Lewis rats were immunized with horseradish peroxidase (HRP) injected intradermally in the hind foot pads; anti-HRP antibodies in the cerebrum were detected with a cytochemical reaction for HRP in sections incubated with the antigen (HRP), or after intravenous injection of HRP a few minutes before sacrifice.
In two groups of animals the blood brain barrier was disrupted by a heat lesion or by a traumatic lesion (insertion of a canula into the lateral ventricle). In animals with a heat or with a traumatic lesion, anti-HRP antibody was detected not only in the vicinity of the lesion but also within the entire cerebral parenchyma and the choroid plexus.
By electron microscopy, anti-HRP antibody was seen on the plasma membranes of endothelial cells and pericytes of the capillaries of the brain parenchyma and of the choroid plexus, and on the plasma membranes of the choroid plexus epithelium. Anti-HRP antibody was not seen in a group of immunized animals with an intact blood brain barrier or in a group of animals immunized 30 or more days after the brain lesion.
These experiments have shown that significant levels of circulating antibody against an antigen (HRP), which is irrelevant to brain antigens, has access to and achieves a widespread distribution into the brains of animals with a disrupted blood brain barrier.