Establishing an Easy-to-Use 3D BM Niche Model for the Co-Culture of Primary Bone Marrow Mesenchymal Stromal Cells (BM-MSC) with Hematopoietic Stem and Progenitor Cells (HSPC)

Background

BM-MSC are a collection of primitive cell populations, a major stromal component of the BM niche and are essential to maintain tissue homeostasis. 2D culture protocols are popular, though not standardized, and effective for BM-MSC expansion, but display limited niche function in co-culture systems. 3D co-culture leverages the importance of cell signaling networks activated by cell–cell and cell–matrix interactions in native tissues. Here, we establish a superior and easy-to-use 3D culture protocol and compare to conventional 2D approaches.

Methods, Results and Discussion

First, we compared commercially available expansion media to the most widely cited investigator-defined media for optimal 2D expansion and 3D culture of ex vivo murine BM-MSC.

Based on this, we developed a five-marker flow cytometry panel to accurately quantify BM-MSC culture purity. To test BM-MSC niche function in 2D vs 3D cultures, we generated 3D spheroid hanging drop (HD) cultures from 2D expanded BM-MSC. In HD spheroids, cells are in direct contact with each other as well as extracellular matrix components and can be used to co-culture multiple different cell types. To assess the niche-like function of 3D HD cultures compared to 2D adherent cultures, we assessed Nestin protein expression since Nestin is a marker of HSC niches within mouse BM in vivo. BM-MSC were cultured in 2D or HD and Nestin protein expression assessed by immunofluorescence imaging, showing a significant increase in percent of Nestin-positive cells in HD-cultured BM-MSC, compared to matched 2D controls, in agreement with previous reports in human BM-MSC 3D culture. To assess 3D in vitro BM-MSC niche (IVBMN) function, we assayed stemness markers and survival of FACS isolated primary HSPC (Lineage^-Sca1⁺cKit⁺, LSK) from mouse BM. LSK cells were cultured as standard (alone) or seeded as co-cultures with BM-MSC within HD with specific cytokine support, in low oxygen (6.5%) conditions (IVBMN) (Figure 1). LSK cells cultured within IVBMN exhibited superior survival as well as retention of stemness markers, demonstrating the benefit of BM-MSC support.

In vivo, the BM niche also includes BM niche resident macrophages (BM-Mɸ), which were recently shown to be a limiting factor for engraftment and HSC reconstitution in irradiated mice, and are part of long term and short term HSC niches. To further extend our IVBMN, BM-Mɸ [F4/80⁺ Ly6G^- VCAM1⁺ CD169⁺] were collected by FACS and added to IVBMN cultures (IVBMN-Mɸ). Notably, the IVBMN-Mɸ co-culture model further enhanced survival and stemness of primary LSK cells, when compared to LSK cultured alone or within IVBMN. Based on these preliminary data, we demonstrate that our IVBMN-Mɸ culture system is a unique tool which mimics niche properties in vitro and is easy-to-use for HSPC manipulation.

Conclusions

Here we describe a standardized flow panel for reliable murine BM-MSC immunophenotyping, and characterize a simple but effective 3D BM-MSC co-culture system which allows niche-like function in vitro.

Fig. 1. In vitro BM niche (IVBMN) experimental workflow: A) Ex vivo expanded primary BM-MSC (20x) (i) -/+ FACS isolated BM niche resident Mɸ (ii) in co-culture with primary BM HPSC (LSK) (iii). B) Cells were cultured in a 3D mixed culture system with specific cytokine support, in 6.5% low oxygen. C) IVBMN-Mɸ [MSC+ BM-Mɸ +LSK] co-culture promotes maintenance of primary viable LSK compared to IVBMN [MSC+LSK] or LSK standard culture conditions alone, after 5 days culture.