Prions: from neurografts to neuroinvasion

Spongiform encephalopathies are infectious neurodegenerative diseases cause d by pathogens that seem to be devoid of any informational nucleic acids. H istopathologically, these diseases are characterized by spongiform degenera tion of the central nervous system. Although the main pathological changes during the course of the disease occur in the brain, the infectious agent a ccumulates early in lymphoid tissue. The consecutive development of clinica l disease depends on the presence of an intact immune system including matu re B-cells and follicular dendritic cells. In this article we review the st ate of knowledge on the routes of neuroinvasion used by the infectious agen t in order to gain access to the central nervous system upon entry into ext racerebral sites.