Pharmacological and genetic inhibition of ARG2 in CXCR2^Hi myeloid-derived suppressor cells combats sepsis-induced lymphopenia

<b>Rationale:</b> 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. <b>Methods:</b> 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. <b>Results:</b> We confirmed the specific enrichment of arginase-2 (ARG2) in CXCR2^Hi 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 CXCR2^Hi MDSCs, particularly in response to lipopolysaccharide (LPS) stimulation. In the human context, we revealed that CXCR2^Hi MDSC increased in peripheral in septic patients and correlated significantly to lymphopenia and elevated ARG2 levels. <b>Conclusions:</b> Sepsis stimulated p38-MAPK signaling and expanded ARG2-enriched CXCR2^Hi 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 CXCR2^Hi MDSC-derived ARG2 as a promising target of immune enhancement therapy in sepsis.