Aß43 levels determine the onset of pathological amyloid deposition.

About 2% of Alzheimer's disease (AD) cases have early onset (FAD) and are caused by mutations in either Presenilins (PSEN1/2) or amyloid-ß precursor protein (APP). PSEN1/2 catalyze production of Aß peptides of different length from APP. Aß peptides are the major components of amyloid plaques, a pathological lesion that characterizes AD. Analysis of mechanisms by which PSEN1/2 and APP mutations affect Aß peptide compositions lead to the implication of the absolute or relative increase in Aß42 in amyloid-ß plaques formation. Here, to elucidate the formation of pathogenic Aß cocktails leading to amyloid pathology, we utilized FAD rat knock-in models carrying the Swedish APP (Apps allele) and the PSEN1 L435F (Psen1LF allele) mutations. To accommodate the differences in the pathogenicity of rodent and human Aß, these rat models are genetically engineered to express human Aß species as both the Swedish mutant allele and the WT rat allele (called Apph) have been humanized in the Aß-coding region. Analysis of the eight possible FAD mutant permutations indicates that the CNS levels of Aß43, rather than absolute or relative increases in Aß42, determine the onset of pathological amyloid deposition in FAD knock-in rats. Notably, Aß43 was found in amyloid plaques in late onset AD and mild cognitive impairment cases, suggesting that the mechanisms initiating amyloid pathology in FAD knock-in rat reflect disease mechanisms driving amyloid pathology in late onset AD. This study helps clarifying the molecular determinants initiating amyloid pathology and supports therapeutic interventions targeting Aß43 in AD.

TNF-α-mediated reduction in inhibitory neurotransmission precedes sporadic Alzheimer's disease pathology in young Trem2R47H rats.

Alzheimer's disease (AD) is a neurodegenerative dementia associated with deposition of amyloid plaques and neurofibrillary tangles, formed by amyloid ß (Aß) peptides and phosphor-tau, respectively, in the central nervous system. Approximately 2% of AD cases are due to familial AD (FAD); ∼98% of cases are sporadic AD (SAD). Animal models with FAD are commonly used to study SAD pathogenesis. Because mechanisms leading to FAD and SAD may be distinct, to study SAD pathogenesis, we generated Trem2R47H knock-in rats, which carry the SAD risk factor p.R47H variant of the microglia gene triggering receptor expressed on myeloid cells 2 (TREM2). Trem2R47H rats produce human-Aß from a humanized-App rat allele because human-Aß is more toxic than rodent-Aß and the pathogenic role of the p.R47H TREM2 variant has been linked to human-Aß-clearing deficits. Using periadolescent Trem2R47H rats, we previously demonstrated that supraphysiological tumor necrosis factor-α (TNF-α) boosts glutamatergic transmission, which is excitatory, and suppresses long-term potentiation, a surrogate of learning and memory. Here, we tested the effect of the p.R47H variant on the inhibitory neurotransmitter γ-aminobutyric acid. We report that GABAergic transmission is decreased in Trem2R47H/R47H rats. This decrease is due to acute and reversible action of TNF-α and is not associated with increased human-Aß levels and AD pathology. Thus, the p.R47H variant changes the excitatory/inhibitory balance, favoring excitation. This imbalance could potentiate glutamate excitotoxicity and contribute to neuronal dysfunction, enhanced neuronal death, and neurodegeneration. Future studies will determine whether this imbalance represents an early, Aß-independent pathway leading to dementia and may reveal the AD-modifying therapeutic potential of TNF-α inhibition in the central nervous system.

Early cortical thickness changes predict ß-amyloid deposition in a mouse model of Alzheimer's disease.

Magnetic resonance imaging (MRI) studies have identified aberrant cortical structure in Alzheimer's disease (AD). The association between MRI-derived cortical morphometry measures and ß-amyloid, however, remains poorly understood. In this study, we explored the potential relationship between early alterations in cortical thickness and later stage ß-amyloid deposition, using a novel approach, in a transgenic AD mouse model. We acquired longitudinal anatomical MRI scans from mutant amyloid precursor protein (APP) transgenic mice and age-matched wild-type mice at 1 and 3.5months-of-age, and employed fully-automated image processing methods to derive objective, quantitative measures of cortical thickness on a region-of-interest basis. We also generated 3D quantitative immunohistochemistry (qIHC) volumes of deposited ß-amyloid burden from 18month-old transgenic mice using an automated, production-level process. These studies revealed thinner cortex in most regions in the 1month-old transgenic mice relative to age-matched wild-types, with the exception of the frontal, perirhinal/entorhinal, posterior cingulate, and retrosplenial cortical regions. Between 1 and 3.5months-of-age, the transgenic mice demonstrated stable or increasing cortical thickness, while the wild-type mice showed cortical thinning. Based on data from co-registered 3D MRI and qIHC volumes, we identified an association between abnormal, early, regional cortical thickness change over 2.5months and later ß-amyloid deposition. These observations suggest that the spatio-temporal pattern of early (pre-plaque) alterations in cerebral cortical structure is indicative of regional predisposition to later ß-amyloid pathology in a transgenic AD mouse model.

Correlation of amyloid PET ligand florbetapir F 18 binding with Aß aggregation and neuritic plaque deposition in postmortem brain tissue.

BACKGROUND: Florbetapir F 18 (F-AV-45) is a positron emission tomography imaging ligand for the detection of amyloid aggregation associated with Alzheimer disease. Earlier data showed that florbetapir F 18 binds with high affinity to ß-amyloid (Aß) plaques in human brain homogenates (Kd=3.7 nM) and has favorable imaging pharmacokinetic properties, including rapid brain penetration and washout. This study used human autopsy brain tissue to evaluate the correlation between in vitro florbetapir F 18 binding and Aß density measured by established neuropathologic methods. METHODS: The localization and density of florbetapir F 18 binding in frozen and formalin-fixed paraffin-embedded sections of postmortem brain tissue from 40 patients with a varying degree of neurodegenerative pathology was assessed by standard florbetapir F 18 autoradiography and correlated with the localization and density of Aß identified by silver staining, thioflavin S staining, and immunohistochemistry. RESULTS: There were strong quantitative correlations between florbetapir F 18 tissue binding and both Aß plaques identified by light microscopy (Silver staining and thioflavin S fluorescence) and by immunohistochemical measurements of Aß using 3 antibodies recognizing different epitopes of the Aß peptide. Florbetapir F 18 did not bind to neurofibrillary tangles. CONCLUSIONS: Florbetapir F 18 selectively binds Aß in human brain tissue. The binding intensity was quantitatively correlated with the density of Aß plaques identified by standard neuropathologic techniques and correlated with the density of Aß measured by immunohistochemistry. As Aß plaques are a defining neuropathologic feature for Alzheimer disease, these results support the use of florbetapir F 18 as an amyloid positron emission tomography ligand to identify the presence of Alzheimer disease pathology in patients with signs and symptoms of progressive late-life cognitive impairment.

Late-long-term potentiation magnitude, but not Aß levels and amyloid pathology, is associated with behavioral performance in a rat knock-in model of Alzheimer disease.

Cleavage of Amyloid precursor protein by ß- and γ-secretases lead to Aß formation. The widely accepted pathogenic model states that these mutations cause AD via an increase in Aß formation and accumulation of Aß in Amyloid plaques. APP mutations cause early onset familial forms of Alzheimer's disease (FAD) in humans. We generated App-Swedish (Apps ) knock-in rats, which carry a pathogenic APP mutation in the endogenous rat App gene. This mutation increases ß-secretase processing of APP leading to both augmented Aß production and facilitation of glutamate release in Apps/s rats, via a ß-secretase and APP-dependent glutamate release mechanism. Here, we studied 11 to 14-month-old male and female Apps/s rats. To determine whether the Swedish App mutation leads to behavioral deficits, Apps/s knock-in rats were subjected to behavioral analysis using the IntelliCage platform, an automated behavioral testing system. This system allows behavioral assessment in socially housed animals reflecting a more natural, less stress-inducing environment and eliminates experimenter error and bias while increasing precision of measurements. Surprisingly, a spatial discrimination and flexibility task that can reveal deficits in higher order brain function showed that Apps/s females, but not Apps/s male rats, performed significantly worse than same sex controls. Moreover, female control rats performed significantly better than control and Apps/s male rats. The Swedish mutation causes a significant increase in Aß production in 14-month-old animals of both sexes. Yet, male and female Apps/s rats showed no evidence of AD-related amyloid pathology. Finally, Apps/s rats did not show signs of significant neuroinflammation. Given that the APP Swedish mutation causes alterations in glutamate release, we analyzed Long-term potentiation (LTP), a long-lasting form of synaptic plasticity that is a cellular basis for learning and memory. Strikingly, LTP was significantly increased in Apps/s control females compared to both Apps/s sexes and control males. In conclusion, this study shows that behavioral performances are sex and App-genotype dependent. In addition, they are associated with LTP values and not Aß or AD-related pathology. These data, and the failures of anti-Aß therapies in humans, suggest that alternative pathways, such as those leading to LTP dysfunction, should be targeted for disease-modifying AD therapy.

Use of florbetapir-PET for imaging beta-amyloid pathology.

CONTEXT: The ability to identify and quantify brain ß-amyloid could increase the accuracy of a clinical diagnosis of Alzheimer disease. OBJECTIVE: To determine if florbetapir F 18 positron emission tomographic (PET) imaging performed during life accurately predicts the presence of ß-amyloid in the brain at autopsy. DESIGN, SETTING, AND PARTICIPANTS: Prospective clinical evaluation conducted February 2009 through March 2010 of florbetapir-PET imaging performed on 35 patients from hospice, long-term care, and community health care facilities near the end of their lives (6 patients to establish the protocol and 29 to validate) compared with immunohistochemistry and silver stain measures of brain ß-amyloid after their death used as the reference standard. PET images were also obtained in 74 young individuals (18-50 years) presumed free of brain amyloid to better understand the frequency of a false-positive interpretation of a florbetapir-PET image. MAIN OUTCOME MEASURES: Correlation of florbetapir-PET image interpretation (based on the median of 3 nuclear medicine physicians' ratings) and semiautomated quantification of cortical retention with postmortem ß-amyloid burden, neuritic amyloid plaque density, and neuropathological diagnosis of Alzheimer disease in the first 35 participants autopsied (out of 152 individuals enrolled in the PET pathological correlation study). RESULTS: Florbetapir-PET imaging was performed a mean of 99 days (range, 1-377 days) before death for the 29 individuals in the primary analysis cohort. Fifteen of the 29 individuals (51.7%) met pathological criteria for Alzheimer disease. Both visual interpretation of the florbetapir-PET images and mean quantitative estimates of cortical uptake were correlated with presence and quantity of ß-amyloid pathology at autopsy as measured by immunohistochemistry (Bonferroni ρ, 0.78 [95% confidence interval, 0.58-0.89]; P <.001]) and silver stain neuritic plaque score (Bonferroni ρ, 0.71 [95% confidence interval, 0.47-0.86]; P <.001). Florbetapir-PET images and postmortem results rated as positive or negative for ß-amyloid agreed in 96% of the 29 individuals in the primary analysis cohort. The florbetapir-PET image was rated as amyloid negative in the 74 younger individuals in the nonautopsy cohort. CONCLUSIONS: Florbetapir-PET imaging was correlated with the presence and density of ß-amyloid. These data provide evidence that a molecular imaging procedure can identify ß-amyloid pathology in the brains of individuals during life. Additional studies are required to understand the appropriate use of florbetapir-PET imaging in the clinical diagnosis of Alzheimer disease and for the prediction of progression to dementia.

Statin therapy inhibits remyelination in the central nervous system.

Remyelination of lesions in the central nervous system contributes to neural repair following clinical relapses in multiple sclerosis. Remyelination is initiated by recruitment and differentiation of oligodendrocyte progenitor cells (OPCs) into myelinating oligodendrocytes. Simvastatin, a blood-brain barrier-permeable statin in multiple sclerosis clinical trials, has been shown to impact the in vitro processes that have been implicated in remyelination. Animals were fed a cuprizone-supplemented diet for 6 weeks to induce localized demyelination in the corpus callosum; subsequent return to normal diet for 3 weeks stimulated remyelination. Simvastatin was injected intraperitoneally during the period of coincident demyelination and OPC maturation (weeks 4 to 6), throughout the entire period of OPC responses (weeks 4 to 9), or during the remyelination-only phase (weeks 7 to 9). Simvastatin treatment (weeks 4 to 6) caused a decrease in myelin load and both Olig2(strong) and Nkx2.2(strong) OPC numbers. Simvastatin treatment (weeks 4 to 9 and 7 to 9) caused a decrease in myelin load, which was correlated with a reduction in Nkx2.2(strong) OPCs and an increase in Olig2(strong) cells, suggesting that OPCs were maintained in an immature state (Olig2(strong)/Nkx2.2(weak)). NogoA+ oligodendrocyte numbers were decreased during all simvastatin treatment regimens. Our findings suggest that simvastatin inhibits central nervous system remyelination by blocking progenitor differentiation, indicating the need to monitor effects of systemic immunotherapies that can access the central nervous system on brain tissue-repair processes.

Improvement of antitumor activity by gene amplification with a replicating but nondisseminating adenovirus.

Gene therapy is a promising approach for cancer treatment; however, efficacy of current vectors remains insufficient. To improve the success of suicide gene therapy, we constructed a replication-competent adenoviral vector that has its protease gene deleted and expresses bacterial cytosine deaminase fused with bacterial uracil phosphoribosyltransferase (CU). The prodrug, 5-fluorocytosine, is transformed into the highly toxic and tissue-diffusible 5-fluorouracil by CU in infected cells. This vector is incapable of producing infectious particles but is able to undergo a single round of replication, thereby increasing transgene copy number and expression. In the presence of 5-FC, compared with the first-generation vector (AdCU), the replication-competent vector, Ad(dPS)CU-IRES-E1A, was significantly more efficacious for in vitro tumor cell killing and in bystander assays, whereas 25-fold fewer viral particles were required in a three-dimensional spheroid model. For in vivo experiments, in which virus was injected into preestablished intracranial glioma xenografts, followed by 5-FC treatment, mice receiving Ad(dPS)CU-IRES-E1A had significantly smaller tumors at 35 days postinjection as well as significantly longer median survival than mice treated with the replication-deficient, protease-deleted vector [Ad(dPS)CU]. In an immunocompetent syngeneic model, Ad(dPS)CU + 5-FC-treated mice had a median survival of only 23 days, whereas Ad(dPS)CU-IRES-E1A + 5-FC-treated animals had a survival of 57.1% at 365 days. In conclusion, Ad(dPS)CU-IRES-E1A in the presence of 5-FC produces more potent tumoricidal effects than its replication-deficient counterparts.

Synergistic tissue counterstaining and image segmentation techniques for accurate, quantitative immunohistochemistry.

Quantitative analysis of digitized IHC-stained tissue sections is increasingly used in research studies and clinical practice. Accurate quantification of IHC staining, however, is often complicated by conventional tissue counterstains caused by the color convolution of the IHC chromogen and the counterstain. To overcome this issue, we implemented a new counterstain, Acid Blue 129, which provides homogeneous tissue background staining. Furthermore, we combined this counterstaining technique with a simple, robust, fully automated image segmentation algorithm, which takes advantage of the high degree of color separation between the 3-amino-9-ethyl-carbazole (AEC) chromogen and the Acid Blue 129 counterstain. Rigorous validation of the automated technique against manual segmentation data, using Ki-67 IHC sections from rat C6 glioma and beta-amyloid IHC sections from transgenic mice with amyloid precursor protein (APP) mutations, has shown the automated method to produce highly accurate results compared with ground truth estimates based on the manually segmented images. The synergistic combination of the novel tissue counterstaining and image segmentation techniques described in this study will allow for accurate, reproducible, and efficient quantitative IHC studies for a wide range of antibodies and tissues.

Constitutive expression of a costimulatory ligand on antigen-presenting cells in the nervous system drives demyelinating disease.

It has been proposed that the activation status of antigen-presenting cells (APCs) plays a significant role in the development of autoimmune disease. Whether expression of costimulatory ligands on tissue-resident APCs controls organ-specific autoimmune responses has not been tested. We here report that transgenic mice constitutively expressing the costimulatory ligand B7.2/CD86 on microglia in the central nervous system (CNS) and on related cells in the proximal peripheral nervous tissue spontaneously develop autoimmune demyelinating disease. Disease-affected nervous tissue in transgenic mice showed infiltration characterized by a predominance of CD8+ memory-effector T cells, as well as CD4+ T cells. Transgenic animals lacking alphabeta TCR+ T cells were completely resistant to disease development. Transgenic T cells induced disease when adoptively transferred into T cell-deficient B7.2 transgenic recipients but not into non-transgenic recipients. These data provide evidence that B7/CD28 interactions within the nervous tissue are critical determinants of disease development. Our findings have important implications for understanding the etiology of nervous system autoimmune diseases such as multiple sclerosis (MS) and Guillain-Barré syndrome (GBS).

Bone marrow-derived versus parenchymal sources of inducible nitric oxide synthase in experimental autoimmune encephalomyelitis.

The role of nitric oxide (NO) in central nervous system (CNS) inflammation is uncertain. Whereas experimental autoimmune encephalomyelitis (EAE) is exacerbated in mice deficient in inducible nitric oxide synthase (iNOS), inhibitor studies have suggested a pro-inflammatory role for NO. These discrepancies may reflect balance between immunoregulatory and neurocytopathologic roles for NO. We investigated selective effects of bone marrow-derived versus CNS parenchymal sources of iNOS in EAE in chimeric mice. Chimeras that selectively expressed or ablated iNOS in leukocytes both showed significant delay in disease onset, with no difference in disease severity. We conclude that bone marrow-derived and CNS parenchymal sources of iNOS-derived NO both play a regulatory role in EAE.

Neuropathologic heterogeneity does not impair florbetapir-positron emission tomography postmortem correlates.

Neuropathologic heterogeneity is often present among Alzheimer disease (AD) patients. We sought to determine whether amyloid imaging measures of AD are affected by concurrent pathologies. Thirty-eight clinically and pathologically defined AD and 17 nondemented patients with quantitative florbetapir F-18 (F-AV-45) positron emission tomography (PET) imaging during life and postmortem histological ß-amyloid quantification and neuropathologic examination were assessed. AD patients were divided on the basis of concurrent pathologies, including those with Lewy bodies (LBs) (n = 21), white matter rarefaction (n = 27), severe cerebral amyloid angiopathy (n = 11), argyrophilic grains (n = 5), and TAR DNA binding protein-43 inclusions (n = 18). Many patients exhibited more than 1 type of concurrent pathology. The ratio of cortical to cerebellar amyloid imaging signal (SUVr) and immunohistochemical ß-amyloid load were analyzed in 6 cortical regions of interest. All AD subgroups had strong and significant correlations between SUVr and histological ß-amyloid measures (p µ 0.001). All AD subgroups had significantly greater amyloid measures versus nondemented patients, and mean amyloid measures did not significantly differ between AD subgroups. When comparing AD cases with and without each pathology, AD cases with LBs had significantly lower SUVr measures versus AD cases without LBs (p = 0.002); there were no other paired comparison differences. These findings indicate that florbetapir-PET imaging is not confounded by neuropathological heterogeneity within AD.

Microglial recruitment, activation, and proliferation in response to primary demyelination.

We have characterized the cellular response to demyelination/remyelination in the central nervous system using the toxin cuprizone, which causes reproducible demyelination in the corpus callosum. Microglia were distinguished from macrophages by relative CD45 expression (CD45(dim)) using flow cytometry. Their expansion occurred rapidly and substantially outnumbered infiltrating macrophages and T cells throughout the course of cuprizone treatment. We used bromodeoxyuridine incorporation and bone marrow chimeras to show that both proliferation and immigration from blood accounted for increased microglial numbers. Microglia adopted an activated phenotype during demyelination, up-regulating major histocompatibility class I and B7.2/CD86. A subpopulation of CD45(dim-high) microglia that expressed reduced levels of CD11b emerged during demyelination. These microglia expressed CD11c and were potent antigen-presenting cells in vitro. T cells were recruited to the demyelinated corpus callosum but did not appear to be activated. Our study highlights the role of microglia as a heterogeneous population of cells in primary demyelination, with the capacity to present antigen, proliferate, and migrate into demyelinated areas.

X-linked inhibitor of apoptosis regulates T cell effector function.

To understand how the balance between pro- and anti-apoptotic signals influences effector function in the immune system, we studied the X-linked inhibitor of apoptosis (XIAP), an endogenous regulator of cellular apoptosis. Real-time PCR showed increased XIAP expression in blood of mice with experimental autoimmune encephalomyelitis, correlating with disease severity. Daily administration (10 mg/kg/day i.p.) of a 19-mer antisense oligonucleotide specific for XIAP (ASO-XIAP) abolished disease-associated XIAP mRNA and protein expression, and given from day of onset, alleviated experimental autoimmune encephalomyelitis and prevented relapses. Prophylactic treatment also reduced XIAP expression and prevented disease. Random or 5-base mismatched ASO was not inhibitory, and ASO-XIAP did not affect T cell priming. In ASO-XIAP-treated animals, infiltrating cells and inflammatory foci were dramatically reduced within the CNS. Flow cytometry showed an 88-93% reduction in T cells. The proportion of TUNEL(+) apoptotic CD4(+) T cells in the CNS was increased from <1.6 to 26% in ASO-XIAP-treated mice, and the proportion of Annexin V-positive CD4(+) T cells in the CNS increased. Neurons and oligodendrocytes were not affected; neither did apoptosis increase in liver, where XIAP knockdown also occurred. ASO-XIAP increased susceptibility of T cells to activation-induced apoptosis in vitro. Our results identify XIAP as a critical controller of apoptotic susceptibility of effector T cell function.

A pathogenic role for CD8+ T cells in a spontaneous model of demyelinating disease.

Transgenic (Tg) mice that overexpress the costimulatory ligand B7.2/CD86 on microglia spontaneously develop a T cell-mediated demyelinating disease. Characterization of the inflammatory infiltrates in the nervous tissue revealed a predominance of CD8+ T cells, suggesting a prominent role of this T cell subset in the pathology. In this study, we show that the same neurological disease occurred in Tg mice deficient in the generation of CD4+ T cells, with an earlier time of onset. Analysis of the CD8+ T cell repertoire at early stage of disease revealed the presence of selected clonal expansions in the CNS but not in peripheral lymphoid organs. We further show that Tg animals deficient in IFN-gamma receptor expression were completely resistant to disease development. Microglia activation that is an early event in disease development is IFN-gamma dependent and thus appears as a key element in disease pathogenesis. Collectively, our data indicate that the spontaneous demyelinating disease in this animal model occurs as a consequence of an inflammatory response initiated through the activation of CNS-specific CD8+ T cells by Tg expression of B7.2 within the target organ. Thus, autoreactive CD8+ T cells can contribute directly to the pathogenesis of neuroinflammatory diseases such as multiple sclerosis.

Elevated interferon gamma expression in the central nervous system of tumour necrosis factor receptor 1-deficient mice with experimental autoimmune encephalomyelitis.

Inflammation in the central nervous system (CNS) can be studied in experimental autoimmune encephalomyelitis (EAE). The proinflammatory cytokines interferon-gamma (IFN-gamma) and tumour necrosis factor (TNF) are implicated in EAE pathogenesis. Signals through the type 1 TNF receptor (TNFR1) are required for severe EAE to develop, whereas deficiency in IFN-gamma or its receptor result in more severe EAE. We investigated IFN-gamma expression in TNFR1-deficient (TNFR1-/-) mice. We describe here that there were more IFN-gamma-secreting T cells present in the CNS of TNFR1-/- mice during EAE compared to wild-type (WT) mice, despite that clinical symptoms were mild, with delayed onset. There was greater expression of IL-12/23p40 by antigen-presenting cells in these mice, and in vitro, TNFR1-/- antigen-presenting cells induced greater secretion of IFN-gamma but not interleukin (IL)-17 when cultured with primed T cells than did WT antigen presenting cells. TNFR1-/- mice with EAE had significantly higher expression of CXCL10 mRNA (but not CCL5 mRNA) in the CNS compared to WT mice with EAE. These data demonstrate that IFN-gamma expression is enhanced in the CNS of TNFR1-/- mice with EAE and suggest that IFN-gamma levels do not necessarily correlate with EAE severity.

Neutrophils that infiltrate the central nervous system regulate T cell responses.

Regulation of inflammatory responses is critical to progression of organ-specific autoimmune disease. Although many candidate cell types have been identified, immunoregulatory activity has rarely been directly assayed and never from the CNS. We have analyzed the regulatory capability of Gr-1high neutrophils isolated from the CNS of mice with experimental autoimmune encephalomyelitis. Proportions of neutrophils were markedly increased in the CNS of IFN-gamma-deficient mice. Strikingly, CNS-derived neutrophils, whether or not they derived from IFN-gamma-deficient mice, were potent suppressors of T cell responses to myelin or adjuvant Ags. Neutrophil suppressor activity was absolutely dependent on IFN-gamma production by target T cells, and suppression was abrogated by blocking NO synthase. These data identify an immunoregulatory capacity for neutrophils, and indicate that interplay between IFN-gamma, NO, and activated Gr-1high neutrophils within the target organ determines the outcome of inflammatory and potentially autoimmune T cell responses.