Targeted-phage internalize into cells in CP explants in a ligand dependant fashion. Choroid plexus was dissected from the lateral and 4th ventricle and incubated with PhD-targeted phage (1 × 1012pfu/ml) for 2 h at 37C. To remove any particles bound to the cell surface, the explants were washed with PBS containing Tween 20, fixed and then immunostained with antibodies against M13 phage. Binding of primary antibodies was detected using Alexa594-goat anti rabbit antibodies. No immunostaining was observed when choroid plexus explants were incubated with Peptide A control phage (Panel A). In contrast, immunoreactive M13 staining was observed in the cytoplasm of selected populations of epithelial cells after incubation with Peptide B (Panel B), Peptide C (Panel C) or Peptide D (Panel D) targeted phage indicating the internalization of these targeted particles by epithelial cells in the choroid plexus. To demonstrate that internalization is ligand-mediated, explants were co-incubated with 200 ng/ml Peptide C (Panel E) or Peptide D (Panel F) and the targeted phage. M13 phage: red; Cell nuclei (DAPI): white.