Abstract The inducible or “immunological” isoform of nitric oxide synthase (iNOS) is induced in many cell types by inflammatory stimuli and synthesizes toxic amounts of NO. In rodent models of focal cerebral ischemia, iNOS is expressed in neutrophils invading the injured brain and in local blood vessels. Studies with iNOS inhibitors and iNOS null mice indicate that NO produced by iNOS contributes to ischemic brain injury. In the present study, we sought to determine whether iNOS is also expressed in the human brain after ischemic stroke. Studies were conducted using immunohistochemistry on autopsy brains with neuropatho- logical evidence of acute cerebral infarction. iNOS im- munoreactivity was observed in neutrophils infiltrating the ischemic brain and in blood vessels within the isch- emic territory. iNOS-positive cells also were immunore- active for nitrotyrosine, reflecting protein nitration by NO-derived peroxynitrite and nitrites. iNOS or nitrotyro- sine immunoreactivity was not detected outside the region of the infarct. These observations provide evidence that iNOS is expressed in the human brain after ischemic in- farction and support the hypothesis that iNOS inhibitors may be useful in the treatment of ischemic stroke in hu- mans. Key words Cerebral ischemia · Inflammation · Immunohistochemistry · Nitrotyrosine Introduction Cerebral ischemia is associated with an inflammatory re- action involving the injured brain. A few hours after in- duction of ischemia, intravascular leukocytes adhere to the cerebral endothelium and infiltrate that ischemic brain [18, 34]. At later times, microglial cells become activated and astrocytes become reactive [17, 19]. Experimental ev- idence suggests that this inflammatory reaction is delete- rious to the ischemic brain [15]. Thus, if the neutrophilic infiltration is prevented, for example, by antibodies against adhesion molecules or by depletion of neutrophils prior to induction of ischemia, ischemic damage is re- duced [8, 9]. Therefore, post-ischemic inflammation is thought to contribute to the evolution of the tissue damage that occurs after cerebral ischemia [15]. Recent evidence suggests that expression of inducible nitric oxide synthase (iNOS) in neutrophils is one of the mechanisms by which post-ischemic inflammation exerts its deleterious effects. iNOS is transcriptionally induced in many cell types during inflammation and generates toxic levels of NO continuously [33]. NO produced by iNOS is a major mechanism of cytotoxicity in models of inflammation [30]. In rats, iNOS is expressed in infiltrat- ing neutrophils and cerebral blood vessels 6–96 h after oc- clusion of the middle cerebral artery (MCA) [25, 26]. In- hibition of iNOS by the relatively selective inhibitor aminoguanidine, administered starting 6–24 h after isch- emia, reduces the ensuing tissue damage and neurological deficits [24, 26, 32]. In addition, mice with a null muta- tion of iNOS do not express the enzyme following MCA occlusion and have smaller infarcts and a better neurolog- ical outcome than wild-type controls [27]. These data in- dicate that iNOS expression is deleterious to the post- ischemic brain, and raise the possibility that iNOS in- hibitors could be used in the treatment of cerebral isch- emia in patients. Colleen Forster · H. Brent Clark · M. Elizabeth Ross · Constantino Iadecola Inducible nitric oxide synthase expression in human cerebral infarcts Acta Neuropathol (1999) 97 : 215–220 © Springer-Verlag 1999 Received: 8 June 1998 / Revised, accepted: 23 September 1998 REGULAR PAPER C. Forster · M. E. Ross · C. Iadecola () Laboratory of Cerebrovascular Biology and Stroke, Department of Neurology, University of Minnesota Medical School, Box 295 UMHC, 420 Delaware Street S.E. Minneapolis, MN 55455, USA e-mail: iadec001@tc.umn.edu, Tel.: +1-612-624-1902, Fax: +1-612-625-7950 M. E. Ross Laboratory of Molecular Biology and Development, Department of Neurology, University of Minnesota Medical School, Minneapolis, Minnesota, USA H. B. Clark Department of Neurology and Laboratory Medicine and Pathology (Neuropathology), University of Minnesota, Minneapolis, MN 55455, USA