Association of Microglial Activation With Spontaneous ARIA-E and Cerebrospinal Fluid Levels of Anti-Aβ Autoantibodies

Background and objectives: Amyloid-related imaging abnormalities suggestive of vasogenic edema or sulcal effusion (ARIA-E) are the most common adverse events complicating Alzheimer's disease (AD) immunotherapy with anti-amyloid-beta (Aβ) monoclonal antibodies (mAbs). ARIA-E can also occur spontaneously in cerebral amyloid angiopathy-related inflammation (CAA-ri), a rare autoimmune encephalopathy associated with increased cerebrospinal fluid (CSF) levels of anti-Aβ autoantibodies. Although the pathophysiological mechanisms of ARIA-E remain to be fully elucidated, experimental evidence from ex-vivo studies suggest that gantenerumab and aducanumab enable microglial activation. However, the in vivo evidence for a direct association between neuroinflammation and ARIA-E in patients with high CSF anti-Aβ (auto)antibody levels has never been demonstrated. Methods: Spatial distribution and temporal variations of microglial activation associated with ARIA-E and CSF anti-Aβ autoantibody levels at (sub)acute presentation and after corticosteroid therapy, in a longitudinal case series of patients with CAA-ri, an increasingly recognized spontaneous model of the iatrogenic ARIA-E in AD immunotherapy. Multimodal and multiparametric magnetic resonance imaging (MRI) for CAA and ARIA-E quantification, as measured with validated MRI scoring systems; CSF testing for anti-Aβ autoantibodies and AD biomarkers; 11C-PK11195 positron emission tomography (PET) for activated microglia. Results: At (sub)acute presentation, we found focal peaks of microglial activation having a greater spatial co-localization with ARIA-E compared to chronic age-related white matter change (ARWMC) imaging abnormalities. The severity of ARIA-E and the magnitude of the associated microglial activation was greater in patients having AD and severe CAA concomitant disease, compared to patients having CAA only. CSF anti-Aβ autoantibodies at presentation were high in all patients and markedly decreased at post-treatment follow-up, in parallel with clinical resolution of acute symptoms, reduced ARIA-E severity, and reduced microglial activation. Discussion: Our findings extend the current notion of ARIA-E by providing the first in vivo 11C-PK11195-PET evidence for an association between microglial activation and the magnitude and severity of ARIA-E in patients with increased CSF concentration of anti-Aβ autoantibodies and comorbid AD and CAA disease..Our results highlight CSF testing for anti-Aβ autoantibodies as a promising diagnostic, prognostic, and therapy response biomarker to help guide future treatment and management decisions in real clinical practice and clinical trials.