Single-cell lineage tracing approaches in hematology research: technical considerations

From Hematopoietic Stem Cells to Platelets: Unifying Differentiation Pathways Identified by Lineage Tracing Mouse Models

Platelets are the terminal progeny of megakaryocytes, primarily produced in the bone marrow, and play critical roles in blood homeostasis, clotting, and wound healing. Traditionally, megakaryocytes and platelets are thought to arise from multipotent hematopoietic stem cells (HSCs) via multiple discrete progenitor populations with successive, lineage-restricting differentiation steps. However, this view has recently been challenged by studies suggesting that (1) some HSC clones are biased and/or restricted to the platelet lineage, (2) not all platelet generation follows the "canonical" megakaryocytic differentiation path of hematopoiesis, and (3) platelet output is the default program of steady-state hematopoiesis. Here, we specifically investigate the evidence that in vivo lineage tracing studies provide for the route(s) of platelet generation and investigate the involvement of various intermediate progenitor cell populations. We further identify the challenges that need to be overcome that are required to determine the presence, role, and kinetics of these possible alternate pathways.

Exploiting somatic mutations to decipher human blood production: a natural lineage-tracing strategy

Lineage tracing using fluorescent proteins, genetic barcodes, and various other strategies has provided critical insights into the dynamics of both fetal and adult hematopoiesis in model organisms. However, these technologies cannot be readily used to study hematopoiesis in human beings. Therefore, there is a critical need to develop strategies to assess cellular dynamics within human hematopoietic tissues in vivo. Recently, researchers have used naturally acquired somatic mutations, coupled with other single-cell technologies, to retrospectively analyze clonal cellular dynamics. In summer 2022, the International Society for Experimental Hematology's New Investigator Committee hosted a webinar focused on novel approaches to dissect fetal and adult hematopoiesis, with presentations from Drs. Ana Cvejic and Vijay Sankaran. Here, we provide an overview of these exciting technological advances and some of the novel insights they have already provided in studying human hematopoiesis.

Engineering human hematopoietic environments through ossicle and bioreactor technologies exploitation

The bone marrow microenvironment contains cellular niches that maintain the pool of hematopoietic stem and progenitor cells and support hematopoietic maturation. Malignant hematopoietic cells also co-opt normal cellular interactions to promote their own growth and evade therapy. In vivo systems used to study human hematopoiesis have been developed through transplantation into immunodeficient mouse models. However, incomplete cross-compatibility between the murine stroma and transplanted human hematopoietic cells limits the rate of engraftment and the study of relevant interactions. To supplement in vivo xenotransplantation models, complementary strategies have recently been developed, including the use of three-dimensional human bone marrow organoids in vivo, generated from bone marrow stromal cells seeded onto osteo-inductive scaffolds, as well as the use of ex vivo bioreactor models. These topics were the focus of the Spring 2020 International Society for Experimental Hematology New Investigator webinar. We review here the latest advances in generating humanized hematopoietic organoids and how they allow for the study of novel microenvironmental interactions.