Role of CD40 ligand in amyloidosis in transgenic Alzheimer's mice

We have shown that interaction of CD40 with CD40L enables microglial activation in response to amyloid-beta peptide (Abeta), which is associated with Alzheimer's disease (AD)-like neuronal tau hyperphosphorylation in vivo. Here we report that transgenic mice overproducing Abeta, but deficient in CD40L, showed decreased astrocytosis and microgliosis associated with diminished Abeta levels and beta-amyloid plaque load. Furthermore, in the PSAPP transgenic mouse model of AD, a depleting antibody against CD40L caused marked attenuation of Abeta/beta-amyloid pathology, which was associated with decreased amyloidogenic processing of amyloid precursor protein (APP) and increased circulating levels of Abeta. Conversely, in neuroblastoma cells overexpressing wild-type human APP, the CD40-CD40L interaction resulted in amyloidogenic APP processing. These findings suggest several possible mechanisms underlying mitigation of AD pathology in response to CD40L depletion, and validate the CD40-CD40L interaction as a target for therapeutic intervention in AD.     

CD40 signaling regulates innate and adaptive activation of microglia in response to amyloid beta-peptide

Although deposition of amyloid beta-peptide (Abeta) as Abeta plaques involves activation of microglia-mediated inflammatory responses, activated microglia ultimately fail to clear Abeta plaques in the brains of either Alzheimer's disease (AD) patients or AD mouse models. Mounting evidence suggests that chronic microglia-mediated immune response during Abeta deposition etiologically contributes to AD pathogenesis by promoting Abeta plaque formation. However, the mechanisms that govern microglia response in the context of cerebral Abeta/beta-amyloid pathology are not well understood. We show that ligation of CD40 by CD40L modulates Abeta-induced innate immune responses in microglia, including decreased microglia phagocytosis of exogenous Abeta(1-42) and increased production of pro-inflammatory cytokines. CD40 ligation in the presence of Abeta(1-42) leads to adaptive activation of microglia, as evidenced by increased co-localization of MHC class II with Abeta. To assess their antigen-presenting cell (APC) function, cultured microglia were pulsed with Abeta(1-42) in the presence of CD40L and co-cultured with CD4(+) T cells. Under these conditions, microglia stimulate T cell-derived IFN-gamma and IL-2 production, suggesting that CD40 signaling promotes the APC phenotype. These data provide a mechanistic explanation for our previous work showing decreased microgliosis associated with diminished cerebral Abeta/beta-amyloid pathology when blocking CD40 signaling in transgenic Alzheimer's mice.     

Microglial overexpression of the M-CSF receptor augments phagocytosis of opsonized Abeta

The role of microglia in Alzheimer's disease (AD) has come under intense scrutiny recently because microglia may clear amyloid beta (Abeta) by phagocytosis after immunization of transgenic mice. Increased expression of the macrophage colony-stimulating factor receptor (M-CSFR) is an important feature of microglia in AD and transgenic mouse models for AD. Increased expression of M-CSFR on mouse and human microglia accelerates phagocytosis of aggregated Abeta in part through macrophage scavenger receptors. We now show that Abeta phagocytosis by microglia overexpressing M-CSFR is further enhanced by antibody opsonization of Abeta. M-CSFR overexpression increased microglial phagocytosis of opsonized aggregated Abeta in culture medium, and accelerated ingestion of native Abeta from AD brain sections. M-CSFR overexpression also increased microglial expression of Fcgamma receptors, and blocking Fcgamma receptors attenuated the enhanced Abeta uptake observed after M-CSFR overexpression and antibody opsonization. Microglia in AD and in AD mouse models with increased expression of M-CSFR are likely to rapidly ingest opsonized Abeta after immunization, making high intracerebral antibody titers unnecessary.     

CD40 deficiency mitigates Alzheimer's disease pathology in transgenic mouse models

We have previously shown that transgenic mice carrying a mutant human APP but deficient in CD40L, display a decrease in astrocytosis and microgliosis associated with a lower amount of deposited Aβ. Furthermore, an anti-CD40L treatment causes a diminution of Aβ pathology in the brain and an improved performance in several cognitive tasks in the double transgenic PSAPP mouse model. Although these data suggest a potential role for CD40L in Alzheimer's disease pathology in transgenic mice they do not cast light on whether this effect is due to inhibition of signaling via CD40 or whether it is due to the mitigation of some other unknown role of CD40L. In the present report we have generated APP and PSAPP mouse models with a disrupted CD40 gene and compared the pathological features (such as amyloid burden, astrocytosis and microgliosis that are typical of Alzheimer's disease-like pathology in these transgenic mouse strains) with appropriate controls. We find that all these features are reduced in mouse models deficient for CD40 compared with their littermates where CD40 is present. These data suggest that CD40 signaling is required to allow the full repertoire of AD-like pathology in these mice and that inhibition of the CD40 signaling pathway is a potential therapeutic strategy in Alzheimer's disease.     

Microglia lacking E Prostanoid Receptor subtype 2 have enhanced Abeta phagocytosis yet lack Abeta-activated neurotoxicity

Experimental therapies for Alzheimer's disease (AD) are focused on enhanced clearance of neurotoxic Abeta peptides from brain. Microglia can be neuroprotective by phagocytosing Abeta; however, this comes at the cost of activated innate immunity that causes paracrine damage to neurons. Here, we show that ablation of E prostanoid receptor subtype 2 (EP2) significantly increased microglial-mediated clearance of Abeta peptides from AD brain sections and enhanced microglial Abeta phagocytosis in cell culture. The enhanced phagocytosis was PKC-dependent and was associated with elevated microglial secretion of the chemoattractant chemokines, macrophage inflammatory protein-1alpha and macrophage chemoattractant protein-1. This suggested that microglial activation is negatively regulated by EP2 signaling through suppression of prophagocytic cytokine secretion. However, despite this enhancement of Abeta phagocytosis, lack of EP2 completely suppressed Abeta-activated microglia-mediated paracrine neurotoxicity. These data demonstrate that blockade of microglial EP2 is a highly desirable mechanism for AD therapy that can maximize neuroprotective actions while minimizing bystander damage to neurons.     

Inflammatory responses to amyloidosis in a transgenic mouse model of Alzheimer's disease

Mutations in the amyloid precursor protein (APP) and presenilin-1 and -2 genes (PS-1, -2) cause Alzheimer's disease (AD). Mice carrying both mutant genes (PS/APP) develop AD-like deposits composed of beta-amyloid (Abeta) at an early age. In this study, we have examined how Abeta deposition is associated with immune responses. Both fibrillar and nonfibrillar Abeta (diffuse) deposits were visible in the frontal cortex by 3 months, and the amyloid load increased dramatically with age. The number of fibrillar Abeta deposits increased up to the oldest age studied (2.5 years old), whereas there were less marked changes in the number of diffuse deposits in mice over 1 year old. Activated microglia and astrocytes increased synchronously with amyloid burden and were, in general, closely associated with deposits. Cyclooxygenase-2, an inflammatory response molecule involved in the prostaglandin pathway, was up-regulated in astrocytes associated with some fibrillar deposits. Complement component 1q, an immune response component, strongly colocalized with fibrillar Abeta, but was also up-regulated in some plaque-associated microglia. These results show: i) an increasing proportion of amyloid is composed of fibrillar Abeta in the aging PS/APP mouse brain; ii) microglia and astrocytes are activated by both fibrillar and diffuse Abeta; and iii) cyclooxygenase-2 and complement component 1q levels increase in response to the formation of fibrillar Abeta in PS/APP mice.     

Intracellular Aβ-oligomers and early inflammation in a model of Alzheimer's disease

Lifelong use of nonsteroidal anti-inflammatory drugs (NSAIDs) has been shown to diminish the incidence of Alzheimer's disease (AD), suggesting a key role of inflammation in early stages of the pathology. While amyloid plaque-associated inflammation has been extensively studied in human and animal models, little is known about the inflammatory process prior to plaque deposition, i.e., in preclinical stages of AD. In this study we investigated microglial and neuronal inflammatory markers in preplaque transgenic McGill-Thy1-APP mice. We found evidence that prior to plaque deposition classical markers of microglial activation such as major histocompatibility complex class II (MHC-II), inducible nitric oxide synthase (i-NOS), and CD40 are already upregulated in the hippocampus of transgenic mice. Microglial cells from transgenic mice in the preplaque stage displayed intermediately activated morphology and appeared to be recruited toward intracellular amyloid-β peptide (Aβ)-oligomer burdened neurons. The inducible, neuron-specific cyclooxygenase 2 (COX-2) enzyme was found to be upregulated and specifically expressed by neurons in close relationship with Aβ-bearing cells, at this early stage of the AD-like pathology. Our study suggests that neuroinflammation might be one of the earliest pathological responses to intracellular accumulation of Aβ-oligomers.     

Dense-core plaques in Tg2576 and PSAPP mouse models of Alzheimer's disease are centered on vessel walls

Occurrence of amyloid beta (Abeta) dense-core plaques in the brain is one of the chief hallmarks of Alzheimer's disease (AD). It is not yet clear what factors are responsible for the aggregation of Abeta in the formation of these plaques. Using Tg2576 and PSAPP mouse models that exhibit age-related development of amyloid plaques similar to that observed in AD, we showed that approximately 95% of dense plaques in Tg2576 and approximately 85% in PSAPP mice are centered on vessel walls or in the immediate perivascular regions. Stereoscopy and simulation studies focusing on smaller plaques suggested that vascular associations for both Tg2576 and PSAPP mice were dramatically higher than those encountered by chance alone. We further identified ultrastructural microvascular abnormalities occurring in association with dense plaques. Although occurrence of gross cerebral hemorrhage was infrequent, we identified considerable infiltration of the serum proteins immunoglobulin and albumin in association with dense plaques. Together with earlier evidence of vascular clearance of Abeta, our data suggest that perturbed vascular transport and/or perivascular enrichment of Abeta leads to the formation of vasocentric dense plaques in Tg2576 and PSAPP mouse models of AD.     

Reduction of amyloid angiopathy and Abeta plaque burden after enriched housing in TgCRND8 mice: involvement of multiple pathways

Diversity and intensity of intellectual and physical activities seem to have an inverse relationship with the extent of cognitive decline in Alzheimer's disease (AD). To study the interaction between an active lifestyle and AD pathology, female TgCRND8 mice carrying human APPswe+ind were transferred into enriched housing. Four months of continuous and diversified environmental stimulation resulted in a significant reduction of beta-amyloid (Abeta) plaques and in a lower extent of amyloid angiopathy. Neither human amyloid precursor protein (APP) mRNA/protein levels nor the level of carboxy-terminal fragments of APP nor soluble Abeta content differed between both groups, making alterations in APP expression or processing unlikely as a cause of reduced Abeta deposition. Moreover, DNA microarray analysis revealed simultaneous down-regulation of proinflammatory genes as well as up-regulation of molecules involved in anti-inflammatory processes, proteasomal degradation, and cholesterol binding, possibly explaining reduced Abeta burden by lower aggregation and enhanced clearance of Abeta. Additionally, immunoblotting against F4/80 antigen and morphometric analysis of microglia (Mac-3) revealed significantly elevated microgliosis in the enriched brains, which suggests increased amyloid phagocytosis. In summary, this study demonstrates that the environment interacts with AD pathology at dif-ferent levels.     

Identification of CD40 ligand in Alzheimer's disease and in animal models of Alzheimer's disease and brain injury.

Chronic neuroinflammatory processes including glial activation may play a role in the pathogenesis of Alzheimer's disease (AD). The immune and inflammatory mediator CD40 ligand (CD40L) can augment the activation of cultured microglia by amyloid beta-protein (Abeta) and promote neuron death. We investigated whether CD40L is increased in AD and in animal models of AD and neuroinflammation. In the frontal cortex of elderly, non-AD controls, CD40L immunoreactivity was found in the glial limiting membrane, astrocytes, and vascular profiles in gray and white matter. In AD, intense CD40L immunoreactivity occurred in hypertrophied astrocytes throughout the frontal cortex. The majority of CD40L-immunoreactive astrocytes in the gray matter occurred within, or at the periphery of, Abeta(1-42)-immunoreactive plaques. A semiquantitative analysis revealed a three-fold elevation in the number of CD40L-immunoreactive astrocytes in AD compared to controls. The cortex and hippocampus from 6 and 12 month-old amyloid precursor protein/presenilin 1 transgenic mice exhibited numerous neuritic plaques and CD40L-positive astrocytes, which were not detected in non-transgenic controls. In adult rats, little or no CD40L staining occurred in astrocytes of the intact brain, whereas intrastriatal excitotoxic or stab wound lesions produced a strong CD40L immunoreactivity that was more segregated than glial fibrillary acidic protein. These findings indicate that astrocytes are the predominant source of CD40L in brain, and are consistent with the proposed role of CD40L-mediated neurotoxic inflammation in AD.     

Microglial EP2 as a new target to increase amyloid beta phagocytosis and decrease amyloid beta-induced damage to neurons

Epidemiologic and animal model data support a role for the prostaglandin pathway in AD pathogenesis. However, unexpected toxicity from protracted use of some nonsteroidal anti-inflammatory drugs (NSAIDs) compels investigation of therapeutic targets in this pathway other than COX inhibitors. Previously, we have shown that mice lacking one specific receptor for PGE2, EP2 (EP2-/-), are protected from the indirect neurotoxic effects of cerebral innate immune response mediated by CD14-dependent activation. Here we review data showing that EP2-/- microglia have a highly desirable combination of features: ablated indirect neurotoxicity following exposure to Abeta(1-42) coupled with enhanced phagocytosis of Abeta peptides, both synthetic and those deposited in human brain. These data point to microglial EP2 as a more focused target within the PG pathway for therapy in AD.     

ApoAI deficiency results in marked reductions in plasma cholesterol but no alterations in amyloid-beta pathology in a mouse model of Alzheimer's disease-like cerebral amyloidosis

Epidemiological studies suggest links between cholesterol metabolism and Alzheimer's disease (AD), with hypercholesterolemia associated with increased AD risk, and use of cholesterol-lowering drugs associated with decreased risk. Animal models using cholesterol-modifying dietary or pharmacological interventions demonstrate similar findings. Proposed mechanisms include effects of cholesterol on the metabolism of amyloid-beta (Abeta), the protein that deposits in AD brain. To investigate the effect of genetic alterations in plasma cholesterol on Abeta pathology, we crossed the PDAPP transgenic mouse model of AD-like cerebral amyloidosis to apolipoprotein AI-null mice that have markedly reduced plasma cholesterol levels due to a virtual absence of high density lipoproteins, the primary lipoprotein in mice. Interestingly and in contrast to models using non-physiological high fat diets or cholesterol-lowering drugs to modify plasma cholesterol, we observed no differences in Abeta pathology in PDAPP mice of the various apoAI genotypes despite robust differences in plasma cholesterol levels between the groups. Absence of apoAI also resulted in reductions in brain but not cerebrospinal fluid cholesterol, but had no effect on brain apolipoprotein E levels. These and other data suggest that it is perhaps the level of brain apolipoprotein E, not cholesterol per se, that plays a primary role in brain Abeta metabolism.     

Elimination of the class A scavenger receptor does not affect amyloid plaque formation or neurodegeneration in transgenic mice expressing human amyloid protein precursors.

The class A scavenger receptor (SR) is expressed on reactive microglia surrounding cerebral amyloid plaques in Alzheimer's disease (AD). Interactions between the SR and amyloid beta peptides (Abeta) in microglial cultures elicit phagocytosis of Abeta aggregates and release of neurotoxins. To assess the role of the SR in amyloid clearance and Abeta-associated neurodegeneration in vivo, we used the platelet-derived growth factor promoter to express human amyloid protein precursors (hAPPs) in neurons of transgenic mice. With increasing age, hAPP mice develop AD-like amyloid plaques. We bred heterozygous hAPP (hAPP(+/-)) mice that were wild type for SR (SR(+/+)) with SR knockout (SR(-/-)) mice. Crosses among the resulting hAPP(+/-)SR(+/-) offspring yielded hAPP(+/-) and hAPP(-/-) littermates that were SR(+/+) or SR(-/-). These second-generation mice were analyzed at 6 and 12 months of age for extent of cerebral amyloid deposition and loss of synaptophysin-immunoreactive presynaptic terminals. hAPP(-/-)SR(-/-) mice showed no lack of SR expression, plaque formation, or synaptic degeneration, indicating that lack of SR expression does not result in significant accumulation of endogenous amyloidogenic or neurotoxic factors. In hAPP(+/-) mice, ablation of SR expression did not alter number, extent, distribution, or age-dependent accumulation of plaques; nor did it affect synaptic degeneration. Our results do not support a critical pathogenic role for microglial SR expression in neurodegenerative alterations associated with cerebral beta amyloidosis.     

Systemic immune challenges trigger and drive Alzheimer-like neuropathology in mice

ABSTRACT: BACKGROUND: Alzheimer's disease (AD) is the most prevalent form of age-related dementia, and its effect on society increases exponentially as the population ages. Accumulating evidence suggests that neuroinflammation, mediated by the brain's innate immune system, contributes to AD neuropathology and exacerbates the course of the disease. However, there is no experimental evidence for a causal link between systemic infection or neuroinflammation and the onset of the disease. METHODS: The viral mimic, polyriboinosinic-polyribocytidilic acid (PolyI:C) was used to stimulate the immune system of experimental animals. Wild-type (WT) and transgenic mice were exposed to this cytokine inducer prenatally (gestation day (GD)17) and/or in adulthood. Behavioral, immunological, immunohistochemical, and biochemical analyses of AD-associated neuropathologic changes were performed during aging. RESULTS: We found that a systemic immune challenge during late gestation predisposes WT mice to develop AD-like neuropathology during the course of aging. They display chronic elevation of inflammatory cytokines, an increase in the levels of hippocampal amyloid precursor protein (APP) and its proteolytic fragments, altered Tau phosphorylation, its mis-sorting to somatodendritic compartments, and significant impairments in working memory in old age. If this prenatal infection is followed by a second immune challenge in adulthood, the phenotype is strongly exacerbated, and mimics AD-like neuropathologic changes. These include deposition of APP and its proteolytic fragments, along with Tau aggregation, microglia activation and reactive gliosis. Whereas Abeta peptides were not significantly enriched in extracellular deposits of double immune-challenged WT mice at 15 months, they dramatically increased in age-matched immune-challenged transgenic AD mice, precisely around the inflammation-induced accumulations of APP and its proteolytic fragments, in striking similarity to the post-mortem findings in human patients with AD. CONCLUSION: Chronic inflammatory conditions induce age-associated development of an AD-like phenotype in WT mice, including the induction of APP accumulations, which represent a seed for deposition of aggregation-prone peptides. The PolyI:C mouse model therefore provides a unique tool to investigate the molecular mechanisms underlying the earliest pathophysiological changes preceding fibrillary Abeta plaque deposition and neurofibrillary tangle formations in a physiological context of aging. Based on the similarity between the changes in immune-challenged mice and the development of AD in humans, we suggest that systemic infections represent a major risk factor for the development of AD.     

Noradrenaline deficiency in brain increases beta-amyloid plaque burden in an animal model of Alzheimer's disease

Loss of Locus coeruleus (LC) noradrenergic (NA) neurons occurs in several neurodegenerative conditions including Alzheimer's disease (AD). In vitro and in vivo studies have shown that NA influences several features of AD disease including inflammation, neurodegeneration, and cognitive function. In the current study we tested if LC loss influenced beta amyloid (Abeta) plaque deposition. LC neuronal degeneration was induced in transgenic mice expressing mutant V717F human amyloid precursor protein (APP) by treatment with the selective neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine DSP4 (5mg/kg every 2 weeks beginning at age 3 months). At 9 months of age, when control mice show low amyloid load, DSP4-treated mice showed an approximately 5-fold increase in the average number of Abeta plaques. This was accompanied by an increase in the levels of APP C-terminal cleavage fragments. DSP4-treatment increased both microglial and astroglial activation. In vivo, DSP4-treatment decreased expression and activity of the Abeta degrading enzyme neprilysin, while in vitro NA increased phagocytosis of Abeta1-42 by microglia. These findings suggest that noradrenergic innervation from LC are needed to maintain adequate Abeta clearance, and therefore that LC degeneration could contribute to AD pathogenesis.     

Modeling microglial activation in Alzheimer’s disease with human postmortem microglial cultures

Alzheimer’s disease (AD) is a uniquely human disorder. Despite intense research, the lack of availability of model systems has hindered AD studies though in recent years transgenic mouse models have been produced, which develop AD-like amyloid beta peptide (Aβ) plaques. For the study of inflammatory changes in AD brains, these transgenic mice may have limitations due to differences in the innate immune system of humans and rodents. Many studies of inflammatory processes in AD have focused on the role of activated microglia. Over the last 8 years, our research has focused on the properties of human microglia cultured from brain tissues of AD and non-demented (ND) individuals. As these are the cells observed to be activated in AD tissues, they represent a useful system for modeling the inflammatory components of AD.

In this review, we summarize data by our group and others on the use of microglia for AD-related inflammatory research, with emphasis on results using human postmortem brain microglia. A range of products have been shown to be produced by human postmortem microglia, both constitutively and in response to treatment with Aβ, including proinflammatory cytokines such as interleukin (IL)-1β, IL-6, tumor necrosis factor (TNF) , and macrophage colony stimulating factor (M-CSF), along with complement proteins, especially C1q, superoxide radicals and neurotoxic factors. In our studies, we have demonstrated that there was a significant difference between AD and ND microglia in terms of their secretion of M-CSF and C1q. We also discuss the role of putative Aβ microglial receptors, particular recent data showing a role for the receptor for advanced glycation endproducts (RAGE) in mediating the responses of human microglia to Aβ. Finally, our studies on the use of an Aβ spot paradigm to model microglia interactions with plaques demonstrated that many of the features of AD inflammation can be modeled with postmortem brain derived microglia.     

Dok2 mediates the CD200Fc attenuation of Abeta-induced changes in glia

ABSTRACT: BACKGROUND: The interaction between the membrane glycoprotein, CD200 and its cognate receptor CD200 receptor (CD200R), has been shown to play a role in maintaining microglia in a quiescent state. There is evidence of increased activation under resting and stimulated conditions in microglia prepared from CD200-deficient mice compared with wild-type mice, whereas activation of the receptor by CD200 fusion protein (CD200Fc) ameliorates inflammatory changes which are evident in the central nervous system (CNS) of the mouse model of multiple sclerosis (MS), experimental autoimmune encephalomyelitis (EAE) and also in the hippocampus of aged rats. Additionally, an inverse relationship between microglial activation and expression of CD200 has been observed in animals treated with lipopolysaccharide (LPS) or amyloid-beta (Abeta). METHODS: We assessed the effect of CD200R activation by CD200Fc on Abeta-induced production of the pro-inflammatory cytokines, interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNFalpha) and the expression of microglial activation markers, CD68 and CD40 in cultured glia. The role played by downstream of tyrosine kinase 2 (Dok2) phosphorylation in mediating the effects of CD200R activation was evaluated by siRNA knockdown of Dok2. To further examine the impact of inflammatory changes on synaptic plasticity, the effect of CD200Fc on Abeta-induced impairment of long-term potentiation (LTP) in the CA1 region of hippocampal slices was also investigated. RESULTS: We demonstrate that Abeta-induced increases in IL-1beta, TNFalpha, CD68 and CD40 were inhibited by CD200Fc. The evidence suggests that Dok2 phosphorylation is a key factor in mediating the effect of CD200Fc, since Dok2 knockdown by siRNA abrogated its effects on microglial activation and inflammatory cytokine production. Consistent with evidence that inflammatory changes negatively impact on LTP, we show that the Abeta-induced impairment of LTP was attenuated by CD200Fc. CONCLUSIONS: The findings suggest that activation of CD200R and Dok2 is a valuable strategy for modulating microglial activation and may have therapeutic potential in neurodegenerative conditions.     

Neuropathology and pathogenesis of encephalitis following amyloid-beta immunization in Alzheimer's disease

Immunizing transgenic PDAPP mice, which overexpress mutant APP and develop beta-amyloid deposition resembling plaques in Alzheimer's disease (AD), results in a decrease of amyloid burden when compared with non-treated transgenic animals. Immunization with amyloid-beta peptide has been initiated in a randomised pilot study in AD. Yet a minority of patients developed a neurological complication consistent with meningoencephalitis and one patient died; the trial has been stopped. Neuropathological examination in that patient showed meningoencephalitis, and focal atypically low numbers of diffuse and neuritic plaques but not of vascular amyloid, nor regression of tau pathology in neurofibrillary tangles and neuropil threads. The present neuropathological study reports the second case of meningoencephalitis following immunization with amyloid-beta peptide in AD, and has been directed toward exploring mechanisms underlying decreased tau pathology in relation with amyloid deposit regression, and possible molecular bases involved in the inflammatory response following immunization. Inflammatory infiltrates were composed of CD8+, CD4+, CD3+, CD5+ and, rarely, CD7+ lymphocytes, whereas B lymphocytes and T cytotoxic cells CD16, CD57, TIA and graenzyme were negative. Characteristic neuropathological findings were focal depletion of diffuse and neuritic plaques, but not of amyloid angiopathy, and the presence of small numbers of extremely dense (collapsed) plaques surrounded by active microglia, and multinucleated giant cells filled with dense Abeta42 and Abeta40, in addition to severe small cerebral blood vessel disease and multiple cortical hemorrhages. Reduced amyloid burden was accompanied by low amyloid-associated oxidative stress responses (reduced superoxide dismutase-1: SOD-1 expression) and by local inhibition of the stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) and p38 kinase which are involved in tau phosphorylation. These results support the amyloid cascade of tau phosphorylation in AD regarding phosphorylation of tau dependent on beta-amyloid deposition in neuritic plaques, but not of tau in neurofibrillary tangles and threads. Furthermore, amyloid reduction was accompanied by increased expression of the PA28a/beta inductor, and of LMP7, LMP2 and MECL1 subunits of the immunoproteasome in microglial and inflammatory cells surrounding collapsed plaques, and in multinucleated giant cells. Immunoproteasome subunit expression was accompanied by local presentation of MHC class I molecules. Release of antigenic peptides derived from beta-amyloid processing may enhance T-cell inflammatory responses accounting for the meningoencephalitis following amyloid-beta peptide immunization.     

The relationship between the morphological subtypes of microglia and Alzheimer’s disease neuropathology

Microglial associations with both the major Alzheimer’s disease (AD) pathognomonic entities, β‐amyloid‐positive plaques and tau‐positive neurofibrillary tangles, have been noted in previous investigations of both human tissue and mouse models. However, the precise nature of their role in the pathogenesis of AD is debated; the major working hypothesis is that pro‐inflammatory activities of activated microglia contribute to disease progression. In contrast, others have proposed that microglial dystrophy with a loss of physiological and neuroprotective activities promotes neurodegeneration. This immunohistochemical study sought to gain clarity in this area by quantifying the morphological subtypes of microglia in the mildly‐affected primary visual cortex (PVC), the moderately affected superior frontal cortex (SFC) and the severely affected inferior temporal cortex (ITC) of 8 AD cases and 15 age and gender‐matched, non‐demented controls with ranging AD‐type pathology. AD cases had increased β‐amyloid and tau levels compared to controls in all regions. Neuronal loss was observed in the SFC and ITC, and was associated with atrophy in the latter. A major feature of the ITC in AD was a decrease in ramified (healthy) microglia with image analysis confirming reductions in arborized area and skeletal complexity. Activated microglia were not associated with AD but were increased in non‐demented controls with greater AD‐type pathology. Microglial clusters were occasionally associated with β‐amyloid‐ and tau‐positive plaques but represented less than 2% of the total microglial population. Dystrophic microglia were not associated with AD, but were inversely correlated with brain pH suggesting that agonal events were responsible for this morphological subtype. Overall these novel findings suggest that there is an early microglial reaction to AD‐type pathology but a loss of healthy microglia is the prominent feature in severely affected regions of the AD brain.