A new model of human lymphopoiesis across development and aging

Over the past decade our research has implemented a multimodal approach to human lymphopoiesis, combining clonal-scale mapping of lymphoid developmental architecture with the monitoring of dynamic changes in the pattern of lymphocyte generation across ontogeny. We propose that lymphopoiesis stems from founder populations of CD127/interleukin (IL)7R- or CD127/IL7R+ early lymphoid progenitors (ELPs) polarized respectively toward the T-natural killer (NK)/innate lymphoid cell (ILC) or B lineages, arising from newly characterized CD117lo multi-lymphoid progenitors (MLPs). Recent data on the lifelong lymphocyte dynamics of healthy donors suggest that, after birth, lymphopoiesis may become increasingly oriented toward the production of B lymphocytes. Stemming from this, we posit that there are three major developmental transitions, the first occurring during the neonatal period, the next at puberty, and the last during aging.

Multimodal cartography of human lymphopoiesis reveals B and T/NK/ILC lineages are subjected to differential regulation

The developmental cartography of human lymphopoiesis remains incompletely understood. Here, we establish a multimodal map demonstrating that lymphoid specification follows independent direct or stepwise hierarchic routes converging toward the emergence of newly characterized CD117lo multi-lymphoid progenitors (MLPs) that undergo a proliferation arrest before entering the CD127- (NK/ILC/T) or CD127+ (B) lymphoid pathways. While the differentiation of CD127- early lymphoid progenitors is mainly driven by Flt3 signaling, emergence of their CD127+ counterparts is regulated cell-intrinsically and depends exclusively on the divisional history of their upstream precursors, including hematopoietic stem cells. Further, transcriptional mapping of differentiation trajectories reveals that whereas myeloid granulomonocytic lineages follow continuous differentiation pathways, lymphoid trajectories are intrinsically discontinuous and characterized by sequential waves of cell proliferation allowing pre-commitment amplification of lymphoid progenitor pools. Besides identifying new lymphoid specification pathways and regulatory checkpoints, our results demonstrate that NK/ILC/T and B lineages are under fundamentally distinct modes of regulation. (149 words).

Human growth hormone supplement promotes human lymphohematopoietic cell reconstitution in immunodeficient mice

Aim: To investigate the potential of human growth hormone (hGH) to improve human hematopoietic reconstitution in humanized mice. Materials & methods: Immunodeficient mice were conditioned by total body irradiation and transplanted with human CD34⁺ fetal liver cells. Peripheral blood, spleen and bone marrow were harvested, and levels of human lymphohematopoietic cells were determined by flow cytometry. Results: Supplementation with hGH elevated human lymphohematopoietic chimerism by more than twofold. Treatment with hGH resulted in significantly increased reconstitution of human B cells and myeloid cells in lymphoid organs, enhanced human erythropoiesis in the bone morrow, and improved engraftment of human hematopoietic stem cells. Conclusion: hGH supplementation promotes human lymphohematopoietic reconstitution in humanized mice.

Modeling Human Fetal Hematopoiesis in Humanized Mice

Due to difficulties to access primary human bone marrow samples and age or donor effects, human hematopoiesis has long remained far less well characterized than in the mouse. Despite recent progresses in single-cell RNA profiling only little is known as to phenotype, function and developmental trajectories of human lymphomyeloid progenitors and precursors. This is especially true regarding the developmental architecture of the lymphoid lineage which has been the subject of persistent controversies over the past decades. Here, we describe an original approach of in vivo modeling of human fetal hematopoiesis immunodeficient NSG mice engrafted with neonatal CD34⁺ hematopoietic progenitor cells (HPCs) allowing for rapid identification and isolation of lymphomyeloid developmental intermediates.

[Bipartite organization of human lymphopoiesis]

Due to difficulties to access primary bone marrow samples, human hematopoiesis has long remained far less characterized than in the mouse. Using an in vivo modeling approach of fetal hematopoiesis in humanized mice, we recently showed that human lymphoid cells stem from two functionally specialized populations of CD127^- and CD127⁺ early lymphoid progenitors (ELP) that differentiate independently, respond differently to growth factors, undergo divergent modes of lineage restriction and generate distinct lymphoid populations. Our results demonstrate that, conversely to the mouse, human lymphopoiesis displays a bipartite developmental architecture.

The functional relationship between hematopoietic stem cells and developing T lymphocytes

In contrast to all other blood and immune cells, T lymphocytes do not develop in the bone marrow (BM), but in the specialized microenvironment provided by the thymus. Similar to the other lineages, however, all T cells arise from multipotent hematopoietic stem cells (HSCs) that reside in the BM. Not all HSCs give rise to T cells; but how many and what kind of developmental checkpoints are located along this intricate differentiation path is the subject of intense research. Traditionally, this process has been studied almost exclusively using mouse cells, but recent advances in immunodeficient mouse models, high-speed cell sorting, lentiviral transduction protocols, and deep sequencing techniques have allowed these questions to be addressed using human cells. Here we review the process of thymic seeding by BM-derived cells and T cell commitment in humans, discussing recent insights into the clonal composition of the thymus and the definition of developmental checkpoints, on the basis of insights from human severe combined immunodeficiency patients.

Identification of checkpoints in human T-cell development using severe combined immunodeficiency stem cells

BACKGROUND

Severe combined immunodeficiency (SCID) represents congenital disorders characterized by a deficiency of T cells caused by arrested development in the thymus. Yet the nature of these developmental blocks has remained elusive because of the difficulty of taking thymic biopsy specimens from affected children.

OBJECTIVE

We sought to identify the stages of arrest in human T-cell development caused by various major types of SCID.

METHODS

We performed transplantation of SCID CD34(+) bone marrow stem/progenitor cells into an optimized NSG xenograft mouse model, followed by detailed phenotypic and molecular characterization using flow cytometry, immunoglobulin and T-cell receptor spectratyping, and deep sequencing of immunoglobulin heavy chain (IGH) and T-cell receptor δ (TRD) loci.

RESULTS

Arrests in T-cell development caused by mutations in IL-7 receptor α (IL7RA) and IL-2 receptor γ (IL2RG) were observed at the most immature thymocytes much earlier than expected based on gene expression profiling of human thymocyte subsets and studies with corresponding mouse mutants. T-cell receptor rearrangements were functionally required at the CD4(-)CD8(-)CD7(+)CD5(+) stage given the developmental block and extent of rearrangements in mice transplanted with Artemis-SCID cells. The xenograft model used is not informative for adenosine deaminase-SCID, whereas hypomorphic mutations lead to less severe arrests in development.

CONCLUSION

Transplanting CD34(+) stem cells from patients with SCID into a xenograft mouse model provides previously unattainable insight into human T-cell development and functionally identifies the arrest in thymic development caused by several SCID mutations.

Small-molecule inhibitor of glycogen synthase kinase 3β 6-Bromoindirubin-3-oxime inhibits hematopoietic regeneration in stem cell recipient mice

Small-molecule inhibitors of glycogen synthase kinase 3β (GSK3β) have demonstrated strong anti-leukemia effects in preclinical studies. Here, we investigated the effect of GSK3β inhibitor 6-Bromoindirubin-3-oxime (BIO) previously shown to inhibit leukemia cell growth in vitro and of animal models on hematopoietic regeneration in recipients of stem cell transplant. BIO administered to immunocompromised mice transplanted with human hematopoietic stem cells inhibited human stem cell engraftment in the bone marrow (BM) and peripheral blood. BIO reduced CD34(+) progenitor cells in the BM, and primitive lymphoid progenitors re-populated host thymus at later stages post-transplant. The development of all T-cell subsets in the thymus was suppressed in BIO-treated mice. Human cell engraftment was gradually restored after discontinuation of BIO treatment; however, T-cell depletion remained until the end of experiment, which correlated with the attenuated thymic function in the host. BIO delayed CD34(+) cell expansion in stroma-supported or cytokine-only cultures. BIO treatment delayed progenitor cell divisions and induced apoptosis in cultures with sub-optimal cytokine support. In addition, BIO inhibited B- and T-cell development in co-cultures with MS5 and OP9-DL1 BM stroma cells, respectively. These data suggest that administration of GKS3β inhibitors may act to delay hematopoietic regeneration in patients who received stem cell transplant.

GSK-3β inhibition preserves naive T cell phenotype in bone marrow reconstituted mice

Hematopoietic stem cell transplantation (HSCT) is used in the treatment of hematologic and nonhematologic disorders. PostHSCT immunologic reconstitution is a critical component for successful outcome. Pretransplant conditioning impairs thymic function, leading to delayed T cell regeneration. Thymus-independent T cell expansion is associated with defective generation of naive T cells and memory T cell skewing, resulting in decreased diversity in the T cell repertoire, thus attenuating the immune responses and increasing the risk of opportunistic infections and leukemia relapse. Wingless (Wnt) signaling has been identified as an important regulator of T cell development and function. Activated Wnt signaling inhibited differentiation of mature T cells in transgenic mouse models. The effect of Wnt activation on T cell regeneration following HSCT was not investigated. In this study, we demonstrate that the GSK-3β inhibitor 6-bromoindirubin 3'-oxime (BIO) activates Wnt/β-catenin signaling, elevates the proportion of naive T cells, and delays T cell differentiation during homeostatic T cell expansion in lymphodepleted mice transplanted with human hematopoietic stem cells. In vitro BIO-treatment promoted naive T cell expansion following mitogenic stimulation and improved proliferative responses of T cells to allogeneic stimuli. Treatment with BIO acts to expand the IL7Rα(+) subset of naive T cells, suggesting the potential mechanism driving T cell expansion during IL-7-dependent T cell proliferation. BIO downregulated expression of genes activated during effector cell differentiation and preserved naive T cell gene expression. We propose that administration of GSK-3β inhibitor increases the potency of T cells in recipients of HSCT by expansion of naive T cell subsets with a diverse T cell receptor repertoire.

Infectious diseases in humanized mice

Despite many theoretical incompatibilities between mouse and human cells, mice with reconstituted human immune system components contain nearly all human leukocyte populations. Accordingly, several human-tropic pathogens have been investigated in these in vivo models of the human immune system, including viruses such as human immunodeficiency virus (HIV) and Epstein-Barr virus (EBV), as well as bacteria such as Mycobacterium tuberculosis and Salmonella enterica Typhi. While these studies initially aimed to establish similarities in the pathogenesis of infections between these models and the pathobiology in patients, recent investigations have provided new and interesting functional insights into the protective value of certain immune compartments and altered pathology upon mutant pathogen infections. As more tools and methodologies are developed to make these models more versatile to study human immune responses in vivo, such improvements build toward small animal models with human immune components, which could predict immune responses to therapies and vaccination in human patients.

Identification of TMEM131L as a novel regulator of thymocyte proliferation in humans

In this study, we identify transmembrane protein 131-like (TMEM131L) as a novel regulator of thymocyte proliferation and demonstrate that it corresponds to a not as yet reported inhibitor of Wnt signaling. Short hairpin RNA-mediated silencing of TMEM131L in human CD34(+) hematopoietic progenitors, which were then grafted in NOD-SCID/IL-2rγ(null) mice, resulted in both thymocyte hyperproliferation and multiple pre- and post-β-selection intrathymic developmental defects. Consistent with deregulated Wnt signaling, TMEM131L-deficient thymocytes expressed Wnt target genes at abnormally high levels, and they displayed both constitutive phosphorylation of Wnt coreceptor LRP6 and β-catenin intranuclear accumulation. Using T cell factor reporter assays, we found that membrane-associated TMEM131L inhibited canonical Wnt/β-catenin signaling at the LRP6 coreceptor level. Whereas membrane-associated TMEM131L did not affect LRP6 expression under basal conditions, it triggered lysosome-dependent degradation of its active phosphorylated form following Wnt activation. Genetic mapping showed that phosphorylated LRP6 degradation did not depend on TMEM131L cytoplasmic part but rather on a conserved extracellular domain proximal to the membrane. Collectively, these data indicate that, during thymopoiesis, stage-specific surface translocation of TMEM131L may regulate immature single-positive thymocyte proliferation arrest by acting through mixed Wnt-dependent and -independent mechanisms.