Splenic PD-L1+ Myeloid-Derived Suppressor Cells Induce Contact-Dependent Alveolar Macrophage Dysfunction in Early Sepsis.

Sepsis frequently leaves patients with persistent immune impairment that contributes to late mortality, yet the anatomical routes that transmit this response to the lung remain poorly defined. Here, we identify the spleen as a key extramedullary hub orchestrating pulmonary immune paralysis through a spleen-lung axis. Mechanistically, splenic programmed death-ligand 1 (PD-L1)+ polymorphonuclear myeloid-derived suppressor cells undergo TFDP1-driven expansion and migrate to the lungs via CXCL2/CXCR2 signaling, establishing an interorgan immunosuppressive circuit. In the lung, these cells reprogram alveolar macrophages through programmed death-1 (PD-1)-dependent checkpoint signaling, inducing a dysfunctional state characterized by impaired antibacterial responses and enrichment of complement and immune checkpoint pathways. Genetic or pharmacologic disruption of the PD-L1-PD-1 axis restores macrophage function and improves pulmonary host defense in experimental sepsis. Together, these findings define a mechanistic link between splenic myelopoiesis and distal lung immune paralysis.