While prion infection of the lymphoreticular system (LRS) is necessary for neuroinvasion in many prion diseases, in bovine spongiform encephalopathy and atypical cases of sheep scrapie there is evidence to challenge that LRS infection is required for neuroinvasion. Here we investigated the role of prion infection of LRS tissues in neuroinvasion following extraneural inoculation with the HY and DY strains of the transmissible mink encephalopathy (TME) agent. DY TME agent infectivity was not detected in spleen or lymph nodes following intraperitoneal inoculation and clinical disease was not observed following inoculation into the peritoneum or lymph nodes, or after oral ingestion. In contrast, inoculation of the HY TME agent by each of these peripheral routes resulted in replication in the spleen and lymph nodes and induced clinical disease. To clarify the role of the LRS in neuroinvasion, the HY and DY TME agents were also inoculated into the tongue because it is densely innervated and lesions on the tongue, which are common in ruminants, increase the susceptibility of hamsters to experimental prion disease. Following intratongue inoculation, the DY TME agent caused prion disease and was detected in both the tongue and brainstem nuclei that innervate the tongue, but the prion protein PrP Sc was not detected in the spleen or lymph nodes. These findings indicate that the DY TME agent can spread from the tongue to the brain along cranial nerves and neuroinvasion does not require agent replication in the LRS. These studies provide support for prion neuroinvasion from highly innervated peripheral tissues in the absence of LRS infection in natural prion diseases of livestock.In scrapie infection of sheep and chronic wasting disease infection of cervids, prion agent infection and replication in the gut-associated lymphoreticular system (LRS) precede entry into the nervous system following oral prion exposure (1,15,29,35). After infection of the LRS, the prion agent enters peripheral nerves and retrogradely spreads to the central nervous system, where it can replicate to high levels. The scrapie agent also retrogradely spreads from the enteric nervous system to the dorsal motor nucleus of the vagus in the brainstem via the vagus nerve (21,28,36). These modes of neuroinvasion are postulated to be dependent on prior agent amplification in the LRS. This is supported by studies using mice that are not susceptible to prion infection via extraneural routes of inoculation as a result of a permanent or transient loss of functional germinal centers in lymphoid follicles (17)(18)(19)(20)22).However, in bovine spongiform encephalopathy (BSE)-infected cattle and atypical scrapie, the role of the LRS in prion neuroinvasion is less clear, and perhaps not essential. In natural cases of BSE, prion infectivity has not been detected in lymph nodes or spleen, and the disease-specific isoform of the prion protein PrP Sc was not found in the distal ileum (31). Following experimental oral exposure of calves to the BSE agent, BSE infectivity w...

Centrifugal spread of the prion agent to peripheral tissues is postulated to occur by axonal transport along nerve fibers. This study investigated the distribution of the pathological isoform of the protein (PrP Sc ) in the tongues and nasal cavities of hamsters following intracerebral inoculation of the HY strain of the transmissible mink encephalopathy (TME) agent. We report that PrP Sc deposition was found in the lamina propria, taste buds, and stratified squamous epithelium of fungiform papillae in the tongue, as well as in skeletal muscle cells. Using laser scanning confocal microscopy, PrP Sc was localized to nerve fibers in each of these structures in the tongue, neuroepithelial taste cells of the taste bud, and, possibly, epithelial cells. This PrP Sc distribution was consistent with a spread of HY TME agent along both somatosensory and gustatory cranial nerves to the tongue and suggests subsequent synaptic spread to taste cells and epithelial cells via peripheral synapses. In the nasal cavity, PrP Sc accumulation was found in the olfactory and vomeronasal epithelium, where its location was consistent with a distribution in cell bodies and apical dendrites of the sensory neurons. Prion spread to these sites is consistent with transport via the olfactory nerve fibers that descend from the olfactory bulb. Our data suggest that epithelial cells, neuroepithelial taste cells, or olfactory sensory neurons at chemosensory mucosal surfaces, which undergo normal turnover, infected with the prion agent could be shed and play a role in the horizontal transmission of animal prion diseases.