Microglia ablation on in vivo CD11b-HSVTK system deteriorated prion disease. (A) Survival curves of RML6- or NBH-inoculated tga20^+/+;TK+ or tga20^+/+;TK− mice implanted with GCV or PBS containing intraventricular osmotic minipumps (tga20^+/+;TK− RML(PBS), n = 5, median survival = 56 dpi; tga20^+/+;TK− RML(GCV), n = 4, median survival = 56.5 dpi; tga20^+/+;TK+ NBH(PBS), n = 6, remained asymptomatic up to 138 dpi; tga20^+/+;TK+ NBH(GCV), n = 6, median survival = 61.5 dpi; tga20^+/+;TK+ RML(PBS), n = 9, median survival = 57 dpi; tga20^+/+;TK+ RML(GCV), n = 10, median survival = 50 dpi; ****, P < 0.0001). Dashed line represents time of pump implantation. Microglia-depleted mice showed an accelerated prion progression as compared with all other groups. (B) Iba1 immunohistochemistry of cortical brain tissue from all experimental groups showing dramatic reduction in the microglial colonization of GCV-treated, RML6-infected tga20^+/+;TK+ mice. Bars, 10 µm. (C and D) Quantification of microglia depletion in (C) cortical (tga20^+/+;TK+ RML(PBS), n = 8; tga20^+/+;TK+ RML(GCV), n = 9; tga20^+/+;TK− RML(GCV), n = 7; tga20^+/+;TK+ NBH(GCV), n = 5; **, P < 0.01; ***, P < 0.001) or (D) cerebellar tissue (tga20^+/+;TK+ RML(PBS), n = 8; tga20^+/+;TK+ RML(GCV), n = 9; tga20^+/+;TK− RML(GCV), n = 8; tga20^+/+;TK+ NBH(GCV), n = 5; ***, P < 0.001; **, P < 0.01). Red, microglia depleted. Error bars represent SEM. Statistical significance in A was determined using Log-rank (Mantel-Cox) test. Statistical significance in C and D was determined using one-way ANOVA with Tukey’s post-test for multicolumn comparison. Histology results represent at least five independent experiments.