Osteopontin promotes fibrosis in dystrophic mouse muscle by modulating immune cell subsets and intramuscular TGF-β

Abstract

Duchenne muscular dystrophy (DMD) is an X-linked, degenerative muscle disease that is exacerbated by secondary inflammation. Here, we characterized the immunological milieu of dystrophic muscle in mdx mice, a model of DMD, to identify potential therapeutic targets. We identified a specific subpopulation of cells expressing the Vβ8.1/8.2 TCR that is predominant among TCR-β + T cells. These cells expressed high levels of osteopontin (OPN), a cytokine that promotes immune cell migration and survival. Elevated OPN levels correlated with the dystrophic process, since OPN was substantially elevated in the serum of mdx mice and muscle biopsies after disease onset. Muscle biopsies from individuals with DMD also had elevated OPN levels. To test the role of OPN in mdx muscle, mice lacking both OPN and dystrophin were generated and termed doublemutant mice (DMM mice). Reduced infiltration of NKT-like cells and neutrophils was observed in the muscle of DMM mice, supporting an immunomodulatory role for OPN in mdx muscle. Concomitantly, an increase in CD4 + and FoxP3 + Tregs was also observed in DMM muscle, which also showed reduced levels of TGF-β, a known fibrosis mediator. These inflammatory changes correlated with increased strength and reduced diaphragm and cardiac fibrosis. These studies suggest that OPN may be a promising therapeutic target for reducing inflammation and fibrosis in individuals with DMD.

OPN‐a induces muscle inflammation by increasing recruitment and activation of pro‐inflammatory macrophages

“…Previously, inhibition of OPN has been shown to reduce skeletal muscle inflammation in dystrophic mice (Vetrone et al . 2009) and promote regeneration following acute injury in aged mice (Paliwal et al . 2012).…”

“…2009). However, we did not observe a direct effect of OPN on TGFβ production in macrophages or myoblasts (not shown).…”

Inhibiting myostatin reverses muscle fibrosis through apoptosis

“…The cells on which these factors and drugs act may be multiple, including various inflammatory cells, fibroblasts and myocytes. For example, osteopontin is produced by a subset of T cells as well as myocytes and genetic deletion of osteopontin in mdx mice was recently shown to reduce TGF-b levels, inflammation and fibrosis compared to controls (Vetrone et al, 2009). Recent studies have demonstrated that skeletal muscle contains multiple progenitor cell types in addition to satellite cells, including pericytes which contribute to skeletal muscle fiber development, and fibro/ adipogenic progenitors (FAPS) which may contribute to fibrosis or fatty infiltration (Dellavalle et al, 2011;Dellavalle et al, 2007;Joe et al, 2010;Minasi et al, 2002).…”

Osteopontin, inflammation and myogenesis: influencing regeneration, fibrosis and size of skeletal muscle

“…As osteopontin promotes inflammation and fibrosis in a wide range of tissues (Mori et al 2008;Pardo et al 2005), Spencer and colleagues tested the hypothesis that osteopontin may influence inflammation and fibrosis in dystrophin deficient muscles by crossing dystrophin-deficient mdx mice with osteopontin-null mice to produce double mutant mice (DMM), lacking both dystrophin and osteopontin (Vetrone et al 2009). This study reported that osteopontin deficiency decreased neutrophils, but not macrophages, natural killer T-cells ( + cells was found to accompany an increase in FoxP3 mRNA expression, suggesting that these cells may represent a population of regulatory T-cells (Vetrone et al 2009). Functionally, young (4 and 8 week old) DMM mice were found to have greater grip strength and a greater proportion of regenerating fibres in the quadriceps muscles than littermate mdx control mice.…”