Early Binding of Anti-Amyloid Antibodies to CAA Drives Complement Activation, Inflammation and ARIA in Mice

Abstract Anti-amyloid antibody treatment for Alzheimer’s disease is linked to Amyloid-Related Imaging Abnormalities (ARIA), including vasogenic edema (ARIA-E) and microhemorrhages (ARIA-H), especially in ApoE ε4/4 carriers. To investigate mechanisms underlying ARIA, we examined the binding and temporal vascular effects of immunization with 3D6, the precursor to the anti-amyloid antibody bapineuzumab, in two aged Alzheimer’s disease amyloid mouse models. Acutely, 3D6 bound to cerebral amyloid angiopathy (CAA), resulting in C1q binding and classical complement activation. Weekly short-term immunization over 7 weeks resulted in elevated CAA- and plaque-associated complement deposition, red blood cell extravasation and microhemorrhages, and was accompanied by significant transcriptomic changes in genes related to complement, inflammation, vascular dysfunction, and endothelial lipid responses. Longer-term dosing over 13-15 weeks further increased complement deposition and was associated with blood-brain barrier disruption, MMP-9 upregulation, and microhemorrhages, accompanied by reduced amyloid burden and modest CAA clearance. C3 levels correlated with microhemorrhage severity. Perivascular macrophages co-localized with complement-decorated CAA in 3D6-treated mice. These findings implicate complement activation as an early key driver of ARIA and suggest that therapeutic targeting of complement may reduce ARIA risk. Competing Interest Statement The authors have declared no competing interest. Abbreviations - 3D6 - Murine IgG2a precursor to bapineuzumab - Aβ - Amyloid-β (protein) - Abca1 - ATP-binding cassette transporter A1 (gene) - AD - Alzheimer’s disease - ANOVA - Analysis of variance - ApoE / APOE - Apolipoprotein E - ApoE ε4 / ε4 - Apolipoprotein E ε4 allele - ARIA - Amyloid-Related Imaging Abnormalities - ARIA-E - ARIA-edema (vasogenic edema or effusion) - ARIA-H - ARIA-hemorrhage (microhemorrhages or superficial siderosis) - BBB - Blood-brain barrier - C1q (C1qa, C1qb, C1qc) - Complement component 1q (and its specific polypeptide chains) - C1r - Complement component 1r (serine protease) - C1s - Complement component 1s (serine protease) - C3 - Complement component 3 - C3a - Complement component 3a (anaphylatoxin) - C3aR - Complement component 3a receptor - C3b / iC3b / C3c - Complement component 3b / inactivated C3b / C3c (cleavage fragments) - C4 / C4b - Complement component 4 / 4b - C4b2b - C3 convertase (classical pathway) - C5 - Complement component 5 - C5a - Complement component 5a (anaphylatoxin) - C5aR - Complement component 5a receptor - C5b - Complement component 5b - C5b-9 - Terminal membrane attack complex (MAC) - CAA - Cerebral amyloid angiopathy - CAA-ri - Cerebral amyloid angiopathy-related inflammation - Cfh - Complement factor H - Ch25h - Cholesterol 25-hydroxylase (gene) - DAPI - 4’,6-diamidino-2-phenylindole (fluorescent DNA stain) - DEGs - Differentially expressed genes - DESeq2 - Differential gene expression analysis tool - df - Degrees of freedom - FACS - Fluorescence-activated cell sorting - Fc - Fragment crystallizable region (of an antibody) - FLAIR - Fluid-attenuated inversion recovery (MRI sequence) - Gfap - Glial fibrillary acidic protein (gene/marker) - Gpnmb - Glycoprotein nonmetastatic melanoma protein B (gene/marker) - H&E - Hematoxylin and eosin (stain) - i.p. - Intraperitoneally (injection method) - IgG2a - Immunoglobulin G subclass 2a - Lcn2 - Lipocalin 2 (gene) - Lgals3 - Galectin-3 (gene) - Lrg1 - Leucine-rich alpha-2-glycoprotein 1 (gene) - Lyz2 - Lysozyme 2 (gene) - mAbs - Monoclonal antibodies - MMP-9 / Mmp12 - Matrix metallopeptidases - MRI - Magnetic resonance imaging - p / padj - P-value / adjusted p-value - PBS - Phosphate-buffered saline - PCA - Principal Component Analysis - qPCR - Quantitative polymerase chain reaction - RBC - Red blood cell - S97 - A specific pan-Aβ antibody - t-PA - Tissue plasminogen activator