BMP-2 generates a functional bone marrow niche by inducing the differentiation of local mesenchymal cells into CAR cells.

Ectopic bone marrow (eBM) holds tremendous potential as an artificial organ, serving not only in stem cell transplantation therapies but also as a controlled experimental system for analyzing cellular dynamics and interactions between cells and the matrix during the formation, maintenance, and aging of BM. Although bone morphogenetic protein-2 (BMP-2) has been reported to induce eBM formation, it remains unproven whether BMP-2-induced eBM (BMP-eBM) can provide a functional BM niche that is comparable with native BM in long bones (LB-BM). In this study, through the use of single-cell RNA sequencing and transplantation models, we demonstrate that BMP-eBM displays a microstructure, cellular composition, and functional hematopoiesis similar to LB-BM. BMP-eBM establishes an optimized microenvironment capable of supporting hematopoietic stem cells and CXC chemokine ligand 12 (CXCL12)-abundant reticular (CAR) cells, which are critical components of the BM niche. BMP-eBM was able to significantly restore survival in irradiated mice. Through parabiosis and cell transplantation experiments, we identified that in situ adipose tissue-derived CD51highCxcl12-GFP- cells are the principal source of CAR cells within BMP-eBM. Furthermore, BMP-eBM can be isolated and after preconditioning, retransplanted as an independent, functional hematopoietic organ. In conclusion, our study confirms that BMP-eBM functions effectively as a hematopoietic organ, capable of supporting and maintaining a functional BM niche. These findings underscore BMP-2 as a crucial molecule for eBM generation and suggest its potential for addressing BM-related diseases and for use as a platform for in vitro and ex vivo biomedical applications.