BACKGROUND Immunosuppression is regarded as the main cause of death induced by sepsis. Anti-programmed death-ligand 1 (PD-L1) therapy is promising in reversing sepsis-induced immunosuppression but no evidence is available on use of commercially available anti-PD-L1 medications for this indication. The present preclinical study was performed to investigate the therapeutic effect of an anti-PD-L1 nanobody (KN035) in sepsis. MATERIAL AND METHODS The level of expression of PD-L1 in PD-L1 humanized mice was confirmed with flow cytometry. Plasma concentrations of KN035 at different dosages at different time points were detected using an enzyme-linked immunosorbent assay. PD-L1 humanized mice were allocated into 4 groups: sham, cecal ligation and puncture (CLP), isotype (isotype+CLP), and PD-L1 (KN035+CLP). The 7-day survival rate was observed to investigate outcomes in CLP mice. Disease severity was assessed with histopathological scoring of mice lungs and livers. Immune status was assessed based on cell apoptosis in the spleen and bacterial clearance. RESULTS PD-L1 levels were significantly elevated in peripheral lymphocytes, monocytes, and neutrophils after CLP surgery. Blood concentrations of KN035 showed that 2.5 mg/kg had potential to be an ideal dosage for KN035 therapy. Survival analysis demonstrated that KN035 was associated with significantly reduced mortality on Day 7 after surgery (P=0.0083). The histopathological tests showed that KN035 alleviated sepsis-induced injury in the lungs and liver. KN035 reduced the number of apoptotic cells in the spleen and almost eliminated bacterial colonies in the peritoneal lavage fluid from the CLP mice. CONCLUSIONS KN035, an anti-PD-L1 antibody, can improve the rate of survival in CLP mice and alleviate sepsis-induced apoptosis in the spleen.