Ly6C⁺Ly6G^- Myeloid-derived suppressor cells play a critical role in the resolution of acute inflammation and the subsequent tissue repair process after spinal cord injury

Acute inflammation is a prominent feature of central nervous system (CNS) insult and is detrimental to the CNS tissue. Although this reaction spontaneously diminishes within a short period of time, the mechanism underlying this inflammatory resolution remains largely unknown. In this study, we demonstrated that an initial infiltration of Ly6C⁺Ly6G^- immature monocyte fraction exhibited the same characteristics as myeloid-derived suppressor cells (MDSCs), and played a critical role in the resolution of acute inflammation and in the subsequent tissue repair by using mice spinal cord injury (SCI) model. Complete depletion of Ly6C⁺Ly6G^- fraction prior to injury by anti-Gr-1 antibody (clone: RB6-8C5) treatment significantly exacerbated tissue edema, vessel permeability, and hemorrhage, causing impaired neurological outcomes. Functional recovery was barely impaired when infiltration was allowed for the initial 24 h after injury, suggesting that MDSC infiltration at an early phase is critical to improve the neurological outcome. Moreover, intraspinal transplantation of ex vivo-generated MDSCs at sites of SCI significantly reduced inflammation and promoted tissue regeneration, resulting in better functional recovery. Our findings reveal the crucial role of an Ly6C⁺Ly6G^- fraction as MDSCs in regulating inflammation and tissue repair after SCI, and also suggests an MDSC-based strategy that can be applied to acute inflammatory diseases. Myeloid-derived suppressor cells (MDSCs) exert immunosuppressive effects in several inflammatory diseases, including cancer and autoimmune disease. We demonstrated that Ly6C⁺Ly6G^- myeloid cells which infiltrated into injured spinal cord had a typical feature of MDSCs and played a critical role in the attenuation of acute inflammation and the subsequent tissue repair process after spinal cord injury (SCI). Our findings clarified the role of MDSCs after traumatic SCI, and suggested a potential MDSC-based therapeutic strategy for the acute phase of central nervous system injury.