Background/Purpose: RA and PsA share various pathogenic features, while also displaying significant differences at the clinical, cellular and molecular levels. In this study, we investigate the inflammatory capacity of RA and PsA monocytes and monocyte-derived macrophages (MO-MACs), in addition to other effector functions.

Methods: Peripheral blood mononuclear cells (PBMCs) were obtained and CD14⁺ monocytes isolated from RA and PsA patients and assessed under ex vivo and LPS-stimulated conditions. MO-MACs were generated via 7-day stimulation with M-CSF (50ng/mL) and polarised to M1 and M2. Inflammatory responses (IL-6, IL-1β, TNF-α, CXCL9-11, SLAMF1-7) were assessed by Real-time PCR (RT-PCR). Frequency of monocyte subsets and expression of activation (CD40) and macrophage signature markers (CD64, CD163, CD206, SLAMF7) were assessed by flow cytometry. Endocytosis assays were performed on monocytes and MO-MACs. Demethylation genes TET1-3 were assessed by RT-PCR. Finally, monocytes were cultured with a methylation inhibitor (RG108) and activator (budesonide), and pro-inflammatory responses assessed.

Results: Significant increases in LPS-induced expression of IL-6, IL-1b and CXCL9-11 (all p< 0.05) were observed in RA compared to PsA monocytes. Expression of SLAMF1 and 2 were significantly increased in LPS-induced RA and PsA monocytes, with SLAMF4 significantly decreased (all p< 0.05). Heightened responses for SLAMF7 were observed in RA vs PsA monocytes (p< 0.05).The increased pro-inflammatory response of LPS-stimulated monocytes in RA vs PsA was paralleled by a significant decrease in monocyte endocytic capacity, an effect that was more pronounced for RA (p< 0.01). Analysis of monocyte-derived macrophages demonstrated that both RA and PsA retain the hyper-inflammatory phenotype of their precursor cell. Expression of IL-6, CXCL9 and CXCL11 were significantly higher in RA MO-M1 (all p< 0.05), whereas IL-1b was higher in PsA (p< 0.05). Heightened responses for SLAMF7 were observed for RA MO-M1 (p< 0.05), in contrast to SLAMF2 which was significantly increased in PsA MO-M1 (p< 0.05). Similarly to monocytes, endocytic capacity was reduced in RA compared to PsA M0 macrophages. RA PBMCs exhibited decreased classical (CD14⁺CD16^–) but increased intermediate (CD14⁺CD16⁺) monocyte frequencies compared to PsA, though non-classical monocytes (CD14^–CD16⁺) frequencies were comparable. Frequencies of CD64, CD163, CD206 and SLAMF7 were higher in all RA monocyte subsets compared to PsA, suggesting that RA monocytes have a primed pro-inflammatory macrophage phenotype. Finally, we demonstrated increased expression of methylation markers TET2 in RA monocytes (p< 0.05) and of TET3 in PsA monocytes (p< 0.05). Budesonide decreased the expression of IL-6, TNF-a and IL-1b in ex vivo and LPS-stimulated monocytes.

Conclusion: Monocytes are inherently more pro-inflammatory and activated than PsA monocytes, an effect that is maintained following differentiation into macrophage. The distinct inflammatory pre-programming of monocytes may also involve altered epigenetic alterations.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/ra-monocytes-and-monocyte-derived-macrophages-display-heightened-inflammatory-responses-reduced-endocytic-capacity-and-distinct-tet-expression-compared-to-psa-monocytes/