Erythropoietin receptor in B cells plays a role in bone remodeling in mice

Abstract

Erythropoietin (EPO) is a key regulator of erythropoiesis. However, EPO receptors (EPO-Rs) are also expressed on non-erythroid cell types, including myeloid and bone cells. Immune cells also participate in bone homeostasis. B cells produce receptor activator of nuclear factor kappa-Β ligand (RANKL) and osteoprotegerin (OPG), two pivotal regulators of bone metabolism. Here we explored the ability of B cells to transdifferentiate into functional osteoclasts and examined the role of EPO in this process in a murine model. <b>Methods:</b> We have combined specifically-designed experimental mouse models and <i>in vitro</i> based osteoclastogenesis assays, as well as PCR analysis of gene expression. <b>Results:</b> (i) EPO treatment <i>in vivo</i> increased RANKL expression in bone marrow (BM) B cells, suggesting a paracrine effect on osteoclastogenesis; (ii) B cell-derived osteoclastogenesis occured <i>in vivo</i> and <i>in vitro,</i> as demonstrated by B cell lineage tracing in murine models; (iii) B-cell-derived osteoclastogenesis <i>in vitro</i> was restricted to Pro-B cells expressing CD115/CSF1-R and is enhanced by EPO; (iv) EPO treatment increased the number of B-cell-derived preosteoclasts (β3⁺CD115⁺), suggesting a physiological rationale for B cell derived osteoclastogenesis; (v) finally, mice with conditional EPO-R knockdown in the B cell lineage (cKD) displayed a higher cortical and trabecular bone mass. Moreover, cKD displayed attenuated EPO-driven trabecular bone loss, an effect that was observed despite the fact that cKD mice attained higher hemoglobin levels following EPO treatment. <b>Conclusions:</b> Our work highlights B cells as an important extra-erythropoietic target of EPO-EPO-R signaling and suggests their involvement in the regulation of bone homeostasis and possibly in EPO-stimulated erythropoietic response. Importantly, we present here for the first time, histological evidence for B cell-derived osteoclastogenesis <i>in vivo</i>.