Andrea Ditadi

The hematopoietic system performs essential functions for living organisms through a wide array of cell types, including immune cells for host defense, erythrocytes for oxygen transport, and megakaryocytes involved in vascular and tissue repair. The adoptive transfer of mature blood cells and the transplantation of hematopoietic stem cells (HSCs) have been extensively exploited in clinical settings for the treatment of inherited blood disorders as well as blood malignancies. In addition, blood cells are also preferential targets in the context of gene therapy, in which they provide revolutionary treatments not only for classical hematologic diseases but also for novel and unexpected indications, such as inherited metabolic disorders. However, clinical bottlenecks remain, including the limited availability of suitably matched donor cells, the need for high cell doses, and the complexity of personalized manufacturing. Given their proliferative ability and capacity to generate any human cell type, human pluripotent stem cells (hPSCs), encompassing both embryonic stem cells (ESCs) and induced PSCs, represent a novel, potentially unlimited, easy-to-engineer source of either patient-specific or allogeneic blood cells for “off-the-shelf” cell therapies. In parallel, hPSC-derived blood cells enable in vitro modeling of diverse hematologic diseases and the identification of novel therapeutic targets. Recent advances describing protocols for the de novo generation of HSCs are opening new and exciting avenues for a broad application of hPSC-derived blood cells in both basic research and clinical medicine.

Pluripotency, the ability to generate all body cell types, emerges in a disorganized embryonic cell mass. After implantation, these cells form a columnar epithelium and initiate lumenogenesis. During gastrulation, some undergo epithelial-to-mesenchymal transition to form the primitive streak (PS). The signals controlling these events in humans are largely unknown. Here, to study them, we developed a chemically defined 3D model where conventional pluripotent stem cells self-organize into a columnar epithelium with a lumen, from which PS-like cells emerge. We show that early TGFβ family inhibition prevents epithelial identity, also in murine 3D embryo models and in embryos. ZNF398 acts downstream of TGFβ1, activating the epithelial master regulator ESRP1 while repressing mesenchymal factors CDH2 and ZEB2. After epithelium formation, TGFβ1 stimulation is dispensable for its maintenance. However, treatment via ACTIVIN—a distinct TGFβ family ligand—induces PS efficiently. Thus, signalling of the TGFβ family dynamically governs pluripotent epiblast epithelial identity.

The successful generation of long-term engrafting hematopoietic stem cells (HSCs) from human-induced pluripotent stem cells (hiPSCs) has long been sought to revolutionize treatments for hematological disorders, eliminating reliance on donors and avoiding immune rejection, and thus has been seen as a major milestone in regenerative medicine. Previous studies, guided by developmental hematopoiesis, made progress in creating blood cells from hiPSCs, but challenges persisted in producing hematopoietic cells with functional properties of genuine HSCs capable of long-term engraftment. In their recent study, Ng and colleagues described an optimized differentiation protocol that manipulates key signaling pathways, including TGF-β, WNT, BMP, and retinoic acid in a stage-specific manner to generate HSCs with multilineage capacity. This strategy yielded hematopoietic cells capable of engrafting long term with high levels of human chimerism in recipient mice. This research provides a blueprint for future studies aiming for personalized HSC-based therapies for various blood disorders.

During embryonic development, blood cells emerge from specialized endothelial cells, named haemogenic endothelial cells (HECs). As HECs are rare and only transiently found in early developing embryos, it remains difficult to distinguish them from endothelial cells. Here we performed transcriptomic analysis of 28- to 32-day human embryos and observed that the expression of Fc receptor CD32 (FCGR2B) is highly enriched in the endothelial cell population that contains HECs. Functional analyses using human embryonic and human pluripotent stem cell-derived endothelial cells revealed that robust multilineage haematopoietic potential is harboured within CD32⁺ endothelial cells and showed that 90% of CD32⁺ endothelial cells are bona fide HECs. Remarkably, these analyses indicated that HECs progress through different states, culminating in FCGR2B expression, at which point cells are irreversibly committed to a haematopoietic fate. These findings provide a precise method for isolating HECs from human embryos and human pluripotent stem cell cultures, thus allowing the efficient generation of haematopoietic cells in vitro.

Omenn Syndrome (OS) is a rare hematological disorder, caused by hypomorphic mutations in genes involved in B-/T-cell receptor (BCR/TCR) rearrangement that result in impaired lymphocyte development and immunodeficiency. Notwithstanding, few T-cell clones enriched in self-reactive specificities expand in peripheral tissues, where they trigger severe inflammation and autoimmune reactions. Interestingly, residual OS lymphocytes display characteristics proper of embryonic lymphocytes that emerge before, and independently from, hematopoietic stem cells (HSCs). This prompted us to hypothesize whether OS autoreactive T-cells are generated in the embryo independently from HSCs. Here we show that in the Rag2 ^R229Q/R229Q OS mouse model, embryonic but not adult bone marrow-derived hematopoietic progenitors can generate T-cells. T-lymphopoiesis can be rescued in adult OS blood progenitors via their Lin28-mediated reprogramming to an embryonic-like state. Remarkably, when transplanted in immunodeficient mice, embryonic-like OS progenitors trigger tissue morphological alterations and inflammation in the large intestine of the recipients, recapitulating the typical OS inflammatory phenotype. Our study describes the previously unappreciated contribution of embryonic progenitors to the pool of autoreactive infilitrating T-cells, providing a novel platform for both the detailed study of human autoimmune disorders and the design of more targeted therapies.

Summary: During development, in the embryo proper blood cells emerge from a subset of specialized endothelial cells, named hemogenic endothelial cells (HECs), via a process known as endothelial-to-hematopoietic transition (EHT) driven by time-specific Notch signaling activation ¹ . HECs represent an elusive cell population as they are rare and transient, rapidly generating blood cells, and specific markers are lacking. Therefore, it remains unclear how and when the hematopoietic fate is specified and how blood cell emergence is molecularly regulated. Notably, thorough characterization of this process is essential to guide the generation of therapeutic blood products in vitro from human pluripotent stem cells (hPSCs). To identify specific human HEC markers, we performed transcriptomic analysis of 28-32-day human embryos, a developmental stage characterized by active hematopoiesis. We observed that the expression of FCGR2B , encoding for the Fc receptor CD32, is highly enriched in the ACE ⁺ CD34 ⁺ endothelial cell population that contains HECs. Functional ex vivo analyses confirmed that multilineage hematopoietic potential is highly enriched in CD32 ⁺ endothelial cells isolated from human embryos. In addition, clonal analysis revealed that 90% of CD32 ⁺ hPSC-derived endothelial cells are bona fide HECs. We leveraged this specificity to study how HECs commit to the blood fate. Remarkably, our analyses indicated that HECs progress through different states culminating with the one identified by CD32 expression. Indeed CD32 ⁺ HECs no longer require Notch to generate hematopoietic progeny and display full commitment to hematopoiesis even before the expression of hematopoietic markers. These findings provide a precise method for isolating HECs primed to the blood fate from human embryos and hPSC cultures, thus allowing the efficient generation of hematopoietic cells in vitro .