Deletion of von-Hippel Lindau in Dmp1-expressing cells impairs B cell development

Abstract

Abstract Our objective is to characterize how altered bone homeostasis influences B lymphocyte development. The von-Hippel Lindau protein (VHL) regulates hypoxia-inducible factor degradation and is involved in responses to low oxygen environments. Osteoblasts (OBs) support B cell development and mature into osteocytes (OCYs). We utilized Dmp1-Cre;Vhl conditional knockout mice (cKO), in which Vhl is deleted in OBs and OCYs. The cKO display dysregulated bone growth, high bone density, smaller bone marrow (BM) cavity volume, and an overall decrease in BM cellularity compared to wild-type (WT) mice. B cell progenitors is significantly increased in frequency and numbers whereas Hardy Fractions B, C, D, E and F were significantly decreased in the BM of cKO mice. Analysis of BM B cells 16 weeks after whole BM transplantation, showed a significant reduction in Fractions B-C and later Fractions in the WT→cKO mice, while B cell development was normal in the cKO→WT chimeras, indicating a cell-extrinsic effect of Vhl on B cells within the cKO BM microenvironment. We hypothesize that B cell development is not supported due to reduction of key niche cells and decreased production of IL7, SCF, CXCL12, and Flt3. To define the defective niche cells, we will use in vitro B cell development assays in which WT B cells will be co-cultured with sorted cKO OBs, MSCs, and OCYs. B cell apoptosis, proliferation and turnover rates will be measured. We have begun preliminary studies using intravital and ex vivo imaging of the cKO BM to identify changes in oxygen tension, blood vessel type and structure, the presence and location of stromal cells. These studies have the potential to reveal novel molecular mechanisms by which Vhl in Dmp1 expressing cells may serve as a B cell niche.