Yolk sac, but not hematopoietic stem cell-derived progenitors, sustain erythropoiesis throughout murine embryonic life.

In the embryo, the first hematopoietic cells derive from the yolk sac and are thought to be rapidly replaced by the progeny of hematopoietic stem cells. We used three lineage-tracing mouse models to show that, contrary to what was previously assumed, hematopoietic stem cells do not contribute significantly to erythrocyte production up until birth. Lineage tracing of yolk sac erythromyeloid progenitors, which generate tissue resident macrophages, identified highly proliferative erythroid progenitors that rapidly differentiate after intra-embryonic injection, persisting as the major contributors to the embryonic erythroid compartment. We show that erythrocyte progenitors of yolk sac origin require 10-fold lower concentrations of erythropoietin than their hematopoietic stem cell-derived counterparts for efficient erythrocyte production. We propose that, in a low erythropoietin environment in the fetal liver, yolk sac-derived erythrocyte progenitors efficiently outcompete hematopoietic stem cell progeny, which fails to generate megakaryocyte and erythrocyte progenitors.

A wave of bipotent T/ILC-restricted progenitors shapes the embryonic thymus microenvironment in a time-dependent manner.

During embryonic development, multiple waves of hematopoietic progenitors with distinct lineage potential are differentially regulated in time and space. Two different waves of thymic progenitors colonize the fetal thymus where they contribute to thymic organogenesis and homeostasis. The origin, the lineage differentiation potential of the first wave, and their relative contribution in shaping the thymus architecture, remained, however, unclear. Here, we show that the first wave of thymic progenitors comprises a unique population of bipotent T and innatel lymphoid cells (T/ILC), generating a lymphoid tissue inducer cells (LTi's), in addition to invariant Vγ5+ T cells. Transcriptional analysis revealed that innate lymphoid gene signatures and, more precisely, the LTi-associated transcripts were expressed in the first, but not in the second, wave of thymic progenitors. Depletion of early thymic progenitors in a temporally controlled manner showed that the progeny of the first wave is indispensable for the differentiation of autoimmune regulator-expressing medullary thymic epithelial cells (mTECs). We further show that these progenitors are of strict hematopoietic stem cell origin, despite the overlap between lymphopoiesis initiation and the transient expression of lymphoid-associated transcripts in yolk sac (YS) erythromyeloid-restricted precursors. Our work highlights the relevance of the developmental timing on the emergence of different lymphoid subsets, required for the establishment of a functionally diverse immune system.

Genetic dissection of Rift Valley fever pathogenesis: Rvfs2 locus on mouse chromosome 11 enables survival to early-onset hepatitis.

Infection of mice with Rift Valley fever virus (RVFV) reproduces major pathological features of severe human disease, notably the early-onset hepatitis and delayed-onset encephalitis. We previously reported that the Rvfs2 locus from the susceptible MBT/Pas strain reduces survival time after RVFV infection. Here, we used BALB/cByJ (BALB) mice congenic for Rvfs2 (C.MBT-Rvfs2) to investigate the pathophysiological mechanisms impacted by Rvfs2. Clinical, biochemical and histopathological features indicated similar liver damage in BALB and C.MBT-Rvfs2 mice until day 5 after infection. However, while C.MBT-Rvfs2 mice succumbed from acute liver injury, most BALB mice recovered and died later of encephalitis. Hepatocytes of BALB infected liver proliferated actively on day 6, promoting organ regeneration and recovery from liver damage. By comparison with C.MBT-Rvfs2, BALB mice had up to 100-fold lower production of infectious virions in the peripheral blood and liver, strongly decreased RVFV protein in liver and reduced viral replication in primary cultured hepatocytes, suggesting that the BALB Rvfs2 haplotype limits RVFV pathogenicity through decreased virus replication. Moreover, bone marrow chimera experiments showed that both hematopoietic and non-hematopoietic cells are required for the protective effect of the BALB Rvfs2 haplotype. Altogether, these results indicate that Rvfs2 controls critical events which allow survival to RVFV-induced hepatitis.

Fermentation Products of Commensal Bacteria Alter Enterocyte Lipid Metabolism.

Despite the recognized capacity of the gut microbiota to regulate intestinal lipid metabolism, the role of specific commensal species remains undefined. Here, we aimed to understand the bacterial effectors and molecular mechanisms by which Lactobacillus paracasei and Escherichia coli regulate lipid metabolism in enterocytes. We show that L-lactate produced by L. paracasei inhibits chylomicron secretion from enterocytes and promotes lipid storage by a mechanism involving L-lactate absorption by enterocytes, its conversion to malonyl-CoA, and the subsequent inhibition of lipid beta-oxidation. In contrast, acetate produced by E. coli also inhibits chylomicron secretion by enterocytes but promotes lipid oxidation by a mechanism involving acetate absorption by enterocytes, its metabolism to acetyl-CoA and AMP, and the subsequent upregulation of the AMPK/PGC-1α/PPARα pathway. Our study opens perspectives for developing specific bacteria- and metabolite-based therapeutic interventions against obesity, atherosclerosis, and malnutrition by targeting lipid metabolism in enterocytes.

Mouse HSA+ immature cardiomyocytes persist in the adult heart and expand after ischemic injury.

The assessment of the regenerative capacity of the heart has been compromised by the lack of surface signatures to characterize cardiomyocytes (CMs). Here, combined multiparametric surface marker analysis with single-cell transcriptional profiling and in vivo transplantation identify the main mouse fetal cardiac populations and their progenitors (PRGs). We found that CMs at different stages of differentiation coexist during development. We identified a population of immature heat stable antigen (HSA)/ cluster of differentiation 24 (CD24)+ CMs that persists throughout life and that, unlike other CM subsets, actively proliferates up to 1 week of age and engrafts cardiac tissue upon transplantation. In the adult heart, a discrete population of HSA/CD24+ CMs appears as mononucleated cells that increase in frequency after infarction. Our work identified cell surface signatures that allow the prospective isolation of CMs at all developmental stages and the detection of a subset of immature CMs throughout life that, although at reduced frequencies, are poised for activation in response to ischemic stimuli. This work opens new perspectives in the understanding and treatment of heart pathologies.

Molecular and Functional Characterization of Lymphoid Progenitor Subsets Reveals a Bipartite Architecture of Human Lymphopoiesis.

The classical model of hematopoiesis established in the mouse postulates that lymphoid cells originate from a founder population of common lymphoid progenitors. Here, using a modeling approach in humanized mice, we showed that human lymphoid development stemmed from distinct populations of CD127- and CD127+ early lymphoid progenitors (ELPs). Combining molecular analyses with in vitro and in vivo functional assays, we demonstrated that CD127- and CD127+ ELPs emerged independently from lympho-mono-dendritic progenitors, responded differently to Notch1 signals, underwent divergent modes of lineage restriction, and displayed both common and specific differentiation potentials. Whereas CD127- ELPs comprised precursors of T cells, marginal zone B cells, and natural killer (NK) and innate lymphoid cells (ILCs), CD127+ ELPs supported production of all NK cell, ILC, and B cell populations but lacked T potential. On the basis of these results, we propose a "two-family" model of human lymphoid development that differs from the prevailing model of hematopoiesis.

Asynchronous lineage priming determines commitment to T cell and B cell lineages in fetal liver.

The molecular events that initiate lymphoid-lineage specification remain unidentified because the stages of differentiation during which lineage commitment occurs are difficult to characterize. We isolated fetal liver progenitor cells undergoing restriction of their differentiation potential toward the T cell-innate lymphoid cell lineage or the B cell lineage. Transcripts that defined the molecular signatures of these two subsets were sequentially upregulated in lympho-myeloid precursor cells and in common lymphoid progenitor cells, respectively, and this preceded lineage restriction; this indicates that T cell-versus-B cell commitment is not a binary fate 'decision'. The T cell-bias and B cell-bias transcriptional programs were frequently co-expressed in common lymphoid progenitor cells and were segregated in subsets biased toward T cell differentiation or B cell differentiation, after interleukin 7 (IL-7) signaling that controlled the number of progenitor cells engaging in T cell differentiation versus B cell differentiation.

Single-Cell Gene Expression Analyses Reveal Heterogeneous Responsiveness of Fetal Innate Lymphoid Progenitors to Notch Signaling.

T and innate lymphoid cells (ILCs) share some aspects of their developmental programs. However, although Notch signaling is strictly required for T cell development, it is dispensable for fetal ILC development. Constitutive activation of Notch signaling, at the common lymphoid progenitor stage, drives T cell development and abrogates ILC development by preventing Id2 expression. By combining single-cell transcriptomics and clonal culture strategies, we characterize two heterogeneous α4ß7-expressing lymphoid progenitor compartments. αLP1 (Flt3(+)) still retains T cell potential and comprises the global ILC progenitor, while αLP2 (Flt3(-)) consists of ILC precursors that are primed toward the different ILC lineages. Only a subset of αLP2 precursors is sensitive to Notch signaling required for their proliferation. Our study identifies, in a refined manner, the diversity of transitional stages of ILC development, their transcriptional signatures, and their differential dependence on Notch signaling.

Characterization of Thymic Settling Progenitors in the Mouse Embryo Using In Vivo and In Vitro Assays.

Characterizing thymic settling progenitors is important to understand the pre-thymic stages of T cell development, essential to devise strategies for T cell replacement in lymphopenic patients. We studied thymic settling progenitors from murine embryonic day 13 and 18 thymi by two complementary in vitro and in vivo techniques, both based on the "hanging drop" method. This method allowed colonizing irradiated fetal thymic lobes with E13 and/or E18 thymic progenitors distinguished by CD45 allotypic markers and thus following their progeny. Colonization with mixed populations allows analyzing cell autonomous differences in biologic properties of the progenitors while colonization with either population removes possible competitive selective pressures. The colonized thymic lobes can also be grafted in immunodeficient male recipient mice allowing the analysis of the mature T cell progeny in vivo, such as population dynamics of the peripheral immune system and colonization of different tissues and organs. Fetal thymic organ cultures revealed that E13 progenitors developed rapidly into all mature CD3(+) cells and gave rise to the canonical γδ T cell subset, known as dendritic epithelial T cells. In comparison, E18 progenitors have a delayed differentiation and were unable to generate dendritic epithelial T cells. The monitoring of peripheral blood of thymus-grafted CD3(-/-) mice further showed that E18 thymic settling progenitors generate, with time, larger numbers of mature T cells than their E13 counterparts, a feature that could not be appreciated in the short term fetal thymic organ cultures.

Neonatal thymectomy favors Helicobacter pylori-promoted gastric mucosa-associated lymphoid tissue lymphoma lesions in BALB/c mice.

Neonatal thymectomy in BALB/c mice has been described as a model of gastric mucosa-associated lymphoid tissue (MALT) lymphoma (GML). By using this experimental system, we screened, for the first time to our knowledge, Helicobacter pylori GML-associated strains for their capacity to promote disease. A cohort of BALB/c mice underwent thymectomy at day 3 after birth (d3Tx). Successful thymic ablation was evaluated by the degree of lymphopenia in blood samples collected at 4 weeks of age. d3Tx and non-thymectomized controls were infected with either GML strains (B38 or B47) or control strains (SS1 or TN2GF4). Gastric samples collected at 6, 12, and 18 months after infection were studied for bacteria content, and submitted to histological, immunochemical, molecular, and immunological analyses. Severe gastric inflammation was only observed in d3Tx mice. In these animals, the gastric lamina propria was infiltrated with lymphoid cells organized in follicles composed of B cells with few infiltrating T cells. PCR of D/J IgH gene segments proved the monoclonality of infiltrating B cells, which strongly correlated with the presence of lymphoepithelial lesions. B-cell infiltrates were particularly prominent in mice infected with the B47-GML strain. No pathological changes were detected in noninfected d3Tx mice. We identified new H. pylori isolates adapted to the mouse stomach with high potential of GML development, which is only revealed in hosts rendered lymphopenic by neonatal thymic ablation.

Two waves of distinct hematopoietic progenitor cells colonize the fetal thymus.

The generation of T cells depends on the migration of hematopoietic progenitor cells to the thymus throughout life. The identity of the thymus-settling progenitor cells has been a matter of considerable debate. Here we found that thymopoiesis was initiated by a first wave of T cell lineage-restricted progenitor cells with limited capacity for population expansion but accelerated differentiation into mature T cells. They gave rise to αß and γδ T cells that constituted Vγ3(+) dendritic epithelial T cells. Thymopoiesis was subsequently maintained by less-differentiated progenitor cells that retained the potential to develop into B cells and myeloid cells. In that second wave, which started before birth, progenitor cells had high proliferative capacity but delayed differentiation capacity and no longer gave rise to embryonic γδ T cells. Our work reconciles conflicting hypotheses on the nature of thymus-settling progenitor cells.

Notchless-dependent ribosome synthesis is required for the maintenance of adult hematopoietic stem cells.

Blood cell production relies on the coordinated activities of hematopoietic stem cells (HSCs) and multipotent and lineage-restricted progenitors. Here, we identify Notchless (Nle) as a critical factor for HSC maintenance under both homeostatic and cytopenic conditions. Nle deficiency leads to a rapid and drastic exhaustion of HSCs and immature progenitors and failure to maintain quiescence in HSCs. In contrast, Nle is dispensable for cycling-restricted progenitors and differentiated cells. In yeast, Nle/Rsa4 is essential for ribosome biogenesis, and we show that its role in pre-60S subunit maturation has been conserved in the mouse. Despite its implication in this basal cellular process, Nle deletion affects ribosome biogenesis only in HSCs and immature progenitors. Ribosome biogenesis defects are accompanied by p53 activation, which causes their rapid exhaustion. Collectively, our findings establish an essential role for Nle in HSC and immature progenitor functions and uncover previously unsuspected differences in ribosome biogenesis that distinguish stem cells from restricted progenitor populations.

Origin, trafficking, and intraepithelial fate of gut-tropic T cells.

The small intestine epithelium (SI-Ep) harbors millions of unconventional (γδ and CD4(-) CD8(-) NK1.1(-) TCRαß) and conventional (CD8αß and CD4) T cells, designated intraepithelial lymphocytes (IELs). Here, we identified the circulating pool of SI-Ep-tropic T cells and studied their capacity to colonize the SI-Ep under steady-state conditions in SPF mice. Developmentally regulated levels of α4ß7 endowed recent thymic emigrants (RTEs) of unconventional types with higher SI-Ep tropism than their conventional homologues. SI-Ep-tropic RTEs, which in all lineages emerged naive, homed to the SI-Ep, but this environment was inadequate to stimulate them to cycle. In contrast, conventional and, unexpectedly, unconventional T cells, particularly Vγ7(+) (hallmark of γδ IELs), previously stimulated to cycle in the gut-associated lymphoid tissue (GALT), proliferated in the SI-Ep. Cycling unconventional SI-Ep immigrants divided far more efficiently than their conventional homologues, thereby becoming predominant. This difference impacted on acquisition of high Granzyme B content, which required extensive proliferation. In conclusion, SI-Ep-tropic T cells follow a thymus-SI-Ep or a GALT-SI-Ep pathway, the latter generating highly competitive immigrants that are the sole precursors of cytotoxic IELs. These events occur continuously as part of the normal IEL dynamics.

Extrathymic induction of Foxp3⁺ regulatory T cells declines with age in a T-cell intrinsic manner.

Extrathymically induced Foxp3⁺ regulatory T (Treg) cells contribute to the pool of Treg cells and are implicated in the maintenance of immune tolerance at environmental interfaces. The impact of T-cell senescence on their generation and function is, however, poorly characterized. We report here that steady-state induction of Foxp3 is impaired in aged T cells in vivo. In vitro assays further revealed that this defective generation of Treg cells was independent from the strength of TCR stimulation and arose before T-cell proliferation. Importantly, they also revealed that this impairment of Foxp3 induction is unrelated to known age-related T-cell defects, such as IL-2 secretion impairment, accumulation of activated T-cell populations, or narrowing of the T-cell repertoire. Finally, a loss of extrathymic induction of Foxp3 and tolerance to minor-mismatched skin graft were observed in aged mice treated by nondepleting anti-CD4 antibody. The T-cell intrinsic impairment of Treg-cell generation revealed here highlights age as a key factor to be considered in immune tolerance induction.

Critical role of TCR specificity in the development of Vγ1Vδ6.3+ innate NKTγδ cells.

A large fraction of innate NKTγδ T cells uses TCRs composed of a semi-invariant Vδ6.3/6.4-Dδ2-Jδ1 chain together with more diverse Vγ1-Jγ4 chains. To address the role of γδTCR specificity in their generation, we analyzed their development in mice transgenic (Tg) for a Vγ1-Jγ4 chain frequently expressed by NKTγδ cells (Tg-γ) and in mice Tg for the same Vγ1-Jγ4 chain together with a Vδ6BDδ2Jδ1 chain not usually found among NKTγδ cells (Tg-γδ). Surprisingly, both promyelocytic leukemia zinc finger (PLZF)(+) and NK1.1(+) NKTγδ cells were found in the thymus of Tg-γδ albeit at lower numbers than in Tg-γ mice, and virtually all of them expressed the Tg TCR. However, the PLZF(+) subset, but not the NK1.1(+) subset, also expressed an endogenous Vδ6.3/6.4 chain, and its size was severely reduced in TCRδ(-/-) Tg-γδ mice. These results could suggest that the PLZF(+) and the NK1.1(+) subsets are developmentally unrelated. However, PLZF(+) and NK1.1(+) NKTγδ cells express identical Vδ6.3/6.4 chains, and NK1.1(+) cells can be obtained upon intrathymic injection of sorted PLZF(+) cells, thus indicating their developmental relationship. In fact, the NK1.1(+) γδ thymocytes present in Tg-γδ mice correspond to a small subset of NK1.1(+) γδ thymocytes in wild-type animals, which express a more diverse repertoire of TCRs and can be recognized by the expression of the CD62L Ag. Collectively, our data demonstrated that TCR specificity is essential for the development of most NKTγδ T cells and revealed a developmental heterogeneity in γδ T cells expressing the NK1.1 marker.

Immature hematopoietic stem cells undergo maturation in the fetal liver.

Hematopoietic stem cells (HSCs), which are defined by their capacity to reconstitute adult conventional mice, are first found in the dorsal aorta after 10.5 days post coitus (dpc) and in the fetal liver at 11 dpc. However, lympho-myeloid hematopoietic progenitors are detected in the dorsal aorta from 9 dpc, raising the issue of their role in establishing adult hematopoiesis. Here, we show that these progenitors are endowed with long-term reconstitution capacity, but only engraft natural killer (NK)-deficient Rag2γc(-/-) mice. This novel population, called here immature HSCs, evolves in culture with thrombopoietin and stromal cells, into HSCs, defined by acquisition of CD45 and MHC-1 expression and by the capacity to reconstitute NK-competent mice. This evolution occurs during ontogeny, as early colonization of fetal liver by immature HSCs precedes that of HSCs. Moreover, organ culture experiments show that immature HSCs acquire, in this environment, the features of HSCs.

The Ets-1 transcription factor controls the development and function of natural regulatory T cells.

Regulatory T cells (T reg cells) constitute a population of CD4(+) T cells that limits immune responses. The transcription factor Foxp3 is important for determining the development and function of T reg cells; however, the molecular mechanisms that trigger and maintain its expression remain incompletely understood. In this study, we show that mice deficient for the Ets-1 transcription factor (Ets-1(-/-)) developed T cell-mediated splenomegaly and systemic autoimmunity that can be blocked by functional wild-type T reg cells. Spleens of Ets-1(-/-) mice contained mostly activated T cells, including Th2-polarized CD4(+) cells and had reduced percentages of T reg cells. Splenic and thymic Ets-1(-/-) T reg cells expressed low levels of Foxp3 and displayed the CD103 marker that characterizes antigen-experienced T reg cells. Thymic development of Ets-1(-/-) T reg cells appeared intrinsically altered as Foxp3-expressing cells differentiate poorly in mixed fetal liver reconstituted chimera and fetal thymic organ culture. Ets-1(-/-) T reg cells showed decreased in vitro suppression activity and did not protect Rag2(-/-) hosts from naive T cell-induced inflammatory bowel disease. Furthermore, in T reg cells, Ets-1 interacted with the Foxp3 intronic enhancer and was required for demethylation of this regulatory sequence. These data demonstrate that Ets-1 is required for the development of natural T reg cells and suggest a role for this transcription factor in the regulation of Foxp3 expression.

Ontogeny, function, and peripheral homeostasis of regulatory T cells in the absence of interleukin-7.

Mice lacking interleukin-7 (IL-7-/- mice) have no signs of autoimmune disease, contrary to other models of lymphopenia. We investigated whether the absence of disease was due to the fact that IL-7 is dispensable for the ontogeny, function, and homeostasis of regulatory CD4+ T cells. We show here that the establishment of the peripheral pool of Foxp3-expressing regulatory cells is IL-7 independent, and the premature involution of the thymus in IL-7-/- mice does not change the representation of the CD4+CD25+ T-cell compartment. In addition, CD4+CD25+ T cells expand in the absence of IL-7, without losing Foxp3 expression. The frequency of activated peripheral CD4+ T cells increases with age in both the CD25- and CD25+ compartments, with the CD4+CD25+ T cells displaying signs of constant activation. IL-7-/- CD4+CD25+ T cells control inflammatory bowel disease induced by IL-7-/- T cells even in hosts lacking IL-7. Depletion of the CD25+ T-cell subset after thymic involution results in a mild form of inflammatory bowel disease (IBD), which resolves concomitantly with the regeneration of this subset. This study shows for the first time that IL-7-/- mice have a robust regulatory Foxp3-expressing CD4+ T-cell compartment that controls T-cell-mediated disease. It also highlights the potential of the regulatory Foxp3-expressing CD4+CD25- T-cell population to restore a functional CD4+CD25+ T-cell compartment through an IL-7-independent pathway.

Regulatory potential and control of Foxp3 expression in newborn CD4+ T cells.

Thymectomy at day 3 after birth leads to autoimmune disease in some genetic backgrounds. Disease is thought to be caused by the lack/paucity of regulatory T cells. We show that 3-day-old mice already contain a significant compartment of Foxp3-expressing CD25(+)CD4(+) splenocytes. Whereas, in adult spleen, the subsets of regulatory T cells (CD25(+) and/or CD103(+)) express high amounts of Foxp3 mRNA, in 3-day-old mice, both thymic and splenic CD25(+)CD4(+) T cell subsets express lower amounts of Foxp3 mRNA, and CD103(+) cells are barely detected. In adult day 3-thymectomized mice, the CD25(+)CD4(+) T cell subset is overrepresented (most of the cells being CD103(+)) and expresses high amounts of Foxp3 mRNA, independent of the development of autoimmune gastritis. These cells control inflammatory bowel disease and the homeostatic expansion of lymphocytes. This study demonstrates that the peripheral immune system of newborn mice is endowed of a remarkable regulatory potential, which develops considerably in the absence of thymic supply.