Figure 6. Infiltrating monocytic cells are increasingly associated with Aβ plaque and cellular uptake of Aβ.

(A–F) Fluorescent micrographs of hippocampal and cortical plaques from 7-month-old mice. Sections were immunolabeled for ACE (green), CD45 (red), human Aβ (4G8; cyan), and nuclei (DAPI; blue). A–D show the composite picture and images from the individual color channels. (A) In AD⁺ACE^WT/WT brains, the plaques were generally larger and composed of highly aggregated Aβ. CD45^hiACE^lo cells were colocalized with amyloid plaques. (B and C) In AD⁺ACE^10/10 mice, the plaques were generally smaller and had a more diffuse morphology. CD45^hiACE^hi infiltrating monocyte–derived macrophages were more frequently present in the vicinity of amyloid plaques. ACE was also expressed in the brain by resident CD45^lo-intermedACE^hi microglia. In AD⁺ACE^10/10 mice, infiltrating monocytes expressed high levels of ACE (arrowheads). (D) Cortical region from an AD⁺ACE^10/WT mouse showed increased CD45^hiACE^hi infiltrating macrophages at the amyloid lesion site. Colocalization of CD45^hiACE^hi cells and intracellular Aβ immunoreactivity (arrows in insert) were observed, indicating cellular uptake of Aβ. (E and F) Enlargements of boxed areas in A and B. (F) In the AD⁺ACE^10/10 plaque region, macrophages (MΦ) and microglia (MG) overexpressed ACE. Internalization of Aβ (white) by plaque-associated CD45^hiACE^hi macrophages was observed. (G) Multichannel image shows recruitment of blood-derived IBA1⁺ (green) CD45^hi (red) macrophages to 6E10⁺ plaque (cyan) in AD⁺ACE^10/10 brain. Scale bars: 10 μm. (H) Quantitation of IBA1⁺CD45^hi immunoreactive area. (I) The ratio of IBA1⁺CD45^hi area to the 6E10-immunoreactive area showed a significant increase (2.3-fold) in the number of macrophages per plaque area in AD⁺ACE^10/10 versus that observed in AD⁺ACE^WT/WT mice. *P < 0.05.