Anti-Aβ immunotherapy is currently one of the leading strategies for AD therapeutic development with several vaccines in human clinical trial
[2, 19]. Numerous pre-clinical studies with mouse models of AD have demonstrated the ability of Aβ vaccination to prevent amyloid deposition in the brain. Thus, active immunization with Aβ
[10, 20], as well as passive immunization
[15–17, 19–27] resulted in a reduction of the amyloid burden in the brain. Importantly, active immunization prevented cognitive decline
[28, 29] and passive immunizations were shown to reverse the memory loss in aged Tg2576 mice
. While these and other studies did not report any adverse events in AD transgenic mouse models immunized with fibrillar Aβ42, data from the AN1792 vaccine trial reported meningoencephalitis in 6% of the patients. These adverse reactions to Aβ42-immunotherapy appear not to be due to the humoral antibody response, but rather to the cell-mediated autoimmune response which induced a Th1 type immune response in patients that received the AN1792 vaccine
[31–37]. Thus, the goal of this pre-clinical study was to investigate the effectiveness of a non human random peptide (oligomer mimic), that induces a Th2-type humoral immune response specific to oligomeric Aβ peptide, but would not be expected to cause Th1 auto inflammatory side effects because the antigen is non-human.
The data presented here indicate that immunization with non-human 3A amyloid random sequence oligomer produces an efficacious immune response in a murine model of AD that is comparable to that of mice immunized with fibrillar Aβ42 and Aβ40 oligomer mimics. We tested escape latency, number of platform crosses in the Morris water maze test (MWM) (which is related to hippocampus), novel object recognition (which is related to cortex) and inhibitory avoidance (which is related to amygdala). It was found that at 14 months, 3A oligomer mimic vaccinated mice have a significant improvement in cognitive function compared to controls. We examined the effect of immunization on the neuropathology in 14 months old mice, and showed a significant reduction in plaques in both cortex and hippocampal region immunized with 3A peptide. The ELISA data shows that the reduction in insoluble Aβ42 is modest (approximately 40%) and mostly absent for Aβ40 in the fibril vaccinated group, while we observed a more robust (approximately 80%) decrease in plaque burden as determined by 6E10 immunoreactivity by IHC. The explanation for this apparent discrepancy is not clear, but it may be due to the fact that the ELISA measures the amount Aß while the immunohistochemistry measures area of the plaque. This may be a reflection of a change of state of the amyloid in the fibril vaccinated group to a more compact or dense state rather than its removal.
These results are consistent with recent findings that vaccination against a random sequence peptide encoded by read through of a stop codon in the pABri mRNA improves cognition and reduces plaques in APP/PS1 mice
. Mice immunized with the ABri random peptide produced antibodies that recognize aggregated Aβ, reduced plaque deposition and improve cognition, similar to the results we report here. However, the immune response to this antigen is broader and includes antibodies that react with neurofibrillary tangles and plaques. Together with our results with the 3A random sequence antigen, these results provide strong support to the concept that targeting generic epitopes associated with Aβ aggregates is therapeutically effective. Small but significant levels of this type of antibodies exist in non-vaccinated humans and the levels of these antibodies are inversely correlated with the incidence of AD
. These conformation-specific, sequence-independent antibodies are the major antibodies against Aβ that exist in normal human plasma
 and these antibodies may account for the reported effectiveness of IVIg in reducing the incidence of AD in humans
. IAPP oligomer and 3A oligomer mimics vaccinated mice showed a relatively low titer as compared to Aβ oligomer and Aβ fibril vaccinated groups, yet retaining their efficacy in preventing cognitive deficits and amyloid deposition. Increasing evidence suggests a role for amyloid oligomers as the toxic species responsible for disease progression and pathogenesis independently of the plaque load
. In this view, our results suggest that effective targeting of toxic species rather than the overall extent of immune response elicited may be relevant. The higher titer observed for either Aβ oligomer and Aβ fibril vaccinated groups may therefore reflect the recognition of a broader range of aggregated conformations.
There is increasing evidence that Aβ oligomers and fibrils are conformationally and structurally diverse
[40–42], which raises the issue of whether the different conformers are differentially associated with pathogenesis
. Because monoclonal antibodies recognize these conformers in a mutually exclusive fashion
[44, 45], a single antibody may not be able to target all of the pathologically significant forms of Aβ. The polyclonal response to active immunization is broad, suggesting that it may be more beneficial in its ability to target more different conformers of Aβ than a single monoclonal
. A potential advantage of vaccination against conformation dependent epitopes is that the resulting antibodies are specific for aggregated forms of Aβ and do not react with Aβ monomer or APP
[13, 40]. This suggests that these antibodies may have a better pharmacological profile than antibodies that recognize APP, which is abundantly expressed or Aβ monomer which is a normal product of APP processing and may have normal roles that should not be interfered with. The lack of reactivity with normal human proteins would also be expected to provide a lower potential for autoimmune complications as well.
We also found that the non-Aβ antigens, 3A and IAPP were associated with a much lower incidence of micro hemorrhage than the Aβ oligomer and Aβ fibril groups. Since the conformation dependent immune response to Aβ oligomer and Aβ fibril antigens are distinct while the immune response to the oligomer mimic antigens is common
[13, 40], this suggests that the increased incidence of micro hemorrhage may be due to sequence specific antibodies that are common to the Aß containing antigens. This reduced incidence of micro hemorrhage may also contribute to a superior safety profile of the random sequence oligomer antigen, like 3A.
These results demonstrate that vaccination against generic amyloid oligomer epitopes is capable of attenuating cognitive impairment and producing a protective immune response without increasing the incidence of micro hemorrhage, suggesting that vaccination against a non-human amyloid oligomer epitope may be a safer therapeutic strategy for developing an effective vaccine that circumvents auto inflammatory immune complications.