3D co-culture of hematopoietic stem and progenitor cells and mesenchymal stem cells in collagen scaffolds as a model of the hematopoietic niche

Here, we propose a collagen-based three-dimensional (3D) environment for hematopoietic stem and progenitor cells (HPC) with mesenchymal stem cells (MSC) derived either from bone marrow (BM) or umbilical cord (UC), to recapitulate the main components of the BM niche. Mechanisms described for HPC homeostasis were systematically analyzed in comparison to the conventional liquid HPC culture. The 3D-cultivation allows dissecting two sub-populations of HPC: (I) HPC in suspension above the collagen gel and (II) migratory HPC in the collagen fibres of the collagen gel. The different sites represent distinct microenvironments with significant impact on HPC fate. HPC in niche I (suspension) are proliferative and a dynamic culture containing HPC (CD34⁺/CD38^-), maturing myeloid cells (CD38⁺, CD13⁺, CAE⁺) and natural killer (NK) cells (CD56⁺). In contrast, HPC in niche II showed clonal growth with significant high levels of the primitive CD34⁺/CD38^- phenotype with starting myeloid (CD13⁺, CAE⁺) differentiation, resembling the endosteal part of the BM niche. In contrast, UC-MSC are not adequate for HSC expansion as they significantly enhance HPC proliferation and lineage commitment. In conclusion, the 3D-culture system using collagen and BM-MSC enables HPC expansion and provides a potential platform to dissect regulatory mechanisms in hematopoiesis.