SDH inhibition quenches rheumatoid arthritis flare-associated IL-1β+IFN-γ+ macrophages via NF-κB/STAT4.

OBJECTIVES

Rheumatoid arthritis (RA) is a chronic autoimmune disease in which distinct macrophage (MΦ) endotypes can guide therapy. TLR5, upregulated during disease flares, drives an IL-1β+NFKB1+IFN-γ+STAT4lo MΦ phenotype in blood and synovium that is refractory to tumour necrosis factor (TNF) and IL-6R blockade. This study aimed to identify an alternative strategy to correct inflammation and metabolic dysfunction in these pathogenic MΦs.

METHODS

Peripheral blood-derived MΦs and synovial tissue from patients with RA, together with a localised arthritis model, were used to assess responses to succinate dehydrogenase (SDH) inhibition following TLR5 stimulation. We performed bulk RNA sequencing, Seahorse extracellular flux assays, flow cytometry, and multiscale immunofluorescence microscopy spanning whole RA synovium to individual mitochondria.

RESULTS

TLR5, SDH, and p65/NFKB1 colocalised in synovial CD14⁺ MΦs and were upregulated in circulation during disease flares. IL-1β+NFKB1+IFN-γ+STAT4lo MΦs elicited by TLR5 stimulation exhibited biphasic metabolic reprogramming, with an early increase in mitochondrial adenosine triphosphate (ATP) production followed by tricarboxylic acid (TCA) cycle disruption, compensatory glycolysis, and SDH upregulation. In contrast to HK2 inhibition, which targets the first step of glycolysis, SDH inhibition within the TCA cycle intercepted TLR5-induced NF-κB/STAT4 signalling and oxidative stress, restoring TCA enzyme expression and mitochondrial function. In localised TLR5-driven synovitis, SDH inhibition attenuated joint inflammation by reducing the population of F4/80⁺IL-1β+iNOS⁺SDH⁺ MΦs and restoring metabolic balance.

CONCLUSIONS

SDH functions as a metabolic-immune checkpoint in pathogenic IL-1β+NFKB1+IFN-γ+STAT4lo MΦs in RA. Targeting SDH may concurrently resolve mitochondrial dysfunction and inflammation in patients experiencing disease flares, with synovial enrichment of TLR5, SDH, IL-1β, NFKB1, and IFN-γ serving as predictive biomarkers.