The role of complement C3 in intracerebral hemorrhage-induced brain injury

Complement activation in the injured central nervous system: another dual-edged sword?

The complement system, a major component of the innate immune system, is becoming increasingly recognised as a key participant in physiology and disease. The awareness that immunological mediators support various aspects of both normal central nervous system (CNS) function and pathology has led to a renaissance of complement research in neuroscience. Various studies have revealed particularly novel findings on the wide-ranging involvement of complement in neural development, synapse elimination and maturation of neural networks, as well as the progression of pathology in a range of chronic neurodegenerative disorders, and more recently, neurotraumatic events, where rapid disruption of neuronal homeostasis potently triggers complement activation. The purpose of this review is to summarise recent findings on complement activation and acquired brain or spinal cord injury, i.e. ischaemic-reperfusion injury or stroke, traumatic brain injury (TBI) and spinal cord injury (SCI), highlighting the potential for complement-targeted therapeutics to alleviate the devastating consequences of these neurological conditions.

The Molecular Mechanisms that Promote Edema After Intracerebral Hemorrhage

Intracerebral hemorrhage (ICH) is a devastating type of stroke with no effective therapies. Clinical advances in ICH treatment are limited by an incomplete understanding of the molecular mechanisms responsible for secondary injury and poor outcome. Increasing evidence suggests that cerebral edema is a major contributor to secondary injury and poor outcome in ICH. ICH activates specific signaling pathways that promote edema and damage neuronal tissue. By increasing our understanding of these pathways, we may be able to target them pharmaceutically to reduce edema in ICH patients. In this review, we focus on three major signaling pathways that promote edema after ICH: (1) the coagulation cascade and thrombin, (2) the inflammatory response and matrix metalloproteinases, and (3) the complement cascade and hemoglobin toxicity. We will describe the experimental evidence that confirms these pathways promote edema in ICH, discuss potential targets for new therapies, and comment on important directions for future research.

Complement Factor H Y402H polymorphism is associated with an increased risk of mortality after intracerebral hemorrhage

Intracerebral hemorrhage (ICH) accounts for 10% to 15% of all strokes and is a major cause of morbidity and mortality. Despite advances in management, numerous clinical trials have failed to demonstrate significant benefit of medical and surgical interventions, underscoring the need for the identification of novel therapeutic targets based on improved understanding of ICH pathophysiology and optimal risk stratification based on reliable and effective prognosticators. The alternative complement cascade has been implicated as an important contributor to neurological injury after ICH. Therefore, common, functionally relevant genetic variants in the key components of this pathway have been associated with greater inflammation post-ictus, further cerebral damage, and ultimately, a worse outcome. We investigated the affects of single-nucleotide polymorphisms (SNP) on mortality in complement component 3 C3 (rs2230199), complement component 5 C5 (rs17611), and Complement Factor H (CFH; rs1061170) genes, which are associated with the onset and progression of several neurological diseases, in a prospective cohort of patients with spontaneous ICH. From February 2009 through May 2010, adult patients with spontaneous ICH were admitted to the Columbia University Neurological Intensive Care Unit and enrolled in the Intracerebral Hemorrhage Outcomes Project. Demographic, clinical, radiographic, and treatment data were prospectively collected. Buccal swabs were obtained, and isolated cells were sequenced for the aforementioned SNP. A total of 103 patients were admitted with ICH, and of these, 82 consented for genetic testing and were included in the analysis. The median age was 61 years and 39% were females. The median Glasgow Coma Scale score on admission was 11.5. The CFH SNP was significantly associated with both discharge (p = 0.01) and 6-month mortality (p = 0.02), while no such association was observed for C3 (p = 0.545 and p = 0.830) or C5 (p = 0.983 and p = 0.536) SNP. Additionally, after controlling for pertinent variables identified in the univariate analysis, the CFH genotype independently predicted mortality at discharge (p = 0.019, odds ratio [OR] 7.62, 95% confidence interval [CI] 1.40-41.6) and at 6 months (p = 0.041, OR 1.822, 95% CI 1.025-3.239). The CFH genotype was also independently predictive of survival duration (p = 0.041, OR 1.822, 95% CI 1.025-3.239). We concluded that CFH Y402H polymorphism independently predicts mortality at discharge and 6-months and survival duration after spontaneous ICH.

Minocycline attenuates photoreceptor degeneration in a mouse model of subretinal hemorrhage microglial: inhibition as a potential therapeutic strategy

Hemorrhage under the neural retina (subretinal hemorrhage) can occur in the context of age-related macular degeneration and induce subsequent photoreceptor cell death and permanent vision loss. Current treatments with the objective of removing or displacing the hemorrhage are invasive and of mixed efficacy. We created a mouse model of subretinal hemorrhage to characterize the inflammatory responses and photoreceptor degeneration that occur in the acute aftermath of hemorrhage. It was observed that microglial infiltration into the outer retina commences as early as 6 hours after hemorrhage. Inflammatory cells progressively accumulate in the outer nuclear layer concurrently with photoreceptor degeneration and apoptosis. Administration of minocycline, an inhibitor of microglial activation, decreased microglial expression of chemotactic cytokines in vitro and reduced microglial infiltration and photoreceptor cell loss after subretinal hemorrhage in vivo. Inflammatory responses and photoreceptor atrophy occurred after subretinal hemorrhage, however, the degree of response and atrophy were similar between C3-deficient and C3-sufficient mice, indicating a limited role for complement-mediated processes. Our data indicate a role for inflammatory responses in driving photoreceptor cell loss in subretinal hemorrhage, and it is proposed that microglial inhibition may be beneficial in the treatment of subretinal hemorrhage.

Exploring neuroprotective drug therapies for intracerebral hemorrhage

Intracerebral hemorrhage (ICH) is a devastating neurological disorder with high mortality and poor prognosis, for which virtually no effective drug therapies are available at present. Experimental animal models, based on intrastriatal injection of collagenase or autologous blood, have enabled great advances in elucidation of cellular/molecular events contributing to brain pathogenesis associated with ICH. Many lines of evidence indicate that blood constituents, including hemoglobin-derived products as well as proteases such as thrombin, play important roles in the pathogenic events. Inflammatory reactions involving neutrophils, activated microglia, and production of proinflammatory cytokines also constitute a critical aspect of pathology leading to neurodegeneration and tissue damage. Efforts are continuing to find drugs that potentially alleviate pathological and neurological outcomes of ICH. Various drugs that possess antioxidative, anti-inflammatory or neurotrophic/neuroprotective properties have been demonstrated to produce therapeutic effects on ICH animal models. Drugs already in clinical use such as minocycline, statins, and several nuclear receptor ligands are among the list of effective drugs, but whether they also show therapeutic efficacy in human ICH patients remains unproven. Here, current knowledge of ICH pathogenesis and problems arising with respect to exploration of new drug candidates are discussed.

Complement factor 3 deficiency attenuates hemorrhagic shock-related hepatic injury and systemic inflammatory response syndrome

Although complement activation is known to occur in the setting of severe hemorrhagic shock and tissue trauma (HS/T), the extent to which complement drives the initial inflammatory response and end-organ damage is uncertain. In this study, complement factor 3-deficient (C3(-/-)) mice and wild-type control mice were subjected to 1.5-h hemorrhagic shock, bilateral femur fracture, and soft tissue injury, followed by 4.5-h resuscitation (HS/T). C57BL/6 mice were also given 15 U of cobra venom factor (CVF) or phosphate-buffered saline injected intraperitoneally, followed by HS/T 24 h later. The results showed that HS/T resulted in C3 consumption in wild-type mice and C3 deposition in injured livers. C3(-/-) mice had significantly lower serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) and circulating DNA levels, together with much lower circulating interleukin (IL)-6, IL-10, and high-mobility group box 1 (HMGB1) levels. Temporary C3 depletion by CVF preconditioning also led to reduced transaminases and a blunted cytokine release. C3(-/-) mice displayed well-preserved hepatic structure. C3(-/-) mice subjected to HS/T had higher levels of heme oxygenase-1, which has been associated with tissue protection in HS models. Our data indicate that complement activation contributes to inflammatory pathways and liver damage in HS/T. This suggests that targeting complement activation in the setting of severe injury could be useful.

Minocycline attenuates brain edema, brain atrophy and neurological deficits after intracerebral hemorrhage

Evidence suggests that microglia activation contributes to brain injury after intracerebral hemorrhage (ICH). The present study aimed to determine if minocycline, an inhibitor of microglia activation, can reduce brain edema, brain atrophy and neurological deficits after ICH.Male Sprague-Dawley rats received an infusion of 100-microL autologous whole blood into the right basal ganglia. Rats received minocycline or vehicle treatment. There were two sets of experiments in this study. In the first set of experiments, the effects of minocycline on ICH-induced brain edema were examined at day 3. In the second set, behavioral tests were performed at days 1, 3, 7, 14 and 28. Rats were killed at day 28 for brain atrophy measurement (caudate and lateral ventricle size).Minocycline reduced perihematomal brain edema in the ipsilateral basal ganglia (78.8 +/- 0.4 vs. 80.9 +/- 1.1% in the vehicle-treated group, p < 0.01). Minocycline also improved functional outcome. In addition, minocycline reduced brain tissue loss in the ipsilateral caudate (p < 0.01) and ventricular enlargement (p < 0.05).In conclusion, minocycline attenuates ICH-induced brain edema formation, neurological deficits and brain atrophy in rats suggesting an important role of microglia in ICH-related brain injury.

Synergistic neuroprotective effects of C3a and C5a receptor blockade following intracerebral hemorrhage

BACKGROUND

Intracerebral hemorrhage (ICH) is associated with neurological injury that may be ameliorated by a neuroprotective strategy targeting the complement cascade. We investigated the role of C5a-receptor antagonist (C5aRA) solely and in combination with C3a-receptor antagonist (C3aRA) following ICH in mice.

METHODS

Adult male C57BL/6J mice were randomized to receive vehicle, C5aRA alone or C3aRA and C5aRA 6 and 12 h after ICH, and every 12 h thereafter. A double injection technique was used to infuse 30 microL of autologous whole blood into the right striatum. A final group of mice received a sham procedure consisting only of needle insertion followed by vehicle injections. Brain water content and flow cytometry analysis for leukocyte and microglia infiltration and activation in both hemispheres were measured on day 3 post ICH. Neurological dysfunction was assessed using a Morris water-maze (MWM), a 28-point scale, and a corner test at 6, 12, 24, 48 and 72 h after ICH induction.

RESULTS

Neurological deficits were present and comparable in all three cohorts 6 h after ICH. Animals treated with C5aRA and animals treated with combined C3aRA/C5aRA demonstrated significant improvements in neurological function assessed by both the corner turn test and a 28-point neurological scale at 24, 48 and 72 h relative to vehicle-treated animals. Similarly, C5aRA and C3aRA/C5aRA-treated mice demonstrated better spatial memory retention in the Morris water-maze test compared with vehicle-treated animals (C3aRA/C5aRA: 23.4+/-2.0 s p< or =0.0001 versus vehicle: 10.0+/-1.7 s). Relative to vehicle-treated mice, the brain water content in C3aRA/C5aRA-treated mice was significantly decreased in the ipsilateral cortex and ipsilateral striatum (ipsilateral cortex: C3aRA/C5aRA: 0.755403+/-0.008 versus 0.773327+/-0.003 p=0.01 striatum: 0.752273+/-0.007 versus 0.771163+/-0.0036 p=0.02). C5aRA-treated mice and C3aRA/C5aRA-treated mice had a decreased ratio of granulocytes (CD45(+)/CD11b(+)/Ly-6G(+)) in the hemorrhagic versus non-hemorrhagic hemispheres relative to vehicle-treated animals (C5aRA: 1.78+/-0.36 p=0.02 C3aRA/C5aRA: 1.59+/-0.22 p=0.005 versus vehicle: 3.01).

CONCLUSIONS

While administration of C5aRA alone provided neuroprotection, combined C3aRA/C5aRA therapy led to synergistic improvements in neurofunctional outcome while reducing inflammatory cell infiltration and brain edema. The results of this study indicate that simultaneous blockade of the C3a and C5a receptors represents a promising neuroprotective strategy in hemorrhagic stroke.

The complement cascade as a therapeutic target in intracerebral hemorrhage

Intracerebral hemorrhage (ICH) is the second most common and deadliest form of stroke. Currently, no pharmacologic treatment strategies exist for this devastating disease. Following the initial mechanical injury suffered at hemorrhage onset, secondary brain injury proceeds through both direct cellular injury and inflammatory cascades, which trigger infiltration of granulocytes and monocytes, activation of microglia, and disruption of the blood-brain barrier with resulting cerebral edema. The complement cascade has been shown to play a central role in the pathogenesis of secondary injury following ICH, although the specific mechanisms responsible for the proximal activation of complement remain incompletely understood. Cerebral injury following cleavage of complement component 3 (C3) proceeds through parallel but interrelated pathways of anaphylatoxin-mediated inflammation and direct toxicity secondary to membrane attack complex-driven erythrocyte lysis. Complement activation also likely plays an important physiologic role in recovery following ICH. As such, a detailed understanding of the variation in functional effects of complement activation over time is critical to exploiting this target as an exciting translational strategy for intracerebral hemorrhage.

Intracerebral haemorrhage

Intracerebral haemorrhage is an important public health problem leading to high rates of death and disability in adults. Although the number of hospital admissions for intracerebral haemorrhage has increased worldwide in the past 10 years, mortality has not fallen. Results of clinical trials and observational studies suggest that coordinated primary and specialty care is associated with lower mortality than is typical community practice. Development of treatment goals for critical care, and new sequences of care and specialty practice can improve outcome after intracerebral haemorrhage. Specific treatment approaches include early diagnosis and haemostasis, aggressive management of blood pressure, open surgical and minimally invasive surgical techniques to remove clot, techniques to remove intraventricular blood, and management of intracranial pressure. These approaches improve clinical management of patients with intracerebral haemorrhage and promise to reduce mortality and increase functional survival.

Microglial activation and brain injury after intracerebral hemorrhage

Microglial activation and thrombin formation contribute to brain injury after intracerebral hemorrhage (ICH). Tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1beta) are 2 major proinflammatory cytokines. In this study, we investigated whether thrombin stimulates TNF-alpha and IL-1beta secretion in vitro, and whether microglial inhibition reduces ICH-induced brain injury in vivo. There were 2 parts to this study. In the first part, cultured rat microglial cells were treated with vehicle, thrombin (5 and 10U/mL), or thrombin plus tuftsin (0.05 microg/mL), an inhibitor of microglia activation. Levels of TNF-alpha and IL-1beta in culture medium were measured by ELISA at 4, 8, and 24 h after thrombin treatment. In the second part of the study, rats received an intracerebral infusion of 100 microL autologous whole blood with or without 25 microg of tuftsin 1-3 fragment. Rats were killed at day 1 or day 3 for immunohistochemistry and brain water content measurement. We found that thrombin receptors were expressed in cultured microglia cells, and TNF-alpha and IL-1beta levels in the culture medium were increased after thrombin treatment. Tuftsin reduced thrombin-induced upregulation of TNF-alpha and IL-1beta. In vivo, microglia were activated after ICH, and intracerebral injection of tuftsin reduced brain edema in the ipsilateral basal ganglia (81.1 +/- 0.7% vs. 82.7 +/- 1.3% in vehicle-treated group; p < 0.05) after ICH. These results suggest a critical role of microglia activation in ICH-related brain injury.

Bilirubin oxidation products, oxidative stress, and intracerebral hemorrhage

Hematoma and perihematomal regions after intracerebral hemorrhage (ICH) are biochemically active environments known to undergo potent oxidizing reactions. We report facile production of bilirubin oxidation products (BOXes) via hemoglobin/Fenton reaction under conditions approximating putative in vivo conditions seen following ICH. Using a mixture of human hemoglobin, physiological buffers, unconjugated solubilized bilirubin, and molecular oxygen and/or hydrogen peroxide, we generated BOXes, confirmed by spectral signature consistent with known BOXes mixtures produced by independent chemical synthesis, as well as HPLC-MS of BOX A and BOX B. Kinetics are straightforward and uncomplicated, having initial rates around 0.002 microM bilirubin per microM hemoglobin per second under normal experimental conditions. In hematomas from porcine ICH model, we observed significant production of BOXes, malondialdehyde, and superoxide dismutase, indicating a potent oxidizing environment. BOX concentrations increased from 0.084 +/- 0.01 in fresh blood to 22.24 +/- 4.28 in hematoma at 72h, and were 11.22 +/- 1.90 in adjacent white matter (nmol/g). Similar chemical and analytical results are seen in ICH in vivo, indicating the hematoma is undergoing similar potent oxidations. This is the first report of BOXes production using a well-defined biological reaction and in vivo model of same. Following ICH, amounts of unconjugated bilirubin in hematoma can be substantial, as can levels of iron and hemoglobin. Oxidation of unconjugated bilirubin to yield bioactive molecules, such as BOXes, is an important discovery, expanding the role of bilirubin in pathological processes seen after ICH.

C3a receptor antagonist attenuates brain injury after intracerebral hemorrhage

Neuroprotective therapy targeting the complement cascade may reduce injury associated with intracerebral hemorrhage (ICH). We investigated the role of C3a-receptor antagonist (C3aRA) after ICH in mice. Autologous whole blood was infused into the right striatum of mice that were treated with C3aRA or vehicle, using both a pre- and postinjury dosing regimen. Hematoma volume, brain water content, and inflammatory cell profile were assessed at 72 h post-ICH. Neurologic dysfunction was assessed by evaluating both spatial memory and sensorimotor capacity. Animals pretreated with C3aRA showed significantly improved neurologic function, brain water content, and granulocyte infiltration relative to vehicle-treated animals when assessed at 72 h. There was no significant difference in hemorrhagic/nonhemorrhagic ratio of microglial activation among all groups. Hematoma volumes were also not significantly different between C3aRA-treated and vehicle-treated animals. Administration of C3aRA beginning 6 h postinjury afforded significant amelioration of neurologic dysfunction as well as a reduction in brain water content. Treatment with C3aRA improved neurologic outcome while reducing inflammatory cell infiltration and brain edema formation after experimental ICH in mice. Results of this study suggest that the C3a receptor may be a promising target for therapeutic intervention in hemorrhagic stroke.

Intracerebral hemorrhage leads to infiltration of several leukocyte populations with concomitant pathophysiological changes

Intracerebral hemorrhage (ICH) is a stroke subtype with high rates of mortality and morbidity. The immune system, particularly complement and cytokine signaling, has been implicated in brain injury after ICH. However, the cellular immunology associated with ICH has been understudied. In this report, we use flow cytometry to quantitatively profile immune cell populations that infiltrate the brain 1 and 4 days post-ICH. At 1 day CD45(hi) GR-1(+) cells were increased 2.0-fold compared with saline controls (P<or=0.05); however, we did not observe changes in any other cell populations analyzed. At 4 days ICH mice presented with a 2.4-fold increase in CD45(hi) cells, a 1.9-fold increase in CD45(hi) GR-1(-) cells, a 3.4-fold increase in CD45(hi) GR-1(+) cells, and most notably, a 1.7-fold increase in CD4(+) cells (P<or=0.05 for all groups), compared with control mice. We did not observe changes in the numbers of CD8(+) cells or CD45(lo) GR-1(-) cells (P=0.43 and 0.49, respectively). Thus, we have shown the first use of flow cytometry to analyze leukocyte infiltration in response to ICH. Our finding of a CD4 T-cell infiltrate is novel and suggests a role for the adaptive immune system in the response to ICH.

Minocycline reduces intracerebral hemorrhage-induced brain injury

OBJECTIVES

Microglial activation and thrombin formation contribute to brain injury after intracerebral hemorrhage. Tumor necrosis factor-alpha and interleukin-1beta are two major pro-inflammatory cytokines. The present study investigated if thrombin stimulates tumor necrosis factor-alpha and interleukin-1beta secretion in vitro and if microglial inhibition reduces intracerebral hemorrhage-induced brain injury in vivo.

METHODS

There were two parts in this study. In the first part, cultured rat microglial cells were treated with vehicle, thrombin (10 U/ml) or thrombin plus minocycline (1 or 10 microM), an inhibitor of microglia activation. Levels of tumor necrosis factor-alpha and interleukin-1beta in culture medium were measured by enzyme-linked immunosorbent assay 24 hours after thrombin treatment. In the second part, rats had an intracerebral injection of 100 microl autologous whole blood. Rats received minocycline or vehicle treatment. Brain edema was measured at day 3 and brain atrophy was determined at day 28 after intracerebral hemorrhage.

RESULTS

Thrombin receptors were expressed in cultured microglia cells, and tumor necrosis factor-alpha and interleukin-1beta levels in the culture medium were increased after thrombin treatment. Minocycline reduced thrombin-induced up-regulation of tumor necrosis factor-alpha and interleukin-1beta. In vivo, minocycline reduced perihematomal brain edema, neurological deficits and brain atrophy.

DISCUSSION

Thrombin stimulates microglia to release the pro-inflammatory cytokines, tumor necrosis factor-alpha and interleukin-1beta, and microglial inhibition with minocycline reduces brain injury after intracerebral hemorrhage, suggesting a critical role of microglia activation in intracerebral hemorrhage-related brain injury.

Effects of aging on complement activation and neutrophil infiltration after intracerebral hemorrhage

Intracerebral hemorrhage (ICH)-induced brain edema and neurological deficits are greater in aged rats than in young rats. Complement activation and neutrophil infiltration contribute to brain injury after ICH. In this study, we investigated the effects of aging on activation of the complement cascade and neutrophil influx following ICH. Male Sprague-Dawley rats (3 or 18 months old) received an infusion of 100 microL autologous blood into right caudate. Rats were killed at 1, 3, 7, and 28 days after ICH and the brains were sampled for immunohistochemistry and Western blot analysis. Levels of complement factor C9 and clusterin were used as markers for complement activation, and myeloperoxidase (MPO) staining was performed to detect neutrophil infiltration. Western blot analysis showed that complement C9 and clusterin levels in ipsilateral basal ganglia after ICH were higher in aged rats than in young rats (p < 0.05). Immunohistochemistry showed there were more C9- and clusterin-positive cells around the hematoma in aged rats. However, MPO-positive cells in ipsilateral basal ganglia were fewer in aged rats (p < 0.05) after ICH. Our results suggest that ICH causes more severe complement activation and less neutrophil infiltration in aged rats. Clarification of the mechanisms of brain injury after ICH in the aging brain should help develop new therapeutic strategies for ICH.

Reduced tissue damage and improved recovery of motor function after traumatic brain injury in mice deficient in complement component C4

Complement component C4 mediates C3-dependent tissue damage after systemic ischemia-reperfusion injury. Activation of C3 also contributes to the pathogenesis of experimental and human traumatic brain injury (TBI); however, few data exist regarding the specific pathways (classic, alternative, and lectin) involved. Using complement knockout mice and a controlled cortical impact (CCI) model, we tested the hypothesis that the classic pathway mediates secondary damage after TBI. After CCI, C4c and C3d immunostaining were detected in cortical vascular endothelial cells in wild-type (WT) mice; however, C4c and C3d immunostaining were also detected in C1q(-/-) mice, and C3d immunostaining was detected in C4(-/-) mice. After CCI, WT and C1q(-/-) mice had similar motor deficits, Morris water maze performance, and brain lesion size. Naive C4(-/-) and WT mice did not differ in baseline motor performance, but C4(-/-) mice had reduced postinjury motor deficits (days 1 to 7, P<0.05) and decreased brain tissue damage (days 14 and 35, P<0.05) versus WT. Reconstitution of C4(-/-) mice with human C4 (hC4) reversed their protection against postinjury motor deficits (P<0.05 versus vehicle), but administration of hC4 did not impair postinjury motor performance (versus vehicle) in WT mice. The protective effects of C4(-/-) were functionally distinct from the classic pathway and terminal complement, as C1q(-/-) and C3(-/-) mice had postinjury tissue damage and motor dysfunction similar to WT. Thus, C4 contributes to motor deficits and brain tissue damage after CCI by mechanism(s) fundamentally different from those involved in experimental systemic ischemia-reperfusion injury.

Time course of increased heme oxygenase activity and expression after experimental intracerebral hemorrhage: correlation with oxidative injury

Heme oxygenase (HO) activity in tissue adjacent to an intracerebral hematoma may modulate cellular vulnerability to heme-mediated oxidative injury. Although HO-1 is induced after experimental intracerebral hemorrhage (ICH), the time course of this induction, its effect on tissue HO activity, and its association with oxidative injury markers has not been defined. We therefore quantified HO activity, HO-1 expression, tissue heme content, and protein carbonylation for 8 days after injection of autologous blood into the mouse striatum. Increased striatal HO-1 protein was observed within 24 h, peaked on day 5 at a level that was 10-fold greater than baseline, and returned to baseline by day 8; HO-2 expression was not altered. HO activity increased by only 1.6-fold at its peak on day 5, and had also returned to baseline by day 8. A significant increase in protein carbonylation was observed at 3-5 days, which also was markedly attenuated by 8 days, concomitant with a return of tissue heme to near-normal levels. These results suggest that the increase in HO activity in tissue surrounding an experimental ICH is considerably less than would be predicted based on an analysis of HO-1 expression per se. As HO-1 expression is temporally associated with increased tissue heme and increased protein carbonylation, it may be more useful as a marker of heme-mediated oxidative stress in ICH models, rather than as an index of HO activity.