Previous studies on Aβ autoantibodies have only muddied the issue of their significance to AD. Using various methods to test blood and cerebrospinal fluid samples, different groups have reported that Aβ antibody titers increase (Nath et al., 2003), decrease (see, e.g., Moir et al., 2005 and ARF related news story), or hardly budge (Hyman et al., 2001) in AD patients relative to healthy people. Wyss-Coray’s team set out to do a more comprehensive analysis looking at sundry forms of Aβ in greater numbers of patients—young and old, healthy or with varying AD severity. “In my opinion, this is the definitive paper because their sample sizes are so substantial, and their methods of measuring the antibodies are as expansive as could be done at this point in time,” said Dave Morgan of the University of South Florida, Tampa, in an interview with ARF.

Using peptide microarrays containing numerous modified and mutated Aβ species, as well as control and other amyloidogenic peptides, first author Markus Britschgi and colleagues analyzed plasma antibodies from 36 cognitively normal people and 75 AD patients. In this screen, and in an independent set of 117 samples (62 normal, 55 AD), Aβ autoantibodies specific to oligomeric, fibrillar and other higher-order Aβ forms were about 12 times as prevalent as antibodies to monomeric Aβ. “This is a very important paper showing that anti-oligomer antibodies naturally occur,” said Michael Agadjanyan of the Institute for Molecular Medicine in Huntington Beach, California.

Many of the study participants had antibodies against pyroglutamate forms of Aβ, which are believed to seed oligomerization (see ARF related news story). Unexpectedly, many individuals had antibodies that recognize mutant forms of Aβ, including Dutch (E22Q), Flemish (A21G), and Arctic (E22G) Aβ1-40 peptides that are only found in familial AD cases. This supports the idea that some antibodies to Aβ, such those raised by Charlie Glabe’s lab at the University of California, Irvine, recognize common three-dimensional structures rather than individual primary peptide sequences (see ARF related news story). Some plasma samples from non-demented individuals also contained antibodies against foreign peptides that are unique to other familial dementias. “People have antibodies against oligomers of the ABri and ADan peptides, for example, but people don’t make these peptide unless they have British or Danish dementia,” Wyss-Coray told ARF. “This is consistent with the concept that cross-reactive oligomer-specific antibodies exist naturally.” As further confirmation of this phenomenon, his team immunized vervet monkeys with full-length Aβ and found that this not only boosted production of anti-Aβ antibodies but also expanded their repertoire of antibodies that cross-react with mutant Aβ and other toxic amyloidogenic peptides.

These findings jibe with more recent work from Glabe’s group, which demonstrated that immune serum from Aβ-immunized rabbits could also recognize islet amyloid in transgenic mouse models of diabetes (Kayed et al., 2007). Glabe’s team went on to show that vaccinating Tg2576 AD mice with different amyloid oligomers improved cognition regardless of the peptide antigen sequence. Similarly, others have reported that mice inoculated with potato virus Y, which has a sequence closely matching the N-terminal region of Aβ, produced antibodies that bind plaques and neurofibrillary tangles in AD brain tissue (Friedland et al., 2008). “The implications of these observations is that it may be possible to develop an effective human vaccine using random peptide sequences that form amyloid oligomers and that this vaccine would avoid the autoinflammatory side effects that doomed earlier vaccines,” Glabe wrote in an email to ARF (see full comment below).

For the time being, Wyss-Coray’s new data leave open a fundamental issue—whether natural Aβ autoantibodies have biological significance. “What would be most satisfying, I guess, is if you found that normal people had really high titers of these antibodies and AD patients were invariably in the lower quartile,” Morgan said. Instead, the study found no difference between plasma antibodies from AD and normal populations. Within the AD group, though, people with moderate to severe AD had lower immunity to oligomeric Aβ1-42 than did patients with mild disease. And in an independent cohort of healthy women, some antibodies from older people (70 or above) had reduced reactivity compared to those from younger (21-44 years) age groups.

To further explore the utility of these natural antibodies, Wyss-Coray and colleagues used their peptide arrays to analyze CSF samples from healthy and mild AD patients. These experiments showed that autoantibodies detected in the periphery were in fact reaching the central nervous system. Antibody reactivity was much lower in CSF than in plasma but had a similar range of specificities to oligomeric and pyroglutamate-modified Aβ forms and to ABri, ADan, and mutant Aβ. In a separate set of analyses, the scientists purified plasma IgGs from three study participants and showed that each prep protected against Aβ toxicity when added to cultured mouse hippocampal neurons. Scientists have seen similar neuroprotection in studies with commercially available intravenous immunoglobulin (IVIg) (Szabo et al., 2008), a mixture of pooled human plasma antibodies that is being tested in a Phase 2 trial of mild to moderate AD (see ARF related conference story).

Based on studies in the current paper, the authors estimate that these natural antibodies exist in human plasma at concentrations around five micrograms per kg body weight. “These concentrations are around 100-1000 times below doses used for passive immunotherapy, but could nevertheless have an impact over a lifetime, or a few years,” Wyss-Coray said, noting a recent review (Lutz et al., 2009) suggesting that low-titer, low-affinity autoantibodies can show functional potency. Ultimately, he said, “it all depends on whether the current trials show any clinical benefit. If the immunotherapy doesn’t work, it’s a moot point to discuss the role of natural autoantibodies.”

To Morgan, the existing data already make a strong case that Aβ titers in unvaccinated human plasma are too low to offer therapeutic benefit. “We know they’re not 100 percent protective because there were people with AD who did have high levels of these antibodies,” he told ARF. He also mentioned several parts of the current study—namely, the primate immunization data, and the antibody reactivities compared with those that recognize common human pathogens—that suggest that natural antibody titers are much lower than the levels normally needed for therapeutic efficacy (see full comment below). Nevertheless, he said “the authors’ conclusion that these could play a role in disease onset and progression is certainly one that merits further analysis and testing.”

Toward this end, Wyss-Corray said his team would love to analyze samples from an active immunization trial to get a closer look at the antibody repertoires of participants who fared better clinically. “Maybe they make antibodies against a specific conformation or post-translational modification of Aβ, and that’s why they had clinical improvement,” he told ARF.—Esther Landhuis.

Reference:
Britschgi M, Olin CE, Johns HT, Takeda-Uchimura Y, Lemieux MC, Rufibach K, Rajadas J, Zhang H, Tomooka B, Robinson WH, Clark CM, Fagan AM, Galasko DR, Holtzman DM, Jutel M, Kaye JA, Lemere CA, Leszek J, Li G, Peskind ER, Quinn JF, Yesavage JA, Ghiso JA, Wyss-Coray T. Neuroprotective natural antibodies to assemblies of amyloidogenic peptides decrease with normal aging and advancing Alzheimer's disease. Proc Natl Acad Sci U S A. 2009 Jul 6. Abstract