CD33 and Alzheimer disease: They also reported that numbers of CD33-immunoreactive microglia correlated positively with both insoluble Aβ42 levels and amyloid plaque burden in AD brain, that CD33 inhibited clearance of Aβ42 in microglial cell cultures, and that brain levels of insoluble Aβ42/plaque burden were markedly reduced in APPSwe/PS1ΔE9/CD33−/− mice.