Researchers at Washington State University have potentially found the reason why tumor necrosis factor inhibitors (TNFis) are an ineffective treatment in as many as 40% of patients with rheumatoid arthritis (RA), by identifying an alternate inflammatory signaling pathway that allows inflammation to persist even when TNF activity is blocked. The study, published in Cellular & Molecular Immunology, found that TNF-α can exploit signaling through the TWEAK/Fn14 axis to amplify inflammation, showing a biological basis for TNFi resistance and identifying a new target that could potentially improve treatment efficacy.
“It’s kind of like a back-door entry or an alternate route,” said senior author Salah-uddin Ahmed, PhD, a professor and associate dean for research and graduate education in the College of Pharmacy and Pharmaceutical Sciences at Washington State University. “If you shut the main door for TNF, it has other ways to cause inflammation.”
TNF inhibitors, which are also used to treat other autoimmune disorders such as Crohn’s disease and ankylosing spondylitis, have provided relief to a broad swath of patients suffering from RA for more than 20 years. Unfortunately, the benefits of TNFi treatments show little improvement or lose their effects over time for some patients, with the reasons for this treatment resistance remaining unknown. The molecular mechanisms behind this resistance have remained unclear.
For their research, the WSU team focused on a specific form of TNF called TNF-α, which is one of the main inflammatory cytokines in the immune system, to find out whether it might affect other inflammatory pathways not addressed by TNFis. In studies using human synovial tissue from patients with RA in conjunction with joint samples from rat models of arthritis, the researchers found elevated expression of Fn14, the receptor for the cytokine TWEAK, in diseased joint tissues. In cell-based experiments, low doses of TNF-α and TWEAK acted together to induce inflammatory responses that were stronger than either signal alone.
“Our study reveals novel molecular crosstalk between TWEAK/Fn14 and TNF-α signaling and its potential implications for therapy resistance,” the researchers wrote. They showed that increased expression of Fn14 caused TNF-α to induce inflammatory chemokines and matrix metalloproteinases, while silencing or blocking Fn14 reduced TNF-driven inflammation.
Searching for the molecular basis of this signaling, the team turned to RNAseq to identify more than 200 genes whose expression changed significantly when Fn14 was knocked down in TNF-α–stimulated synovial fibroblasts. Gene set enrichment analysis further showed that interferon-related pathways were less active when Fn14 signaling was suppressed, tying the pathway to immune signatures previously associated with poor responses to TNF inhibitors.
Using this information, the investigators then used the Fn14 agonist L524-0366 to see if it could quell its influence on inflammation. They found that blocking Fn14 reduced activation of kinases involved in inflammatory signaling and decreased production of chemokines and tissue-degrading enzymes in vitro. In a mouse model of arthritis, daily administration of L524-0366 beginning at disease onset reduced disease severity.
The research built upon earlier studies that had implicated TWEAK and Fn14 in affecting tissue remodeling and inflammation in RA, with other work establishing that TWEAK belongs to the same protein superfamily as TNF. Clinical studies had also shown that blocking TWEAK could reduce inflammatory markers, and that interferon signatures were associated with poor TNF inhibitor response.
“This is the first study to reveal novel crosstalk between TWEAK/Fn14 and TNF-α signaling and shed light on the potential molecular mechanism of anti-TNF therapy resistance in RA,” the researchers wrote.
The findings have potential implications for clinical care, suggesting the potential strategy of targeting Fn14 and TNF together could resistance in patients who do not respond adequately to TNFis alone. The data also showed that measuring serum TWEAK levels could help guide treatment decisions, as reductions in TWEAK were observed in patients receiving TNF inhibitors.
Ahmed said the team’s will now examine potential therapeutic strategies that either target both pathways together or focus specifically on Fn14. Additional studies are planned to test the mechanism in other immune cell types, examine receptor interactions in more detail, and evaluate the approach in additional preclinical models.
“This is almost like a partner in crime for TNF,” Ahmed said, who noted that understanding this interaction could also inform treatment strategies for other TNF-driven autoimmune diseases.
