Discovery and engineering of an anti-TREM2 antibody to promote amyloid plaque clearance by microglia in 5XFAD mice

Triggering receptor expressed on myeloid cells 2 (TREM2) plays a crucial role in regulating microglial functions and removal of amyloid plaques in Alzheimer's disease (AD). However, therapeutics based on this knowledge have not been developed due to the low antibody brain penetration and weak TREM2 activation. In this study, we engineered a TREM2 bispecific antibody to potently activate TREM2 and enter the brain. To boost TREM2 activation, we increased the valency of bivalent anti-TREM2 Ab2 IgG to tetra-variable domain immunoglobulin (TVD-Ig), thus improving the EC₅₀ of amyloid-β oligomer (oAβ)-lipid microglial phagocytosis by more than 100-fold. Ab2 TVD-Ig treatment also augmented both microglia migration toward oAβ and microglia survival by 100-fold over the bivalent IgG antibody. By targeting the transferrin receptor (TfR), the brain-penetrating Ab2 TVD-Ig/αTfR bispecific antibody realized broad brain parenchyma distribution with a 10-fold increase in brain antibody concentration. Ab2 TVD-Ig/αTfR treatment of 5-month-old 5XFAD mice significantly boosted microglia-plaque interactions and enhanced amyloid plaque phagocytosis by microglia. Thus, potent TREM2 activation by a multivalent agonist antibody coupled with TfR-mediated brain entry can boost microglia clearance of amyloid plaques, which suggests the antibody has potential as an AD treatment.List of abbreviations AD: Alzheimer's disease; Ab: antibody; APOE: apolipoprotein E; Aβ: amyloid beta; BBB: blood-brain barrier; BLI: bio-layer interferometry; CNS: central nervous system; CSF: colony-stimulating factor; CytoD: cytochalasin d; DAM: microglia type associated with neurodegenerative diseases; DAP12: DNAX-activation protein 12; TVD-Ig: tetra-variable domain immunoglobulin; ECD: extracellular domain; ELISA: enzyme-linked immunoassay; ESC: embryonic stem cell; hMGLs: human embryonic stem cell-derived microglia-like lines; IBA1: ionized calcium-binding adaptor molecule 1; ITAM: immunoreceptor tyrosine-based activation motif; KiH: knob-into-hole; NFAT: nuclear factor of activated t-cells; PC: phosphatidylcholine; PK: pharmacokinetics; PS: phosphatidylserine; pSYK: phosphorylated spleen tyrosine kinase; scFv: single-chain variable fragment; SEC: size-exclusion chromatography; sTREM2: soluble triggering receptor expressed on myeloid cells 2; SYK: spleen tyrosine kinase; TfR: transferrin receptor; TREM2: triggering receptor expressed on myeloid cells 2.