Idiopathic inflammatory myopathies: polymyositis and dermatomyositis

The idiopathic inflammatory myopathies (IIM) are a heterogeneous group of systemic autoimmune syndromes characterized by chronic muscle weakness and skeletal muscle inflammation. Although initially described more than a century ago, these diseases are rare and heterogeneous, that to date there is only a limited understanding of their molecular cause and treatment. Of the IIM, the best recognized subsets of diseases are polymyositis (PM); dermatomyositis (DM) and inclusion body myositis (IBM); together these have an incidence of 0.001 % (DM>IBM>PM), among which females are more affected [94, 95].

Polymyositis affects predominantly adults who present subacute or chronic proximal muscle weakness, elevated creatine kinase and mononuclear cell infiltration [96] (see Figure 1.9 A-B). Dermatomyositis, affecting both children and adults, causes a purple discoloration of the eye lids, edema around the eyes and the mouth, skin rashes on the face and upper body extremities, muscle pain and weakness.

Figure 1.9: (A) Muscle biopsy in polymyositis (PM) showing the muscle fibers (pink) being attacked by inflammatory cells (purple) ; (B) Lymphocyte infiltration in myofiber in PM; (C) Muscle biopsy in dermatomyositis (DM); (D) Necrotic capillary in DM.

The key pathological feature in DM is a vasculitis which attacks the capillaries and arterioles in the connective tissue around and within the muscle fibres. Capillary loss is derived from endothelial swelling and necrosis (see Figure 1.9). This inflammatory condition is caused by circulating anti-endothelial antibodies which interact with vascular antigens, activating the complement and leading to formation of the membranolytic attack complex [96] that ultimately destroys endothelial cells.

The borders separating myositis from related syndromes within the category of rheumatologic diseases are not sharply defined. For example, systemic lupus erythematosus (SLE) shares both clinical and serological manifestations with myositis;

furthermore, several conditions treated as myositis represent rare, or as-yet unrecognized genetic metabolic disorders, characterized by deficient production of enzymes vital to the energy production of a muscle cell [96]. The diagnosis relies on histological features of muscle biopsy indicating muscle degeneration [97], such as mononuclear inflammatory infiltrates in PM, whereas in case of DM, additional heliotrope sign or papules represent characteristic manifestations. Furthermore, a strong family history of autoimmune diseases [98] and a clinical response to immunosuppressive therapy [99] may help confirm the specificities, referred to as myositis specific autoantibodies (MSA), which occur almost exclusively in myositis. By contrast, myositis associated autoantibodies (MAA) lack disease specificity, as they are present in overlapping syndromes and also in healthy individuals; hence their clinical utility is limited [100, 101]. Myositis autoantibodies are usually present from the earliest stage of the disease and persist over time even when the disease is controlled or in remission [102, 103].

The most common established MSA is anti-Jo-1, present in approximately 20 % of patients with myositis [104]. Anti-Jo-1 reacts with the cytoplasmic enzyme histidyl-transfer-ribonucleic acid (-tRNA) synthetase, which catalyzes binding of histidine to its tRNA [105].

A recent study using a mouse model of myositis suggests that production of anti-Jo-1 is a secondary event, appearing as a consequence of myositis, rather than requiring a primary stimulus, such as a virus, directing the immune response to Jo-1 antigen [106]. Other MSAs include autoantibodies to the signal recognition particle, anti-SRP, detected in 4 % of the myositis patients [107], and anti-Mi-2, in 5-10 % of the patients [107-109]. Anti-Mi-2 autoantibodies have high myositis specificity, and 90-95 % of the patients have the characteristic rashes of dermatomyositis. Mi-2 is a nuclear protein, involved in the chromosomally mediated regulation of transcription by an ATP-dependent mechanism [110]. Clinical data suggests that patients with different MSAs show distinct features of muscle histology [108], although it is unclear whether myositis specific autoantibodies play

a role in disease pathogenesis. One hypothesis for the occurrence of these autoantibodies against cellular components is that autoantigens undergoing proteolytic cleavage during apoptosis may generate fragments containing new epitopes which may be displayed on the cell surface in apoptotic blebs inducing an autoimmune response [111, 112].

Cell-mediated immune responses are believed to play a pathogenic role in myositis [113].

Histopathologic examination of muscle biopsy in patients with dermatomyositis showed increased number of T helper cells (CD4+) and B lymphocytes in perivascular regions of the muscle. In contrast, muscle biopsy of individuals with polymyositis showed infiltration of cytotoxic T cells (CD8+) and macrophages in the endomysial region [114]. Recent studies documented a characteristic pattern of T-cell receptor gene expression in clinical groups with myositis, suggesting that an antigen-driven immune response might account for the restricted repertoire of the expressed genes [115].

The pathogenesis of myositis is largely unknown at the present. Recent findings suggest that environmental agents acting on genetically susceptible persons lead to physiologic responses involving immunological activation and subsequent tissue damage in myositis syndromes [96]. Primary therapy, aimed at reducing the muscle inflammation, involves the use of corticosteroids and immunomodulatory agents; however, many factors have to be considered in order to achieve successful therapy responses [116]. Hence, further research is necessary in this field for a better understanding of both genetics and environmental risk factors of autoimmunity. A clinical study recently initiated at the National Institutes of Environmental Health Sciences in Bethesda is aimed at delineating contributing factors from these two categories, by analyzing myositis patients, their unaffected twins/siblings, as well as unrelated controls. The design of the twin-sibling study is presented in Chapter 2.3. Driven by the current state of knowledge in the field of myositis, and in an effort to compare its immunological features with other autoimmune disorders, part of the clinical study, and a major goal of this thesis was to determine the subclass specific glycosylation profiles of total plasma IgG in myositis patients.

1.7 Analysis of biopolymer structure and post-translational modifications by