(C, D) Representative immunofluorescence images (left) of (C) Iba1 (green) and KI67 (red) or (D) CD74 (red) on cortical sections from SPF and GF mice. Nuclei were stained with DAPI (blue). Scale bars: 25 μm. White arrows indicate KI67+/Iba1+ or CD74+/Iba1+ macrophages and arrowheads show KI67-/Iba1+ or CD74-/Iba1+ macrophages. Quantification thereof (right). Each symbol represents one mouse. At least three slides were examined per individual mouse. Data are presented as mean ± s.e.m. Significant differences were evaluated by two-way ANOVA followed by Tukey's post-hoc comparison test and marked with asterisks (*P < 0.05, **P < 0.01, ***P < 0.001). Data are representative of two independent experiments. 