Rheumatoid arthritis macrophages are primed for inflammation and display bioenergetic and functional alterations.

OBJECTIVES: Myeloid cells with a monocyte/macrophage phenotype are present in large numbers in the Rheumatoid Arthritis (RA) joint, significantly contributing to disease, however distinct macrophage functions have yet to be elucidated. This study investigates the metabolic activity of infiltrating polarised macrophages and their impact on pro-inflammatory responses in RA.

METHODS: CD14+ monocytes from RA and HC bloods were isolated and examined ex-vivo or following differentiation into 'M1/M2' macrophages. Inflammatory responses and metabolic analysis +/- specific inhibitors were quantified by RT-PCR, western-blot, Seahorse-XFe-technology, phagocytosis assays and transmission electron microscopy along with RNA-seq transcriptomic analysis.

RESULTS: Circulating RA monocytes are hyper-inflammatory upon stimulation, with significantly higher expression of key cytokines compared with HC (p< 0.05) a phenotype which is maintained upon differentiation into mature ex vivo polarised macrophages. This induction in pro-inflammatory mechanisms is paralleled by cellular bioenergetic changes. RA macrophages are highly metabolic, with a robust boost in both oxidative phosphorylation and glycolysis in RA along with altered mitochondrial morphology compared with healthy control. RNA-sequencing analysis revealed divergent transcriptional variance between pro and anti-inflammatory RA macrophages revealing a role for STAT3 and NAMPT in driving macrophage activation states. STAT3 and NAMPT inhibition results in significant decrease in pro-inflammatory gene expression observed in RA macrophages. Interestingly NAMPT inhibition specifically, restores macrophage phagocytic function and results in reciprocal STAT3 inhibition linking these two signalling pathways.

CONCLUSION: This study demonstrates a unique inflammatory and metabolic phenotype of RA monocyte-derived macrophages and identifies a key role for NAMPT and STAT3 signalling in regulating this phenotype.