Expression of C/EBPβ in Bone Marrow Mesenchymal Stem Cells Is Mandatory for Early-Stage B Cell Lymphopoiesis
Authors: Yoshioka Satoshi , Miura Yasuo , Yao Hisayuki , Hayashi Yoshihiro , Tamura Akihiro , Ichinohe Tatsuo , Hirai Hideyo , Takaori-Kondo Akifumi , Maekawa Taira 
Published: October 2019

Abstract
Abstract 3457
The interplay between hematopoietic cells and bone marrow microenvironment organized by mesenchymal stem cells is important for the maintenance of hematopoiesis. With respect to B cell lymphopoiesis, several constituents of bone marrow microenvironment specific for B cells (“B cell niche”) have been identified, including CXCL12/stromal cell-derived factor-1 (SDF-1)-abundant reticular cells as cellular factors, and CXCL12/SDF-1, interleukin (IL)-7, stem cell factor (SCF), fms-related tyrosine kinase 3 ligand (Flt3-L), and nuclear factor kappa-B ligand (RANKL), as essential humoral factors. However, the precise mechanism through which mesenchymal stem cells in the bone marrow microenvironment support B cell lymphopoiesis, especially the role of transcription factors, remains unknown. We show that the mesenchymal stem cells lacking a transcription factor, CCAAT enhancer binding protein (C/EBP) b, are functionally abnormal, which contribute to the impairment of B cell lymphopoiesis in C/EBPb knockout mice.
In C/EBPb knockout mice, the number of B cells, in particular, B220+CD43+ precursor B cells, was significantly decreased in bone marrow compared with that in wild-type littermates (Figure 1A and 1B). As shown in Fig. 1A, the percentage of total B220+ B cells was decreased at 19.1 ± 7.1% in the bone marrow of C/EBPb knockout mice (KO, n = 13) compared to wild-type mice (WT, n = 14, 26.5 ± 7.3%: *P<0.05). The percentage of B220+CD43+ precursor B cells was also decreased at 5.2 ± 1.5% in the bone marrow of C/EBPb knockout mice (KO, n = 13) compared to wild-type mice (WT, n = 14, 7.4 ± 1.6%: **P<0.01). Intriguingly, in vivo bone marrow transplantation experiments demonstrated that the bone marrow cells derived from C/EBPb knockout mice were engrafted in lethally-irradiated (10 Gy) wild-type mice with equivalently B cell recovery compared to the bone marrow cells from normal wild-type mice. Conversely, when normal wild-type c-kit+ Sca-1+ lineages− hematopoietic stem cells (KSL cells) were co-cultured with C/EBPb deficient mesenchymal stem cells in vitro (KO), they showed impaired B cell differentiation compared to the co-culture with normal wild-type mesenchymal stem cells (WT, Figure 1C). Mechanistically, the CXCL12/SDF-1 production by C/EBPb deficient mesenchymal stem cells was reduced compared with that by wild-type mesenchymal stem cells (KO, n = 5, 4.47 ± 1.16 ng/mL; WT, n = 5, 9.90 ± 1.93 ng/mL; **P < 0.01). These results suggest a possibility that abnormal C/EBPƒÀ deficient mesenchymal stem cells in bone marrow microenvironment contribute to impaired B cell lymphopoiesis in C/EBPb knockout mice.
We further found that C/EBPb deficient mesenchymal stem cells displayed several functional abnormalities. First, calcium accumulation was significantly reduced in 4 week osteogenesis-inducing cultures of C/EBPb-deficient mesenchymal stem cells compared to cultures of wild-type mesenchymal stem cells. This occurred along with the down-regulated expression of the principal osteogenic master molecule runt-related transcription factor 2 (Runx2). Second, lipid deposition was significantly reduced in 1 week adipogenesis-inducing cultures of C/EBPb-deficient mesenchymal stem cells. The expression of adipogenic markers, including peroxisome proliferator-activated receptor b (PPARb) was significantly reduced in adipogenic cultures of C/EBPb-deficient mesenchymal stem cells compared with cultures of wild-type mesenchymal stem cells Finally, the number of colony-forming unit fibroblast (CFU-F) was higher in the bone marrow of C/EBPb knockout mice than in that of wild-type mice. Collectively, C/EBPb-deficient mesenchymal stem cells have aberrant multi-differentiation capability and increased proliferation activity compared with wild-type mesenchymal stem cells, further supporting that C/EBPb-deficient mesenchymal stem cells were functionally abnormal.

Altogether, this work demonstrates that impaired B cell lymphopoiesis in C/EBPb knockout mice is attributed to abnormal mesenchymal stem cells in bone marrow microenvironment, at least in a steady-state, an effect that is due in part to the impaired CXCL12/SDF-1 production.

Disclosures:
No relevant conflicts of interest to declare.