RA patients may benefit from the inhibition of Th17 cells, either by neutralizing their main effector cytokine IL-17 22, or by targeting further upstream via its transcription factor RORγt 23 or Th17-inducing cytokines 24,25. In this study, we explored the potential of targeting TGF-β signaling to inhibit Th17 cells, this in view of the importance of this growth factor in the differentiation of these cells.
Previous studies in experimental arthritis models showed contradictory results with pro- or anti-inflammatory roles for TGF-β. For instance, recombinant TGF-β1 injection in naïve rat and mouse joints induced joint inflammation with synovial infiltration of T lymphocytes and neutrophils 26–28. Treatment of arthritic rats with anti-TGF-β antibodies inhibited inflammation and bone resorption 29. Additionally, intraperitoneal (IP) injected HTS466284 (TGF-β type 1 receptor kinase inhibitor) prevented experimental arthritis in mice 30. IP treatment with the specific TGF-β blocking peptide p17 reduced severity of CIA, however these effects were not statistically significant 31.
In contrast, other studies showed that IP administration of TGF-β1 reduced the incidence and severity of CIA, especially when injected in late disease 32,33. In rats with SCW-induced arthritis, IP 34 and intramuscular injection 35 of TGF-β at peak of inflammation suppressed the development of arthritis, whereas anti-TGF-β in CIA mice increased pro-inflammatory cytokines and arthritis severity 36,37. Furthermore, an increase in TGF-β during the remission phase of CIA suggests an important role for TGF-β in regulating the disease 38. These opposing findings indicate a differential role for TGF-β, probably depending on the type of animal model, route of administration, disease stage, and site of inflammation.
In our in vitro studies, we observed that blocking TGF-β during Th17 differentiation efficiently and significantly reduced Rorc mRNA expression, but not IL-17 protein production. TGF-β is important in T cell differentiation, but also in dampening of the immune response of various T and B lymphocytes 39. TGF-β inhibits differentiation and activation of specific T helper subsets by suppressing their lineage-specific transcription factors such as T-Bet and GATA-3, which are critical for Th1 and Th2 responses, respectively 40 The importance of TGF-β in regulating T cell responses in vivo has been strengthened by the observation that mice lacking TGF-βRII specifically on T cells develop lethal multi-organ inflammation 15. Also TGF-β1−/− mice develop multi-organ inflammation due to an impaired control of T cell activation and differentiation 41. The phenotype of the TGF-β1−/− mice is completely rescued if mice are crossed to an MHCII knockout background, highlighting a crucial role for TGF-β in regulating pathological CD4 + T cell responses 42. Furthermore, subsequent studies using mice with T cell-specific deletions of Smad2 and Smad3 showed that intracellular signaling via Smad2/3 is essential for the TGF-β-mediated inhibition of effector T cells 43. In our in vivo experiment, intra-articular SB-505124 treatment during experimental arthritis caused a clear reduction in Th17 levels,. However, this did not result in suppression of arthritis, suggesting that (1) the Th17 pathway is not that important in this model, (2) the remaining (Th17) cells are highly active due to loss of TGF-β-mediated dampening, or (3) that the treatment with SB-505124 had a too limited duration in time on TGF- β signaling to suppress the arthritis measured at end stage of our in vivo experiment. Unfortunately, our study design did not enable us to investigate the activation of the local Th17 cells and other immune cells by checking cytokine levels in the joints. However, this would be in line with our in vitro studies where we observed that blocking TGF-β during Th17 differentiation efficiently and significantly reduced the Rorc mRNA expression but not the IL-17 production. From the potent effects of the anti-IL-17 treatment group as reference control, we can conclude that IL-17 is an important cytokine to block during this phase of this arthritis model.
TGF-β is often considered as an immunosuppressive cytokine, inhibiting for instance TNF-α, IFN-γ and IL-1β 10. Interestingly, when inhibiting the TGF-β signaling pathway by SB-505124 on human RA synovium explants, IL-6 protein production by RA synovium was significantly reduced, suggesting that TGF-β is an important inducer of IL-6 in arthritic synovium. In line with this observation, we have shown that TGF-β stimulates the production of IL-6 by primary articular chondrocytes 44 and by the chondrocyte G6 cell line 44. TGF-β also stimulates IL-6 production in PBMCs 45. The other way around, TGF-β down-regulates IL-6 signaling in intestinal epithelial cells 46, showing that TGF-β regulation of IL-6 signaling is cell type, tissue and context dependent. In our ex vivo studies, synovial explants are originating from end-stage RA patients undergoing total knee or hip replacement, and thereby less active in secreting cytokines than early RA tissue. However, even with this less inflamed tissue, we observe an interesting suppression of the proinflammatory cytokine IL-6 in all donors after inhibition of TGF-β signaling by SB-505124.
In our approach we chose to use the small molecule inhibitor SB-505124 to inhibit TGF-β signaling, which has an advantage over antibodies in tissue and cell penetration 47,48. Disadvantage is the short half-life 49, therefore SB-505124 must be administered frequently. With daily intra-articular injections, we observed that this compound highly efficiently blocked TGF-β-mediated Smad2/3 phosphorylation (supplementary Figs. 1 and 2).
In our proof-of-concept study to demonstrate the potential of controlling Th17 cells by targeting TGF-β signalling, we included the positive control anti-IL17 treatment, since the arthritis model we used is known to be dependent on CD4 + T cells and IL-17 19,50. Depletion of CD4 + T lymphocytes in mBSA/IL-1-induced arthritis led to dose-dependent T cell proliferation in draining lymph nodes in response to mBSA. Anti–IL17 antibody administered during our mBSA/IL-1-induced arthritis markedly reduced disease, confirming that the model is indeed IL-17 dependent 19,50. However, our blocking of TGF-β signalling apparently did not sufficiently suppress the formation or activity of Th17 cells, as arthritis pathology was not affected by SB-505124 treatment, showing that our strategy to target the Th17 pathway upstream is not as effective as blocking the main effector cytokine IL-17 itself.