Abeta vaccination effects on plaque pathology in the absence of encephalitis in Alzheimer disease

Antibody responses, amyloid-beta peptide remnants and clinical effects of AN-1792 immunization in patients with AD in an interrupted trial

Post mortem examinations of AN-1792-vaccinated humans revealed this therapy produced focal senile plaque disruption. Despite the dispersal of substantial plaque material, vaccination did not constitute even a partial eradication of brain amyloid as water soluble amyloid-beta (Abeta) 40/42 increased in the gray matter compared to sporadic Alzheimer's disease (AD) patients and total brain Abeta levels were not decreased. Significant aspects of AD pathology were unaffected by vaccination with both vascular amyloid and hyper-phosphorylated tau deposits appeared refractory to this therapy. In addition, vaccination resulted in the consequential and drastic expansion of the white matter (WM) amyloid pool to levels without precedent in sporadic AD patients. Although vaccination disrupted amyloid plaques, this therapy did not enhance long-term cognitive function or necessarily halt neurodegeneration. The intricate involvement of vascular pathology in AD evolution and the firm recalcitrance of vessel-associated amyloid to antibody-mediated disruption suggest that immunization therapies might be more effective if administered on a prophylactic basis before vascular impairment and well ahead of any clinically evident cognitive decline. Amyloid-beta is viewed as pathological based on the postmortem correlation of senile plaques with an AD diagnosis. It remains uncertain which of the various forms of this peptide is the most toxic and whether Abeta or senile plaques themselves serve any desirable or protective functions. The long-term cognitive effects of chronic immunotherapy producing a steadily accumulating and effectively permanent pool of disrupted Abeta peptides within the human brain are unknown. In addition, the side effects of such therapy provided on a chronic basis could extend far beyond the brain. Eagerly seeking new therapies, critical knowledge gaps should prompt us to take a more wholistic perspective viewing Abeta and the amyloid cascade as aspects of complex and many-faceted physiological processes that sometimes end in AD dementia.

Microvasculature changes and cerebral amyloid angiopathy in Alzheimer's disease and their potential impact on therapy

The introduction of immunotherapy and its ultimate success will require re-evaluation of the pathogenesis of Alzheimer's disease particularly with regard to the role of the ageing microvasculature and the effects of APOE genotype. Arteries in the brain have two major functions (a) delivery of blood and (b) elimination of interstitial fluid and solutes, including amyloid-beta (Abeta), along perivascular pathways (lymphatic drainage). Both these functions fail with age and particularly severely in Alzheimer's disease and vascular dementia. Accumulation of Abeta as plaques in brain parenchyma and artery walls as cerebral amyloid angiopathy (CAA) is associated with failure of perivascular elimination of Abeta from the brain in the elderly and in Alzheimer's disease. High levels of soluble Abeta in the brain correlate with cognitive decline in Alzheimer's disease and reflect the failure of perivascular drainage of solutes from the brain and loss of homeostasis of the neuronal environment. Clinically and pathologically, there is a spectrum of disease related to functional failure of the ageing microvasculature with "pure" Alzheimer's disease at one end of the spectrum and vascular dementia at the other end. Changes in the cerebral microvasculature with age have a potential impact on therapy with cholinesterase inhibitors and especially on immunotherapy that removes Abeta from plaques in the brain, but results in an increase in severity of CAA and no clear improvement in cognition. Drainage of Abeta along perivascular pathways in ageing artery walls may need to be improved to maximise the potential for improvement of cognitive function with immunotherapy.

Imaging microglial activation during neuroinflammation and Alzheimer's disease

Microglial activation is an important pathogenic component of neurodegenerative disease processes. This state of increased inflammation is associated not only with neurotoxic consequences but also neuroprotective effects, e.g., phagocytosis and clearance of amyloid in Alzheimer's disease. In addition, activation of microglia appears to be one of the major mechanisms of amyloid clearance following active or passive immunotherapy. Imaging techniques may provide a minimally invasive tool to elucidate the complexities and dynamics of microglial function and dysfunction in aging and neurodegenerative diseases. Imaging microglia in vivo in live subjects by confocal or two/multiphoton microscopy offers the advantage of studying these cells over time in their native environment. Imaging microglia in human subjects by positron emission tomography scanning with translocator protein-18 kDa ligands can offer a measure of the inflammatory process and a means of detecting progression of disease and efficacy of therapeutics over time.

Alzheimer's disease and blood-brain barrier function-Why have anti-beta-amyloid therapies failed to prevent dementia progression?

Proteopathies of the brain are defined by abnormal, disease-inducing protein deposition that leads to functional abrogation and death of neurons. Immunization trials targeting the removal of amyloid-beta plaques in Alzheimer's disease have so far failed to stop the progression of dementia, despite autopsy findings of reduced plaque load. Here, we summarize current knowledge of the relationship between AD pathology and blood-brain barrier function, and propose that the activation of the excretion function of the blood-brain barrier might help to achieve better results in trials targeting the dissolution of cerebral amyloid-beta aggregates. We further discuss a possible role of oligomers in limiting the efficacy of immunotherapy.

Immunotherapy, vascular pathology, and microhemorrhages in transgenic mice

Alzheimer's disease (AD) is a progressive, neurodegenerative disorder that results in severe cognitive decline. Amyloid plaques are a principal pathology found in AD and are composed of aggregated amyloid-beta (Abeta) peptides. According to the amyloid hypothesis, Abeta peptides initiate the other pathologies characteristic for AD including cognitive deficits. Immunotherapy against Abeta is a potential therapeutic for the treatment of humans with AD. While anti-Abeta immunotherapy has been shown to reduce amyloid burden in both mouse models and in humans, immunotherapy also exacerbates vascular pathologies. Cerebral amyloid angiopathy (CAA), that is, the accumulation of amyloid in the cerebrovasculature, is increased with immunotherapy in humans with AD and in mouse models of amyloid deposition. CAA persists in the brains of clinical trial patients that show removal of parenchymal amyloid. Mouse model studies also show that immunotherapy results in multiple small bleeds in the brain, termed microhemorrhages. The neurovascular unit is a term used to describe the cerebrovasculature and its associated cells-astrocytes, neurons, pericytes and microglia. CAA affects brain perfusion and there is now evidence that the neurovascular unit is affected in AD when CAA is present. Understanding the type of damage to the neurovascular unit caused by CAA in AD and the underlying cause of microhemorrhage after immunotherapy is essential to the success of therapeutic vaccines as a treatment for AD.

Immunotherapy for Alzheimer's disease

Patients with Alzheimer's disease (AD) express severe cognitive deficiencies with a concurrent increase in brain deposits of aggregated amyloid-beta (Abeta), a catabolic derivative of the ubiquitous amyloid precursor protein (APP). Interference in the homeostasis of Abeta has been suggested as a treatment for AD patients. In AD murine models it has been shown that active and passive immunization against Abeta alters the equilibrium of the different forms of Abeta in brain and serum, leading to a concomitant cognitive improvement. Generally, the clinical trials that followed the study of the murine AD model confirmed the results of the AD models, although safety issues advocate the passive vaccination approach rather than the active one. However, passive vaccination of patients with monoclonal antibodies derived from nonhuman sources is limited. Anti-Abeta IgM and IgG antibodies, which are present in the serum of every healthy individual and probably play a role in the homeostasis of Abeta in healthy subjects, might be beneficial to AD patients, as shown for the effect exerted by the commercial preparation of intravenous immunoglobulin. Human monoclonal anti-Abeta antibodies, which correspond to the ubiquitous anti-Abeta antibodies, are plausible candidates for future immunotherapy of AD patients.

Long-term follow-up of patients immunized with AN1792: reduced functional decline in antibody responders

BACKGROUND

Immunization of patients with Alzheimer's disease (AD) with synthetic amyloid-beta peptide (Abeta(42)) (AN1792) was previously studied in a randomized, double-blind, placebo-controlled phase 2a clinical trial, Study AN1792(QS-21)-201. Treatment was discontinued following reports of encephalitis. One year follow-up revealed that AN1792 antibody responders showed improvements in cognitive measures as assessed by the neuropsychological test battery (NTB) and a decrease in brain volume compared with placebo.

METHODS

A follow-up study, Study AN1792(QS-21)-251, was conducted to assess the long-term functional, psychometric, neuroimaging, and safety outcomes of patients from the phase 2a study 4.6 years after immunization with AN1792. The results were analyzed by comparing patients originally identified as antibody responders in the AN1792 phase 2a study with placebo-treated patients.

RESULTS

One hundred and fifty-nine patients/caregivers (30 placebo; 129 AN1792) participated in this follow-up study. Of the 129 AN1792-treated patients, 25 were classified in the phase 2a study as antibody responders (anti-AN1792 titers > or = 1:2,200 at any time after the first injection). Low but detectable, sustained anti-AN1792 titers were found in 17 of 19 samples obtained from patients classified as antibody responders in the phase 2a study. No detectable anti-AN1792 antibodies were found in patients not classified as antibody responders in the phase 2a study. Significantly less decline was observed on the Disability Assessment for Dementia scale among antibody responders than placebo-treated patients (p=0.015) after 4.6 years. Significant differences in favor of responders were also observed on the Dependence Scale (p=0.033). Of the small number of patients who underwent a follow-up MRI, antibody responders showed similar brain volume loss during the follow-up period subsequent to the AN1792 phase 2a study compared with placebo-treated patients.

CONCLUSIONS

Approximately 4.6 years after immunization with AN1792, patients defined as responders in the phase 2a study maintained low but detectable, sustained anti-AN1792 antibody titers and demonstrated significantly reduced functional decline compared with placebo-treated patients. Brain volume loss in antibody responders was not significantly different from placebo-treated patients approximately 3.6 years from the end of the original study. No further cases of encephalitis were noted. These data support the hypothesis that Abeta immunotherapy may have long-term functional benefits.

Is passive immunization for Alzheimer's disease 'alive and well' or 'dead and buried'?

BACKGROUND

Passive immunization strategies are under investigation as potential disease-modifying therapies for Alzheimer's disease (AD). Current approaches, based on data demonstrating behavioral improvement and reduced pathology in transgenic animal models, have focused exclusively on immune targeting of beta-amyloid.

OBJECTIVE

To examine immunization strategies for AD.

METHODS

A review of relevant publications.

RESULTS/CONCLUSIONS

Preliminary results from three Phase II trials suggest both the promise and the need to exercise caution with this method of immunotherapy. The strategies used were distinct, using monoclonal N-terminal, central epitope, and polyclonal antibodies to maximize the efficacy and safety of each approach. The tested compounds are moving into Phase III trials for mild to moderate AD. We await the discoveries that from these studies that may yield the first disease-modifying therapy for AD.

Induction of toll-like receptor 9 signaling as a method for ameliorating Alzheimer's disease-related pathology

The pathogenesis of Alzheimer's disease (AD) is thought to be related to the accumulation of amyloid beta (Abeta) in amyloid deposits and toxic oligomeric species. Immunomodulation is emerging as an effective means of shifting the equilibrium from Abeta accumulation to clearance; however, excessive cell mediated inflammation and cerebral microhemorrhages are two forms of toxicity which can occur with this approach. Vaccination studies have so far mainly targeted the adaptive immune system. In the present study, we have stimulated the innate immune system via the Toll-like receptor 9 (TLR9) with cytosine-guanosine-containing DNA oligodeoxynucleotides in Tg2576 AD model transgenic mice. This treatment produced a 66% and 80% reduction in the cortical (p = 0.0001) and vascular (p = 0.0039) amyloid burden, respectively, compared with nontreated AD mice. This was in association with significant reductions in Abeta42, Abeta40, and Abeta oligomer levels. We also show that treated Tg mice performed similarly to wild-type mice on a radial arm maze. Our data suggest that stimulation of innate immunity via TLR9 is highly effective at reducing the parenchymal and vascular amyloid burden, along with Abeta oligomers, without apparent toxicity.

Immunization in Alzheimer's disease: naïve hope or realistic clinical potential?

There has been considerable recent interest in vaccination of patients by immunotherapy as a potentially clinically useful methodology for combating histopathological changes in Alzheimer's disease (AD). The focus of the majority of this research has been on (1) active immunotherapy using the pre-aggregated synthetic beta-amyloid (Abeta) 42 preparation AN1792 vaccine (QS-21), or (2) passive immunization using injections of already prepared polyclonal anti-Abeta antibodies (intravenous immunoglobulin). These two clinical approaches to the treatment of patients with AD represent the focus of this review. We conclude here that, with certain caveats, immunization offers further potential as a technique for the treatment (and possible prevention) of AD. New studies are seeking to develop and apply safer vaccines that do not result in toxicity and neuroinflammation. Nevertheless, caution is warranted, and future clinical investigations are required to tackle key outstanding issues. These include the need to demonstrate efficacy in humans as well as animal models (especially with respect to the potentially toxic side effects of immunotherapy), and fine-tuning in safely guiding the immune response. The issue of defining necessary and sufficient criteria for determining clinical efficacy remains an additional important issue for future immunization trials. The vaccination methodology appears to offer substantial current promise for clearing both soluble and aggregated amyloid in AD. However, it remains to be determined whether this approach will help to repair already damaged neural systems in the disease, and the extent to which vaccination-driven amyloid clearance will impact beneficially on patients' neurocognitive capacity and their functional status. The outcomes of future studies will be important both clinically and scientifically: an important further test of the validity of the amyloid hypothesis of AD is to evaluate the impact of an effective anti-amyloid strategy on the functional status of patients with this disease.

Development of AFFITOPE vaccines for Alzheimer's disease (AD)--from concept to clinical testing

Based on the notion that cerebral accumulation of certain Abeta species is central to AD pathogenesis and endowed with the knowledge that emerged during clinical testing of the first human Alzheimer vaccine, AN1792, we designed a new generation of Alzheimer vaccines. Rather than relying on full-length Abeta itself or fragments thereof, AFFITOPE vaccines use short peptides, mimicking parts of the native Abeta sequence, as their antigenic component. The technology created to identify these peptides, termed AFFITOPE-technology, at the same time provides the basis for the multi-component safety concept realized in AFFITOPE vaccines. First, as they are nonself, AFFITOPES don't need to break tolerance typically established against self proteins. This allows us to use aluminium hydroxide, the agent first approved as immunological adjuvant for human use and, thus, exhibiting an excellent safety profile. Second, AFFITOPES employed in Alzheimer vaccines are only 6 amino acids in length, which precludes the activation of Abeta-specific autoreactive T cells. Third, and above all, the AFFITOPE technology allows for controlling the specificity of the vaccine-induced antibody response focusing it exclusively on Abeta and preventing crossreactivity with APP. In a program based on two AFFITOPES allowing neoepitope targeting of Abeta (free N-terminus), this approach was taken all the way from concept to clinical application. Early clinical data support the safety concept inherent to AFFITOPE Alzheimer vaccines. Further clinical testing will focus on the identification of the optimal vaccine dose and immunization schedule. Together, result of these trials will provide a solid basis for clinical POC studies.

Intracerebroventricular amyloid-beta antibodies reduce cerebral amyloid angiopathy and associated micro-hemorrhages in aged Tg2576 mice

Although immunization against amyloid-beta (Abeta) holds promise as a disease-modifying therapy for Alzheimer disease (AD), it is associated with an undesirable accumulation of amyloid in the cerebrovasculature [i.e., cerebral amyloid angiopathy (CAA)] and a heightened risk of micro-hemorrhages. The central and peripheral mechanisms postulated to modulate amyloid with anti-Abeta immunotherapy remain largely elusive. Here, we compared the effects of prolonged intracerebroventricular (i.c.v.) versus systemic delivery of anti-Abeta antibodies on the behavioral and pathological changes in an aged Tg2576 mouse model of AD. Prolonged i.c.v. infusions of anti-Abeta antibodies dose-dependently reduced the parenchymal plaque burden, astrogliosis, and dystrophic neurites at doses 10- to 50-fold lower than used with systemic delivery of the same antibody. Both i.c.v. and systemic anti-Abeta antibodies reversed the behavioral impairment in contextual fear conditioning. More importantly, unlike systemically delivered anti-Abeta antibodies that aggravated vascular pathology, i.c.v.-infused antibodies globally reduced CAA and associated micro-hemorrhages. We present data suggesting that the divergent effects of i.c.v.-delivered anti-Abeta antibodies result from gradually engaging the local (i.e., central) mechanisms for amyloid clearance, distinct from the mechanisms engaged by high doses of anti-Abeta antibodies that circulate in the vasculature following systemic delivery. With robust efficacy in reversing AD-related pathology and an unexpected benefit in reducing CAA and associated micro-hemorrhages, i.c.v.-targeted passive immunotherapy offers a promising therapeutic approach for the long-term management of AD.

Developing novel immunogens for a safe and effective Alzheimer's disease vaccine

Alzheimer's disease (AD) is the most prevalent form of neurodegeneration; however, therapies to prevent or treat AD are inadequate. Amyloid-beta (Abeta) protein accrues in cortical senile plaques, one of the key neuropathological hallmarks of AD, and is elevated in brains of early onset AD patients in a small number of families that bear certain genetic mutations, further implicating its role in this devastating neurological disease. In addition, soluble Abeta oligomers have been shown to be detrimental to neuronal function. Therapeutic strategies aimed at lowering cerebral Abeta levels are currently under development. One strategy is to immunize AD patients with Abeta peptides so that they will generate antibodies that bind to Abeta protein and enhance its clearance. As of 1999, Abeta immunotherapy, either through active immunization with Abeta peptides or through passive transfer of Abeta-specific antibodies, has been shown to reduce cerebral Abeta levels and improve cognitive deficits in AD mouse models and lower plaque load in nonhuman primates. However, a Phase II clinical trial of active immunization using full-length human Abeta1-42 peptide and a strong Th1-biased adjuvant, QS-21, ended prematurely in 2002 because of the onset of meningoencephalitis in approximately 6% of the AD patients enrolled in the study. It is possible that T cell recognition of the human full-length Abeta peptide as a self-protein may have induced an adverse autoimmune response in these patients. Although only approximately 20% of immunized patients generated anti-Abeta titers, responders showed some general slowing of cognitive decline. Focal cortical regions devoid of Abeta plaques were observed in brain tissues of several immunized patients who have since come to autopsy. In order to avoid a deleterious immune response, passive Abeta immunotherapy is under investigation by administering monthly intravenous injections of humanized Abeta monoclonal antibodies to AD patients. However, a safe and effective active Abeta vaccine would be more cost-effective and more readily available to a larger AD population. We have developed several novel short Abeta immunogens that target the Abeta N-terminus containing a strong B cell epitope while avoiding the Abeta mid-region and C-terminus containing T cell epitopes. These immunogens include dendrimeric Abeta1-15 (16 copies of Abeta1-15 on a lysine antigen tree), 2xAbeta1-15 (a tandem repeat of two lysine-linked Abeta1-15 peptides), and 2xAbeta1-15 with the addition of a three amino acid RGD motif (R-2xAbeta1-15). Intranasal immunization with our short Abeta fragment immunogens and a mucosal adjuvant, mutant Escherichia coli heat-labile enterotoxin LT(R192G), resulted in reduced cerebral Abeta levels, plaque deposition, and gliosis, as well as increased plasma Abeta levels and improved cognition in a transgenic mouse model of AD. Preclinical trials in nonhuman primates, and human clinical trials using similar Abeta immunogens, are now underway. Abeta immunotherapy looks promising but must be made safer and more effective at generating antibody titers in the elderly. It is hoped that these novel immunogens will enhance Abeta antibody generation across a broad population and avoid the adverse events seen in the earlier clinical trial.

DNA epitope vaccine containing complement component C3d enhances anti-amyloid-beta antibody production and polarizes the immune response towards a Th2 phenotype

We have engineered a DNA epitope vaccine that expresses 3 self-B cell epitopes of Abeta(42) (3Abeta(1-11)), a non-self T helper (Th) cell epitope (PADRE), and 3 copies of C3d (3C3d), a component of complement as a molecular adjuvant, designed to safely reduce CNS Abeta. Immunization of mice with 3Abeta(1-11)-PADRE epitope vaccine alone generated only moderate levels of anti-Abeta antibodies and a pro-inflammatory T helper (Th1 phenotype) cellular immune response. However, the addition of 3C3d to the vaccine construct significantly augmented the anti-Abeta humoral immune response and, importantly, shifted the cellular immune response towards the potentially safer anti-inflammatory Th2 phenotype.

The coming of age of virus-like particle vaccines

Virus-like particles are supra-molecular assemblages, usually icosahedral or rod-like structures. They incorporate key immunologic features of viruses which include repetitive surfaces, particulate structures and induction of innate immunity through activation of pathogen-associated molecular-pattern recognition receptors. They carry no replicative genetic information and can be produced recombinantly in large scale. Virus-like particles thus represent a safe and effective vaccine platform for inducing potent B- and T-cell responses. In addition to being effective vaccines against the corresponding virus from which they are derived, virus-like particles can also be used to present foreign epitopes to the immune system. This can be achieved by genetic fusion or chemical conjugation. This technological innovation has greatly broadened the scope of their use, from immunizing against microbial pathogens to immunotherapy for chronic diseases. Towards this end, virus-like particles have been used to induce autoantibodies to disease-associated self-molecules involved in chronic diseases, such as hypertension and Alzheimer's disease. The recognition of the potent immunogenicity and commercial potential for virus-like particles has greatly accelerated research and development activities. During the last decade, two prophylactic virus-like particle vaccines have been registered for human use, while another 12 vaccines entered clinical development.

Consequence of Abeta immunization on the vasculature of human Alzheimer's disease brain

A major feature of Alzheimer's disease is the accumulation of amyloid-beta peptide (Abeta) in the brain both in the form of plaques in the cerebral cortex and in blood vessel as cerebral amyloid angiopathy (CAA). Experimental models and human clinical trials have shown that accumulation of Abeta plaques can be reversed by immunotherapy. In this study, we hypothesized that Abeta in plaques is solubilized by antibodies generated by immunization and drains via the perivascular pathway, detectable as an increase in cerebrovascular Abeta. We have performed a follow up study of Alzheimer's disease patients immunized against Abeta42. Neuropathological examination was performed on nine patients who died between four months and five years after their first immunization. Immunostaining for Abeta40 and Abeta42 was quantified and compared with that in unimmunized Alzheimer's disease controls (n = 11). Overall, compared with these controls, the group of immunized patients had approximately 14 times as many blood vessels containing Abeta42 in the cerebral cortex (P<0.001) and seven times more in the leptomeninges (P = 0.013); among the affected blood vessels in the immunized cases, most of them had full thickness and full circumference involvement of the vessel wall in the cortex (P = 0.001), and in the leptomeninges (P = 0.015). There was also a significantly higher level of cerebrovascular Abeta40 in the immunized cases than in the unimmunized cases (cortex: P = 0.009 and leptomeninges: P = 0.002). In addition, the immunized patients showed a higher density of cortical microhaemorrhages and microvascular lesions than the unimmunized controls, though none had major CAA-related intracerebral haemorrhages. The changes in cerebral vascular Abeta load did not appear to substantially influence the structural proteins of the blood vessels. Unlike most of the immunized patients, two of the longest survivors, four to five years after first immunization, had virtually complete absence of both plaques and CAA, raising the possibility that, given time, Abeta is eventually cleared from the cerebral vasculature. The findings are consistent with the hypothesis that Abeta immunization results in solubilization of plaque Abeta42 which, at least in part, exits the brain via the perivascular pathway, causing a transient increase in the severity of CAA. The extent to which these vascular alterations following Abeta immunization in Alzheimer's disease are reflected in changes in cognitive function remains to be determined.

Mannan-Abeta28 conjugate prevents Abeta-plaque deposition, but increases microhemorrhages in the brains of vaccinated Tg2576 (APPsw) mice

Background

New pre-clinical trials in AD mouse models may help to develop novel immunogen-adjuvant configurations with the potential to avoid the adverse responses that occurred during the clinical trials with AN-1792 vaccine formulation. Recently, we have pursued an alternative immunization strategy that replaces QS21 the Th1 type adjuvant used in the AN-1792 clinical trial with a molecular adjuvant, mannan that can promote a Th2-polarized immune response through interactions with mannose-binding and CD35/CD21 receptors of the innate immune system. Previously we established that immunization of wild-type mice with mannan-Aβ₂₈ conjugate promoted Th2-mediated humoral and cellular immune responses. In the current study, we tested the efficacy of this vaccine configuration in amyloid precursor protein (APP) transgenic mice (Tg2576).

Methods

Mannan was purified, activated and chemically conjugated to Aβ₂₈ peptide. Humoral immune responses induced by the immunization of mice with mannan-Aβ₂₈ conjugate were analyzed using a standard ELISA. Aβ₄₂ and Aβ₄₀ amyloid burden, cerebral amyloid angiopathy (CAA), astrocytosis, and microgliosis in the brain of immunized and control mice were detected using immunohistochemistry. Additionally, cored plaques and cerebral vascular microhemorrhages in the brains of vaccinated mice were detected by standard histochemistry.

Results

Immunizations with low doses of mannan-Aβ₂₈ induced potent and long-lasting anti-Aβ humoral responses in Tg2576 mice. Even 11 months after the last injection, the immunized mice were still producing low levels of anti-Aβ antibodies, predominantly of the IgG1 isotype, indicative of a Th2 immune response. Vaccination with mannan-Aβ₂₈ prevented Aβ plaque deposition, but unexpectedly increased the level of microhemorrhages in the brains of aged immunized mice compared to two groups of control animals of the same age either injected with molecular adjuvant fused with an irrelevant antigen, BSA (mannan-BSA) or non-immunized mice. Of note, mice immunized with mannan-Aβ₂₈ showed a trend toward elevated levels of CAA in the neocortex and in the leptomeninges compared to that in mice of both control groups.

Conclusion

Mannan conjugated to Aβ₂₈ provided sufficient adjuvant activity to induce potent anti-Aβ antibodies in APP transgenic mice, which have been shown to be hyporesponsive to immunization with Aβ self-antigen. However, in old Tg2576 mice there were increased levels of cerebral microhemorrhages in mannan-Aβ₂₈ immunized mice. This effect was likely unrelated to the anti-mannan antibodies induced by the immunoconjugate, because control mice immunized with mannan-BSA also induced antibodies specific to mannan, but did not have increased levels of cerebral microhemorrhages compared with non-immunized mice. Whether these anti-mannan antibodies increased the permeability of the blood brain barrier thus allowing elevated levels of anti-Aβ antibodies entry into cerebral perivascular or brain parenchymal spaces and contributed to the increased incidence of microhemorrhages remains to be investigated in the future studies.

Active and passive immunotherapy for neurodegenerative disorders

Immunotherapeutic strategies to combat neurodegenerative disorders have galvanized the scientific community since the first dramatic successes in mouse models recreating aspects of Alzheimer disease (AD) were reported. However, initial human trials of active amyloid-beta (Abeta) vaccination were halted early because of a serious safety issue: meningoencephalitis in 6% of subjects. Nonetheless, some encouraging preliminary data were obtained, and rapid progress has been made toward developing alternative, possibly safer active and passive immunotherapeutic approaches for several neurodegenerative conditions. Many of these are currently in human trials for AD. Despite these advances, our understanding of the essential mechanisms underlying the effects seen in preclinical models and human subjects is still incomplete. Antibody-induced phagocytosis of pathological protein deposits, direct antibody-mediated disruption of aggregates, neutralization of toxic soluble proteins, a shift in equilibrium toward efflux of specific proteins from the brain, cell-mediated immune responses, and other mechanisms may all play roles depending on the specific immunotherapeutic scenario.

Immunotherapy reduces vascular amyloid-beta in PDAPP mice

In addition to parenchymal amyloid-beta (Abeta) plaques, Alzheimer's disease (AD) is characterized by Abeta in the cerebral vasculature [cerebral amyloid angiopathy (CAA)] in the majority of patients. Recent studies investigating vascular Abeta (VAbeta) in amyloid precursor protein transgenic mice have suggested that passive immunization with anti-Abeta antibodies may clear parenchymal amyloid but increase VAbeta and the incidence of microhemorrhage. However, the influences of antibody specificity and exposure levels on VAbeta and microhemorrhage rates have not been well established, nor has any clear causal relationship been identified. This report examines the effects of chronic, passive immunization on VAbeta and microhemorrhage in PDAPP mice by comparing antibodies with different Abeta epitopes (3D6, Abeta(1-5); 266, Abeta(16-23)) and performing a 3D6 dose-response study. VAbeta and microhemorrhage were assessed using concomitant Abeta immunohistochemistry and hemosiderin detection. 3D6 prevented or cleared VAbeta in a dose-dependent manner, whereas 266 was without effect. Essentially complete absence of VAbeta was observed at the highest 3D6 dose, whereas altered morphology suggestive of ongoing clearance was seen at lower doses. The incidence of microhemorrhage was increased in the high-dose 3D6 group and limited to focal, perivascular sites. These colocalized with Abeta deposits having altered morphology and apparent clearance in the lower-dose 3D6 group. Our results suggest that passive immunization can reduce VAbeta levels, and modulating antibody dose can significantly mitigate the incidence of microhemorrhage while still preventing or reducing VAbeta. These observations raise the possibility that Abeta immunotherapy can potentially slow or halt the course of CAA development in AD that is implicated in vascular dysfunction.

Developing injectable immunoglobulins to treat cognitive impairment in Alzheimer's disease

BACKGROUND

Alzheimer's disease is a devastating disorder, clinically characterized by a comprehensive cognitive decline. The novel strategy of anti-amyloid-beta immunotherapy has been suggested following encouraging results obtained in murine models of Alzheimer's disease, in non-human primates, and in small-scale clinical trials.

OBJECTIVE

To examine the choice between active or passive anti-amyloid-beta immunization and the choice of the molecule to which the immune machinery should be targeted, which are central issues in future immune therapy of Alzheimer's disease.

METHODS

Research into the new area of Alzheimer's disease immune therapy is primarily based on in vivo and in vitro studies of murine models of Alzheimer's disease. The studies are hence limited to defined genetic deficiencies.

RESULTS/CONCLUSIONS

In humans, infusion of anti-amyloid-beta antibodies is considered a safer approach than active anti-amyloid-beta vaccination. Alzheimer's-disease-protective anti-amyloid-beta monoclonal antibodies should target specific epitopes within the amyloid beta(1 42) peptide, avoiding possibly harmful binding to the ubiquitous normal amyloid precursor protein. Since Alzheimer's disease immunotherapy requires repeated infusion of antibodies over a prolonged period of time, Alzheimer's disease patients will tolerate such antibodies provided the latter are exclusively of human origin. Human monoclonal antibodies that correspond to ubiquitous anti-amyloid-beta, present in all healthy humans, might bear important protective characteristics.

Biomarkers for early diagnosis of Alzheimer disease: 'ALZheimer ASsociated gene'--a new blood biomarker?

Simple, non-invasive tests for an early detection of degenerative dementia by use of biomarkers are urgently required. However, up to the present, no validated extracerebral diagnostic markers (plasma/serum, platelets, urine, connective tissue) for the early diagnosis of Alzheimer disease (AD) are available. In disease stages with evident cognitive disturbances, the clinical diagnosis of probable AD is made with around 90% accuracy using modern clinical, neuropsychological and imaging methods. Diagnostic sensitivity and specificity even in early disease stages are improved by CSF markers, in particular combined tau and amyloid beta peptides (Abeta) and plasma markers (eg, Abeta-42/Abeta-40 ratio). Recently, a novel gene/protein--ALZAS (Alzheimer Associated Protein)--with a 79 amino acid sequence, containing the amyloid beta-42 fragment (Abeta-42), the amyloid precursor protein (APP) transmembrane signal and a 12 amino acid C-terminal, not present in any other known APP alleles, has been discovered on chromosome 21 within the APP region. Reverse transcriptase-PCR revealed the expression of the transcript of this protein in the cortex and hippocampal regions as well as in lymphocytes of human AD patients. The expression of ALZAS is mirrored by a specific autoimmune response in AD patients, directed against the ct-12 end of the ALZAS-peptide but not against the Abeta-sequence. ELISA studies of plasma detected highest titers of ALZAS in patients with mild cognitive impairment (presymptomatic AD), but only moderately increased titers in autopsy-confirmed AD, whereas low or undetectable ct-12 titers were found in cognitively intact age-matched subjects and young controls. The antigen, ALZAS protein, was detected in plasma in later clinical stages of AD. It is suggested that ALZAS represents an indicator in a dynamic equilibrium between both peripheral and brain degenerative changes in AD and may become a useful "non-invasive" diagnostic marker via a simple blood test.

Present and prospective clinical therapeutic regimens for Alzheimer's disease

Alzheimer's disease (AD) is an incurable neurodegenerative disorder that produces cognitive impairments that increase in severity as the disease progresses. The clinical symptoms are related to the presence of neuritic plaques and neurofibrillary tangles in the cerebral cortex which represent the pathophysiological hallmarks of AD. The debilitating nature of the disease can result in clinical burden for the patient, emotional strain for those that care for patients with Alzheimer's, and significant financial burden to society. The goals of current treatments, such as cholinesterase inhibitors and N-methyl-D-aspartate receptor antagonist, are to reduce the severity or slow the progression of cognitive symptoms. Although these treatments have demonstrated modest clinical benefit, they are unable to prevent, prohibit, or reverse the underlying pathophysiology of AD. Considerable progress has been made toward the development of disease-modifying treatments. Treatments currently under development mainly target the production, aggregation, and removal of existing amyloid beta-peptide aggregates which are believed to instigate the overall development of the neuropathology. Additional strategies that target tau pathology are being studied to promote neural protection against AD pathology. The current research has continued to expand our knowledge toward the development of disease modifying Alzheimer's therapies; however, no specific treatment strategy capable of demonstrating empirical efficacy and safety has yet to emerge.

Amyloid-beta immunisation for Alzheimer's disease

Alzheimer's disease is the main cause of dementia in elderly people and is becoming an ever greater problem as societies worldwide age. Treatments that stop or at least effectively modify disease course do not yet exist. In Alzheimer's disease, the conversion of the amyloid-beta peptide (Abeta) from a physiological water-soluble monomeric form into neurotoxic oligomeric and fibrillar forms rich in stable beta-sheet conformations is an important event. The most toxic forms of Abeta are thought to be oligomers, and dimers might be the smallest neurotoxic species. Numerous immunological approaches that prevent the conversion of the normal precursor protein into pathological forms or that accelerate clearance are in development. More than ten new approaches to active and passive immunotherapy are under investigation in clinical trials with the aim of producing safe methods for immunological therapy and prevention. A delicate balance between immunological clearance of an endogenous protein with acquired toxic properties and the induction of an autoimmune reaction must be found.

Parenchymal and vascular Abeta-deposition and its effects on the degeneration of neurons and cognition in Alzheimer's disease

The deposition of the amyloid beta-protein (Abeta) is one of the pathological hallmarks of Alzheimer's disease (AD). Abeta-deposits show the morphology of senile plaques and cerebral amyloid angiopathy (CAA). Senile plaques and vascular Abeta-deposits occur first in neocorti-cal areas. Then, they expand hierarchically into further brain regions. The distribution of Abeta plaques throughout the entire brain, thereby correlates with the clinical status of the patients. Imaging techniques for Abeta make use of the hierarchical distribution of Abeta to distinguish AD patients from non-AD patients. However, pathology seen in AD patients represents a late stage of a pathological process starting 10-30 years earlier in cognitively normal individuals. In addition to the fibrillar amyloid of senile plaques, oligomeric and monomeric Abeta is found in the brain. Recent studies revealed that oligomeric Abeta is presumably the most toxic Abeta-aggregate, which interacts with glutamatergic synapses. In doing so, dendrites are presumed to be the primary target for Abeta-toxicity. In addition, vascular Abeta-deposits can lead to capillary occlusion and blood flow disturbances presumably contributing to the alteration of neurons in addition to the direct neurotoxic effects of Abeta. All these findings point to an important role of Abeta and its aggregates in the neurodegenerative process of AD. Since there is already significant neuron loss in AD patients, treatment strategies aimed at reducing the amyloid load will presumably not cure the symptoms of dementia but they may stop disease progression. Therefore, it seems to be necessary to protect the brain from Abeta-toxicity already in stages of the disease with minor neuron loss before the onset of cognitive symptoms.

The role of the immune system in clearance of Abeta from the brain

In Alzheimer's disease (AD), there is abnormal accumulation of Abeta and tau proteins in the brain. There is an associated immunological response, but it is still unclear whether this is beneficial or harmful. Inflammation in AD, specifically in the form of microglial activation, has, for many years, been considered to contribute to disease progression. However, two types of evidence suggest that it may be appropriate to revise this view: first, the disappointing results of prospective clinical trials of anti-inflammatory agents and, second, the observation that microglia can clear plaques in AD following Abeta immunization. Although Abeta immunization alters AD pathology, there is limited evidence so far of benefit to cognitive function. Immunization against microorganisms is almost always used as a method of disease prevention rather than to treat a disease process that has already started. In animal models, immunotherapy at an early age can protect against Abeta accumulation and it will be interesting to see if this can usefully be applied to humans to prevent AD.

Targeting beta-amyloid pathology in Alzheimer's disease with Abeta immunotherapy

More than 10 clinical trials of Abeta immunotherapy are currently underway in patients with Alzheimer's disease (AD). The aim is to identify safe approaches for the efficacious antibody-mediated removal of brain beta-amyloid or its neurotoxic oligomeric precursors consisting of aggregated amyloid beta-peptide (Abeta). Initial experimental and neuro-pathological evidence for clearance of brain beta-amyloid in response to Abeta immunotherapy is associated with structural and functional rescue of neurons, as well as initial signs of clinical stabilization and reduced rates of dementia progression. For the next steps in the future improvement of Abeta immunotherapy, major challenges in pharmacokinetics, safety, and tolerability need to be addressed. These include the low penetrations rates of IgG molecules through the blood-brain barrier, possible reductions in brain volume, the possibility of autoimmune disease related to unwanted cross-reactivity with endogenous antigens on physiological structures, micro-hemorrhages related to cross-reaction with pre-existing vascular amyloid pathology, possible relocalization of Abeta from beta-amyloid plaques to brain blood vessels resulting in increased amyloid angiopathy, and the lacking activity of Abeta antibodies on pre-existing neurofibrillary tangle pathology, as well as the lacking molecular identification of the forms of Abeta to be therapeutically targeted. The solutions to these problems will be guided by the fine lines between tolerance and immunity against physiological and pathological structures, respectively, as well as by the understanding of the pathogenic transition of soluble Abeta into toxic oligomeric aggregation intermediates in the dynamic equilibrium of beta-amyloid fibril assembly. Provided that the ongoing and planned clinical trials address these issues in a timely manner, there is a good chance for Abeta immunotherapy to be one of the first disease-modifying therapies of Alzheimer's disease to be introduced into clinical practice.

Therapeutic strategies for Alzheimer's disease

Therapeutic approaches for Alzheimer's disease (AD) are guided by four disease characteristics: amyloid plaques, neurofibrillar tangles (NFT), neurodegeneration, and dementia. Amyloid plaques are composed largely of 4 kDa beta-amyloid (Abeta) peptides, with the more amyloidogenic, 42 amino acid form (Abeta42) as the primary species. Because multiple, rare mutations that cause early-onset, familial AD lead to increased production or aggregation of Abeta42, amyloid therapeutics aim to reduce the amount of toxic Abeta42 aggregates. Amyloid-based therapies include gamma-secretase inhibitors and modulators, BACE inhibitors, aggregation blockers, catabolism inducers, and anti-Abeta biologics. Tangles are composed of paired helical filaments of hyperphosphorylated tau protein. Tau-based therapeutics include kinase inhibitors, microtubule stabilizers, and catabolism inducers. Therapeutic strategies for neurodegeneration target multiple mechanisms, including excitotoxicity, mitochondrial dysfunction, oxidative damage, and inflammation or stimulation of neuronal viability. Although not disease modifying, cognition enhancers are important to treat the symptom of dementia. Strategies for cognition enhancement include cholinesterase inhibitors, and other approaches to enhance the signaling of cholinergic and glutamatergic neurons. In summary, plaques, tangles, neurodegeneration and dementia guide the development of multiple therapeutic approaches for AD and are the subject of this review.

Can the immune system be harnessed to repair the CNS?

Experimental and clinical data have demonstrated that activating the immune system in the CNS can be destructive. However, other studies have shown that enhancing an immune response can be therapeutic, and several clinical trials have been initiated with the aim of boosting immune responses in the CNS of individuals with spinal cord injury, multiple sclerosis and Alzheimer's disease. Here, we evaluate the controversies in the field and discuss the remaining scientific challenges that are associated with enhancing immune function in the CNS to treat neurological diseases.

Amyloid plaque imaging in vivo: current achievement and future prospects

INTRODUCTION

Alzheimer's disease (AD) is a very complex neurodegenerative disorder, the exact cause of which is still not known. The major histopathological features, amyloid plaques and neurofibrillary tangles, already described by Alois Alzheimer, have been the focus in research for decades. Despite a probable whole cascade of events in the brain leading to impairment of cognition, amyloid is still the target for diagnosis and treatment.

DISCUSSION

The rapid development of molecular imaging techniques now allows imaging of amyloid plaques in vivo in Alzheimer patients by PET amyloid ligands such as Pittsburgh compound B (PIB). Studies so far have revealed high (11)C-PIB retention in brain at prodromal stages of AD and a possibility to discriminate AD from other dementia disorders by (11)C-PIB. Ongoing studies are focussing to understand the relationship between brain and CSF amyloid processes and cognitive processes.

CONCLUSION

In vivo imaging of amyloid will be important for early diagnosis and evaluation of new anti-amyloid therapies in AD.

Reducing AD-like pathology in 3xTg-AD mouse model by DNA epitope vaccine - a novel immunotherapeutic strategy

BACKGROUND

The development of a safe and effective AD vaccine requires a delicate balance between providing an adequate anti-Abeta antibody response sufficient to provide therapeutic benefit, while eliminating an adverse T cell-mediated proinflammatory autoimmune response. To achieve this goal we have designed a prototype chemokine-based DNA epitope vaccine expressing a fusion protein that consists of 3 copies of the self-B cell epitope of Abeta(42) (Abeta(1-11)) , a non-self T helper cell epitope (PADRE), and macrophage-derived chemokine (MDC/CCL22) as a molecular adjuvant to promote a strong anti-inflammatory Th2 phenotype.

METHODS AND FINDINGS

We generated pMDC-3Abeta(1-11)-PADRE construct and immunized 3xTg-AD mouse model starting at age of 3-4 months old. We demonstrated that prophylactic immunizations with the DNA epitope vaccine generated a robust Th2 immune response that induced high titers of anti-Abeta antibody, which in turn inhibited accumulation of Abeta pathology in the brains of older mice. Importantly, vaccination reduced glial activation and prevented the development of behavioral deficits in aged animals without increasing the incidence of microhemorrhages.

CONCLUSIONS

Data from this transitional pre-clinical study suggest that our DNA epitope vaccine could be used as a safe and effective strategy for AD therapy. Future safety and immunology studies in large animals with the goal to achieve effective humoral immunity without adverse effects should help to translate this study to human clinical trials.

A two-year study with fibrillar beta-amyloid (Abeta) immunization in aged canines: effects on cognitive function and brain Abeta

Aged canines (dogs) accumulate human-type beta-amyloid (Abeta) in diffuse plaques in the brain with parallel declines in cognitive function. We hypothesized that reducing Abeta in a therapeutic treatment study of aged dogs with preexisting Abeta pathology and cognitive deficits would lead to cognitive improvements. To test this hypothesis, we immunized aged beagles (8.4-12.4 years) with fibrillar Abeta(1-42) formulated with aluminum salt (Alum) for 2.4 years (25 vaccinations). Cognitive testing during this time revealed no improvement in measures of learning, spatial attention, or spatial memory. After extended treatment (22 vaccinations), we observed maintenance of prefrontal-dependent reversal learning ability. In the brain, levels of soluble and insoluble Abeta(1-40) and Abeta(1-42) and the extent of diffuse plaque accumulation was significantly decreased in several cortical regions, with preferential reductions in the prefrontal cortex, which is associated with a maintenance of cognition. However, the amount of soluble oligomers remained unchanged. The extent of prefrontal Abeta was correlated with frontal function and serum anti-Abeta antibody titers. Thus, reducing total Abeta may be of limited therapeutic benefit to recovery of cognitive decline in a higher mammalian model of human brain aging and disease. Immunizing animals before extensive Abeta deposition and cognitive decline to prevent oligomeric or fibrillar Abeta formation may have a greater impact on cognition and also more directly evaluate the role of Abeta on cognition in canines. Alternatively, clearing preexisting Abeta from the brain in a treatment study may be more efficacious for cognition if combined with a second intervention that restores neuron health.

Stabilization of a beta-hairpin in monomeric Alzheimer's amyloid-beta peptide inhibits amyloid formation

According to the amyloid hypothesis, the pathogenesis of Alzheimer's disease is triggered by the oligomerization and aggregation of the amyloid-beta (Abeta) peptide into protein plaques. Formation of the potentially toxic oligomeric and fibrillar Abeta assemblies is accompanied by a conformational change toward a high content of beta-structure. Here, we report the solution structure of Abeta(1-40) in complex with the phage-display selected affibody protein Z(Abeta3), a binding protein of nanomolar affinity. Bound Abeta(1-40) features a beta-hairpin comprising residues 17-36, providing the first high-resolution structure of Abeta in beta conformation. The positions of the secondary structure elements strongly resemble those observed for fibrillar Abeta. Z(Abeta3) stabilizes the beta-sheet by extending it intermolecularly and by burying both of the mostly nonpolar faces of the Abeta hairpin within a large hydrophobic tunnel-like cavity. Consequently, Z(Abeta3) acts as a stoichiometric inhibitor of Abeta fibrillation. The selected Abeta conformation allows us to suggest a structural mechanism for amyloid formation based on soluble oligomeric hairpin intermediates.

Enhanced Th2 immunity after DNA prime-protein boost immunization with amyloid beta (1-42) plus CpG oligodeoxynucleotides in aged rats

Generation and accumulation of fibrillar amyloid beta (Abeta) is widely considered as the pathogenic basis of neurodegeneration in Alzheimer's disease (AD). Both active immunization with fibrillar Abeta and passive immunization with anti-Abeta antibodies in transgenic mouse models of AD result in prevention/dissociation of Abeta plaque formation and restoration of cognitive functions. However, similar immunization studies in humans had to be halted because 6% of the AD patients developed acute meningoencephalitis, likely due to anti-Abeta specific autoimmune Th1 cells. Hence, making Abeta immunotherapy successful requires production of strong antibody responses without Th1-type immunity. In an attempt to develop safer vaccines, we examined the influence of oligodeoxynucleotides as adjuvant on the Th1 and Th2 immune response to Abeta in aged rats. We further investigated whether a DNA prime-protein boost strategy could elicit a more robust Th2 response. The results of the present study showed that all the animals injected with either Abeta peptide alone or Abeta encoding plasmid alone or plasmid DNA prime followed by peptide boost have elicited specific anti-Abeta antibodies. When co-administered, synthetic oligodeoxynucleotides (ODN) further enhanced the anti-Abeta titres. More importantly, the IgG subclasses of the antibodies generated by DNA prime-peptide boost regimen with ODN as adjuvant were primarily of IgG2b and IgG1 isotypes, suggesting that heterologous immunization strategy along with ODN would be advantageous in eliciting more beneficial Th2-type humoral immune response.

Developing preventive therapies for chronic diseases: lessons learned from Alzheimer's disease

A remarkable rise in life expectancy during the past century has made Alzheimer's disease (AD) the most common form of progressive intellectual failure in humans. Patients with AD lose their most human qualities-reasoning, abstraction, language, and memory. The brain plaques that Alois Alzheimer first described 100 years ago have inspired the search for genetic alterations that underlie AD. Four genes have been unequivocally implicated to date in inherited forms of AD, where mutations or natural variations in these genes cause excessive accumulation of the amyloid beta-protein, the building block of amyloid plaques. This aggregation leads to subsequent neuronal degeneration in brain regions important for memory and cognition. The discovery of the genes involved in the mechanisms of amyloid beta-protein build-up in AD, coupled with cell culture and animal models of their involved pathways, has led to the development of specific pharmacological strategies to lower amyloid beta-protein levels as a way of treating or preventing all forms of the disease. While hard work lies ahead, the movement from basic research to the clinic in AD represents a triumph of reductionist biology applied to the most complex of all biological systems, the human cerebral cortex.

PK11195 labels activated microglia in Alzheimer's disease and in vivo in a mouse model using PET

Activated microglia may promote neurodegeneration in Alzheimer's disease (AD) and may also help in amyloid clearance in immunization therapies. In vivo imaging of activated microglia using positron emission tomography (PET) could assist in defining the role of activated microglia during AD progression and therapeutics. We hypothesized that PK11195, a ligand that binds activated microglia, could label these cells in postmortem AD tissues and in vivo in an animal model of AD using PET. [(3)H](R)-PK11195 binding was significantly higher in AD frontal cortex compared to controls and correlated mainly with the abundance of immunohistochemically labeled activated microglia. With age, the brains of APP/PS1 transgenic mice showed progressive increase in [(3)H](R)-PK11195 binding and [(11)C](R)-PK11195 retention in vivo assessed using microPET, which correlated with the histopathological abundance of activated microglia. These results suggest that PK11195 binding in AD postmortem tissue and transgenic mice in vivo correlates with the extent of microglial activation and may help define the role of activated microglia in the pathogenesis and treatment of AD.

Immunotherapy as treatment for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is characterized pathologically by the deposition of beta-amyloid (A beta)-containing extracellular neuritic plaques, intracellular neurofibrillary tangles and neuronal loss. Much evidence supports the hypothesis that A beta peptide aggregation contributes to AD pathogenesis, however, currently approved therapeutic treatments do nothing to stop or reverse A beta deposition. The success of active and passive anti-A beta immunotherapies in both preventing and clearing parenchymal amyloid in transgenic mouse models led to the initiation of an active anti-A beta vaccination (AN1792) trial in human patients with mild-to-moderate AD, but was prematurely halted when 6% of inoculated patients developed aseptic meningoencephalitis. Autopsy results from the brains of four individuals treated with AN1792 revealed decreased plaque burden in select brain areas, as well as T-cell lymphocytes in three of the patients. Furthermore, antibody responders showed some improvement in memory task measures. These findings indicated that anti-A beta therapy might still be a viable option for the treatment of AD, if potentially harmful proinflammatory processes can be avoided. Over the past 6 years, this target has led to the development of novel experimental immunization strategies, including selective A beta epitope targeting, antibody and adjuvant modifications, as well as alternative routes and mechanisms of vaccine delivery, to generate anti-A beta antibodies that selectively target and remove specific A beta species without evoking autoimmunity. Results from the passive vaccination AD clinical trials that are currently underway will provide invaluable information about both the effectiveness of newly improved anti-A beta vaccines in clinical treatment, as well as the role of the A beta peptide in the pathogenesis of the disease.

Experimental investigation of antibody-mediated clearance mechanisms of amyloid-beta in CNS of Tg-SwDI transgenic mice

Novel amyloid precursor protein transgenic mice, which contain the Swedish as well as the vasculotropic Dutch and Iowa mutations (Tg-SwDI), were used to investigate the mechanisms of antibody-mediated clearance of amyloid-beta (Abeta) from the brain. Export of the Abeta-DI peptide across the blood-brain barrier is severely reduced because of the vasculotropic mutations. Therefore, antibody-mediated clearance of Abeta-DI is dependent on antibodies entering the brain. In this report, we immunized Tg-SwDI mice with various peptide antigens, including Abeta40-DI, Abeta42, and an Abeta epitope vaccine. Immunization of Tg-SwDI mice with substantial cortical diffuse and vascular fibrillar deposits failed to promote clearance of parenchymal or vascular amyloid deposits. We then immunized young Tg-SwDI mice before the accumulation of Abeta and saw no evidence that anti-Abeta antibodies could diminish deposition of parenchymal or vascular amyloid deposits. However, injection of anti-Abeta antibodies, affinity-purified from immunized Tg-SwDI mice, into the hippocampus induced a rapid clearance of diffuse Abeta deposits but not vascular amyloid deposits. These results further support the "peripheral sink hypothesis" as a legitimate mechanism of antibody-mediated clearance of Abeta when the blood-brain barrier remains intact. Thus, approaches that deliver immunotherapy to the brain may be more effective at clearing Abeta than immunization strategies in which the majority of the antibodies are in the periphery.

Alzheimer's disease peptide epitope vaccine reduces insoluble but not soluble/oligomeric Abeta species in amyloid precursor protein transgenic mice

Active vaccination of elderly Alzheimer's disease (AD) patients with fibrillar amyloid-beta peptide (Abeta42), even in the presence of a potent Th1 adjuvant, induced generally low titers of antibodies in a small fraction (approximately 20% responders) of those that received the AN-1792 vaccine. To improve the immunogenicity and reduce the likelihood of inducing adverse autoreactive T-cells specific for Abeta42, we previously tested in wild-type mice an alternative approach for active immunization: an epitope vaccine that selectively initiate B cell responses toward an immunogenic self-epitope of Abeta in the absence of anti-Abeta T cell responses. Here, we describe a second generation epitope vaccine composed of two copies of Abeta(1-11) fused with the promiscuous nonself T cell epitope, PADRE (pan human leukocyte antigen DR-binding peptide) that completely eliminates the autoreactive T cell responses and induces humoral immune responses in amyloid precursor protein transgenic 2576 mice with pre-existing AD-like pathology. Based on the titers of anti-Abeta(1-11) antibody experimental mice were divided into low, moderate and high responders, and for the first time we report a positive correlation between the concentration of anti-Abeta(1-11) antibody and a reduction of insoluble, cerebral Abeta plaques. The reduction of insoluble Abeta deposition was not associated with adverse events, such as CNS T cell or macrophage infiltration or microhemorrhages. Surprisingly, vaccination did not alter the levels of soluble Abeta. Alternatively, early protective immunization before substantial neuropathology, neuronal loss and cognitive deficits have become firmly established may be more beneficial and safer for potential patients, especially if they can be identified in a preclinical stage by the development of antecedent biomarkers of AD.

Anti-inflammatory and immune therapy for Alzheimer's disease: current status and future directions

From the initial characterizations of inflammatory responses in Alzheimer's disease (AD) affected brains, namely the demonstration of activated microglia and reactive astrocytes, complement system activation, increased production of proinflammatory cytokines, and evidence for microglial-produced neurotoxins, there was hope that reducing inflammation might be a feasible treatment for this memory-robbing disease. This hope was supported by a number of epidemiology studies demonstrating that patients who took non-steroidal anti-inflammatory drugs had significantly lower risk of developing AD. However, clinical trials of anti-inflammatories have not shown effectiveness, and in recent years, the concept of immune therapy has become a treatment option as animal studies and clinical trials with Abeta vaccines have demonstrated enhanced amyloid removal through stimulation of microglial phagocytosis.This review will examine the current status of whether inhibiting inflammation is a valid therapeutic target for treating AD; what lessons have come from the clinical trials; what new pathways and classes of agents are being considered; and how this field of research can progress towards new therapeutics. We will examine a number of agents that have shown effectiveness in reducing inflammation amongst other demonstrated mechanisms of action. The major focus of much AD drug discovery has been in identifying agents that have anti-amyloid properties; however, a number of these agents were first identified for their anti-inflammatory properties. As drug development and clinical testing is a costly and lengthy endeavor, sound justification of new therapeutic targets is required. Possible future directions for AD anti-inflammatory or immune clearance therapy will be discussed based on recent experimental data.

Advances on the understanding of the origins of synaptic pathology in AD

Although Alzheimer's disease (AD) was first discovered a century ago, we are still facing a lack of definitive diagnosis during the patient's lifetime and are unable to prescribe a curative treatment. However, the past 10 years have seen a "revamping" of the main hypothesis about AD pathogenesis and the hope to foresee possible treatment. AD is no longer considered an irreversible disease. A major refinement of the classic beta-amyloid cascade describing amyloid fibrils as neurotoxins has been made to integrate the key scientific evidences demonstrating that the first pathological event occurring in AD early stages affects synaptic function and maintenance. A concept fully compatible with synapse loss being the best pathological correlate of AD rather than other described neuropathological hallmarks (amyloid plaques, neurofibrillary tangles or neuronal death). The notion that synaptic alterations might be reverted, thus offering a potential curability, was confirmed by immunotherapy experiments targeting beta-amyloid protein in transgenic AD mice in which cognitive functions were improved despite no reduction in the amyloid plaques burden. The updated amyloid cascade now integrates the synapse failure triggered by soluble Abeta-oligomers. Still no consensus has been reached on the most toxic Abeta conformations, neither on their site of production nor on their extra- versus intra-cellular actions. Evidence shows that soluble Abeta oligomers or ADDLs bind selectively to neurons at their synaptic loci, and trigger major changes in synapse composition and morphology, which ultimately leads to dendritic spine loss. However, the exact mechanism is not yet fully understood but is suspected to involve some membrane receptor(s).

Increased T-cell reactivity and elevated levels of CD8+ memory T-cells in Alzheimer's disease-patients and T-cell hyporeactivity in an Alzheimer's disease-mouse model: implications for immunotherapy

Neuroinflammation is observed in neurodegenerative diseases like Alzheimer's disease (AD). However, a little is known about the mechanisms of neural-immune interactions. The involvement of peripheral T-cell function in AD is still far from clear, though it plays an important role in immunotherapy. The aim of this study was to determine peripheral T-cell reactivity in AD patients and in an AD mouse model. Mitogenic activation via ligation of the T-cell receptor (TCR) with PHA-L was measured in T lymphocytes from AD patients and Thy1(APP 751SL) x HMG(PS1 M146L)-transgenic mice (APP x PS1). In order to uncover failures in TCR signaling, the TCR was also bypassed by PMA and ionomycin treatment. All patients were sporadic late onset cases and the transgenic mice expressed no mutant APP in lymphocytes, so that direct interactions of mutant APP on T-cell function can be excluded. CD4+ and CD8+ T-cell showed increased reactivity (tyrosine phosphorylation, CD69 expression, and proliferation) in AD, while APP x PS1 transgenic mice displayed hyporeactive CD8+ T-cells after TCR ligation. Increased levels of CD8+ T memory cells and down regulation of CD8 receptor were found in AD and the animal model. Anergic TCR uncoupling was associated with loss of MAPK signaling (p38, ERK1 and ERK2) in APP x PS1. Our data implicate the generation of reactive memory T-cell in AD and of anergic memory T-cells in transgenic mice and should be taken into concern when designing immunotherapy.