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 (G-CSF) deletion coupled with STAT3 hyperactivation in bone cells. Here, we tested the existence of this protective effect by manipulating neutrophil progenitors in bone marrow by Ly6G antibody (aLy6G) treatment. Two protocols revealed an inverse relationship between marrow neutrophil progenitors and osteoclasts. Two weeks of aLy6G treatment increased marrow immature neutrophils by 25% and halved osteoclast mRNA markers in cortical bone. In contrast, coupling six weeks of aLy6G with anti-rat IgG2a to maintain antigenicity reduced marrow pre-neutrophils by 50%. This 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 pre-neutrophils 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.