ReviewAnti-Scl-70 (topo-I) antibodies in SLE: Myth or reality?
Introduction
Systemic autoimmune rheumatic diseases (SARD) such as systemic sclerosis (SSc; scleroderma, Scl) or systemic lupus erythematosus (SLE) are characterized by circulating autoantibodies (aabs) to a variety of defined intracellular targets. In SSc these include topoisomerase I (topo-I, also referred to as Scl-70), centromere proteins (CENP), RNA polymerase I, fibrillarin, the PM/Scl complex, Ro52 or U1-ribonucleoproteins [1], [2], [3], [4]. SSc is a multi-system and clinically heterogeneous disorder that affects the connective tissue of skin, internal organs and blood vessels. During the last decade, clinicians and researchers have used the disease classification criteria wherein SSc is segregated into limited cutaneous (lcSSc) and diffuse cutaneous SSc (dcSSc) as suggested by Le Roy et al., the American College of Rheumatology, and two other more recently proposed by Senécal, et al. [5] and Hudson, et al. [6].
Akin to SSc, SLE is a multi-system and clinically heterogeneous disorder that is characterized by aabs directed against more than one hundred different autoantigens [7] that include anti-dsDNA, anti-Sm, SS-A/Ro60 and anti-ribosomal P antibodies [1]. Only 2% of SLE patients are reported to develop an overlap SSc/SLE disease [8].
Aabs to topo-I have historically been considered a specific marker for SSc and can be detected using a broad range of technologies such as immunofluorescence (IIF), western blot, line immunoassay (LIA), ELISA and addressable laser bead assays (ALBIA) [2], [9], [10], [11], [12]. In this context, the lack of widely applied standardization of autoantibodies assays has to be taken into consideration [13]. The terminology Scl-70 was based on early studies indicating that the target antigen was a 70 kDa nuclear protein [9], but later evidence indicated that the presumed 70 kDa antigen was a breakdown product of the labile 110 kDa DNA topoisomerase I (topo-I) [9]. Hence, the preferred nomenclature is topo-I, although the Scl-70 terminology persists in the literature. Historically, ATA have been reported to generate a nuclear and nucleolar fine speckled staining of interphase cells and a fine speckled staining of the metaphase plate in mitotic cells in IIF on HEp-2 cells. However, with an increasing number of commercially available HEp-2 cell slides from various suppliers, significant variations in the ATA staining pattern have been observed (unpublished observations). Just recently, the staining pattern produced by ATA has been redefined as a staining of five cellular regions: nucleus, nucleolus and cytoplasm in interphase cells; nucleolar organizing region (NOR) and chromosomes in mitotic cells [14].
ATA were most commonly associated with the diffuse form of SSc attended by progressive disease and high mortality rates that were often attributed to right heart failure in association with pulmonary fibrosis and restrictive lung disease [8]. One study of Japanese SSc patients suggested that ATA were associated with renal crisis [2], but no convincing association with this clinical feature has been found in other studies. ATA have been reported in rare cases of SLE with and without clinical features of SSc [15], [16]. In the majority of those patients, ATA were accompanied by other SLE associated or specific antibodies such as anti-Sm antibodies [16]. The presence of ATA in a patient initially evaluated for Raynaud's phenomenon has been shown to predict the future development of SSc [17]. Anti-CENP and ATA are rarely found in the same serum, being present in less than 0.5% of a cohort of SSc patients [18]. Although found in 10–40% of SSc sera, some groups reported that ATA occur in up to 25% of SLE, which questions the presumed high disease specificity of ATA [19], [20]. Therefore, the objective of our study 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 own experimental investigations.
Section snippets
Specimens
Sera from patients with SLE (n = 168) and SSc (n = 100) were collected at the University of Calgary (Calgary, Canada). Specimens from apparently healthy subjects were taken from the serum bank of Dr. Fooke Laboratorien GmbH (n = 139). Patients with SSc were diagnosed according to the criteria for SSc as described previously [21]. SLE patients were diagnosed according to the American College of Rheumatology (ACR) criteria for SLE [22]. All samples were collected in accordance with the local ethics
Literature review (meta-analysis)
PubMed was queried for studies that investigated the prevalence of ATA in SLE. In total, eleven studies were identified that provided sufficient information on the patient cohort and the method used for ATA detection [19], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]. The reported prevalence of ATA in SLE ranged from 0% to 25% and a broad range of technologies were used. Cumulatively, ATA were detected in 98/2366 (4.1%) SLE patients. The data is summarized
Discussion
Autoantibodies reacting with DNA topo-I are widely regarded as specific serological markers of SSc. They are observed in 10–40% of the unselected SSc patients and are associated with diffuse skin involvement and a more aggressive disease expression [2], [3], [4], [5]. A controversy arose when studies indicated that up to 25% of SLE patients without any clinical evidence of SSc had ATA [19], [20]. There have been attempts to reconcile these data with earlier data on the basis of the sensitivity
Conclusion
Based on literature review and experimental data, we found, using three different assay platforms, that the point prevalence of ATA in a SLE cohort derived from four centers was less than 5%, a finding that is less than that reported in some earlier studies. The different prevalence might be attributed to methodological aspects of the assays and/or unintended selection bias in the patient cohorts studied. Nevertheless, we found significantly more ATA in SLE patients compared to healthy
Competing interest
MM was employed at Dr. Fooke Laboratorien GmbH selling the Scl-70 ELISA and is now employed INOVA Diagnostics, Inc. MJF is a consultant to ImmunoConcepts Inc and INOVA Diagnostics, Inc.
Take-home messages
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High titre ATA are highly specific for SSc
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ATA occur in SLE but only in low titres and significantly less prevalent than previously reported
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The average prevalence of ATA in SLE is < 5%
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Although ATA were significantly associated with anti-dsDNA antibodies in SLE, the reactivity is unlikely only dependent on binding of dsDNA to topo-I.
Acknowledgement
The authors acknowledge the technical assistance of Haiyan Hou and Jane Yang.
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