Ever since linking loss-of-function mutations in the microglial gene TREM2 to a higher risk of Alzheimer’s disease, scientists have been developing antibodies to boost TREM2 signaling. The latest? A human antibody bestowed with enhanced ability to cross the blood-brain barrier. Kathryn Monroe, Pascal Sanchez, and colleagues at Denali Therapeutics, South San Francisco, report the preclinical characterization of the antibody in the January 12 Nature Neuroscience. It drove TREM2 signaling in cultured human microglia, inducing their proliferation and enhancing their phagocytosis. In a mouse model of amyloidosis, the antibody slid into the brain, increased microglial activity, and boosted brain glucose metabolism. A first Phase 1 trial is ongoing.
- TREM2 antibody sports transferrin receptor binding domain to ease entry into brain.
- It activated microglia and boosted brain glucose metabolism.
- The antibody is in a first Phase 1 trial.
“This is a nifty and elegant approach to get an antibody into the brain by promoting transcytosis,” John Lukens of the University of Virginia, Charlottesville, told Alzforum. Christian Haass of the German Center for Neurodegenerative Diseases in Munich co-authored this work. “The brain shuttle is very important because it allows for dramatically lower antibody concentrations in the periphery, which may decrease the risk of side effects,” Haass wrote to Alzforum.
TREM2 signaling prompts microglia to move, proliferate, and engulf debris such as amyloid plaques. Previously, Haass' group had developed the anti-TREM2 mouse antibody 4D9, which roused microglia to clear amyloid plaques in mice (Mar 2020 news). Haass partnered with Denali to develop a similar antibody against human TREM2 for clinical use that would efficiently slide into the brain. The currently used therapeutic antibodies have a hard time crossing the blood-brain barrier (BBB), such that less than 1 percent of what is infused or injected under the skin gets into the brain.
Co-first authors Bettina van Lengerich, Lihong Zhan, and Dan Xia tapped their brain shuttle program (May 2020 news). It involves hitching a human transferrin receptor binding domain onto cargo of interest, such as an antibody or protein. Transferrin receptors (TfRs) then carry vehicle cum cargo across the BBB. In this case, the scientists inserted the TfR domain, aka antibody transport vehicle (ATV), into the Fc region of 4D9 (see image below). Other companies are trying a similar approach. For example, Roche does it with its brain-shuttle version of gantenerumab (Dec 2021 conference news) and BioArctic collaborates with researchers at Uppsala University, Sweden, on their TfR-based brain transporter technology platform (Hultqvist et al., 2017).
After intravenous injection into 2- to 3-month-old mice expressing human TfR, sixfold more ATV:4D9 than 4D9 reached the mouse brain. More ATV:4D9 than 4D9 ended up in TfR-rich bone marrow as well, but no immune-cell changes were detected after 12 weekly injections.
ATV:4D9 latched onto microglia that expressed Iba-1 and had stubby branches, two signs of reactivity. One day after ATV:4D9 injection, microglia had shifted their gene expression from homeostatic to reactive, upregulating genes involved in oxidative phosphorylation. One week after treatment, freshly isolated microglia had settled back into their homeostatic state and shape.
Confident that the ATV facilitated brain entry as expected, the scientists generated a human counterpart to 4D9. They injected mice and rats with the extracellular domain of human TREM2, isolated antibodies, and chose the one that best activated TREM2, as measured by increased phosphorylation of the downstream kinase Syk. The researchers then inserted the ATV sequence into the Fc region of this bivalent IgG1, which has a different Fab sequence than 4D9. The resulting antibody, dubbed ATV:TREM2, bound the TREM2 extracellular domain near the ADAM17 protease cleavage site, where 4D9 binds.
ATV:TREM2 doubled TREM2 signaling in cultured human macrophages treated with the lipid phosphatidylserine, a physiological ligand for TREM2, as judged by Syk phosphorylation. Curiously, regardless of its BBB shuttle function, the ATV seemed to boost antibody activity. In cultured human kidney cells, ATV:TREM2 worked better than a version without the ATV, and bivalent antibodies outdid their monovalent equivalents.
Even though the 1.4 μM affinity of ATV:TREM2 for TfR dimers on the cell surface pales in comparison to the 2 nM affinity for TREM2, the authors believe that TfR binding helps cluster TREM2, which is known to amplify signaling (see image below). In essence, the ATV turns a bivalent antibody into a tetravalent one. An anti-TREM2 tetravalent antibody was recently reported to boost TREM2 signaling 100-fold (Sep 2022 news).
The More, The Merrier. The monovalent TREM2 antibody (anti-TREM2 MV) did not prompt TREM2 (green) to phosphorylate Syk. The bivalent (anti-TREM2) and ATV:TREM2 monovalent antibody activated TREM2 equally. Surprisingly, bivalent ATV:TREM2 boosted TREM2 signaling even more. [Courtesy of van Lengerich et al., Nature Neuroscience, 2023.]
Indeed, in kidney cells expressing a TREM2/mini biotin ligase chimera in addition to normal TREM2, ATV:TREM2 induced more TREM2 biotinylation than did the antibody sans ATV, suggesting the ATV improved clustering. Immunoprecipitation of TREM2 from the treated cells pulled out TfR, suggesting the two proteins had formed a complex.
Monroe and Sanchez were surprised that the ATV enhanced TREM2 signaling. So was Nimansha Jain of Washington University in St. Louis. “It is very intriguing that the ATV:TREM2-activating antibody enhances the brain exposure and pharmacodynamic microglial responses compared to an anti-TREM2 antibody,” she wrote (comment below). “Moving forward, it will be interesting to see how this increased brain exposure affects Aβ pathology and tau pathology in various mouse models.”
To see how it performed in vivo, the researchers injected ATV:TREM2 into the veins of transgenic mice expressing human TREM2 and human TfR. Compared to the bare TREM2 antibody, more ATV:TREM2 entered the brain. Not only that, it permeated the entire parenchyma, as seen on in vivo SPECT imaging of antibodies labeled with indium-111 (image below).
Accessing the Brain. As seen by SPECT imaging, much more ATV:TREM2 flooded the mouse brain (right) after intravenous injection than anti-TREM2 without the antibody transport vehicle (left). [Courtesy of van Lengerich et al., Nature Neuroscience, 2023.]
ATV:TREM2 increased the number of proliferating microglia as measured by uptake of the modified nucleoside EdU. When taken out of the mice, these microglia engulfed more fluorescently labeled Aβ and myelin than microglia from mice given a control antibody. They also upregulated genes involved in oxidative phosphorylation and glycolysis. Likewise, cultured human microglia given ATV:TREM2 ramped up glucose metabolism.
How would a mouse model of amyloidosis respond to ATV:TREM2? The scientists crossed mice expressing human TREM2/TfR with the 5xFAD line, then injected the antibody into 4.5-month-old offspring. At this age, the mice have widespread amyloid plaques and gliosis. Up to eight days after injection, antibody-treated mice had higher TSPO- and FDG-PET signals in their cortices than did untreated mice, indicating more active microglia and higher brain glucose metabolism, respectively.
The scientists did not report whether ATV:TREM2 cleared plaques. “The question remains whether [antibody-induced changes in microglia are] enough to modify disease in any way,” wrote Renzo Mancuso, VIB-Center for Molecular Neurology, Belgium (comment below).
Lukens noted that the antibody’s effects on tauopathy would be critical, because a TREM2 agonistic antibody was recently shown to worsen tangle load in mice (Oct 2022 news). “We are only scratching the surface of how agonist [TREM2] antibodies may be customized for CNS applications,” wrote Chris Bennett, University of Pennsylvania, Philadelphia (comment below). Sanchez told Alzforum that they plan to test the antibody in a dual amyloid/tau model using the APP-SAA knock-in being developed at Jackson labs.
Denali partners with Takeda to test the antibody, now called DNL919, in a Phase 1 study of 80 healthy people in the Netherlands. Denali's Laura Hansen told Alzforum that results should be in by the end of the year. Sanchez is also interested in exploring whether ATV:TREM2 has additive of synergistic effects with anti-amyloid antibody treatments.—Chelsea Weidman Burke
Mutation Interactive Images Citations
- Paper Alert: Mouse TREM2 Antibody Boosts Microglial Plaque Clean-Up
- Molecular Transport Vehicle Shuttles Therapies into Brain
- Brain Shuttle Could Halve Amount of Gantenerumab Needed
- Potent TREM2 Antibody Stirs Microglia to Prune Plaques in Mice
- In Mice, TREM2 Antibody Mobilizes Microglia, Yet Worsens Tangles
Research Models Citations
- Hultqvist G, Syvänen S, Fang XT, Lannfelt L, Sehlin D. Bivalent Brain Shuttle Increases Antibody Uptake by Monovalent Binding to the Transferrin Receptor. Theranostics. 2017;7(2):308-318. PubMed.
- Potent TREM2 Antibody Stirs Microglia to Prune Plaques in Mice
- In Mice, Activating TREM2 Tempers Plaque Toxicity, not Load
- Can Transferrin Antibodies Safely Open the Blood-Brain Barrier?
- Less Is More: High-Affinity Antibodies Block Blood-Brain Barrier Conduit
- Antibody Ferry Looks Safe in Monkeys, Charts Course for Human Studies
- van Lengerich B, Zhan L, Xia D, Chan D, Joy D, Park JI, Tatarakis D, Calvert M, Hummel S, Lianoglou S, Pizzo ME, Prorok R, Thomsen E, Bartos LM, Beumers P, Capell A, Davis SS, de Weerd L, Dugas JC, Duque J, Earr T, Gadkar K, Giese T, Gill A, Gnörich J, Ha C, Kannuswamy M, Kim DJ, Kunte ST, Kunze LH, Lac D, Lechtenberg K, Leung AW, Liang CC, Lopez I, McQuade P, Modi A, Torres VO, Nguyen HN, Pesämaa I, Propson N, Reich M, Robles-Colmenares Y, Schlepckow K, Slemann L, Solanoy H, Suh JH, Thorne RG, Vieira C, Wind-Mark K, Xiong K, Zuchero YJ, Diaz D, Dennis MS, Huang F, Scearce-Levie K, Watts RJ, Haass C, Lewcock JW, Di Paolo G, Brendel M, Sanchez PE, Monroe KM. A TREM2-activating antibody with a blood-brain barrier transport vehicle enhances microglial metabolism in Alzheimer's disease models. Nat Neurosci. 2023 Jan 12; PubMed.