Antibodies Against Microglial Receptors TREM2 and CD33 Head to Trials
As neuroinflammation becomes ever more deeply implicated in Alzheimer’s disease, therapies targeting this process are starting to enter clinical trials. At the 14th International Conference on Alzheimer’s and Parkinson’s Diseases, held March 27–31 in Lisbon, Portugal, speakers presented several approaches under investigation. Two groups have generated anti-TREM2 antibodies that they claim activate the receptor and stimulate microglia to remove amyloid. One of these has entered a Phase 1 trial. One company is taking an antibody against CD33 into human studies, while another group is investigating an ApoE antibody in mice. Meanwhile, a small molecule that started out being tried in cardiovascular disease a decade ago turns out to douse inflammation via an epigenetic mechanism. Could it boost cognition in aging?
- Two groups claim to have found antibodies that activate TREM2.
- An antibody against CD33 has entered trials.
- The epigenetic regulator apabetalone dampens neuroinflammation.
It’s too early to tell if any of these approaches will work. Even so, at a time when Alzheimer’s researchers are finally ready to turn the page on NSAIDs (Apr 2019 news), the buzz in Lisbon reflected their eagerness to start minting their intense interest in inflammation into new therapeutic approaches.
Is TREM2 a Viable Therapeutic Target?
First, the poster child for microglial AD risk. The receptor TREM2 prods microglia to police amyloid plaques, and people with high levels of its soluble portion in their CSF decline more slowly at all stages of Alzheimer’s disease (May 2017 news; Part 4 of this series). These findings would appear to make TREM2 an attractive target—except therapies would have to boost it. Activators are generally harder to design than inhibitors.
Now, two groups claim to have done it. One is the biotech company Alector, based in South San Francisco, which generated a monoclonal antibody against TREM2. In Lisbon, Donna Wilcock of the University of Kentucky, Lexington, said the mouse version of the antibody, AL002a, binds and activates TREM2. In cell culture, AL002a treatment increased phosphorylation of Syk, a downstream effector of TREM2 signaling.
Wilcock’s group previously reported efficacy for this antibody in APPPS1 mice (Dec 2016 conference news). In Lisbon, she added new in vivo data. The researchers injected 10 μg of AL002a or IgG isotype control into the frontal cortices and hippocampi of 5-month-old 5XFAD mice. Three days later, immunostaining indicated Aβ levels had been halved, while pro-inflammatory and anti-inflammatory cytokines had spiked, suggesting microglial activation.
The researchers then injected 50 μg/kg AL002a into 4-month-old 5XFAD mice intraperitoneally for 14 weeks. After this, treated transgenics had 50 percent more activated CD11b+ microglia than untreated ones, along with elevated pro-inflammatory and anti-inflammatory cytokines. Treatment doubled the number of microglial cells surrounding amyloid plaques, and halved the amount of Aβ. Treated brains harbored fewer diffuse plaques than untreated, but compact amyloid remained in place, Wilcock said.
What about behavior? Mice receiving AL002a recognized new objects and navigated a radial arm water maze as well as wild-types did; untreated transgenics performed poorly. Future studies will examine if AL002a affects tau pathology, and test it in older mice with more advanced pathology, Wilcock said. In answer to audience questions, she said she has seen no adverse vascular effects so far.
The human version of the antibody, AL002, last November entered its first Phase 1 trial for AD patients, conducted at multiple sites in Australia as well as one in London and one in Orlando, Florida. Researchers will infuse a single dose of AL002 or placebo to 51 healthy adults, stepping up the dose after the prior, lower dose appears safe. If volunteers tolerate these single doses well, 16 people with Alzheimer’s will be randomized to multiple doses of AL002 or placebo. Alector is developing AL002 in collaboration with Abbvie.
The Alector antibody is not the only one to activate TREM2. In Lisbon, Christian Haass of the German Center for Neurodegenerative Diseases in Munich described a similar monoclonal antibody his lab has developed against the mouse receptor. He said his antibody also boosts phospho-Syk signaling in cultured cells in a dose-dependent fashion, and that functional assays support the idea that it activates TREM2 signaling. Treatment helped cultured macrophages survive growth-factor deprivation, Haass said. In primary microglial cultures, the antibody stimulated phagocytosis of myelin debris. Haass presented more data at the Federation of European Neuroscience Societies (FENS) conference on AD, held May 5–8 in Rungstedgaard, Denmark.
Others are developing anti-TREM2 antibodies as well. In a conference poster, Hyun Jung Kim of the Korea Brain Research Institute in Daegu described the generation of two different monoclonal antibodies raised in mouse against purified human TREM2. Both antibodies reportedly immunoprecipitated TREM2 from transfected HEK293 cells, forming a stable complex with the receptor. Each antibody recognized a different region of TREM2. The poster did not describe the functional effects of these proteins.
Anti-TREM2 antibodies could perhaps serve as biomarkers, as well as therapeutics. Researchers led by Silvio Meier of Uppsala University, Sweden, are developing a version that could be used for PET imaging. They conjugated an experimental anti-TREM2 monoclonal antibody, mAb1729, to a moiety that binds the transferrin receptor to enable its uptake into brain. They radiolabeled this complex with iodine 125, then injected these bispecific antibodies into 16-month-old Tg-ArcSwe, Tg-Swe, and wild-type mice. One day later, uptake was about 50 percent higher in transgenic mice than in wild-types. This reflected the higher levels of TREM2 in transgenic brain, the researchers found. The tracer eventually could be used to measure microglial activation in AD brain, they suggested.
Other Ways to Tweak Microglial Activation
Of course microglial activation entails far more than TREM2, and researchers are exploring other targets. For example, Alector just began its first trial of the anti-CD33 antibody AL003. The microglial receptor CD33 opposes the effects of TREM2 signaling, and may make a more amenable target because it would be inhibited rather than activated (see Part 5 of this series). In the first phase of the trial, 42 healthy adults will receive a single treatment of either placebo or one of seven different AL003 doses. The second, multiple-dose phase will enroll 12 patients clinically diagnosed with Alzheimer’s, two of whom will receive placebo. The outcomes are to learn about safety and tolerability, and to determine the highest achievable concentration of AL003 in serum and cerebrospinal fluid. The trial is enrolling in Melbourne, Australia.
ApoE, too, is becoming an intriguing target. Amyloid plaques are loaded with this protein, and much of it seems to come from microglia (Jan 2019 news). David Holtzman, Washington University in St. Louis, previously described an antibody, HAE-4, that binds preferentially to aggregated, nonlipidated ApoE. ApoE4 is poorly lipidated, a feature that seems to promote amyloid pathology (Dec 2011 news; May 2014 news; Part 6 of this series). When the researchers injected HAE-4 into APPPS1 mice expressing human ApoE4, it triggered microglial activation and halved plaque load after six weeks (Apr 2018 news).
In Lisbon, Monica Xiong in Holtzman’s group presented new data on this antibody. She noted that many anti-amyloid approaches cause microhemorrhages and swelling in the brain, called amyloid-related imaging abnormalities, or ARIA, particularly in people who have extensive cerebral amyloid angiopathy (CAA). To find out if HAE-4 has this effect, Xiong and colleagues tested it in 5XFAD mice that have human ApoE4. This model develops mostly CAA with a few parenchymal plaques (Liao et al., 2015). The researchers compared HAE-4 to a chimeric mouse version of aducanumab as well as to control antibody. The chimeric antibody conjugates aducanumab to a mouse Fc domain to enable activation of mouse microglia. They administered 50 mg/kg of each antibody weekly for eight weeks, starting two months after plaques formed. The anti-ApoE antibody reduced plaque load by 40 percent without causing microhemorrhages. Chimeric aducanumab, on the other hand, nudged plaque load down by 20 percent, which was statistically insignificant in this study, and induced microbleeds.
These data suggest ApoE might be a safer target than Aβ itself for reducing plaques, Xiong concluded. Why might this be? Poorly lipidated ApoE4 makes up but a small component of plaques, whereas anti-Aβ antibodies glom on all over them. Xiong suspects the heightened plaque binding by the latter may excessively trigger microglia and astrocytes, leading to neuroinflammation and microhemorrhages. She is currently testing this theory. HAE-4 was licensed to the South San Francisco-based biotech company Denali Therapeutics (see FierceBiotech news; April 2018 news), but is no longer being developed there, according to Joe Lewcock of Denali.
Mechanism of Action. BET proteins bind acetylated lysines (ac) in histone and recruit transcription factors (TF); apabetalone (yellow) competes for this binding site, preventing transcription. [Courtesy of Ewelina Kulikowski.]
An Epigenetic Way to Douse Inflammation
Some therapeutic approaches come from other areas of research, but unexpectedly turn out to have neuroinflammatory effects. Take apabetalone (RVX-208), an investigational small-molecule drug that binds and inhibits bromodomain and extra-terminal (BET) proteins. BET proteins normally recognize acetylated lysines in histones, and then recruit other transcription factors in order to regulate gene expression. Because apabetalone competes for acetylated histone binding within the BET binding region, it prevents BET proteins from fastening to chromatin. Apabetalone was developed by the biotech company Resverlogix, based in Calgary, Canada, and company researchers have reported various treatment benefits for it. According to published papers, it pumps up expression of ApoA-1, the main component of HDL or “good” cholesterol, prevents calcification of blood vessels, and suppresses gene expression pathways associated with kidney disease (McLure et al., 2013; Wasiak et al., 2018; Gilham et al., 2019).
Resverlogix has tested apabetalone for numerous chronic diseases. The furthest advanced is the Phase 3 BETonMACE trial, which enrolls 2,425 high-risk cardiovascular patients with diabetes and low HDL. Apabetalone is also entering a Phase 2a trial of dialysis patients with end-stage kidney disease, and starting a pilot study for pulmonary arterial hypertension.
In these studies, as well as in preclinical mouse work, the researchers noticed that apabetalone calmed peripheral inflammation. Ewelina Kulikowski and colleagues at Resverlogix found that apabetalone turns down expression of complement proteins, part of the innate immune system (Wasiak et al., 2017). This observation led them to test the molecule in cellular and mouse models of neuroinflammation.
In Lisbon, Kulikowski reported that apabetalone inhibits expression of adhesion proteins by endothelial cells, preventing monocytes from sticking to them. This is a crucial step for monocytes to infiltrate into brain (Jul 2018 conference news). In a microglial cell line stimulated with lipopolysaccharide and IFNγ, apabetalone suppressed expression of pro-inflammatory cytokines such as IL-6 and IL-1β, along with complement proteins C3 and C1q. In a wild-type mouse injected intraperitoneally with the inflammatory agent LPS, seven days of treatment with 150 mg/kg apabetalone halved expression of the endothelial inflammation markers E-selectin and ICAM, as well as macrophage and microglial markers CCR2 and CD68. The researchers did not present behavioral data, or experiments in AD mouse models.
Resverlogix researchers wondered if apabetalone treatment could bolster cognition in aging. To test this idea, they decided to include a cognitive substudy in the BETonMACE trial. Jeffrey Cummings of the Cleveland Clinic Lou Ruvo Center for Brain Health in Las Vegas, an academic consultant to the study, described the experimental paradigm in Lisbon. BETonMACE participants receive 100 mg apabetalone daily for about two years. The primary endpoint is the time until a cardiovascular event such as stroke, heart attack, or death, and the trial continues until 250 such events have occurred. The drug will be deemed a success if it significantly delays adverse cardiac events.
In the cognitive substudy, 467 participants age 70 or older take the Montreal Cognitive Assessment (MoCA) at baseline, every year thereafter, and at study termination. The researchers will compare change from baseline in people on apabetalone and placebo to look for a slowing of cognitive decline. A MoCA of 26 or higher is considered normal cognition, and the average MoCA score at baseline for the whole cohort was 25, Cummings said in Lisbon. Researchers will also analyze subgroups of people who started the trial with mild cognitive impairment; among the 246 people who scored below 26 at baseline, the average MoCA was 22. Topline data from the study are expected this summer.—Madolyn Bowman Rogers
- Closing the Book on NSAIDs for Alzheimer’s Prevention
- Paper Alert: TREM2 Crucial for Microglial Activation
- Parsing How Alzheimer’s Genetic Risk Works Through Microglia
- Inflammation Helps Microglia Clear Amyloid from AD Brains
- Could CD33 Be the Microglial Target for Stimulating Phagocytosis?
- Without TREM2, Plaques Grow Fast in Mice, Have Less ApoE
- Lowering ApoE Brings Down Amyloid in Mice
- Has ApoE’s Time Come as a Therapeutic Target?
- Could Greasing the Wheels of Lipid Processing Treat Alzheimer’s?
- Human ApoE Antibody Nips Mouse Amyloid in the Bud
- VCAM1: Gateway to the Aging Brain?
Research Models Citations
- Liao F, Zhang TJ, Jiang H, Lefton KB, Robinson GO, Vassar R, Sullivan PM, Holtzman DM. Murine versus human apolipoprotein E4: differential facilitation of and co-localization in cerebral amyloid angiopathy and amyloid plaques in APP transgenic mouse models. Acta Neuropathol Commun. 2015 Nov 10;3:70. PubMed.
- McLure KG, Gesner EM, Tsujikawa L, Kharenko OA, Attwell S, Campeau E, Wasiak S, Stein A, White A, Fontano E, Suto RK, Wong NC, Wagner GS, Hansen HC, Young PR. RVX-208, an inducer of ApoA-I in humans, is a BET bromodomain antagonist. PLoS One. 2013;8(12):e83190. Epub 2013 Dec 31 PubMed.
- Wasiak S, Tsujikawa LM, Halliday C, Stotz SC, Gilham D, Jahagirdar R, Kalantar-Zadeh K, Robson R, Sweeney M, Johansson JO, Wong NC, Kulikowski E. Benefit of Apabetalone on Plasma Proteins in Renal Disease. Kidney Int Rep. 2018 May;3(3):711-721. Epub 2017 Dec 8 PubMed.
- Gilham D, Tsujikawa LM, Sarsons CD, Halliday C, Wasiak S, Stotz SC, Jahagirdar R, Sweeney M, Johansson JO, Wong NC, Kalantar-Zadeh K, Kulikowski E. Apabetalone downregulates factors and pathways associated with vascular calcification. Atherosclerosis. 2019 Jan;280:75-84. Epub 2018 Nov 14 PubMed.
- Wasiak S, Gilham D, Tsujikawa LM, Halliday C, Calosing C, Jahagirdar R, Johansson J, Sweeney M, Wong NC, Kulikowski E. Downregulation of the Complement Cascade In Vitro, in Mice and in Patients with Cardiovascular Disease by the BET Protein Inhibitor Apabetalone (RVX-208). J Cardiovasc Transl Res. 2017 Aug;10(4):337-347. Epub 2017 May 31 PubMed.
- Cut Loose, Soluble TREM2 Beckons Microglia to Mop Up Plaques
- Clotting Protein from Blood Incites Microglia, and Synapses Die
- ApoE Binds Complement Protein, Keeps Inflammatory Cascade in Check
- ApoE: Common Microglial Culprit in Aging, Alzheimer’s, and Tauopathy?
- TREM2 Binds Aβ, Reprograms Microglia to Curb Plaques
McGill University Faculty of Medicine
My colleagues and I are encouraged by attention directed at modifiers of innate immune activity for treatment of AD. However, we urge caution in the application of anti-immune (including anti-TREM2) treatments in humans. The proposed Phase 1 work should proceed with awareness of the differences in pathogenetic mechanisms in mouse transgenic "AD" and human sporadic disease.
Recent data, some unpublished, suggest that CNS immune activity may modify not only the development of human AD pathology (Meyer et al., 2018; Suárez-Calvet et al., 2019), but also the symptomatic expression of such pathology (Meyer et al. in “CSF proteins suggest biological pathways for symptomatic resilience to AD pathology,” a talk presented at AD/PD 2019, Lisbon, Portugal). In some instances at least, innate immune activation may improve, and not impair, cognitive performance. Thus, the RH47 TREM-2 mutation, which is a potent risk factor for expression of AD dementia, incurs loss—not gain—of TREM2 function. The forthcoming trials should monitor carefully for possible deleterious effects of anti-TREM2 interventions.
Meyer PF, Savard M, Poirier J, Labonté A, Rosa-Neto P, Weitz TM, Town T, Breitner J, Alzheimer’s Disease Neuroimaging Initiative, PREVENT-AD Research Group. Bi-directional Association of Cerebrospinal Fluid Immune Markers with Stage of Alzheimer's Disease Pathogenesis. J Alzheimers Dis. 2018;63(2):577-590. PubMed.
Suárez-Calvet M, Morenas-Rodríguez E, Kleinberger G, Schlepckow K, Araque Caballero MÁ, Franzmeier N, Capell A, Fellerer K, Nuscher B, Eren E, Levin J, Deming Y, Piccio L, Karch CM, Cruchaga C, Shaw LM, Trojanowski JQ, Weiner M, Ewers M, Haass C, Alzheimer’s Disease Neuroimaging Initiative. Early increase of CSF sTREM2 in Alzheimer's disease is associated with tau related-neurodegeneration but not with amyloid-β pathology. Mol Neurodegener. 2019 Jan 10;14(1):1. PubMed.
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