Background/Purpose: Monocyte-derived dendritic cells  (MoDCs) differentiate from monocytes under inflammatory conditions like RA, and are very potent in presenting antigen to T cells. GM-CSF-responsive MoDCs have been shown to promote Th17 cell differentiation during experimental autoimmune encephalomyelitis (EAE). Since the pathogenicity of Th17 cells in EAE has been associated with the production of both IL-17 and GM-CSF, this introduces an interesting feedback-loop within the suggested MoDC-Th17 pathway. In this study, we aimed to further elucidate this GM-CSF/Th17 pathway by determining the contribution of GM-CSF-producing cells in driving synovial MoDC levels and promoting Th17 differentiation during experimental arthritis. In addition, we analyzed the GM-CSF expression by CD4+ T cells under Th17 differentiation conditions to further unravel the interaction between these two cytokines.  

Methods: Arthritis was elicited in C57Bl/6 mice by intra-articular injections of methylated BSA in combination with subcutaneous injection of IL-1β. To investigate the in vivo role of GM-CSF in synovial MoDC and Th17 development, the mice were treated with anti-GM-CSF or control antibodies on day 0, 2, and 4 after arthritis induction. Synovial MoDC and Th17 levels were determined 7 days after arthritis onset. To elucidate which GM-CSF-producing cellular compartment is mainly responsible for synovial MoDC and Th17 development, bone marrow (BM) chimeras were created by transferring BM of GM-CSF knockout mice to irradiated wild-type mice or vice versa, and MoDC and Th17 levels were determined by FACS on synovial tissue 7 days after arthritis induction. In addition, GM-CSF expression by Th17 cells was studied ex vivo using naïve murine CD4+ T cells differentiated under various culturing conditions, and both Th17 differentiation efficacy and GM-CSF levels were assessed.

Results: Unexpectedly, inhibiting the GM-CSF pathway during murine experimental arthritis using neutralizing antibodies increased MoDC levels in the arthritic synovium. Interestingly, the percentage of synovial Th17 cells was also increased by this anti-GM-CSF treatment, suggesting a negative feedback of GM-CSF on Th17 development. Our study using bone marrow chimera mice showed that particularly mice deficient for BM-derived GM-CSF had increased levels of synovial MoDCs as well as Th17 cells, confirming the antagonistic regulation of MoDCs and Th17 cells by GM-CSF. Finally, our ex vivo studies showed that, while Th17 cells did not produce much GM-CSF, differentiation of naïve T cells under conditions suboptimal for Th17 development resulted in increased levels of GM-CSF, indicating deviating differentiation conditions and antagonistic regulation of IL-17 and GM-CSF by CD4+ T cells.

Conclusion: Our experiments are the first to demonstrate increased levels of synovial MoDCs and Th17 cells after anti-GM-CSF treatment, confirming a loop between GM-CSF and differentiation of MoDCs and Th17 cells. However, contradictory to what has been shown in EAE, GM-CSF was shown to have a negative impact on this loop. This negative regulation between GM-CSF and Th17 cells provides further rationale for a combination blocking strategy of IL-17 and GM-CSF in the treatment of RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/anti-gm-csf-treatment-promotes-synovial-monocyte-derived-dendritic-cells-and-increases-th17-cells-during-experimental-arthritis/