Modeling the human bone marrow niche in mice: From host bone marrow engraftment to bioengineering approaches

Abstract

Xenotransplantation of patient-derived samples in mouse models has been instrumental in depicting the role of hematopoietic stem and progenitor cells in the establishment as well as progression of hematological malignancies. The foundations for this field of research have been based on the development of immunodeficient mouse models, which provide normal and malignant human hematopoietic cells with a supportive microenvironment. Immunosuppressed and genetically modified mice expressing human growth factors were key milestones in patient-derived xenograft (PDX) models, highlighting the importance of developing humanized microenvironments. The latest major improvement has been the use of human bone marrow (BM) niche-forming cells to generate human-mouse chimeric BM tissues in PDXs, which can shed light on the interactions between human stroma and hematopoietic cells. Here, we summarize the methods used for human hematopoietic cell xenotransplantation and their milestones and review the latest approaches in generating humanized BM tissues in mice to study human normal and malignant hematopoiesis.

Figure 1.

The hematopoietic BM niche. The BM is a heterogeneous environment composed of different types of cells. The two main architectural scaffolds of the tissue are the bone and the vessels, integrated in a complex network connected to nerve fibers. Associated with these structures are different types of cells, as depicted in the figure, regulating the tissue homeostasis and the normal HSC fate in healthy and disease states.

Figure 2.

Ectopic bone “ossicle.” (A) Whole body micro–computerized tomography image showing bone tissue in a mouse. The red circle and the arrow show the location of a subcutaneous ossicle structure. (B) Gross morphology of a mouse-harvested ossicle. (C) Hematoxylin/eosin histological staining of an ossicle based on an implant of hMSC carrier gelatin sponge with BMP-2. Note bone tissue (B and black arrows) forming a ring on the surface of the ossicle and a core resembling adult BM tissue with trabecular bone, hematopoietic cells, adipocytes, and vascular structures. (D) Masson’s trichrome histological staining of an ossicle based on hMSC carrier ceramic implant. Note the remaining ceramic material in pale blue (Cer), newly formed bone in the surface of the ceramics in dark blue, and mature BM tissue with hematopoietic cells, adipocytes, and vascular structures with erythrocytes in red.

Figure 3.

Different approaches to bioengineer humanized hematopoietic niche. All approaches are based on an in vitro step to prepare an implantable structure with hMSCs and a cell carrier material. Some approaches include an in vitro cell differentiation step, the co-seeding of hECs, or the addition of osteogenic factors such as BMPs before implantation in mice. Following these first step, the human cell carrier devices are then implanted in mice aiming to generate subcutaneous humanized niches in vivo. Human hematopoietic cells can be integrated in the system at different steps. They can be seeded in vitro (1), before the in vivo implantation of the device. They can be i.v. injected in the mouse before (2) or after the implantation of the device (3), and they can also be injected directly inside the device after implantation (4). Bone formation in vivo may be promoted via systemic PTH injection. Red symbols on top of the mice represent sublethal irradiation. HCs, human BM cells.

Figure 4.

Timeline of PDX in human hematopoietic context. (Bottom) Milestones in human hematopoietic PDX approaches are reported, based on engraftment in host mouse BM niche. (Top) Milestones in human hematopoietic PDX approaches are reported, based on the generation of implantable humanized microenvironment with hMSCs. Refer to Table 1 and text for more details and references.