Transcriptomes and metabolism define mouse and human MAIT cell populations

Mucosal-associated invariant T (MAIT) cells are a subset of T lymphocytes that respond to microbial metabolites. We defined MAIT cell populations in different organs and characterized the developmental pathway of mouse and human MAIT cells in the thymus using single-cell RNA sequencing and phenotypic and metabolic analyses. We showed that the predominant mouse subset, which produced IL-17 (MAIT17), and the subset that produced IFN-γ (MAIT1) had not only greatly different transcriptomes but also different metabolic states. MAIT17 cells in different organs exhibited increased lipid uptake, lipid storage, and mitochondrial potential compared with MAIT1 cells. All these properties were similar in the thymus and likely acquired there. Human MAIT cells in lung and blood were more homogeneous but still differed between tissues. Human MAIT cells had increased fatty acid uptake and lipid storage in blood and lung, similar to human CD8 T resident memory cells, but unlike mouse MAIT17 cells, they lacked increased mitochondrial potential. Although mouse and human MAIT cell transcriptomes showed similarities for immature cells in the thymus, they diverged more strikingly in the periphery. Analysis of pet store mice demonstrated decreased lung MAIT17 cells in these so-called "dirty" mice, indicative of an environmental influence on MAIT cell subsets and function.

A regulatory circuit controlled by extranuclear and nuclear retinoic acid receptor α determines T cell activation and function

Ligation of retinoic acid receptor alpha (RARα) by RA promotes varied transcriptional programs associated with immune activation and tolerance, but genetic deletion approaches suggest the impact of RARα on TCR signaling. Here, we examined whether RARα would exert roles beyond transcriptional regulation. Specific deletion of the nuclear isoform of RARα revealed an RARα isoform in the cytoplasm of T cells. Extranuclear RARα was rapidly phosphorylated upon TCR stimulation and recruited to the TCR signalosome. RA interfered with extranuclear RARα signaling, causing suboptimal TCR activation while enhancing FOXP3+ regulatory T cell conversion. TCR activation induced the expression of CRABP2, which translocates RA to the nucleus. Deletion of Crabp2 led to increased RA in the cytoplasm and interfered with signalosome-RARα, resulting in impaired anti-pathogen immunity and suppressed autoimmune disease. Our findings underscore the significance of subcellular RA/RARα signaling in T cells and identify extranuclear RARα as a component of the TCR signalosome and a determinant of immune responses.

CD4+-mediated colitis in mice is independent of the GPR183 and GPR18 pathways

Colitis is characterized by an exacerbated intestinal immune response, but the genetic and other mechanisms regulating immune activation remain incompletely understood. In order to identify new pathways leading to colitis, we sought to identify genes with increased expression in the colons of patients that also are near loci identified by genome wide association studies (GWAS) associated with IBD risk. One such SNP, rs9557195 was of particular interest because it is within an intron of G-protein-coupled receptor (GPR) 183, known to be important for lymphocyte migration. Furthermore, this SNP is in close proximity to the gene encoding another G-protein coupled receptor, GPR18. Analyzing publicly available datasets, we found transcripts of GPR183 and GPR18 to be increased in colon biopsies from ulcerative colitis and Crohn's disease patients, and GPR183 was even more increased in patients resistant to TNF treatment. Expression of both genes also was increased in mouse models of colitis. Therefore, our aim was to understand if increased expression of these GPRs in the intestine is related to disease severity in colitis models. Here we investigated the role of these receptors in the T cell transfer model and the dextran sulfate sodium model. In the T cell transfer model, GPR183 expression on donor T cells, as well as on other cell types in the Rag-/- recipients, was not essential for severe colitis induction. Furthermore, deficiency in Rag-/- mice for the enzyme that synthesizes a cholesterol metabolite that is a major ligand for GPR183 also did not affect disease. Similarly, lack of GPR18 expression in T cells or other cell types did not affect colitis pathogenesis in the T cell transfer or in the dextran sulfate sodium model. Therefore, despite increased expression of transcripts for these genes in the intestine during inflammation in humans and mice, they are not required for disease severity in mouse models of colitis induced by chemical injury or T cell cytokines, perhaps due to redundancy in mechanisms important for homing and survival of lymphocytes to the inflamed intestine.

Dietary Antigens Drive Protective Innate-like CD4 T Cell Immunity at the Mucosal Barrier of the Small Intestine

CD4 T helper (Th) cells reprogram to cytotoxic T lymphocytes (CTLs) in the intestinal epithelium during steady state. The unique localization of mucosal CD4 CTLs, in the absence of a pathogenic threat, suggests local non-pathogen antigens and organ-specific properties direct the CTL conversion.

Similar to myeloid antigen presenting cells, intestinal epithelial cells (IECs) constitutively express MHC class II (MHCII), indicating crosstalk with nearby CD4 T cells. IECs constantly absorb and process luminal digested proteins and present these dietary antigens to neighboring CD4 T cells on their basolateral side. We have generated solid evidence with murine conditional knock-out and bone marrow chimera models that MHCII+ IECs and the dietary epitopes they present are instrumental in the development of small intestinal CD4 CTLs in a homeostatic, non-inflammatory environment.

These dietary antigen-specific CD4 CTLs accumulate over time as innate-like protective cells that fortify the epithelium with a pre-existing protective arm. As fully differentiated and functional resident T cells, CD4 CTLs are aptly designed to rapidly respond, independent of pathogen specificity, and contain infections. Using Salmonella as an enteric pathogen model, the data show that dietary antigen-specific CD4 CTLs have the capacity to rapidly and effectively kill infected epithelial cells, thereby preserving the integrity of the barrier and blocking systemic spreading of the pathogen. The double-pronged ability for CD4 CTLs to protect against invaders while also mitigating immunopathology, by differentiating inflammatory CD4 T cells into excellently trained CD4 CTLs, reveal previously unknown mechanisms of mucosal lymphocyte biology.

The Transcription Factors Thpok And Runx3 Identify Two Distinct CD4−CD8− Double Negative T Cell Populations And Respectively Define Their Commitment In The Helper Or Cytotoxic Fate

In the small intestine epithelium, the major TCRab population is the double negative (DNT) cells lacking the expression of either CD4 or CD8. In contrast to the conventional selected cells, the DNT cells undergo an agonist selection, which leads to their functional education and capacity to migrate to intestine.

DNT cells also possess the capacity to reach the liver, where we observed a heterogeneity of the DNT cells populations, based on expression of ThPOK and Runx3. In the gut, the latter is only expressed by the DNT cells. GSEA analysis and cytokines intra-cellular staining have shown that ThPOK+ DNT cells and Runx3 DNT cells are strongly divergent in their biological function. ThPOK+ DNT cells signature is enriched in genes expressed by CD4 T helper cells with a strong production of the inflammatory cytokine Il-17 whereas Runx3+ DNT cells signature is enriched in gene upregulated in CD8 T cytotoxic cells. In the thymus, based on the expression of these two transcription factors, we also found distinct DNT populations. Adoptive transfer, as well as hanging drop fetal thymus culture have demonstrated that solely Runx3+ DNT cells in the thymus are the precursors of the Runx3+ DNT cells in the gut. Furthermore, we demonstrated in the thymus both DNT cells populations are selected upon an agonist interaction with a self-antigen presented by developing thymocytes, for the development of ThPOK+ DNT cells, and TECs for Runx3+ DNT cells. RNA-seq experiment of thymic DNT populations revealed that the agonist selection process results in their differential functional education since T helper pathways are upregulated in ThPOK+ DNT cells whereas pathways involved in function and differentiation of cytotoxic cells are upregulated in Runx3+ DNT cells.

THEMIS functions as a cytoplasmic adaptor and nuclear transcription factor in developing thymocytes and mature T cells

THEMIS functions in the cytoplasm as an adaptor molecule which dampens TCR signaling. During thymic development, THEMIS sets the affinity threshold for activation, enabling the selection of conventional T cells in response to low affinity antigens. Although these events occur in the cytoplasm, THEMIS possess a nuclear localization signal (NLS) and a significant amount of THEMIS resides in the nucleus.

To investigate the importance of THEMIS in the nucleus, we deleted the Themis NLS to force THEMIS in the cytoplasm. Surprisingly, T cell development was impaired similar to the Themis null mice, suggesting that THEMIS may have important roles in the nucleus as well. Alternatively, we established Themis mutant mice, in which THEMIS localized exclusively in the nucleus or where THEMIS was constantly present in both compartments. Normal conventional T cells were absent in both mutant mice. Since lack of THEMIS or the constant presence of THEMIS in the cytoplasm or nucleus or both, all resulted in aberrant T cell development it indicated that the translocation of THEMIS to or/and from the nucleus is essential for its function.

To examine the role of THEMIS in T cells in the periphery, we generated mutant mice where THEMIS is expressed normal during thymic development but altered post-thymically. Adoptive transfer of naïve CD4 T cells with altered THEMIS expression into RAG deficient host induced enhanced sever colitis and furthermore, we observed that forced localization of THEMIS in the nucleus redirects the polarization of CD4 T cells toward the pathogenic TH17 helper lineage.

These findings identify a dual function for THEMIS as a cytoplasmic adaptor during TCR signaling and a nuclear factor directing gene expression in thymocytes and mature T cells.

TCR signal strength shapes functional imprinting during CD4 T cell development

Naïve CD4 and CD8ab T cells are selected in the thymus in response to low-avidity TCR interactions. Such weak TCR signals install a quiescent program that guides naïve T cells to the lymph nodes where they differentiate upon encounter of cognate antigen. Thymocytes expressing self-specific TCRs can be agonist-selected in response to a strong TCR signal and differentiate in the thymus to functionally committed double negative T cells, NKT cells or regulatory T cells. Although the role for strength of TCR activation is clear in instructing these different outcomes, the impact of self-recognition in the thymus on “naïve” T cells is unclear. Previous work demonstrated that negative selection of self-specific T cells is not all encompassing and that affinity for self could even enhance immune protection against pathogens.

We have used Stim1fl/fl Stim2fl/fl conditional deletion mice that fail to sustain calcium entry into the cytosol upon TCR activation to assess the role of TCR strength during selection. We confirmed that agonist selection is drastically affected in these mice whereas numbers of conventional T cells are not. Interestingly, viSNE representation of multi-parameter flow cytometry revealed single cell heterogeneity among the CD4SP thymocyte subset that is controlled by Stim1/Stim2. This indicates that differential TCR activation levels give rise to phenotypically different CD4 T cells. Purification of a strongly signaled (Stim1/2-dependent) subset of CD4 thymocytes showed functional differences in terms of their ability to proliferate in response to TCR activation. Our work points to a role for self-recognition during T cell selection that shapes the response of individual CD4 T cells to TCR activation in the periphery.

Cytotoxic CD4 T cells control host immunity during viral persistence

The CD4 T helper cell lineage is initially established and controlled in the thymus by the transcription factor, ThPOK. Sustained expression of ThPOK by mature T cells in the periphery maintains the CD4 T helper commitment and allows for plasticity and differentiation to different helper fates (Th1, Th2, Th17...) in response to stimulation with cognate antigens. Our group demonstrated before that in the intestine, repeated stimulation of mature CD4 T cells results in the loss of ThPOK expression followed by the reprogramming of the T helper cells to CD4 cytotoxic T lymphocytes (CTL), expressing notably CD8 T cells attributes (such as Runx3, granzyme B).

Continuous activation is also characteristic of chronic viral infections. We therefore investigated if viral infections lead to the generation of reprogrammed CD4 CTL and moreover if these reprogrammed CD4 CTL play essential roles for the organism’s protection. Using the well-defined lymphocytic choriomeningitis virus (LCMV) acute (strain LCMV-Amstrong) versus chronic (LCMV-Clone 13) infection model, we observed that solely chronic infection enables activated CD4 T helper cells to lose ThPOK and convert to CTL with potent killer activity. Moreover, viral-induced CD4 CTL precursors produce various cytokines, notably Il-10, which is required for the induction of the reprogramming process. Finally, we engineered two unique mouse model systems, in which either all the CD4 T cells are forced to reprogram to CTL (gain of function) or else, prevented from reprogramming to CTL (loss of function). Using these models, we show that the reprogrammed CD4 CTL generated under chronic infection conditions, play critical roles in protective immunity.

Thymic selection of innate self-specific T cells: Both Timing and Signal Strength matter

Naïve CD4 and CD8 T cells are selected in the thymus in response to low-avidity TCR interactions. Such weak TCR signals are crucial for installing a quiescent program that guides them to the lymph nodes yet allows for immune activation upon encounter of high-affinity ligands in the periphery. In contrast, agonist-selected T cells receive strong self-based TCR signals in the thymus that imprint them to guard the mucosal borders and maintain quiescence at steady state. Although the TCR signal strength is a decisive factor in instructing these different outcomes, the mechanism that generates protective self-specific T cells remains largely enigmatic. Previous work suggests that the self-specific double negative (DN) T cell lineage already diverges early during T cell development. We have explored the role of developmental timing by Cre-mediated deletion of components of the TCR signaling pathway. The Lck proximal promoter drives Cre expression before b-selection and the CD4 promoter drives Cre expression after b-selection but before positive selection. As TCR targets we investigated the lack of Themis expression, essential for the generation of conventional naïve T cells, but dispensable for agonist selection. Vice versa, we analyzed the requirement for Stim1 and Stim2, driving agonist selection, but not conventional selection. Both Lck prox Cre- and CD4 Cre-driven deletion of Themis lead to a drastic decrease in naïve T cells. However, DN T cells are particularly dependent on early strong TCR signals at the Pre-TCR stage, as only Lck prox Cre- but not CD4 Cre-driven deletion of Stim1/2 eliminates this subset. Our results suggest that strong signals at the Pre-TCR checkpoint are crucial for the generation of innate self-specific T cells.

A novel double edged sword in T cell development and function: Receptor-Interacting Protein Kinase 1 (RIPK1)

Ripk1 is a key regulator of survival and death. This dual function is controlled by post-translational modifications: ubiquitination of RIPK1 leads to activation and survival whereas its deubiquitination or phosphorylation triggers death. In addition to its involvement in downstream signaling by many death receptors RIPK1 also functions in TCR-induced signaling cascades that either lead to activation-induced death or survival and NFkb activation. The role of Ripk1 in T cells development and functions are still poorly understood mainly because of the perinatal lethality of Ripk1 genomic deletion. To circumvent this problem, we generated T cell specific conditional KO mice by crossed Ripk1fl/fl mice with CD4-Cre. This deletion during development greatly affected the generation of TCRab thymocytes and in particular the agonist selected T cell subsets, including the NKT cells and the TCRab DN T cells. Similarly, TCRab T cells in the periphery were greatly reduced, in particular the NK T cell and DN subsets, whereas as expected TCRgd T cells were not affected. Most remaining TCRab T cells displayed a CD44+ activated phenotype and differentiation to cytotoxic cells, including the CD4 T cells. A similar effect was seen among the mucosal T cells in the gut, where few DN T cells were present and most of the intraepithelial TCRab T cells were cytotoxic CD8 T cells with high levels of Granzyme expression. All CD4 IETs were negative for ThPOK and expressed granzyme similar to their CD8 counterparts.

These observations highlight an important role for RIPK1 in thymic development of agonist selected T cells as well as conventional CD4 and CD8 thymocytes and in the periphery for the functional maturation and survival of lymphoid and tissue resident T cell subsets

Natural ThPOK− CD4 cytotoxic T lymphocytes : Thymic development of a new distinct self-specific CD4 T cell population

The lineage commitment to either CD4 or CD8 T cells is made in the thymus, where thymocytes with specificity for MHC-II molecules differentiates into the CD4 “helper” linage whereas ones specific for MHC-I molecules commit to the CD8 “cytotoxic” lineage. The molecular regulation is controlled by the action of key transcription factors. Notably ThPOK, which promotes the CD4 fate and suppress the cytotoxic fate of CD4 T cells. Recently, we made a major breakthrough and showed that these lineages is not fixed and the mature peripheral CD4 T cells have the plasticity to lose ThPOK expression and reprogram to ThPOKneg CD4 CTL (cytotoxic T lymphocytes) marked by the expression of CD8a, Granzyme B and potent killer capacity.

Investigating the origin of CD4 CTL, we observed in ThPOK fate maping mouse a strong proportion of CD4 CTL that never expressed ThPOK, suggesting that some CD4 CTL are natural cytotoxic cells that develop in the thymus. Based on a recent works allowing the analysis of mature CD4 cells in the thymus, we observed a substantial number of mature CD4 T cells that do not express ThPOK and display an active-like phenotype. Moreover, once injected, thymic ThPOKneg mature CD4 T cells remain ThPOKneg in the periphery and start to acquire CD8a expression demonstrating that natural CD4 CTL arise from the thymus. We also demonstrated that this population requires a strong TCR signal during their development, suggesting that natural CD4 CTL have a repertoire specific for self-antigens.

We have identified a new distinct population of mature CD4 T cells that develop in the thymus: the natural cytotoxic CD4 T lymphocytes. We also established mechanism underlying these process by demonstrating that CD4 CTL needs a strong TCR signal for their differentiation.

The checkpoint for agonist selection precedes conventional selection in human thymus

The thymus plays a central role in self-tolerance, partly by eliminating precursors with a T cell receptor (TCR) that binds strongly to self-antigens. However, the generation of self-agonist-selected lineages also relies on strong TCR signaling. How thymocytes discriminate between these opposite outcomes remains elusive. Here, we identified a human agonist-selected PD-1⁺ CD8αα⁺ subset of mature CD8αβ⁺ T cells that displays an effector phenotype associated with agonist selection. TCR stimulation of immature post-β-selection thymocyte blasts specifically gives rise to this innate subset and fixes early T cell receptor alpha variable (TRAV) and T cell receptor alpha joining (TRAJ) rearrangements in the TCR repertoire. These findings suggest that the checkpoint for agonist selection precedes conventional selection in the human thymus.

Antigen receptor-redirected T cells derived from hematopoietic precursor cells lack expression of the endogenous TCR/CD3 receptor and exhibit specific antitumor capacities

Recent clinical studies indicate that adoptive T-cell therapy and especially chimeric antigen receptor (CAR) T-cell therapy is a very potent and potentially curative treatment for B-lineage hematologic malignancies. Currently, autologous peripheral blood T cells are used for adoptive T-cell therapy. Adoptive T cells derived from healthy allogeneic donors may have several advantages; however, the expected occurrence of graft versus host disease (GvHD) as a consequence of the diverse allogeneic T-cell receptor (TCR) repertoire expressed by these cells compromises this approach. Here, we generated T cells from cord blood hematopoietic progenitor cells (HPCs) that were transduced to express an antigen receptor (AR): either a CAR or a TCR with or without built-in CD28 co-stimulatory domains. These AR-transgenic HPCs were culture-expanded on an OP9-DL1 feeder layer and subsequently differentiated to CD5⁺CD7⁺ T-lineage precursors, to CD4⁺ CD8⁺ double positive cells and finally to mature AR⁺ T cells. The AR⁺ T cells were largely naive CD45RA⁺CD62L⁺ T cells. These T cells had mostly germline TCRα and TCRβ loci and therefore lacked surface-expressed CD3/TCRαβ complexes. The CD3^- AR-transgenic cells were mono-specific, functional T cells as they displayed specific cytotoxic activity. Cytokine production, including IL-2, was prominent in those cells bearing ARs with built-in CD28 domains. Data sustain the concept that cord blood HPC derived, in vitro generated allogeneic CD3^- AR⁺ T cells can be used to more effectively eliminate malignant cells, while at the same time limiting the occurrence of GvHD.

In vitro human embryonic stem cell hematopoiesis mimics MYB-independent yolk sac hematopoiesis

Although hematopoietic precursor activity can be generated in vitro from human embryonic stem cells, there is no solid evidence for the appearance of multipotent, self-renewing and transplantable hematopoietic stem cells. This could be due to short half-life of hematopoietic stem cells in culture or, alternatively, human embryonic stem cell-initiated hematopoiesis may be hematopoietic stem cell-independent, similar to yolk sac hematopoiesis, generating multipotent progenitors with limited expansion capacity. Since a MYB was reported to be an excellent marker for hematopoietic stem cell-dependent hematopoiesis, we generated a MYB-eGFP reporter human embryonic stem cell line to study formation of hematopoietic progenitor cells in vitro. We found CD34(+) hemogenic endothelial cells rounding up and developing into CD43(+) hematopoietic cells without expression of MYB-eGFP. MYB-eGFP(+) cells appeared relatively late in embryoid body cultures as CD34(+)CD43(+)CD45(-/lo) cells. These MYB-eGFP(+) cells were CD33 positive, proliferated in IL-3 containing media and hematopoietic differentiation was restricted to the granulocytic lineage. In agreement with data obtained on murine Myb(-/-) embryonic stem cells, bright eGFP expression was observed in a subpopulation of cells, during directed myeloid differentiation, which again belonged to the granulocytic lineage. In contrast, CD14(+) macrophage cells were consistently eGFP(-) and were derived from eGFP-precursors only. In summary, no evidence was obtained for in vitro generation of MYB(+) hematopoietic stem cells during embryoid body cultures. The observed MYB expression appeared late in culture and was confined to the granulocytic lineage.

Notch Induces T Cell Receptor γδ Thymocytes to Differentiate along a Bipotent CD4 CD8 Double Positive Pathway (111.6)

It is well documented that in wildtype mice TCRγδ+ cells differentiate along a double negative (DN) pathway whereas TCRαβ+ cells differentiate along the double positive (DP) pathway, suggesting that the TCR itself induces lineage differentiation. Under experimental conditions and in genetically modified mice, however, evidence was presented suggesting that rather than TCR itself, TCR signal strength and notch determine lineage choice. In the human thymus, a “chimeric” DP TCRγδ+ cell population is present and constitutes a sizeable fraction of the γδ population. We asked the question whether these cells belong to the αβ or γδ lineage or whether these cells are bipotent. We found that TCRγδ DP cells are bipotent cells since strong TCR signals induces differentiation to TCRγδ cells, whereas Notch activation induces TCRαβ lineage differentiation. We furthermore could show that Notch signaling diverts TCRγδ DN cells to the DP pathway and induces strong proliferation. In line with these findings, TCRγδ+ acute lymphoblastic leukemias (ALL) with activating Notch1 mutations follow the DP differentiation pathway, whereas the DN ALL cells are devoid of these activating Notch1 mutations. We were able to confirm that also in vivo TCRγδ DP have rearranged TCRβ chains, actively rearrange the TCRα locus and delete the TCRδ locus (αβ lineage). Using TCRα rearrangements as a lineage marker, we could show that a subpopulation of mature TCRγδ cells is derived from DP cells.

RHAMM/HMMR (CD168) is not an ideal target antigen for immunotherapy of acute myeloid leukemia

BACKGROUND

Criteria for good candidate antigens for immunotherapy of acute myeloid leukemia are high expression on leukemic stem cells in the majority of patients with acute myeloid leukemia and low or no expression in vital tissues. It was shown in vaccination trials that Receptor for Hyaluronic Acid Mediated Motility (RHAMM/HMMR) generates cellular immune responses in patients with acute myeloid leukemia and that these responses correlate with clinical benefit. It is not clear however whether this response actually targets the leukemic stem cell, especially since it was reported that RHAMM is expressed maximally during the G2/M phase of the cell cycle. In addition, tumor specificity of RHAMM expression remains relatively unexplored.

DESIGN AND METHODS

Blood, leukapheresis and bone marrow samples were collected from both acute myeloid leukemia patients and healthy controls. RHAMM expression was assessed at protein and mRNA levels on various sorted populations, either fresh or after manipulation.

RESULTS

High levels of RHAMM were expressed by CD34(+)CD38(+) and CD34(-) acute myeloid leukemia blasts. However, only baseline expression of RHAMM was measured in CD34(+)CD38(-) leukemic stem cells, and was not different from that in CD34(+)CD38(-) hematopoietic stem cells from healthy controls. RHAMM was significantly up-regulated in CD34(+) cells from healthy donors during in vitro expansion and during in vivo engraftment. Finally, we demonstrated an explicit increase in the expression level of RHAMM after in vitro activation of T cells.

CONCLUSIONS

RHAMM does not fulfill the criteria of an ideal target antigen for immunotherapy of acute myeloid leukemia. RHAMM expression in leukemic stem cells does not differ significantly from the expression in hematopoietic stem cells from healthy controls. RHAMM expression in proliferating CD34+ cells of healthy donors and activated T cells further compromises RHAMM-specific T-cell-mediated immunotherapy.

Notch induces human T-cell receptor γδ+ thymocytes to differentiate along a parallel, highly proliferative and bipotent CD4 CD8 double-positive pathway

In wild-type mice, T-cell receptor (TCR) γδ(+) cells differentiate along a CD4 CD8 double-negative (DN) pathway whereas TCRαβ(+) cells differentiate along the double-positive (DP) pathway. In the human postnatal thymus (PNT), DN, DP and single-positive (SP) TCRγδ(+) populations are present. Here, the precursor-progeny relationship of the various PNT TCRγδ(+) populations was studied and the role of the DP TCRγδ(+) population during T-cell differentiation was elucidated. We demonstrate that human TCRγδ(+) cells differentiate along two pathways downstream from an immature CD1(+) DN TCRγδ(+) precursor: a Notch-independent DN pathway generating mature DN and CD8αα SP TCRγδ(+) cells, and a Notch-dependent, highly proliferative DP pathway generating immature CD4 SP and subsequently DP TCRγδ(+) populations. DP TCRγδ(+) cells are actively rearranging the TCRα locus, and differentiate to TCR(-) DP cells, to CD8αβ SP TCRγδ(+) cells and to TCRαβ(+) cells. Finally, we show that the γδ subset of T-cell acute lymphoblastic leukemias (T-ALL) consists mainly of CD4 SP or DP phenotypes carrying significantly more activating Notch mutations than DN T-ALL. The latter suggests that activating Notch mutations in TCRγδ(+) thymocytes induce proliferation and differentiation along the DP pathway in vivo.

Functionally mature CD4 and CD8 TCRalphabeta cells are generated in OP9-DL1 cultures from human CD34+ hematopoietic cells

Human CD34(+) hematopoietic precursor cells cultured on delta-like ligand 1 expressing OP9 (OP9-DL1) stromal cells differentiate to T lineage cells. The nature of the T cells generated in these cultures has not been studied in detail. Since these cultures do not contain thymic epithelial cells which are the main cell type mediating positive selection in vivo, generation of conventional helper CD4(+) and cytotoxic CD8(+) TCRalphabeta cells is not expected. Phenotypically mature CD27(+)CD1(-) TCRgammadelta as well as TCRalphabeta cells were generated in OP9-DL1 cultures. CD8 and few mature CD4 single-positive TCRalphabeta cells were observed. Mature CD8 single-positive cells consisted of two subpopulations: one expressing mainly CD8alphabeta and one expressing CD8alphaalpha dimers. TCRalphabeta CD8alphaalpha and TCRgammadelta cells both expressed the IL2Rbeta receptor constitutively and proliferated on IL-15, a characteristic of unconventional T cells. CD8alphabeta(+) and CD4(+) TCRalphabeta cells were unresponsive to IL-15, but could be expanded upon TCR stimulation as mature CD8alphabeta(+) and CD4(+) T cells. These T cells had the characteristics of conventional T cells: CD4(+) cells expressed ThPOK, CD40L, and high levels of IL-2 and IL-4; CD8(+) cells expressed Eomes, Runx3, and high levels of granzyme, perforin, and IFN-gamma. Induction of murine or human MHC class I expression on OP9-DL1 cells had no influence on the differentiation of mature CD8(+) cells. Similarly, the presence of dendritic cells was not required for the generation of mature CD4(+) or CD8(+) T cells. These data suggest that positive selection of these cells is induced by interaction between T precursor cells.