Apolipoprotein E4 is the strongest genetic risk for late-onset Alzheimer’s disease, yet efforts to make a therapy against it have not paid off. That may be about to change. David Holtzman and colleagues at Washington University in St. Louis have generated monoclonal antibodies that selectively bind to aggregated ApoE. While ignoring the lipidated forms of ApoE that are abundant in blood and brain, these antibodies reduced amyloid plaques in mice. This clearance seems to require activation of microglia. The findings, reported in the March 30 Journal of Clinical Investigation, suggest that plaque-targeting ApoE immunotherapy might work for AD.
- New antibody binds aggregated, plaque-associated ApoE.
- In mice it stimulates microglia to suppress amyloid accumulation.
- It leaves lipidated ApoE alone.
“It’s exciting that they found an antibody selective for an apparently aggregated form of ApoE,” said Cynthia Lemere, Brigham and Women’s Hospital in Boston. “Previously, targeting ApoE seemed to be a shot in the dark, and one with potentially serious side effects. Targeting what appears to be a more pathologic form seems to be a wiser, safer strategy, and I’m excited to see they’re moving forward,” Lemere told Alzforum. Gary Landreth, Indiana University, Indianapolis, agreed. “The ability to selectively target deposited, non-lipidated forms of ApoE as a means of lowering plaque burden is therapeutically attractive, given the importance of ApoE-mediated lipid transport in the brain,” he wrote to Alzforum (see full comment below).
Apolipoprotein E plays a major role in shuttling fats around the body and cholesterol in brain, and interfering with those functions could have catastrophic consequences. Nevertheless, the E4 isoform accelerates aggregation of Aβ, impedes its clearance, and co-deposits with Aβ, making it an obvious target for a therapeutic. The Holtzman lab previously found that injecting an antibody to mouse ApoE prevented plaque formation, shrank existing plaques, and improved neuronal function and behavior in APP/PS1 mice (May 2014 news).
Live Action. The HAE-4 anti-ApoE antibody (red, left panel) stains amyloid plaques (blue, center) in the brain of a living mouse. Third panel shows merged image. [Courtesy of Liao et al., JCI 2018.]
To see if the same held true for human ApoE, joint first authors Fan Liao and Aimin Li generated four new monoclonal antibodies to recombinant ApoE4, then tested their ability to prevent amyloid deposition in APP/PS1 animals that had been crossed with ApoE knock-in mice. The latter express the human E4 allele in place of mouse ApoE. Liao and colleagues injected the antibodies intraperitoneally into female APPPS1/APOE4 mice once a week for seven weeks, starting when they were two months old. When the mice reached 3.5 months, the researchers measured amyloid deposition. The best of the antibodies, HAE-4, caused a 50 percent reduction in Aβ plaque load compared with an isotype-matched IgG2ab control. The HAE-4 treated mice also had reductions, albeit smaller, in insoluble brain Aβ40 and Aβ42, but no change in plasma or brain ApoE4 levels.
Continuously infusing HAE-4 into the brain reduced plaque load, as well. Given this chronic dosing, the researchers were surprised to find overall ApoE levels held steady. They soon understood why. HAE-4, which recognizes recombinant ApoE3 and E4, did not bind lipidated human ApoE in plasma from E4 or E3 knock-in mice. Instead, the antibody preferentially reacted with aggregated species of non-lipidated ApoeE4 or E3 in vitro, and with ApoE localized to amyloid plaques in the APPS1/APOE4 crosses [see image above].
How does HAE-4 reduce plaques? Intraperitoneal injection of the antibody every three days for two weeks increased the number of cells in the brain expressing CD45, a marker of activated microglia. When the authors used adenovirus to express HAE-4 in the brain with and without its microglia-targeting Fc effector domain, only the intact antibody caused reductions in plaque, fibrillar Aβ, and insoluble Aβ40/42. “We found that for antibodies to work, they need to be able to activate microglia to gobble up the plaques,” said Holtzman. In an accompanying editorial in JCI, David Borchelt, University of Florida, Gainesville, speculated that the antibody may slow or prevent plaque formation by clearing small ApoE/Aβ aggregates that seed plaques.
Mary Jo LaDu of the University of Illinois, Chicago, said the work raises many questions. Most importantly, does the antibody improve behavior? “They show reduced amyloid deposition, which may or may not be bad, and microglial activation, which may or may not be good. But in the end, we need to know if the antibody’s making these mice better,” said LaDu. And although HAE-4 binds to plaques in the mice, LaDu would like to know if it clears them. In Holtzman’s previous work, mouse ApoE antibodies both reduced existing plaque and improved behavior in APP-PS1 mice, but it remains to be seen if human antibodies do the same, he said.
Yadong Huang, Gladstone Institutes, San Francisco, was intrigued by the study. “It offers proof of concept that we can target ApoE for AD,” he said. But he, too, had questions. What epitope does the antibody recognize? And will it bind plaques in the human brain, or ApoE in plasma? Holtzman told Alzforum that HAE-4 binds plaques in human brain sections in the few cases they have examined so far.
Could ApoE antibodies offer advantages over the Aβ antibodies currently in Phase 3 trials? For one, they might have fewer side effects, such as inflammation and microhemorrhages [ARIA], although that remains to be seen, said Holtzman. It is also possible that removing non-lipidated ApoE may have benefits beyond amyloid reduction. For example, Holtzman’s group recently showed that ApoE4 contributes to tau toxicity, independent of its effects on amyloid (Sep 2017 news).
For next steps, Holtzman said they will study other mouse models, comparing HAE-4 and anti-Aβ antibodies head to head to see which can best improve behavior, neuronal and vascular function, and to look for potential side effects. Washington University has licensed the antibodies to Denali Therapeutics of South San Francisco, and Holtzman is collaborating with the company to further characterize them.—Pat McCaffrey
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- Liao F, Li A, Xiong M, Bien-Ly N, Jiang H, Zhang Y, Finn MB, Hoyle R, Keyser J, Lefton KB, Robinson GO, Serrano JR, Silverman AP, Guo JL, Getz J, Henne K, Leyns CE, Gallardo G, Ulrich JD, Sullivan PM, Lerner EP, Hudry E, Sweeney ZK, Dennis MS, Hyman BT, Watts RJ, Holtzman DM. Targeting of nonlipidated, aggregated apoE with antibodies inhibits amyloid accumulation. J Clin Invest. 2018 Mar 30; PubMed.
- Borchelt DR. Targeting the accomplice to thwart the culprit: a new target for the prevention of amyloid deposition. J Clin Invest. 2018 Mar 30; PubMed.