Upregulation of citrullination pathway: From Autoimmune to Idiopathic Lung Fibrosis

Pathogenic role and clinical significance of neutrophils and neutrophil extracellular traps in idiopathic inflammatory myopathies

Idiopathic inflammatory myopathies (IIM) are a heterogeneous group of chronic autoimmune diseases characterized by muscle damage and extramuscular symptoms, including specific skin rash, arthritis, interstitial lung disease, and cardiac involvement. While the etiology and pathogenesis of IIM are not yet fully understood, emerging evidence suggests that neutrophils and neutrophil extracellular traps (NETs) have a role in the pathogenesis. Recent research has identified increased levels of circulating and tissue neutrophils as well as NETs in patients with IIM; these contribute to the activation of the type I and type II interferons pathway. During active IIM disease, myositis-specific antibodies are associated with the formation and incomplete degradation of NETs, leading to damage in the lungs, muscles, and blood vessels of patients. This review focuses on the pathogenic role and clinical significance of neutrophils and NETs in IIM, and it includes a discussion of potential targeted treatment strategies.

Identification and validation of mutual hub genes in idiopathic pulmonary fibrosis and rheumatoid arthritis-associated usual interstitial pneumonia

Objectives

The study aims at exploring common hub genes and pathways in idiopathic pulmonary fibrosis (IPF) and rheumatoid arthritis-associated usual interstitial pneumonia (RA-UIP) through integrated bioinformatics analyses.

Methods

The GSE199152 dataset containing lung tissue samples from IPF and RA-UIP patients was acquired from the Gene Expression Omnibus (GEO) database. The identification of overlapping differentially expressed genes (DEGs) in IPF and RA-UIP was carried out through R language. Protein-protein interaction (PPI) network analysis and module analysis were applied to filter mutual hub genes in the two diseases. Enrichment analyses were also conducted to analyze the possible biological functions and pathways of the overlapped DEGs and hub genes. The diagnostic value of key genes was assessed with R language, and the expressions of these genes in pulmonary cells of IPF and rheumatoid arthritis-associated interstitial lung disease (RA-ILD) patients were analyzed with single cell RNA-sequencing (scRNA-seq) datasets. The expression levels of hub genes were validated in blood samples from patients, specimens of human lung fibroblasts, lung tissue samples from mice, as well as external GEO datasets.

Results

Four common hub genes (THBS2, TIMP1, POSTN, and CD19) were screened. Enrichment analyses showed that the abnormal expressions of DEGs and hub genes may be connected with the onset of IPF and RA-UIP by regulating the progression of fibrosis. ScRNA-seq analyses illustrated that for both IPF and RA-ILD patients, THBS2, TIMP1, and POSTN were mainly expressed in lung fibroblasts, while CD19 was uniquely high-expressed in B cells. The qRT-PCR and immunohistochemistry (IHC) results verified that the expression levels of hub genes were mostly in accordance with the findings obtained from the bioinformatics analyses.

Conclusion

Though IPF and RA-UIP are distinct diseases, they may to some extent have mutual pathogenesis in the development of fibrosis. THBS2, TIMP1, POSTN, and CD19 may be the potential biomarkers of IPF and RA-UIP, and intervention on related pathways of these genes could offer new strategies for the precision treatment of IPF and RA-UIP.

Diabetes mellitus and idiopathic pulmonary fibrosis: a Mendelian randomization study

BACKGROUND

The question as to whether or not diabetes mellitus increases the risk of idiopathic pulmonary fibrosis (IPF) remains controversial. This study aimed to investigate the causal association between type 1 diabetes (T1D), type 2 diabetes (T2D), and IPF using Mendelian randomization (MR) analysis.

METHODS

We used two-sample univariate and multivariate MR (MVMR) analyses to investigate the causal relationship between T1D or T2D and IPF. We obtained genome-wide association study (GWAS) data for T1D and T2D from the European Bioinformatics Institute, comprising 29,652 T1D samples and 101,101 T1D single nucleotide polymorphisms (SNPs) and 655,666 T2D samples and 5,030,727 T2D SNPs. We also used IPF GWAS data from the FinnGen Biobank comprising 198,014 IPF samples and 16,380,413 IPF SNPs. All cases and controls in these datasets were derived exclusively from European populations. In the univariate MR analysis, we employed inverse variance-weighted (IVW), weighted median (WM), and MR-Egger regression methods. For the MVMR analysis, we used the multivariate IVW method primarily, and supplemented it with multivariate MR-Egger and multivariate MR- least absolute shrinkage and selection operator methods. Heterogeneity tests were conducted using the MR-IVW and MR-Egger regression methods, whereas pleiotropic effects were assessed using the MR-Egger intercept. The results of MR and sensitivity analyses were visualized using forest, scatter, leave-one-out, and funnel plots.

RESULTS

Univariate MR revealed a significant causal relationship between T1D and IPF (OR = 1.118, 95% CI = 1.021-1.225, P = 0.016); however, no significant causal relationship was found between T2D and IPF (OR = 0.911, 95% CI = 0.796-1.043, P = 0.178). MVMR analysis further confirmed a causal association between T1D and IPF (OR = 1.133, 95% CI = 1.011-1.270, P = 0.032), but no causal relationship between T2D and IPF (OR = 1.009, 95% CI = 0.790-1.288, P = 0.950). Sensitivity analysis results validated the stability and reliability of our findings.

CONCLUSION

Univariate and multivariate analyses demonstrated a causal relationship between T1D and IPF, whereas no evidence was found to support a causal relationship between T2D and IPF. Therefore, in clinical practice, patients with T1D should undergo lung imaging for early detection of IPF.

Gut microbiota does not play a mediating role in the causal association between inflammatory bowel disease and several its associated extraintestinal manifestations: a Mendelian randomization study

Background and objectives

Observational study has found inflammatory bowel disease to be associated with multiple extraintestinal manifestations. To this end, we characterized the causal association between inflammatory bowel disease and extraintestinal manifestations through a Mendelian randomization study and further explored the role of intestinal flora in inflammatory bowel disease and the extraintestinal manifestations associated with it.

Materials and methods

We genetically predicted the causal relationship between inflammatory bowel disease and twenty IBD-related extraintestinal manifestations (including sarcoidosis, iridocyclitis, interstitial lung disease, atopic dermatitis, ankylosing spondylitis, psoriatic arthropathies, primary sclerosing cholangitis, primary biliary cholangitis). We used the full genome-wide association study (GWAS) summary statistics on gut microbiota in 18,340 participants from 24 cohorts to explore its role in the casual relationships between IBD and IBD-related extraintestinal manifestations. Inverse variance weighting (IVW) was used as the main analytical method to assess the causal associations. We performed Cochran's Q test to examine the heterogeneity. To assess the robustness of the IVW results, we further performed sensitivity analyses including the weighted median method, MR-Egger regression, and Mendelian Randomization Pleiotropy RESidual Sum and Outlier (MR-PRESSO) test. The leave-one-out sensitivity analysis was further performed to monitor if significant associations were dominated by a single nucleotide polymorphism (SNP).

Result

A total of eight extraintestinal manifestations were found to be at elevated risk of development due to inflammatory bowel diseases. A total of 11 causal relationships were found between IBD and gut microbiota, four of which were stable. Between gut microbiota and these eight extraintestinal manifestations, a total of 67 nominal causal associations were identified, of which 13 associations were stable, and notably 4 associations were strongly correlated.

Conclusion

Through the two-sample MR analysis, we identified extraintestinal manifestations that were causally associated with inflammatory bowel disease and obtained multiple associations from inflammatory bowel disease and gut microbiota, and gut microbiota and extraintestinal manifestations in further analyses. These associations may provide useful biomarkers and potential targets for pathogenesis and treatment.

Peptidyl Arginine Deiminases in Chronic Diseases: A Focus on Rheumatoid Arthritis and Interstitial Lung Disease

Protein citrullination is accomplished by a broad enzyme family named Peptidyl Arginine Deiminases (PADs), which makes this post-translational modification in many proteins that perform physiological and pathologic mechanisms in the body. Due to these modifications, citrullination has become a significant topic in the study of pathological processes. It has been related to some chronic and autoimmune diseases, including rheumatoid arthritis (RA), interstitial lung diseases (ILD), multiple sclerosis (MS), and certain types of cancer, among others. Antibody production against different targets, including filaggrin, vimentin, and collagen, results in an immune response if they are citrullinated, which triggers a continuous inflammatory process characteristic of autoimmune and certain chronic diseases. PAD coding genes (PADI1 to PADI4 and PADI6) harbor variations that can be important in these enzymes' folding, activity, function, and half-life. However, few studies have considered these genetic factors in the context of chronic diseases. Exploring PAD pathways and their role in autoimmune and chronic diseases is a major topic in developing new pharmacological targets and valuable biomarkers to improve diagnosis and prevention. The present review addresses and highlights genetic, molecular, biochemical, and physiopathological factors where PAD enzymes perform a major role in autoimmune and chronic diseases.

Plasma LTBP2 as a potential biomarker in differential diagnosis of connective tissue disease-associated interstitial lung disease and idiopathic pulmonary fibrosis: a pilot study

Few biomarkers distinguish connective tissue disease-associated interstitial lung disease (CTD-ILD) from idiopathic pulmonary fibrosis (IPF). Latent transforming growth factor-β binding protein-2 (LTBP2), a secreted extracellular matrix protein, is involved in pulmonary fibrosis. However, the role of LTBP2 in differentially diagnosing CTD-ILD and IPF is unclear. In this study, enzyme-linked immunosorbent assays quantified plasma LTBP2 concentrations in 200 individuals (35 healthy controls, 42 CTD patients without ILD, 89 CTD-ILD patients, and 34 IPF patients). CTD-ILD and IPF were further classified based on chest imaging pattern and pulmonary function test results. Plasma LTBP2 levels were significantly elevated in the IPF group compared with the CTD-ILD group. ROC analysis further suggested the possible value of LTBP2 in differentially diagnosing CTD-ILD and IPF. Additionally, CTD-ILD patients with progressive lung fibrosis had higher plasma LTBP2 concentrations than those who did not. Similarly, patients with IPF developing acute exacerbation showed higher plasma LTBP2 levels than those with stable IPF. This is the first study showing that LTBP2 was closely associated with the usual interstitial pneumonia (UIP) pattern in rheumatoid arthritis-associated ILD (RA-ILD). Moreover, the optimal cutoff values of LTBP2 for distinguishing IPF from CTD-UIP/RA-UIP were 33.75 and 38.33 ng/mL with an AUC of 0.682 and 0.681, respectively. Our findings suggest that plasma LTBP2 levels may differentially diagnose CTD-ILD and IPF, and assess their fibrotic activity. Additionally, clinical LTBP2 evaluation may be a great aid to identifying the presence of the UIP pattern in RA-ILD and to discriminating IPF from CTD-UIP, particularly RA-UIP.

Peptidylarginine deiminase 2 citrullinates MZB1 and promotes the secretion of IgM and IgA

Introduction

MZB1 is an endoplasmic reticulum residential protein preferentially expressed in plasma cells, marginal zone and B1 B cells. Recent studies on murine B cells show that it interacts with the tail piece of IgM and IgA heavy chain and promotes the secretion of these two classes of immunoglobulin. However, its role in primary human B cells has yet to be determined and how its function is regulated is still unknown. The conversion of peptidylarginine to peptidylcitrulline, also known as citrullination, by peptidylarginine deiminases (PADs) can critically influence the function of proteins in immune cells, such as neutrophils and T cells; however, the role of PADs in B cells remains to be elucidated.

Method

An unbiased analysis of human lung citrullinome was conducted to identify citrullinated proteins that are enriched in several chronic lung diseases, including rheumatoid arthritis-associated interstitial lung disease (RA-ILD), chronic obstructive pulmonary disease, and idiopathic pulmonary fibrosis, compared to healthy controls. Mass spectrometry, site-specific mutagenesis, and western blotting were used to confirm the citrullination of candidate proteins. Their citrullination was suppressed by pharmacological inhibition or genetic ablation of PAD2 and the impact of their citrullination on the function and differentiation of human B cells was examined with enzyme-linked immunosorbent assay, flow cytometry, and co-immunoprecipitation.

Results

Citrullinated MZB1 was preferentially enriched in RA-ILD but not in other chronic lung diseases. MZB1 was a substrate of PAD2 and was citrullinated during the differentiation of human plasmablasts. Ablation or pharmacological inhibition of PAD2 in primary human B cells attenuated the secretion of IgM and IgA but not IgG or the differentiation of IgM or IgA-expressing plasmablasts, recapitulating the effect of ablating MZB1. Furthermore, the physical interaction between endogenous MZB1 and IgM/IgA was attenuated by pharmacological inhibition of PAD2.

Discussion

Our data confirm the function of MZB1 in primary human plasmablasts and suggest that PAD2 promotes IgM/IgA secretion by citrullinating MZB1, thereby contributing to the pathogenesis of rheumatoid arthritis and RA-ILD.

Citrullination of matrisomal proteins in health and diseases

Proteins once translated are subjected to post-translational modifications (PTMs) that can critically modify their characteristics. Citrullination is a unique type of PTM that is catalysed by peptidylarginine deiminase (PAD) enzymes, which regulate a multitude of physiological functions such as apoptosis, gene expression and immune response by altering the structure and function of cellular proteins. However, emerging data have unravelled compelling evidence to support that PAD-mediated citrullination is not exclusive to cellular proteins; rather citrullination of extracellular matrix (ECM) proteins also plays a major contributing role in various physiological/pathological conditions. Here, we discuss putative mechanisms for citrullination-induced alterations in the function of ECM proteins. Further, we put emphasis on influential roles of ECM citrullination in various pathological scenarios to underscore the clinical potential of its manipulation in human diseases. This article is part of the Theo Murphy meeting issue 'The virtues and vices of protein citrullination'.

PADI6: What we know about the elusive fifth member of the peptidyl arginine deiminase family

Peptidyl arginine deiminase 6 (PADI6) is a maternal factor that is vital for early embryonic development. Deletion and mutations of its encoding gene in female mice or women lead to early embryonic developmental arrest, female infertility, maternal imprinting defects and hyperproliferation of the trophoblast. PADI6 is the fifth and least well-characterized member of the peptidyl arginine deiminases (PADIs), which catalyse the post-translational conversion of arginine to citrulline. It is less conserved than the other PADIs, and currently has no reported catalytic activity. While there are many suggested functions of PADI6 in the early mouse embryo, including in embryonic genome activation, cytoplasmic lattice formation, maternal mRNA and ribosome regulation, and organelle distribution, the molecular mechanisms of its function remain unknown. In this review, we discuss what is known about the function of PADI6 and highlight key outstanding questions that must be answered if we are to understand the crucial role it plays in early embryo development and female fertility. This article is part of the Theo Murphy meeting issue 'The virtues and vices of protein citrullination'.

Presence of anti-modified protein antibodies in idiopathic pulmonary fibrosis

BACKGROUND AND OBJECTIVE

Studies of autoimmunity and anti-citrullinated protein antibodies (ACPA) in idiopathic pulmonary fibrosis (IPF) have been confined to investigations of anti-cyclic citrullinated peptide (anti-CCP) antibodies which utilize synthetic peptides as surrogate markers for in vivo citrullinated antigens. We studied immune activation by analysing the prevalence of in vivo anti-modified protein antibodies (AMPA) in IPF.

METHODS

We included patients with incident and prevalent IPF (N = 120), sex and smoking-matched healthy controls (HC) (N = 120) and patients with RA (N = 104). Serum (median time: 11 months [Q1-Q3: 1-28 months] from diagnosis) was analysed for presence of antibodies towards native and posttranslational modified (citrullinated [Cit, N = 25]; acetylated [Acet, N = 4] and homocitrullinated [Carb, N = 1]) peptides derived from tenascin (TNC, N = 9), fibrinogen (Fib, N = 11), filaggrin (Fil, N = 5), histone (N = 8), cathelicidin (LL37, N = 4) and vimentin (N = 5) using a custom-made peptide microarray.

RESULTS

AMPA were more frequent and in increased levels in IPF than in HC (44% vs. 27%, p < 0.01), but less than in RA (44% vs. 79%, p < 0.01). We specifically observed AMPA in IPF towards certain citrullinated, acetylated and carbamylated peptides versus HC: tenascin (Cit(2033) -TNC2025-2040 ; Cit(2197) -TNC2177-2200 ; Cit(2198) -TNC2177-2200 ), fibrinogen (Cit(38,42) -Fibα36-50 ; Cit(72) -Fibβ60-74 ) and filaggrin (Acet-Fil307-324 , Carb-Fil307-324 ). No differences in survival (p = 0.13) or disease progression (p = 0.19) between individuals with or without AMPA was observed in IPF. However, patients with incident IPF had better survival if AMPA were present (p = 0.009).

CONCLUSION

A significant proportion of IPF patients present with specific AMPA in serum. Our results suggest autoimmunity as a possible characteristic for a subgroup of IPF that may affect disease outcome.

Etiology and Pathogenesis of Rheumatoid Arthritis-Interstitial Lung Disease

Interstitial lung disease (ILD) is one of the most serious extra-articular complications of rheumatoid arthritis (RA), which increases the mortality of RA. Because the pathogenesis of RA-ILD remains poorly understood, appropriate therapeutic strategies and biomarkers have not yet been identified. Thus, the goal of this review was to summarize and analyze the reported data on the etiology and pathogenesis of RA-ILD. The incidence of RA-ILD increases with age, and is also generally higher in men than in women and in patients with specific genetic variations and ethnicity. Lifestyle factors associated with an increased risk of RA-ILD include smoking and exposure to pollutants. The presence of an anti-cyclic citrullinated peptide antibody, high RA disease activity, and rheumatoid factor positivity also increase the risk of RA-ILD. We also explored the roles of biological processes (e.g., fibroblast-myofibroblast transition, epithelial-mesenchymal transition, and immunological processes), signaling pathways (e.g., JAK/STAT and PI3K/Akt), and the histopathology of RA involved in RA-ILD pathogenesis based on published preclinical and clinical models of RA-ILD in animal and human studies.

Molecular Factors in PAD2 (PADI2) and PAD4 (PADI4) Are Associated with Interstitial Lung Disease Susceptibility in Rheumatoid Arthritis Patients

Around 50% of rheumatoid arthritis (RA) patients show some extra-articular manifestation, with the lung a usually affected organ; in addition, the presence of anti-citrullinated protein antibodies (ACPA) is a common feature, which is caused by protein citrullination modifications, catalyzed by the peptidyl arginine deiminases (PAD) enzymes. We aimed to identify single nucleotide variants (SNV) in PADI2 and PADI4 genes (PAD2 and PAD4 proteins, respectively) associated with susceptibility to interstitial lung disease (ILD) in RA patients and the PAD2 and PAD4 levels. Material and methods: 867 subjects were included: 118 RA-ILD patients, 133 RA patients, and 616 clinically healthy subjects (CHS). Allelic discrimination was performed in eight SNVs using qPCR, four in PADI2 and four in PADI4. The ELISA technique determined PAD2 and PAD4 levels in serum and bronchoalveolar lavage (BAL) samples, and the population structure was evaluated using 14 informative ancestry markers. Results: The rs1005753-GG (OR = 4.9) in PADI2 and rs11203366-AA (OR = 3.08), rs11203367-GG (OR = 2.4) in PADI4 are associated with genetic susceptibility to RA-ILD as well as the ACTC haplotype (OR = 2.64). In addition, the PAD4 protein is increased in RA-ILD individuals harboring the minor allele homozygous genotype in PADI4 SNVs. Moreover, rs1748033 in PADI4, rs2057094, and rs2076615 in PADI2 are associated with RA susceptibility. In conclusion, in RA patients, single nucleotide variants in PADI4 and PADI2 are associated with ILD susceptibility. The rs1748033 in PADI4 and two different SNVs in PADI2 are associated with RA development but not ILD. PAD4 serum levels are increased in RA-ILD patients.

Integrating bioinformatic resources to identify characteristics of rheumatoid arthritis-related usual interstitial pneumonia

BACKGROUND

Rheumatoid arthritis (RA) is often accompanied by a common extra-articular manifestation known as RA-related usual interstitial pneumonia (RA-UIP), which is associated with a poor prognosis. However, the mechanism remains unclear. To identify potential mechanisms, we conducted bioinformatics analysis based on high-throughput sequencing of the Gene Expression Omnibus (GEO) database.

RESULTS

Weighted gene co-expression network analysis (WGCNA) analysis identified 2 RA-positive related modules and 4 idiopathic pulmonary fibrosis (IPF)-positive related modules. A total of 553 overlapped differentially expressed genes (DEG) were obtained, of which 144 in the above modules were further analyzed. The biological process of "oxidative phosphorylation" was found to be the most relevant with both RA and IPF. Additionally, 498 up-regulated genes in lung tissues of RA-UIP were screened out and enriched by 7 clusters, of which 3 were closely related to immune regulation. The analysis of immune infiltration showed a characteristic distribution of peripheral immune cells in RA-UIP, compared with IPF-UIP in lung tissues.

CONCLUSIONS

These results describe the complex molecular and functional landscape of RA-UIP, which will help illustrate the molecular pathological mechanism of RA-UIP and identify new biomarkers and therapeutic targets for RA-UIP in the future.

Factors associated with interstitial lung disease in patients with rheumatoid arthritis: A systematic review and meta-analysis

OBJECTIVES

Interstitial lung disease (ILD) is frequent in patients with rheumatoid arthritis (RA) and is a potentially life-threatening complication with significant morbidity and mortality. This meta-analysis aims to systematically determine the factors associated with the development of rheumatoid arthritis-related interstitial lung disease (RA-ILD).

MATERIALS AND METHODS

All primary studies which reported the factors associated with of RA-ILD were eligible for the review except case reports. The Cochrane Library, PubMed, Embase, Web of Science, Chinese Biological Medicine Database (CBM), China National Knowledge Infrastructure (CNKI), and WANFANG electronic databases were searched through to December 30, 2022, for studies investigating the factors associated with RA-ILD. The methodological quality assessment of the eligible studies was performed using the Newcastle-Ottawa Scale (NOS). 2 reviewers extracted relevant data independently. Then, weighed mean differences (WMDs) or pooled odds ratios (ORs) and corresponding 95% confidence intervals (CIs) were obtained for the relationships between the factors and RA-ILD. The statistical meta-analysis, subgroup and sensitivity analyses were performed using the Review Manager 5.3, and publication bias with Egger's test were performed using the Stata12.0 software.

RESULTS

A total of 22 articles were screened for a meta-analysis which involved 1887 RA-ILD patients and 8066 RA without ILD patients. Some identified factors that were associated with an increased risk of RA-ILD included male sex (OR = 1.92, 95% CI: 1.54-2.39; P < 0.00001), older age (WMD = 5.77 years, 95% CI: 3.50-8.04; P < 0.00001), longer duration of RA (WMD = 0.80 years, 95% CI 0.12-1.47; P = 0.02), older age at onset of RA (WMD = 6.41 years, 95% CI: 3.17-9.64; P = 0.0001), smoking (OR = 1.69, 95% CI: 1.30-2.18; P < 0.0001). Five factors of laboratory items associated with the development of RA-ILD were evaluated in the meta-analysis. Compared with RA without ILD patients, positive rheumatoid factor (RF) (OR = 1.72, 95% CI: 1.47-2.01; P < 0.00001) and positive anti-citrullinated protein antibodies (ACPA) (OR = 1.58, 95% CI: 1.31-1.90; P < 0.00001) increased the risk of RA-ILD. Meanwhile, RF titer (WMD = 183.62 (IU/mL), 95% CI: 66.94-300.30; P = 0.002) and ACPA titer (WMD = 194.18 (IU/mL), 95% CI: 115.89-272.47; P < 0.00001) were significantly associated with increased risk of RA-ILD. Elevated erythrocyte sedimentation rate (ESR) (WMD = 7.41 (mm/h), 95% CI: 2.21-12.61; P = 0.005) and C-reactive protein (CRP) (WMD = 4.98 (mg/L), 95% CI: 0.76-9.20; P = 0.02) were also significantly associated with the development of the RA-ILD, whereas antinuclear antibody (ANA) positive status was not significantly associated with increased risk of RA-ILD (OR = 1.27, 95% CI: 1.00-1.60; P = 0.05).

CONCLUSIONS

This meta-analysis showed that male gender, older age, longer duration of RA, older age at onset of RA, smoking, positive RF, positive ACPA, elevated RF titer, elevated ACPA titer, higher ESR and higher CRP were associated with RA-ILD.

Comparison of the diagnostic criteria for progressive pulmonary fibrosis in connective tissue disease related interstitial lung disease

BACKGROUND

Progressive pulmonary fibrosis (PPF) is possible among patients with connective tissue disease (CTD) related interstitial lung disease (ILD). Our aim herein was to compare the prevalence and clinical characteristics of patients with CTD-ILD, with and without PPF, according to the different diagnostic criteria currently used in practice.

METHODS

This retrospective study included patients diagnosed with CTD-ILD, with a ≥1-year follow-up of their lung function, at a single tertiary hospital in South Korea. Diagnostic criteria from two clinical trials (RELIEF and TRAIL1) and from a recently updated guideline (ATS/ERS/JRS/ALAT) were applied.

RESULTS

Of the 107 patients included, 80% tested positive for Sjogren's disease, rheumatoid arthritis, and systemic sclerosis. The prevalence of CTD-ILD with PPF for the different diagnostic criteria was as follows: RELIEF, 25.2%; TRAIL1, 20.6%; and ATS/ERS/JRS/ALAT, 38.3%. A previous history of pulmonary tuberculosis and less positivity for antinuclear antibodies were identified in the PPF group. The radiologic pattern of ILD did not differ between patients with and without PPF, with a usual interstitial pneumonia pattern identified in 34.6% of the patients, independent of the diagnostic criteria used. Systemic steroids and immunomodulatory agents were used in about 80% of patients with PPF and 50% without PPF, irrespective of the diagnostic criteria used. Antifibrotic therapy was used in a limited number of patients in both groups.

CONCLUSIONS

The proportion of patients with CTD-ILD and PPF was higher for the ATS/ERS/JRS/ALAT guideline criteria, without a between-group difference in clinical characteristics.

Role of the PADI family in inflammatory autoimmune diseases and cancers: A systematic review

The peptidyl arginine deiminase (PADI) family is a calcium ion-dependent group of isozymes with sequence similarity that catalyze the citrullination of proteins. Histones can serve as the target substrate of PADI family isozymes, and therefore, the PADI family is involved in NETosis and the secretion of inflammatory cytokines. Thus, the PADI family is associated with the development of inflammatory autoimmune diseases and cancer, reproductive development, and other related diseases. In this review, we systematically discuss the role of the PADI family in the pathogenesis of various diseases based on studies from the past decade to provide a reference for future research.

Nicotine and Cotinine Induce Neutrophil Extracellular Trap Formation-Potential Risk for Impaired Wound Healing in Smokers

Smoking undoubtedly affects human health. Investigating 2318 representative patients at a level 1 trauma center identified delayed wound healing, tissue infections, and/or sepsis as main complications in smokers following trauma and orthopedic surgery. Therefore, smoking cessation is strongly advised to improve the clinical outcome in these patients, although smoking cessation often fails despite nicotine replacement therapy raising the need for specific interventions that may reduce the complication rate. However, the underlying mechanisms are still unknown. In diabetics, delayed wound healing and infections/sepsis are associated with increased neutrophilic PADI4 expression and formation of neutrophil extracellular traps (NETs). The aim was to investigate if similar mechanisms hold for smokers. Indeed, our results show higher PADI4 expression in active and heavy smokers than non-smokers, which is associated with an increased complication rate. However, in vitro stimulation of neutrophils with cigarette smoke extract (CSE) only moderately induced NET formation despite accumulation of reactive oxygen species (ROS). Physiological levels of nicotine and its main metabolite cotinine more effectively induced NET formation, although they did not actively induce the formation of ROS, but interfered with the activity of enzymes involved in anti-oxidative defense and NET formation. In summary, we propose increased formation of NETs as possible triggers for delayed wound healing, tissue infections, and/or sepsis in smokers after a major trauma and orthopedic surgery. Smoking cessation might reduce this effect. However, our data show that smoking cessation supported by nicotine replacement therapy should be carefully considered as nicotine and its metabolite cotinine effectively induced NET formation in vitro, even without active formation of ROS.

Pulmonary fibrosis associated with rheumatoid arthritis: from pathophysiology to treatment strategies

INTRODUCTION

Rheumatoid arthritis (RA) is the most common inflammatory autoimmune disease, characterised by symmetric destructive arthritis and synovitis. Lung involvement is frequent, including in the form of interstitial lung disease (ILD). RA-ILD often presents with a radiologic and pathologic pattern of usual interstitial pneumonia, similar to idiopathic pulmonary fibrosis, highlighting the similarities between the two diseases, but other patterns and pathological associations are described.

AREAS COVERED

This article reviews the pathogenesis of pulmonary fibrosis in the setting of rheumatoid arthritis as well as the current and future therapeutic options.

EXPERT OPINION

Pulmonary fibrosis in the setting of RA-ILD is an example of genotype-environment interaction and involves multiple mechanisms including autoimmunity, inflammation and fibrogenesis. Despite that ILD conveys most of the exceeding mortality in RA patients, there are no official guidelines for the management of RA-ILD. Attention should be paid to potential lung toxicity of RA treatment even though some of them might help stabilise the ILD. Current standard of care is often composed of glucocorticoids that may be associated with immunosuppressive therapy. Following the approval of antifibrotic therapy for ILDs with a progressive fibrosing phenotype, current works are evaluating the benefit of such treatment in RA-ILD.

Novel Biomarkers, Diagnostic and Therapeutic Approach in Rheumatoid Arthritis Interstitial Lung Disease-A Narrative Review

Rheumatoid arthritis (RA) is considered a systemic inflammatory disease marked by polyarthritis which affects the joints symmetrically, leading to progressive damage of the bone structure and eventually joint deformity. Lung involvement is the most prevalent extra-articular feature of RA, affecting 10-60% of patients with this disease. In this review, we aim to discuss the patterns of RA interstitial lung disease (ILD), the molecular mechanisms involved in the pathogenesis of ILD in RA, and also the therapeutic challenges in this particular extra-articular manifestation. The pathophysiology of RA-ILD has been linked to biomarkers such as anti-citrullinated protein antibodies (ACPAs), MUC5B mutation, Krebs von den Lungen 6 (KL-6), and other environmental factors such as smoking. Patients at the highest risk for RA-ILD and those most likely to advance will be identified using biomarkers. The hope is that finding biomarkers with good performance characteristics would help researchers better understand the pathophysiology of RA-ILD and, in turn, lead to the development of tailored therapeutics for this severe RA manifestation.

Syndecan-2 regulates PAD2 to exert antifibrotic effects on RA-ILD fibroblasts

Rheumatoid arthritis (RA)-associated interstitial lung disease (RA-ILD) is the most common pulmonary complication of RA, increasing morbidity and mortality. Anti-citrullinated protein antibodies have been associated with the development and progression of both RA and fibrotic lung disease; however, the role of protein citrullination in RA-ILD remains unclear. Here, we demonstrate that the expression of peptidylarginine deiminase 2 (PAD2), an enzyme that catalyzes protein citrullination, is increased in lung homogenates from subjects with RA-ILD and their lung fibroblasts. Chemical inhibition or genetic knockdown of PAD2 in RA-ILD fibroblasts attenuated their activation, marked by decreased myofibroblast differentiation, gel contraction, and extracellular matrix gene expression. Treatment of RA-ILD fibroblasts with the proteoglycan syndecan-2 (SDC2) yielded similar antifibrotic effects through regulation of PAD2 expression, phosphoinositide 3-kinase/Akt signaling, and Sp1 activation in a CD148-dependent manner. Furthermore, SDC2-transgenic mice exposed to bleomycin-induced lung injury in an inflammatory arthritis model expressed lower levels of PAD2 and were protected from the development of pulmonary fibrosis. Together, our results support a SDC2-sensitive profibrotic role for PAD2 in RA-ILD fibroblasts and identify PAD2 as a promising therapeutic target of RA-ILD.

Latent Transforming Growth Factor-β Binding Protein-2 Regulates Lung Fibroblast-to-Myofibroblast Differentiation in Pulmonary Fibrosis via NF-κB Signaling

Despite past extensive studies, the mechanisms underlying pulmonary fibrosis (PF) still remain poorly understood. The aberrantly activated lung myofibroblasts, predominantly emerging through fibroblast-to-myofibroblast differentiation, are considered to be the key cells in PF, resulting in excessive accumulation of extracellular matrix (ECM). Latent transforming growth factor-β (TGFβ) binding protein-2 (LTBP2) has been suggested as playing a critical role in modulating the structural integrity of the ECM. However, its function in PF remains unclear. Here, we demonstrated that lungs originating from different types of patients with PF, including idiopathic PF and rheumatoid arthritis-associated interstitial lung disease, and from mice following bleomycin (BLM)-induced PF were characterized by increased LTBP2 expression in activated lung fibroblasts/myofibroblasts. Moreover, serum LTBP2 was also elevated in patients with COVID-19-related PF. LTBP2 silencing by lentiviral shRNA transfection protected against BLM-induced PF and suppressed fibroblast-to-myofibroblast differentiation in vivo and in vitro. More importantly, LTBP2 overexpression was able to induce differentiation of lung fibroblasts to myofibroblasts in vitro, even in the absence of TGFβ1. By further mechanistic analysis, we demonstrated that LTBP2 silencing prevented fibroblast-to-myofibroblast differentiation and subsequent PF by suppressing the phosphorylation and nuclear translocation of NF-κB signaling. LTBP2 overexpression-induced fibroblast-to-myofibroblast differentiation depended on the activation of NF-κB signaling in vitro. Therefore, our data indicate that intervention to silence LTBP2 may represent a promising therapy for PF.

Real-life prevalence of progressive fibrosing interstitial lung diseases

The concept of progressive fibrosing interstitial lung disease (PF-ILD) has recently emerged. However, real-life proportion of PF-ILDs outside IPF is still hard to evaluate. Therefore, we sought to estimate the proportion of PF-ILD in our ILD cohort. We also determined the proportion of ILD subtypes within PF-ILD and investigated factors associated with PF-ILDs. Finally, we quantified interobserver agreement between radiologists for the assessment of fibrosis. We reviewed the files of ILD patients discussed in multidisciplinary discussion between January 1st 2017 and December 31st 2019. Clinical data, pulmonary function tests (PFTs) and high-resolution computed tomography (HRCTs) were centrally reviewed. Fibrosis was defined as the presence of traction bronchiectasis, reticulations with/out honeycombing. Progression was defined as a relative forced vital capacity (FVC) decline of ≥ 10% in ≤ 24 months or 5% < FVC decline < 10% and progression of fibrosis on HRCT in ≤ 24 months. 464 consecutive ILD patients were included. 105 had a diagnosis of IPF (23%). Most frequent non-IPF ILD were connective tissue disease (CTD)-associated ILD (22%), hypersensitivity pneumonitis (13%), unclassifiable ILD (10%) and sarcoidosis (8%). Features of fibrosis were common (82% of CTD-ILD, 81% of HP, 95% of uILD). After review of HRCTs and PFTs, 68 patients (19% of non-IPF ILD) had a PF-ILD according to our criteria. Interobserver agreement for fibrosis between radiologists was excellent (Cohen’s kappa 0.86). The main diagnosis among PF-ILD were CTD-ILD (36%), HP (22%) and uILD (20%). PF-ILD patients were significantly older than non-F-ILD (P = 0.0005). PF-ILDs represent about 20% of ILDs outside IPF. This provides an estimation of the proportion of patients who might benefit from antifibrotics. Interobserver agreement between radiologists for the diagnosis of fibrotic ILD is excellent.

Citrullinated fibrinogen is a target of auto-antibodies in interstitial lung disease in mice with collagen-induced arthritis

Interstitial lung disease (ILD) is a very common and lethal complication of rheumatoid arthritis (RA), yet its pathogenesis is not well understood, in part due to the lack of adequate animal models. Although collagen-induced arthritis (CIA) is the most widely used animal model for RA, the lung involvement occurring in this model has scarcely been studied. To evaluate the suitability of CIA as a model for RA-associated ILD (RA-ILD), we immunized DBA/1 mice with bovine type II collagen and characterized lung disease in this model. Histologic analyses revealed patchy interstitial infiltration of inflammatory cells in the peripheral regions of the lung, notably in the subpleural region, in mice with CIA. This pattern resembled usual interstitial pneumonia in humans, which is the most prevalent pattern in RA-ILD. Among infiltrates in the lung, CD11bhi macrophages of the M2 phenotype were most prominently increased. IgG and C3 were deposited in the subpleural region where inflammatory cells infiltrated. The sera from CIA mice contained auto-antibodies against citrullinated proteins, which are specific and predictive markers for RA. Protein citrullination was enhanced in the lung of CIA mice compared with naive mice, and citrullinated fibrinogen was primarily targeted by these auto-antibodies. The elevation of auto-antibodies against citrullinated proteins and their deposition in the lung with patchy subpleural preponderance suggest that CIA can serve as a model to study the pathogenesis of RA-ILD.

Causal risk and protective factors in rheumatoid arthritis: A genetic update

The characterization of risk and protective factors in complex diseases such as rheumatoid arthritis (RA) has evolved from epidemiological studies, which test association, to the use of Mendelian randomization approaches, which test direct relationships. Indeed, direct associations with the mucosal origin of RA are retrieved with periodontal disease (Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans predominantly), interstitial lung involvement, tobacco smoking and air pollutants. Next, factors directly associated with an acquired immune response include genetic factors (HLA DRB1, PTPN22), capacity to produce anti-modified protein antibodies (AMPA), and relatives with a history of autoimmune diseases. Finally, factors can be also classified according to their direct capacity to interfere with the IL-6/CRP/sIL-IL6R proinflammatory pathway as risk factor (body fat, cardiometabolic factors, type 2 diabetes, depressive syndrome) or either as protective factors by controlling of sIL-6R levels (higher education level, and intelligence). Although some co-founders have been characterized (e.g. vitamin D, physical activity, cancer) the direct association with sex-discrepancy, pregnancy, and infections among other factors remains to be better explored.

Citrullinated vimentin mediates development and progression of lung fibrosis

The mechanisms by which environmental exposures contribute to the pathogenesis of lung fibrosis are unclear. Here, we demonstrate an increase in cadmium (Cd) and carbon black (CB), common components of cigarette smoke (CS) and environmental particulate matter (PM), in lung tissue from subjects with idiopathic pulmonary fibrosis (IPF). Cd concentrations were directly proportional to citrullinated vimentin (Cit-Vim) amounts in lung tissue of subjects with IPF. Cit-Vim amounts were higher in subjects with IPF, especially smokers, which correlated with lung function and were associated with disease manifestations. Cd/CB induced the secretion of Cit-Vim in an Akt1- and peptidylarginine deiminase 2 (PAD2)-dependent manner. Cit-Vim mediated fibroblast invasion in a 3D ex vivo model of human pulmospheres that resulted in higher expression of CD26, collagen, and α-SMA. Cit-Vim activated NF-κB in a TLR4-dependent fashion and induced the production of active TGF-β1, CTGF, and IL-8 along with higher surface expression of TLR4 in lung fibroblasts. To corroborate ex vivo findings, mice treated with Cit-Vim, but not Vim, independently developed a similar pattern of fibrotic tissue remodeling, which was TLR4 dependent. Moreover, wild-type mice, but not PAD2^-/- and TLR4 mutant (MUT) mice, exposed to Cd/CB generated high amounts of Cit-Vim, in both plasma and bronchoalveolar lavage fluid, and developed lung fibrosis in a stereotypic manner. Together, these studies support a role for Cit-Vim as a damage-associated molecular pattern molecule (DAMP) that is generated by lung macrophages in response to environmental Cd/CB exposure. Furthermore, PAD2 might represent a promising target to attenuate Cd/CB-induced fibrosis.

Immune cells in lens injury repair and fibrosis

Immune cells, both tissue resident immune cells and those immune cells recruited in response to wounding or degenerative conditions, are essential to both the maintenance and restoration of homeostasis in most tissues. These cells are typically provided to tissues by their closely associated vasculatures. However, the lens, like many of the tissues in the eye, are considered immune privileged sites because they have no associated vasculature. Such absence of immune cells was thought to protect the lens from inflammatory responses that would bring with them the danger of causing vision impairing opacities. However, it has now been shown, as occurs in other immune privileged sites in the eye, that novel pathways exist by which immune cells come to associate with the lens to protect it, maintain its homeostasis, and function in its regenerative repair. Here we review the discoveries that have revealed there are both innate and adaptive immune system responses to lens, and that, like most other tissues, the lens harbors a population of resident immune cells, which are the sentinels of danger or injury to a tissue. While resident and recruited immune cells are essential elements of lens homeostasis and repair, they also become the agents of disease, particularly as progenitors of pro-fibrogenic myofibroblasts. There still remains much to learn about the function of lens-associated immune cells in protection, repair and disease, the knowledge of which will provide new tools for maintaining the core functions of the lens in the visual system.

Characterization of Citrullination Sites in Neutrophils and Mast Cells Activated by Ionomycin via Integration of Mass Spectrometry and Machine Learning

Citrullination is an important post-translational modification implicated in many diseases including rheumatoid arthritis (RA), Alzheimer's disease, and cancer. Neutrophil and mast cells have different expression profiles for protein-arginine deiminases (PADs), and ionomycin-induced activation makes them an ideal cellular model to study proteins susceptible to citrullination. We performed high-resolution mass spectrometry and stringent data filtration to identify citrullination sites in neutrophil and mast cells treated with and without ionomycin. We identified a total of 833 validated citrullination sites on 395 proteins. Several of these citrullinated proteins are important components of pathways involved in innate immune responses. Using this benchmark primary sequence data set, we developed machine learning models to predict citrullination in neutrophil and mast cell proteins. We show that our models predict citrullination likelihood with 0.735 and 0.766 AUCs (area under the receiver operating characteristic curves), respectively, on independent validation sets. In summary, this study provides the largest number of validated citrullination sites in neutrophil and mast cell proteins. The use of our novel motif analysis approach to predict citrullination sites will facilitate the discovery of novel protein substrates of protein-arginine deiminases (PADs), which may be key to understanding immunopathologies of various diseases.

Protein expression profiling suggests relevance of noncanonical pathways in isolated pulmonary embolism

Patients with isolated pulmonary embolism (PE) have a distinct clinical profile from those with deep vein thrombosis (DVT)-associated PE, with more pulmonary conditions and atherosclerosis. These findings suggest a distinct molecular pathophysiology and the potential involvement of alternative pathways in isolated PE. To test this hypothesis, data from 532 individuals from the Genotyping and Molecular Phenotyping of Venous ThromboEmbolism Project, a multicenter prospective cohort study with extensive biobanking, were analyzed. Targeted, high-throughput proteomics, machine learning, and bioinformatic methods were applied to contrast the acute-phase plasma proteomes of isolated PE patients (n = 96) against those of patients with DVT-associated PE (n = 276) or isolated DVT (n = 160). This resulted in the identification of shared molecular processes between PE phenotypes, as well as an isolated PE-specific protein signature. Shared processes included upregulation of inflammation, response to oxidative stress, and the loss of pulmonary surfactant. The isolated PE-specific signature consisted of 5 proteins: interferon-γ, glial cell line-derived neurotrophic growth factor, polypeptide N-acetylgalactosaminyltransferase 3, peptidyl arginine deiminase type-2, and interleukin-15 receptor subunit α. These proteins were orthogonally validated using cis protein quantitative trait loci. External replication in an independent population-based cohort (n = 5778) further validated the proteomic results and showed that they were prognostic for incident primary isolated PE in individuals without history of VTE (median time to event: 2.9 years; interquartile range: 1.6-4.2 years), supporting their possible involvement in the early pathogenesis. This study has identified molecular overlaps and differences between VTE phenotypes. In particular, the results implicate noncanonical pathways more commonly associated with respiratory and atherosclerotic disease in the acute pathophysiology of isolated PE.

A proposed role of neutrophil extracellular traps and their interplay with fibroblasts in ANCA-associated vasculitis lung fibrosis

ANCA-associated vasculitides (AAV) comprise three diseases: granulomatosis with polyangiitis, microscopic polyangiitis (MPA), and eosinophilic granulomatosis with polyangiitis. They are characterised by small vessel inflammation and have a broad range of clinical manifestations and multiorgan involvement which endanger the patient's life. An increasingly recognised complication of AAV, especially in MPA is lung fibrosis, for which no clearcut therapy in this context is available. The release of neutrophil extracellular traps (NETs) in these diseases has been related to the development of fibrosis, but the precise mechanisms are not fully unravelled. This review provides an overview of some of the important proteins known to compose NETs, and proposes some mechanisms by which these remarkable components may exert an impact on the different fibroblastic phenotypes leading to lung fibrosis.

PAD4 Deficiency Improves Bleomycin-induced Neutrophil Extracellular Traps and Fibrosis in Mouse Lung

Excessive release of neutrophil extracellular traps (NETs) has been implicated in several organ fibrosis, including pulmonary fibrosis. NETs constitute a phenomenon in which decorated nuclear chromatin with cytosolic proteins is released into the extracellular space. PAD4 (peptidylarginine deiminase 4) plays an important role in the formation of NETs. However, the role of NETs in the pathogenesis of pulmonary fibrosis remains undefined. Here, we identified NETs in the alveolar and interstitial lung space of mice undergoing bleomycin (BLM)-induced lung fibrosis, which was suppressed by a pan-PAD inhibitor, Cl-amidine. In vitro, BLM directly induced NETs in blood neutrophils, which was also inhibited by Cl-amidine. Furthermore, Padi4 gene knockout (PAD4-KO) in mice led to the alleviation of BLM-induced NETs and pulmonary fibrosis and to the expression of inflammatory and fibrotic genes. PAD4 deficiency prevented decreases in alveolar epithelial and pulmonary vascular endothelial cell numbers and increases in ACTA2-positive mesenchymal cells and S100A4-positive fibroblasts in the lung. Hematopoietic cell grafts from PAD4-KO mice, not wild-type mice, resolved BLM-induced lung fibrosis and fibrotic gene expression in wild-type and PAD4-KO mice, suggesting that expression of PAD4 in hematopoietic cells may be involved in the development of lung fibrosis. These data suggest that PAD4 deficiency could ameliorate BLM-induced formation of NETs and lung fibrosis, suggesting that this pathway could serve as a therapeutic target for pulmonary fibrosis treatment.

The Role of Citrullination in Inflammatory Bowel Disease: A Neglected Player in Triggering Inflammation and Fibrosis?

Citrullination is a posttranslational modification of proteins mediated by a specific family of enzymes called peptidylarginine deiminases (PAD). Dysregulation of these enzymes is involved in the etiology of various diseases, from cancer to autoimmune disorders. In inflammatory bowel disease (IBD), data for a role of citrullination in the disease process are starting to accumulate at different experimental levels including gene expression analyses, RNA, and protein quantifications. Most data have been generated in ulcerative colitis, but data in Crohn disease are lacking so far. In addition, the citrullination of histones is the fundamental process promoting inflammation through the formation of neutrophil extracellular traps (NETs). Interestingly, NETs have also been shown to activate fibroblasts into myofibroblasts in fibrotic interstitial lung disease. Therefore, citrullination merits more thorough study in the bowel to determine its role in driving disease complications such as fibrosis. In this review we describe the process of citrullination and the different players in this pathway, the role of citrullination in autoimmunity with a special focus on IBD, the emerging role for citrullination and NETs in triggering fibrosis, and, finally, how this process could be therapeutically targeted.

Histone Deacetylase 3-Mediated Inhibition of microRNA-19a-3p Facilitates the Development of Rheumatoid Arthritis-Associated Interstitial Lung Disease

Histone deacetylase (HDAC) has been implicated in rheumatoid arthritis (RA) progression. We investigated the roles of histone deacetylase 3 (HDAC3) involved in RA-associated interstitial lung disease (ILD) fibrosis. Firstly, we measured the expression of HDAC3 and interleukin 17 receptor A (IL17RA) in lung tissue samples from normal controls, idiopathic pulmonary fibrosis (IPF) patients, and RA-ILD patients. Next, chromatin immunoprecipitation (ChIP) and dual luciferase reporter assay were employed to detect the interaction between HDAC3 and microRNA-19a-3p (miR-19a-3p) and between miR-19a-3p and IL17RA. Further, immunohistochemistry was used to localize HDAC3 and IL17RA expression in lung tissues. Additionally, functional assays were conducted followed by expression determination of HDAC3, miR-19a-3p, and IL17RA with reverse transcription quantitative PCR (RT-qPCR) and Western blot analysis. The effect of HDAC3 on RA-ILD in the constructed RA-ILD mouse model was also studied based on arthritis assessment. We found overexpressed HDAC3 and IL17RA as well as silenced miR-19a-3p in RA-ILD mouse model and RA-ILD patients. In the mouse model, HDAC3 downregulated miR-19a-3p in lung fibroblasts to promote the progression of RA-ILD fibrosis. In lung fibroblasts of RA-ILD mice, IL17RA was a target gene of miR-19a-3p. miR-19a-3p negatively regulated IL17RA, thereby increasing the expression of fibrosis markers, COL1A1, COL3A1, and FN, in lung fibroblasts of mice. Taken together, HDAC3 upregulated IL17RA expression by targeting miR-19a-3p to facilitate the RA-ILD fibrosis development, which sheds light on a new HDAC3/miR-19a-3p/IL17RA axis functioning in RA-ILD fibrosis.

Reinterpreting Evidence of Rheumatoid Arthritis-Associated Interstitial Lung Disease to Understand Etiology

Interstitial Lung Disease (ILD) is a well-known complication of rheumatoid arthritis (RA) which often results in significant morbidity and mortality. It is often diagnosed late in the disease process via descriptive criteria. Multiple subtypes of RA-ILD exist as defined by chest CT and histopathology. In the absence of formal natural history studies and definitive diagnostics, a conventional dogma has emerged that there are two major subtypes of RA-ILD (nonspecific interstitial pneumonia (NSIP) and Usual Interstitial Pneumonia (UIP)). These subtypes are based on clinical experience and correlation studies. However, recent animal model data are incongruous with established paradigms of RA-ILD and beg reassessment of the clinical evidence in order to better understand etiology, pathogenesis, prognosis, and response to therapy. To this end, here we: 1) review the literature on epidemiology, radiology, histopathology and clinical outcomes of the various RAILD subtypes, existing animal models, and current theories on RA-ILD pathogenesis; 2) highlight the major gaps in our knowledge; and 3) propose future research to test an emerging theory of RAILD that posits initial rheumatic lung inflammation in the form of NSIP-like pathology transforms mesenchymal cells to derive chimeric disease, and subsequently develops into frank UIP-like fibrosis in some RA patients. Elucidation of the pathogenesis of RA-ILD is critical for the development of effective interventions for RA-ILD.

Exacerbation of symptomatic arthritis by cigarette smoke in experimental arthritis

Introduction

Epidemiologically, cigarette smoking is a well-known risk factor for the pathogenesis of rheumatoid arthritis (RA). However, there has been few plausible explanations why cigarette smoking aggravated RA. We investigated the causal effect of smoking in experimental model of arthritis development.

Methods

During induction of experimental arthritis with collagen challenge, mice were exposed to a smoking environment with 3R4F cigarettes. Generated smoke was delivered to mice through a nose-only exposure chamber (ISO standard 3308). Human cartilage pellet was challenged by cigarette smoke extract to identify citrullinating potential in vitro.

Results

Cigarette smoke exacerbated arthritis in a collagen-induced arthritis (CIA) model. Exposure to smoke accelerated the onset of arthritis by 2 weeks compared to the conventional model without smoke. Citrullination of lung tissue as well as tarsal joints were revealed in smoke-aggravated CIA mice. Interestingly, tracheal cartilage was a core organ regarding intensity and area size of citrullination. The trachea might be an interesting organ in viewpoint of sharing cartilage with joint and direct smoke exposure. Anti-CCP antibodies were barely detected in the serum of CIA mice, they were significantly elevated in cigarette smoke group. Citrullinated antigens were increased in the serum of smoke-exposed mice. Lastly, a cigarette smoke extract enhanced human cartilage citrullination in vitro.

Conclusions

Missing link of arthritic mechanism between smoke and RA could be partially explained by tracheal citrullination. To control tracheal cartilage citrullination may be beneficial for preventing arthritis development or aggravation if cigarette smoke is becoming a risk factor to pre-arthritic individual.

Peptidyl Arginine Deiminase 2 (PADI2)-Mediated Arginine Citrullination Modulates Transcription in Cancer

Protein arginine deimination leading to the non-coded amino acid citrulline remains a key question in the field of post-translational modifications ever since its discovery by Rogers and Simmonds in 1958. Citrullination is catalyzed by a family of enzymes called peptidyl arginine deiminases (PADIs). Initially, increased citrullination was associated with autoimmune diseases, including rheumatoid arthritis and multiple sclerosis, as well as other neurological disorders and multiple types of cancer. During the last decade, research efforts have focused on how citrullination contributes to disease pathogenesis by modulating epigenetic events, pluripotency, immunity and transcriptional regulation. However, our knowledge regarding the functional implications of citrullination remains quite limited, so we still do not completely understand its role in physiological and pathological conditions. Here, we review the recently discovered functions of PADI2-mediated citrullination of the C-terminal domain of RNA polymerase II in transcriptional regulation in breast cancer cells and the proposed mechanisms to reshape the transcription regulatory network that promotes cancer progression.

Insights into the study and origin of the citrullinome in rheumatoid arthritis

The presence of autoantibodies and autoreactive T cells to citrullinated proteins and citrullinating enzymes in patients with rheumatoid arthritis (RA), together with the accumulation of citrullinated proteins in rheumatoid joints, provides substantial evidence that dysregulated citrullination is a hallmark feature of RA. However, understanding mechanisms that dysregulate citrullination in RA has important challenges. Citrullination is a normal process in immune and non-immune cells, which is likely activated by different conditions (eg, inflammation) with no pathogenic consequences. In a complex inflammatory environment such as the RA joint, unique strategies are therefore required to dissect specific mechanisms involved in the abnormal production of citrullinated proteins. Here, we will review current models of citrullination in RA and discuss critical components that, in our view, are relevant to understanding the accumulation of citrullinated proteins in the RA joint, collectively referred to as the RA citrullinome. In particular, we will focus on potential caveats in the study of citrullination in RA and will highlight methods to precisely detect citrullinated proteins in complex biological samples, which is a confirmatory approach to mechanistically link the RA citrullinome with unique pathogenic pathways in RA.

IgA Antibodies Directed Against Citrullinated Protein Antigens Are Elevated in Patients With Idiopathic Pulmonary Fibrosis

BACKGROUND

The etiology of idiopathic pulmonary fibrosis (IPF) is unknown. Because it shares genetic, histopathologic, and radiographic features with the fibrosing interstitial lung disease seen in rheumatoid arthritis (RA), the goal of this study was to investigate RA-related autoantibodies in IPF.

METHODS

The study included patients with IPF from two separate cohorts at National Jewish Health and Brigham Women's Hospital (n = 181), general population control subjects (n = 160), and control subjects with disease (n = 86 [40 with RA-usual interstitial pneumonia and 46 with hypersensitivity pneumonitis]). Serum was tested for RA-associated antibodies (including IgG and IgA) to citrullinated protein antigens (ACPA). Lung tissue in 11 patients with IPF was examined for ectopic lymphoid aggregates.

RESULTS

An increased prevalence of ACPA positivity was found in two separate IPF cohorts. In particular, positivity for IgA-ACPA was increased in these two IPF cohorts compared with general population control subjects (21.3% and 24.8% vs 5.6%; P < .01). Patients with IPF were more likely to be IgA-ACPA-positive than IgG-ACPA-positive (23.2% vs 8.3%; P < .01), whereas patients with RA were more likely to be IgG-ACPA-positive than IgA-ACPA-positive (72.5% vs 52.5%; P = .04). There was a strong correlation between IgA-ACPA level and the number of ectopic lymphoid aggregates on lung histologic examination in IPF (r = 0.72; P = .01).

CONCLUSIONS

In this study, IgA-ACPA was elevated in patients with IPF and correlated with lymphoid aggregates in the lung, supporting the theory that IgA-ACPA may play a role in lung disease pathogenesis in a subset of individuals with IPF. Future studies are needed to determine whether this subset of ACPA-positive patients with IPF is distinct from patients with IPF but without antibodies.

Accumulation of damaged mitochondria in alveolar macrophages with reduced OXPHOS related gene expression in IPF

BACKGROUND

Impaired mitochondria homeostasis and function are established hallmarks of aging and increasing evidence suggests a link with lung fibrosis. Mitochondria homeostasis may be also affected in alveolar macrophages (AMs) in idiopathic pulmonary fibrosis (IPF). In this study, we used bronchoalveolar lavage (BAL), a tool for both clinical and research purposes, and a rich source of AMs.

METHODS

BAL samples were examined from 52 patients with IPF and 19 healthy individuals. Measurements of mitochondria reactive oxygen species (mtROS), mitochondria morphology and related gene expression were performed. Additionally, autophagy and mitophagy levels were analysed.

RESULTS

Mitochondria in AMs from IPF patients had prominent morphological defects and impaired transcription paralleled to a significant reduction of mitochondria homeostasis regulators PINK1, PARK2 and NRF1. mtROS, was significantly higher in IPF and associated with reduced expression of mitochondria-encoded oxidative phosphorylation (OXPHOS) genes. Age and decline in lung function correlated with higher mtROS levels. Augmentation of damaged, oxidised mitochondria in IPF AMs however was not coupled to increased macroautophagy and mitophagy, central processes in the maintenance of healthy mitochondria levels.

CONCLUSION

Our results suggest a perturbation of mitochondria homeostasis in alveolar macrophages in IPF.

Citrullination of fibronectin alters integrin clustering and focal adhesion stability promoting stromal cell invasion

The extracellular matrix (ECM) microenvironment is increasingly implicated in the instruction of pathologically relevant cell behaviors, from aberrant transdifferentation to invasion and beyond. Indeed, pathologic ECMs possess a panoply of alterations that provide deleterious instructions to resident cells. Here we demonstrate the precise manner in which the ECM protein fibronectin (FN) undergoes the posttranslational modification citrullination in response to peptidyl-arginine deiminase (PAD), an enzyme associated with innate immune cell activity and implicated in systemic ECM-centric diseases, like cancer, fibrosis and rheumatoid arthritis. FN can be citrullinated in at least 24 locations, 5 of which reside in FN's primary cell-binding domain. Citrullination of FN alters integrin clustering and focal adhesion stability with a concomitant enhancement in force-triggered integrin signaling along the FAK-Src and ILK-Parvin pathways within fibroblasts. In vitro migration and in vivo wound healing studies demonstrate the ability of citrullinated FN to support a more migratory/invasive phenotype that enables more rapid wound closure. These findings highlight the potential of ECM, particularly FN, to "record" inflammatory insults via post-translational modification by inflammation-associated enzymes that are subsequently "read" by resident tissue fibroblasts, establishing a direct link between inflammation and tissue homeostasis and pathogenesis through the matrix.

Molecular Mechanisms of Pulmonary Fibrogenesis and Its Progression to Lung Cancer: A Review

Idiopathic pulmonary fibrosis (IPF) is defined as a specific form of chronic, progressive fibrosing interstitial pneumonia of unknown cause, occurring primarily in older adults, and limited to the lungs. Despite the increasing research interest in the pathogenesis of IPF, unfavorable survival rates remain associated with this condition. Recently, novel therapeutic agents have been shown to control the progression of IPF. However, these drugs do not improve lung function and have not been tested prospectively in patients with IPF and coexisting lung cancer, which is a common comorbidity of IPF. Optimal management of patients with IPF and lung cancer requires understanding of pathogenic mechanisms and molecular pathways that are common to both diseases. This review article reflects the current state of knowledge regarding the pathogenesis of pulmonary fibrosis and summarizes the pathways that are common to IPF and lung cancer by focusing on the molecular mechanisms.

Investigation of key autophagy-and mitophagy-related proteins and gene expression in BALF cells from patients with IPF and RA-ILD

Idiopathic pulmonary fibrosis (IPF) is a chronic and irreversible interstitial lung disease with a poor prognosis and limited therapeutic options. Over the past decade, research efforts have focused on the pathogenetic mechanisms involved in this enigmatic lung disease. Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease often complicated by the development of interstitial lung disease (ILD), leading to high mortality and morbidity. Autophagy is a process regulating the turnover of subcellular components and organelles, and represents a major cellular homeostatic mechanism. Recent evidence suggests a role of autophagy and mitochondrial dysfunction in the development of IPF, focusing on lung fibroblasts and epithelial cells. The aim of this study was to examine the mRNA levels of molecules involved inthe autophagy pathway in bronchoalveolar lavage fluid (BALF)-derived cellsfrom patients with IPF in comparison topatients with RA demonstrating lung involvement (ILD) by RT-qPCR. The significant upregulation of BECLIN1 was observed in patients with RA-ILD compared with those with IPF. Other genes involved in the autophagy pathway were also examined, such as Unc-51 like autophagy activating kinase 1 (ULK1), BCL2 interacting protein 3 (BNIP3) and p62. No differences in the mRNA expression levels of these genes were observed. As regards the selective degradation of mitochondria and mitophagy, similar PTEN-induced putative kinase 1 (PINK1) and PARKIN; E3 ubiquitin ligase (PRKN) expression, as well as PINK1 protein levels, were observed. On the whole, the findings of this study demonstrate an increased expression of BECLIN1 in BALF cells from patients with RA-ILD compared with those from patients with IPF, while similar levels in other key molecules implicating in the autophagy pathway were observed in patients with IPF and RA-ILD.

Accumulation of citrullinated glial fibrillary acidic protein in a mouse model of bile duct ligation-induced hepatic fibrosis

Hepatic stellate cells (HSCs) play pivotal roles in hepatic fibrosis as they synthesize glial fibrillary acidic protein (GFAP), which is increased in activated HSCs. GFAP-expressing HSCs and myofibroblasts accumulate in and around hepatic fibrosis lesions. Peptidylarginine deiminase 2 (PAD2) is responsible for the citrullination of GFAP (cit-GFAP). However, the involvement of PAD2 and cit-GFAP in hepatic fibrosis remains unclear. To determine the expression of PAD2 and cit-GFAP in hepatic fibrosis, C57BL/6 mice underwent bile duct ligation (BDL) or a sham operation. In BDL livers, the expression of PAD2 and its enzyme activity were significantly increased compared with controls. In addition, PAD2-postitive cells were rarely observed in only the portal vein and the small bile duct in sham-operated livers, whereas an increased number of PAD2-positive cells were detected in the bile duct and Glisson’s sheath in BDL livers. Interestingly, PAD2 was colocalized with α-SMA-positive cells and CK19-positive cells in BDL livers, indicating upregulated PAD2 in activated HSCs and portal fibroblasts of the livers of BDL mice. We also found that citrullinated proteins were highly accumulated in the livers of BDL mice compared with controls. Moreover, the expression level of GFAP and the amount of cit-GFAP were higher in BDL livers than in control livers. In correlation with PAD2 localization, cit-GFAP was observed in α-SMA-positive and CK19-positive cells in the livers of BDL mice. These results suggest that the increased expression and activation of PAD2 along with increased citrullinated proteins, specifically cit-GFAP, may play important roles in the pathogenesis of hepatic fibrosis.