IL-6 Signaling Attenuates TNF-α Production by Plasmacytoid Dendritic Cells in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is characterized by autoimmune joint destruction with debilitating consequences. Despite treatment advancements with biologic therapies, a significant proportion of RA patients show an inadequate clinical response, and restoration of immune self-tolerance represents an unmet therapeutic need. We have previously described a tolerogenic phenotype of plasmacytoid dendritic cells (pDCs) in RA patients responding to anti-TNF-alpha agents. However, the molecular mechanisms involved in tolerogenic reprogramming of pDCs in RA remain elusive. In this study, guided by transcriptomic analysis of CD303(+) CD123(+) pDCs from RA patients in remission, we revealed enhanced expression of IL-6R and its downstream signaling compared with healthy pDCs. Functional assessment demonstrated that IL-6R engagement resulted in marked reduction of TNF-alpha secretion by pDCs whereas intracellular TNF-alpha was significantly increased. Accordingly, pharmacologic inhibition of IL-6R signaling restored TNF-alpha secretion levels by pDCs. Mechanistic analysis demonstrated impaired activity and decreased lysosomal degradation of ADAM17 (a disintegrin and metalloproteinase 17) sheddase in pDCs, which is essential for TNF-alpha cleavage. Importantly, reduction of TNF-alpha secretion by IL-6-treated pDCs attenuated the inflammatory potential of RA patient-derived synovial fibroblasts. Collectively, these findings position pDCs as an important source of TNF-alpha in RA pathogenesis and unravel an anti-inflammatory mechanism of IL-6 by limiting the pDC-derived TNF-alpha secretion.