Background and objectives Early investigations into the in-vivo functions of TNF in the huTNF-Tg mice established the pivotal role of this cytokine in the aetiopathogenesis of rheumatoid arthritis. We have previously demonstrated that restricted expression of p55TNFR in synovial fibroblasts (SFs) from huTNF-Tg mice is sufficient to drive the full pathogenic process. Here, we investigated whether expression of p55TNFR on SFs is necessary for the development of arthritis in either the huTNF-Tg or in Collagen-Antibody-Induced-Arthritis (CAIA) models, and how downstream SF- specific signals, such as NFkB, contribute to pathogenesis.
Methods ColVI-Cre mice, providing SF-specific recombination, were crossbred into p55TNFR (p55TNFRSF-KO) or ikk2 conditional knock-out backgrounds (ikk2SF-KO) and/or into hTNF-Tg background. CAIAwas induced by injection of monoclonal antibodies against cartilage antigens, and LPS. The TNF-mediated responses of ikk2-null SFs were examined by RT-PCR, western blot and FACS analysis.
Results SF-specific p55TNFR function is necessary for the development of arthritis in the huTNF-Tg model as well as in CAIA. The huTNF-Tg ikk2SF-KO and ikk2SF-KO mice subjected to CAIA exhibit ameliorated joint pathology indicating that SF ikk2 signals are required for the efficient formation of pannus. Mechanistically, ikk2-ko SFs were extremely susceptible to TNF-induced death while the huTNF-Tg ikk2-ko SFs displayed necroptotic death, suggesting a novel property of chronically TNF-exposed ikk2-deficient SFs in shifting towards necroptosis. Inhibition of necroptosis by the genetic ablation of RIPK3 kinase fully restored the residual synovial inflammation persisting in the huTNF-Tg ikk2SF-KO mice, validating in vivo the physiological role played by necroptosis in chronically TNF-exposed SFs in the absence of NFkB signals.
Conclusions Our results demonstrate a physiologically significant role of the p55TNFR/NFkB pathway in modelled arthritis by identifying the p55TNFR/ikk2/ripk3 axis as regulator of arthritogenic responses downstream of TNF, controlling gene expression and preventing programmed cell death.