Phagocytic characterization and therapeutic efficacy of an anti-PyroGlutamate-3 Av IgG2a antibody in aged APP/PS1dE9 mice

Pyroglutamate-3 Av (pGlu-3 Av) is a highly pathogenic Av species in Alzheimer's disease (AD) brain. The Av N-terminus is truncated and the 3rd residue (glu) cyclized by glutaminyl cyclase, leading to increased aggregation and neurotoxicity. Previously, we reported that chronic treatment in young AD transgenic (Tg) mice with 07/1, an anti-pGlu-3 Av IgG1 mAb, staved off AD-like changes (Frost et al., 2012, 2015). Here, we compared different Ig isotypes of an anti-pGlu-3 Av mAb using ex vivo phagocytosis, longitudinal microglial PET imaging with TSPO-specific 18F-GE180, and therapeutic passive immunotherapy (IT) in plaque-bearing, aged APP/PS1dE9 Tg mice. 07/2a, an IgG2a version of the 07 mAb, was more effective than the 07/1 mAb at microglia-mediated Av clearance in mouse brain by ex vivo phagocytosis. Next, 4 and 16 mo Tg mice underwent baseline 18F-GE180 PET scans on Day 0, followed by a single i.p. injection of 500 mg of 07/1, 07/2a, or 3A1 (a pan-Av IgG1 mAb), or PBS. Hippocampal uptake of 18F-GE180 was increased in 07/2a-treated mice at Days 3 and 30 in 4 mo Tg mice and was increased at Day 3 but reduced at Day 30 in 16 mo Tg mice; 07/1-treatment resulted in increased 18F-GE180 uptake only at Day 30 at both ages, while 3A1-treatment resulted in reduced uptake at Days 3 and 30 at both ages. Therapeutic IT was performed in 12 mo-old male Tg mice (n = 15/group) given weekly i.p. injections with 300 mg of 07/1, 07/2a, or 3A1, or PBS, for 16 wk. Mice underwent Open Field (OF), Water T Maze (WTM) and Context Fear Conditioning (CFC) testing at 15 mo and sacrifice at 16 mo. Mice treated with 07/2a showed significant benefit in spatial learning and memory in acquisition (p < 0.05) and reversal (cognitive flexibility; p < 0.05) WTM compared to Tg-PBS mice. CFC and microhemorrhages were similar between groups. 07/2a-treated mice had significantly less pGlu-3 Av immunoreactivity (p < 0.01), and 07/1-treated mice showed a trend for a reduction, vs. PBS-treated mice. Thus, selective targeting of pGlu-3 Av aggregates with an IgG2a mAb is effective in lowering plaque burden in the absence of microhemorrhage, while improving cognitive performance in aged APP/PS1dE9 Tg mice.