What is the role of inflammation in the pathophysiology of sporadic inclusion body myositis (s-IBM)?

Updated: Jun 08, 2018
  • Author: Michael P Collins, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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s-IBM is characterized by the presence of non-necrotic myofibers invaded by mononuclear inflammatory cells, which, as a pathologic phenomenon, is significantly more common than vacuolated, congophilic, and necrotic fibers. [5, 6] It is found at all stages of the disease in both treated and untreated patients.

The endomysial infiltrates in patients with s-IBM are composed primarily of CD8+ T cells and macrophages in a 2:1 ratio. [7] Myeloid dendritic (antigen-presenting) and CD138+ plasma cells are also present in substantial numbers, [8, 9] while B cells and natural killer (NK) cells are rare. T cells, macrophages, and myeloid dendritic cells all have the potential to invade non-necrotic muscle fibers. [10] The autoinvasive CD8+ T cells surround major histocompatibility complex (MHC) class I-immunoreactive myofibers and express perforin and other markers of activation. [11, 12, 13]

Identical autoinvasive T-cell clones can persist over time, even in different muscles, [14, 15] but the amplified subfamilies sometimes change, which suggests of epitope spreading. [16] Collectively, these observations implicate an antigen-driven, MHC class I-restricted, cytotoxic T-cell–mediated process directed against myofibers. The specific antigens responsible for this reaction are unknown.

Various chemokines, cytokines, and chemokine receptors are upregulated in the inflammatory cell infiltrates, blood vessels, and myofibers in s-IBM. [17] In microarray experiments, cytokine and chemokine genes are differentially upregulated to a significantly greater degree in s-IBM and polymyositis than in dermatomyositis. [18, 19]

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