Prion diseases are characterised by the accumulation of PrPSc, an abnormally folded isoform of the cellular prion protein (PrPC), in affected tissues. Following peripheral exposure high levels of prion-specific PrPSc accumulate first upon follicular dendritic cells (FDC) in lymphoid tissues before spreading to the CNS. Expression of PrPC is mandatory for cells to sustain prion infection and FDC appear to express high levels. However, whether FDC actively replicate prions or simply acquire them from other infected cells is uncertain. In the attempts to-date to establish the role of FDC in prion pathogenesis it was not possible to dissociate the Prnp expression of FDC from that of the nervous system and all other non-haematopoietic lineages. This is important as FDC may simply acquire prions after synthesis by other infected cells. To establish the role of FDC in prion pathogenesis transgenic mice were created in which PrPC expression was specifically “switched on” or “off” only on FDC. We show that PrPC-expression only on FDC is sufficient to sustain prion replication in the spleen. Furthermore, prion replication is blocked in the spleen when PrPC-expression is specifically ablated only on FDC. These data definitively demonstrate that FDC are the essential sites of prion replication in lymphoid tissues. The demonstration that Prnp-ablation only on FDC blocked splenic prion accumulation without apparent consequences for FDC status represents a novel opportunity to prevent neuroinvasion by modulation of PrPC expression on FDC.

Summary. Platelet interactions with adhesive ligands exposed at sites of vascular injury initiate the normal hemostatic response but may also lead to arterial thrombosis. Platelet membrane glycoprotein (GP)VI is a key receptor for collagen. Impairment of GPVI function in mice results in a long-term antithrombotic protection and prevents neointimal hyperplasia following arterial injury. On the other hand, GPVI de®ciency in humans or mice does not result in serious bleeding tendencies. Blocking GPVI function may thus represent a new and safe antithrombotic approach, but no speci®c, potent anti-GPVI directed at the human receptor is yet available. Our aim was to produce accessible antagonists of human GPVI to evaluate the consequences of GPVI blockade. Amongst several monoclonal antibodies to the extracellular domain of human GPVI, one, 9O12.2, was selected for its capacity to disrupt the interaction of GPVI with collagen in a puri®ed system and to prevent the adhesion of cells expressing recombinant GPVI to collagen and collagen-related peptides (CRP). While 9O12.2 IgGs induced platelet activation by a mechanism involving GPVI and FcgRIIA, 9O12.2 Fab fragments completely blocked collageninduced platelet aggregation and secretion from 5 mg mL À1 and fully prevented CRP-induced activation from 1.5 mg mL À1 . 9O12.2 Fabs also inhibited the procoagulant activity of collagen-stimulated platelets and platelet adhesion to collagen in static conditions. Furthermore, 9O12.2 Fabs impaired platelet adhesion, and prevented thrombi formation under arterial¯ow conditions. We thus describe here for the ®rst time a functional monoclonal antibody to human GPVI and demonstrate its effect on collagen-induced platelet aggregation and procoagulant activity, and on thrombus growth.

We have ablated peripheral lymph nodes in sheep and subsequently cannulated the pseudo-afferent lymphatic vessel that arises as a consequence of afferent lymphatic vessels reanastomosing with the former efferent duct. This technique allows the collection of lymph with a cellular composition that resembles true afferent fluid, and in particular, containing 1-10% dendritic cells. A 16-h collection of this lymph may contain between 10(6) and 10(7) dendritic cells. This dendritic cell population may be enriched to greater than 75% by a single-density gradient centrifugation step. We have generated a mAb that recognizes sheep CD1. This monoclonal not only reacts with afferent dendritic cells, but with dendritic cells in the skin and paracortical T cell areas of lymph nodes. The expression of CD1 suggests afferent dendritic cells are related to skin Langerhans' cells and other dendritic cells that act as accessory cells for T cell responses. Consistent with this is the high level of expression by dendritic cells of molecules involved in antigen recognition by T cells, including MHC class I and class II. Afferent dendritic cells express high levels of the cellular adhesion molecule LFA-3, and at the same time express a ligand for this molecule, namely CD2. The accessory functions of afferent dendritic cells resemble those displayed by mature Langerhans' cells and by lymph node interdigitating cells. These include clustering with resting T cells and stimulating their proliferation in a primary response to antigen. Afferent dendritic cells are capable of acquiring soluble protein antigen in vivo or in vitro and presenting the material directly to autologous T cells in an antigen-specific manner. We conclude that afferent dendritic cells represent a lymph-borne Langerhans' cell involved in antigen carriage to the lymph node.

Teladorsagia circumcincta has a widespread deleterious impact on sheep welfare and economic production. In order to devise effective methods of integrated control and selective breeding, it is necessary to understand the interaction between sheep and T. circumcincta. In this study, female Blackface lambs expected to be genetically variable for resistance to gastrointestinal nematodes were either exposed to a continuous experimental infection of T. circumcincta or sham dosed to monitor the phenotypic response to infection. As a measure of parasitism and host response, faecal eggs were counted over a three month period and post-mortem burdens ascertained. The host response to the infection was also described by differential count of white blood cells, IgA antibody level, and variation in body weight.Results suggest that nematode abundance (mean number of parasites per host) and prevalence (proportion of infected animals) were maximal shortly after the beginning of infection when virtually all the flock was infected and shed worm eggs. The host response was associated with increasing IgA antibody levels and eosinophil concentrations which, the data suggest, caused a reduction of total adult worms and an increase in the frequency of EAL4 (early arrested L4) worms.