Background/Purpose: Synovial macrophages play a key role in RA pathogenesis.  Their numbers are greatly increased in RA synovium, their phenotype is consistent with a pro-inflammatory function, and clinical data indicate all efficacious RA therapies lead to a significant reduction of this cell type in the diseased joint.  Studies probing global gene expression in RA synovial macrophages have been limited in scope.  We set out to further our understanding of synovial macrophage biology — both in RA and additional arthropathies — through gene expression profiling, gene mining and pathway analysis in synovial macrophages from patients undergoing disease flare. 

Methods: Synovial fluids were collected  from 10 RA patients in disease flare (treatments included oral DMARDs and/or biologics), and CD14+ cells  isolated by positive selection.  Control macrophage samples were generated from CD14+ peripheral blood monocytes from healthy donors, and differentiated with M-CSF.  Macrophages were also isolated from synovial fluid from patients with Psoriatic Arthritis and other arthropathies to allow for comparison of dysregulated genes and pathways across related diseases.  RNAs were isolated, processed, and transcriptional profiling of the samples was performed using Affymetrix arrays.  Statistical analysis was performed using principle component analysis (PCA) and upregulated genes defined as >2.0-fold increase in expression and an FDR p-value of <0.05.  Internal genesets from cytokine-stimulated macrophages, and M1 or  M2 polarized macrophages, were used as comparators to further characterize upregulated genes and pathways in RA macrophages.

Results: Approximately 1900 genes were significantly upregulated in RA synovial macrophages, and 70% demonstrated overlap with Psoriatic arthritis and other arthropathies.  Forty percent of RA-upregulated genes were upregulated in M1 polarized macrophages, vs. 7% in M2 macrophages.  Only 10% of RA-regulated genes were upregulated by chronic treatment of control macrophages with TNFa, indicating other factors impact gene expression to a greater extent in RA macrophages.  Gene Set Enrichment and Ingenuity Pathway Analysis indicated that several immunoregulatory pathways previously linked to RA pathogenesis were upregulated in RA macrophages (including CD40, and FcR), independent of treatment.  Interestingly, the LTb signaling pathway, which was highly upregulated in M1 macrophages was not significantly induced in RA macrophages indicating a unique inflammatory phenotype of these disease macrophages.

Conclusion: We performed a comprehensive study focused on identifying differentially regulated genes and pathways in synovial macrophages from patients with RA and related arthropathies.  These results suggest a unique inflammatory phenotype of RA macrophages and maintenance of key inflammatory pathways during flare that is independent of treatment.  These observations may shed light on novel intervention points for treatment of RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/analysis-of-gene-expression-patterns-in-rheumatoid-arthritis-ra-synovial-macrophages-from-patients-undergoing-disease-flare/