PROJECT SUMMARY My long-term objective is to characterize the microenvironment that influences hematopoiesis in the context of altered bone homeostasis. Hematopoietic stem cells (HSCs) continuously replenish blood cells by differentiation and maturation into either the lymphoid or myeloid lineage. Lymphoid lineage cells include B lymphocytes are required for the production of antibodies that are crucial for a robust adaptive immune response, while myeloid lineage cells include erythrocytes (red blood cells), platelets essential to stop bleeding, granulocytes and macrophages which are crucial components of innate immunity. The microenvironment in the bone marrow (BM) that supports conventional B cell (also called “B2” cells) development and myeloid development has been described (3-9), but the detailed contributions of the microenvironment that maintain and that regulate B cell development and myeloid-erythroid development in the bone marrow remain unknown. My aim is to fill the scientific knowledge gap on the microenvironmental niches that influence the maintenance and function of B cells, myeloid cells and erythrocytes. In the course of this study to examine the effects of altered bone homeostasis on immune cell development, my laboratory discovered that B cell development was severely impaired in mice in which a hypoxia response pathway gene, von-Hippel Lindau (Vhl), is conditionally deleted in osteocytes (10). I hypothesize that Vhl-deficiency in osteocytes results in structural and molecular changes in the BM microenvironments that insufficiently produces niche cells and cytokines required to appropriately support hematopoiesis in the BM. In support of this hypotheses, my preliminary analyses suggest that deletion of Vhl in osteocytes results in a decrease of endothelial cells, mesenchymal stem cells and osteoblasts. My project objective is to further characterize the osteocyte- regulated microenvironmental influences on the development and maintenance of 1) B lymphocytes and 2) myeloid-erythroid development due to Vhl deficiency. I plan to utilize immunoassays, co-culture methods, and transplantation strategies to characterize the Vhl deficient bone marrow microenvironment. This information could be applied to future studies of the effects of irradiation, myeloablative conditioning, or bone-building drugs on bone marrow niches and immune cell development.