Background and Objectives Both IL-6 and IL-21 have been described to drive in vitro Th17 differentiation in the presence of TGFbeta. We explored whether also in vivo IL-6 and IL-21 play an exchangeable and redundant role in Th17 differentiation during experimental arthritis, and to what extent combined blocking of these cytokines inhibits Th17 differentiation and suppresses arthritis development.
Materials and Methods To investigate the synergistic effects of combined blocking of the IL-6 and IL-21 pathways, arthritis development and Th17 cells were first studied in IL-6 -/-xIL-21R-/- mice and their wild-type (WT) and single-knockout controls during antigen-induced arthritis (AIA). In addition, the kinetic effects of combined IL-6R and IL-21 neutralisation was studied in DBA-1J mice treated at various stages of collagen-induced arthritis (CIA).
Results Mice deficient for either IL-6 or IL-21R showed suppressed antigen-induced arthritis compared to WT controls. This disease reduction was accompanied by a significant reduction in CD4+IL17+ T cells in the draining lymph nodes as determined by FACS. However, mice lacking both the IL-6 and IL-21 signalling pathways showed an even stronger disease suppression than the IL-6-/- and IL-21R-/- mice, and a striking reduction in Th17 levels was observed in these mice.
Based upon our findings in gene-knockout mice, we aimed to confirm the synergistic effects of IL-6/IL-21 with a cytokine-neutralisation approach using anti-IL-6R antibodies and sIL-21R-Fc treatment during CIA. Antibodies were given as single treatment or in combination, and was started at immunisation (day 0) or around the booster (day 21).
Combined blocking of IL-6R and IL-21 early during arthritis development (day 0) was a very potent approach to prevent arthritis development, reaching a disease incidence of only 40% at day 35 (isotype control 100%, sIL-21R-Fc 100%, anti-IL-6R 60%). Analyzing the mice that did develop arthritis, we observed that the anti-IL-6R/sIL-21R-Fc combination was also clearly more potent in suppressing the arthritis severity in comparison to the single treatments. Interestingly, blocking the IL-6/IL-21 pathways at a later stage during arthritis development (day 21) was clearly less effective and did not show any additional effects to anti-IL-6R treatment alone.
Conclusions Combined blocking of the IL-6 and IL-21 pathways suppresses Th17 differentiation in vivo as demonstrated by our IL-6/IL-21R-deficient mice. However, our neutralisation study during CIA shows that to influence arthritis development this IL-6/IL-21 blocking approach only has a limited therapeutic window. These findings suggests that to target Th17-driven joint pathology, blocking Th17 effector cytokines like IL-17 and IL-22 might be more effective than attempting to reduce Th17 cell numbers during active disease.