To examine whether synovial interleukin-17 (IL-17) expression promotes tumor necrosis factor (TNF)–induced joint pathologic processes in vivo, and to analyze the surplus ameliorative value of neutralizing IL-17 in addition to TNF during collagen-induced arthritis (CIA).
Adenoviral vectors were used to induce overexpression of IL-17 and/or TNF in murine knee joints. In addition, mice with CIA were treated, at different stages of arthritis, with soluble IL-17 receptor (sIL-17R), TNF binding protein (TNFBP), or the combination.
Overexpression of IL-17 and TNF resulted in joint inflammation and bone erosion in murine knees. Interestingly, IL-17 strikingly enhanced both the joint-inflammatory and joint-destructive capacity of TNF. Further analysis revealed a strongly enhanced up-regulation of S100A8, IL-1β, and matrix metalloproteinase (MMP) messenger RNA, only when both TNF and IL-17 were present. Moreover, the increase in irreversible cartilage destruction was not merely the result of enhanced inflammation, but also was associated with a direct synergistic effect of these cytokines in the joint. S100A9 deficiency in mice protected against IL-17/TNF–induced expression of cartilage NITEGE neoepitopes. During established arthritis, the combination of sIL-17R and TNFBP was more effective than the anticytokine treatments alone, and significantly inhibited further joint inflammation and cartilage destruction.
Local synovial IL-17 expression enhances the role of TNF in joint destruction. Synergy between TNF and IL-17 in vivo results in striking exaggeration of cartilage erosion, in parallel with a synergistic up-regulation of S100A8, IL-1β, and erosive MMPs. Moreover, neutralizing IL-17 in addition to TNF further improves protection against joint damage and is still effective during late-stage CIA. Therefore, compared with anti-TNF alone, combination blocking of TNF and IL-17 may have additional therapeutic value for the treatment of destructive arthritis.