted from single-cell transcriptome sequencing data analysis. This allowed us to overcome technical effects by batch correction, analyze multiple samples simultaneously, limit computational cost by subsampling, and to rely on KNN-graphs for graph-based clustering. As a result, we successfully identified rare, distinct and gradually developing immune subsets within the human thymus tissues. Although the thymus undergoes early involution from infanthood onwards, our data suggests that this does not affect human T-cell development as we did not observe significant alterations in the proportions of T-lineage developmental intermediates from birth to puberty. Thus, in addition to providing an interesting novel strategy to analyze conventional flow cytometry data for the thymus, our work shows that the early phase of human thymic involution mainly limits the overall T cell output since no obvious changes in thymocyte subsets could be observed.

A cell atlas of human thymic development defines T cell repertoire formation.

The thymus provides a nurturing environment for the differentiation and selection of T cells, a process orchestrated by their interaction with multiple thymic cell types. We used single-cell RNA sequencing to create a cell census of the human thymus across the life span and to reconstruct T cell differentiation trajectories and T cell receptor (TCR) recombination kinetics. Using this approach, we identified and located in situ CD8αα+ T cell populations, thymic fibroblast subtypes, and activated dendritic cell states. In addition, we reveal a bias in TCR recombination and selection, which is attributed to genomic position and the kinetics of lineage commitment. Taken together, our data provide a comprehensive atlas of the human thymus across the life span with new insights into human T cell development.