Figure 1.

Peripherally-derived myeloid cells rapidly repopulate the microglia-depleted brain and adopt a microglia-like phenotype in non-AD mice. (a) Experimental time line. Mice were irradiated, and injected with tdRFP bone marrow cells. Four weeks after BM-transfer, a cranial window was installed and a mini-osmotic pump was implanted to deliver Ganciclovir for microglia ablation in TK+ animals. Imaging using 2-photon microscopy was started six days after surgery, and the mice were subsequently imaged once a week for six weeks. Mouse graphic designed by Gwilz [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons. (b,c) Representative pictures from 2-photon imaging sessions displaying Frac-GFP;TK− (b) and Frac-GFP;TK+ (c) mice at indicated time points after surgery. GFP-positive cells represent endogenous microglia and RFP-positive cells represent peripherally-derived myeloid cells (PDMCs); scale bar = 100 µm. (d) Number of PDMCs per field of view over time; n = 6; degrees of freedom (df) = 34; 1-way ANOVA with Tukey post-hoc test, *p < 0.05. (e) Post mortem stereological quantification of microglia (FracGFP;TK−) and PDMC (FracGFP;TK+) cell density per mm³; n = 7; df = 12; Unpaired t-test ns. (f) Cell-to-cell distance of endogenous microglia (green bars) and PDMCs (red bars). Each field of view of the first minute of each imaging session was analysed in Imaris with the spot recognition algorithm. The xyz coordinates of spots were exported and the Euclidian distances between cells were measured for every detected cell with a custom written algorithm; n = 6, 3 fields of view per animal; df = 426; 2-way ANOVA with Sidaks post-hoc test; interaction <0.0001; ****p < 0.0001 (g) Distribution of PDMCs and microglia relative to total Iba1+ cells based on post mortem stereological quantification; FracGFP;TK− n = 3, FracGFP;TK+ n = 4.