Polymorphisms in the gene encoding the transcription factor IRF5 that lead to higher mRNA expression are associated with many autoimmune diseases. Here we show that IRF5 expression in macrophages was reversibly induced by inflammatory stimuli and contributed to the plasticity of macrophage polarization. High expression of IRF5 was characteristic of M1 macrophages, in which it directly activated transcription of the genes encoding interleukin 12 subunit p40 (IL-12p40), IL-12p35 and IL-23p19 and repressed the gene encoding IL-10. Consequently, those macrophages set up the environment for a potent T helper type 1 (TH1)-TH17 response. Global gene expression analysis demonstrated that exogenous IRF5 upregulated or downregulated expression of established phenotypic markers of M1 or M2 macrophages, respectively. Our data suggest a critical role for IRF5 in M1 macrophage polarization and define a previously unknown function for IRF5 as a transcriptional repressor.

Irf5 promotes inflammatory macrophage polarization and th1/th17 response

TNF and ROS Crosstalk in Inflammation

Recent studies have demonstrated dramatic shifts in metabolic supply-and-demand ratios during inflammation, a process resulting in localized tissue hypoxia within inflammatory lesions (“inflammatory hypoxia”). As part of the adaptive immune response, T cells are recruited to sites of inflammatory hypoxia. Given the profound effects of hypoxia on gene regulation, we hypothesized that T-cell differentiation is controlled by hypoxia. To pursue this hypothesis, we analyzed the transcriptional consequences of ambient hypoxia (1% oxygen) on a broad panel of T-cell differentiation factors. Surprisingly, these studies revealed selective, robust induction of FoxP3, a key transcriptional regulator for regulatory T cells (Tregs). Studies of promoter binding or loss- and gain-of-function implicated hypoxia-inducible factor (HIF)-1α in inducing FoxP3. Similarly, hypoxia enhanced Treg abundance in vitro and in vivo. Finally, Treg-intrinsic HIF-1α was required for optimal Treg function and Hif1a–deficient Tregs failed to control T-cell–mediated colitis. These studies demonstrate that hypoxia is an intrinsic molecular cue that promotes FoxP3 expression, in turn eliciting potent anti-inflammatory mechanisms to limit tissue damage in conditions of reduced oxygen availability.

https://www.pnas.org/content/pnas/109/41/E2784.full.pdf

Hypoxia-inducible factor-1 alpha–dependent induction of FoxP3 drives regulatory T-cell abundance and function during inflammatory hypoxia of the mucosa

Macrophages are very important in the pathogenesis of rheumatoid arthritis (RA). The increase in the number of sublining macrophages in the synovium is an early hallmark of active rheumatic disease, and high numbers of macrophages are a prominent feature of inflammatory lesions. The degree of synovial macrophage infiltration correlates with the degree of joint erosion, and depletion of these macrophages from inflamed tissue has a profound therapeutic benefit. Research has now uncovered an unexpectedly high level of heterogeneity in macrophage origin and function, and has emphasized the role of environmental factors in their functional specialization. Although the heterogeneous populations of macrophages in RA have not been fully characterized, preliminary results in mouse models of arthritis have contributed to our understanding of the phenotype and ontogeny of synovial macrophages, and to deciphering the properties of monocyte-derived infiltrating and tissue-resident macrophages. Elucidating the molecular mechanisms that drive polarization of macrophages towards proinflammatory or anti-inflammatory phenotypes could lead to identification of signalling pathways that inform future therapeutic strategies.

/pdf/macrophage-heterogeneity-in-the-context-of-rheumatoid-2y285i44h0.pdf

Macrophage heterogeneity in the context of rheumatoid arthritis

Mitochondrial dysfunction and the inflammatory response

Introduction Hypoxia is a microenvironmental feature in the inflamed joint, which promotes survival advantage for cells. The aim of this study was to examine the relationship of partial oxygen pressure in the synovial tissue (tPO 2 ) in patients with inflammatory arthritis with macroscopic/microscopic inflammation and local levels of proinflammatory mediators. Methods Patients with inflammatory arthritis underwent full clinical assessment and video arthroscopy to quantify macroscopic synovitis and measure synovial tPO 2 under direct visualisation. Cell specific markers (CD3 (T cells), CD68 (macrophages), Ki67 (cell proliferation) and terminal deoxynucleotidyl transferase dUTP nick end labelling (cell apoptosis)) were quantified by immunohistology. In vitro migration was assessed in primary and normal synoviocytes (synovial fibroblast cells (SFCs)) using a wound repair scratch assay. Levels of tumour necrosis factor α (TNFα), interleukin 1β (IL1β), interferon γ (IFNγ), IL6, macrophage inflammatory protein 3α (MIP3α) and IL8 were quantified, in matched serum and synovial fluid, by multiplex cytokine assay and ELISA. Results The tPO 2 was 22.5 (range 3.2–54.1) mm Hg and correlated inversely with macroscopic synovitis (r=−0.421, p=0.02), sublining CD3 cells (−0.611, p 2 in vivo) induced cell migration. This was coupled with significantly higher levels of synovial fluid tumour necrosis factor α (TNFα), IL1β, IFNγ and MIP3α in patients with tPO 2 Conclusions This is the first study to show a direct in vivo correlation between synovial tPO 2 , inflammation and cell migration, thus it is proposed that hypoxia is a possible primary driver of inflammatory processes in the arthritic joint.

/pdf/synovial-tissue-hypoxia-and-inflammation-in-vivo-4q59ovs2ok.pdf

Synovial tissue hypoxia and inflammation in vivo

Synovial macrophages are one of the resident cell types in synovial tissue and while they remain relatively quiescent in the healthy joint, they become activated in the inflamed joint and, along with infiltrating monocytes/macrophages, regulate secretion of pro-inflammatory cytokines and enzymes involved in driving the inflammatory response and joint destruction. Synovial macrophages are positioned throughout the sub-lining layer and lining layer at the cartilage-pannus junction and mediate articular destruction. Sub-lining macrophages are now also considered as the most reliable biomarker for disease severity and response to therapy in rheumatoid arthritis (RA). There is a growing understanding of the molecular drivers of inflammation and an appreciation that the resolution of inflammation is an active process rather than a passive return to homeostasis, and this has implications for our understanding of the role of macrophages in inflammation. Macrophage phenotype determines the cytokine secretion profile and tissue destruction capabilities of these cells. Whereas inflammatory synovial macrophages have not yet been classified into one phenotype or another it is widely known that TNFα and IL-l, characteristically released by M1 macrophages, are abundant in RA while IL-10 activity, characteristic of M2 macrophages, is somewhat diminished. Here we will briefly review our current understanding of macrophages and macrophage polarization in RA as well as the elements implicated in controlling polarization, such as cytokines and transcription factors like NFκB, IRFs and NR4A, and pro-resolving factors, such as LXA4 and other lipid mediators which may promote a non-inflammatory, pro-resolving phenotype, and may represent a novel therapeutic paradigm.

/pdf/macrophages-in-synovial-inflammation-2h1qyuuija.pdf

Macrophages in synovial inflammation.

Objective

To assess blood vessel stability in inflammatory synovial tissue (ST) and to examine neural cell adhesion molecule (NCAM), oxidative DNA damage, and hypoxia in vivo.



Methods

Macroscopic vascularity and ST oxygen levels were determined in vivo in patients with inflammatory arthritis who were undergoing arthroscopy. Vessel maturity/stability was quantified in matched ST samples by dual immunofluorescence staining for factor VIII (FVIII)/α-smooth muscle actin (α-SMA). NCAM and 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG) were examined by immunohistochemistry. Angiogenesis was assessed in vitro, using human dermal endothelial cells (HDECs) in a Matrigel tube formation assay.



Results

A significant number of immature vessels (showing no pericyte recruitment) was observed in tissue from patients with inflammatory arthritis (P < 0.001), in contrast to osteoarthritic and normal tissue, which showed complete recruitment of pericytes. Low in vivo PO2 levels in the inflamed joint (median [range] 22.8 [3.2–54.1] mm Hg) were inversely related to increased macroscopic vascularity (P < 0.04) and increased microscopic expression of FVIII and α-SMA (P < 0.04 and P < 0.03, respectively). A significant proportion of vessels showed focal expression of NCAM and strong nuclear 8-oxodG expression, implicating a loss of EC–pericyte contact and increased DNA damage, levels of which were inversely associated with low in vivo PO2 (P = 0.04 for each comparison). Circulating cells were completely negative for 8-oxodG. Exposure of HDEC to 3% O2 (reflecting mean ST in vivo measurements) significantly increased EC tube formation (P < 0.05).



Conclusion

Our findings indicate the presence of unstable vessels in inflamed joints associated with hypoxia, incomplete EC–pericyte interactions, and increased DNA damage. These changes may further contribute to persistent hypoxia in the inflamed joint to further drive this unstable microenvironment.

Angiogenesis and blood vessel stability in inflammatory arthritis.

Objectives To assess levels of oxidative DNA damage (8-oxo-7,8-dihydro-2′-deoxyguanine; 8-oxo-dG) and lipid peroxidation (4-hydroxy-2-nonenal; 4-HNE) in serum, synovial fluid and tissue of patients with inflammatory arthritis in relation to in vivo hypoxia levels, disease activity and angiogenic markers. Methods Oxygen levels in synovial tissue were assessed using an oxygen/temperature probe. Nuclear and cytoplasmic 8-oxo-dG and 4-HNE levels were assessed in synovial tissue from 23 patients by immunohistochemistry. 8-Oxo-dG and 4-HNE levels in serum and synovial fluid were determined using 8-oxo-dG and hexanoyl-Lys (HEL) adduct ELISAs, respectively. Serum vascular endothelial growth factor (VEGF) and angiopoietin 2 (Ang2) levels were also measured by ELISA. Results The median oxygen tension in synovial tissue was profoundly hypoxic at 19.35 mm Hg (2.5%). Nuclear 8-oxo-dG levels were significantly higher than nuclear 4-HNE levels in the lining and sublining layers (all p Conclusions Lipid peroxidation is associated with low oxygen tension in vivo, disease activity and angiogenic marker expression in inflammatory arthritis.

https://ard.bmj.com/content/annrheumdis/69/6/1172.full.pdf

Oxidative damage in synovial tissue is associated with in vivo hypoxic status in the arthritic joint

Objective

To investigate the relationship between acute-phase serum amyloid A (A-SAA) and joint destruction in inflammatory arthritis



Methods

Serum A-SAA and C-reactive protein (CRP) levels, the erythrocyte sedimentation rate (ESR), and levels of matrix metalloproteinase 1 (MMP-1), MMP-2, MMP-3, MMP-9, MMP-13, tissue inhibitor of metalloproteinases 1 (TIMP-1), vascular endothelial growth factor (VEGF), and type I and type II collagen–generated biomarkers C2C and C1,2C were measured at 0–3 months in patients with inflammatory arthritis commencing anti–tumor necrosis factor α (anti-TNFα) therapy and were correlated with 1-year radiographic progression The effects of A-SAA on MMP/TIMP expression on RA fibroblast-like synoviocytes (FLS), primary human chondrocytes, and RA/psoriatic arthritis synovial explant cultures were assessed using real-time polymerase chain reaction, enzyme-linked immunosorbent assay, antibody protein arrays, and gelatin zymography



Results

Serum A-SAA levels were significantly (P < 005) correlated with MMP-3, the MMP-3:TIMP-1 ratio, C1,2C, C2C, and VEGF The baseline A-SAA level but not the ESR or the CRP level correlated with the 28-joint swollen joint count and was independently associated with 1-year radiographic progression (P = 0038) A-SAA increased MMP-1, MMP-3, MMP-13, and MMP/TIMP expression in RA FLS and synovial explants (P < 005) In chondrocytes, A-SAA induced MMP-1, MMP-3, and MMP-13 messenger RNA and protein expression (all P < 001), resulting in a significant shift in MMP:TIMP ratios (P < 005) Gelatin zymography revealed that A-SAA induced MMP-2 and MMP-9 activity Blockade of the A-SAA receptor SR-B1 (A-SAA receptor scavenger receptor-class B type 1) inhibited MMP-3, MMP-2, and MMP-9 expression in synovial explant cultures ex vivo Importantly, we demonstrated that A-SAA has the ability to induce TNFα expression in RA synovial explant cultures (P < 005)



Conclusion

A-SAA may be involved in joint destruction though MMP induction and collagen cleavage in vivo The ability of A-SAA to regulate TNFα suggests that A-SAA signaling pathways may provide new therapeutic strategies for the treatment of inflammatory arthritis

https://onlinelibrary.wiley.com/doi/pdf/10.1002/art.33455

Acute-phase serum amyloid A regulates tumor necrosis factor α and matrix turnover and predicts disease progression in patients with inflammatory arthritis before and after biologic therapy.

Aisling Kennedy

Papers

Synovial tissue hypoxia and inflammation in vivo

Macrophages in synovial inflammation.

Angiogenesis and blood vessel stability in inflammatory arthritis.

Oxidative damage in synovial tissue is associated with in vivo hypoxic status in the arthritic joint

Acute-phase serum amyloid A regulates tumor necrosis factor α and matrix turnover and predicts disease progression in patients with inflammatory arthritis before and after biologic therapy.