Activin A-Expressing Polymorphonuclear Myeloid-Derived Suppressor Cells Infiltrate Skeletal and Cardiac Muscle and Promote Cancer Cachexia.

Cachexia is a major cause of death in cancer and leads to wasting of cardiac and skeletal muscle, as well as adipose tissue. Various cellular and soluble mediators have been postulated in driving cachexia; however, the specific mechanisms behind this muscle wasting remain poorly understood. In this study, we found polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) to be critical for the development of cancer-associated cachexia. Significant expansion of PMN-MDSCs was observed in the cardiac and skeletal muscles of cachectic murine models. Importantly, the depletion of this cell subset, using depleting anti-Ly6G Abs, attenuated this cachectic phenotype. To elucidate the mechanistic involvement of PMN-MDSCs in cachexia, we examined major mediators, that is, IL-6, TNF-α, and arginase 1. By employing a PMN-MDSC-specific Cre-recombinase mouse model, we showed that PMN-MDSCs were not maintained by IL-6 signaling. In addition, PMN-MDSC-mediated cardiac and skeletal muscle loss was not abrogated by deficiency in TNF-α or arginase 1. Alternatively, we found PMN-MDSCs to be critical producers of activin A in cachexia, which was noticeably elevated in cachectic murine serum. Moreover, inhibition of the activin A signaling pathway completely protected against cardiac and skeletal muscle loss. Collectively, we demonstrate that PMN-MDSCs are active producers of activin A, which in turn induces cachectic muscle loss. Targeting this immune/hormonal axis will allow the development of novel therapeutic interventions for patients afflicted with this debilitating syndrome.