Dickson, Dennis W.

Abstract INTRODUCTION Anti‐amyloid beta (Aβ) monoclonal antibodies are effective at lowering amyloid in Alzheimer's disease (AD). However, whether Fc‐mediated effector function is absolutely required for efficacy is not completely understood. This is important for optimizing therapeutic efficacy and mitigating side effects such as amyloid‐related imaging abnormalities (ARIA). Antibodies lacking Fc effector function, like single‐domain antibodies (nanobodies), offer a unique tool to dissect these mechanisms, as their small size facilitates blood–brain barrier (BBB) penetration and allows Fc‐mediated functions to be studied independently. METHODS We immunized a llama with Aβ aggregates and constructed a phage display library to screen for aggregate‐specific nanobodies. Lead candidates were characterized by epitope mapping and binding affinity to amyloid plaques in both murine and human AD brain tissues. We further assessed their BBB permeability and evaluated their efficacy in clearing pre‐existing plaques in amyloid precursor protein (APP)/presenilin 1 (PS1) mice. RESULTS We identified two lead nanobodies, 3A11 and 2D10, that bind distinct epitopes and specifically bind Aβ plaques in murine and human AD brain tissues. Following systemic administration, the monovalent, unmodified (Fc‐less) 2D10 nanobody, but not 3A11, successfully crossed the BBB and engaged amyloid plaques in APP/PS1 mice. However, despite robust target engagement, the Fc‐less 2D10 failed to recruit microglia or reduce plaque burden. In contrast, an engineered 2D10‐Fc fusion antibody potently cleared amyloid plaques, achieving a reduction in pathology comparable to aducanumab treatment. This efficacy was directly correlated with Fc‐mediated microglial recruitment and activation, demonstrating that the Fc domain is essential for phagocytic plaque removal. DISCUSSION Our findings demonstrate that Fc effector function is indispensable for microglial‐mediated amyloid clearance in vivo. By clarifying this fundamental mechanism, this study provides a framework for the rational design of next‐generation immunotherapies. Furthermore, 2D10‐Fc represents a promising therapeutic candidate, combining the high‐affinity targeting of nanobodies with the effector power necessary for robust plaque clearance.

Abstract INTRODUCTION Globular glial tauopathy (GGT) is a rare type of frontotemporal lobar degeneration (FTLD) characterized by deposition of 4‐repeat tau. Detecting GGT with tau positron emission tomography (tau‐PET) is challenging. We aim to determine the associations between tau‐PET, autoradiography, and neuropathology methods in GGT. METHODS We identified three patients with GGT who had completed antemortem tau‐PET. Healthy control and two patients with Alzheimer's disease (AD) were also included for comparison. We analyzed gray matter (GM) and white matter (WM) from the superior/middle frontal gyrus (S/M‐FG) and the superior/middle temporal gyrus (S/M‐TG). Immunohistochemical staining was performed with antibodies against phospho‐tau (AT8, PHF‐1, RD4), glial fibrillary acidic protein (GFAP), and resting microglia (ionized calcium‐binding adaptor molecule 1; Iba1). Immunofluorescence with a fluorescent tau‐PET analog (T726) was performed. Regional tau‐PET standardized uptake value ratios (SUVRs) were calculated for comparative GM and WM regions. Autoradiographic studies with 18F‐AV1451 were also conducted. RESULTS Tau‐PET showed increased uptake in the WM regions of the S/M‐FG and S/M‐TG in GGT, and the GM regions in AD. Co‐localization was observed between T726 and PHF‐1, RD4, and GFAP in the WM in the patients with GGT, whereas co‐localization was observed with PHF‐1 predominantly in the GM in AD. Little co‐localization was observed with Iba1. In vitro 18F‐AV1451 autoradiography studies demonstrated minimal binding in GGT. Tau‐PET differentiated underlying GGT from AD based on the relative involvement of the WM and GM. CONCLUSION Histopathologic findings suggest that some flortaucipir uptake in GGT may represent underlying 4R tau, whereas autoradiographic analysis suggests that uptake is likely due to off‐target binding. Further studies with larger cohorts are needed to determine the pathological basis of flortaucipir uptake in GGT.

Dominant-inactivating mutations in the colony stimulating factor-1 receptor ( CSF1R ) cause CSF-1R–related leukoencephalopathy (CRL), an adult-onset neurodegenerative disease that is modeled in the Csf1r +/– mouse. CRL is caused by microglial dysfunction. However, the primary microglial deficit is unknown. To address this question, we employed single-nucleus RNA sequencing of brains from young Csf1r +/– mice without pathological or behavioral alterations. Reduction of CSF-1R signaling caused metal ion accumulation in brain macrophages, with concomitant activation of cell death and stress response pathways in oligodendrocytes and neuronal subpopulations. Reduction of metallothionein 1 ( Mt1 ) and 3 ( Mt3 ) gene expression was a common feature in glial and neuronal cells of Csf1r +/– mice. Overexpression of Mt1 restored metal ion homeostasis, normalized ROS production in microglia, and prevented the development of behavioral deficits, while Mt3 deletion had disease-enhancing effects. These findings demonstrate CSF-1R regulation of metal ion homeostasis via metallothioneins in the brain.