Tumour necrosis factor α augments the inhibitory effects of CTLA‐4‐Ig on osteoclast generation from human monocytes via induction of CD80 expression

Summary Cytotoxic T lymphocyte antigen‐4‐immunoglobulin (CTLA‐4‐Ig) exerts anti‐rheumatic action via negative regulation of the co‐stimulation process between antigen‐presenting cells and T cells. CTLA‐4‐Ig also binds to CD80/CD86 on monocytes of osteoclast precursors. However, little is known about the effect of CTLA‐4‐Ig on osteoclastogenesis in rheumatoid arthritis (RA). In this study we evaluated the effects of CTLA‐4‐Ig on osteoclast generation from human blood monocytes (PBM) and rheumatoid synovial fluid monocytes (RSFM). Highly purified monocytes were cultured with receptor activator of nuclear factor kappa‐B ligand (RANKL) and macrophage colony‐stimulating factor (M‐CSF) in the presence of CTLA‐4‐Ig. CTLA‐4‐Ig inhibited RANKL‐induced osteoclast generation in PBM and RSFM, as determined by tartrate‐resistant acid phosphatase (TRAP) staining and bone resorption assay using osteo assay surface plates. In addition, CTLA‐4‐Ig reduced the gene and protein expressions of nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) and cathepsin K during osteoclastogenesis. Furthermore, CTLA‐4‐Ig significantly inhibited cell proliferation during osteoclastogenesis. Interestingly, the gene expression of indoleamine 2,3‐dioxygenase‐1, an inducer of apoptosis, was enhanced by CTLA‐4‐Ig. We next examined the effect of tumour necrosis factor (TNF)‐α, a major inflammatory cytokine in rheumatoid synovium, on the expression of CD80 and CD86 by flow cytometric analysis. TNF‐α potently induced the surface expression of CD80, which is known to have much higher affinity to CTLA‐4‐Ig than CD86, and this induction was observed at mRNA levels. Interestingly, freshly prepared rheumatoid synovial monocytes also expressed CD80 as much as TNF‐α‐treated PBM. Furthermore, TNF‐α enhanced CTLA‐4‐Ig‐induced inhibition of osteoclastogenesis and cell proliferation. Taken together, the TNF‐α‐induced CD80 may augment CTLA‐4‐Ig‐induced inhibition of osteoclastogenesis, suggesting that CTLA‐4‐Ig potently inhibits osteoclast differentiation and protects bone destruction in rheumatoid inflamed joints.