Fig 2: S100A9 knockout suppresses CLP-induced p38/STAT3/ERK signalling activation and apoptosis in the lungs. (A) Western blotting analysis of p38/STAT3//ERK signalling in the lung tissues of mice 12 h after CLP surgery (n = 6). The statistical results are presented in (B-D). (E) Western blot analysis of Bax and Bcl-2 expressions in the lung tissues of mice 12 h after CLP surgery (n = 6). The statistical results are presented in (F). S100, S100 calcium-binding protein; CLP, cecal ligation and puncture; STAT3, signal transducer and activator of transcription 3; ERK, extracellular signal-regulated kinase; Bax, B-cell leukaemia/lymphoma 2 protein (Bcl-2)-associated X protein. The data are presented as the mean ± standard error of the mean. *P < 0.05; ***P < 0.001

Fig 3: S100A9 is a candidate biomarker for sepsis diagnosis. (A) The plasma S100A9 levels in individuals with sepsis (n = 81) compared with healthy controls (n = 22). (B) The ROC curve for the diagnostic tests. S100, S100 calcium-binding protein; ROC, receiver operating characteristic. The data are presented as the mean ± standard error of the mean. ****P < 0.0001

Fig 4: Elevated S100 calcium-binding protein (S100) A8/A9 levels are associated with pulmonary vascular hyperpermeability and disease severity in polymicrobial sepsis. (A) S100A8/A9, occludin, and VE-cadherin protein expression levels in the lungs of WT mice were assessed using Western blotting 12 h after CLP surgery. The statistical results of S100A8/A9, occludin, and VE-cadherin are illustrated in (B), (C), and (D) respectively (n = 5). (E) The body temperatures were assessed using a rectal thermometer 12 h after CLP/sham surgery (n = 6). (F) Survival rates among the WT and S100A9 KO mice after CLP surgery were compared using the Kaplan–Meier test (n = 15, three independent experiments). (G) Carotid pressures were measured 12 h after surgery and the related statistical results are presented in (H-I) (n = 5). S100, S100 calcium-binding protein; VE, vascular endothelial; WT, wide-type. KO, knockout. CLP, cecal ligation and puncture. VE-cad, VE-cadherin. The data are presented as the mean ± standard error of the mean. ns, no significant difference. *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001

Fig 5: S100A9 knockout attenuates CLP-induced decrease in occludin and VE-cadherin levels in the lungs. (A) Representative immunofluorescent staining images of occludin (green) and DAPI (blue) in lung tissues of mice 12 h after CLP surgery (n = 6). The statistical results are presented in (B). (C) Representative immunofluorescent staining images of VE-cadherin (red) and DAPI (blue) in lung tissues of mice 12 h after CLP surgery (n = 6). The statistical results are illustrated in (D). (E) The occludin and VE-cadherin protein expressions in mice lungs were determined using Western blotting 12 h after CLP surgery (n = 6). The statistical results are presented in (F) and (G). S100, S100 calcium-binding protein; CLP, cecal ligation and puncture; VE, vascular endothelial; DAPI, 4′,6-diamidino-2-phenylindole. Data are presented shown as the mean ± standard error of the mean. ns, no significant difference. *P < 0.05; **P < 0.01