Inducible Nitric Oxide Synthase in Neutrophils and Endothelium Contributes to Ischemic Brain Injury in Mice

Abstract

NO produced by inducible NO synthase (iNOS) contributes to ischemic brain injury, but the cell types expressing iNOS and mediating tissue damage have not been elucidated. To examine the relative contribution of iNOS in resident brain cells and peripheral leukocytes infiltrating the ischemic brain, we used bone marrow (BM) chimeric mice in which the middle cerebral artery was occluded and infarct volume was determined 3 d later. iNOS−/− mice engrafted with iNOS+/+ BM exhibited larger infarcts (44 ± 2 mm3; n = 13; mean ± SE) compared with autologous transplanted iNOS−/− mice (24 ± 3 mm3; n = 10; p < 0.01), implicating blood-borne leukocytes in the damage. Furthermore, iNOS+/+ mice transplanted with iNOS−/− BM had large infarcts (39 ± 6 mm3; n = 13), similar to those of autologous transplanted iNOS+/+ mice (39 ± 4 mm3; n = 14), indicating the resident brain cells also play a role. Flow cytometry and cell sorting revealed that iNOS is highly expressed in neutrophils and endothelium but not microglia. Surprisingly, postischemic iNOS expression was enhanced in the endothelium of iNOS+/+ mice transplanted with iNOS−/− BM and in leukocytes of iNOS−/− mice with iNOS+/+ BM, suggesting that endothelial iNOS suppresses iNOS expression in leukocytes and vice versa. To provide independent evidence that neutrophils mediate brain injury, neutrophils were isolated and transferred to mice 24 h after stroke. Consistent with the result in chimeric mice, transfer of iNOS+/+, but not iNOS−/−, neutrophils into iNOS−/− mice increased infarct volume. The findings establish that iNOS in both neutrophils and endothelium mediates tissue damage and identify these cell types as putative therapeutic targets for stroke injury.