Intracerebroventricular amyloid-beta antibodies reduce cerebral amyloid angiopathy and associated micro-hemorrhages in aged Tg2576 mice

Abstract

Although immunization against amyloid-beta (Abeta) holds promise as a disease-modifying therapy for Alzheimer disease (AD), it is associated with an undesirable accumulation of amyloid in the cerebrovasculature [i.e., cerebral amyloid angiopathy (CAA)] and a heightened risk of micro-hemorrhages. The central and peripheral mechanisms postulated to modulate amyloid with anti-Abeta immunotherapy remain largely elusive. Here, we compared the effects of prolonged intracerebroventricular (i.c.v.) versus systemic delivery of anti-Abeta antibodies on the behavioral and pathological changes in an aged Tg2576 mouse model of AD. Prolonged i.c.v. infusions of anti-Abeta antibodies dose-dependently reduced the parenchymal plaque burden, astrogliosis, and dystrophic neurites at doses 10- to 50-fold lower than used with systemic delivery of the same antibody. Both i.c.v. and systemic anti-Abeta antibodies reversed the behavioral impairment in contextual fear conditioning. More importantly, unlike systemically delivered anti-Abeta antibodies that aggravated vascular pathology, i.c.v.-infused antibodies globally reduced CAA and associated micro-hemorrhages. We present data suggesting that the divergent effects of i.c.v.-delivered anti-Abeta antibodies result from gradually engaging the local (i.e., central) mechanisms for amyloid clearance, distinct from the mechanisms engaged by high doses of anti-Abeta antibodies that circulate in the vasculature following systemic delivery. With robust efficacy in reversing AD-related pathology and an unexpected benefit in reducing CAA and associated micro-hemorrhages, i.c.v.-targeted passive immunotherapy offers a promising therapeutic approach for the long-term management of AD.

Fig. 1.

Behavioral improvement upon icv infusion of low-dose anti-Aβ IgG₁ or systemic delivery of a relatively high dose of the same IgG in aged Tg2576 mice. Aged (16–18 months) Tg2576 mice received control IgG₁ (white bars) or 6E10 (gray bars) via systemic or icv routes as indicated. Numbers within bars denote the total IgG₁ dose in milligrams. Relative to WT, the transgenic mice treated with either systemic or icv control IgG₁ display a substantial deficit (P < 0.01) in contextual memory, as measured by their freezing response to contextual fear conditioning (context previously paired with an aversive foot-shock stimulus). Prolonged systemic or icv treatment with 6E10 significantly reversed this deficit in the Tg2576 mice. *, P < 0.05, **, P < 0.01; one-way ANOVA followed by Tukey post-hoc test.

Fig. 2.

Decrease in parenchymal plaques following prolonged icv or systemic treatment with anti-Aβ IgG₁ in aged Tg2576 mice. Transgenic mice received control IgG₁ (white bars) or 6E10 (gray bars) via systemic or icv routes. Numbers within bars denote the total dose in milligrams. Quantitative analysis of the percent cortical(A) or hippocampal (B) area occupied by parenchymal plaques demonstrated a decrease in the systemic as well as icv 6E10 groups; the latter was dose-dependent (P < 0.001). Analysis involved 18 to 20 serial sections per mouse for the cerebral cortex, and 7 to 9 serial sections per mouse for the hippocampus, with 8 mice per treatment group. Low-magnification (×4) images of brain sections demonstrate parenchymal plaque burden in the cerebral cortex and hippocampus of a WT mouse (C) or Tg2576 mouse treated with systemic control IgG₁ (2 mg; D), systemic 6E10 (2 mg; E), icv control IgG₁ (0.2 mg; F), icv 6E10 (0.04 mg; G), or icv 6E10 (0.2 mg; H). Parenchymal plaques are stained black against the light brown-orange myelin staining. **, P < 0.01, ***, P < 0.001; 2-tailed t test (systemic groups) or one-way ANOVA with Tukey post-hoc test (icv groups).

Fig. 3.

Contrasting effects of icv versus systemic anti-Aβ IgG₁ on CAA in aged Tg2576 mice. Mice chronically received control IgG₁ (white bars) or 6E10 (gray bars) via systemic or icv routes. Numbers within bars denote the total dose in milligrams. Quantitative analysis of the percent cortical (A) or hippocampal (B) area occupied by CAA revealed an increase with systemic 6E10 treatment but a decrease with icv 6E10 treatment. Images represent CAA in the cerebral cortex of Tg2576 mice treated with systemic control IgG₁ (C), systemic 6E10 (D), or icv 6E10 (0.2 mg; E). Arrows mark the distinct stain for CAA compared with the parenchymal plaques (black stain). (F) The high-magnification image shows multifocal, circumferential bands of amyloid deposited in a blood vessel. *, P < 0.05, **, P < 0.01, ***, P < 0.001; 2-tailed t test (systemic groups) or one-way ANOVA with Tukey post-hoc test (icv groups). (Scale bars, C–E, 200 μm; F, 50 μm.)

Fig. 4.

Central and/or peripheral actions of icv versus systemically delivered anti-Aβ IgG₁. Tg2576 mice received control IgG₁ (white bars) or 6E10 (gray bars) via systemic or icv routes. Numbers within or above bars denote the total dose in milligrams. Images represent IgG₁ binding to amyloid plaques after systemic or icv delivery of 6E10 (B) but not control IgG₁ (A), as revealed by the HRP immunohistochemical product for antibody counterstained with Congo red. (C) Treatment-induced changes in microglial activation around plaques was analyzed as the ratio of percent cortical area occupied by active microglia to the percent cortical area occupied by plaques. Both systemic and icv 6E10 significantly induced the activation of microglia around plaques. Images represent microglial induction in the hippocampal CA1 area of the Tg2576 mice that received icv infusions of control IgG₁ (D) or 6E10 (0.2 mg; E). (F) ELISA revealed an increase in plasma Aβ with systemic, but not icv, delivery of 6E10. *, P < 0.05, **, P < 0.01, ***, P < 0.001; 2-tailed t test (systemic groups) or one-way ANOVA with Tukey post-hoc test (icv groups). (Scale bars: A and B, 50 μm; D and E, 200 μm.)