1. Prion cell tropism significantly varies among animal species, depending on both the agent strain and host-specific factors. For example, prions show high lymphotropism in scrapie infected sheep and in vCJD, but little, if any, in sCJD or BSE. In particular, the BSE strain is associated with significant PrP-res accumulation in tonsils, spleen and appendix in humans, whereas, it is largely confined to the nervous system in infected cattle. So, it appears that, at least in the case of BSE and vCJD, host properties can influence the accumulation of the infectious agent in lymphoid organs. Given that the normal cellular prion protein (PrPc), is sine qua non for PrP-res formation and the development of TSE, it appears reasonable to hypothesize that tissue-specific PrPc properties may represent one of the host factors influencing the cell tropism of the infectious agent in human or bovine. Characterization of bovine and human cellular prion protein expressed in the central nervous system and in lymphoid organs V. Defaweux 1, S. Capellari 2, S. Stramiello 2, N. Antoine 3, G. Dorban 1, C. Demonceau 1, O. Jolois 1, E. Heinen 1 and P. Parchi 2. 1 Dpt of Morphology and Immunology, Institute of Human Histology, Faculty of Medecine, University of Liège, Belgium–www.ulg.ac.be/histohum. 2 Department of Neurological Sciences, Faculty of Medicine, University of Bologna, Italy. 3 Laboratoy of Animal Histology, Department of Morphology and Pathology, Faculty of Veterinary Medecine, University of Liège, Belgium. SAF32 SAF60  PrPc glycoform ratios are significantly different between cerebellum and medulla in bovine and human.  Only the unglycosylated PrPc is distributed like wise in medulla and in the cerebellum of bovine and human. Western blot analysis to compare the ratio of PrPc glycoforms expressed in the CNS of bovine and human Western blot analysis of truncated PrPc forms expressed in bovine CNS  The expression of truncated forms of PrPc (i.e. 21 and 18 kDa PrPc) is also significantly heterogenous according to the brain region investigated. Western blot analysis to compare the PrPc glycoform ratios and the truncated PrPc forms expressed in bovine lymphoid tissuesin bovine lymhpoid cells SAF32 SAF60 SAF32 Isolation of PrPc expressing follicular dendritic cells (FDC) FDC ultrastructure SAF32+ FDC  PrPc is highly glycosylated in spleen and in lymphoid follicles isolated from bovine lymphoid tissues as well as in their FDC and lymphocytes.  After deglycosylation, a novel PrPc truncated form with a relative molecular mass of about 25 kDa was detected in bovine lymphoid organs beside the typical 18 and 21 kDa forms. Western blot analysis to compare the PrPc glycoform ratios and the truncated PrPc forms expressed in human lymphoid tissues Our results highlight variation in the profile expression of PrPc in peripheral and central tissues of bovine and human. Such differences may have an implication for PrPc function and may represent critical factors influencing the accumulation of the infectious agent in these areas. SAF32  No difference in WB PrPc profile was seen in follicles, lymphocytes and FDC of human tissues  Immune PrPc is highly glycosylated and, after deglycosylation, the 25 kDa is also expressed in human lymphoid tissues and cells. Supported by the EU contract QLG3-CT-2002-81030