Background:
Alzheimer's disease (AD) is the most common neurodegenerative disease in the elderly, leading to progressive cognitive impairment and ultimately death. AD pathological hallmarks include extracellular amyloid-β (Aβ) aggregates, intracellular Tau neurofibrillary tangles, and neuronal cell death.
(Data source: Shajahan SR, et al. Front Aging Neurosci. 2024)
Microglia are brain-resident macrophages that respond to AD pathology, acquiring specific transcriptional and functional states. The microglial response to AD and other brain pathologies is controlled by a balance of activating and inhibitory receptors. Among these activating receptors is TREM2, which plays a key role in promoting the clearance of pathological Aβ. Alector 's TREM2 monoclonal antibody, AL002, has reached Phase II clinical trials.
(Data source: Alector official website)
LILRB4 (also known as ILT3) is an inhibitory receptor expressed on monocytes, macrophages, conventional dendritic cells, and plasmacytoid dendritic cells. It inhibits the ability of microglia to clear amyloid-β plaques, and gain-of-function mutations in this immunosuppressive receptor are associated with an increased risk of Alzheimer's disease . On April 3, 2024, Jinchao Hou et al. published an article in Science Translational Medicine titled, "Antibody-mediated targeting of human microglial leukocyte Ig-like receptor B4 attenuates amyloid pathology in a mouse model," investigating the therapeutic potential of targeting human leukocyte Ig-like receptor B4 ( LILRB4 ). LILRB4 inhibits microglial phagocytosis and is highly expressed in microglia from patients with Alzheimer's disease. Expression of human LILRB4 in an AD mouse model reduced the association between microglia and amyloid plaques and increased amyloid pathology. These effects were reversed by treatment with a LILRB4-specific antibody, suggesting LILRB4 as a potential therapeutic target for Alzheimer's disease.
LILRB4 is highly expressed in the brain tissue of AD patients
LILRB4 expression is associated with the pathological process of AD, with its expression level positively correlated with the content of phosphorylated Tau protein (pTau). Furthermore, LILRB4 expression is strongly correlated with the expression level of APOE, a marker of microglial activation. Therefore, researchers believe that LILRB4 plays a role in the pathogenesis of AD, possibly related to the pathological process and the activation state of microglial cells.
LILRB4 overexpression mouse model
Using the transgenic mouse model ILT-Telo Tg x 5XFAD that expresses the human LILRB4 receptor, the research team further confirmed that LILRB4 is abundantly expressed in microglia and that LILRB4 inhibits the ability of microglia to clear beta-amyloid plaques.
Anti-LILRB4 monoclonal antibody development
The research team constructed an anti-human LILRB4 monoclonal antibody, ZM3.1, with a mutation in the Fc region. This antibody undergoes mutations in the Fc fragment to eliminate binding to FcRs and its effector function, while maintaining its ability to bind to LILRB4. ELISA analysis revealed that the LILRB4 antibody has a high affinity for the human LILRB4 protein and can specifically block LILRB4 pathway activation. Using labeling for tracking, the researchers confirmed that the anti-LILRB4 monoclonal antibody ZM3.1 is specifically retained in the brain of the ILT-Telo Tg x 5XFAD mouse model.
LILRB4 -blocking antibody alleviates amyloid pathology
Treatment of 4-month-old ILT-Telo Tg x 5XFAD mice with anti-LILRB4 reduced the total area and number of Aβ plaques in the brain, particularly in the cortex and hippocampus. BACE1+ regions, associated with neuronal degeneration around Aβ plaques, were also reduced, mitigating Aβ-induced synaptic dystrophy , confirming that anti-LILRB4 can alleviate Aβ burden and neuronal dystrophy.
LILRB4 -blocking antibodies promote microglial activation and Aβ phagocytosis
The researchers found that the effects of LILRB4 -blocking antibodies on microglial phenotype occurred exclusively through human LILRB4 signaling, which improved microglial aggregation, enhanced microglial activation and Aβ phagocytosis, and inhibited IFN-dependent pathways.
LILRB4 antibody blocking mechanism:
Computer simulation predicted the potential binding site between human LILRB4 and ApoE, and further designed four different mutant recombinant LILRB4 proteins, demonstrating that the binding site between mApoE and LILRB4 is located between the D1 - D2 domains between the two Ig-like domains of LILRB4. Anti-LILRB4 antibodies competitively bind to the D1-D2 region between the two Ig-like domains of the LILRB4 extracellular region, blocking the binding of ApoE and LILRB4 and inhibiting the LILRB4-mediated immunosuppressive signal.
Summary
This study used an anti-human LILRB4 monoclonal antibody to treat a mouse model of Alzheimer's disease (AD). The results showed that the anti-human LILRB4 monoclonal antibody reduced Aβ deposition, improved Aβ-related behavioral abnormalities, enhanced microglial activity, and attenuated interferon-induced gene expression. Therefore, LILRB4 -blocking antibodies may be a potential therapeutic approach for Alzheimer's disease.
