Applications of the Human Bone Marrow Organoid System to Study Hematopoiesis and Anemia Stress.

Hematopoietic stem and progenitor cells (HSPCs) are precisely organized within specialized bone marrow niches containing hematopoietic and nonhematopoietic cell types. Physiologically accurate human models of this niche environment that include functional HSPCs have been lacking until recently. Using induced pluripotent stem cells (iPSCs) grown in mixed-matrix hydrogels, vascularized three-dimensional human bone marrow organoids (hBMOs) can now be generated that contain mesenchymal, endothelial, and hematopoietic cells, all vital components of the bone marrow niche. While hBMOs have been shown to support engraftment of normal and malignant cells, there are numerous opportunities to use hBMOs for developing a wider experimental toolkit to test gene functions and stress responses. Here, we establish two new applications for the hBMO platform. First, we demonstrate successful engraftment of gene-edited CD34+ cells from healthy donors, enabling direct investigation of gene-specific effects on human hematopoiesis in a defined microenvironment. Gene-edited engrafted HSPCs are maintained in hBMOs for 7 days and undergo multilineage differentiation. Second, we adapted a method to induce stress erythropoiesis-a response to acute anemia in mice and humans-in hBMOs, resulting in robust expansion of immunophenotypically defined hematopoietic progenitors and erythroid populations. These two advances expand the laboratory uses for hBMOs and establish this system as a versatile platform for studying human hematopoiesis and erythropoiesis, stress responses, and gene functions within a physiologically relevant bone marrow microenvironment.