PAD enzymes in rheumatoid arthritis: pathogenic effectors and autoimmune targets

Molecular mimicry in the pathogenesis of autoimmune rheumatic diseases

Autoimmune rheumatic diseases (ARDs) are a heterogeneous group of conditions characterized by excessive and misdirected immune responses against the body's own musculoskeletal tissues. Their exact aetiology remains unclear, with genetic, demographic, behavioural and environmental factors implicated in disease onset. One prominent hypothesis for the initial breach of immune tolerance (leading to autoimmunity) is molecular mimicry, which describes structural or sequence similarities between human and microbial proteins (mimotopes). This similarity can lead to cross-reactive antibodies and T-cell receptors, resulting in an immune response against autoantigens. Both commensal microbes in the human microbiome and pathogens can trigger molecular mimicry, thereby potentially contributing to the onset of ARDs. In this review, we focus on the role of molecular mimicry in the onset of rheumatoid arthritis and systemic lupus erythematosus. Moreover, implications of molecular mimicry are also briefly discussed for ankylosing spondylitis, systemic sclerosis and myositis.

Therapeutic potential of phytocompounds in rheumatoid arthritis: Molecular insights and clinical applications

Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by systemic involvement, inflammation, and the destruction of synovial joints. RA can be categorized as anti-citrullinated protein antibodies-positive or negative based on genetic risk factors and autoantibodies. This review systematically sourced articles related to RA, phytocompounds, signaling pathways, and clinical insights from primary medical databases, including Scopus, PubMed, and Web of Science. This review explores the therapeutic potential of phytocompounds in treating RA by targeting key inflammation and immunological response signaling pathways. Phytocompounds such as curcumin, resveratrol, and flavonoids alter essential molecular pathways in RA pathophysiology, including nuclear factor kappa-light-chain-enhancer of activated B cells, mitogen-activated protein kinases, janus kinase-signal transducer and activator of transcription, and the inflammasome. These substances possess pro-resolving, anti-apoptotic, and antioxidant properties, which enhance their therapeutic efficacy. Alternative medicine, including dietary, herbal, and nutritional supplements, may help reduce RA symptoms. In vitro, in vivo, and clinical studies have demonstrated the effectiveness of these treatments. Phytocompounds have potential as a treatment for RA by altering signaling pathways, reducing oxidative stress, and protecting cartilage and bone. However, few clinical trials confirm its long-term safety, bioavailability, and effectiveness. Further clinical trials and translational research are needed to validate the effectiveness, safety, and pharmacokinetics of phytocompounds, while identifying novel plant-derived bioactive chemicals could improve patient outcomes.

Biomarkers for early diagnosis of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune disease that primarily affects the joints and can even lead to disability. No treatment can completely cure RA, and early intervention is the most effective way to improve the prognosis. Autoantibodies are important in disease diagnosis, treatment, and prognosis assessment, especially rheumatoid factor (RF) and anti-citrullinated protein antibodies (ACPAs), which have been widely used in clinical practice. Several studies have fully discussed the critical roles of RF and ACPA in the pathophysiology of RA, but fewer reviews focus on advances in RF and ACPA-related assays for early diagnosis. Therefore, this article outlines the clinical applications and predictive value of RF and ACPA in early RA diagnosis, along with emerging immunoassays published in the last five years. Additionally, it discusses the advantages and challenges of multiplex immunoassay techniques for simultaneous RF and ACPA detection. Finally, some suggestions for improving the performance of combined testing were summarized to achieve early diagnosis of RA, thereby delaying disease progression.

A patent review of peptidylarginine deiminase 4 (PAD4) inhibitors (2014-present)

INTRODUCTION

PAD4 mediates the post-translational modification of arginine residues into citrulline which can have profound effects on protein structure, function and interactions. Protein citrullination and neutrophil extracellular trap (NET) formation associated with increased PAD4 activity have been implicated in the development of autoimmune conditions, cardiovascular diseases, neurodegenerative disorders, and cancer. PAD4 inhibitors have been shown to suppress citrullination and NETs formation.

AREAS COVERED

This review covers 10 years of industrial drug discovery campaigns reported in 28 patent applications from 10 companies. Cortellis, the World Intellectual Property Organization website, Scopus and SciFinder were used to search the patent literature using the keywords 'PAD4' and 'PAD4 inhibitor.' Most of the reported inhibitors share the same core scaffold with varied decoration of different complexity, including highly functionalized macrocycles, with some in vivo and pharmacokinetic (PK) data reported for selected examples.

EXPERT OPINION

Despite PAD4's clear involvement in multiple disease pathways, its detailed mechanism remains insufficiently understood. Selective and potent compounds with improved PK properties have been provided but most research on PAD4 is still at the experimental stage or preclinical development; the most promising is JBI-1044, at the IND stage, while some companies have turned to antibodies despite considerable previous investment in small molecules.

Enzyme-activating B-cell receptors boost antigen presentation to pathogenic T cells in gluten-sensitive autoimmunity

Autoantibodies against the enzyme transglutaminase 3 (TG3) are characteristic to the gluten-sensitive skin disorder dermatitis herpetiformis (DH), which is an extraintestinal manifestation of celiac disease. We here demonstrate that TG3-specific B cells can activate gluten-specific CD4+ T cells through B-cell receptor (BCR)-mediated internalization of TG3-gluten enzyme-substrate complexes. Stereotypic anti-TG3 antibodies using IGHV2-5/IGKV4-1 gene segments enhance the catalytic activity of TG3, and this effect translates into increased gluten presentation to T cells when such antibodies are expressed as BCRs. The crystal structure of TG3 bound to an IGHV2-5/IGKV4-1 Fab shows that antibody binding to a β-sheet in the catalytic core domain causes the enzyme to adopt the active conformation. This mechanism explains the production of stereotypic anti-TG3 autoantibodies in DH and highlights a role for TG3-specific B cells as antigen-presenting cells for gluten-specific T cells. Similar boosting effects of autoreactive BCRs could be relevant for other autoimmune diseases, including rheumatoid arthritis.

Ferrostatin-1 reduces the inflammatory response of rheumatoid arthritis by decreasing the antigen presenting function of fibroblast-like synoviocytes

Rheumatoid arthritis (RA) is a systemic chronic autoimmune disease with complex mechanism. Currently, ferroptosis is believed to play a role in it, but the specific mechanism is unknown, especially in immune response. In this study, we demonstrated that the high expression of major histocompatibility complex I (MHC-I) molecules in RA fibroblast-like synoviocytes (FLSs) is an antigen-presenting cell property and that this property is closely related to the increase in antigens after citrullination. Moreover, we detected higher levels of ferroptosis among FLSs from RA patient than among FLSs from OA patients. Ferroptosis can increase the expression of citrullinated histone H3 (cit-h3) by promoting the production of peptidyl arginine deiminase 4 (PAD4), which further promotes the expression of MHC-I molecules. We cocultured RA-FLSs treated with ferroptosis drugs with selected CD8 + T cells to assess the effect of ferroptosis on the endogenous antigen-presenting function of RA-FLSs. Ferroptosis promoted the proliferation of CD8 + T cells and the release of the inflammatory factors Tumor necrosis factor-α (TNF-α) and Interferon-gamma (IFN-γ), which enhanced the inflammatory effect. This phenomenon was also observed in a collagen-induced arthritis (CIA) mouse model. Finally, ferrostatin-1 (fer-1), a ferroptosis inhibitor, inhibited the above effects and reduced the release of inflammatory factors, indicating that ferroptosis may play a therapeutic role in RA and providing new ideas for the treatment of RA in the field of immunity.

From a better knowledge of periodontal disease to Porphyromonas gingivalis target for rheumatoid arthritis disease activity

Periodontal disease (PD) and rheumatoid arthritis (RA) are both inflammatory diseases affecting the tooth and joint, with local inflammation associated with bone loss. Bacterial infections by oral bacteria are involved in periodontal inflammation, and the best known to be associated with PD is Porphyromonas gingivalis (Pg). A large body of recent data suggests a strong involvement of this specific bacteria, Pg, in PD outcomes, but also in RA. The aim of this review is to discuss the association between PD and Pg, RA and its mechanisms, and to determine whether targeting Pg bacteria could improve RA. Numerous epidemiological studies have already confirmed the association between PD and Pg, as well as between PD and RA, which is mainly associated with a common genetic background, the shared epitope. The involvement of Pg in pathogenesis of RA is supported by the induction of gingival citrullinated proteins and therefore of anti-citrullinated proteins antibodies, which constitute the most specific biomarker of RA. The prevalence of Pg in RA is still controversial, but studies should include patients with preclinical and early RA. Finally, recent data confirmed that targeting Pg is highly effective in improving RA.

A Multifaceted Computational Approach to Identify PAD4 Inhibitors for the Treatment of Rheumatoid Arthritis (RA)

BACKGROUND/OBJECTIVES

Neutrophil cells' lysis forms the extracellular traps (NETs) to counter the foreign body during insults to the body. Peptidyl arginine deiminase (PAD) participates in this process and is then released into the extracellular fluid with the lysed cell components. In some diseases, patients with abnormal function of PADs, especially PAD 4, tend to form autoantibodies against the abnormal citrullinated proteins that are the result of PAD activity on arginine side chains. Those antibodies, which are highly distinct in RA, are distinctly anti-citrullinated protein antibodies (ACPA). This study used an in-silico drug repurposing approach of FDA-approved medications to identify potential alternative medications that can inhibit this process and address solutions to the current limitations of existing therapies.

METHODS

We utilized Maestro Schrödinger as a computational tool for preparing and docking simulations on the PAD 4 enzyme crystal structure that is retrieved from RCSB Protein Data Bank (PDB ID: 4X8G) while the docked FDA-approved medications are obtained from the Zinc 15 database. The protein was bound to GSK 199-an investigational compound-as a positive control for the docked molecules. Preparation of the protein was performed by Schrödinger Protein Preparation Wizard tool. Binding pocket determination was performed by Glide software (Schrödinger Release 2021-3:Schrödinger, LLC., New York, NY, USA, 2021). and validation of molecular docking was carried out through the redocking of GSK 199 and superimposition. After that, standard and induced fit docking were performed.

RESULTS/CONCLUSIONS

Among the four obtained hits Pemetrexed, Leucovorin, Chlordiazepoxide, and Ioversol, which showed the highest XP scores providing favorable binding interactions. The induced-fit docking (IFD) results displayed the strong binding affinities of Ioversol, Pemetrexed, Leucovorin, Chlordiazepoxide in the order IFD values -11.617, -10.599, -10.521, -9.988, respectively. This research investigates Pemetrexed, Leucovorin, Chlordiazepoxide, and Ioversol as potential repurposing agents in the treatment of rheumatoid arthritis (RA) as they are identified as PAD4 inhibitors.

Rheumatoid arthritis unmasked: the power of B cell depletion therapy

Rheumatoid arthritis (RA) is an enduring autoimmune illness characterized by persistent inflammation and joint damage. Recent advancements in B cell depletion therapies (BCDTs) have provided new avenues for managing RA. This review article delves into the pathophysiology of RA, highlighting the pivotal role of B cells in disease progression. We explore the mechanisms underlying B cell depletion, focusing on monoclonal antibodies such as rituximab as well as innovative approaches like chimeric antigen receptor (CAR) T cell therapies. An in-depth analysis of clinical studies reveals the efficacy and limitations of these therapies, including success rates, side effects, and cost implications for patients. Despite promising outcomes, the incomplete depletion of B cells and associated risks underscore the need for further research. This review aims to provide a comprehensive understanding of BCDTs in RA, shed light on their potential and challenges, and guide future therapeutic strategies.

Helicobacter pylori and rheumatoid arthritis: Investigation of relation from traditional Chinese medicine

Rheumatoid arthritis (RA) is an autoimmune condition that predominantly affects synovial joints, manifesting with joint swelling, pain, and stiffness. In advanced stages, unchecked inflammation can inflict damage on bone and cartilage, resulting in disabilities and deformities of the joints. Additionally, systemic and extra-articular complications may arise due to the consequences of uncontrolled inflammation. Helicobacter pylori (H. pylori) is one of the most prevalent chronic bacterial infections in humans. This microorganism is a spiral-shaped, flagellated, microaerophilic gram-negative bacterium. Prolonged exposure leads to the activation of the immune system, with infected gastric mucosa epithelial cells continuously producing cytokines. This production, in turn, triggers the generation of antibodies as well as T Helper 1 and T Helper 2 effector T cells. The persistent antigenic stimulation resulting from H. pylori infection could lead to the progression of autoimmune diseases. Numerous clinical and pharmacological trials have illustrated the efficacy of traditional Chinese medicine against H. pylori. This review aims to delve into the connection between H. pylori and rheumatoid arthritis so as understand the pathogenesis. The concluding section of this review explores the interplay of Chinese medicine and Helicobacter pylori concerning rheumatoid arthritis.

Large-scale multicenter study reveals anticitrullinated SR-A peptide antibody as a biomarker and exacerbator for rheumatoid arthritis

Current diagnosis and treatment of rheumatoid arthritis (RA) is still challenging. More than one-third of patients with RA could not be accurately diagnosed because of lacking biomarkers. Our recent study reported that scavenger receptor-A (SR-A) is a biomarker for RA, especially for anticyclic citrullinated peptide antibody (anti-CCP)-negative RA. Here, we further identified the B cell autoantigenic epitopes of SR-A. By a large-scale multicenter study including one training and three validation cohorts of 1954 participants, we showed that anticitrullinated SR-A peptide antibody (anti-CSP) was exclusively elevated in RA as a biomarker, particularly useful for seronegative RA. Combination of anti-CSP with anti-CCP demonstrated superior diagnostic value for RA, with sensitivity of 84.83% and specificity of 92.43%. Moreover, RA anti-CSP revealed distinct glycosylation patterns, capable of provoking inflammation in cartilage organoids and exacerbating disease progression in experimental arthritis. Together, these data identify anti-CSP as an RA autoantibody clinically applicable and actively involved in disease pathogenesis.

The Role and Research Progress of ACPA in the Diagnosis and Pathological Mechanism of Rheumatoid Arthritis

An autoimmune condition known as rheumatoid arthritis (RA) is characterized by persistent polyarticular inflammation. Within two years of the disease's onset, irreparable bone and joint deterioration can occur as a result of the inflammatory course of the illness, leading to joint deformity and loss of function. In most patients diagnosed with RA, a range of autoantibodies, particularly anti-citrullinated protein antibodies (ACPA), can be detected months to years before the onset of clinically recognizable disease. Additionally, an increasing number of studies suggest that ACPA is involved in the pathogenesis of RA and may play a direct pathogenic role in the disease. This paper focuses on the role of ACPA in the pathomechanism of RA and discusses its unique clinical applications for the early identification and prediction of RA, as well as the influencing factors. Moreover, this article outlines the association of ACPA-positive (ACPA+) RA with other autoimmune diseases.

The advent of chimeric antigen receptor T Cell therapy in recalibrating immune balance for rheumatic autoimmune disease treatment

CAR-T cell therapy, a cutting-edge cellular immunotherapy with demonstrated efficacy in treating hematologic malignancies, also exhibits significant promise for addressing autoimmune diseases. This innovative therapeutic approach holds promise for achieving long-term remission in autoimmune diseases, potentially offering significant benefits to affected patients. Current targets under investigation for the treatment of these conditions include CD19, CD20, and BCMA, among others. However, CAR-T therapy faces difficulties such as time-consuming cell manufacturing, complex and expensive process, and the possibility of severe adverse reactions complicating the treatment, etc. This article examines CAR-T therapy across various rheumatic autoimmune diseases, including systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), Sjögren's syndrome (SS), systemic sclerosis (SSc), antisynthetase syndrome (ASS), and ANCA-associated vasculitis (AAV), highlighting both therapeutic advancements and ongoing challenges.

Exploration of Potential Anti-Inflammatory Cum Anti-Rheumatoid-Arthritis Phyto-Molecule Through Integrated Green Approach: Network Pharmacology, Molecular Docking, Molecular Dynamics, In-Vitro and Ex-Vivo Study

Rheumatoid arthritis (RA) and associated inflammatory complications are the most prevalent illnesses and can turn into fatal conditions if left untreated. Allopathic medicine is not satisfactory for curing RA. Scientific literature reports reveal that several phyto-compounds viz. flavonoids, saponins, and terpenoids, can heal joints and organs from auto-inflammatory rheumatoid arthritis and pain. Gene ontology, gene network analysis, molecular clustering, and literature review were used to optimise RA-specific highly expressed genes. In-silico molecular docking was performed to short-out potential phytomolecules (Neohesperidin dihydrochalcone (NHDC)) from 1000 datasets-library against RA and validate using MD simulation running at 100 ns. In-vitro anti-inflammatory assays of NHDC inhibited egg-albumin denaturation, IC50 of 47.739±0.51 μg/ml. The ex-vivo MTT assay with NHDC rendered 67.209 % inhibition at 100 μM against fd-FLS-cells. NHDC downregulated pro-inflammatory cytokine IL-17 A production by 61.11 % and 50 % at 300 and 200 μM, respectively. Thus, this Studies recommend that NHDC may be highlighted as a novel multi-target PADI4 and JAK3 inhibitor with better efficacy and minimal toxicity in RA warranted to In-Vivo and clinical investigation. The current findings have uncovered remarkable genes and signalling pathways linked to RA, which could enhance our existing comprehension of the molecular mechanisms that drive its development and progression.

Screening and identification of peptidyl arginine deiminase 4 inhibitors from herbal plants extracts and purified natural products by a trypsin assisted sensitive immunoassay based on streptavidin magnetic beads

Peptidyl arginine deiminase 4 (PAD4) plays a critical role in many autoimmune diseases including rheumatoid arthritis. Herein, a trypsin assisted highly immunoassay method was established to determine PAD4 activity and screen potent inhibitors from herbal plants extracts and purified natural products. The method was applied to determine endogenous PAD4 activity in both cell and tissue lysates, as well as the inhibitory effects of 20 herbal plants and 50 purified natural products. The Cinnamomi ramulus extract showed strongest inhibitory potency with IC50 value lower than 5 μg/mL. Meanwhile, pyrroloquinoline quinone (PQQ), widely used as a dietary supplement, was discovered as a promising PAD4 inhibitor with an IC50 value lower than 4 μM. The inhibition kinetic analysis, drug affinity response target stability (DARTS) and molecular docking were performed to confirm the interaction between PQQ and PAD4. This method has great potential for researchers to monitor activities and discover potential inhibitors of PAD4.

Oral Pathobiont-Derived Outer Membrane Vesicles in the Oral-Gut Axis

Oral pathobionts are essential in instigating local inflammation within the oral cavity and contribute to the pathogenesis of diseases in the gastrointestinal tract and other distant organs. Among the Gram-negative pathobionts, Porphyromonas gingivalis and Fusobacterium nucleatum emerge as critical drivers of periodontitis, exerting their influence not only locally but also as inducers of gut dysbiosis, intestinal disturbances, and systemic ailments. This dual impact is facilitated by their ectopic colonization of the intestinal mucosa and the subsequent mediation of distal systemic effects by releasing outer membrane vesicles (OMVs) into circulation. This review elucidates the principal components of oral pathobiont-derived OMVs implicated in disease pathogenesis within the oral-gut axis, detailing virulence factors that OMVs carry and their interactions with host epithelial and immune cells, both in vitro and in vivo. Additionally, we shed light on the less acknowledged interplay between oral pathobionts and the gut commensal Akkermansia muciniphila, which can directly impede oral pathobionts' growth and modulate bacterial gene expression. Notably, OMVs derived from A. muciniphila emerge as promoters of anti-inflammatory effects within the gastrointestinal and distant tissues. Consequently, we explore the potential of A. muciniphila-derived OMVs to interact with oral pathobionts and prevent disease in the oral-gut axis.

Peptidylarginine deiminase (PAD): A promising target for chronic diseases treatment

In 1958, the presence of citrulline in the structure of the proteins was discovered for the first time. Several years later they found that Arginine converted to citrulline during a post-translational modification process by PAD enzyme. Each PAD is expressed in a certain tissue developing a series of diseases such as inflammation and cancers. Among these, PAD2 and PAD4 play a role in the development of rheumatoid arthritis (RA) by producing citrullinated autoantigens and increasing the production of inflammatory cytokines. PAD4 is also associated with the formation of NET structures and thrombosis. In the crystallographic structure, PAD has several calcium binding sites, and the active site of the enzyme consists of different amino acids. Various PAD inhibitors have been developed divided into pan-PAD and selective PAD inhibitors. F-amidine, Cl-amidine, and BB-Cl-amidine are some of pan-PAD inhibitors. AFM-30a and JBI589 are selective for PAD2 and PAD4, respectively. There is a need to evaluate the effectiveness of existing inhibitors more accurately in the coming years, as well as design and production of novel inhibitors targeting highly specific isoforms.

A Chemical Approach to Assess the Impact of Post-translational Modification on MHC Peptide Binding and Effector Cell Engagement

The human major histocompatibility complex (MHC) plays a pivotal role in the presentation of peptidic fragments from proteins, which can originate from self-proteins or from nonhuman antigens, such as those produced by viruses or bacteria. To prevent cytotoxicity against healthy cells, thymocytes expressing T cell receptors (TCRs) that recognize self-peptides are removed from circulation (negative selection), thus leaving T cells that recognize nonself-peptides. Current understanding suggests that post-translationally modified (PTM) proteins and the resulting peptide fragments they generate following proteolysis are largely excluded from negative selection; this feature means that PTMs can generate nonself-peptides that potentially contribute to the development of autoreactive T cells and subsequent autoimmune diseases. Although it is well-established that PTMs are prevalent in peptides present on MHCs, the precise mechanisms by which PTMs influence the antigen presentation machinery remain poorly understood. In the present work, we introduce chemical modifications mimicking PTMs on synthetic peptides. This is the first systematic study isolating the impact of PTMs on MHC binding and also their impact on TCR recognition. Our findings reveal various ways PTMs alter antigen presentation, which could have implications for tumor neoantigen presentation.

Neutrophil extracellular traps and citrullinated fibrinogen contribute to injury in a porcine model of limb ischemia and reperfusion

Background

Ischemia/reperfusion injury (IRI) is a complex pathological process, triggered by the restoration of blood flow following an interrupted blood supply. While restoring the blood flow is the only option to salvage the ischemic tissue, reperfusion after a prolonged period of ischemia initiates IRI, triggering a cascade of inflammatory responses ultimately leading to neutrophil recruitment to the inflamed tissue, where they release neutrophil extracellular traps (NETs). NETs are web-like structures of decondensed chromatin and neutrophilic proteins, including peptidyl-arginine deiminase 2 and 4 (PAD2, PAD4), that, once outside, can citrullinate plasma proteins, irreversibly changing their conformation and potentially their function. While the involvement of NETs in IRI is known mainly from rodent models, we aimed to determine the effect of NET formation and especially PADs-mediated extracellular protein citrullination in a porcine model of limb IRI.

Methods

We conducted our study on amputated pig forelimbs exposed to 1 h or 9 h of ischemia and then reperfused in vivo for 12 h. Limb weight, edema formation, compartmental pressure were measured, and skeletal muscle was analyzed by immunofluorescence (TUNEL assay and dystrophin staining) to evaluate tissue damage. Fibrin tissue deposition, complement deposition and NETs were investigated by immunofluorescence. Citrullinated plasma proteins were immunoprecipitated and citrullinated fibrinogen was identified in the plasma by Western blot and in the tissue by immunofluorescence and Western blot.

Results

Our data consolidate the involvement of NETs in a porcine model of limb IRI, correlating their contribution to damage extension with the duration of the ischemic time. We found a massive infiltration of NETs in the group subjected to 9 h ischemia compared to the 1 h and citrullinated fibrinogen levels, in plasma and tissue, were higher in 9 h ischemia group. We propose fibrinogen citrullination as one of the mechanisms contributing to the worsening of IRI. NETs and protein citrullination represent a potential therapeutic target, but approaches are still a matter of debate. Here we introduce the idea of therapeutic approaches against citrullination to specifically inhibit PADs extracellularly, avoiding the downstream effects of hypercitrullination and keeping PADs' and NETs' intracellular regulatory functions.

Citrullination modulation stabilizes HIF-1α to promote tumour progression

Citrullination plays an essential role in various physiological or pathological processes, however, whether citrullination is involved in regulating tumour progression and the potential therapeutic significance have not been well explored. Here, we find that peptidyl arginine deiminase 4 (PADI4) directly interacts with and citrullinates hypoxia-inducible factor 1α (HIF-1α) at R698, promoting HIF-1α stabilization. Mechanistically, PADI4-mediated HIF-1αR698 citrullination blocks von Hippel-Lindau (VHL) binding, thereby antagonizing HIF-1α ubiquitination and subsequent proteasome degradation. We also show that citrullinated HIF-1αR698, HIF-1α and PADI4 are highly expressed in hepatocellular carcinoma (HCC) tumour tissues, suggesting a potential correlation between PADI4-mediated HIF-1αR698 citrullination and cancer development. Furthermore, we identify that dihydroergotamine mesylate (DHE) acts as an antagonist of PADI4, which ultimately suppresses tumour progression. Collectively, our results reveal citrullination as a posttranslational modification related to HIF-1α stability, and suggest that targeting PADI4-mediated HIF-1α citrullination is a promising therapeutic strategy for cancers with aberrant HIF-1α expression.

Multi-Autoantibody Testing Identifies Expansion of Reactivity to Targeted Antigens Before a Diagnosis of Rheumatoid Arthritis

OBJECTIVE

Rheumatoid arthritis (RA) has a "pre-RA" period in which multiple autoantibodies, including antibodies to citrullinated (cit) proteins (ACPA), rheumatoid factor (RF), anti-peptidyl arginine deiminase (anti-PAD), among others, have been described; however, few studies have tested all autoantibodies in a single pre-RA cohort. This study aims to evaluate the prevalence of multiple autoantibodies in pre-RA and potentially identify an autoantibody profile in pre-RA that indicates imminent onset of clinical RA.

METHODS

We evaluated 148 individuals with two pre- and one post-RA diagnosis samples available from the Department of Defense Serum Repository and matched controls. Samples were tested for immuglobulin (Ig) G anti-cyclic cit peptide-3 (anti-CCP3), five ACPA fine specificities, five anti-PAD isoforms, as well as RF IgA and RF IgM using commercial platforms; cutoffs were determined using levels present in <1% of controls.

RESULTS

Positivity of anti-CCP3, RF IgA and RF IgM, anti-PAD1, anti-cit-vimentin 2, anti-cit-fibrinogen, and anti-cit-histone 1 increased over time in pre-RA, although anti-PAD and ACPA fine specificities were predominately present within anti-CCP3-positive individuals. Within anti-CCP3-positive samples from the pre-RA period, positivity for RFs as well as anti-PAD and ACPA fine specificities classified samples as being closer to the time of RA diagnosis.

CONCLUSION

Multiple autoantibodies are present in pre-RA and increase in positivity as the time of RA diagnosis approaches. These results confirm previous findings predicting imminent RA and provide a pathway using commercial-grade assays to assess the risk for and timing of development of clinical RA.

The molecular basis underlying T cell specificity towards citrullinated epitopes presented by HLA-DR4

CD4+ T cells recognising citrullinated self-epitopes presented by HLA-DRB1 bearing the shared susceptibility epitope (SE) are implicated in rheumatoid arthritis (RA). However, the underlying T cell receptor (TCR) determinants of epitope specificity towards distinct citrullinated peptide antigens, including vimentin-64cit59-71 and α-enolase-15cit10-22 remain unclear. Using HLA-DR4-tetramers, we examine the T cell repertoire in HLA-DR4 transgenic mice and observe biased TRAV6 TCR gene usage across these two citrullinated epitopes which matches with TCR bias previously observed towards the fibrinogen β-74cit69-81 epitope. Moreover, shared TRAV26-1 gene usage is evident in four α-enolase-15cit10-22 reactive T cells in three human samples. Crystal structures of mouse TRAV6+ and human TRAV26-1+ TCR-HLA-DR4 complexes presenting vimentin-64cit59-71 and α-enolase-15cit10-22, respectively, show three-way interactions between the TCR, SE, citrulline, and the basis for the biased selection of TRAV genes. Position 2 of the citrullinated epitope is a key determinant underpinning TCR specificity. Accordingly, we provide a molecular basis of TCR specificity towards citrullinated epitopes.

The role of anti-citrullinated protein antibody in pathogenesis of RA

Rheumatoid arthritis (RA) is a common autoimmune rheumatic disease that causes chronic synovitis, bone erosion, and joint destruction. The autoantigens in RA include a wide array of posttranslational modified proteins, such as citrullinated proteins catalyzed by peptidyl arginine deiminase4a. Pathogenic anti-citrullinated protein antibodies (ACPAs) directed against a variety of citrullinated epitopes are abundant both in plasma and synovial fluid of RA patients. ACPAs play an important role in the onset and progression of RA. Intensive and extensive studies are being conducted to unveil the mechanisms of RA pathogenesis and evaluate the efficacy of some investigative drugs. In this review, we focus on the formation and pathogenic function of ACPAs.

Reciprocal regulation of protein arginine deiminase 2 and 4 expression in the colonic mucosa of ulcerative colitis

Neutrophils express protein arginine deiminase 2 and PAD4, both of which mediate the citrullination of target proteins to induce production of neutrophil extracellular traps. Although PAD-dependent NETs trigger inflammatory bowel disease, the mechanisms governing the expression of PAD2 and PAD4 are poorly understood. In this study, we tried to clarify expression mechanisms of PAD2 and PAD4 in the colonic mucosa of patients with ulcerative colitis and Crohn's disease. Administration of Cl-amidine, a pan PAD-inhibitor, attenuated the development of dextran sodium sulfate-induced colitis, the effects of which were accompanied by reduced IL-6 and TNF-α production by colonic lamina propria mononuclear cells upon exposure to Toll-like receptor ligands. The mRNA expression of colonic PAD2 and PAD4 was negatively and positively correlated with disease activity and pro-inflammatory cytokine responses in patients with UC, respectively. Reciprocal regulation of PAD2 and PAD4 mRNA expression was observed in the colonic mucosa of UC patients, but not in those of CD patients. PAD4 mRNA expression was correlated with disease activity and pro-inflammatory cytokine responses in patients with CD. Collectively, these data suggest that reciprocal regulation of PAD2 and PAD4 expression is associated with disease activity in UC patients.

Nanomedicines targeting activated immune cells and effector cells for rheumatoid arthritis treatment

Rheumatoid arthritis (RA) is a chronic systemic autoimmune disease characterized by synovial inflammation and inflammatory cellular infiltration. Functional cells in the RA microenvironment (RAM) are composed of activated immune cells and effector cells. Activated immune cells, including macrophages, neutrophils, and T cells, can induce RA. Effector cells, including synoviocytes, osteoclasts, and chondrocytes, receiving inflammatory stimuli, exacerbate RA. These functional cells, often associated with the upregulation of surface-specific receptor proteins and significant homing effects, can secrete pro-inflammatory factors and interfere with each other, thereby jointly promoting the progression of RA. Recently, some nanomedicines have alleviated RA by targeting and modulating functional cells with ligand modifications, while other nanoparticles whose surfaces are camouflaged by membranes or extracellular vesicles (EVs) of these functional cells target and attack the lesion site for RA treatment. When ligand-modified nanomaterials target specific functional cells to treat RA, the functional cells are subjected to attack, much like the intended targets. When functional cell membranes or EVs are modified onto nanomaterials to deliver drugs for RA treatment, functional cells become the attackers, similar to arrows. This study summarized how diversified functional cells serve as targets or arrows by engineered nanoparticles to treat RA. Moreover, the key challenges in preparing nanomaterials and their stability, long-term efficacy, safety, and future clinical patient compliance have been discussed here.

Sequential immunotherapy: towards cures for autoimmunity

Despite major progress in the treatment of autoimmune diseases in the past two decades, most therapies do not cure disease and can be associated with increased risk of infection through broad suppression of the immune system. However, advances in understanding the causes of autoimmune disease and clinical data from novel therapeutic modalities such as chimeric antigen receptor T cell therapies provide evidence that it may be possible to re-establish immune homeostasis and, potentially, prolong remission or even cure autoimmune diseases. Here, we propose a 'sequential immunotherapy' framework for immune system modulation to help achieve this ambitious goal. This framework encompasses three steps: controlling inflammation; resetting the immune system through elimination of pathogenic immune memory cells; and promoting and maintaining immune homeostasis via immune regulatory agents and tissue repair. We discuss existing drugs and those in development for each of the three steps. We also highlight the importance of causal human biology in identifying and prioritizing novel immunotherapeutic strategies as well as informing their application in specific patient subsets, enabling precision medicine approaches that have the potential to transform clinical care.

Antibody discovery identifies regulatory mechanisms of protein arginine deiminase 4

Unlocking the potential of protein arginine deiminase 4 (PAD4) as a drug target for rheumatoid arthritis requires a deeper understanding of its regulation. In this study, we use unbiased antibody selections to identify functional antibodies capable of either activating or inhibiting PAD4 activity. Through cryogenic-electron microscopy, we characterized the structures of these antibodies in complex with PAD4 and revealed insights into their mechanisms of action. Rather than steric occlusion of the substrate-binding catalytic pocket, the antibodies modulate PAD4 activity through interactions with allosteric binding sites adjacent to the catalytic pocket. These binding events lead to either alteration of the active site conformation or the enzyme oligomeric state, resulting in modulation of PAD4 activity. Our study uses antibody engineering to reveal new mechanisms for enzyme regulation and highlights the potential of using PAD4 agonist and antagonist antibodies for studying PAD4-dependency in disease models and future therapeutic development.

Comprehensive insights into rheumatoid arthritis: Pathophysiology, current therapies and herbal alternatives for effective disease management

Rheumatoid arthritis is a chronic autoimmune inflammatory disease characterized by immune response overexpression, causing pain and swelling in the synovial joints. This condition is caused by auto-reactive antibodies that attack self-antigens due to their incapacity to distinguish between self and foreign molecules. Dysregulated activity within numerous signalling and immunological pathways supports the disease's development and progression, elevating its complexity. While current treatments provide some alleviation, their effectiveness is accompanied by a variety of adverse effects that are inherent in conventional medications. As a result, there is a deep-rooted necessity to investigate alternate therapeutic strategies capable of neutralizing these disadvantages. Medicinal herbs display a variety of potent bioactive phytochemicals that are effective in the complementary management of disease, thus generating an enormous potency for the researchers to delve deep into the development of novel phytomedicine against autoimmune diseases, although additional evidence and understanding are required in terms of their efficacy and pharmacodynamic mechanisms. This literature-based review highlights the dysregulation of immune tolerance in rheumatoid arthritis, analyses the pathophysiology, elucidates relevant signalling pathways involved, evaluates present and future therapy options and underscores the therapeutic attributes of a diverse array of medicinal herbs in addressing this severe disease.

Evaluating the docetaxel effect in an animal model of polyarthritis

Rheumatoid arthritis (RA) is immune-mediated, inflammatory disease that affects synovial joints, and characterized by inflammatory changes in synovial tissue, cartilage, bone, and less commonly in extra-articular structures. Docetaxel (DTX) is a semi-synthetic anti-neoplastic medication. Peptidyl-arginine deiminase type 4 (PAD4) is expressed in macrophages and neutrophils in RA synovial membrane. Their effectiveness is in producing anti-cyclic citrullinated peptide antibodies (ACPA)-targeted citrullinated neoepitopes.

AIM

To evaluate the anti-inflammatory effects of DTX in RA and the effect of methotrexate on PAD4 to investigate its potential as an RA biomarker.

METHODS

Forty male Wistar rats were divided into five groups of eight rats. Healthy rats formed the control group. The Second Group to Fifth group were induced with Complete Freund's adjuvant. The third group received DTX at a dosage of 1 mg/kg on alternate days, as determined by a preliminary experiment. The fourth group was given 1 mg/kg/week of methotrexate intraperitoneally. The fifth group was treated with a half dose of DTX and methotrexate simultaneously.

RESULTS

Significant Arthritis index and knee joint circumference decrease in the DTX group. No significant difference in body weight, platelet-lymphocyte ratio, and white blood cell count between the groups. Neutrophile lymphocyte ratio showed weak correlation with ACPA, while PAD4 showed good correlation with RA markers. Level of ACPA, PAD4, TNF-α, IL-1β, and VEGF significantly decreased in the DTX group than induction group (p < 0.05).

CONCLUSION

DTX reduces the progression and joint destruction in rats induced by Complete Freund's Adjuvant which may due to inhibition of PAD4, TNF-α, IL-1β, VEGF, and ACPA. Also, methotrexate exhibited anti PAD4 effect.

Role of Myostatin in Rheumatoid Arthritis: A Review of the Clinical Impact

Rheumatoid arthritis (RA) is a chronic inflammatory disease that affects synovial joints and that frequently involves extra-articular organs. A multiplicity of interleukins (IL) participates in the pathogenesis of RA, including IL-6, IL-1β, transforming growth factor-beta (TGF-β), and tumor necrosis factor (TNF)-α; immune cells such as monocytes, T and B lymphocytes, and macrophages; and auto-antibodies, mainly rheumatoid factor and anti-citrullinated protein antibodies (ACPAs). Skeletal muscle is also involved in RA, with many patients developing muscle wasting and sarcopenia. Several mechanisms are involved in the myopenia observed in RA, and one of them includes the effects of some interleukins and myokines on myocytes. Myostatin is a myokine member of the TGF-β superfamily; the overproduction of myostatin acts as a negative regulator of growth and differentiates the muscle fibers, limiting their number and size. Recent studies have identified abnormalities in the serum myostatin levels of RA patients, and these have been found to be associated with muscle wasting and other manifestations of severe RA. This review analyzes recent information regarding the relationship between myostatin levels and clinical manifestations of RA and the relevance of myostatin as a therapeutic target for future research.

A comprehensive review on the role of mesenchymal stromal/stem cells in the management of rheumatoid arthritis

INTRODUCTION

Rheumatoid arthritis (RA) is a chronic autoimmune inflammatory disease with systemic manifestations. Although the success of immune modulatory drug therapy is considerable, about 40% of patients do not respond to treatment. Mesenchymal stromal/stem cells (MSCs) have been demonstrated to have therapeutic potential for inflammatory diseases.

AREAS COVERED

This review provides an update on RA disease and on pre-clinical and clinical studies using MSCs from bone marrow, umbilical cord, adipose tissue, and dental pulp, to regulate the immune response. Moreover, the clinical use, safety, limitations, and future perspective of MSCs in RA are discussed. Using the PubMed database and ClincalTrials.gov, peer-reviewed full-text papers, abstracts and clinical trials were identified from 1985 through to April 2023.

EXPERT OPINION

MSCs demonstrated a satisfactory safety profile and potential for clinical efficacy. However, it is mandatory to deepen the investigations on how MSCs affect the proinflammatory deregulated RA patients' cells. MSCs are potentially good candidates for severe RA patients not responding to conventional therapies but a long-term follow-up after stem cells treatment and standardized protocols are needed. Future research should focus on well-designed multicenter randomized clinical trials with adequate sample sizes and properly selected patients satisfying RA criteria for a valid efficacy evaluation.

Iguratimod inhibits protein citrullination and inflammation by downregulating NBCe2 in patients with rheumatoid arthritis

BACKGROUND

Bicarbonate has recently been identified as a crucial factor affecting peptidylarginine deiminase (PAD) activity; however, the mechanism underlying its role in rheumatoid arthritis (RA) remains unclear. Iguratimod (IGU), a small-molecule disease-modifying anti-rheumatic drug, requires further investigation. This study aimed to explore the mechanism by which bicarbonate affects citrullination and inflammation in RA and identify new targets for IGU.

METHODS

We enrolled 20 patients with RA in the study. Sodium bicarbonate cotransporter 2 (NBCe2) was detected in the peripheral blood neutrophils and peripheral blood mononuclear cells (PBMCs) of these patients. The effects of varying concentrations of IGU, methotrexate (MTX), dexamethasone (DXM), and S0859 (an NBCe2 inhibitor) on NBCe2, PAD2, PAD4, and citrullinated histone H3 (cit-H3) levels in, migration ability of, and cytokine production from neutrophils and PBMCs were examined.

RESULTS

Our findings showed that in patients with RA, citrullinated protein production by peripheral blood neutrophils instead of PBMCs, which showed higher NBCe2 expression levels, increased with an increase in the bicarbonate concentration. In addition, tumor necrosis factor-alpha (TNF-α) promoted NBCe2 expression in neutrophils from patients with RA. Furthermore, we revealed that the inhibitory effects of IGU on neutrophil NBCe2 and cit-H3 levels, degrees of inhibition of neutrophil and PBMC migration, and suppression of interleukin 6, TNF-α, and metalloproteinase-9 secretion from neutrophil-like differentiated HL-60 cells did not substantially differ from those of MTX, DXM, and S0859 at specific doses.

CONCLUSIONS

Bicarbonate promotes protein citrullination and inflammation in RA via NBCe2, and IGU can downregulate NBCe2.

Targeting the vivid facets of apolipoproteins as a cardiovascular risk factor in rheumatoid arthritis

Mostly, cardiovascular diseases are blamed for casualties in rheumatoid arthritis (RA) patients. Customarily, dyslipidemia is probably the most prevalent underlying cause of untimely demise in people suffering from RA as it hastens the expansion of atherosclerosis. The engagement of inflammatory cytokines like tumor necrosis factor-α (TNF-α), interleukin-1 (IL-1), interleukin-6 (IL-6), etc., is crucial in the progression and proliferation of both RA and abnormal lipid parameters. Thus, lipid abnormalities should be monitored frequently in patients with both primary and advanced RA stages. An advanced lipid profile examination, i.e., direct role of apolipoproteins associated with various lipid molecules is a more dependable approach for better understanding of the disease and selecting suitable therapeutic targets. Therefore, studying their apolipoproteins is more relevant than assessing RA patients' altered lipid profile levels. Among the various apolipoprotein classes, Apo A1 and Apo B are primarily being focused. In addition, it also addresses how calculating Apo B:Apo A1 ratio can aid in analyzing the disease's risk. The marketed therapies available to control lipid abnormalities are associated with many other risk factors. Hence, directly targeting Apo A1 and Apo B would provide a better and safer option.

Exhaled Nitric Oxide Reflects the Immune Reactions of the Airways in Early Rheumatoid Arthritis

Patients with rheumatoid arthritis (RA) have altered levels of exhaled nitric oxide (NO) compared with healthy controls. Here, we investigated whether the clinical features of and immunological factors in RA pathogenesis could be linked to the NO lung dynamics in early disease. A total of 44 patients with early RA and anti-citrullinated peptide antibodies (ACPAs), specified as cyclic citrullinated peptide 2 (CCP2), were included. Their exhaled NO levels were measured, and the alveolar concentration, the airway compartment diffusing capacity and the airway wall concentration of NO were estimated using the Högman-Meriläinen algorithm. The disease activity was measured using the Disease Activity Score for 28 joints. Serum samples were analysed for anti-CCP2, rheumatoid factor, free secretory component, secretory component containing ACPAs, antibodies against Porphyromonas gingivalis (Rgp) and total levels of IgA, IgA1 and IgA2. Significant negative correlations were found between the airway wall concentration of NO and the number of swollen joints (Rho -0.48, p = 0.004), between the airway wall concentration of NO and IgA rheumatoid factor (Rho -0.41, p = 0.017), between the alveolar concentration and free secretory component (Rho -0.35, p = 0.023) and between the alveolar concentration and C-reactive protein (Rho -0.36, p = 0.016), but none were found for anti-CCP2, IgM rheumatoid factor or the anti-Rgp levels. In conclusion, altered NO levels, particularly its production in the airway walls, may have a role in the pathogenesis of ACPA-positive RA.

The monocyte cell surface is a unique site of autoantigen generation in rheumatoid arthritis

Although anti-citrullinated protein autoantibodies (ACPAs) are a hallmark serological feature of rheumatoid arthritis (RA), the mechanisms and cellular sources behind the generation of the RA citrullinome remain incompletely defined. Peptidylarginine deiminase IV (PAD4), one of the key enzymatic drivers of citrullination in the RA joint, is expressed by granulocytes and monocytes; however, the subcellular localization and contribution of monocyte-derived PAD4 to the generation of citrullinated autoantigens remain underexplored. In this study, we demonstrate that PAD4 displays a widespread cellular distribution in monocytes, including expression on the cell surface. Surface PAD4 was enzymatically active and capable of citrullinating extracellular fibrinogen and endogenous surface proteins in a calcium dose-dependent manner. Fibrinogen citrullinated by monocyte-surface PAD4 could be specifically recognized over native fibrinogen by a panel of eight human monoclonal ACPAs. Several unique PAD4 substrates were identified on the monocyte surface via mass spectrometry, with citrullination of the CD11b and CD18 components of the Mac-1 integrin complex being the most abundant. Citrullinated Mac-1 was found to be a target of ACPAs in 25% of RA patients, and Mac-1 ACPAs were significantly associated with HLA-DRB1 shared epitope alleles, higher C-reactive protein and IL-6 levels, and more erosive joint damage. Our findings implicate the monocyte cell surface as a unique and consequential site of extracellular and cell surface autoantigen generation in RA.

Diagnostic values, association with disease activity and possible risk factors of anti-PAD4 in rheumatoid arthritis: a meta-analysis

OBJECTIVE

Anti-peptidyl arginine deaminase 4 (anti-PAD4) antibody has been a subject of investigation in RA in the last two decades. This meta-analysis investigated the diagnostic values, association with disease activity and possible risk factors of anti-PAD4 antibody in rheumatoid arthritis.

METHOD

We searched studies from five databases up to 1 December 2022. Bivariate mixed-effect models were used to pool the diagnostic accuracy indexes, and the summary receiver operating characteristics (SROC) curve was plotted. The quality of diagnostic studies was assessed using QUADAS-2. Non-diagnostic meta-analyses were conducted using the random-effects model. Sensitivity analysis, meta-regression, subgroup analyses and Deeks' funnel plot asymmetry test were used to address heterogeneity.

RESULT

Finally, 24 journal articles and one letter were included. Anti-PAD4 antibody had a good diagnostic value between RA and healthy individuals, but it might be lower between RA and other rheumatic diseases. Moreover, anti-PAD4 could slightly enhance RA diagnostic sensitivity with a combination of ACPA or ACPA/RF. Anti-PAD4 antibody was positively correlated with HLA-SE and negatively correlated with ever or current smoking in patients with RA. RA patients with anti-PAD4 antibody had higher DAS28, ESR, swollen joint count (SJC) and the possibility of having interstitial lung disease (ILD) and pulmonary fibrosis compared with those without.

CONCLUSION

Our study suggests that anti-PAD4 antibody is a potentially useful diagnostic biomarker and clinical indicator for RA. Further mechanistic studies are required to understand the impact of HLA-SE and smoking on the production of anti-PAD4 antibody.

Exosomes and exosomal miRNAs: A new avenue for the future treatment of rheumatoid arthritis

Rheumatoid arthritis is a chronic systemic autoimmune disease that involves mainly synovitis and joint injury and is one of the main causes of disability. The pathogenesis of rheumatoid arthritis is complicated, and the treatment cycle is long. The traditional methods of inhibiting inflammation and immunosuppression are no longer sufficient for treatment of the disease, so there is an urgent need to seek new treatments. The exocrine microenvironment is a kind of microvesicle with a lipid bilayer membrane structure that can be secreted by most cells in the body. This structure contains cell-specific proteins, lipids and nucleic acids that can transmit this information from one cell to another. To achieve cell-to-cell communication. Exocrine microRNAs can be contained in exocrine cells and can be selectively transferred to target receptor cells via exocrine signaling, thus regulating the physiological function of target cells. This article focuses on the pathological changes that occur during the development of rheumatoid arthritis and the biological regulation of exocrine and exocrine microRNAs in rheumatoid joints. Research on the roles of exocrine and exocrine microRNAs in regulating the inflammatory response, cell proliferation/apoptosis, autophagy, effects on fibroblast-like synoviocytes and immune regulation in rheumatoid arthritis was reviewed. In addition, the challenges faced by this new treatment are discussed.

Association of PADI2 and PADI4 polymorphisms in COVID-19 host severity and non-survival

Background

Enzymes of the peptidylarginine deiminase family (PADs) play a relevant role in the pathogenesis of COVID-19. However, the association of single nucleotide polymorphisms (SNPs) in their genes with COVID-19 severity and death is unknown.

Methodology

We included 1045 patients who were diagnosed with COVID-19 between October 2020 and December 2021. All subjects were genotyped for PADI2 (rs1005753 and rs2235926) and PADI4 (rs11203366, rs11203367, and rs874881) SNPs by TaqMan assays and their associations with disease severity, death, and inflammatory biomarkers were evaluated.

Results

291 patients presented had severe COVID-19 according to PaO2/FiO2, and 393 had a non-survival outcome. Carriers of the rs1005753 G/G genotype in the PADI2 gene presented susceptibility for severe COVID-19, while the heterozygous carriers in rs11203366, rs11203367, and rs874881 of the PADI4 gene showed risk of death. The GTACC haplotype in PADI2-PADI4 was associated with susceptibility to severe COVID-19, while the GCACC haplotype was a protective factor. The GCGTG haplotype was associated with severe COVID-19 but as a protective haplotype for death. Finally, the GTACC haplotype was associated with platelet-to-lymphocyte ratio (PLR), the GCACC haplotype with neutrophil-to-hemoglobin and lymphocyte and the GCGTG haplotype as a protective factor for the elevation of procalcitonin, D-dimer, CRP, LCRP, NHL, SII, NLR, and PLR.

Conclusions

Our results suggest that the haplotypic combination of GTACC and some individual genotypes of PADI2 and PADI4 contribute to the subjects' susceptibility for severity and death by COVID-19.

PAD4 controls tumor immunity via restraining the MHC class II machinery in macrophages

Tumor-associated macrophages (TAMs) shape tumor immunity and therapeutic efficacy. However, it is poorly understood whether and how post-translational modifications (PTMs) intrinsically affect the phenotype and function of TAMs. Here, we reveal that peptidylarginine deiminase 4 (PAD4) exhibits the highest expression among common PTM enzymes in TAMs and negatively correlates with the clinical response to immune checkpoint blockade. Genetic and pharmacological inhibition of PAD4 in macrophages prevents tumor progression in tumor-bearing mouse models, accompanied by an increase in macrophage major histocompatibility complex (MHC) class II expression and T cell effector function. Mechanistically, PAD4 citrullinates STAT1 at arginine 121, thereby promoting the interaction between STAT1 and protein inhibitor of activated STAT1 (PIAS1), and the loss of PAD4 abolishes this interaction, ablating the inhibitory role of PIAS1 in the expression of MHC class II machinery in macrophages and enhancing T cell activation. Thus, the PAD4-STAT1-PIAS1 axis is an immune restriction mechanism in macrophages and may serve as a cancer immunotherapy target.

The immunopathogenesis of narcolepsy type 1

Narcolepsy type 1 (NT1) is a chronic sleep disorder resulting from the loss of a small population of hypothalamic neurons that produce wake-promoting hypocretin (HCRT; also known as orexin) peptides. An immune-mediated pathology for NT1 has long been suspected given its exceptionally tight association with the MHC class II allele HLA-DQB1*06:02, as well as recent genetic evidence showing associations with polymorphisms of T cell receptor genes and other immune-relevant loci and the increased incidence of NT1 that has been observed after vaccination with the influenza vaccine Pandemrix. The search for both self-antigens and foreign antigens recognized by the pathogenic T cell response in NT1 is ongoing. Increased T cell reactivity against HCRT has been consistently reported in patients with NT1, but data demonstrating a primary role for T cells in neuronal destruction are currently lacking. Animal models are providing clues regarding the roles of autoreactive CD4+ and CD8+ T cells in the disease. Elucidation of the pathogenesis of NT1 will allow for the development of targeted immunotherapies at disease onset and could serve as a model for other immune-mediated neurological diseases.

A variety of death modes of neutrophils and their role in the etiology of autoimmune diseases

Neutrophils are important in the context of innate immunity and actively contribute to the progression of diverse autoimmune disorders. Distinct death mechanisms of neutrophils may exhibit specific and pivotal roles in autoimmune diseases and disease pathogenesis through the orchestration of immune homeostasis, the facilitation of autoantibody production, the induction of tissue and organ damage, and the incitement of pathological alterations. In recent years, more studies have provided in-depth examination of various neutrophil death modes, revealing nuances that challenge conventional understanding and underscoring their potential clinical utility in diagnosis and treatment. This review explores the multifaceted processes and characteristics of neutrophil death, with a focus on tailored investigations within various autoimmune diseases. It also highlights the potential interplay between neutrophil death and the landscape of autoimmune disorders. The review encapsulates the pertinent pathways implicated in various neutrophil death mechanisms across diverse autoimmune diseases while also charts possible avenues for future research.

Targeting glycolytic pathway in fibroblast-like synoviocytes for rheumatoid arthritis therapy: challenges and opportunities

INTRODUCTION

Rheumatoid arthritis (RA) is a chronic autoimmune disorder characterized by hyperplastic synovium, pannus formation, immune cell infiltration, and potential articular cartilage damage. Notably, fibroblast-like synoviocytes (FLS), especially rheumatoid arthritis fibroblast-like synoviocytes (RAFLS), exhibit specific overexpression of glycolytic enzymes, resulting in heightened glycolysis. This elevated glycolysis serves to generate ATP and plays a pivotal role in immune regulation, angiogenesis, and adaptation to hypoxia. Key glycolytic enzymes, such as hexokinase 2 (HK2), phosphofructose-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3), and pyruvate kinase M2 (PKM2), significantly contribute to the pathogenic behavior of RAFLS. This increased glycolysis activity is regulated by various signaling pathways.

MATERIALS AND METHODS

A comprehensive literature search was conducted to retrieve relevant studies published from January 1, 2010, to the present, focusing on RAFLS glycolysis, RA pathogenesis, glycolytic regulation pathways, and small-molecule drugs targeting glycolysis.

CONCLUSION

This review provides a thorough exploration of the pathological and physiological characteristics of three crucial glycolytic enzymes in RA. It delves into their putative regulatory mechanisms, shedding light on their significance in RAFLS. Furthermore, the review offers an up-to-date overview of emerging small-molecule candidate drugs designed to target these glycolytic enzymes and the upstream signaling pathways that regulate them. By enhancing our understanding of the pathogenic mechanisms of RA and highlighting the pivotal role of glycolytic enzymes, this study contributes to the development of innovative anti-rheumatic therapies.

An unbiased proteomic analysis of PAD4 in human monocytes: novel substrates, binding partners and subcellular localizations

Peptidylarginine deiminase IV (PAD4) post-translationally converts arginine residues in proteins to citrullines and is implicated in playing a central role in the pathogenesis of several diseases. Although PAD4 was historically thought to be a nuclear enzyme, recent evidence has revealed a more complex localization of PAD4 with evidence of additional cytosolic and cell surface localization and activity. However, the mechanisms by which PAD4, which lacks conventional secretory signal sequences, traffics to extranuclear localizations are unknown. In this study, we show that PAD4 was enriched in the organelle fraction of monocytes with evidence of citrullination of organelle proteins. We also demonstrated that PAD4 can bind to several cytosolic, nuclear and organelle proteins that may serve as binding partners for PAD4 to traffic intracellularly. Additionally, cell surface expression of PAD4 increased with monocyte differentiation into monocyte-derived dendritic cells and co-localized with several endocytic/autophagic and conventional secretory pathway markers, implicating the use of these pathways by PAD4 to traffic within the cell. Our results suggest that PAD4 is expressed in multiple subcellular localizations and may play previously unappreciated roles in physiological and pathological conditions. This article is part of the Theo Murphy meeting issue 'The virtues and vices of protein citrullination'.

Aberrant Activation of Immune and Non-Immune Cells Contributes to Joint Inflammation and Bone Degradation in Rheumatoid Arthritis

Abnormal activation of multiple immune and non-immune cells and proinflammatory factors mediate the development of joint inflammation in genetically susceptible individuals. Although specific environmental factors like smoking and infections are associated with disease pathogenesis, until now, we did not know the autoantigens and arthritogenic factors that trigger the initiation of the clinical disease. Autoantibodies recognizing specific post-translationally modified and unmodified antigens are generated and in circulation before the onset of the joint disease, and could serve as diagnostic and prognostic markers. The characteristic features of autoantibodies change regarding sub-class, affinity, glycosylation pattern, and epitope spreading before the disease onset. Some of these antibodies were proven to be pathogenic using animal and cell-culture models. However, not all of them can induce disease in animals. This review discusses the aberrant activation of major immune and non-immune cells contributing to joint inflammation. Recent studies explored the protective effects of extracellular vesicles from mesenchymal stem cells and bacteria on joints by targeting specific cells and pathways. Current therapeutics in clinics target cells and inflammatory pathways to attenuate joint inflammation and protect the cartilage and bones from degradation, but none cure the disease. Hence, more basic research is needed to investigate the triggers and mechanisms involved in initiating the disease and relapses to prevent chronic inflammation from damaging joint architecture.

Myelodysplastic neoplasm-associated U2AF1 mutations induce host defense defects by compromising neutrophil chemotaxis

Myelodysplastic neoplasm (MDS) is a hematopoietic stem cell disorder that may evolve into acute myeloid leukemia. Fatal infection is among the most common cause of death in MDS patients, likely due to myeloid cell cytopenia and dysfunction in these patients. Mutations in genes that encode components of the spliceosome represent the most common class of somatically acquired mutations in MDS patients. To determine the molecular underpinnings of the host defense defects in MDS patients, we investigated the MDS-associated spliceosome mutation U2AF1-S34F using a transgenic mouse model that expresses this mutant gene. We found that U2AF1-S34F causes a profound host defense defect in these mice, likely by inducing a significant neutrophil chemotaxis defect. Studies in human neutrophils suggest that this effect of U2AF1-S34F likely extends to MDS patients as well. RNA-seq analysis suggests that the expression of multiple genes that mediate cell migration are affected by this spliceosome mutation and therefore are likely drivers of this neutrophil dysfunction.

Extracellular aaRSs drive autoimmune and inflammatory responses in rheumatoid arthritis via the release of cytokines and PAD4

OBJECTIVES

Recent studies demonstrate that extracellular-released aminoacyl-tRNA synthetases (aaRSs) play unique roles in immune responses and diseases. This study aimed to understand the role of extracellular aaRSs in the pathogenesis of rheumatoid arthritis (RA).

METHODS

Primary macrophages and fibroblast-like synoviocytes were cultured with aaRSs. aaRS-induced cytokine production including IL-6 and TNF-α was detected by ELISA. Transcriptomic features of aaRS-stimulated macrophages were examined using RNA-sequencing. Serum and synovial fluid (SF) aaRS levels in patients with RA were assessed using ELISA. Peptidyl arginine deiminase (PAD) 4 release from macrophages stimulated with aaRSs was detected by ELISA. Citrullination of aaRSs by themselves was examined by immunoprecipitation and western blotting. Furthermore, aaRS inhibitory peptides were used for inhibition of arthritis in two mouse RA models, collagen-induced arthritis and collagen antibody-induced arthritis.

RESULTS

All 20 aaRSs functioned as alarmin; they induced pro-inflammatory cytokines through the CD14-MD2-TLR4 axis. Stimulation of macrophages with aaRSs displayed persistent innate inflammatory responses. Serum and SF levels of many aaRSs increased in patients with RA compared with control subjects. Furthermore, aaRSs released PAD4 from living macrophages, leading to their citrullination. We demonstrate that aaRS inhibitory peptides suppress cytokine production and PAD4 release by aaRSs and alleviate arthritic symptoms in a mouse RA model.

CONCLUSIONS

Our findings uncovered the significant role of aaRSs as a novel alarmin in RA pathogenesis, indicating that their blocking agents are potent antirheumatic drugs.

Global research trends and focus on the link between rheumatoid arthritis and neutrophil extracellular traps: a bibliometric analysis from 1985 to 2023

Rheumatoid arthritis (RA) is an autoimmune disease that currently has an unknown cause and pathogenesis, and is associated with many complications and a high disability rate. The neutrophil extracellular trap network (NETs) is a newly discovered mechanism that allows neutrophils to capture and kill pathogens. Multiple studies in recent years have highlighted its relevance to the progression of rheumatoid arthritis. Despite the growing number of studies indicating the crucial role of NETs in RA, there has been no bibliometric review of research hotspots and trends in this area. In this study, we retrieved articles related to NETs in RA from the Web of Science Core Collection (WoSCC) database from 1985 to 2023 and used visualization tools such as Citespace, VOSviewer, Tableau Public, and Microsoft Office Excel 2021 to analyze the data. After screening, we included a total of 416 publications involving 2,334 researchers from 1,357 institutions in 167 countries/regions, with relevant articles published in 219 journals. The U.S., China, and Germany are the top 3 countries/regions with 124, 57, and 37 publications respectively. Mariana J. Kaplan is the most published author, and journals such as Frontiers in Immunology and International Journal of Molecular Sciences have had a significant impact on research in this field. The clinical application of PAD enzymes and their inhibitors, and the drug development of NETs as therapeutic targets for RA is a trend for future research. Our study provides a comprehensive bibliometric analysis and summary of NETs in RA publications, which will aid researchers in conducting further scientific research.

Highly-tumor-targeted PAD4 inhibitors with PBA modification inhibit tumors in vivo by specifically inhibiting the PAD4-H3cit-NETs pathway in neutrophils

As a new target for tumor therapy, PAD4 protein, shows excellent antitumor activity, and phenylboronic acid (PBA) could combine with sialic acid on the tumor surface to achieve dual targeting in situ and for metastatic tumors. The purpose of this study was therefore to modify PAD4 protein inhibitors with different phenylboronic acid groups in order to obtain highly-targeted PAD4 inhibitors. The activity and mechanism of these PBA-PAD4 inhibitors were studied in vitro by MTT assay, laser confocal analysis, and flow cytometry. The effects of the compounds on primary tumor and lung metastasis in mice were evaluated in vivo using a S180 sarcoma model and a 4T1 breast cancer model. In addition, cytometry mass (CyTOF) was used to analyze the immune microenvironment, and the results show that the PAD4 inhibitor 5i modified by m-PBA at the carboxyl terminal of ornithine skeleton had the best antitumor activity. In vitro evaluation of this activity revealed that 5i could not directly kill tumor cells but had a significant inhibitory effect on tumor cell metastasis. Further mechanism studies showed that 5i could be taken up by 4T1 cells in a time-dependent manner and distributed around the cell membrane but could not be taken up by normal cells. In addition, although 5i was distributed in the cytoplasm of tumor cells while in the nucleus of neutrophils, it could both decrease the histone 3 citrullination (H3cit) in the nucleus. In vivo 4T1 tumor-bearing mouse models, 5i inhibited breast cancer growth and metastasis in a concentration-dependent manner, and NET formation in tumor tissues was significantly reduced. In conclusion, PBA-PAD4 inhibitors show high targeting of tumor cells and good safety in vivo. By specifically inhibiting PAD4 protein in the neutrophil nucleus, PBA-PAD4 inhibitors also show excellent antitumor activity toward growth and metastasis in vivo, which provides a new idea for the design of highly-targeted PAD4 inhibitors.