Fig. 2. Increased BBB permeability in adult germ-free versus pathogen-free mice.

(A) In vivo PET imaging of [¹¹C]raclopride. Average coronal, sagittal, and horizontal PET summation images (brain area encircled in purple ellipse) in pathogen-free (PF) or germ-free (GF) adult mice 2 to 3 min after [¹¹C]raclopride injection. (B) Average whole-brain time-activity curves of [¹¹C]raclopride uptake expressed as % standardized uptake value (% SUV) in the two groups. *P < 0.05 and **P < 0.05 by one-way ANOVA. (C) Values (% SUV) obtained at 1-min intervals during the first 5 min. Data are expressed as means ± SEM (five to six mice per group). (D) Representative images showing Evans blue dye extravasation (red) in three brain regions (cortex - upper row, striatum - middle row, and hippocampus - lower row) of germ- and pathogen-free mice and pathogen-free mice treated with TNFα (n = 3 mice per group). Blue, 4′,6-diamidino-2-phenylindole (DAPI) (nuclear staining). Scale bar, 50 µm. (E and F) Neuronal loss in the hippocampus of germ-free adult mice receiving R4A antibody. (E) Photomicrographs of the CA1 region of the hippocampus (middorsal, matched sections) from PBS-treated germ-free adult mice (left) and germ-free mice treated with different concentrations of R4A antibody (100 and 250 µg) (right). Yellow arrows in the right panel indicate neuronal loss and dying neurons in R4A-treated germ-free mice. Scale bar, 10 µm. (F) Quantitative analysis. Percent neuronal survival in the PBS-treated germ-free mouse group was set at 100%. **P < 0.01 by one-way ANOVA compared with the PBS-treated germ-free mouse group (n = 3 mice per group).