Secretory clusterin (sCLU)/apolipoprotein J is a multifunctional glycoprotein that is ubiquitously expressed in various tissues. Reduced sCLU in the joints of patients with bone erosive disease is associated with disease activity; however, its exact role has yet to be elucidated. Here, we report that CLU is expressed and secreted during osteoclastogenesis in mouse bone marrow-derived macrophages (BMMs) that are treated with receptor activator of nuclear factor kappa-B ligand (RANKL) and macrophage colony-stimulating factor (M-CSF). CLU-deficient BMMs obtained from CLU-/- mice exhibited no significant alterations in OC differentiation in comparison with BMMs obtained from wild-type mice. In contrast, exogenous sCLU treatment significantly inhibited DC formation in both BMMs and DC precursor cultures. The inhibitory effect of sCLU was more prominent in BMMs than DC precursor cultures. Interestingly, treating BMMs with sCLU decreased the proliferative effects elicited by M-CSF and suppressed M-CSF-induced ERR activation of OC precursor cells without causing apoptotic cell death. This study provides the first evidence that sCLU reduces DC formation by inhibiting the actions of M-CSF, thereby suggesting its protective role in bone erosion. (C) 2014 Elsevier Inc. All rights reserved.