Science sub-journal: Alzheimer's disease antibody therapy innovation! An Zhiqiang's team developed a quadrivalent TREM2 agonist antibody

*For medical professionals only

Alzheimer's disease (AD), commonly known as Alzheimer's disease, is currently the most common neurodegenerative disease, with approximately 9.

The pathology of AD is characterized by amyloid (Aβ) deposition and abnormal phosphorylation of Tau protein resulting in nerve plaques and nerve fiber entanglements[2

TREM2 is a single transmembrane receptor protein expressed in microglia (an immune phagocyte) whose natural ligands are oligomerized Aβ and phospholipids

At present, there are some studies using the inflammatory antibody of TREM2 to treat AD-mode mice [4–6], but due to the low potency and difficulty in crossing the blood-brain barrier, further antibody engineering and related mechanism research

Recently, the research team of Professor An Zhiqiang and Professor Ningyan Zhang from the University of Texas at the Houston Health Science Center in the United States published the research results of quadrivalent TREM2 agonist antibodies in AD-mode mice in the internationally renowned medical journal Science and Translational Medicine[7].

They screened for an inflammatory monoclonal antibody to TREM2, Ab18, and obtained an Ab18 TVD-Ig with a 100-fold increase in TREM2 activation capacity by swapping the divalent immunoglobulin G1 (IgG1) for quadrivalent variable domain immunoglobulin (TVD-Ig

After injecting Ab18 TVD-Ig/αTfR (once a week) into AD-mode mice, they found that they could significantly enhance the migration of microglia to amyloid plaques and phagocytic ability, reduce Aβ deposition and excessive phosphorylation of tau protein, and improve cognitive function

Overall, this study demonstrates that it is feasible to increase the titer of TREM2 agonist antibodies and the ability of the brain to deliver them through antibody engineering, and can also facilitate the development

Screenshot of the first page of the article

To obtain an inflammatory antibody to TREM2, the researchers first screened the human single-stranded variable fragment (scFv) library shown by the phage and validated it on the human embryonic kidney (HEK)293T cell line expressing TREM2, and finally obtained 8 antibodies
.

TREM2 agonist antibody screening procedure

Since the binding of TREM2 and its natural ligand is critical to the function of microglia, in order not to affect the normal physiological function of microglia, the researchers selected 4 of the receptors that do not compete with the TREM2 natural ligand for further study
.

Of these 4 antibodies, only Ab18 activates TREM2 signaling
in a concentration-dependent manner.

Although Ab18 can activate TREM2, the median effective concentration (EC₅₀) at which it activates TREM2 signaling is 79.
4 nM, which is difficult to reach in the brain because antibodies have difficulty crossing the blood-brain barrier
.

So the researchers next engineered the antibody to increase the activator activity
of Ab18.

It has been reported that raising a divalent antibody to tetravalent (e.
g.
, IgG-scFv form) can improve the antibody's ability to crosslink with the receptor, thereby increasing activity [8].

So they built 6 different forms of tetravalent Ab18 and found that the activity increased by 10-100 times, with Ab18 TVD-Ig having the highest
potency.

Pattern diagram of the engineered Ab18

Next, the researchers examined Ab18 TVD-Ig's ability to engulf oligomerized Aβ-lipids in microglia and found that EC₅₀ was 6.
9 nM, a 33-fold improvement over the original Ab18 IgG, and the migratory ability and viability of microglia were also significantly improved
.

Further research found that quadrivalent antibodies do not affect the content of TREM2 on the cell, but promote TREM2 aggregation
.

Although the concentration range of Ab18 TVD-Ig activation of TREM2 is 1-10 nM, the concentration of periinjected antibodies to reach the brain parenchyma is usually less than 1 nM [9].

Therefore, the researchers hope to further modify the Ab18 TVD-Ig to improve its ability to
cross the blood-brain barrier.

Since the TfR-mediated transcytosis pathway can cross the blood-brain barrier [10], they combined Ab18 TVD-Ig to an antibody (αTfR) against mouse transferrin receptors to obtain a bispecific antibody, Ab18 TVD-Ig/αTfR
.

Structural pattern diagram of bispecific antibodies

They performed intraperitoneal injection of Ab18 TVD-Ig/αTfR into mice at a single injection at a dose of 20 mg/kg, and after 4 hours, the concentration of Ab18 TVD-Ig/αTfR in the brain was already 5 times that of the control antibody, and after 24 hours, it reached 10 times, and then the ratio began to decline, and the difference disappeared
after 7 days.

Due to transcytosis, Ab18 TVD-Ig/αTfR clears faster in plasma than control antibodies, and the researchers calculated the brain/plasma antibody concentration ratio, and found that 24 hours later, Ab18 TVD-Ig/αTfR reached 30 times that of the control antibody, and until 7 days later, it was still 10 times that
of the control antibody.

As a result, Ab18 TVD-Ig/αTfR significantly improves the ability of antibodies to
cross the brain barrier.

So does this antibody work in AD-mode mice?

The researchers injected Ab18 TVD-Ig/αTfR into AD-mode mice (5XFAD) weekly for a total of 14 weeks and found that the strength, number, and size of plaques in the cerebral cortex and hippocampus of mice in the experimental group were significantly reduced
compared with the control group.

Ab18 TVD-Ig/αTfR reduces Aβ deposition in the brain

Because TREM2 can promote microglia migration to plaques and engulf and clear plaques, the researchers examined the effects of Ab18 TVD-Ig/αTfR injection on
microglia.

The experimental results found that Ab18 TVD-Ig/αTfR promotes microglia aggregation near plaques and strengthens the phagocytic activity of microglia without affecting astrocytes
.

So do these phenotypes affect neurons in the brains of AD-mode mice?

Experimental results showed that Ab18 TVD-Ig/αTfR significantly improved the synaptic function of neurons in the brains of AD-mode mice and alleviated neuronal death
caused by AD.

Further, through a series of behavioral experiments, they found that Ab18 TVD-Ig/αTfR could improve anxiety-related behaviors and enhance memory and cognitive function
in AD-mode mice.

Ab18 TVD-Ig/αTfR enhances cognitive function in AD-mode mice

Overall, this study shows that through antibody engineering combined with TfR-mediated brain delivery, multivalent TREM2 agonist antibodies that can pass through the blood-brain barrier can be obtained, which strengthens microglial function, reduces Aβ pathological deposition, and promotes the development of clinically effective therapeutic measures
targeting TREM2.

It is worth noting that AD has a variety of mode mice, such as 5XFAD, PS19, 3xTg-AD, etc.
, the pathological characteristics of different mode mice may not be the same, and the function of TREM2 in the early and late stages of AD may also be different, therefore, in order to achieve the best therapeutic effect, future research may need to focus on the role of TREM2 agonist antibodies in different AD mode mice, as well as the role of AD in different periods of
AD.

In addition, this study only used female mice for the study, and usually female 5XFAD mice have faster pathological progression of Aβ, so future studies should also be conducted on male mice and analyze their differences
in response to antibodies.

It is hoped that further research in the future can bring such therapies into the clinic and bring dawn
to countless AD patients.

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Responsible editor| Zhang Jinxu