Antinucleosome antibodies in SLE: a two-year follow-up study of 101 patients

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Abstract

We prospectively analyzed the diagnostic sensitivity and specificity as well as the clinical relevance of antinucleosome antibodies in SLE.

One hundred and one consecutive SLE patients were followed for 3 years. Three serial serum samples from each patient were tested for antinucleosome antibodies by ELISA (optimum cut-off value 10 U/ml), and for anti-dsDNA antibody (by ELISA and IIF on Crithidia luciliae), and anti-dsDNA avidity (by Scatchard plot analysis). Sera from 100 healthy individuals (HI), 35 patients with systemic sclerosis (SSc), 30 with primary Sjögren's syndrome (SS), 20 with rheumatoid arthritis (RA) and 48 with infectious diseases (ID), were assayed as controls. SLE activity and damage were evaluated using the ECLAM score and the SLICC/ACR index.

At baseline, antinucleosome antibodies were found in 87 patients with SLE (86.1%), in 8 patients with SSc (22.8%), in 2 HI (2%), and in 1 ID (2.1%). The sensitivity and specificity of antinucleosome testing for SLE were 86.1% and 95.3%, respectively. The prevalence of antinucleosome antibodies in SLE was significantly higher than that of anti-dsDNA antibodies, with a correlation between them. No relevant relationship was found between antinucleosome antibodies and disease features, including renal involvement, disease activity, and disease damage. During follow-up, no significant variation was observed in antinucleosome level, nor in anti-dsDNA antibody level or avidity.

We conclude that antinucleosome antibodies are commonly found in SLE. Low antibody levels can be detected in SSc, whereas medium/high levels are highly specific for SLE. Their clinical relevance during the disease course and utility for monitoring the individual patient seem to be poor.

Introduction

Systemic lupus erythematosus (SLE) is considered the prototypic autoimmune disease for the wide spectrum of clinical and serological manifestations which include the presence of many different autoantibodies in patients' sera [1], [2], [3].

Double stranded DNA (dsDNA) and histones are the most widely targeted autoantigens in both murine and human SLE. However, there is no experimental evidence of their immunopathogenic role in the disease. In general, both naked DNA and free histones per se are weak immunogens [4]. The respective antibodies share common immunochemical and structural features [5]and only some of them have peculiar properties that increase their potential to be pathogenic [6], [7], [8]. In contrast, bacterial DNA can induce anti-DNA antibodies by injection [9]and native mammalian DNA becomes antigenic when complexed with charged proteins such as histones [10]. Even histones acquire immunostimulating properties when linked to nucleic acids.

Chromatin in the cell nucleus is structurally packed in DNA-histone complexes named nucleosomes which could probably represent the in vivo driving antigen of the autoimmune response in SLE. Evidence is growing on the fundamental role of nucleosomes as in vivo major immunopathogenic particles in SLE. Increased levels of circulating nucleosomes are found in patients with SLE [11]. Nucleosomes are more strongly antigenic compared to DNA or histones alone [12], and become immunogenic when massively released from necrotic or apoptotic cells [13], [14], and/or modified by non-self determinants [15]. In this way they stimulate the production of antibodies initially to nucleosomes and then to DNA and histones [16], and could promote tissue damage, particularly nephritis, by mediating DNA or nucleosome-specific antibody binding to basement membrane and deposition in glomeruli [17], [18], [19]. In addition, it has been recently demonstrated that this nucleosome-mediated autoantibody binding also occurs in the skin of SLE patients with active nephritis [20]. Since protein-DNA complexes rather than free DNA are likely the in vivo target antigens for anti-DNA binding, laboratory tests which use such native antigens should improve our ability to assess the antibody pathogenicity and diagnosis. Many ELISA assays using differently purified nucleosome extracts to detect antinucleosome antibodies are now available and proposed as a potential new diagnostic tool in SLE. However, their relevance for SLE diagnosis has not yet been definitely validated.

The aim of our study was to prospectively assess the diagnostic sensitivity and specificity as well as the clinical relevance of antinucleosome antibodies in SLEby investigating their relationship with anti-dsDNA antibodies, disease activity and damage, and clinical manifestations.

Section snippets

Patients and methods

We prospectively investigated 101 consecutive patients with SLE, who met at least four of theAmerican College of Rheumatology (ACR) criteria [1], [2]. The M/F ratio was 1/9, mean age 29.8±9.0 years, and mean disease duration 80.2±69.6 months. All patients were strictly monitored for a mean follow-up period of 27.6±9.4 months.

For definitions of SLE specific features, including renal involvement, we used those included in the ACR criteria [1]. During the 2 year follow-up, the patients were

Results

Data regarding clinical and immunological findings of 101 SLE patients at the beginning of the study and during follow-up are detailed in a previous paper [25].

At baseline, serum antinucleosome antibodies were positive in 87 patients with SLE (86.1%), in 8 patients with SSc (22.8%), in 2 HI (2%), in one patient with ID (2.1%), and in no SS or RA controls. The sensitivity and specificity of antinucleosome antibodies for the diagnosis of SLE were 86.1% and 95.3%, respectively.

The antinucleosome

Discussion

Anti-dsDNA antibodies play a key role in the diagnosis of SLE because they are specific markers of the disease and generally related to disease activity and organ damage, primarily nephritis. However, in recent years, the diagnostic relevance and clinical impact of anti-dsDNA antibodies for SLE has been questioned mainly because the antibodies measured by most available tests are heterogeneous in terms of avidity and immunochemical properties related to their pathogenic role. Laboratory tests

Acknowledgements

This work was supported by ‘Ministero Italiano dell'Università e della Ricerca Scientifica e Tecnologica’, project 9906242552-003 ‘Autoanticorpi e danno d'organo nelle malattie autoimmuni sistemiche’.

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