Background TNF is one of the major cytokines involved in the pathogenesis of rheumatoid arthritis (RA) and anti-TNF therapies are effective at preventing inflammation and structural damage in this disease. However, it is not clear whether TNF destroys cartilage directly or via induction of other cytokines including IL-1β. Furthermore, the cellular sources for TNF within the inflamed joints remain controversial.
Objectives Here, we studied effects of TNF on cartilage and compared TNF production by different cell types to gain deeper insights into the mechanism of action for anti-TNF biologics in RA.
Methods In vitro studies were performed using primary human synovial fibroblasts (SF), chondrocytes or cartilage explants, and human peripheral blood mononuclear cells (PBMC) from individuals without history of joint disease. Cells and tissues were stimulated with recombinant human TNF, IL-1β, LPS, or members of alarmin family (S100A8 and S100A9). Adalimumab, etanercept, or certolizumab pegol were used to block TNF. Levels of cytokines and matrix metalloproteinases (MMPs) were detected using mesoscale discovery (MSD) assay, and gene expression was measured by real-time PCR. For in vitro cartilage destruction studies, bovine explants were co-cultured with activated human PBMC followed with proteoglycan quantification. In vivo effects of adalimumab on cartilage were evaluated in human TNF transgenic (hTNF-Tg) mouse model of arthritis.
Results We first examined whether TNF blockade provide cartilage protection in vivo and found that adalimumab treatment significantly decreased proteoglycan depletion from the articular cartilage of hTNF-Tg mice. In vitro, TNF induced strong up-regulation of IL-6, MMP1, and MMP3 in isolated human chondrocytes and cartilage explants. Additionally, TNF treatment resulted in significant proteoglycan depletion from bovine cartilage explants confirming previous findings; showing that bovine cartilage can respond to human cytokines. In an effort to identify cellular sources of TNF, we challenged chondrocytes, PBMC and SF with inflammatory stimuli, including alarmins, known to be involved in joint pathology and found that PBMC produced significantly higher levels of TNF. In sharp contrast with SF and chondrocytes, TNF failed to induce high amounts of IL-6, MMP1, and MMP3 in PBMC; suggesting that different cell populations play a distinct role in triggering and propagating joint inflammation. Furthermore, blocking both endogenous and exogenous TNF with anti- TNF biologics protected cartilage from proteoglycan depletion in vitro.
Conclusions This work demonstrates that TNF directly triggers the catabolic program in human chondrocytes leading to cartilage damage. Our findings also suggest that neutralization of TNF, produced by PBMC infiltrating inflamed joint, decreases catabolic activity of chondrocytes and fibroblasts, which contributes to cartilage protective effects of anti-TNF biologics in RA.
Acknowledgement We thank Jochen Salfeld, Melanie Ruzek and Sarah Sirin for critical review of the manuscript.
Disclosure of Interest A. Yarilina Shareholder of: Abbvie, Employee of: Abbvie, T. Melim Shareholder of: Abbvie, Employee of: Abbvie, Z. Kaymakcalan Shareholder of: Abbvie, Employee of: Abbvie