Bone marrow neutrophil progenitors suppress osteoclast formation in murine cortical and trabecular bone

In inflammation, circulating neutrophils indirectly damage the skeleton by inducing formation of bone-resorbing osteoclasts. However, neutrophil progenitors in marrow have no known physiological function. A bone-protective role for the neutrophil lineage was recently suggested when a profound defect in bone structure was observed in mice with neutropenia due to granulocyte colony-stimulating factor deletion coupled with STAT3 hyperactivation in bone cells. Here, we tested the existence of this protective effect by manipulating neutrophil progenitors in bone marrow using anti-Ly6G (αLy6G) treatment. Two protocols revealed an inverse relationship between marrow neutrophil progenitors and osteoclasts. Two weeks of αLy6G treatment increased marrow immature neutrophils by 25%, and halved osteoclast markers in cortical bone. In contrast, 6 weeks of αLy6G, combined with anti-rat immunoglobulin G2a to maintain antigenicity, reduced marrow preneutrophils by 50%. This latter treatment doubled trabecular osteoclast surface, halved trabecular bone mass, and significantly reduced high-density bone mass, both in control mice and in mice with bone-specific STAT3 hyperactivation. In culture, isolated preneutrophils dose-dependently inhibited osteoclastogenesis, independent of direct contact. We conclude that neutrophil progenitors directly inhibit osteoclast formation by releasing soluble factors. This identifies a novel action of hematopoietic cells in marrow to protect bone structure.