Exploring the B cell microenvironment in von-Hippel Lindau (Vhl) Dmp1-specific knockout mice

Abstract

Abstract Osteoblasts (OBs) and osteocytes (OCYs) have been shown to support B cell development. The von-Hippel Lindau protein (VHL) regulates hypoxia-inducible factor degradation and is involved in responses to low oxygen environments. 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 overall decrease in BM cellularity compared to wild-type (WT) mice. B cell progenitors were significantly increased in frequency and numbers whereas Hardy Fractions B, C, D, E and F were significantly decreased in the cKO BM. cKO Fraction B-C cells showed an increase in apoptosis and decreased proliferation. Reciprocal BM chimeras showed significant reduction in Fractions B-C and later Fractions in the WT→cKO, 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 such as stromal cells, OBs, mesenchymal stem cells and decreased production of key cytokines. In preliminary studies, we found that CXCL12 and SCF, key cytokines for B cell development, were reduced in the cKO BM serum. We have begun longitudinal studies using intravital and ex vivo imaging of the cKO BM to identify temporally dependent changes in oxygenation, blood vessel type, structure, permeability, and location of stromal cells. Ex vivo long bone imaging of cKO BM in 6-month-old mice revealed an increase in blood vessel diameter compared to WT. These studies will reveal details and novel molecular mechanisms by which Vhl in Dmp1 expressing cells may serve as a B cell niche.