Background/Purpose: Rheumatoid arthritis (RA) is an autoimmune disease characterised by synovial inflammation and joint destruction. Prior research has revealed perturbation in the myeloid compartment of individuals with RA compared to healthy subjects. Furthermore, initial findings suggest that there is altered metabolic activity in the RA monocyte compartment. Given the right stimuli, monocytes can differentiate into multi-nucleated bone-resorbing osteoclasts (OCs), which play a key role in RA joint degradation. In this study, we focus on the differences in metabolism of healthy donors and patients with active RA in monocytes and monocyte-derived OCs.

Methods: Blood samples from patients (n=9) with active RA (DAS28 > 2.6) treated with conventional DMARDs and naive to biologic therapy, and age and sex matched healthy donors, were collected. CD14+ monocytes were isolated from peripheral blood and differentiated into OCs with 25 ng/ml M-CSF and 25 ng/ml RANKL for 12 days in the presence or absence of TNF to mimic inflammation. Monocytes and differentiated OCs were stained with 4 different metabolic dyes, namely 2-NBDG (glucose uptake), Bodipy FL C16 (fatty acid uptake), MitoTracker Deep Red FM (mitochondrial mass) and TMRM (mitochondrial membrane potential). Cells were also fixed and stained with antibodies targeting key metabolic enzymes including glucose transporter 1 (GLUT1), Acetyl-CoA carboxylase 1 (ACC1), Succinate Dehydrogenase Complex Flavoprotein Subunit A (SDHA), Glucose-6-phosphate dehydrogenase (G6PD), pyruvate kinase muscle isozyme (PKM), carnitine palmitoyltransferase 1A (CPT1A), and cytochrome C (CytC). Samples were measured using a Cytek Aurora flow cytometer and results were analysed using FlowJo software.

Results: We observed a significant increase in the metabolism of RA compared to healthy monocytes via all of the metabolic assays: 2-NBDG (p=0.009), Bodipy FL C16 (p=< 0.0001), MitoTracker Deep Red FM (p=< 0.0001) and TMRM (p=0.004). In contrast, preliminary data in an n=4, showed no significant differences in metabolic enzyme expression. To understand whether the metabolic differences noted in monocytes were maintained in differentiated mature cells, we initially evaluated the metabolism of OCs. Preliminary results in OCs showed a significantly higher uptake of glucose (2-NBDG, p=0.0003) and fatty acid (Bodipy FL C16, p=0.04) in RA OCs compared to healthy. Further evaluation of monocyte-derived macrophages (M-CSF driven) also revealed a significantly higher uptake of glucose and fatty acid in RA-derived cells (p=0.02 and p=0.006, respectively). Finally, no major differences in mitochondrial mass and membrane potential were observed in RA-derived osteoclasts or macrophages.

Conclusion: In summation, RA monocytes have increased uptake of glucose and fatty acid which is maintained in differentiated macrophages and OCs. This combined suggest an overall higher metabolic activity in RA myeloid compartment which might contribute to bone resorption and inflammation in RA. Further studies are now required to determine the drivers of this altered state and reveal potential novel pathways that could be therapeutically targeted to simultaneously decrease inflammation and bone erosion.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/altered-metabolic-state-in-myeloid-precursors-and-mature-cells-within-rheumatoid-arthritis/