IL-27 has stimulatory and regulatory immune functions and is expressed in rheumatoid arthritis synovium. We investigated the effects of IL-27 on human osteoclastogenesis to determine whether IL-27 can stimulate or attenuate osteoclast-mediated bone resorption that is a hallmark of rheumatoid arthritis.
Osteoclasts were generated from blood-derived human CD14+ cells. The effects of IL-27 on osteoclast formation were evaluated by counting the number of TRAP+ multinucleated cells and measuring expression of osteoclast-related genes. The induction of NFATc1 and the activation of signaling pathways downstream of RANK were measured by immunoblotting. The expression of key molecules implicated in osteoclastogenesis (NFATc1, RANK, costimulatory receptors, ITAM-harboring adaptors) was measured by real time RT-PCR. Murine osteoclast precursors were obtained from bone marrow. Responsiveness to IL-27 of synovial fluid macrophages derived from RA patients was also tested.
IL-27 inhibited human osteoclastogenesis, suppressed the induction of NFATc1, downregulated expression of RANK and TREM-2, and inhibited RANKL-mediated activation of ERK, p38 and NF-κB in osteoclast precursors. Synovial fluid macrophages derived from RA patients were refractory to the effects of IL-27. In contrast to humans, IL-27 only moderately suppressed murine osteoclastogenesis, likely due to low expression of the IL-27 receptor subunit WSX-1 on murine osteoclast precursors.
IL-27 inhibits human osteoclastogenesis by a direct mechanism suppressing responses of osteoclast precursors to RANKL. Our findings suggest that in addition to its well-known anti-inflammatory effects, IL-27 plays a homeostatic role in restraining bone erosion. This homeostatic function is compromised under conditions of chronic inflammation such as RA synovitis.