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Anti-dsDNA antibody avidity determination by a simple reliable ELISA method for SLE diagnosis and monitoring

Servizio di Microbiologia e Immunologia, Azienda Ospedaliera ‘S.Maria degli Angeli’, Pordenone, Italy

P B Romelli

Bouty Laboratories, Milano, Italy

C Savina

Bouty Laboratories, Milano, Italy

N Bizzaro

Laboratorio di Patologia Clinica, Ospedale Civile, S. Donàdi Piave (VE), Italy, nbizzaro{at}dacos.it

R Tozzoli

Laboratorio Analisi Chimico-cliniche e Microbiologia, Ospedale Civile, Latisana (UD), Italy

E Tonutti

Instituto di Chimica Clinica, Azienda Ospedaliera ‘S.Maria della Misericordia’, Udine, Italy

A Ghirardello

Divisione di Reumatologia, Universitàdi Padova, Padova, Italy

A Doria

Divisione di Reumatologia, Universitàdi Padova, Padova, Italy

High avidity anti-dsDNA antibodies are more specific for SLE diagnosis, and more closely associated with renal involvement than intermediate or low-affinity anti-dsDNA antibodies. ELISA methods are largely used to detect anti-dsDNA, but their high sensitivity is inversely related to specificity because they also detect low avidity antibodies.We developed an ELISA assay based on the law of mass action and the competitive binding of dsDNA in solution and coated to microwells with anti-dsDNA antibodies. A simplified Scatchard plot analysis system was used to measure anti-dsDNA antibody avidity which was expressed as apparent affinity constant (K_aa), and quantified in liters per unit (l=U).

We prospectively studied 101 consecutive SLE patients, who were followed for 3 years; three serum samples were sequentially collected from each patient during follow-up for determination of IgG anti-dsDNA antibody concentration, and anti-dsDNA avidity. SLE disease activity was estimated using the European Consensus Lupus Activity Measure (ECLAM) index. Sera from 100 healthy subjects and 133 patients with other connective tissue diseases or infectious diseases were also assayed as controls.

The mean K_aa in SLE patients was 65.2±47.3 l=U, with no variations over time. Anti-dsDNA-positive SLE patients had higher K_aa values (79.1±46.8) than anti-dsDNA negative patients (27.2±20.1; P < 0.001). No correlation emerged between anti-dsDNA avidity and the ECLAM activity index score. Avidity was significantly higher in patients with renal involvement vs patients without this complication (78.2±50 vs 59.9±45.6 l=U; P = 0.0013).

This simple ELISA method could be very useful in the diagnostic phase to differentiate high avidity anti-dsDNA autoantibodiesthat are characteristicallyfound in SLE patients from low avidity antibodies that can also be found in other inflammatory diseases. Moreover, our data confirm the predictive value of high avidity anti-dsDNA antibodies for the development of lupus nephritis.

Key Words: systemic lupus erythematosus • anti-DNA antibodies • avidity • nephritis • ELISA

Lupus, Vol. 12, No. 1, 31-36 (2003)
DOI: 10.1191/0961203303lu277oa


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