Background/Purpose: Myeloid Dendritic Cells (DC) are potent antigen presenting cells that can be subdivided into CD141⁺ and CD1c⁺ DC. We have previously reported an unacknowledged role of CD141⁺DC in the RA synovium. However, the identification and function of CD1c⁺ DC in the RA synovium has yet to be fully elucidated. Therefore our aim was to investigate if CD1c⁺DC reside in the RA synovium and ascertain if they represent a unique population, distinct from peripheral blood CD1c⁺DC.

Methods: Synovial tissue (ST) biopsies and synovial fluid mononuclear cells (SFMC) were obtained via arthroscopy and healthy control (HC) ST was obtained during ACL surgery. Synovial biopsies were enzymatically and mechanically dissociated to yield a single cell suspension. SFMC and peripheral blood mononuclear cells (PBMC) in addition to synovial tissue digests were stained with a panel of fluorochrome conjugated antibodies and single cell analysis was performed by multicolour flow cytometry. CD1c⁺DC were sorted from RA SF and peripheral blood (PB) and RNA-sequencing was performed. Principal Component Analysis (PCA), Enriched pathway analysis, heatmaps and hierarchical clustering were identified using the DeSeq2 R package and Ingenuity^® Pathway Analysis (IPA). For functional experiments CD1c⁺DC were magnetically sorted from RA SFMC and RA PBMC and phagocytosis was assessed using flow cytometry. Finally allogeneic T cell cocultures were performed with SF and PB CD1c⁺DC in the presence or absence of the STAT3 inhibitor Stattic.

Results: CD1c⁺DC are significantly decreased in RA PB compared to HC PB (p< 0.01) and express significantly higher levels of the chemokine receptors CCR7 (p< 0.05) and CXCR3 (p=0.08) compared to HC DC - suggestive of DC migration to the synovium.  In support of this hypothesis, CD1c⁺DC were identified in RA ST by multicolour flow cytometry and were significantly increased in RA ST compared to RA PB (p< 0.01). RA ST CD1c⁺DC express significantly higher expression of the DC activation marker CD80 compared to RA PB (p< 0.01) or HC ST (p< 0.05). CD1c⁺DC are also enriched in RA synovial fluid compared to RA PB (p< 0.05), express significantly higher levels of the maturation markers CD80,CD83,CD40,PD-L1 and BTLA (all p< 0.05) and are less phagocytic compared to matched PB. RNA-sequencing and PCA analysis revealed distinct transcriptional variation between synovial and PB CD1c⁺DC. Moreover, IPA analysis revealed an enrichment in the STAT3 pathway in synovial DC. Synovial CD1c⁺DC induce proinflammatory cytokines (TNFα, GMCSF, IFNγ) from CD4⁺ T-cells in allogeneic cocultures, and effect which is abrogated in the presence of a STAT3 inhibitor.

Conclusion: CD1c⁺DC are present in the RA synovium in a mature state, have distinct tissue specific characteristics and mediate their T cell stimulatory capabilities via STAT3.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/functionally-mature-cd1c-dendritic-cells-contribute-to-synovial-inflammation-in-rheumatoid-arthritis-via-stat3/