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

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 that -Cre; conditional knockout mice ( cKO), which deletes in 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 the cKO BM, B cell progenitors were increased in frequency and number, whereas Hardy Fractions B-F were decreased. cKO Fractions B-C cells showed increased apoptosis and quiescence. Reciprocal BM chimeras confirmed a B cell-extrinsic source of the cKO B cell defects. In support of this, cKO BM supernatant contained reduced CXCL12 and elevated EPO levels. Intravital and imaging revealed cKO BM blood vessels with increased diameter, volume, and a diminished blood-BM barrier. Staining of cKO 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 in cKO 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.