Comparison between multiplex assays for autoantibody detection in systemic lupus erythematosus

Performance of a multiplex assay compared to enzyme and precipitation methods for anti-ENA testing in systemic lupus and systemic sclerosis

INTRODUCTION

Testing for autoantibodies to extractable nuclear antigens (ENA) is essential in the investigation of connective tissue disease. Counterimmunoelectrophoresis is an early described testing methodology for antibodies to ENAs, but is labour-intensive, only moderately sensitive, and reliant on high-quality reference sera. Enzyme-linked immunosorbent assay (ELISA) is automatable for relatively high sample throughput, but has issues with false positives. The addressable laser bead immunoassay (ALBIA) is a multiplex technology which can assess several antibody specificities simultaneously on a small serum sample. We report performance of an ALBIA system compared with CIE and ELISA.

METHODS

Samples from 100 systemic sclerosis patients attending Royal Free Hospital in 2007 and 99 SLE patients attending St Thomas's Hospital in 2007-2008 were studied. All samples were tested for antibodies to RNP, Sm, Ro, La, Scl-70, Jo-1 by in-house CIE, FIDIS™ ALBIA (BMD, France), and ELISAs (Phadia, Germany). Cohen's kappa coefficient was used to examine agreement of the different assay methods for the same antibody. McNemar's test was used to detect differences between methodologies.

RESULTS

One sample was positive for anti-Jo-1 by CIE, & confirmed by ALBIA & ELISA. All 198 remaining samples were anti-Jo-1 negative by all 3 methods. With respect to RNP, Ro, La, Scl-70 antibodies, there was good agreement in assay performance between CIE, ALBIA, and ELISA. For Sm, agreement was less good between CIE and ELISA (kappa 0.491), and ALBIA and ELISA (kappa 0.403). Using McNemar's test performance was no different between the 3 assays, with the following exceptions: between CIE and ELISA for Ro-60 (p<0.01) and RNP (p<0.05), and between ALBIA and ELISA for RNP (p<0.01).

CONCLUSIONS

The FIDIS™ ALBIA produced similar level of performance as CIE, but with advantages of automation, and less dependence on highly skilled operators. ALBIA represents a potential advancement applicable to routine Immunology diagnostics.

Sm peptides in differentiation of autoimmune diseases

Systemic lupus erythematosus (SLE) and related diseases are characterized by circulating autoantibodies to defined intracellular targets. Among the earliest identified autoantibodies were those directed to components of U2-U6 small nuclear ribonucleoproteins (snRNPs) known as Smith (Sm) antigen, which are highly specific for SLE. The Sm-antigen is composed of at least nine different polypeptides with molecular weights ranging from 9 to 29.5 kDa (B (B1, 28 kDa), B' (B2, 29 kDa), N (B3, 29.5 kDa), D1 (16 kDa), D2 (16.5 kDa), D3 (18 kDa), E (12 kDa), F (11 kDa), and G (9 kDa)). All of the nine core proteins, but most frequently the B and D polypeptides, are targets of the anti-Sm autoimmune response. However, since SmBB' and U1 specific RNPs share the cross-reactive epitope motif PPPGMRPP, SmD is regarded as the most SLE specific Sm-antigen. It has been shown that the polypeptides D1, D3, and BB' contain symmetrical dimethylarginine, constituting a major autoepitope within the C-terminus of SmD1 and SmD3. Several synthetic peptides have been used for the detection of anti-Sm antibodies and thus for the diagnosis of SLE. Anti-Sm antibodies have been reported to occur later than other SLE associated autoantibodies and, on average, around 1 year before the clinical onset of SLE. The present review provides a comprehensive summary on the history of anti-Sm antibodies and their use as biochemical tools to study cellular processes and as biomarker in the diagnosis of SLE. Additionally, a meta-analysis focused on recent data analyzes the prevalence of anti-Sm antibodies in SLE.

Anti-Scl-70 (topo-I) antibodies in SLE: Myth or reality?

Autoantibodies to topoisomerase I (topo-I, also referred to as Scl-70, ATA) have historically been considered a specific marker for systemic sclerosis (SSc). Although found in approximately 20% of SSc sera, other evidence indicated that ATA also occur in up to 25% of systemic lupus erythematosus (SLE). Given that SLE is approximately 100 times more prevalent than SSc, the number of ATA positive SLE patients is at least 2-3 orders of magnitude greater than the prevalence of ATA in SSc. These observations have raised questions about the disease specificity and clinical value of ATA. Therefore, our objective was to analyse the prevalence of ATA in different disease conditions with a special focus on SLE employing a systematic literature review (meta-analysis) in combination with our experimental investigations.