Background/Purpose: Fibroblast-like synoviocytes (FLS) are targets of the pro-inflammatory IL-17 cytokines and a major source of inflammatory mediators in the inflamed synovium in rheumatoid arthritis (RA). We have determined the activity of IL-17A and IL-17AF (the heterodimer of IL-17A and IL-17F) to augment the effects of TNF or IL-1β on FLS derived from RA patients (RA-FLS) as described in a separate study. Since the newly developed anti-IL-17A antibody secukinumab has shown promise in various clinical trials including RA, we provide here further evidence how it interferes with IL-17 pathways by investigating the in vitro neutralizing activity of secukinumab on the release of key pro-inflammatory FLS products upon costimulation with TNF combined with IL-17A or IL-17AF and upon costimulation with IL-1β and IL-17A.  

Methods: RA-FLS were stimulated for 18h with IL-1β (300 pM) combined with IL-17A, or with TNF (60 pM) combined with either IL-17A (30 pM) or IL-17AF (1 nM). Secukinumab or control antibody was given shortly before cytokine stimulation of RA-FLS. The release of IL-6, IL-8, CXCL-1 and CCL2 was determined by homogeneous time resolved fluorescence technology or AlphaLISA. RA-FLS cell lines were obtained from Cell Application Inc.

Results: While IL-17A was weak as a single stimulus, co-stimulation of primary human RA-FLS with IL-17A and TNF synergistically potentiated the effect of TNF on the release of the pro-inflammatory cytokines IL-6, IL-8 and CXCL-1 (Gro-α). Interestingly, CCL2 (MCP-1) release was not potentiated but was only dependent on TNF, indicating a different mechanism or regulation of this chemokine. Secukinumab potently inhibited the potentiated release of IL-6 induced by IL-17A/TNF costimulation at picomolar concentrations with an IC50 value of 0.14 ± 0.02 nM, but had no effect on basal TNF-induced release of IL-6. Similar results were obtained for the inhibition of IL-8 and CXCL-1 release. CCL2 release was not attenuated by secukinumab, which is in line with CCL2’s dependency on the TNF pathway. As we had observed a lower potency of IL-17AF compared to IL-17A to co-stimulate mediator release, IL-17AF was administered at 34 higher concentrations to induce similar maximum IL-6 levels. The effect of secukinumab on IL-17AF/TNF stimulated mediator release were similar to those observed for IL-17A but were less potent, consistent with the lower binding affinity of secukinumab to IL-17AF and the higher IL-17AF cytokine levels required in the in vitro experiment. Combination of IL-1β with IL-17A showed no potentiating effect on pro-inflammatory cytokines. In line with this, there was no significant effect of secukinumab on mediator release from IL-17/IL-1β co-stimulated FLS. 

Conclusion: The data show that secukinumab completely neutralizes the amplifying effect of IL-17 on TNF-stimulated RA synovial fibroblasts, one of the key target cells in rheumatoid arthritis. It is anticipated that this blockade contributes to the therapeutic effect of secukinumab seen in RA clinical trials.  

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-anti-il-17a-monoclonal-antibody-secukinumab-ain457-inhibits-pro-inflammatory-mediator-release-from-human-primary-synoviocytes-costimulated-with-il-17-and-tnf/