CD33 Alzheimer's disease locus: altered monocyte function and amyloid biology

Abstract

In our functional dissection of the CD33 Alzheimer's disease susceptibility locus, we found that the rs3865444(C) risk allele was associated with greater cell surface expression of CD33 in the monocytes (t50 = 10.06, P(joint) = 1.3 × 10(-13)) of young and older individuals. It was also associated with diminished internalization of amyloid-β 42 peptide, accumulation of neuritic amyloid pathology and fibrillar amyloid on in vivo imaging, and increased numbers of activated human microglia.

Figure 1. The CD33 risk allele is associated with increased CD33 expression and decreased uptake

The MFI of CD33 protein expression on circulating monocytes of young healthy adults is increased with the rs3865444^C risk allele (a). There is a sevenfold increase in CD33 expression in rs3865444^CC vs. rs3865444^AA subjects. Monocytes from older adults also have differential CD33 expression based on genotype (b). The MFI of FITC-labeled dextran (c) and Aβ(42) (d) uptake by monocytes from young healthy adults is decreased with the rs3865444^C risk allele. Data shown is the change in MFI between samples incubated at 4°C and 37°C. Each circle represents an individual. The mean of the distribution is denoted by a horizontal line.

Figure 2. The susceptiblity allele of rs3865444 is associated with an increase in Pittsburgh Compound B (PiB) imaging

PiB-PET images from a PiB− rs3865444^AA subject (a) and a PiB+ rs3865444^CC subject (b) demonstrating a typical pattern of binding of PiB to amyloid plaque in the PiB positive (DVR > 2.0) subjects and lack of binding (DVR ~= 1.0) in the PiB negative subjects. The PiB values from the HAB (c) and ADNI (d) cohorts demonstrate an increased frequency of PiB+ subjects with the rs3865444^C risk allele. PiB+ and PiB− values are separated by a dotted line.

Figure 3. Immunohistochemistry reveals CD33 expression in the brain and an increase frequency of stage III microglia/macrophages associated with rs3865444^C

(a) CD33+ microglia/macrophages (brown) are seen in sections of the dorsolateral prefrontal cortex from a subject with pathologically confirmed AD; they are found in the vicinity of amyloid deposits (red). (b) Adjacent sections labeled with anti-CR3/43 (top panel), a marker for microglia/macrophage, and anti-CD33 (bottom panel) demonstrate that the cells staining with anti-CD33 have the morphologic attributes of microglia/macrophage. (c) CD33 is expressed at all 3 stages of activation in microglia/macrophages in the brain. The density of stage 3 CR3/43+ microglia is increased in the inferior temporal region (d) and posterior putamen region (e) with the rs3865444^C risk allele. Each circle represents a subject. The line represents the mean.