Presence of CD4+ and CD8+ T cells and expression of MHC class I and MHC class II antigen in horses with Borna disease virus-induced encephalitis.

Tissues from 9 horses and 1 donkey suffering from natural Borna disease were investigated immunomorphologically. Lymphocytic inflammatory reactions and increased expressions of MHC class I and class II antigen were found in the brain as well as in the trigeminal and olfactory system. Perivascular inflammatory infiltrates were dominated by CD4+ T cells, whereas the majority of CD8+ T cells were disseminated intraparenchymally. No evidence of inflammation was found in the retina. Borna disease virus proteins and nucleic acids were present in the hippocampus, thalamus and medulla oblongata in all 10 animals, in the cerebral cortex, retina, trigeminal ganglion and nerve in 9, in the olfactory epithelium in 6 and in roots and proximal parts of large peripheral nerves in 3. No evidence of infection was found in the autonomic nervous system, lung, heart, liver, kidney or gut. BDV- proteins and nucleic acids were even more abundant in the trigeminal system than in the olfactory system, suggesting that infection may have occurred via the trigeminal nerve.