Abstract
Rationale: Myeloid-derived suppressor cells (MDSCs) play a critical role in inducing T-cell lymphopenia in sepsis, and the highly heterogeneous MDSCs necessitate the identification of key molecules within these cells. Methods: By integrating bulk and single-cell transcriptomic sequences, we identified the critical molecular and MDSC subpopulation in pneumonia-induced sepsis (PIS) models. Through fluorescence-activated cell sorting (FACS) technology, we isolated the primary target subset to evaluate its immunosuppressive potential via T-cell proliferation assays, and investigate the underlying cellular and molecular mechanisms. To assess the immunological consequences of molecular interventions (pharmacologic blockade and shRNA-mediated knockdown), we employed a "two-hit" experimental model to monitor T-cell-aassociated immune responses and hosts' outcomes following secondary infection. Futhermore, we collected and analyzed clinical samples to support of translating the cellular and molecular concept to human context. Results: We confirmed the specific enrichment of arginase-2 (ARG2) in CXCR2Hi MDSCs, which expanded during sepsis and drove immunosuppression via ARG2-mediated arginine depletion. The blockade of ARG2 and arginine supplements improved the proliferation and decreased apoptosis of CD4+ T cells. In PIS models, both ARG2 inhibition and knockdown regained CD4+ T cells in lung and bone marrow sites, thus enhancing host's resistance to secondary infections caused by opportunistic pathogens. Further mechanistic investigations indicated p38-MAPK as a critical regulator of the protein stability of the immunosuppressive molecule ARG2 in CXCR2Hi MDSCs, particularly in response to lipopolysaccharide (LPS) stimulation. In the human context, we revealed that CXCR2Hi MDSC increased in peripheral in septic patients and correlated significantly to lymphopenia and elevated ARG2 levels. Conclusions: Sepsis stimulated p38-MAPK signaling and expanded ARG2-enriched CXCR2Hi MDSCs to mediate septic lymphopenia via arginine depletion. The ARG2 inhibition restored T-cell immunity against secondary infection in septic immunosuppressed hosts. These findings identified CXCR2Hi MDSC-derived ARG2 as a promising target of immune enhancement therapy in sepsis.