Inflammation induces osteoclast differentiation from peripheral mononuclear cells in chronic kidney disease patients: Crosstalk between the immune and bone systems

Background. Inflammation and immune system alterations contribute to bone damage in many pathologies by inducing the differentiation of osteoclasts (OCs), the bone resorbing cells. This link is largely unexplored in chronic kidney disease (CKD) and haemodialysis (HD) patients, in which reduced renal function is accompanied by an increased inflammatory state and skeletal abnormality. Methods. We used ex vivo culture experiments to investigate the osteoclastogenic potential of peripheral blood mononuclear cells (PBMCs) of CKD and HD patients, focusing on immune cell subsets and inflammatory cytokines such as LIGHT and receptor activator of nuclear factor jB ligand (RANKL). Results. We observed spontaneous osteoclastogenesis with a significant increase in OC formation and bone resorbing activity in late-stage CKD and HD patients when compared with earlystage CKD patients and healthy donors, likely due to an increased expression of RANKL and LIGHT (homologous to Lymphotoxins exhibiting Inducible expression and competing with herpes simplex virus Glycoprotein D for herpes virus entry mediator [HVEM], a receptor expressed by T lymphocytes) in PBMCs. Specific inhibition of these cytokines in PBMCs isolated from CKD stages 3b-5 and HD patients induced the reduction of OC formation in vitro. The phenotypic characterization of peripheral blood cells revealed a significant increase of OC precursors (CD14CD11bCD51/61) and CD14CD16 monocytes in advanced CKD and HD patients compared with the control group. Conclusions. Our results suggest that circulating inflammatory monocytes fromadvanced CKD or HD patients trans differentiate into OCs in vitro and play a relevant role in mineral bone disorders and that LIGHT and RANKL represent new potential therapeutic targets in these settings.