Reliable and cost-effective serodiagnosis of rheumatoid arthritis

Absolute Quantitation of Serum Antibody Reactivity Using the Richards Growth Model for Antigen Microspot Titration

In spite of its pivotal role in the characterization of humoral immunity, there is no accepted method for the absolute quantitation of antigen-specific serum antibodies. We devised a novel method to quantify polyclonal antibody reactivity, which exploits protein microspot assays and employs a novel analytical approach. Microarrays with a density series of disease-specific antigens were treated with different serum dilutions and developed for IgG and IgA binding. By fitting the binding data of both dilution series to a product of two generalized logistic functions, we obtained estimates of antibody reactivity of two immunoglobulin classes simultaneously. These estimates are the antigen concentrations required for reaching the inflection point of thermodynamic activity coefficient of antibodies and the limiting activity coefficient of antigen. By providing universal chemical units, this approach may improve the standardization of serological testing, the quality control of antibodies and the quantitative mapping of the antibody-antigen interaction space.

Comparison of the analytical and clinical performances of four anti-cyclic citrullinated peptide antibody assays for diagnosing rheumatoid arthritis

INTRODUCTION/OBJECTIVES

Anti-cyclic citrullinated peptide antibody (anti-CCP) is one of the most important serologic markers for diagnosing rheumatoid arthritis (RA). This study aimed to compare the analytical and clinical performances of the second- and third-generation anti-CCP assays.

METHODS

Four automated anti-CCP assays were evaluated: Chorus anti-CCP (Diesse Diagnostica), Elecsys anti-CCP (Roche Diagnostics), Atellica® IM anti-CCP IgG (Siemens Healthineers), and Quanta Flash® CCP3 (Inova Diagnostics Inc.). Analytical performance included the precision, linearity, correlation, and concordance rate. For evaluating the clinical performance, 240 patient samples (120 positive and 120 negative samples, determined by the Chorus anti-CCP assay) were used, including those with a diagnosis of RA (n = 132) and non-RA (n = 108). Using receiver operating characteristic (ROC) curve analysis, the sensitivity and specificity were evaluated.

RESULTS

All four assays that were evaluated showed good precision and linearity, and their correlation and concordance rates were in acceptable ranges. The area under the curve (AUC) values ranged from 0.888 to 0.914, showing a good diagnostic performance. The sensitivity and specificity of all assays were similar (88.0-97.2%).

CONCLUSIONS

All four anti-CCP assays showed good analytical and diagnostic performances for diagnosing RA. After adjusting the cutoff values, these assays are expected to show enhanced sensitivity and specificity. Key Points • Previous studies have described the diagnostic performance of a few immunologic markers in RA diagnosis, but nothing has been proven to be sufficiently good in clinical practice. • All four automated anti-CCP assays showed good analytical and diagnostic performances for diagnosing RA in clinical practice. • After adjusting the cutoff values, these assays are expected to show enhanced sensitivity and specificity. • The present study provides reassuring evidence that any of the studied commercially available anti-CCP tests for detecting rheumatoid arthritis provide similar diagnostic information to institutions that adopt these specific testing systems.

Antibody modified gold nanoparticles for fast colorimetric screening of rheumatoid arthritis

Rheumatoid arthritis (RA) is a systemic autoimmune disease characterized by chronic joint inflammation and one of the main causes of chronic disability worldwide with high prevalence in the ageing population. RA is characterized by autoantibody production, synovial inflammation and bone destruction, and the most accepted biomarker is rheumatoid factor (RF) autoantibodies. In this work, we developed a low-cost approach for the detection and quantification of the RF marker. This colorimetric immunosensor is based on gold nanoprobe crosslinking that results in extensive aggregation in the presence of the pentameric IgM RF. Aggregation of the nanoconjugates yields a color change from red to purple that can be easily observed by the naked eye. The interaction between nanoconjugates and the specific target was confirmed via dynamic light scattering (DLS), Raman spectroscopy and atomic force microscopy (AFM) imaging. This conceptual system shows a LOD of 4.15 UA mL^-1 IgM RF (clinical threshold is set for 20 IU mL^-1). The one-step biosensor strategy herein proposed is much faster than conventional detection techniques, without the need for secondary antibodies, additional complex washing or signal amplification protocols. To the best of our knowledge this is the first report on target induced aggregation of gold nanoprobes for quantitative colorimetric autoantibody detection.

Diagnostic utility of, and influence of tobacco usage and genetic predisposition on, immunoglobulin A, rheumatoid factor and anti-citrullinated peptide auto-antibodies in South African rheumatoid arthritis patients

Background

The immunoglobulin A isotypes of anti-cyclic citrullinated peptide antibodies (ACPA) and rheumatoid factor (RF) are associated with disease severity and progression in Caucasian rheumatoid arthritis (RA) patients, as well as with genetic predisposition and tobacco use.

Objectives

To compare levels of ACPA-IgA and RF-IgA with those of ACPA-IgG and cRF in a cohort of black South African RA patients and healthy controls.To investigate the relationship between IGA autoantibodies and disease severity, genetic predisposition and tobacco use

Methods

RF-IgA and ACPA-IgA were determined in a cohort of predominantly black South African RA patients (n=75) in relation to serodiagnostic and prognostic potential, as well as tobacco use and genetic predisposition. Healthy control subjects were included to determine sensitivity, specificity and predictive values.

ACPA-IgG/IgA and RF-IgA were determined by enzyme immunoassay and hs-CRP and cRF by nephelometry. Cotinine levels were determined by ELISA.

Results

The frequencies of ACPA-IgA and RF-IgA were 31% and 88% respectively compared to 88% for both types of traditional autoantibody procedures. ACPA-IgA was significantly higher (p=0.007) in patients with short disease duration, while linear regression analysis revealed a positive relationship with baseline disease activity scores. Levels of ACPA-IgG and ACPA-IgA were significantly higher in tobacco users who carried the HLA shared epitope.

Conclusion

Although lacking in serodiagnostic superiority over cRF and ACPA-IgG, inclusion of RF-IgA and ACPA-IgA in autoantibody panels may provide insights into disease pathogenesis, interactions between tobacco usage and HLA genotype in the production of potentially disease-triggering ACPA-IgA antibodies.