Background/Purpose: Myeloid cells – including dendritic cells (DCs), monocytes, and macrophages – are critical to the pathogenesis of rheumatoid arthritis (RA) through production of pro-inflammatory cytokines, recruitment of inflammatory cells, and activation of T cells. Various studies have shown that these cells exhibit activation markers and increased numbers in the joint synovium, but the relationship with the activity of their circulating counterparts has not been fully elucidated. Here, we perform RNA-seq on these in vivo cell populations from RA patients in order to compare their genome-wide transcriptional profile within and across individuals.

Methods: We obtained blood samples and ultrasound-guided minimally invasive synovial biopsy tissue (as described in Mandelin et al, A&R 2018¹) from RA patients with active disease. Using Fluorescence-Activated Cell Sorting (FACS), we isolated classical monocytes (MHCII+CD14++CD16-), non-classical monocytes (MHCII+CD14+CD16+), and dendritic cells (MHCII+CD1c+) from blood. After processing synovial tissue for single-cell suspension, we isolated macrophages (MHCII+CD14+CD11b+CD206+) and dendritic cells (MHCII+CD1c+) by FACS. We extracted RNA from these cell populations and prepared libraries for RNA-seq. Although the cell numbers were low, particularly for the synovial tissue populations, we have previously demonstrated our ability to reliably generate RNA-seq libraries from sorted populations down to tens of cells. These libraries were sequenced on an Illumina NextSeq 500 and assessed for quality of RNA, sequencing, and gene detection.

Results: For each cell population, we assessed the variability of gene expression across patients. As expected, the DCs were highly variable across individuals: this is likely due to the heterogeneity of subtypes within this population. In circulating monocytes, we observed varying levels of common cytokines and chemokines, such as TNF and CCL1. We also compared gene expression across cell populations to characterize transcriptional signatures that were distinctive to a given cell population. In addition to the genes previously known to be unique to dendritic cells vs. monocytes/macrophages in health, we also identified potential pathogenic factors that varied in their expression across cell types. Finally, to explore the relationship between circulating and tissue cell populations, we asked whether there were pathways that were turned on in the blood prior to extravasation into the synovium. For example, we identified genes that maintained their expression across monocytes and synovial macrophages in RA patients supporting the differentiation of the former into the latter.

Conclusion: Together, these results provide a survey of myeloid cells in the blood and synovial tissue of RA patients. We aim to understand how these cells vary across patients and what clinical variables and medication status influence their transcriptional profile across individuals. Our long-term goal is to use these studies to better understand the underlying mechanisms of pathogenesis and response to current treatments of RA as well as to identify potential targets for future therapies.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/a-survey-of-blood-and-synovial-tissue-myeloid-cells-in-ra-patients-by-transcriptional-profiling/