Pathogenesis of mouse scrapie: effect of route of inoculation on infectivity titres and dose-response curves

A crucial role for B cells in neuroinvasive scrapie

Although prions are most efficiently propagated via intracerebral inoculation, peripheral administration has caused kuru [Gajdusek et al, 1966], iatrogenic Creutzfeldt-Jakob disease (CJD) [Gibbs et al, 1997], bovine spongiform encephalitis (BSE), and new variant CJD [Hill et al, 1997; Bruce et al, 1997]. Neurological disease after peripheral inoculation depends on prion expansion within cells of the lymphoreticular system (LRS) [Lasmezas et al. 1996; Wilesmith et al, 1992]. In order to identify the nature of the latter cells, we inoculated a panel of immune deficient mice with prions intraperitoneally. While defects affecting only T lymphocytes had no apparent effect, all mutations affecting differentiation and responses of B lymphocytes prevented development of clinical scrapie. Since absence of B cells and of antibodies correlates with severe defects in follicular dendritic cells (FDCs), the lack of any of these three components may prevent clinical scrapie. Yet, mice expressing immunoglobulins exclusively of the M subclass without detectable specificity for PrPc, and mice with differentiated B cells but lacking functional FDCs, developed scrapie after peripheral inoculation: therefore, differentiated B cells appear to play a crucial role in neuroinvasion of scrapie regardless of B-cell receptor specificity.

Late treatment with polyene antibiotics can prolong the survival time of scrapie-infected animals

Amphotericin B (AmB) is one of the few drugs able to prolong survival times in experimental scrapie and delays the accumulation of PrPres, a specific marker of this disease in the brain in vivo. Previous reports showed that the AmB effect is observed only if the drug is administered around the time of infection. In the present study, intracerebrally infected mice were treated with AmB or one of its derivatives, MS-8209, between 80 and 140 days postinoculation. We observed an increased incubation time and a delay in PrPres accumulation and glial fibrillary acidic protein gene expression. Treatment starting at 80 days postinoculation was as efficient as long-term treatment starting the day of inoculation. Our results indicate that polyene antibiotics may interfere, throughout the course of the experimental disease, with the propagation of the scrapie agent.

A cohort study to examine maternally-associated risk factors for bovine spongiform encephalopathy

This long-term cohort study, initiated in July 1989, was designed to examine maternally-associated risk factors for bovine spongiform encephalopathy (BSE), forming part of the epidemiological research programme to assess the risks of non-feedborne transmission of BSE. In this study, the incidence of BSE in offspring of cows which developed clinical signs of BSE is compared with that in offspring, born in the same calving season and herd, of cows which had reached at least six years of age and had not developed BSE. All offspring were allowed to live to seven years of age. The results indicate a statistically significant risk difference between the two cohorts of 9.7 per cent and a relative risk of 3.2 for offspring of cows which developed clinical BSE. However, there is some evidence that this enhanced risk for offspring of BSE cases declined the later the offspring was born, but was increased the later the offspring was born in relation to the stage of the incubation period of the dam. The results presented cannot distinguish between a genetic component and true maternal transmission or a combination of both risks, but they do not indicate either that the BSE epidemic will be unduly prolonged or that the future incidence of BSE in Great Britain will increase significantly.

Normal host prion protein (PrPC) is required for scrapie spread within the central nervous system

Mice devoid of PrPC (Prnp%) are resistant to scrapie and do not allow propagation of the infectious agent (prion). PrPC-expressing neuroectodermal tissue grafted into Prnp% brains but not the surrounding tissue consistently exhibits scrapie-specific pathology and allows prion replication after inoculation. Scrapie prions administered intraocularly into wild-type mice spread efficiently to the central nervous system within 16 weeks. To determine whether PrPC is required for scrapie spread, we inoculated prions intraocularly into Prnp% mice containing a PrP-overexpressing neurograft. Neither encephalopathy nor protease-resistant PrP (PrPSc) were detected in the grafts for up to 66 weeks. Because grafted PrP-expressing cells elicited an immune response that might have interfered with prion spread, we generated Prnp% mice immunotolerant to PrP and engrafted them with PrP-producing neuroectodermal tissue. Again, intraocular inoculation did not lead to disease in the PrP-producing graft. These results demonstrate that PrP is necessary for prion spread along neural pathways.

How to Limit the Spread of Creutzfeldt-Jakob Disease

Transmissible spongiform encephalopathies are rare lethal diseases induced in humans and animals by unconventional agents called transmissible spongiform encephalopathy agents (TSEAs), virions, or prions. Several cases of iatrogenic Creutzfeldt-Jakob disease (CJD) have been reported in the literature after neuro-surgery, treatment with pituitary-derived hormones, corneal grafting, and use of dura mater lyophilisates. In a given infected individual, TSEA-associated infectiousness depends on the nature of the organ: the central nervous system has the highest infectiousness, spleen and lymph nodes a medium infectiousness, and organs such as bone, skin, or skeletal muscles do not harbor any detectable infectiousness in experimental models. Transmissible spongiform encephalopathy/prions have unconventional properties; in particular, they resist almost all the chemical and physical processes that inactivate conventional viruses. Therefore, prevention of CJD agent transmission must be taken into account in daily hospital practice. Efficient sterilization procedures should be determined. In tissue and blood donation, donors with a neurologic history must be excluded, and patients treated with pituitary-derived hormones should be considered potentially infected with TSEA and excluded.

[Evaluation of transmission of unconventional agents by human albumin]

Transmissible spongiform encephalopathy agents (TSA) or prions induce neurodegenerative diseases in humans and animals. Their nature is still unknown, even if the main component of infectivity is identified as an abnormal isoform of a host-encoded protein, the prion protein (PrP). Today, no diagnostic test is available routinely for the detection of infected patients. TSA are resistant to most of the physical and chemical procedures that are efficient against other micro-organisms. Iatrogenic transmissions of TSA have been reported in the past: they always involved either brain derivatives or instruments that have been in contact with infected central nervous system. In an infected individual, infectivity is mostly detectable in brain. However, a persistent low-level viremia can be demonstrated in association with the white blood cells; infectivity is never found in plasma, serum or in red blood cells. Epidemiological data do not evidence any relationship between spongiform encephalopathies and blood transfusion. Therefore, in 1995, TSA transmission trough albumin is only a theoretical risk.

Experimental drug treatment of scrapie: a pathogenetic basis for rationale therapeutics

Pharmacological treatment with polyanions or amphotericin B in hamsters with experimental scrapie reveals that it is possible to delay the appearance of the disease only when the drug is given before the invasion of the agent into the clinical target areas of the brain. We suggest such early treatment may be possible for individuals at high risk of acquiring the disease, such as healthy mutation-positive relatives of patients with familial Creutzfeldt-Jakob disease or Gerstmann-Sträussler syndrome, or recipients of potentially contaminated pituitary-extracted human growth hormone.

Beef and bovine spongiform encephalopathy: the risk persists

Bovine spongiform encephalopathy (BSE) is one of the transmissible spongiform encephalopathies (TSE) that are currently known to the authors to affect only mammals, including man. The diseases are progressive, fatal paralyses and dementias, for which there are no methods of certain diagnosis and no treatment. In this review the disease in cattle, the mode of transfer of these TSEs between animals by mouth, the possible presence of infective agents in the food that we eat, the resistance of BSE to cooking, and the likelihood that humans may become infected are discussed. The origins of BSE, whether from sheep, from cows, or as a mutation are considered. Whatever the origin of BSE, a substantial danger for man exists. Creutzfeld-Jakob disease (CJD), a TSE of man, may have been derived from eating infected animal tissue in the past. The possibility that this was of bovine origin and the implications that this would have for BSE infected meat in human food are discussed.

Transgenetic studies implicate interactions between homologous PrP isoforms in scrapie prion replication

Transgenic (Tg) mice expressing both Syrian hamster (Ha) and mouse (Mo) prion protein (PrP) genes were used to probe the mechanism of scrapie prion replication. Four Tg lines expressing HaPrP exhibited distinct incubation times ranging from 48 to 277 days, which correlated inversely with HaPrP mRNA and HaPrPC. Bioassays of Tg brain extracts showed that the prion inoculum dictates which prions are synthesized de novo. Tg mice inoculated with Ha prions had approximately 10(9) ID50 units of Ha prions per gram of brain and less than 10 units of Mo prions. Conversely, Tg mice inoculated with Mo prions synthesized Mo prions but not Ha prions. Similarly, Tg mice inoculated with Ha prions exhibited neuropathologic changes characteristic of hamsters with scrapie, while Mo prions produced changes similar to those in non-Tg mice. Our results argue that species specificity of scrapie prions resides in the PrP sequence and prion synthesis is initiated by a species-specific interaction between PrPSc in the inoculum and homologous PrPC.

Intraperitoneal infection with scrapie is established within minutes of injection and is non-specifically enhanced by a variety of different drugs

Single intraperitoneal (i.p.) doses of 16 different drugs were given to mice 2 h before injecting scrapie i.p. Scrapie was injected as serial ten-fold dilutions of standard inocula and the effective titres obtained were used as a measure of the relative efficiency of infection in treated compared to saline injected mice. Despite the wide variety of drugs tested, most of them increased, non-specifically, the efficiency of infection by 0.6 to 2.1 log10 i.p. LD50 units (i.e., 4 to 126-fold), but only when both drug and scrapie were given i.p. The effect was greatest with a 2 h or a 6 h interval suggesting an involvement either of resident peritoneal cells or of elicited cells such as polymorphonuclear neutrophils. There was no increase in the efficiency of infection after intervals of 2 or 7 days when induced macrophages would predominant. The reverse sequence of injections (scrapie-0.5 h-drug) had no effect despite the persistence of high scrapie titre in the peritoneum at the time of drug injection. However, the effect was restored by a second injection of scrapie in the sequence, scrapie-drug-scrapie. It is concluded that scrapie infection is established within minutes of injection but much of the inoculum is associated with peritoneal cells which are irrelevant to pathogenesis. Drugs may enhance the infection of relevant peritoneal cells or their targeting to the visceral lymphoreticular tissues where early replication takes place.

The role of the spleen in the neuroinvasion of scrapie in mice

The pathogenesis of 139A scrapie has been studied in CW mice infected intraperitoneally (i.p.), intravenously (i.v.) or subcutaneously (s.c.). In mice splenectomised before i.p. infection, the evidence points to a neuroinvasive pathway from visceral lymph nodes (and other sites of scrapie replication in the peritoneum) to the thoracic spinal cord. However, in non-splenectomised mice, the major neuroinvasive pathway is clearly from spleen to thoracic cord because i.p. incubation periods are shorter and replication in the thoracic cord starts correspondingly earlier than in splenectomised mice. Studies of splenectomy at different times after i.p. infection show that pathogenesis becomes independent of the spleen once infection has initiated scrapie replication in the spinal cord. The simplest interpretation of all the evidence favours the spread of scrapie infection along splenic nerve fibres to the thoracic spinal cord. The same neuroinvasive pathway is suggested by the findings using the s.c. and i.v. routes of infection. In addition it was found that the 100-fold greater efficiency of infection by the i.v. compared to the i.p. route was entirely dependent on the spleen, because splenectomy before i.v. infection reduced its efficiency to the same as that found in i.p. infected (non-splenectomised) mice.

Effects of the antiserum against a fraction enriched in scrapie-associated fibrils on the scrapie incubation period in mice

An antiserum against a fraction enriched for scrapie-associated fibrils (SAF), was examined for its effects on scrapie incubation period by inoculating mice either intraperitoneally or intracerebrally with various dilutions of the serum mixed with scrapie-infected mouse brain homogenate. After intraperitoneal inoculation the mean time of the incubation period increased with increasing concentrations of the antiserum in a statistically significant fashion, when the serum dilutions were made with phosphate-buffered saline. After intracerebral inoculation, however, there were no statistically significant differences between the control group and any of the antiserum-groups. When the antiserum dilutions were made with pre-immune serum, the mice inoculated intraperitoneally also showed no significant differences between the two groups. These results indicate that the specific antibodies to SAF have no effect on the scrapie infectivity.

Temporal distribution of transmissible mink encephalopathy virus in mink inoculated subcutaneously

Information was sought on the temporal distribution of transmissible mink encephalopathy virus in royal pastel mink inoculated subcutaneously with 10(3.0) 50% intracerebral lethal doses of the Idaho strain. As determined by intracerebral assay in mink, extremely little replication of the virus occurred during the preclinical stage of infection. It seemed largely limited to lymph nodes draining the site of inoculation. Virus first appeared in the central nervous system (CNS) at 20 weeks, when all mink were still clinically normal. Early spongiform degeneration, limited to the posterior sigmoid gyrus of the frontal cortex, was first found at 28 weeks, or a few weeks before onset of clinical disease in most of the mink. Once virus reached the CNS, where greater concentrations occurred than elsewhere, it appeared in many extraneural sites (spleen, liver, kidney, intestine, mesenteric lymph node, and submandibular salivary gland). These seemingly anomalous findings, especially the limited extraneural replication of virus as a prelude to infection of the CNS, suggest that mink are not natural hosts of the virus. The results of this study support the generally held view that transmissible mink encephalopathy arises from chance or inadvertent infection of ranch mink with an exogenous virus, most likely feed-borne wild scrapie virus.

Ionising radiation has no influence on scrapie incubation period in mice

Scrapie has an early non-clinical stage when replication of agent occurs in lymphoreticular organs. Whole-body irradiation failed to alter the incubation or neuropathology of the disease. Many experiments were carried out with different strains of scrapie agent and host, doses and timing of irradiation. The results suggest that mitotically quiescent cells are involved in agent replication.

Targeting of scrapie lesions and spread of agent via the retino-tectal projection

Scrapie infectivity and degenerative vacuolation was initially localized within the contralateral superior colliculus following intraocular injection. The time course of these events was prolonged. With the ME7 strain of scrapie in Sincs7 genotype mice, infectivity began to rise in the superior colliculus from about 70 days, followed by the earliest asymmetrical lesions there from 120 days, with death occurring at about 250 days, at which time vacuolar degeneration was widespread in the brain. With other mouse Sinc genotype mouse/agent strain combinations the process was even further prolonged. With 87V scrapie strain in Sincp7 genotype mice the first lesions to appear were in the contralateral tectum at 300 days. It is concluded that scrapie agent can spread within ganglion cell axons.

Attempts to establish the scrapie agent in cell lines

Attempts were made to establish a persistent infection with scrapie agent in four murine cell lines. Of the four lines tested, viz. P388D1, P3-NS1-Ag-1 (NS1), L cells and an NS1 spleen cell hybrid, only the NS1 cell line showed any evidence of agent replication. Ten per cent dimethylsulphoxide (DMSO) was included in the culture media of L cells inoculated with scrapie agent. This treatment raised the initial levels of scrapie agent associating with the L cells but did not result in a persistently infected cell line. An effect of DMSO in the inoculum was observed in mice inoculated intraperitoneally, the incubation period of the disease was considerably shortened.

Pathogenesis of mouse scrapie. Evidence for direct neural spread of infection to the CNS after injection of sciatic nerve

Previous studies of peripherally injected mouse scrapie suggested that invasion of the CNS occurs initially in mid-thoracic cord by neural spread of infection from spleen and other visceral sites of extraneural replication. We now show that infection of the left sciatic nerve leads to direct spread of infection to brain (at a rate of approximately 1.0-2.0 mm/day), bypassing the need for extraneural replication and thus producing shorter incubation periods. However, the efficiency of intraneural infection is low. It can be increased by crush injury or by the injection of lysophosphatidyl choline, both of which temporarily increase the surface area of axolemma exposed to inoculum. Once infection is established, agent seems to spread throughout the nervous system but, at the clinical stage of disease, the titres in the PNS are much lower than in the CNS.

Disinfection studies with two strains of mouse-passaged scrapie agent. Guidelines for Creutzfeldt-Jakob and related agents

A variety of disinfection procedures were tested on two strains of scrapie agent, treated either as brain macerates (autoclaving) or as 10% homogenates (chemical treatments). It is suggested that a given treatment should produce a titre loss, of both strains of scrapie, of at least 10(4) units before it be regarded as useful for the disinfection of the agents of scrapie and Creutzfeldt-Jakob disease (CJD). By this criterion, treatment at room temperature with about 4% Hycolin (0.6% chlorinated phenols), 0.2% permanganate, 5% Tego (dodecyl-di(aminoethyl)-glycine) or 5% sodium dodecyl sulphate (SDS) are unsuitable. However, data indicate that SDS might be used to reduce the heat stability of scrapie agent. Hypochlorite (Sterilex) was the only satisfactory chemical reagent tested. At least 10(4)-10(5) units of infectivity were lost by treatment with hypochlorite containing 1,000 ppm available chlorine after a 4-16 h exposure, or containing 10,000 ppm available chlorine after a half-hour exposure. The latter result points to the use of concentrated hypochlorite (about 2% available chlorine; approximately 20% Sterilex) to decontaminate surfaces. We suggest that the cleaning action of SDS, or other strong detergents, might also help to decontaminate surfaces, but studies on this are needed. Autoclaving at 126 degrees C for 1-2 h reduced titres by 10(3)-10(7) units, depending on the strain of agent. However, total disinfection of brain containing high titres of infectivity was approached only at 136 degrees C when titre losses of about 10(6) units were obtained by autoclaving for 4-32 min. Further studies are needed before we can make simple, general recommendations for the disinfection of CJD agents in hospital practice.

The antiviral compound HPA-23 can prevent scrapie when administered at the time of infection

The effects of up to 12 daily doses of HPA-23 (ammonium 5-tungsto-2-antimoniate) on scrapie were studied using five experimental models of the disease, some with widely different incubation times. Treatment of animals with HPA-23, starting just before injecting scrapie, produced survivors. In some experiments, animals were fully protected against several hundred infectious units of scrapie. Treatment of animals after infection was far less effective. Prolonging the treatment for several weeks was no more effective than treating for a few days. In a very long incubation model of scrapie, the course of 12 daily doses of HPA-23 represented less than 3 per cent of the total incubation period. HPA-23 may interact with certain cells of the lymphoreticular system, preventing infection and, perhaps, the early replication of agent.

Effect of route of infection on the frequency and distribution of cerebral amyloid plaques in scrapie mice

Cerebral amyloid plaques are a conspicuous pathological feature in mice infected with certain strains of scrapie. The origin of the amyloid protein in these plaques, whether it is locally or systemically synthesized, and the mechanisms leading to its deposition are not known. The frequency of plaques and their distribution in the brain are greatly influenced by the route of injection of the scrapie inoculum. Intracerebral injection consistently results in greater numbers of plaques than are obtained if the same inoculum is introduced peripherally. Following intracerebral injection, plaques are commonly seen in areas close to the lateral ventricles, for example the corpus callosum and hippocampus, whereas they are absent from these areas with peripheral routes. Furthermore, when left- and right- sided intracerebral injections are compared, plaques are more frequent on the side of injection. These results suggest that the distribution of amyloid plaques is influenced either by the localization of some component of the inoculum or by traumatic damage at injection. The most plausible explanation is that amyloid deposition is associated with local concentrations of scrapie infectivity and that the amyloid protein originates in the brain.