Possible Roles for Astroglia and Microglia in the Pathogenesis of Unconventional Slow Infections

Scrapie is the archetype of a group of infectious diseases that have been termed variously unconventional slow infections, subacute spongiform encephalopathies, transmissible spongiform encephalopathies and prion diseases. We will use the term unconventional slow infections. The diseases are caused by transmissible agents which have a number of remarkable characteristics; these include relatively high levels of resistance to a variety of inactivating agents such as high temperature, X-ray and UV irradiation, beta propriolactone, formalin, RNase and DNase (Carp et al., 1989; Prusiner, 1989). The interaction of host and agent also present some unusual findings: 1) If host, strain and dose of agent, route of injection and physical chemical state of the inoculum are kept constant the disease manifestations, such as incubation period, the presence of obesity, etc., are remarkably predictable (Outram, 1976; Carp and Rubenstein, 1991). The precision is particularly surprising considering the extremely long incubation period; 2) One reason for this precision may be related to another interesting characteristic of the agent-host interaction. The host does not appear to respond to the infectious agent: the body does not mount an immunological response to the agent, nor is interferon produced, and in the main organ that is affected, the brain, there is no evidence of an inflammatory response (Carp, 1992).