Cardiosphere‐Derived Cell Exosomes Modulate mdx Macrophage Phenotype and Alter Their Secretome

Duchenne muscular dystrophy (DMD) is a lethal myodegenerative disease associated with chronic inflammation. Standard of care is currently limited to anti‐inflammatory steroids, which marginally delay disease progression. Cardiosphere‐derived cells (CDCs) are cardiac progenitor/stromal cells with immunomodulatory, anti‐fibrotic, and pro‐myogenic properties. We have recently demonstrated CDCs reverse many of the abnormalities of heart and skeletal muscle in mdx mice, which motivated two clinical trials (HOPE‐Duchenne and HOPE‐2). The bioactivity of CDCs is primarily mediated by secreted lipid bilayer nanoparticles called exosomes (CDC‐exos), which mediate cell‐cell transfer of genetic material to modify cell behavior. A single intravenous dose of CDC‐exos dramatically improves contractile function of the mdx mouse soleus and restores muscle architecture by enhancing endogenous muscle repair. While investigating this effect, we discovered a major role for the innate immune response. Downhill running exacerbates skeletal muscle damage in mdx mice. When macrophages are depleted, such mice do not recover their baseline exercise capacity, as reflected by reduced contractile function of the isolated soleus. These data suggest a population of macrophages may support innate muscle repair, albeit incompletely. Because paracrine factors derived from macrophages are known to orchestrate muscle repair following acute injury, we sought to determine if CDC‐exos skew mdx macrophage phenotype and alter their secretome. Macrophages derived from bone marrow of mdx mice (BMDMs) were cultured in macrophage‐colony stimulating factor. Unstimulated macrophages (neither M1 or M2 activated) were then treated with CDC‐exos or vehicle for 48 hours and collected for RNA and qPCR analysis, while the conditioned media was analyzed by antibody chip arrays of secreted cytokines and chemokines. Relative to unstimulated mdx macrophages, CDC‐exo primed macrophages exhibited upregulated gene expression of several C‐C and C‐X‐C motif ligands, and several interleukins (IL). Media conditioned by CDC‐exo primed macrophages contained many of the upregulated cytokines and chemokines, which are known to figure prominently in inflammatory cell recruitment and modulation. In particular, macrophage secretion of IL‐10 was enhanced by CDC‐exo priming, consistent with an M2‐like phenotype supportive of muscle growth and regeneration. Moreover, IL‐6, also upregulated, acts directly on muscle stem cells to induce their proliferation. Interestingly, CDC‐exo primed mdx macrophages robustly secrete tissue inhibitor of matrix metalloproteinase‐2 (TIMP‐2), unlike unstimulated mdx or wild‐type macrophages. In addition to its canonical role in remodeling the extracellular matrix (ECM), TIMP‐2 has been shown to promote differentiation of C2C12 myoblasts and may function in both ECM remodeling and myogenesis in vivo . Further experimentation is required to dissect the specific effects of CDC‐exo primed mdx macrophage secreted factors on skeletal muscle tissue remodeling and repair in mdx mice. Support or Funding Information [R01 HL124074] to E.M. Coalition Duchenne Muscular Dystrophy Association This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .