Background/Purpose: Increased expression of Toll-like Receptor (TLR) 4 and its endogenous agonists in rheumatoid joints suggest involvement in rheumatoid arthritis (RA). The aim of this study was to assess the therapeutic efficacy and downstream effects of TLR4 blockade in murine and humanized models compared with interleukin-1 (IL-1) and tumor necrosis factor (TNF) inhibitors.

Methods: Mice with established collagen-induced arthritis (CIA) systemically received TNF, IL-1 and TLR4 inhibitors (Enbrel, Anakinra and purified B. quintana LPS, resp., each 2 mg/kg/day) using osmotic minipumps. Severe combined immunodeficient (SCID) mice engrafted with active RA synovial tissue, and ex vivo synovial cultures served to translate the findings into RA.

Results: TLR4 blockade significantly suppressed clinical and histopathological manifestations of ongoing CIA to the same extent as IL-1 and TNF inhibitors. Targeting TLR4 substantially reduced serum IL-1β and IL-6, and was the only treatment capable of lowering serum TNFa. High-dose TLR4 inhibitor (8 mg/kg) was found significantly more effective than high-dose Enbrel (10 mg/kg; P < 0.05 using Bonferroni’s multiple comparison test). Importantly, TLR4 inhibition exceeded beneficial effects of TNF blocker by reducing serum IL-17 along with synovial gene expression of IL-23p19, IL-17 and the Th17-related transcription factor RORγt, while Th1 markers and type II collagen-directed T cell proliferation and antibody responses remained unaffected.

In intact RA synovial biopsies, TLR4 stimulation (100 ng/ml E. coli LPS) potently induced TNFα, IL-1β, IL-6 and IL-8 production ex vivo, indicating its functional relevance. When transplanted into SCID mice, one single i.p. injection of TLR4 agonist significantly increased IL-6 production by RA synovium in vivo and sustained the otherwise declining IL-8 levels for up to 7 days. Importantly, TLR4 activation clearly reversed the therapeutic efficacy of anti-TNF treatment in the humanized RA synovium-SCID model, suggesting involvement in anti-TNF non-responsiveness in subgroups of patients.

Therapeutic value of TLR4 targeting in RA was revealed by inhibition of TLR4 in the RA synovium-SCID model. Blocking endogenous TLR4 activation in this model resulted in substantial reduction of spontaneous IL-6 and IL-8 release and synovial inflammation, thereby equaling anti-TNF. In RA synovial explant cultures ex vivo, TLR4 blockade by either B. quintana LPS or the small molecule inhibitor TAK242 suppressed several inflammatory cytokines. High-density (phospho)protein microarray of synovial protein lysates showed 27% reduction in NFκBp65 phospho-Ser536, but not NFκBp105/p50, levels by TLR4 blockade. Interestingly, TLR4 inhibition suppressed synovial expression of multiple other key signaling molecules including TAK1 phospho-Thr187 and -Ser412 (26 and 23%, resp.), JAK1 phospho-Tyr1022 (22%), IRAK1 (22%), IRF3 (51%), Foxo3a (39%) and Btk (24%).

Conclusion: Data in murine and humanized models position TLR4 upstream to a number of inflammatory and pathogenic pathways. The findings including impact on IL-17 production and anti-TNF responsiveness collectively impel future research on TLR4 as a potential therapeutic target in RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/high-efficacy-of-toll-like-receptor-4-targeting-in-murine-and-humanized-models-of-rheumatoid-arthritis-in-comparison-with-il-1-and-tnf-inhibitors/