Neutrophils contribute significantly to ALI (acute lung injury) through adhesion to pulmonary microvascular endothelial cells (PMEC), trans-PMEC migration and alveolar infiltration. Trans-PMEC migration delays expression of neutrophil apoptosis, which promotes intra-alveolar neutrophil survival and neutrophil mediated ALI. We assessed the role of neutrophil vs PMEC inducible nitric oxide (NO) synthase (iNOS) in modulating neutrophil apoptosis. Apoptosis of wild-type vs iNOS-/- neutrophils was quantified by microscopy and FACS annexin-V binding. In a murine model of ALI, neutrophils isolated by BAL(broncho-alveolar lavage) from iNOS-/- mice had increased expression of apoptosis after 24 h culture ex vivo than wild-type neutrophils (15.2 +/- 3.3 vs 3.0 +/- 0.4%, mean +/- sd, p < 0.01). Apoptosis rates of isolated bone marrow iNOS+/+ vs iNOS-/- neutrophils were similar under basal and LPS/IFN-gamma stimulation, and following LPS/IFN-gamma-stimulated trans-PMEC migration. Apoptosis of both iNOS+/+ and iNOS-/- neutrophils was inhibited by trans-PMEC migration only across iNOS+/+ PMEC (1.6 +/- 0.3 and 1.5 +/- 0.3%, respectively; p < 0.05 for each vs non-migrated neutrophils) but not across iNOS-/-PMEC (4.3 +/- 1 and 3.1 +/- 0.6%, respectively). PMEC iNOS-dependent inhibition of neutrophil apoptosis was independent of changes in neutrophil caspase-3 activity. We conclude that PMEC iNOS, but not neutrophil iNOS, has an important inhibitory effect on neutrophil apoptosis during trans-PMEC neutrophil migration, which is independent of caspase-3 activity. Further studies will define the mechanism of PMEC iNOS-dependent inhibition of neutrophil apoptosis and assess the potential relevance of this phenomenon in human neutrophils and ALI. (C) 2010 Elsevier Inc. All rights reserved.