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Tissue injury from mechanical trauma often leads to secondary organ failure. Local accumulation of neutrophils and excessive release of toxic metabolites through inhibition of neutrophil apoptosis may be responsible for capillary leakage and irreversible damage of resident cells of injured tissues.The purpose of this study was to examine the presence of apoptosis-inhibiting factors at the local site of tissue injury. Cerebrospinal fluid (CSF) from patients with severe head injury (n = 10; Abbreviated Injury Scale score, 4.5 +/- 0.2 points) and bronchoalveolar lavage fluid (BALF) from patients with serious chest trauma (n = 10; Abbreviated Injury Scale score, 4.1 +/- 0.1 points) were collected on days 1 and 3 after injury and compared with CSF (n = 5) and BALF (n = 16) obtained from patients undergoing elective orthopedic surgery. Neutrophils from healthy humans were incubated with 10% of CSF or BALF for 16 hours. Neutrophil apoptosis was determined by flow cytometric analysis of propidium iodide nuclear staining, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling, and May-Grunwald-Giemsa staining. Levels of granulocyte colony-stimulating factor (G-CSF) in CSF and BALF were measured with enzyme-linked immunosorbent assay.CSF and BALF from injured patients significantly inhibited spontaneous neutrophil apoptosis of healthy humans compared with control samples, whereas respiratory burst activity was enhanced (p < 0.05). Moreover, CSF and BALF from injured patients contained increased (p < 0.05) amounts of G-CSF. Neutralization of G-CSF in CSF and BALF from injured patients using monoclonal anti-G-CSF antibody markedly (p < 0.05) reduced the apoptosis-inhibiting effect of those body fluids and decreased the respiratory burst.In patients with severe head or chest injury, G-CSF acts locally as a strong inhibitor of spontaneous neutrophil apoptosis, which may cause an increased destructive potential of neutrophils present in injured tissues.