Either young WT (A, B) or old APPPS1 (C-E) tissue was treated with clodronate (Clo) to remove CD68 positive cells or vehicle Ctr from 1 until 7 DIV. Treatment was stopped and subsequently old (A, B) or young (C-E) tissue was added to the culture as schematically indicated in A and C. and analyzed 14 days after. Removal of CD68 positive cells in either young WT or old APPPS1 tissue prevents amyloid plaque clearance in the co-culture model.(A) Immunofluorescence analysis of the old APPPS1 slice co-cultured with the young WT slice pre-treated with Clo and Ctr and immunostained with CD68 (red) and M3.2 (green). Scale bar: 50 µm.(B) Quantitative analysis of core-only plaques in the old APPPS1 tissue co-cultured with the young WT tissue pre-treated with Clo and Ctr as indicated in A reveals a decreased number of core-only plaques upon Clo treatment. The values are expressed as percentages of core-only plaques from the total number of amyloid plaques and represent mean ± SEM from 3 independent experiments, including total of 6 independent slice culture dishes (***P < 0.001).(C) Immunofluorescence analysis of the old APPPS1 slice treated with Clo and Ctr and subsequently co-cultured with the young WT slice and immunostained with CD68 (red) and M3.2 (green). Scale bar: 50 µm.(D) Area of CD68 positive cells (CD68 coverage) in the old APPPS1 tissue treated with Clo and Ctr and subsequently co-cultured with the young WT tissue as indicated in C. CD68 coverage is reduced upon Clo treatment. The values are normalized to CD68 coverage of the Ctr and represent mean ± SEM from 3 independent experiments, including total of 6 independent slice culture dishes (***P < 0.001).(E) Quantitative analysis of core-only plaques in the old APPPS1 tissue treated with Clo and Ctr and subsequently co-cultured with the young WT tissue as indicated in C reveals a decreased number of core-only plaques upon Clo treatment. The values are expressed as percentages of core-only plaques from the total number of amyloid plaques and represent mean ± SEM from 3 independent experiments, including total of 6 independent slice culture dishes (**P < 0.01). 