9 April 2002. The first clinical trial may be dead, but AD vaccination research is alive and kicking. Yesterday, researchers led by Steven Paul at Lilly Research Laboratories in Indianapolis, reported in the online Nature Neuroscience that a single shot of an Aβ-binding antibody reversed memory deficits in transgenic mice even while these mice's brains contained copious amyloid plaques.
At first glance, the study is surprising. For one, it appears paradoxical that memory would improve overnight in a model of a disease thought to be due to a decade-long pathologic process that eventually kills populations of neurons and thus robs the brain of its abilities. For another, the study runs counter to the prevailing notion that amyloid deposits are the chief cause of neural toxicity, instead lending support to a growing view that a changeable, soluble pool of non-fibrillar Aβ may do some damage, as well. Finally, it suggests functional improvement might be gained from limited applications of passive immunization, a type of therapy not often used for chronic diseases.
The scientists, working with colleagues at Washington University School of Medicine in St. Louis, Missouri, and elsewhere, extended their previous studies of the m266 monoclonal antibody in PDAPP transgenic mice, which had shown that repeated immunization over five months could lower brain Aβ levels by acting as a peripheral "sink" (see ARF news item). In today's paper, first author Jean-Cosme Dodart et al. treated 2 year-old PDAPP mice with m266 for six weeks and found that aged old mice performed almost as well in an object recognition memory task as did young wild-type control mice, even though they had as many Aβ deposits in their hippocampus and cerebral cortex as did untreated PDAPP mice, which perform poorly in this test (Dodart et al, 1999).
Then the authors injected a single dose of m266 into middle-aged PDAPP mice and tested them the next day. Again, the treated PDAPP mice performed almost as well as did age-matched wild-type controls. Roughly the same was true with the holeboard task, a learning test. The effect was dose-dependent, as was the concentration of Aβ peptides detected in plasma. The scientists detected complexes of Aβ bound to antibody in all plasma samples but only in cerebrospinal fluid of mice treated with the highest doses of m266.
The Dodart et al. paper appears to contradict previous research on active Aβ immunization studies, which tended to report that behavioral improvement occurred along with a reduced amyloid burden. The study does not settle the question whether these antibodies work from the periphery or by entering the brain.
This study does, however, bolster a line of evidence arguing that soluble Aβ may be as neurotoxic as plaques, though in a different way. For example, one study found that soluble Aβ levels, measured in postmortem brains, could distinguish between people who had plaque pathology but no dementia and those who also had clinical dementia (Lue et al. 1999). Another study found that soluble Aβ levels correlate better with disease severity than do plaque levels (McLean et al, 1999), and a recent paper implicated soluble Aβ, but not plaques, in spatial learning deficits in another AβPP-transgenic mouse strain (Koistinaho et al. 2001.) A paper in the current issue of Nature reports that soluble Aβ oligomers interfere with long-term potentiation in rats (see ARF news item) and another one recently reported similar data in rat hippocampal slices (Wang et al. 2002). Finally, Bill Klein and others at Northwestern University Medical School reported Aβ oligomers, or ADDLs, cause neurological dysfunction in the hippocampus long before neurons degenerate (Lambert et al. 1998)
It is fascinating to wonder how, mechanistically, such marked improvement in memory can occur overnight. Moreover, it remains to be seen if any of this effect-seen in mice whose abnormal production of human AβPP is not generally accompanied by massive neuronal death-can be replicated in human patients, who do lose significant populations of hippocampal and entorhinal cortex neurons before they notice the first symptoms. As famed angiogenesis researcher Judah Folkman likes to quip: "If you are a mouse, we can take good care of you."-Gabrielle Strobel.
Q & A with Steven Paul-Posted 9 April 2002
Dodart JD, Bales K, Gannon KS, Greene S, DeMattos RB, Mathis C, DeLong C, Wu S, Holtzman DM, Paul SM. Immunization reverses memory deficits without reducing brain Aβ burden in Alzheimer's disease model. Nature Neuroscience 2002, 8 April. Abstract
Q: Memory improvement after one day at first blush seems paradoxical when one thinks of AD as a slow, degenerative process that eventually kills off populations of neurons. How does it work?
A: This transgenic mouse model does not develop a lot of neurodegeneration. The difference between the mouse model and human AD is that while the mouse develops abundant neuritic plaques, we do not see frank loss of neurons, certainly not the massive neurodegeneration that occurs in late-stage human AD. The soluble forms of Aβ are toxic to some forms of memory; we only tested two learning/memory tasks. The antibody can somehow acutely block those toxic effects on whatever biological processes underlie those forms of memory.
Last week, Walsh et al. showed that small oligomers of Aβ were particularly effective in blocking LTP, suggesting again, much like our data, that these small oligomers may themselves disrupt a molecular substrate for memory formation (see related ARF news items).
This antibody also prevents the deposition of amyloid plaques (see related ARF news items).
Q: So there must be a separate mechanism from what is generally thought, namely that Aβ ultimately induces cell death by forming fibrils and aggregating?
A: It would be different, yes. That highlights the complexity of how this little peptide might contribute to the pathogenesis of AD. I suspect that if this paper were to have any relevance to human AD, it would be very early in the disease-perhaps when people are having mild cognitive impairment-that the peptide is disrupting memory, quite apart from its effect of depositing aggregate-forming fibrils and killing neurons.
Q: How long does the one-shot improvement last? Did you test again after a week, or a month?
A: We have not tested that yet. I expect it may last as long as the antibody is interacting with the peptide, and that probably relates to the half-life of the antibody in the plasma or brain, which is a few days.
Q: Dominic Walsh and Bill Klein have antibodies for non-fibrillar species of Aβ. How about testing if their levels correlate with cognitive function in these PDAPP mice?
A: We have not done so, that would be a worthwhile experiment.
Q: What exactly does m266 bind? Monomers, oligomers, protofibrils?
A: We know it binds to monomeric Aβ but don't know what other forms it may recognize. It could bind to other low-n oligomers, as well. We do not seem to be able to decorate plaques with this antibody in vivo, so it does not bind well to more complex, aggregated forms of Aβ.
Q: Some of the results of other vaccination studies were contradictory because they varied with the genetic background of the strains used. Is yours a PDAPP-specific effect, and have you tried it on other strains?
A: We have not tried it on other strains yet. How well this can be extrapolated to other mouse strains is not clear at this point. How it will translate to humans is even less clear.
Q: With regard to treating humans, eventually, how much of a risk with passive immunization is serum sickness, and does this infrequent vaccination scheme encourage you because it raises the specter that AD could be treated more like an acute, not a chronic, disease.
A: I do not think serum sickness is not going to be a huge problem at the doses given in this disease. You would not be giving huge doses like we do against virus infections. We have been giving humanized antibodies safely now for a variety of conditions. In light of the Elan data I think the question becomes, can an antibody to the peptide be safe without producing an "inflammatory" response? We do not know the answer to that, but with a well-defined antibody recognizing a well-defined epitope, you improve your chance of avoiding cellular immunity, which is what happens when you actively immunize with peptide.
Q: A T cell response is said to have occurred in the Elan trial.
A: Right. Whether that response caused the problems I do not know.
Q: Finally, papers are always a step behind the real research front. Where is this in terms of future clinical trials?
A: We are continuing to explore it as a possible avenue. But there are still so many complexities, including making sure we have the right antibody that would be safe and effective in the animal model. All the preclinical toxicology work still needs to be done, so we are a ways off from being able to test this in humans. Whether human trials will occur will depend on the results of a whole series of tests that still need to be done. But I think the general strategy of immunotherapy is still interesting.
Q: Will this paper give a boost to those who believe soluble Aβ deserves more attention?
A: Let me emphasize that we are in no way saying that deposited forms of Aβ that are fibrillized into amyloid plaques are harmless. We are quite certain that that will disrupt memory, too. We are just saying that the soluble form can be damaging, as well. We like the fact that this antibody, when administered chronically, also reduces deposited Aβ as well.-Steven Paul, Lilly Research Laboratories, Indianapolis.