Studies on autoantibodies and inflammatory markers in patients with idiopathic inflammatory myopathies

Sammanfattning: Idiopathic Inflammatory myopathies (IIM), commonly known as myositis, are chronic autoimmune diseases characterized by low muscle strength and low muscle endurance as main features. However, a high number of patients may develop extra muscular manifestations such as interstitial lung disease, skin rash or arthritis. There are known genetic and environmental risk factors for developing these conditions, but the cause is not fully understood. To date, it is considered that myositis is driven by an autoimmune component. In this sense, the production of autoantibodies is characteristic in most patients and each of these autoantibodies is strongly linked to specific clinical manifestations. However, the implications of positive autoantibodies as prognostics tools beyond the association with clinical features has not been fully studied. Moreover, different methods have been used to detect these autoantibodies and new methods, usually employed in the daily clinical setting, require validation. In addition to the autoantibodies, the role of inflammatory markers as predictors of subjective health perception has been overlooked in patients with myositis. The aim of this thesis was to validate an autoantibody assay commonly used in the clinic, a line blot assay, as well as to explore the usefulness of autoantibodies as predictors of response to treatment and organ damage, and to investigate the association of inflammatory markers with patient reported outcomes. A validation of a line bot test was conducted using an immunoprecipitation-based algorithm as comparator in a cohort of well-characterized patients with myositis. The prevalence of relevant clinical associations with both assays was compared between patients. A moderate agreement between the assays was found, and the clinical features of patients with detected autoantibodies by the line blot assay were consistent with known clinical phenotypes (Paper I). By using a longitudinal Swedish electronic database (SweMyoNet), dermatomyositis specific autoantibodies (DMSA) were found as predictors for moderate level of response to treatment; initial doses of glucocorticoids and shorter time lag from first symptoms to diagnosis were also predictors of response to treatment (Paper II). In a longitudinal study, based on a large international electronic database (MyoNet), patients with anti-PM/Scl autoantibodies accumulated damage more pronounced than seronegative patients, and patients with DMSA accumulated less pronounced damage than seronegative patients. Furthermore, a strong correlation between severity of muscle damage and functional disability was found, especially in patients with immune-mediated necrotizing myopathies (Paper III). By analyzing data from MyoNet, we found a longitudinal correlation between Patient Global Assessment (PatGA) and inflammatory markers, namely C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) (Paper IV). Altogether, the findings of this thesis indicate that the line blot assay is useful to detect the most frequent autoantibodies in patients with a known myositis diagnosis. The results of this thesis also suggest that autoantibodies are useful as predictors of response to treatment and of organ damage, and that inflammatory markers are associated with subjective health perception status measured by the Patient Global Assessment scale in patients with myositis.