Deletion of Vhl in Dmp1-Expressing Cells Causes Microenvironmental Impairment of B Cell Lymphopoiesis

Abstract

The contributions of skeletal cells to the processes of B cell development in the bone marrow (BM) have not been completely described. The von-Hippel Lindau protein (VHL) plays a key role in cellular responses to hypoxia. Previous work showed thatDmp1-Cre;Vhlconditional knockout mice (VhlcKO), which deletesVhlin subsets of mesenchymal stem cells, late osteoblasts and osteocytes, display dysregulated bone growth and reduction in B cells. Here, we investigated the mechanisms underlying the B cell defects using flow cytometry and high-resolution imaging. In theVhlcKO BM, B cell progenitors were increased in frequency and number, whereas Hardy Fractions B-F were decreased.VhlcKO Fractions B-C cells showed increased apoptosis and quiescence. Reciprocal BM chimeras confirmed a B cell-extrinsic source of theVhlcKO B cell defects. In support of this,VhlcKO BM supernatant contained reduced CXCL12 and elevated EPO levels. Intravital andex vivoimaging revealedVhlcKO BM blood vessels with increased diameter, volume, and a diminished blood-BM barrier. Staining ofVhlcKO B cells with an intracellular hypoxic marker indicated the natural existence of distinct B cell microenvironments that differ in local oxygen tensions and that the B cell developmental defects inVhlcKO BM are not initiated by hypoxia. Our studies identify novel mechanisms linking altered bone homeostasis with drastic BM microenvironmental changes that dysregulate B cell development.