Macrophage migration inhibitory factor is essential for osteoclastogenic mechanisms in vitro and in vivo mouse model of arthritis

• MIF and CD74 deficiency is associated with reduced K/BxN serum transfer arthritis.
• MIF and CD74 facilitate RANKL-induced osteoclastogenesis in vitro.
• MIF and CD74 facilitate RANKL-induced osteoclastogenesis relevant signalling events in vitro.

Macrophage migration inhibitory factor (MIF) enhances activation of leukocytes, endothelial cells and fibroblast-like synoviocytes (FLS), thereby contributing to the pathogenesis of rheumatoid arthritis (RA). A MIF promoter polymorphism in RA patients resulted in higher serum MIF concentration and worsens bone erosion; controversially current literature reported an inhibitory role of MIF in osteoclast formation. The controversial suggested that the precise role of MIF and its putative receptor CD74 in osteoclastogenesis and RA bone erosion, mediated by locally formed osteoclasts in response to receptor activator of NF-κB ligand (RANKL), is unclear. We reported that in an in vivo K/BxN serum transfer arthritis, reduced clinical and histological arthritis in MIF−/− and CD74−/− mice were accompanied by a virtual absence of osteoclasts at the synovium–bone interface and reduced osteoclast-related gene expression. Furthermore, in vitro osteoclast formation and osteoclast-related gene expression were significantly reduced in MIF−/− cells via decreasing RANKL-induced phosphorylation of NF-κB-p65 and ERK1/2. This was supported by a similar reduction of osteoclastogenesis observed in CD74−/− cells. Furthermore, a MIF blockade reduced RANKL-induced osteoclastogenesis via deregulating RANKL-mediated NF-κB and NFATc1 transcription factor activation. These data indicate that MIF and CD74 facilitate RANKL-induced osteoclastogenesis, and suggest that MIF contributes directly to bone erosion, as well as inflammation, in RA.