Use of a commercial line blot assay as a screening test for autoantibodies in inflammatory myopathies
Introduction
Inflammatory myopathies constitute a heterogenous group of rare rheumatic disorders characterized by muscle weakness and infiltrations of inflammatory cells in muscle tissue. Based on clinical and histopathological features they are subclassified into dermatomyositis (DM), polymyositis (PM) and inclusion body myositis (IBM) [1]. The etiology is currently unknown, and both genetic and environmental factors are supposed to contribute to disease susceptibility. Poly- and dermatomyositis also have other organ involvement and are therefore regarded as systemic inflammatory diseases. These two latter diagnoses are also regarded as autoimmune diseases in contrast to inclusion body myositis, due to the described association to certain HLA types [2], [3] and to the fact that autoantibodies can be detected in the majority of patients, in most cases with defined specificities (Table 1 and [4], [5], [6]). It is however not settled whether these autoantibodies have a pathogenic role or are merely epiphenomena. A pathogenic role for anti-Jo1 autoantibodies has recently been suggested, because these antibodies can form immune complexes that induce type I interferon production by plasmacytoid dendritic cells [7].
Autoantibodies found in inflammatory myopathies are generally subdivided into myositis specific autoantibodies (MSA) and myositis-associated autoantibodies (MAA). In Table 1 we have summarized the frequency range of known MSA and MAA in different populations as well as their clinical associations and the biological functions of the corresponding autoantigens.
The most unobjectionable way to characterize antibody reactivities is to search for antibody-antigen reactions when the antigen is present in a fully native state. This is the case in different immune precipitation techniques. Such techniques are, however, laborious and their execution requires specific technical laboratory skills. They are also developed to detect individual antigens, whereas the demands from clinicians often concern the screening of individual patients for the occurrence of a number of individually rare autoantibodies associated with a certain suspected diagnosis.
We have here evaluated a newly developed line-blot assay for the detection of MSA and MAA in a group of clinically well-characterized myositis patients and compared them with disease controls.
Section snippets
Materials and methods
Serum samples from 230 patients were investigated. The myositis group consisted of 50 patients with dermatomyositis, 89 patients with polymyositis, 4 patients with juvenile dermatomyositis diagnosed according to Bohan and Peter criteria [8], [9] and 10 patients with inclusion body myositis [10]. All myositis patients were recruited from the Rheumatology Unit, Karolinska University Hospital, Solna, Stockholm. As disease controls, 26 patients with primary Sjögren´s syndrome, 26 patients with
Results
Most autoantibody reactivities were clearly positive, only 11% (14/127) of the detected antibody reactivities were in the borderline region. Results are summarized in Table 2, where the borderline reactivities are recorded as positive. Anti-Jo-1 was found in 18 myositis and one systemic sclerosis patient. Antibodies against Mi-2 were found in 5 patients with dermato- or polymyositis, whereas anti-Ku was found in 4 myositis and 2 primary Sjögren´s syndrome patients. Eleven patients with poly- or
Conclusions
In this study we have investigated the sensitivity and specificity of a commercial line blot technique for the detection of a number of MSA and MAA. The investigated group of myositis patients was sizeable (n = 153), given the rareness of the diseases, whereas the number of disease controls was limited (n = 77). We found that most of the MSA and MAA detected by the line blot assay were rather specific, except the ubiquitous anti-SSA/Ro52 reactivity which was also common in the investigated control
Take-home messages
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Both myositis-specific autoantibodies (MSA) and myositis-associated autoantibodies (MAA) are important in the diagnosis of inflammatory myopathies. The sensitivity of MSA and MAA is low, and negative laboratory analysis responses do not exclude these diagnoses.
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Commercial line blot assays based on purified myositis-specific and myositis-associated autoantigens seem to constitute a valuable and easily employed tool for the detection of diagnostically and prognostically important autoantibodies.
Acknowledgments
This study was supported by grants from the Swedish Research Council, The Groschinsky foundation and the Capio foundation.
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