The use of synthetic peptides for detection of anti-citrullinated protein antibodies in rheumatoid arthritis

Antibody Cross-Reactivity in Auto-Immune Diseases

Autoimmunity is defined by the presence of antibodies and/or T cells directed against self-components. Although of unknown etiology, autoimmunity commonly is associated with environmental factors such as infections, which have been reported to increase the risk of developing autoimmune diseases. Occasionally, similarities between infectious non-self and self-tissue antigens may contribute to immunological cross-reactivity in autoimmune diseases. These reactions may be interpreted as molecular mimicry, which describes cross-reactivity between foreign pathogens and self-antigens that have been reported to cause tissue damage and to contribute to the development of autoimmunity. By focusing on the nature of antibodies, cross-reactivity in general, and antibody-antigen interactions, this review aims to characterize the nature of potential cross-reactive immune reactions between infectious non-self and self-tissue antigens which may be associated with autoimmunity but may not actually be the cause of disease onset.

Anti-citrullinated protein antibodies as biomarkers in rheumatoid arthritis

INTRODUCTION

The serological biomarker anti-citrullinated protein antibodies (ACPAs) may have several functions but is especially important for the diagnosis of rheumatoid arthritis (RA) along with clinical symptoms.

AREAS COVERED

This review provides an overview of ACPAs, which are useful in RA diagnostics and may improve our understanding of disease etiology. PubMed was searched with combinations of words related to antibodies recognizing epitopes containing the post-translationally modified amino acid citrulline in combination with rheumatoid arthritis; cyclic citrullinated peptide, CCP, anti-CCP, anti-citrullinated protein antibodies, ACPA, citrullination, peptide/protein arginine deiminase, PAD, filaggrin, vimentin, keratin, collagen, perinuclear factor, EBNA1, EBNA2, and others. From this search, we made a qualitative extract of publications relevant to the discovery, characterization, and clinical use of these antibodies in relation to RA. We highlight significant findings and identify areas for improvement.

EXPERT OPINION

ACPAs have high diagnostic sensitivity and specificity for RA and recognize citrullinated epitopes from several proteins. The best-performing single epitope originates from Epstein-Barr Virus nuclear antigen 2 and contains a central Cit-Gly motif, which is recognized by ACPAS when located in a flexible peptide structure. In addition, ACPAs may also have prognostic value, especially in relation to early treatment, although ACPAs' main function is to aid in the diagnosis of RA.

Reactivity of Rheumatoid Arthritis-Associated Citrulline-Dependent Antibodies to Epstein-Barr Virus Nuclear Antigen1-3

Rheumatoid arthritis (RA) is a chronic disease which causes joint inflammation and, ultimately, erosion of the underlying bone. Diagnosis of RA is based on the presence of biomarkers, such as anti-citrullinated protein antibodies (ACPA) and rheumatoid factors, along with clinical symptoms. Much evidence points to a link between the Epstein-Barr virus and RA. In this study, we analyzed ACPA reactivity to citrullinated peptides originating from Epstein-Barr nuclear antigens (EBNA1, EBNA2, and EBNA3) in order to elaborate the diagnostic potential of citrullinated EBNA peptides. Moreover, ACPA cross-reactivity to citrullinated peptides from myelin basic protein (MBP) was analyzed, as citrullinated MBP recently was described to be associated with multiple sclerosis, and some degree of sequence homology between MBP and citrullinated EBNA exists. A peptide from EBNA2, (EBNA2-A, GQGRGRWRG-Cit-GSKGRGRMH) reacted with approximately 70% of all RA sera, whereas only limited reactivity was detected to EBNA1 and EBNA3 peptides. Moreover, screening of ACPA reactivity to hybrid peptides of EBNA3-A (EPDSRDQQS-Cit-GQRRGDENRG) and EBNA2-A and peptides containing citrulline close to the N-terminal confirmed that ACPA sera contain different populations of ACPAs. No notable ACPA reactivity to MBP peptides was found, confirming that ACPAs are specific for RA, and that other factors than the presence of a central Cit-Gly motif are crucial for antibody binding. Collectively, these findings illustrate that citrullinated EBNA2 is an optimal candidate for ACPA detection, supporting current evidence that EBV is linked to RA onset.

Specificity of Anti-Citrullinated Protein Antibodies to Citrullinated α-Enolase Peptides as a Function of Epitope Structure and Composition

Rheumatoid arthritis (RA) is an autoimmune disease affecting approximately 1-2% of the world population. In addition to the first discovered serologic markers for RA, the rheumatoid factors (RFs), anti-citrullinated protein antibodies (ACPAs) are even more specific for the disease compared to RFs and are found in 70-80% of RA patient sera. RA etiopathogenesis still needs to be elucidated, as different factors are proposed to be involved, such as Epstein-Barr virus infection. Hence, understanding the interaction between ACPAs and their citrullinated peptide targets is relevant for a better knowledge of RA pathophysiology and for diagnostic purposes. In this study, a cohort of RA sera, healthy control sera and multiple sclerosis sera were screened for reactivity to a variety of citrullinated peptides originating from α-enolase, pro-filaggrin, proteoglycan and Epstein-Barr nuclear antigen-2 by enzyme-linked immunosorbent assay. ACPA reactivity to citrullinated α-enolase peptides was found to depend on peptide length and peptide conformation, favouring cyclic (disulfide bond) conformations for long peptides and linear peptides for truncated ones. Additional investigations about the optimal peptide conformation for ACPA detection, employing pro-filaggrin and EBNA-2 peptides, confirmed these findings, indicating a positive effect of cyclization of longer peptides of approximately 20 amino acids. Moreover, screening of the citrullinated peptides confirmed that ACPAs can be divided into two groups based on their reactivity. Approximately 90% of RA sera recognize several peptide targets, being defined as cross-reactive or overlapping reactivities, and whose reactivity to the citrullinated peptide is considered primarily to be backbone-dependent. In contrast, approximately 10% recognize a single target and are defined as nonoverlapping, primarily depending on the specific amino acid side-chains in the epitope for a stable interaction. Collectively, this study contributed to characterize epitope composition and structure for optimal ACPA reactivity and to obtain further knowledge about the cross-reactive nature of ACPAs.

Preparation and characterization of cyclic citrullinated peptide-immobilized latex beads for measurement of anti-citrillinated protein antibody through latex particle-enhanced turbidimetric immunoassay

The anti-citrillinated protein antibody (ACPA) plays an important role in early diagnosis of rheumatoid arthritis (RA), and is usually detected by using cyclic citrullinated peptide (CCP) as antigen. The ACPA against CCP test is usually performed utilizing enzyme-linked immunosorbent assay (ELISA), but the ELISA is expensive and time-consuming. Here, latex particle-enhanced turbidimetric immunoassay (LTIA) based on CCP-immobilized latex bead was proposed for fast measurements of ACPA of RA patients. CCP-immobilized latex bead was fabricated through three methods, including direct coupling, overall coupling and layer by layer coupling. According to the optimized experiments, layer-by-layer coupling was the best method with advantages of time-saving, simple operation and good repeatability. In addition, a spacer arm of appropriate length between latex beads and CCP could avoid stereoscopic obstacles and make ACPA closer to CCP. The CCP-immobilized latex bead based on layer by layer coupling (CCP-LB-LLC) was used for assembling the homemade kit, which was applied in fast measurements of ACPA through LTIA. The homemade kit possessed a low limit of detection (0.2 U/mL) and an acceptable the batch-to-batch reproducibility. In addition, the homemade kit can be stored at 4 °C for at least one month. When used to detect 20 clinical samples, the results of homemade kit were consistent with commercial ELISA. Furthermore, LTIA based on the homemade kit was simpler and cheaper than ELISA. These results demonstrated that the homemade kit could be useful for diagnosis of RA patients.

Peptides, Antibodies, Peptide Antibodies and More

The applications of peptides and antibodies to multiple targets have emerged as powerful tools in research, diagnostics, vaccine development, and therapeutics. Antibodies are unique since they, in theory, can be directed to any desired target, which illustrates their versatile nature and broad spectrum of use as illustrated by numerous applications of peptide antibodies. In recent years, due to the inherent limitations such as size and physical properties of antibodies, it has been attempted to generate new molecular compounds with equally high specificity and affinity, albeit with relatively low success. Based on this, peptides, antibodies, and peptide antibodies have established their importance and remain crucial reagents in molecular biology.

Label-free electrochemical impedimetric immunosensor for sensitive detection of IgM rheumatoid factor in human serum

The authors report a label-free and direct detection of rheumatoid factor- Immunoglobulin M (IgM-RF) based on an impedimetric-interdigitated wave type microelectrode array (IDWμE). IDWμE was functionalized with a self-assembled monolayer (SAM) of thioctic acid for antigen immobilization. The SAM functionalized IDWμE were characterized by atomic force microscopy, Energy Dispersive X-Ray Spectroscopy, and X-ray photoelectron spectroscopy. The covalent immobilization of IgG-Fc onto the SAM modified electrode arrays was characterized morphologically via AFM and electrochemically via cyclic voltammetry and electrochemical impedance spectroscopy. Impedimetric measurements in the presence of redox probe (Potassium ferrocyanide and potassium ferricyanide) showed a significant change in both the impedance spectrum and charge transfer resistance upon IgM-RF binding. Impedance measurements were target specific and linear with an increase in IgM-RF concentrations between 1 and 200 IU mL^-1 in redox probe and human serum, respectively. The sensor showed detection limits of 0.6 IU mL^-1 in the presence of redox probe and 0.22 IU mL^-1 in the human serum. The biosensor exhibited good reproducibility (relative standard deviation (RSD), 4.96%) and repeatability (RSD, 2.31%) with an acceptable selectivity towards IgM-RF detection. The sensor also showed a good stability for 3 weeks at 4 °C in 1X PBS. Therefore, the sensitive and stable immunosensor exhibiting IDWμE features can be integrated with a miniaturized potentiostat to develop a sensing system that detects IgM-RF for point-of-care applications.

Use of a Citrullinated Peptide Panel for Detection of Anti-Citrullinated Protein Antibodies by Enzyme-Linked Immunosorbent Assay

Anti-citrullinated protein antibodies (ACPA)s are a hallmark of rheumatoid arthritis (RA) and are essential for serological diagnosis of RA.ACPAs are not specific for a single citrullinated target; in fact, several citrullinated ACPA target proteins have been described. As a consequence, ACPAs are primarily detected by enzyme-linked immunosorbent assays, where several citrullinated peptides are used as target antigens.This chapter focuses on the detection of ACPAs using a recently developed peptide panel in enzyme-linked immunosorbent assays.

EBNA1 IgM-Based Discrimination Between Rheumatoid Arthritis Patients, Systemic Lupus Erythematosus Patients and Healthy Controls

Epstein-Barr Virus (EBV) has been associated with development of rheumatic connective tissue diseases like rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) in genetically susceptible individuals. Diagnosis of RA and SLE relies on clinical criteria in combination with the presence of characteristic autoantibodies. In addition, antibodies to several EBV antigens have been shown to be elevated in patients with these diseases compared to healthy controls (HC). Here, we elaborated improved enzyme-linked immunosorbent assays for antibodies (IgM, IgA, IgG) to the EBV proteins Epstein-Barr Virus nuclear antigen (EBNA)1 and early antigen diffuse (EAD) in order to determine their potential diagnostic role. We showed that especially EBNA1 IgM distinguished RA from SLE and HCs and also distinguished SLE from HCs. EBNA1 IgA was almost as effective in differentiating RA from SLE and HC, while EAD IgG and IgA were able to discern SLE patients from RA patients and HCs. Collectively, these findings illustrate the potential diagnostic use of antibodies to EBV proteins to diagnose RA and to differentiate SLE from RA.

Reducing the risk of misdiagnosis of indirect ELISA by normalizing serum-specific background noise: The example of detecting anti-FGFR3 autoantibodies

Indirect enzyme-linked immunosorbent assay (ELISA) is an important diagnostic method as it enables the quantification of the presence of autoantibodies in human blood sera. However, unspecific binding of antibodies to the solid phase causes considerable serum-specific background noise (SSBN), involving the risk of false positive diagnosis. Therefore, we present a simple and concise, yet obvious proof-of-principle of a recently suggested normalization method. The method is based on subtracting SSBN by using non-coated ELISA wells as a control for each serum-of-interest. We performed ELISA to quantify anti-fibroblast growth factor receptor 3 (FGFR3) antibody levels in three positive controls (two anti-FGFR3-positive patients and a rabbit antiserum against FGFR3) and 58 negative controls (healthy blood donors). In all subjects, we found considerable unspecific reactivity which strongly varied among subjects. The conventional normalization method was not able to balance this strong SSBN, as demonstrated by 2/58 false positive healthy controls and one FGFR3-positive patient that was hidden in the noise (false negative). SSBN normalization reduced the frequency of false-positives to 0/58. Further, all three anti-FGFR3-positive sera were successfully detected and even doubled their z-score used to determine positivity. Albeit occupying more space on the ELISA plate, we strongly recommend considering this normalization method when working with blood sera. To better put the idea across to the community, we depict the SSBN issue and its solution in a graphic scheme. We conclude that SSBN normalization increases the sensitivity and specificity of indirect ELISA and thereby reduces the risk of false positive and false negative diagnosis. © 2019. Licensed under the Creative Commons [CC BY-NC 4.0 licence, https://doi.org/10.1016/j.jim.2019.01.004].