Post-Translational Modifications of Proteins: Novel Insights in the Autoimmune Response in Rheumatoid Arthritis

Incidence of seropositive and seronegative rheumatoid arthritis in Denmark: a nationwide population-based study

OBJECTIVE

To investigate and compare trends in incidence rates (IRs) of seropositive and seronegative rheumatoid arthritis (RA) in Denmark using various data sources for serostatus definition.

METHOD

This nationwide population-based cohort study was based on data from Danish healthcare and clinical quality registries between 2000 and 2018. Information on anti-cyclic citrullinated peptide and immunoglobulin M rheumatoid factor was obtained, and definitions of seropositivity according to the number of applied data sources were prespecified. Annual age- and sex-standardized IRs were calculated as the number of incident seropositive and seronegative cases, divided by the number of person-years (PY) in the general population in that given year.

RESULTS

An increasing temporal trend in IR of seropositive RA and a decreasing trend in seronegative RA were observed. The IRs were higher for seropositive RA than for seronegative RA from 2009 onwards, with a widening of the IR gap between 2009 and 2016 regardless of the definition of seropositivity. When combining laboratory- and physician-reported autoantibody information and ICD-10 codes, the IR of seropositive RA in 2018 was approximately twice that of seronegative RA, at 19.0 and 9.0 per 100 000 PY, respectively. The level of antibody testing increased significantly during the study period.

CONCLUSIONS

The IR of seropositive RA increased over time, whereas the IR of seronegative RA decreased. Temporal IR changes may be caused by a real change in the RA serology subtypes, an increase in autoantibody testing and availability, changes in registration practice over time, or a combination of these factors.

Epidemiology and Risk Factors for Rheumatoid Arthritis Development

Rheumatoid arthritis (RA) is a prevalent chronic inflammatory arthritis worldwide, significantly impacting patients and population health. The disease affects women primarily, with a female-to-male ratio of three to one. Its pathogenesis is multifactorial, including genetic and environmental risk factors. Epidemiological studies highlight the link between the environment and genetic susceptibility to RA. The so-called shared epitope is the most significant risk factor that seems to act synergetic with other environmental factors in the disease occurrence. In addition, recent findings suggest a potential role of new substantial environmental factors, such as the observed pollution of the planet's natural resources, on the susceptibility and progression of the disease. This review summarises the most decisive evidence on epidemiology and genetic, environmental, and lifestyle risk factors for RA. It shows that studying genetic and environmental factors in correlation could lead to prevention strategies that may impact the natural history of the disease.

Anticitrullinated antibodies recognize rheumatoid arthritis associated T-cell epitopes modified by bacterial L-asparaginase

Citrullinated proteins and anti-citrullinated protein antibodies (ACPAs) play an important role in the pathogenesis of rheumatoid arthritis (RA). It has been suggested that during inflammation or dysbiosis, bacteria could initiate production of ACPAs. Most patients with RA are seropositive for ACPAs, but these antibodies have overlapping reactivity to different posttranslational modifications (PTMs). For initiation and development of RA, T lymphocytes and T cell epitopes are still required. In this study, we evaluated the ability of bacterial L-asparaginase to modify RA-related T cell epitopes within type II collagen (CII259-273 and CII311-325), as well as whether these modified epitopes are recognized by ACPAs from RA patients. We included 12 patients with early RA and 11 healthy subjects selected according to predefined specific criteria. LC-MS/MS analyses revealed that the bacterial L-asparaginase can modify investigated T cell epitopes. ELISA tests showed cross-reactivity of ACPA positive sera from early RA patients towards the enzymatically modified immunodominant T cell epitopes within type II collagen (CII), but not to the modified irrelevant peptides. These data suggest that the cross-reactive ACPAs recognize the "carbonyl-Gly-Pro" motif in CII. Moreover, the T cell recognition of the modified major immunodominant T cell epitope Gal264-CII259-273 was not affected. This epitope was still able to activate autoreactive T cells from early RA patients. It is likely that such modifications are the missing link between the T cell priming and the development of anti-modified protein antibodies (AMPAs). Our results provide additional information on the etiology and pathogenesis of RA.

Citrullinated and malondialdehyde-acetaldehyde modified fibrinogen activates macrophages and promotes an aggressive synovial fibroblast phenotype in patients with rheumatoid arthritis

Objective

Post-translational protein modifications with malondialdehyde-acetaldehyde (MAA) and citrulline (CIT) are implicated in the pathogenesis of rheumatoid arthritis (RA). Although precise mechanisms have not been elucidated, macrophage-fibroblast interactions have been proposed to play a central role in the development and progression of RA. The purpose of our study was to evaluate the downstream effects of macrophage released soluble mediators, following stimulation with fibrinogen (FIB) modified antigens, on human fibroblast-like synoviocytes (HFLS).

Methods

PMA-treated U-937 monocytes (Mϕ) and macrophage-differentiated peripheral blood mononuclear cells (MP) were stimulated with FIB, FIB-MAA, FIB-CIT, or FIB-MAA-CIT. HFLS-RA cells were stimulated directly with FIB antigens or with supernatants (SN) from macrophages (Mϕ-SN or MP-SN) stimulated with FIB antigens. Genes associated with an aggressive HFLS phenotype, extracellular matrix proteins, and activated signaling pathways were evaluated.

Results

HFLS-RA cells treated with Mϕ-SNFIB-CIT and Mϕ-SNFIB-MAA-CIT demonstrated significant increases in mRNA expression of genes associated with an aggressive phenotype at 24-h as compared to direct stimulation with the same antigens. Similar results were obtained using MP-SN. Cellular morphology was altered and protein expression of vimentin (p<0.0001 vs. Mϕ-SNFIB) and type II collagen (p<0.0001) were significantly increased in HFLS-RA cells treated with any of the Mϕ-SN generated following stimulation with modified antigens. Phosphorylation of JNK, Erk1/2, and Akt were increased most substantially in HFLS-RA treated with Mϕ-SNFIB-MAA-CIT (p<0.05 vs Mϕ-SNFIB). These and other data suggested the presence of PDGF-BB in Mϕ-SN. Mϕ-SNFIB-MAA-CIT contained the highest concentration of PDGF-BB (p<0.0001 vs. Mϕ-SNFIB) followed by Mϕ-SNFIB-CIT then Mϕ-SNFIB-MAA. HFLS-RA cells treated with PDGF-BB showed similar cellular morphology to the Mϕ-SN generated following stimulation with modified FIB, as well as the increased expression of vimentin, type II collagen, and the phosphorylation of JNK, Erk1/2 and Akt signaling molecules.

Conclusion

Together, these findings support the hypothesis that in response to MAA-modified and/or citrullinated fibrinogen, macrophages release soluble factors including PDGF-BB that induce fibroblast activation and promote an aggressive fibroblast phenotype. These cellular responses were most robust following macrophage activation with dually modified fibrinogen, compared to single modification alone, providing novel insights into the combined role of multiple post-translational protein modifications in the development of RA.

The Role of Autophagy as a Trigger of Post-Translational Modifications of Proteins and Extracellular Vesicles in the Pathogenesis of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic systemic autoimmune disease, characterized by persistent joint inflammation, leading to cartilage and bone destruction. Autoantibody production is directed to post-translational modified (PTM) proteins, i.e., citrullinated or carbamylated. Autophagy may be the common feature in several types of stress (smoking, joint injury, and infections) and may be involved in post-translational modifications (PTMs) in proteins and the generation of citrullinated and carbamylated peptides recognized by the immune system in RA patients, with a consequent breakage of tolerance. Interestingly, autophagy actively provides information to neighboring cells via a process called secretory autophagy. Secretory autophagy combines the autophagy machinery with the secretion of cellular content via extracellular vesicles (EVs). A role for exosomes in RA pathogenesis has been recently demonstrated. Exosomes are involved in intercellular communications, and upregulated proteins and RNAs may contribute to the development of inflammatory arthritis and the progression of RA. In RA, most of the exosomes are produced by leukocytes and synoviocytes, which are loaded with PTM proteins, mainly citrullinated proteins, inflammatory molecules, and enzymes that are implicated in RA pathogenesis. Microvesicles derived from cell plasma membrane may also be loaded with PTM proteins, playing a role in the immunopathogenesis of RA. An analysis of changes in EV profiles, including PTM proteins, could be a useful tool for the prevention of inflammation in RA patients and help in the discovery of personalized medicine.

Oxidative Stress as a Regulatory Checkpoint in the Production of Antiphospholipid Autoantibodies: The Protective Role of NRF2 Pathway

Oxidative stress is a well-known hallmark of Antiphospholipid Antibody Syndrome (APS), a systemic autoimmune disease characterized by arterial and venous thrombosis and/or pregnancy morbidity. Oxidative stress may affect various signaling pathways and biological processes, promoting dysfunctional immune responses and inflammation, inducing apoptosis, deregulating autophagy and impairing mitochondrial function. The chronic oxidative stress and the dysregulation of the immune system leads to the loss of tolerance, which drives autoantibody production and inflammation with the development of endothelial dysfunction. In particular, anti-phospholipid antibodies (aPL), which target phospholipids and/or phospholipid binding proteins, mainly β-glycoprotein I (β-GPI), play a functional role in the cell signal transduction pathway(s), thus contributing to oxidative stress and thrombotic events. An oxidation-antioxidant imbalance may be detected in the blood of patients with APS as a reflection of disease progression. This review focuses on functional evidence highlighting the role of oxidative stress in the initiation and progression of APS. The protective role of food supplements and Nuclear Factor Erythroid 2-Related Factor 2 (NRF2) activators in APS patients will be summarized to point out the potential of these therapeutic approaches to reduce APS-related clinical complications.

A study of the roles of some immunological biomarkers in the diagnosis of rheumatoid arthritis

Autoimmune rheumatoid arthritis (RA) is a systemic condition closely correlated with a variety of autoantibodies (Abs) that could be considered diagnostic and prognostic markers. The current research was designed to detect the diagnostic values for a number (n) of these auto-Abs in RA detection and to evaluate the accuracy of a combined diagnostic scheme. This prospective study was conducted between September 2021 and August 2022 and included 110 subjects with RA, 70 individuals with other autoimmune disorders as positive controls (PC), and 50 unrelated, apparently healthy individuals as healthy controls (HC). The eligibility criteria for all study groups were followed stringently. An enzyme-linked immunosorbent assay (ELISA) was employed to measure rheumatoid factors (RF), cyclic citrullinated peptide antibodies (CCP-Abs), mutated citrullinated vimentin antibodies (MCV-Abs), anti-perinuclear factor antibodies (APF-Abs), and anti-keratin antibodies (AKA). We calculated the specificity, sensitivity, and predictive values of all auto-Abs. Significantly higher levels of anti-CCP-Abs, anti-MCV-Abs, APF-Abs, and AKAs were reported in the RA patients compared to the HC and PC subjects. RF levels, however, were only statistically elevated when compared to the HC individuals. Anti-APF-Abs had a higher sensitivity rate (70.9%), and anti-CCP-Abs had a higher specificity rate (94.16%) compared to other auto-Abs, whereas the combined detection scheme revealed a higher sensitivity (81.81%) and excellent specificity (90.83%) compared to the two former auto-Abs. Anti-perinuclear factor-Ab was a highly sensitive test, and CCP-Ab was a surpassingly specific assay for identifying RA. Furthermore, the combined detection scheme is an essential serological approach for RA diagnosis and crucial in differentiating this disease from other autoimmune diseases, thus promoting early diagnosis and treatment.

Tailored therapeutic decision of rheumatoid arthritis using proteomic strategies: how to start and when to stop?

Unpredictable treatment responses have been an obstacle for the successful management of rheumatoid arthritis. Although numerous serum proteins have been proposed, there is a lack of integrative survey to compare their relevance in predicting treatment outcomes in rheumatoid arthritis. Also, little is known about their applications in various treatment stages, such as dose modification, drug switching or withdrawal. Here we present an in-depth exploration of the potential usefulness of serum proteins in clinical decision-making and unveil the spectrum of immunopathology underlying responders to different drugs. Patients with robust autoimmunity and inflammation are more responsive to biological treatments and prone to relapse during treatment de-escalation. Moreover, the concentration changes of serum proteins at the beginning of the treatments possibly assist early recognition of treatment responders. With a better understanding of the relationship between the serum proteome and treatment responses, personalized medicine in rheumatoid arthritis will be more achievable in the near future.

Significance of Type II Collagen Posttranslational Modifications: From Autoantigenesis to Improved Diagnosis and Treatment of Rheumatoid Arthritis

Collagen type II (COL2), the main structural protein of hyaline cartilage, is considerably affected by autoimmune responses associated with the pathogenesis of rheumatoid arthritis (RA). Posttranslational modifications (PTMs) play a significant role in the formation of the COL2 molecule and supramolecular fibril organization, and thus, support COL2 function, which is crucial for normal cartilage structure and physiology. Conversely, the specific PTMs of the protein (carbamylation, glycosylation, citrullination, oxidative modifications and others) have been implicated in RA autoimmunity. The discovery of the anti-citrullinated protein response in RA, which includes anti-citrullinated COL2 reactivity, has led to the development of improved diagnostic assays and classification criteria for the disease. The induction of immunological tolerance using modified COL2 peptides has been highlighted as a potentially effective strategy for RA therapy. Therefore, the aim of this review is to summarize the recent knowledge on COL2 posttranslational modifications with relevance to RA pathophysiology, diagnosis and treatment. The significance of COL2 PTMs as a source of neo-antigens that activate immunity leading to or sustaining RA autoimmunity is discussed.

Phosphocholine-Functionalized Zwitterionic Highly Branched Poly(β-amino ester)s for Cytoplasmic Protein Delivery

Proteins have tremendous potential for vaccine development and disease treatment, but multiple extracellular and intracellular biological barriers must be overcome before they can exert specific biological functions in the target tissue. The use of polymers as carriers would greatly improve their bioavailability and therapeutic efficiency. Nevertheless, effective protein packaging and cell membrane penetration without causing cytotoxicity is particularly challenging, due largely to the simultaneous distribution of positive and negative charges on protein surface. Here, phosphocholine-functionalized zwitterionic poly(β-amino ester)s, HPAE-D-(±), are developed for cytoplasmic protein delivery. The zwitterionic phosphocholine is capable of binding to both proteins and the cell membrane to facilitate protein packaging and nanoparticle cellular uptake. Compared to amine-functionalized HPAE-E-(+) and carboxylic acid-functionalized HPAE-C-(-), HPAE-D-(±) exhibits much higher cytoplasmic protein delivery efficiency and lower cytotoxicity. In addition, HPAE-D-(±) are readily degraded in aqueous solution. This strategy may be extended to other zwitterions and polymers, thus having profound implications for the development of safe and efficient protein delivery systems.

The regulation of self-tolerance and the role of inflammasome molecules

Inflammasome molecules make up a family of receptors that typically function to initiate a proinflammatory response upon infection by microbial pathogens. Dysregulation of inflammasome activity has been linked to unwanted chronic inflammation, which has also been implicated in certain autoimmune diseases such as multiple sclerosis, rheumatoid arthritis, type 1 diabetes, systemic lupus erythematosus, and related animal models. Classical inflammasome activation-dependent events have intrinsic and extrinsic effects on both innate and adaptive immune effectors, as well as resident cells in the target tissue, which all can contribute to an autoimmune response. Recently, inflammasome molecules have also been found to regulate the differentiation and function of immune effector cells independent of classical inflammasome-activated inflammation. These alternative functions for inflammasome molecules shape the nature of the adaptive immune response, that in turn can either promote or suppress the progression of autoimmunity. In this review we will summarize the roles of inflammasome molecules in regulating self-tolerance and the development of autoimmunity.

The Role of Autoantibody Testing in Modern Personalized Medicine

Personalized medicine (PM) aims individualized approach to prevention, diagnosis, and treatment. Precision Medicine applies the paradigm of PM by defining groups of individuals with akin characteristics. Often the two terms have been used interchangeably. The quest for PM has been advancing for centuries as traditional nosology classification defines groups of clinical conditions with relatively similar prognoses and treatment options. However, any individual is characterized by a unique set of multiple characteristics and therefore the achievement of PM implies the determination of myriad demographic, epidemiological, clinical, laboratory, and imaging parameters. The accelerated identification of numerous biological variables associated with diverse health conditions contributes to the fulfillment of one of the pre-requisites for PM. The advent of multiplex analytical platforms contributes to the determination of thousands of biological parameters using minute amounts of serum or other biological matrixes. Finally, big data analysis and machine learning contribute to the processing and integration of the multiplexed data at the individual level, allowing for the personalized definition of susceptibility, diagnosis, prognosis, prevention, and treatment. Autoantibodies are traditional biomarkers for autoimmune diseases and can contribute to PM in many aspects, including identification of individuals at risk, early diagnosis, disease sub-phenotyping, definition of prognosis, and treatment, as well as monitoring disease activity. Herein we address how autoantibodies can promote PM in autoimmune diseases using the examples of systemic lupus erythematosus, antiphospholipid syndrome, rheumatoid arthritis, Sjögren syndrome, systemic sclerosis, idiopathic inflammatory myopathies, autoimmune hepatitis, primary biliary cholangitis, and autoimmune neurologic diseases.

Role of Citrullinated Collagen in Autoimmune Arthritis

Citrullination of proteins plays an important role in protein function and it has recently become clear that citrullinated proteins play a role in immune responses. In this study we examined how citrullinated collagen, an extracellular matrix protein, affects T-cell function during the development of autoimmune arthritis. Using an HLA-DR1 transgenic mouse model of rheumatoid arthritis, mice were treated intraperitoneally with either native type I collagen (CI), citrullinated CI (cit-CI), or phosphate buffered saline (PBS) prior to induction of autoimmune arthritis. While the mice given native CI had significantly less severe arthritis than controls administered PBS, mice receiving cit-CI had no decrease in the severity of autoimmune arthritis. Using Jurkat cells expressing the inhibitory receptor leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1), Western blot analysis indicated that while CI and cit-CI bound to LAIR-1 with similar affinity, only CI induced phosphorylation of the LAIR ITIM tyrosines; cit-CI was ineffective. These data suggest that cit-CI acts as an antagonist of LAIR-1 signaling, and that the severity of autoimmune arthritis can effectively be altered by targeting T cells with citrullinated collagen.

Increased histone citrullination in juvenile idiopathic arthritis

Objective

Posttranslational modifications (PTMs) of proteins are crucial for regulating various biological processes. However, protein alteration via PTMs, and consequently, the creation of new epitopes, can induce abnormal autoimmune responses in predisposed individuals. Immunopathogenesis of several rheumatic diseases, including the most common childhood form, juvenile idiopathic arthritis (JIA), is associated with the generation of autoantibodies against such modified proteins. Dysregulated generation of neutrophil extracellular traps (NETs) can be a source of post-translationally altered proteins. Thus, we investigated the role of PTMs and the presence of NET-associated markers in JIA patients.

Methods

We recruited 30 pediatric patients with JIA (20 with active disease and 10 in remission) and 30 healthy donors. The serum concentrations of citrullinated histone H3 (citH3), peptidyl arginine deiminases (PADs), and NET-related products were detected using ELISA, and the number of citH3+ neutrophils was assessed using flow cytometry.

Results

The serum levels of citH3 and PADs were higher in active as well as in remission JIA patients than in healthy donors. Similarly, the number of citH3+ neutrophils was higher in the peripheral blood of patients with JIA, implying an enhanced process of NETosis. This was effectively reflected by elevated serum levels of NET-associated products, such as neutrophil elastase, LL37, and cell-free DNA-histone complexes. Additionally, 16.7% of active JIA patients were seropositive for carbamylated autoantibodies, the levels of which declined sharply after initiation of anti-TNFα therapy.

Conclusion

Collectively, our data suggest that the accelerated process of NETosis and PTMs in JIA may result in the generation of anti-citrullinated/carbamylated autoantibodies against various epitopes later in life, which could be prevented by effectively regulating inflammation using immune therapy.

Open Search-Based Proteomics Reveals Widespread Tryptophan Modifications Associated with Hypoxia in Lung Cancer

The tryptophan residue has a large hydrophobic surface that plays a unique role in the folded protein conformation and functions. Tryptophan modifications are presumably to be readily detected in proteins due to the vulnerability of the indole structure to electrophilic attacks. In this study, we report a systematic identification of sequence variations at tryptophan, termed tryptophan variants, from the proteome of patients with nonsmall cell lung cancer (NSCLC). Using shotgun proteomics and a modified open search algorithm, 25 tryptophan variants on 2481 sites in over 858 proteins were identified. Among these, 6 tryptophan variants are previously identified, 15 are newly annotated, and 4 are still unknown, most of which are involved in the cascade of oxidation in the blood microparticle. Remarkably, Trp313 of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was up-oxidized whereas Trp16 and Trp38 of hemoglobin (HBB) were down-oxidized in NCSLC tissues. The results were further supported by an independent cohort of 103 lung adenocarcinoma samples, reflecting a negative feedback and potential detoxification mechanism against tumor glycolysis and hypoxia. Overall, the study reports a quick approach to explore tryptophan variants at the proteomic scale. Our findings highlight the predominant role of tryptophan oxidation in regulating the redox balance of cancer cells and its potential role as prognostic biomarker for patients with NSCLC.

Integrated proteome and malonylome analyses reveal the neutrophil extracellular trap formation pathway in rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune inflammatory disease of unknown etiology in which the posttranslational modifications (PTMs) of proteins play an important role. PTMs, such as those involved in the formation of neutrophil extracellular traps (NETs), have been well studied. The excessive formation and release of NETs can mediate inflammation and joint destruction in RA. It has been gradually recognized that lysine malonylation (Kmal) can regulate some biological processes in some prokaryotes and eukaryotes. However, less is known about the role of Kmal in RA. We therefore performed proteome and malonylome analyses to explore the proteomic characteristics of the peripheral blood mononuclear cells from 36 RA patients and 82 healthy subjects. In total, 938 differentially expressed proteins (DEPs) and 42 differentially malonylated proteins (DMPs) with 55 Kmal sites were detected through a liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based analysis. Functional analysis showed that two DEPs with four malonylated sites and one DMP with a malonylated site were identified in the neutrophil extracellular trap formation (NETosis) pathway. Altogether, this study not only describes the characteristics of the malonylome in RA for the first time, but it also reveals that malonylation may be involved in the NETosis pathway. SIGNIFICANCE: This is the first report that reveals the proteomic features of Kmal in RA through a LC-MS/MS-based method. In this study, we found that several key DMPs were associated with the NETosis pathway, which contributes to the development of RA. The present results provide an informative dataset for the future exploration of Kmal in RA.

Characterization of Monoclonal Antibodies Recognizing Citrulline-Modified Residues

Background

Citrullination is a post-translational protein modification linked to the occurrence and development of a variety of diseases. The detection of citrullinated proteins is predominately based on antibody detection although currently available reagents demonstrate detection bias according to the environmental context of the citrullinated residues. This study aimed to develop improved antibody reagents capable of detecting citrullinated residues in proteins in an unbiased manner.

Methods

BALB/c mice were sequentially immunized using citrulline conjugates with different carrier proteins, and specific monoclonal antibodies (mAbs) identified by primary screening using citrulline-conjugated proteins unrelated to the immunogen. Secondary screening was performed to identify mAbs whose reactivity could be specifically blocked by free citrulline, followed by identification and performance assessment.

Results

Two mAbs, 22F1 and 30G2, specifically recognizing a single citrulline residue were screened from 22 mAbs reacting with citrulline conjugates. Compared with commercially available anti-citrulline antibodies (AB6464, AB100932 and MABN328), 22F1 and 30G2 demonstrated significantly higher reactivity as well as a broader detection spectrum against different citrullinated proteins. 22F1 and 30G2 also had higher specificity than commercial antibodies and overall better applicability to a range of different immunoassays.

Conclusion

Two mAbs specifically recognizing a single citrulline residue were successfully produced, each possessing good specificity against different citrullinated proteins. The improved utility of these reagents is expected to make a strong contribution to protein citrullination-related research.

The Genetic, Environmental, and Immunopathological Complexity of Autoantibody-Negative Rheumatoid Arthritis

Differences in clinical presentation, response to treatment, and long-term outcomes between autoantibody-positive and -negative rheumatoid arthritis (RA) highlight the need for a better comprehension of the immunopathogenic events underlying the two disease subtypes. Whilst the drivers and perpetuators of autoimmunity in autoantibody-positive RA have started to be disclosed, autoantibody-negative RA remains puzzling, also due its wide phenotypic heterogeneity and its possible misdiagnosis. Genetic susceptibility appears to mostly rely on class I HLA genes and a number of yet unidentified non-HLA loci. On the background of such variable genetic predisposition, multiple exogeneous, endogenous, and stochastic factors, some of which are not shared with autoantibody-positive RA, contribute to the onset of the inflammatory cascade. In a proportion of the patients, the immunopathology of synovitis, at least in the initial stages, appears largely myeloid driven, with abundant production of proinflammatory cytokines and only minor involvement of cells of the adaptive immune system. Better understanding of the complexity of autoantibody-negative RA is still needed in order to open new avenues for targeted intervention and improve clinical outcomes.

Detection of Posttranslational Modification Autoantibodies Using Peptide Microarray

Autoantibodies are humoral antibodies against self-proteins and play vital roles in maintaining the homeostasis. Autoantibodies can also target posttranslational modifications (PTMs) of proteins and the identification of new PTM autoantibodies is important to identify biomarkers for the early diagnosis of cancer and autoimmune diseases. In this chapter, we describe a method to detect PTM autoantibodies using citrullinated peptide microarray as an example. This method can be used to screen serum autoantibodies for different human diseases.

PRDX2 promotes the proliferation of colorectal cancer cells by increasing the ubiquitinated degradation of p53

Colorectal cancer is the most common gastrointestinal cancer and causes severe damage to human health. PRDX2 is a member of the peroxiredoxin family reported to have a high level of expression in colorectal cancer. However, the mechanisms by which PRDX2 promotes the proliferation of colorectal cancer are still unclear. Here, the results indicated that PRDX2 expression was upregulated in colorectal cancer and closely correlated with poor prognosis. Functionally, PRDX2 promoted the proliferation of colorectal cancer cells. Mechanistically, PRDX2 could bind RPL4, reducing the interaction between RPL4 and MDM2. These findings demonstrate that the oncogenic property of PRDX2 may be attributed to its regulation of the RPL4-MDM2-p53 pathway, leading to p53 ubiquitinated degradation.

Objective and noninvasive biochemical markers in rheumatoid arthritis: where are we and where are we going?

INTRODUCTION

Rheumatoid arthritis (RA) is a chronic autoimmune disease that affects approximately 1% of the adult population. RA is multi-factorial, and as such our understanding of the molecular pathways involved in the disease is currently limited. An increasing number of studies have suggested that several molecular phenotypes (i.e. endotypes) of RA exist, and that different endotypes respond differently to various treatments. Biochemical markers may be an attractive means for achieving precision medicine, as they are objective and easily obtainable.

AREAS COVERED

We searched recent publications on biochemical markers in RA as either diagnostic or prognostic markers, or as markers of disease activity. Here, we provide a narrative overview of different classes of markers, such as autoantibodies, citrulline products, markers of tissue turnover and cytokines, that have been tested in clinical cohorts or trials including RA patients.

EXPERT OPINION

Although many biochemical markers have been identified and tested, few are currently being used in clinical practice. As more treatment options are becoming available, the need for precision medicine tools that can aid physicians and patients in choosing the right treatment is growing.

Metabolic Control of Autoimmunity and Tissue Inflammation in Rheumatoid Arthritis

Like other autoimmune diseases, rheumatoid arthritis (RA) develops in distinct stages, with each phase of disease linked to immune cell dysfunction. HLA class II genes confer the strongest genetic risk to develop RA. They encode for molecules essential in the activation and differentiation of T cells, placing T cells upstream in the immunopathology. In Phase 1 of the RA disease process, T cells lose a fundamental function, their ability to be self-tolerant, and provide help for autoantibody-producing B cells. Phase 2 begins many years later, when mis-differentiated T cells gain tissue-invasive effector functions, enter the joint, promote non-resolving inflammation, and give rise to clinically relevant arthritis. In Phase 3 of the RA disease process, abnormal innate immune functions are added to adaptive autoimmunity, converting synovial inflammation into a tissue-destructive process that erodes cartilage and bone. Emerging data have implicated metabolic mis-regulation as a fundamental pathogenic pathway in all phases of RA. Early in their life cycle, RA T cells fail to repair mitochondrial DNA, resulting in a malfunctioning metabolic machinery. Mitochondrial insufficiency is aggravated by the mis-trafficking of the energy sensor AMPK away from the lysosomal surface. The metabolic signature of RA T cells is characterized by the shunting of glucose toward the pentose phosphate pathway and toward biosynthetic activity. During the intermediate and terminal phase of RA-imposed tissue inflammation, tissue-residing macrophages, T cells, B cells and stromal cells are chronically activated and under high metabolic stress, creating a microenvironment poor in oxygen and glucose, but rich in metabolic intermediates, such as lactate. By sensing tissue lactate, synovial T cells lose their mobility and are trapped in the tissue niche. The linkage of defective DNA repair, misbalanced metabolic pathways, autoimmunity, and tissue inflammation in RA encourages metabolic interference as a novel treatment strategy during both the early stages of tolerance breakdown and the late stages of tissue inflammation. Defining and targeting metabolic abnormalities provides a new paradigm to treat, or even prevent, the cellular defects underlying autoimmune disease.

Insights of rheumatoid arthritis biomarkers

Rheumatoid arthritis is a chronic, autoimmune connective tissue disease. In addition to joint involvement, extra-articular changes and organ complications also occur in the course of the disease. Untreated disease leads to disability and premature death. Therefore, it is important to recognise and begin treatment early. Based on the presence of rheumatoid factor and antibodies against citrullinated peptides, we can distinguish two forms of the disease: seropositive and seronegative. Research continues to elucidate the mechanisms of the onset of the disease, as well as to uncover factors that induce and influence the activity of the disease. The presence of markers that initially appear and affect the course of the disease can potentially aid in patient treatment. In this article, we have collected biomarkers of rheumatoid arthritis that are well understood as well as those that have been recently described.

From Rheumatoid Factor to Anti-Citrullinated Protein Antibodies and Anti-Carbamylated Protein Antibodies for Diagnosis and Prognosis Prediction in Patients with Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic systemic inflammatory disease mainly involving synovial inflammation and articular bone destruction. RA is a heterogeneous disease with diverse clinical presentations, prognoses and therapeutic responses. Following the first discovery of rheumatoid factors (RFs) 80 years ago, the identification of both anti-citrullinated protein antibodies (ACPAs) and anti-carbamylated protein antibodies (anti-CarP Abs) has greatly facilitated approaches toward RA, especially in the fields of early diagnosis and prognosis prediction of the disease. Although these antibodies share many common features and can function synergistically to promote disease progression, they differ mechanistically and have unique clinical relevance. Specifically, these three RA associating auto-antibodies (autoAbs) all precede the development of RA by years. However, while the current evidence suggests a synergic effect of RF and ACPA in predicting the development of RA and an erosive phenotype, controversies exist regarding the additive value of anti-CarP Abs. In the present review, we critically summarize the characteristics of these autoantibodies and focus on their distinct clinical applications in the early identification, clinical manifestations and prognosis prediction of RA. With the advancement of treatment options in the era of biologics, we also discuss the relevance of these autoantibodies in association with RA patient response to therapy.

Interplay and roles of oxidative stress, toll-like receptor 4 and Nrf2 in trichloroethene-mediated autoimmunity

Previous studies in MRL+/+ mice suggest involvement of oxidative stress (OS) in trichloroethene (TCE)-mediated autoimmunity. However, molecular mechanisms underlying the autoimmunity remain to be fully elucidated. Even though toll-like receptors (TLRs) and Nuclear factor (erythroid-derived 2)-like2 (Nrf2) pathways are implicated in autoimmune diseases (ADs), interplay of OS, TLR and Nrf2 in TCE-mediated autoimmune response remains unexplored. This study was, therefore, undertaken to clearly establish a link among OS, TLR4 and Nrf2 pathways in TCE-induced autoimmunity. Groups of female MRL+/+ mice were treated with TCE, sulforaphane (SFN, an antioxidant) or TCE + SFN (TCE, 10 mmol/kg, i.p., every 4th day; SFN, 8 mg/kg, i.p., every other day) for 6 weeks. TCE exposure led to greater formation of serum 4-hydroxynonenal (HNE)-protein adducts, HNE-specific circulating immune complexes (CICs) and protein carbonyls which were associated with significant increases in serum antinuclear antibodies (ANAs). Moreover, incubation of splenocytes from TCE-treated mice with HNE-modified proteins resulted in enhanced splenocyte proliferation and cytokine release evidenced by increased expression of cyclin D3, Cyclin-dependent kinase 6 (CDK6) and phospho-pRb as well as increased release of IL-6, TNF-α and INF-γ. More importantly, TCE exposure resulted in increased expression of TLR4, MyD88, IRAK4, NF-kB and reduced expression of Nrf2 and HO-1 in the spleen. Remarkably, SFN supplementation not only attenuated TCE-induced OS, upregulation in TLR4 and NF-kB signaling and downregulation of Nrf2, but also ANA levels. These results, in addition to providing further support to a role of OS, also suggest that an interplay among OS, TLR4 and Nrf2 pathways contributes to TCE-mediated autoimmune response. Attenuation of TCE-mediated autoimmunity by SFN provides an avenue for preventive and/or therapeutic strategies for ADs involving OS.

The metabolic signature of T cells in rheumatoid arthritis

PURPOSE OF REVIEW

Rheumatoid arthritis (RA) is a prototypic autoimmune disease manifesting as chronic inflammation of the synovium and leading to acceleration of cardiovascular disease and shortening of life expectancy. The basic defect causing autoimmunity has remained elusive, but recent insights have challenged the notion that autoantigen is the core driver.

RECENT FINDINGS

Emerging data have added metabolic cues involved in the proper maintenance and activation of immune cells as pathogenic regulators. Specifically, studies have unveiled metabolic pathways that enforce T cell fate decisions promoting tissue inflammation; including T cell tissue invasiveness, T cell cytokine release, T cell-dependent macrophage activation and inflammatory T cell death. At the center of the metabolic abnormalities lies the mitochondria, which is consistently underperforming in RA T cells. The mitochondrial defect results at least partially from insufficient DNA repair and leads to lipid droplet accumulation, formation of invasive membrane ruffles, inflammasome activation and pyroptotic T cell death.

SUMMARY

T cells in patients with RA, even naïve T cells never having been involved in inflammatory lesions, have a unique metabolic signature and the changes in intracellular metabolites drive pathogenic T cell behavior. Recognizing the role of metabolic signals in cell fate decisions opens the possibility for immunomodulation long before the end stage synovial inflammation encountered in clinical practice.

Mechanisms of Environment-Induced Autoimmunity

Although numerous environmental exposures have been suggested as triggers for preclinical autoimmunity, only a few have been confidently linked to autoimmune diseases. For disease-associated exposures, the lung is a common site where chronic exposure results in cellular toxicity, tissue damage, inflammation, and fibrosis. These features are exacerbated by exposures to particulate material, which hampers clearance and degradation, thus facilitating persistent inflammation. Coincident with exposure and resulting pathological processes is the posttranslational modification of self-antigens, which, in concert with the formation of tertiary lymphoid structures containing abundant B cells, is thought to promote the generation of autoantibodies that in some instances demonstrate major histocompatibility complex restriction. Under appropriate gene-environment interactions, these responses can have diagnostic specificity. Greater insight into the molecular and cellular requirements governing this process, especially those that distinguish preclinical autoimmunity from clinical autoimmunedisease, may facilitate determination of the significance of environmental exposures in human autoimmune disease.

GlycA Levels during the Earliest Stages of Rheumatoid Arthritis: Potential Use as a Biomarker of Subclinical Cardiovascular Disease

This study aimed at evaluating the clinical relevance of glycoprotein profiles during the earliest phases of rheumatoid arthritis (RA) as biomarkers of cardiovascular (CV) risk and treatment response. Then, GlycA and GlycB serum levels were measured using 1H-nuclear magnetic resonance in 82 early RA patients, 14 clinically-suspect arthralgia (CSA), and 28 controls. Serum glycosyltransferase activity was assessed by a colorimetric assay. Subclinical CV disease was assessed by Doppler-ultrasound. We found that GlycA and GlycB serum levels were increased in RA (both p < 0.001), but not in CSA, independently of cardiometabolic risk factors. Increased serum glycosyltransferase activity paralleled GlycA (r = 0.405, p < 0.001) and GlycB levels (r = 0.327, p = 0.005) in RA. GlycA, but not GlycB, was associated with atherosclerosis occurrence (p = 0.012) and severity (p = 0.001). Adding GlycA to the mSCORE improved the identification of patients with atherosclerosis over mSCORE alone, increasing sensitivity (29.7 vs. 68.0%) and accuracy (55.8 vs. 76.6%) and allowing reclassification into more appropriate risk categories. GlycA-reclassification identified patients with impaired lipoprotein metabolism. Finally, baseline GlycA levels predicted poor clinical response upon anti-rheumatic treatment at 6 and 12 months in univariate and multivariate analysis. In sum, increased GlycA levels during the earliest stage of RA can be considered a powerful biomarker for CV risk stratification and treatment response.

Catalytic activity regulation through post-translational modification: the expanding universe of protein diversity

Protein composition is restricted by the genetic code to a relatively small number of natural amino acids. Similarly, the known three-dimensional structures adopt a limited number of protein folds. However, proteins exert a large variety of functions and show a remarkable ability for regulation and immediate response to intracellular and extracellular stimuli. To some degree, the wide variability of protein function can be attributed to the post-translational modifications. Post-translational modifications have been observed in all kingdoms of life and give to proteins a significant degree of chemical and consequently functional and structural diversity. Their importance is partly reflected in the large number of genes dedicated to their regulation. So far, hundreds of post-translational modifications have been observed while it is believed that many more are to be discovered along with the technological advances in sequencing, proteomics, mass spectrometry and structural biology. Indeed, the number of studies which report novel post translational modifications is getting larger supporting the notion that their space is still largely unexplored. In this review we explore the impact of post-translational modifications on protein structure and function with emphasis on catalytic activity regulation. We present examples of proteins and protein families whose catalytic activity is substantially affected by the presence of post translational modifications and we describe the molecular basis which underlies the regulation of the protein function through these modifications. When available, we also summarize the current state of knowledge on the mechanisms which introduce these modifications to protein sites.

Peptidyl ω-Asp Selenoesters Enable Efficient Synthesis of N-Linked Glycopeptides

Chemical synthesis is an attractive approach allows for the assembly of homogeneous complex N-linked glycopeptides and glycoproteins, but the limited coupling efficiency between glycans and peptides hampered the synthesis and research in the related field. Herein we developed an alternative glycosylation to construct N-linked glycopeptide via efficient selenoester-assisted aminolysis, which employs the peptidyl ω-asparagine selenoester and unprotected glycosylamine to perform rapid amide-bond ligation. This glycosylation strategy is highly compatible with the free carboxylic acids and hydroxyl groups of peptides and carbohydrates, and readily available for the assembly of structure-defined homogeneous N-linked glycopeptides, such as segments derived from glycoprotein EPO and IL-5.

Effects of Methotrexate Alone or Combined With Arthritis-Related Biotherapies in an in vitro Co-culture Model With Immune Cells and Synoviocytes

Background: Methotrexate (MTX) at low dose is a key drug for rheumatoid arthritis (RA). MTX is widely used alone or combined with biologics or steroids. The aim was to study its effects on cytokine production using an in vitro model with synoviocytes interacting with peripheral blood mononuclear cells (PBMC) to reproduce the interactions in RA synovium.

Methods: Activated-PBMC were co-cultured with RA synoviocytes during 48 h. A dose-response of MTX was tested and different biotherapies (Infliximab, Tocilizumab, Abatacept and Rituximab) were added alone or in combination with MTX. Cytokine production (IL-17, IL-6, IL-1β, IFN-γ, and IL-10) was measured by ELISA. These results were compared with those obtained with steroids.

Results: MTX alone had a modest inhibitory effect on cytokine production compared to steroids. The most effective concentration was one of the lowest, 0.01 μg/ml, as for steroids. Infliximab was the most active biotherapy (p ≤ 0.05 for all cytokines) followed by Tocilizumab (p ≤ 0.05 for all cytokines except IL-6). Abatacept and Rituximab had a more restricted effect on cytokines (p ≤ 0.05 for IL-1β and IFN-γ). The combination MTX/biotherapies did not increase significantly the inhibition of cytokine production but some specific inhibitory effects were observed with Infliximab on IL-17 and IL-6, and with Abatacept and Rituximab on IL-1β.

Conclusion: Low dose of MTX was at least as effective as high dose. The effects of the combination with biotherapies showed an important level of heterogeneity between the levels of some specific cytokines and the degree of inhibition with drugs.

An Overview of the Intrinsic Role of Citrullination in Autoimmune Disorders

A protein undergoes many types of posttranslation modification. Citrullination is one of these modifications, where an arginine amino acid is converted to a citrulline amino acid. This process depends on catalytic enzymes such as peptidylarginine deiminase enzymes (PADs). This modification leads to a charge shift, which affects the protein structure, protein-protein interactions, and hydrogen bond formation, and it may cause protein denaturation. The irreversible citrullination reaction is not limited to a specific protein, cell, or tissue. It can target a wide range of proteins in the cell membrane, cytoplasm, nucleus, and mitochondria. Citrullination is a normal reaction during cell death. Apoptosis is normally accompanied with a clearance process via scavenger cells. A defect in the clearance system either in terms of efficiency or capacity may occur due to massive cell death, which may result in the accumulation and leakage of PAD enzymes and the citrullinated peptide from the necrotized cell which could be recognized by the immune system, where the immunological tolerance will be avoided and the autoimmune disorders will be subsequently triggered. The induction of autoimmune responses, autoantibody production, and cytokines involved in the major autoimmune diseases will be discussed.