The Role of Innate Immunity in Dystrophic Diaphragm Pathology

The pathophysiology of Duchenne muscular dystrophy (DMD) is complex and incompletely understood. However it is clear that the primary defect, lack of dystrophin, is necessary but not sufficient on its own to fully account for the onset of muscle fiber destruction, leading to eventual replacement by fibrotic tissue. Dystrophin restoration strategies are currently under clinical development, but the only treatment clearly shown to slow disease progression thus far in human DMD patients is the use of anti‐inflammatory corticosteroids. This presentation will focus on the role of the innate immune system in promoting the dysregulated inflammatory and aberrant muscle repair responses found in DMD. Using the mdx mouse diaphragm as a preclinical model due to its resemblance to the human DMD phenotype, the importance of monocyte‐derived macrophages in driving early disease progression will be outlined. This is exemplified by the therapeutic benefits (improved force generation and reduced fibrosis) noted when monocyte/macrophage recruitment to the dystrophic diaphragm is prevented through genetic or pharmacologic inhibition of the chemokine receptor CCR2. In addition, preventing activation of Toll‐like receptors (TLRs) is similarly beneficial and suggests significant involvement of damage‐associated molecular patterns (DAMPs) in DMD pathogenesis. Data will also be presented indicating that epigenetic alterations occur at the level of macrophage precursors in the bone marrow, which are likely stimulated by TLR4 recognition of DAMPs, and that subsequently play a role in dictating monocyte/macrophage phenotypic characteristics following their entry into the diseased muscles. Although the ideal treatment for DMD would be restoration of dystrophin to all muscles of the body including the diaphragm, significant hurdles remain for this ultimate therapy. In the meanwhile, there are serious drawbacks to the current practice of treating DMD children with corticosteroids, which have major side effects in DMD such as obesity, diabetes and osteoporosis. Future development of a more targeted therapeutic approach based on innate immune system modulation could represent an important advance in the field. Support or Funding Information Canadian Institutes of Health Research