OP0075 FIBROBLAST/MACROPHAGE CROSSTALK VIA LACTATE: NEW THERAPEUTIC TARGET IN RHEUMATOID ARTHRITIS

Abstract

BackgroundThe synovial membrane is the principal site of inflammation in rheumatoid arthritis (RA) and distinct subsets of fibroblasts and macrophages, with different effector functions, have been described within it1,2,3. Inflammation renders the RA synovial microenvironment hypoxic and acidic, with increased levels of lactate, the end product of glycolysis. Lactate acts as an immunomodulatory molecule within the synovium4, interacting with lactate transporters present on fibroblasts and macrophages to regulate their function, movement and metabolism.ObjectivesTo test whether dysfunctional crosstalk between fibroblasts and macrophages, driven by lactate, promotes the persistence of synovial inflammation.MethodsSynovial tissues (n = 8) from patients fulfilling the 2010 ACR/EULAR RA criteria were obtained by ultrasound-guided synovial biopsy. Osteoarthritis (OA) synovial tissues of subjects undergoing joint replacement were used as control group. Monocarboxylate transporter 1 (MCT)1 and MCT4 expression on fibroblasts and macrophages was assessed via confocal microscopy. We used RA synovial fibroblasts and monocyte-derived macrophages to test the effect of lactate in vitro. Migration was assessed in trans-well plates or via scratch test assays. Seahorse was used to evaluate metabolic pathways. IL6 production was measured by ELISA. Bioinformatic data were confirmed on publicly available scRNAseq datasets.ResultsWe showed that: i) The expression of MCT1 and MCT4 which regulate lactate import and export respectively, is up-regulated upon inflammation. ii) Fibroblasts preferentially express MCT1, while MCT4 is more highly expressed by macrophages. iii) Lactate, at the concentration found in RA synovial fluid (10 mM), has divergent effects on the effector functions of these two cell types. In fibroblasts, lactate promotes IL6 production and cell motility; these effects are reduced by pre-treatment with a pan-lactate transporter inhibitor. In contrast macrophages respond to lactate by reducing migration, IL6 secretion and glycolysis.ConclusionThe contrasting effects of lactate on macrophage and fibroblast migration, IL6 production and metabolism suggest that lactate represents a key metabolite ensuring linked choreography between fibroblast and macrophage movement in the synovium which may become uncoupled in disease. We propose that dysfunctional crosstalk between these two cell types due to high lactate levels, promotes inflammation and the establishment of persistent disease in RA. Targeting lactate/MCTs pathway may provide a novel therapeutic strategy, to restore cellular crosstalk and to reduce inflammation in RA patients.