Background/Purpose: The pro-inflammatory cytokine interleukin-17A (IL-17A) activates fibroblast-like synoviocytes (FLS) and other mesenchymal cells via the IL-17RA/RC receptor. FLS are a major source of inflammatory cytokines, chemokines and growth factors in the inflamed synovium in rheumatoid arthritis (RA) and thus contribute to disease perpetuation. Since the macrophage products TNF and IL-1β play an important role in the pathogenesis of RA, we wanted to better understand if IL-17 cytokines augment the effects of TNF or IL-1β on FLS derived from RA patients (RA-FLS) in modulating the expression of inflammatory FLS products and IL-17RA and RC expression. Furthermore, we compared the co-stimulatory effects of IL-17A, the founding member of the IL-17 cytokine family, with the less characterized but closely related cytokines IL-17AF and IL-17F.   

Methods: The ability of IL-17A, IL-17-AF and IL-17F to co-stimulate mediator expression in comparison to single TNF or IL-1β stimulation was determined in criss-cross titrations of RA-FLS, and a panel of pro-inflammatory mediators including IL-6, IL-8, CXCL-1, CCL2, PGE2 and GM-CSF was determined by protein (18h) or mRNA levels (RT-PCR or Affymetrix chips analyses, upon 2, 6, 18 or 24h stimulation). Basal IL-17RA and RC expression and the effects of cytokine stimulation on receptor expression were measured by flow cytometry and quantitative RT-PCR. RA-FLS cell lines were obtained from Cell Application Inc.

Results: When tested as single stimulus in vitro, IL-1β was the most potent cytokine to induce the release of IL-6 and other mediators, followed by TNF. IL-17 cytokines caused only minor elevation over basal levels if used alone. Combination of IL-1β and IL-17A showed no potentiating effect on either protein or mRNA levels of IL-6, IL-8, CXCL-1, CCL2, PGE2/Cox2 and GM-CSF. In contrast, combining IL-17A with TNF, resulted in significant potentiation of IL-6, IL-8, CXCL-1, PGE2 and GM-CSF. Interestingly, this potentiation is selective as CCL2 (MCP-1) release was only dependent on TNF, indicating a different mechanism for regulation of the latter. The closely related cytokine IL-17F and the heterodimer IL-17AF  showed strikingly different potencies for co-stimulation of RA-FLS with TNF, with ranking IL-17A> IL-17AF>IL-17F. To further explore the underlying mechanism of potentiation of TNF+IL-17, we examined IL-17 receptor expression on RA-FLS. Interestingly, cytokine stimulation did not upregulate the expression of IL-17RA or IL-17RC.  

Conclusion: Our data show that IL-17 cytokines synergistically potentiate the effects of TNF on RA-FLS to release pro-inflammatory mediators. All three IL-17 cytokines have pro-inflammatory effects but differ in the respective potencies (IL-17A>IL-17AF>IL-17F). Combination of IL-1β and IL-17 showed no potentiation but was only additive.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/potentiating-effects-of-il-17a-il-17af-il-17f-in-combination-with-tnf-but-not-with-il-1beta-in-human-primary-fibroblast-like-synoviocytes-from-rheumatoid-arthritis-patients/