Functionally Mature CD1c+ Dendritic Cells Preferentially Accumulate in the Inflammatory Arthritis Synovium

Objective: To examine the role of synovial CD1c+DC in patients with Inflammatory Arthritis (IA) with a specific focus on the transcriptional and maturation signatures which govern their function. Methods. RNA Sequencing was performed on healthy control (HC) peripheral blood (PB), IA PB and IA synovial fluid (SF) CD1+DC. Multiparametric flow-cytometry and SPICE analysis were used to examine site (SF and Synovial Tissue (ST) CD1+DC) and disease specific characteristics of CD1+DC while functional assays such as antigen processing, activation and MMP production were also performed. Results. Increased frequency of CD1c+DC (p<0.01) with a concomitant increase in CD80, CCR7 (p<0.01) and CXCR3 (p<0.05) expression was identified in IA PB compared to HC PB. Enrichment of CD1c+DC was identified in IA synovial tissue (ST) (p<0.01) and IA SF (p<0.0001) compared to IA PB, while RNAseq revealed distinct transcriptional variation between PB and SF CD1c+DC. Flow cytometry revealed increased expression of CD83, CD80, PD-L1 and BTLA (all p<0.05) in IA SF CD1c+DC compared PB, while SPICE identified synovial cells with unique co-expression patterns, expressing multiple DC maturation markers simultaneously. Functionally synovial CD1c+DC are hyper-responsive to TLR7/8 ligation (p<0.05), have decreased antigen processing capacity (p=0.07) and dysregulated production of MMPs. Finally, examination of both synovial CD1c+DC and synovial CD141+DC, revealed distinct maturation and transcriptomic profiles. Conclusion: Synovial CD1c+DC accumulate in the inflamed IA synovium in a variety of distinct poly maturational states, distinguishing them transcriptionally and functionally from CD1c+DC in the periphery and synovial CD141+DC.