Aβ plaques were surrounded by activated microglia expressing TLR4. A brain section from a TLR4W Tg mouse was stained with thioflavin S for fibrillar amyloid deposits (B) and immunostained with anti-CD11b antibody using avidin-biotin immuno-peroxidase method and DAB (A). (C) is a superimposition of (A) and (B), demonstrating that fibrillar Aβ deposits are closely accompanied by CD11b-positive microglia. Brain sections from a TLR4W Tg mouse were labeled with anti-TLR4 antibody (green) (E and H) followed by staining with anti-CD11b (red) (D) or anti-CD45 (red) (G) antibody. The yellow colors in the superimposed images of (F) and (I) between (D) and (E) and between (G) and (H) indicate co-localization of TLR4 with activated microglia (CD11b and CD45), respectively. Overlap between astrocytes detected by anti-GFAP antibody (red) (J) and TLR4 (green) (K) is occasionally found in the merged picture (L). Scale bars 50 μm.Asterisks indicate amyloid plaques.