Enlisting the immune system to fight Alzheimer disease can take the form of immunization against Aβ, or other attempts to modulate the inflammatory response that plays a role in disease. In a paper out in the April 2 issue of the Journal of Neuroscience, Elizabeth Head and colleagues of the University of California at Irvine present results of a two and a half year study of immunization of aged beagles with fibrillar Aβ. The study suggests that while immunization of the dogs was very efficient at clearing of cortical amyloid, that did not translate into improvements in tests of learning and memory. The authors suggest that prevention of amyloid buildup may be a better way to affect cognition.

A second study shows that modulation of the immune response by infusing human umbilical cord blood cells into AD mice can result in clearance of amyloid from both the brain and the vascular system. That study, from Jun Tan and colleagues at the University of South Florida in Tampa, is published in the March 26 issue of Stem Cells and Development, where they show that the treatment seems to work by blocking the CD40-mediated inflammatory response and increasing Aβ phagocytosis by microglia.

In the vaccination study, Head and her colleagues compared eight- to 12-year-old beagles, nine of which got a fibrillar Aβ1-42 vaccine monthly for two years, and 11 of which did not. The investigators saw a decline in soluble and insoluble Aβ peptides in the brain of immunized animals, but levels of soluble oligomers remained unchanged. They found no lessening of cognitive decline by most measures of learning, spatial attention, or spatial learning. They did see maintenance of prefrontal-dependent learning in a reversal trial. “One of the simplest interpretations of these data are [sic] that reducing preexisting brain Aβ is insufficient to restore neuronal and cognitive functions,” the authors write. They speculate that the one reason for the lack of improvement could be that the treatment did not affect levels of oligomers.

“The limited functional benefit suggests that prevention of Aβ accumulation by initiating treatment in middle-aged animals may be more efficacious as has been suggested in transgenic mouse models,” they conclude. However, questions remain about how faithfully the dogs, who accumulate diffuse amyloid plaques during aging, reflect Alzheimer disease as it manifests in people (see comment below by David Morgan, University of South Florida).

A second approach to immune modulation is presented by Tan, with University of South Florida coauthor Paul Sanberg, and Terrence Town of the Cedars-Sinai Medical Center in Los Angeles. First authors William Nikolic and Huayan Hou led the study, which involved infusing human umbilical cord blood cells (HUCBCs) into either PS/APP or Tg2576 mouse models for AD. In both cases, they saw a reduction of brain Aβ levels, amyloid plaques, and, in the Tg2576, a reduction in vascular amyloid deposits, along with a drop in inflammatory markers around plaques. With no behavioral data, it is unknown if the changes in pathology translate to cognitive improvements.

How does cord blood have its effect? The investigators provide evidence that the infusion leads to suppression of the CD40-CD40L pathway, a proinflammatory signaling pathway that they showed contributes to amyloid accumulation (see ARF related news story). CD40 is an accessory molecule on microglia and other immune cells that, when activated by its ligand CD40L, stimulates an inflammatory response. In microglia, CD40 stimulation promotes a proinflammatory response to Aβ and inhibits the Aβ phagocytosis. The researchers showed that infusion of cord blood cells led to an inhibition of the CD40-CD40L pathway in AD mice, associated with diminished blood levels of soluble CD40L (sCD40L). Serum Aβ levels were increased, indicating brain-to-blood efflux, which correlated with lowered brain Aβ. The mice also showed a shift in their blood cytokines from a proinflammatory to anti-inflammatory profile, and microglia from the treated mice showed enhanced uptake of Aβ in vitro. The effect of cord blood cells on microglia did not appear to be due to peripheral cells entering the brain, but rather was caused by a soluble factor that could be found in serum from treated mice. Finally, the in vivo anti-inflammatory and Aβ-mobilizing effects depended on CD40, since the infusion had no effect on inflammatory markers or serum Aβ in PS/APP mice lacking CD40.

“We propose that infused HUCBCs exert their effect on reducing cerebral amyloidosis by causing the host to secrete a soluble factor that acts by reducing sCD40L-CD40 interaction on microglia, which then promotes microglial clearance of Aβ,” the authors write. “Taken together, our results provide the basis for a novel immunomodulatory strategy for AD using HUCBCs,” they conclude. HUCBCs have shown promise in preclinical models of neuroinflammatory diseases, including stroke (see ARF related news story) and one previous study in AD mice (Ende et al., 2001 and see review by El-Badri et al., 2006), but whether all these effects are due to immunomodulation or other beneficial actions of the cells remains to be seen.—Pat McCaffrey

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  1. I have a couple of thoughts. First, there is no good evidence, other than correlative, that age-related memory declines in canines are secondary to amyloid accumulation. Rats and mice show age-associated loss of memory without amyloid accumulation. Perhaps the aging-related changes causing memory loss in these mammals also lead to Aβ accumulation in dogs.

    Second, neither dogs nor mice/rats show much neuron loss with age. In AD the neuron loss leads to severe cognitive deterioration requiring institutionalization. Animals in this state would succumb (or, more humanely, be euthanized).

    Third and most importantly, there is a critical difference in the amyloid deposits found in dogs versus Alzheimer humans and APP transgenic mice. They are not senile plaques, but only diffuse deposits (which the authors are careful to indicate in the text, although still referring to the deposits as plaques). The diffuse deposits do not have dystrophic neurites, microglial activation, astrocyte reactivity, or a host of other clear manifestations of pathology. Diffuse deposits appear, histologically, to be quite benign. Hence it is conceivable that clearing them may have little impact on cognitive function, because they are relatively innocuous to begin with.

    The absence of changes on the dot blots with A11 are intriguing, but it would be important to know whether this form of Aβ increases over the lifespan of the dogs and whether this measurement correlates with cognitive function in the dogs (it evidently did not). Lacking these data, it is difficult to put much interpretive zeal into the absence of an effect.

    On the other hand, this may be one more piece of evidence to suggest that removing amyloid alone may not be sufficient to produce memory improvement in AD cases. Data reported by James Nicoll at the NYAS meeting last march showed that despite complete clearance of amyloid, patients from the Phase 1 trials of the Elan vaccine (who received 5-10 inoculations) had no cognitive benefits compared to control AD cases. Although the numbers are very small (two), this suggests that additional approaches, perhaps focusing on neuronal survival and/or tau pathology, may be essential to thwart progression of the disease.

    We all anxiously await the results from the Phase 2 trial of bapineuzumab from Elan-Wyeth to inform us more regarding the role of amyloid in the cognitive symptoms of AD.

    View all comments by Dave Morgan
  2. Comment by Mark A. Smith, Rudy J. Castellani, Hyoung-gon Lee, Akihiko Nunomura, Xiongwei Zhu, and George Perry

    Amyloid-β: The Beginning of the End and the End of the Beginning
    The paper by Head and colleagues (2008) should serve as a major warning for those hoping that removing amyloid will be effective in the treatment of Alzheimer disease (AD) or age-related cognitive impairment. So far, aside from transgenic mice engineered to overproduce amyloid, the record for amyloid immunotherapy has been one of abject failure: 1) trial suspension (human) and 2) no improvement (dogs). At minimum, these studies indicate that our current mouse models of AD are inadequate and, being amyloidocentric, naturally respond to amyloidocentric therapies. By contrast, rather than being driven by a transgene, the amyloid in aging or AD is there for a physiological or pathological reason (Nunomura et al., 2001), and removal of amyloid will not remove these precipitating factors. Moreover, we suspect that the production of amyloid, in response to a primary disease etiology, may serve a beneficial function such that the removal of amyloid would be detrimental and exacerbate any underlying pathological processes (Perry et al., 2000; Rottkamp et al., 2002; Smith et al., 2002; Castellani et al., 2006; Lee et al., 2006a). We previously noted the existence of amyloid spin doctors (Castellani et al., 2007) and wonder whether the ever-increasing inconsistencies in the amyloid hypothesis (Lee et al., 2006b), together with recent therapeutic failures, will finally lead to the realization that it is time to stop spinning. Perhaps the time has come for this to be the beginning of the end for amyloid and the end of the beginning in our quest to understand and treat this disease.

    See also:
    Castellani R, Lee HG, Perry G, Smith MA, Zhu X (2007) ARF Comment: Amyloid spin doctors.

    References:

    . Neuropathology of Alzheimer disease: pathognomonic but not pathogenic. Acta Neuropathol. 2006 Jun;111(6):503-9. PubMed.

    . A two-year study with fibrillar beta-amyloid (Abeta) immunization in aged canines: effects on cognitive function and brain Abeta. J Neurosci. 2008 Apr 2;28(14):3555-66. PubMed.

    . Amyloid-beta vaccination: testing the amyloid hypothesis?: heads we win, tails you lose!. Am J Pathol. 2006 Sep;169(3):738-9. PubMed.

    . Amyloid beta: the alternate hypothesis. Curr Alzheimer Res. 2006 Feb;3(1):75-80. PubMed.

    . Oxidative damage is the earliest event in Alzheimer disease. J Neuropathol Exp Neurol. 2001 Aug;60(8):759-67. PubMed.

    . Amyloid-beta junkies. Lancet. 2000 Feb 26;355(9205):757. PubMed.

    . The state versus amyloid-beta: the trial of the most wanted criminal in Alzheimer disease. Peptides. 2002 Jul;23(7):1333-41. PubMed.

    . Dangers of the amyloid-beta vaccination. Acta Neuropathol. 2002 Jul;104(1):110. PubMed.

  3. We appreciate the thought-provoking comments by Dave Morgan. We would like to clarify some of the features of the canine model and agree conceptually with some of the comments provided.

    We agree with Dr. Morgan that there is a positive correlation between the extent of diffuse Aβ and learning and memory error scores in aged dogs (Cummings, 1996; Head et al., 1998). The current study was originally designed to selectively reduce Aβ in order to test the hypothesis that Aβ causes cognitive dysfunction in aged dogs. It would appear then that reducing Aβ in dogs can have a cognitive benefit over the long term through maintenance of function. However, immediate effects of Aβ removal on cognition appear to be minimal. It is difficult to tease out with the current experimental design whether the cognitive impairments we see when Aβ is already present in old beagles is directly due to the Aβ or the consequence of Aβ being present for extended periods of time (in our case, Aβ can show up in canine prefrontal cortex as young as eight years of age, and thus may have been present in the study dogs for between one and four years (Head et al., 2000).

    Alternatively, as Dr. Morgan points out, cognitive dysfunction in aged dogs may be due to events completely independent of Aβ. The perfect test of the Aβ hypothesis in the canine model is to prevent accumulation of Aβ rather than reverse it. It is also possible that if we combined immunotherapy with an intervention that restores neuron health, and the effects are truly additive, this outcome would provide further proof that Aβ causes both neuronal dysfunction and cognitive impairments.

    As in all other animal models studied to date, there is not the overwhelming neuron loss as occurs in the AD brain. However, we would like to point out that there is evidence of select neuron loss in vulnerable brain regions in the canine. Aged canines exhibit hilar neuron loss in the hippocampus (Siwak-Tapp et al., 2006) and significant atrophy (prefrontal cortex and hippocampus) seen by MRI in association with Aβ and cognition in vulnerable brain regions (Tapp et al., 2004).

    Given the apparent protective effects of immunotherapy and Aβ reduction on frontal function in canines, it is difficult to completely discount diffuse Aβ (without associated tau pathology and glial reaction) as being innocuous. Similar “less worsening” of clinical outcomes was reported in the AN1792 clinical trial (Gilman et al., 2005) along with Aβ reduction but with no effect on neurofibrillary tangle pathology (Nicoll et al., 2003; Ferrer et al., 2004; Masliah et al., 2005). Further, in middle-aged individuals with Down syndrome, a primary pathology is diffuse Aβ with neuritic pathology developing almost a decade later (Hof et al., 1995; Hyman, 1995). If diffuse Aβ is the precursor to downstream pathology (development of neuritic plaques, neurofibrillary tangles, neuron loss, etc.), or makes neurons vulnerable to further insults, then this is the time when we would want to intervene in human clinical trials.

    With respect to our oligomer data, we concur with Dr. Morgan. We have some preliminary evidence to suggest that Aβ oligomers, specifically, do increase with age in the canine temporal cortex but this is obviously an area that needs further work. Measuring oligomers in a system that does not overexpress APP and accumulates endogenous levels of Aβ can be a challenge, although not impossible.

    As is Dr. Morgan, we are also looking forward to what are hopefully going to be positive outcomes from the new clinical trials with immunotherapy. Immunotherapy still is one of the most promising approaches for slowing or potentially halting AD progression. Our work using the canine model illustrates the need to test Aβ-reducing therapies in different animal model systems, each with their own unique advantages, including those that do not overexpress mutant human APP (such as non-human primates), as the outcomes may provide some novel insights into the development of future treatments.

    References:

    . Beta-amyloid accumulation correlates with cognitive dysfunction in the aged canine. Neurobiol Learn Mem. 1996 Jul;66(1):11-23. PubMed.

    . Neuropathology and pathogenesis of encephalitis following amyloid-beta immunization in Alzheimer's disease. Brain Pathol. 2004 Jan;14(1):11-20. PubMed.

    . Clinical effects of Abeta immunization (AN1792) in patients with AD in an interrupted trial. Neurology. 2005 May 10;64(9):1553-62. PubMed.

    . Visual-discrimination learning ability and beta-amyloid accumulation in the dog. Neurobiol Aging. 1998 Sep-Oct;19(5):415-25. PubMed.

    . Region-specific age at onset of beta-amyloid in dogs. Neurobiol Aging. 2000 Jan-Feb;21(1):89-96. PubMed.

    . Age-related distribution of neuropathologic changes in the cerebral cortex of patients with Down's syndrome. Quantitative regional analysis and comparison with Alzheimer's disease. Arch Neurol. 1995 Apr;52(4):379-91. PubMed.

    . Neuropathological changes in Down's syndrome hippocampal formation. Effect of age and apolipoprotein E genotype. Arch Neurol. 1995 Apr;52(4):373-8. PubMed.

    . Abeta vaccination effects on plaque pathology in the absence of encephalitis in Alzheimer disease. Neurology. 2005 Jan 11;64(1):129-31. PubMed.

    . Neuropathology of human Alzheimer disease after immunization with amyloid-beta peptide: a case report. Nat Med. 2003 Apr;9(4):448-52. PubMed.

    . Region specific neuron loss in the aged canine hippocampus is reduced by enrichment. Neurobiol Aging. 2008 Jan;29(1):39-50. PubMed.

    . Frontal lobe volume, function, and beta-amyloid pathology in a canine model of aging. J Neurosci. 2004 Sep 22;24(38):8205-13. PubMed.

    View all comments by Elizabeth Head
  4. The experiments described by Head et al. may not elucidate the role played by senile plaques in AD dementia, but they do bolster the idea that amyloid disruption therapy is likely to be most efficacious when administered early in the course of the disease or provided on a preventative basis.

    The intimate association of vascular disease and AD suggests that in many patients with early signs of dementia, amyloid disruption therapy may be complicated by impaired perfusion and drainage. Such conditions may explain the outcomes observed following the first human vaccination trial in which senile plaques were disrupted, but the amyloid remnants did not exit the brain. That makes the data of Nikolic et al. exciting, as these experiments suggest another means to actually eliminate and/or effectively alleviate the effects of toxic Aβ molecules.

    Successful disruption of amyloid plaques will mitigate and perhaps eliminate the cascade of consequential destruction associated with these deposits. But great uncertainties remain regarding the ultimate risks posed by the chronic disturbance of the immobilized amyloid sequestered within plaques. The relative stress resilience of animal model and human neurons may be quite different, making the extrapolation of laboratory results to clinical outcomes such as improvements in cognitive status a challenging endeavor.

    We cannot yet delineate precise pathological role(s) of Aβ in AD dementia, nor can we assign with certainty a normal-state function for this evolutionarily conserved molecule. Inducing or infusing high titers of antibody in the periphery to effect a CNS compartment change is still in an experimental stage. Present immunization trials employing this stratagem must be supported by further analyses beyond antibody titers and plaque removal.

  5. I'd like to discuss the similarities between data generated in APP/Tg mice vaccinated with an AD vaccine, AN1792 trials, and results reported here in canines by Head and coauthors. Experimental dogs immunized 25 times with a high dose (500 mg/injection/dog) of fibrillar human Aβ42 peptide formulated in Th2-type adjuvant Alum induced high (n = 3), moderate (n = 3), and low (n = 3) titers of anti-Aβ42 antibodies. Although the authors did not detect concentrations of antibodies specific to Aβ42 peptide, they demonstrated that higher titers of these antibodies (which are likely specific to the N-terminus of Aβ42, see [1]) correlated with lower levels of prefrontal insoluble and soluble and insoluble Aβ42 and insoluble Aβ40. These data were somewhat similar to results reported in the AN1792 studies (2) and data generated in APP/Tg 2576 mice immunized with a peptide epitope vaccine (3). This indicates that to be effective, any AD vaccine should induce high titers of antibodies even in elderly people with immunosenescence, without generating autoreactive T cells and microhemorrhages.

    Previously it was shown that the NTB composite z-scores were regressed on the geometric mean antibody titers (2) and the AN1792 vaccine reduced the number of Ab plaques, but not the level of soluble Aβ in brain (4). Interestingly, Head and coauthors reported that reduced levels of Aβ in the prefrontal cortex correlated with cognitive improvement, but vaccination did not reduce the binding of A11 antibodies to Aβ42 species in dot-blot assays.

    Unfortunately, the authors did not show the levels of the different forms of Aβ42 (monomers, oligomers, and fibrils) by Western blot or combination of Western blot and immunoprecipitation, and did not demonstrate directly the association of these forms of β amyloid with memory impairment in aged beagles. This is important, because the lack of a significant therapeutic benefit of the AN1792 vaccine (2,5,6) may be connected to its inability to decrease the levels of soluble (oligomeric) Aβ in the brains (4). Recently the same kind of results were reported when a therapeutic vaccination strategy was tested in APP/Tg 2576 mice immunized with a novel peptide epitope vaccine (3).

    In sum, data from this canine study, results generated with Aβ-immunotherapy of APP/Tg mice (3,7), and new data coming from AN1792 trials (4,8) and data reported by James Nicoll just this month at the NYAS meeting, 2008 suggest that early intervention in the disease process, pre-symptomatic if possible, is likely to be significantly more beneficial than attempting to intervene in the disease process after clinical diagnosis of the disease. Such a protective vaccination strategy may not only be more effective, but also safer than the therapeutic vaccination approach (both active and passive), because it could significantly reduce the probability of adverse events, such as microhemorrhages and subsequent possible infiltration of lymphocytes into the brain.

    The rationale for focusing on the development of protective vaccine for treatment of pre-symptomatic people rather than AD patients is also based on the data demonstrating the pathologic correlates of mild cognitive impairment and early-stage AD. More specifically, Morris and Price (9) have reported that at the time dementia was only minimally apparent clinically, AD was already firmly established histopathologically. Moreover, they reported that entorhinal neuronal number, tissue volume and cortical synaptic integrity decreased notably by the time very mild AD is clinically expressed.

    Recent breakthroughs in the development of biomarkers for AD provide hope that patients can be accurately identified while they are still in the preclinical stages of AD (10-13) which should facilitate delivery of new approaches, including anti-Aβ immunotherapy, before extensive neuronal damage and cerebral amyloid angiopathy has occurred in the brain.

    See also:

    Fagan, A. M., Roe, C. M., Xiong, C., Mintun, M. A., Morris, J. C. & Holtzman, D. M. in 8th International Conference "Alzheimer's and Parkinson's Disease: Progress and New Perspectives" (Salzburg, Austria, 2007).

    References:

    . Immunization with fibrillar Abeta(1-42) in young and aged canines: Antibody generation and characteristics, and effects on CSF and brain Abeta. Vaccine. 2006 Apr 5;24(15):2824-34. PubMed.

    . Clinical effects of Abeta immunization (AN1792) in patients with AD in an interrupted trial. Neurology. 2005 May 10;64(9):1553-62. PubMed.

    . Alzheimer's disease peptide epitope vaccine reduces insoluble but not soluble/oligomeric Abeta species in amyloid precursor protein transgenic mice. J Neurosci. 2007 Nov 14;27(46):12721-31. PubMed.

    . Amyloid-beta peptide remnants in AN-1792-immunized Alzheimer's disease patients: a biochemical analysis. Am J Pathol. 2006 Sep;169(3):1048-63. PubMed.

    . Evaluation of the safety and immunogenicity of synthetic Abeta42 (AN1792) in patients with AD. Neurology. 2005 Jan 11;64(1):94-101. PubMed.

    . Effects of Abeta immunization (AN1792) on MRI measures of cerebral volume in Alzheimer disease. Neurology. 2005 May 10;64(9):1563-72. PubMed.

    . A specific amyloid-beta protein assembly in the brain impairs memory. Nature. 2006 Mar 16;440(7082):352-7. PubMed.

    . The role of the immune system in clearance of Abeta from the brain. Brain Pathol. 2008 Apr;18(2):267-78. PubMed.

    . Pathologic correlates of nondemented aging, mild cognitive impairment, and early-stage Alzheimer's disease. J Mol Neurosci. 2001 Oct;17(2):101-18. PubMed.

    . Cerebrospinal fluid amyloid beta42/phosphorylated tau ratio discriminates between Alzheimer's disease and vascular dementia. J Gerontol A Biol Sci Med Sci. 2006 Jul;61(7):755-8. PubMed.

    . Cerebrospinal fluid tau/beta-amyloid(42) ratio as a prediction of cognitive decline in nondemented older adults. Arch Neurol. 2007 Mar;64(3):343-9. PubMed.

    . Imaging brain amyloid in Alzheimer's disease with Pittsburgh Compound-B. Ann Neurol. 2004 Mar;55(3):306-19. PubMed.

    View all comments by Michael G. Agadjanyan
  6. I want to bring to the attention of Alzforum readers my view that continuing support for the amyloid theory, which currently dominates AD research fields, is unnecessary and should be stopped. I believe this issue should be taken into consideration by scientists as well as the National Institutes of Health. I hope this audience has heard recent news concerning treatment strategies based on the amyloid theory. Vaccination of humans against amyloid-β, able to remove amyloid deposition from the brain, induced brain shrinkage and inflammation at great risk to the patient. I do not think we need to continue down this road.

    In addition, Phase 2 clinical trials with Dimebon indicate that alternate drugs based on the non-amyloid theory have more success. I believe the field should stop animal experimentation for amyloid-based studies, especially the use of dogs, who should not be subjected to this fruitless research. Finally, requesting additional funding to continue research based on the "golden amyloid" theory appears to me to be wasting public money and blocking alternate research and clinical trials that are able to provide necessary tools to fight this devastating disease in the near future.

    View all comments by Gjumrakch Aliev

References

News Citations

  1. Orlando Conference: The Chicken <I>and</I> the Egg
  2. Remote Healing by Stem Cell-Derived GDNF?

Paper Citations

  1. . Human umbilical cord blood cells ameliorate Alzheimer's disease in transgenic mice. J Med. 2001;32(3-4):241-7. PubMed.
  2. . Cord blood mesenchymal stem cells: Potential use in neurological disorders. Stem Cells Dev. 2006 Aug;15(4):497-506. PubMed.

Further Reading

Primary Papers

  1. . A two-year study with fibrillar beta-amyloid (Abeta) immunization in aged canines: effects on cognitive function and brain Abeta. J Neurosci. 2008 Apr 2;28(14):3555-66. PubMed.
  2. . Peripherally administered human umbilical cord blood cells reduce parenchymal and vascular beta-amyloid deposits in Alzheimer mice. Stem Cells Dev. 2008 Jun;17(3):423-39. PubMed.