The Immunomodulatory Role And Mechanism Of Muscle Stem Cells

Beside myogenic cells,healthy skeletal muscle tissue also contains abundant immune cells.These immune cells located in muscle stem cell（MuSC）niche directly determine damage repair and regeneration outcomes.Mounting evidences have confirmed the critical roles of immune cells in directing the fate of MuSCs.However,it is unclear whether there is a bidirectional interaction between immune cells and MuSCs and MuSCs can in turn regulate the functions of immune cells.Skeletal muscle is not only an important endocrine tissue,but also the main immunomodulatory organ.Myocytes participate in the innate immune response and affect the progress of adaptive immunity through secreting myokines.Muscle trauma is accompanied by serious myocyte necrosis.It is unclear whether MuSCs,the main myogenic cells in the damaged tissue microenvironment,can direct the progress of inflammation through immunomodulatory functions.Disturbance of immune cell metabolism is an important cause of inflammatory diseases.It is a new way to treat inflammatory diseases by adjusting the metabolic pattern to guide the phenotypes of immune cells.It is unclear whether MuSCs can change the phenotypes of immune cells by modifying metabolic patterns and affect the outcomes of inflammatory diseases.Based on the above scientific questions and hypotheses,we used dextran sodium sulfate-induced colitis as an experimental model to study the immunomodulatory function of MuSCs.A single intravenous infusion of MuSCs significantly ameliorated colitis,improved clinical parameters such as weight loss,diarrhea,bloody stool,depression and disease activity index.In addition,the shortening of colons in mice were also improved.MuSCs reduced the extent of bowel wall thickening,crypt damage,the infiltration of inflammatory cells and histological scores as shown by histopathologic examination.The reduction of tissue-infiltrated inflammatory macrophages and the decrease in serum IL-6 levels have also demonstrated the anti-inflammatory therapeutic effect of MuSCs.MuSC supernatant as a therapeutic agent to treat colitic mice obtained similar beneficial effects.However,in chlodronate liposome-treated colitic mice,MuSC supernatant had no detectable therapeutic efficacy,indicating that the beneficial effect of MuSCs in colitic mice requires macrophages.Furthermore,MuSCs endowed maturing macrophages with anti-inflammatory immune memory,increased the expression levels of anti-inflammatory genes,and reduced IL-1βand IL-6 after LPS and IFN-γstimulation.In addition,MuSC-preprogrammed macrophages possessed higher levels of PD-L1.Through cell metabolism comparison,lactate accumulation was found to be decreased in macrophages treated with MuSC supernatant.There were lower mitochondrial ROS and higher membrane potential in MuSC-preprogrammed macrophages.Oligomycin A,a mitochondrial OXPHOS inhibitor,abolished the suppression of IL-6 expression and the induction of PD-L1 expression in inflammatory macrophages educated by MuSC supernatant,further demonstrating that MuSCs endow maturing macrophages with metabolic preference for OXPHOS,thus inhibiting inflammatory responses and programing macrophages with anti-inflammatory properties.IGF-2 produced by MuSCs is a key molecule for the immunomodulatory functions of MuSCs.Knockdown or neutralization of IGF-2 abolished the metabolic preference for OXPHOS and anti-inflammatory properties in MuSC-preprogrammed macrophages.Blocking IGF-2 eliminated the therapeutic effect of MuSCs on colitic mice.In addition to the regulatory functions of innate immunity,MuSCs also have adaptive immunomodulatory effects.The proliferation of activated splenocytes was inhibited when co-cultured with MuSCs.The use of i NOS inhibitors or i NOS^-/-MuSCs blocked the inhibitory effect of wild-type MuSCs on the proliferation of splenocytes,further confirming that a novel adaptive immunomodulatory function in MuSCs that is based on their expression of i NOS.By producing high concentrations of NO,MuSCs inhibit the proliferation of activated T cell.The adaptive immunomodulatory function of MuSCs may help to alleviate autoimmune diseases with T cells as the main pathogenic cell population.By inhibiting the proliferative response of activated T cells,MuSCs can alleviate inflammatory damages and recover immune homeostasis under pathological conditions.In this study,we demonstrated that MuSCs have important immunoregulatory functions.They can secrete regulatory factors to affect immune cells,specially regulate the phenotypes of macrophages by secreting IGF-2 and express i NOS to inhibit the proliferation of T cells,thus ameliorating inflammatory diseases.Moreover,the immunomodulatory functions of MuSCs can be applied in the therapies of inflammatory diseases except in the skeletal muscle repair and regeneration.Our findings may help to pave the way for a new era of distinctive MuSCs-based medicines for inflammatory diseases.