Prion contamination in the emergency room

prion conversionHere is a follow-up to last week’s article that described a case of variant Creutzfeldt-Jacob disease in a Texas resident caused by ingestion of BSE-contaminated beef 14 years ago.

A 59 year old male patient was admitted to the trauma unit in Lancaster, PA with a self-inflicted gunshot wound to the head. There was substantial bleeding and brain tissue extrusion from the bullet exit wound. While the patient was intubated, examination of his electronic health records revealed a previous diagnosis of Creutzfeldt-Jacob disease (CJD). After discussion with his family, the breathing tube was removed and the patient expired.

After discovering that the patient had CJD, TSE (transmissible spongiform encephalopathy) decontamination protocols were initiated. Equipment and surfaces that had been exposed to highly infectious brain tissues were identified. Because prions are extremely difficult to destroy, it was decided to incinerate many pieces of equipment costing tens of thousands of dollars. This decision was taken to protect workers in the trauma unit and future hospital patients from hospital-acquired CJD.

The usual sterilization conditions (121 degrees Celsius for 20 minutes under high pressure) do not destroy prion protein infectivity. Consequently the World Health Organization recommends incineration of potentially contaminated materials. While environmental transmission of prion diseases has not been reported, WHO suggests rinsing surfaces with sodium hydroxide or sodium hypochlorite for 1 hour, followed by flooding with water, to remove prions.

This case illustrates the problems associated with an unusual infectious agent, the prion, that is difficult to inactivate. It also shows the value of electronic health records. Without such readily accessible information, the discovery that the patient had CJD would have been substantially delayed, leading to further contamination.

Creutzfeldt-Jacob associated deaths have increased slowly but steadily in the US since 1979. The number of cases will likely continue to increase until early diagnosis tests become routinely available, and drugs are developed that can cure the disease.

A case of prion disease acquired from contaminated beef

prion conversionSpongiform encephalopathies are neurodegenerative diseases caused by misfolding of normal cellular prion proteins. A 2014 case of variant Creutzfeldt-Jacob prion disease in the United States was probably caused by eating beef from animals with bovine spongiform encephalopathy (BSE), or mad cow disease.

Human spongiform encephalopathies are placed into three groups: infectious, familial or genetic, and sporadic, distinguished by how the disease is acquired initially. In the mid 1980s, a prion disease called bovine spongiform encephalopathy appeared in cows in the United Kingdom. It is believed to have been transmitted to cows by feeding them meat and bone meal, a high protein supplement prepared from the offal of sheep, cattle, pigs, and chicken. Some of the animals prepared for feed likely had a prion disease. Cases of variant Creutzfeld-Jakob disease, a new spongiform encephalopathy of humans, began to appear in 1994 in Great Britain. They were characterized by a lower mean age of the patients (26 years), longer duration of illness, and differences in other clinical and pathological characteristics. Variant Creutzfeldt-Jakob disease (vCJD) is caused by prions transmitted by the consumption of cattle with bovine spongiform encephalopathy.

In late 2012 a male Texas resident began showing symptoms of depression and anxiety, followed by delusions, hallucinations, and other changes in behavior. Over the next 18 months the patient’s condition deteriorated, leading to inability to ambulate or speak, and after several episodes of aspiration pneumonia and sepsis the patient died. During the illness prion disease was suspected, but tests for this condition were negative. After death, examination of brain biopsies revealed typical prion plaques, and misfolded prion proteins were found in urine, confirming the diagnosis of variant Creutzfeldt-Jacob disease.

The source of the patient’s prion disease was likely consumption of contaminated beef from cows with bovine spongiform encephalopathy. The patient probably acquired the infection in Russia, Lebanon, or Kuwait, three countries that had received BSE-contaminated beef from the UK, and and where he had previously lived. He resided in the US for 14 years before developing symptoms, an incubation period consistent with models of the vCJD epidemic.

This Texas patient is the fourth worldwide since 2012 to be diagnosed with vCJD; the others were from the UK and France. There are likely to be additional cases of vCJD in the future: surveys of archived appendix tissues in the UK show that 1 in 2,000 persons born during 1941-1985 have asymptomatic vCJD infection. These individuals could transmit the misfolded prion proteins to others via transplantation, blood transfusion, or surgical instruments (prion infectivity is not destroyed by autoclaving).

The good news is that vCJD is rare: there have been 230 reported cases of vCJD worldwide caused by consumption of BSE beef. The bad news is that vCJD will probably continue to appear in humans for many years, not only from the aftermath of the BSE epidemic. Rare cows spontaneously develop BSE, and because cattle are slaughtered before disease symptoms are evident, contaminated meat could enter the food supply.

TWiV 343: The silence of the turnips

On episode #343 of the science show This Week in Virology, the TWiVerinoes discuss the potential for prion spread by plants, global circulation patterns of influenza virus, and the roles of Argonautes and a viral protein in RNA silencing in plants.

You can find TWiV #343 at

Prions in plants

prions in plants

Chronic wasting disease is a prion disease of cervids (deer, elk, moose) that is potentially a threat to human health. A role for environmental prion contamination in transmission is supported by the finding that plants can take up prions from the soil and transmit them to animals.

A concern is that prions of chronic wasting disease could be transmitted to cows grazing in pastures contaminated by cervids. Consumption of infected cows would then pass the disease on to humans. When deer are fed prions they excrete them in the feces before developing clinical signs of infection, and prions can also be detected in deer saliva. In the laboratory, brain homogenates from infected deer can transmit the disease to cows.

To determine whether prions can enter plants, wheat grass roots and leaves were exposed to brain homogenates from hamsters that had died of prion disease. The plant materials were then washed and amounts of prions were determined by protein misfolding cyclic amplification. Prions readily bound these plant tissues, at low concentrations and after as little as 2 minutes of incubation. Mouse, cervid, and human prions also bound to plant roots and leaves. When living wheat grass leaves were sprayed with a 1% hamster brain homogenate, prions could attach to the leaves and be detected for 49 days.

To determine if prions in plants could infect animals, plants were exposed to brain homogenates, washed thoroughly, and then fed to hamsters. The positive control for this experiment was to feed hamsters the brain homogenates. All animals fed infected plants or brain homogenates succumbed to prion disease.

Plants can also take up prions from animal waste. This conclusion was reached by incubating leaves and roots for 1 hour with urine or feces obtained from prion-infected hamsters or cervids. Prions were readily detected in these samples, even after extensive washing.

Experiments were also done to examine whether plants could take up prions from the soil. Barley grass plants were grown on soil that had been mixed with hamster brain homogenate, and then 1-3 weeks later, stem and leaves were assayed for the presence of prions. Small amounts of prions were detected in stems from all plants, while 1 in 4 plants contained prions in leaves, at levels that should be able to infect an animal.

These results show that prions can bind to plants and be taken into the roots, where they may travel to the stem and leaves. Therefore it is possible that prions excreted by deer could pass on to other animals, such as grazing cows, or even humans consuming contaminated plants (illustrated – image credit). Cooking plants will not eliminate infectivity, just as cooking contaminated beef did not halt the spread of bovine spongiform encephalopathy. Keeping cervids out of grazing or growing fields should be considered as a way to manage the risk of prions entering the human food chain.

Resistance to prion disease in humans

prion conversionTransmissible spongiform encephalopathies (TSEs) are rare human neurodegenerative disorders that are caused by infectious proteins called prions. A naturally occurring variant of the human prion has been found that completely protects against the disease.

A protective variant of the prion protein was discovered in the Fore people of Papua New Guinea. Beginning in the early 1900s, the prion disease kuru spread among Fore women and children as a result of ritual cannibalism of the brains of deceased relatives. When cannibalism stopped in the late 1950s, kuru disappeared.

Survivors of the kuru epidemic are heterozygous for a prion protein gene (prnp) with a unique amino acid change not seen in other populations, a change at position 127 from glycine to valine (G127V). The G127V change was always seen together with methionine at 129. Heterozygosity for M and V at amino acid 129, which is protective against prion disease, is found in humans all over the world.

Transgenic mice were used to determine if the G127V change in prion protein protects against disease. These mice lack the murine prnp gene (which encodes the normal prion protein) and contain a copy of either the wild type human prnp gene, or one with changes at amino acids 127 and 129. The mice were then inoculated intracerebrally with brain extracts from individuals who died of kuru. Mice with wild type human prnp were susceptible to infection. In contrast, transgenic mice heterozygous for the variant prnp (G127M129/V127M129) were completely resistant to infection. The mice were also resistant to infection with prions from cases of another human TSE, Creutzfeldt-Jacob disease.

Prnp transgenic mice were also challenged with variant Creutzfeldt-Jacob disease prions. This novel TSE arose after consumption of beef from animals with the prion disease bovine spongiform encephalopathy (BSE). These mice were susceptible to infection with vCJD prions, not a surprising result given that the Fore people were never exposed to BSE prions. However, mice homogygous for the altered prnp (V127M129/V127M129) were completely resistant to infection with vCJD prions – as resistant as mice with no prnp genes.

The protective effect of the M129V polymorphism is thought to be a consequence of inhibition of protein-protein interactions during prion propagation (i.e. the conversion of normal prion to pathogenic prion). How the G127V change confers protection is unknown.

These results show that the G127V change confers resistance to kuru and was likely selected as a consequence of the epidemic. If kuru had not been stopped by the abolition of cannibalism, it likely would have been self-limiting, as individuals with resistance to the disease, caused by the G127V change, repopulated the Fore people.

Is chronic wasting disease a threat to humans?

Areas with CWDChronic wasting disease (CWD) is a prion disease of cervids (deer, elk, moose). It was first detected in Wyoming and Colorado, and has since spread rapidly throughout North America (illustrated; image credit). Because prions that cause bovine spongiform encephalopathy (BSE, mad cow disease) are known to infect humans, there is concern that CWD might also cross the species barrier and cause a novel spongiform encephalopathy. Recent experimental results suggest that CWD prions are not likely to directly infect humans.

The prion protein PrPC is encoded by the prnp gene, which is essential for the pathogenesis of transmissible spongiform encephalopathies (TSEs). Transgenic mice have been used to understand the species barrier to prion transmission. When mice are inoculated with human prions, few animals develop disease and the incubation periods are over 500 days. When the mouse prnp gene is replaced with the human gene, the mice become uniformly susceptible to infection with human prions and the incubation period is shorter. The species barrier to prion transmission is therefore associated with differences in the prion protein sequence between host and target species.

Mice have been used to understand whether CWD prions might be transmitted to humans. Mice are not efficiently infected with CWD prions unless they are made transgenic for the cervid prnp gene. Four different research groups have found that  mice transgenic for the human prnp gene are not infected by CWD prions. These findings suggest that CWD prions are not likely to be transmitted directly to humans. However, changing four amino acids in human prnp to the cervid sequence allows efficient infection of transgenic mice with cervid prions.

Another concern is that prions of chronic wasting disease could be transmitted to cows grazing in pastures contaminated by cervids. Prions can be detected in deer saliva and feces, and contamination of grass could pass the agent on to cows. In the laboratory, brain homogenates from infected deer can transmit the disease to cows. Therefore it is possible that cervid prions could enter the human food chain through cows.

A further worry is that BSE prions shed by cows in pastures might infect cervids, which would then become a reservoir of the agent. BSE prions do not infect mice that are transgenic for the cervid prnp gene. However, intracerebral inoculation of deer with BSE prions causes neurological disease,  and the prions from these animals can infect mice that are transgenic for the cervid prnp gene. Therefore caution must be used when using transgenic mice to predict the abilities of prions to cross species barriers.

Although the risk of human infection with CWD prions appears to be low, hunters should not shoot or consume an elk or deer that is acting abnormally or appears to be sick, to avoid the brain and spinal cord when field dressing game, and not to consume brain, spinal cord, eyes, spleen, or lymph nodes. No case of transmission of chronic wasting disease prions to deer hunters has yet been reported.

Detecting prions by quaking and shaking

prion conversionThe human prion disease, Creutzfeld-Jacob, is diagnosed by a variety of criteria, including clinical features, electroencephalograms, and magnetic resonance imaging. Until recently there was no non-invasive assay to detect PrPSc, the only specific marker for the disease. This challenge has been overcome using amplification procedures to detect Creutzfeldt-Jakob prions in nasal brushings and in urine.

These assays utilize two different methods for amplifying the quantity of prions in vitro. In real-time quaking-induced conversion, PrPC (produced in E. coli) is mixed with a small amount of PrPSc. The mixtures are subjected to cycles of shaking and rest at 42°C for 55-90 hours, which leads to the formation of amyloid fibrils that can be detected by fluorescence. The assay can detect femtogram levels of PrPSc in brain homogenates from humans with Creutzfeld-Jacob disease. In protein misfolding cyclic amplification, samples are incubated for 30 minutes at 37°- 40°C, followed by a pulse of sonication, and this cycle is repeated 96 times. Prions are detected by western blot analysis after treatment with proteinase K. This process can detect a single oligomeric PrPSC.

Two non-invasive assays using these amplification approaches were developed. The first is a nasal-brushing procedure to sample the olfactory epithelium, where PrPSc is known to accumulate in patients with the disease. The real-time quaking-induced conversion assay was positive in 30 of 31 patients with Creutzfeld-Jacob disease, and negative in 43 of 43 healthy controls (a sensitivity of 97%). Furthermore, nasal brushings gave stronger and faster positive results than cerebrospinal fluid in this assay. The high concentrations of PrPSc detected in nasal brushings suggest that prions can contaminate nasal discharge of patients with the disease, a possible source of iatrogenic transmission, which has implications for infection control.

Protein misfolding cyclic amplification was used to assay for the presence of PrPSc in the urine of patients with variant Creutzfeldt-Jakob disease (caused by ingestion of beef from cows with bovine spongiform encephalopathy), which had been previously shown to contain prions. PrPSc was detected in 13 of 14 urine samples from patients with the disease, but not in 224 urine samples from healthy controls and patients with other neurologic diseases, including other TSEs. The estimated concentration of PrPSc in urine was 40-100 oligomeric particles per ml.

Because Creutzfeldt-Jakob disease is so rare, any assay for the disease must have near-perfect specificity. A problem with both cycling assays is that PrPC converts into oligomers and fibrils in the absence of PrPSc. Additional work is needed to address this problem. Nevertheless it is possible that these assays could one day lead to earlier diagnosis and treatments, if the latter become available.

Infectious agents with no genome

prion conversionIf the reader does not believe that viroids and satellites are distinctive, then surely prions, infectious agents composed only of protein, must impress.

The question of whether infectious agents exist without genomes arose with the discovery and characterization of infectious agents associated with a group of diseases called transmissible spongiform encephalopathies (TSEs). These diseases are rare, but always fatal, neurodegenerative disorders that afflict humans and other mammals. They are characterized by long incubation periods, spongiform changes in the brain associated with loss of neurons, and the absence of host responses. TSEs are caused by infectious proteins called prions.

The first TSE recognized was scrapie, so called because infected sheep tend to scrape their bodies on fences so much that they rub themselves raw. Scrapie has been recognized as a disease of European sheep for more than 250 years. It is endemic in some countries, for example, the United Kingdom, where it affects 0.5 to 1% of the sheep population each year.

Sheep farmers discovered that animals from affected herds could pass the disease to a scrapie-free herd, implicating an infectious agent. Infectivity from extracts of scrapie-affected sheep brains was shown to pass through filters with pores small enough to retain everything but viruses. As early as 1966, scrapie infectivity was shown to be considerably more resistant than that of most viruses to ultraviolet (UV) and ionizing radiation. Other TSE agents exhibit similar UV resistance. On the basis of this relative resistance to UV irradiation, some investigators argued that TSE agents are viruses well shielded from irradiation, whereas others claimed that TSE agents have little or no nucleic acids.

Several lines of evidence indicated that human spongiform encephalopathies might be caused by an infectious agent. Carleton Gajdusek and colleagues studied the disease kuru, found in the Fore people of New Guinea. This disease is characterized by cerebellar ataxia (defective motion or gait) without loss of cognitive functions. Kuru spread among women and children as a result of ritual cannibalism of the brains of deceased relatives. When cannibalism stopped in the late 1950s, kuru disappeared. Others observed that lesions in the brains of humans with kuru were similar to lesions in the brains of animals with scrapie. It was soon demonstrated that kuru and other human TSEs can be transmitted to chimpanzees and laboratory animals.

Human spongiform encephalopathies are placed into three groups: infectious, familial or genetic, and sporadic, distinguished by how the disease is acquired initially. An infectious (or transmissible) spongiform encephalopathy is exemplified by kuru and iatrogenic spread of disease to healthy individuals by transplantation of infected corneas, the use of purified hormones, or transfusion with blood from patients with the TSE Creutzfeldt-Jakob disease (CJD). Over 400 cases of iatrogenic Creutzfeldt-Jakob disease have been reported worldwide. The epidemic spread of bovine spongiform encephalopathy (mad cow disease, see below) among cattle in Britain can be ascribed to the practice of feeding processed animal by-products to cattle as a protein supplement. Similarly, the new human disease, variant CJD, arose after consumption of beef from diseased cattle. Sporadic CJD is a disease affecting one to five per million annually, usually late in life (with a peak at 68 years). As the name indicates, the disease appears with no warning or epidemiological indications. Kuru may have been originally established in the small population of Fore people in New Guinea when the brain of an individual with sporadic CJD was eaten. Familial spongiform encephalopathy is associated with an autosomal dominant mutation in the prnp gene. Together familial and sporadic forms of prion disease account for ~99% of all cases.

Clinical signs of infection commonly include cerebellar ataxia, memory loss, visual changes, dementia, and akinetic mutism, with death occurring after months or years. Once the infectious agent is in the central nervous system, the characteristic pathology includes severe astrocytosis, vacuolization (hence the term spongiform), and loss of neurons. There are no inflammatory, antibody, or cellular immune responses.

The unconventional physical attributes and slow infection pattern originally prompted many to argue that TSE agents are not viruses at all. In 1967 it was suggested that scrapie could be caused by a host protein, not by a nucleic acid-carrying virus.

An important breakthrough occurred in 1981, when characteristic fibrillar protein aggregates were visualized in infected brains. These aggregates could be concentrated by centrifugation and remained infectious. Stanley Prusiner and colleagues isolated a protein with unusual properties from scrapie-infected tissue. This protein is insoluble and relatively resistant to proteases. He named the scrapie infectious agent a prion, from the words protein and infectious.

Prusiner’s unconventional proposal was that an altered form of a normal cellular protein, called PrPC, causes the fatal encephalopathy characteristic of scrapie. This controversial protein-only hypothesis caused a firestorm among those who study infectious disease. The hypothesis was that the essential pathogenic component is the host-encoded PrPC protein with an altered conformation, called PrPsc (“PrP-scrapie”). Furthermore, in the simplest case, PrPSc was proposed to have the property of converting normal PrPC protein into more copies of the pathogenic form (illustrated). In recognition of his work on prions, Prusiner was awarded the Nobel Prize in physiology or medicine in 1997.

Sequence analysis of this protein led to the identification of the prnp gene, which is highly conserved in the genomes of many mammals, including humans. Expression of the prnp gene is now known to be essential for the pathogenesis of TSEs. The prnp gene encodes a 35-kDa membrane-associated neuronal glycoprotein, PrPC. The function of this protein has been difficult to determine because mice lacking both copies of the prnp gene develop normally and have few obvious defects. However, these mice are resistant to TSE infection, showing that PrPC is essential for prion propagation.

The discovery of the prnp gene has helped explain the basis of familial TSE diseases such as Creutzfeldt-Jakob disease, Gerstmann-Straussler-Scheinker disease, and fatal familial insomnia. Gerstmann-Straussler-Scheinker disease is associated with a change at PrPC amino acid 102 from proline to leucine. Introduction of this amino acid change into mice gives rise to a spontaneous neurodegenerative disease characteristic of a TSE. Familial Creutzfeldt-Jakob disease may be associated with an insertion of 144 base pairs at codon 53, or changes at amino acids 129, 178, or 200. In fatal familial insomnia, adults develop a progressive sleep disorder and typically die within one year. Development of this disease is strongly linked to the D178N amino acid change.

In the mid 1980s, a new disease appeared in cows in the United Kingdom: bovine spongiform encephalopathy, also called mad cow disease. It is believed to have been transmitted to cows by feeding them meat and bone meal, a high protein supplement prepared from the offal of sheep, cattle, pigs, and chicken. In the late 1970s the method of preparation of meat and bone meal was changed, resulting in material with a higher fat content. It is believed that this change allowed prions, from either a diseased sheep or cow, to retain infectivity and pass on to cattle. Before the disease was recognized in 1985, it was amplified by feeding cows the remains of infected bovine tissues: the incubation period for bovine spongiform encephalopathy is 5 years, but disease was not observed because most cattle are slaughtered between 2-3 years of age. Three years later, as the number of cases of mad cow disease increased, a ban on the use of meat and bone meal was put in place, a practice that together with culling infected cattle has stopped the epidemic. Over 180,000 cattle, mostly dairy cows, died of bovine spongiform encephalopathy from 1986-2000.

Cases of variant Creutzfeld-Jakob disease, a new TSE of humans, began to appear in 1994 in Great Britain. These were characterized by a lower mean age of the patients (26 years), longer duration of illness, and differences in other clinical and pathological characteristics. The results of epidemiologic and experimental studies indicate that variant Creutzfeld-Jakob disease is caused by prions transmitted by the consumption of cattle with bovine spongiform encephalopathy. As of 2011 there had been 175 cases of variant Creutzfeld-Jakob disease in the United Kingdom, and 215 globally.

Bovine spongiform encephalopathy continues to be detected in cattle. As of April 2012, 4 cases have been identified in the United States and 19 in Canada. These cases may arise sporadically, or through consumption of contaminated feed. Because cattle are slaughtered before disease symptoms are evident, there is concern that variant Creutzfeldt-Jakob might increase as contaminated meat enters the food supply. These concerns are being addressed by imposing bans on animal protein-containing feed, and increased surveillance of cows for the disease, for which diagnostic tests are being developed.

Chronic wasting disease is a transmissible spongiform encephalopathy of cervids such as deer, elk, and moose. It is the only known TSE to occur in free-ranging animals. The disease has been reported in the United States, Canada, and South Korea. In captive herds in the US and Canada up to 90% of mule deer and 60% of elk are infected, and the incidence in wild cervids is as high as 15%. Hunters are advised not to shoot or consume an elk or deer that is acting abnormally or appears to be sick, to avoid the brain and spinal cord when field dressing game, and not to consume brain, spinal cord, eyes, spleen, or lymph nodes. No case of transmission of chronic wasting disease prions to deer hunters has yet been reported.

It is not known how the disease is spread among cervids, but transmission by grass contaminated with saliva and feces is one possibility. When deer are fed prions they excrete them in the feces before developing clinical signs of infection, and prions can also be detected in deer saliva. In the laboratory, brain homogenates from infected deer can transmit the disease to cows. A concern is that prions of chronic wasting disease could be transmitted to cows grazing in pastures contaminated by cervids.

Since prions were discovered it has become clear that they cause a wider spectrum of neurodegenerative diseases. For example, the amyloid fibrils in Alzheimer’s disease contain the amyloid-beta peptide that is processed from the amyloid precursor protein; familial disease is caused by mutations in the gene for this protein. Mutations in the tau gene are responsible for heritable tauopathies including familial frontotemporal dementia and inherited progressive supranuclear palsy. Self-propagating tau aggregates pass from cell to cell. The prion-like spread of misfolded alpha-synuclein is believed to be involved in Parkinson’s disease. In these cases there is good evidence that the causative protein, like PrPSc, adopts a conformation that becomes self-propagating.

Despite the involvement of prions in human neurological diseases, in other organisms such proteins are not pathogenic but rather impart diverse functions through templated conformational change of a normal cellular protein. Such prions have been described in fungi where they do not form infectious particles and do not spread from cell to cell. These proteins change conformation in response to an environmental stimulus and acquire a new, beneficial function. An example is the Saccharomyces cerevisiae Ure2p protein, which normally is a nitrogen catabolite repressor when cells are grown in the presence of a rich source of nitrogen. In the aggregated prion state, called [URE3], the protein allows growth on poor nitrogen sources. These findings prompt the question of whether the conversion of PrPC to PrPSC once had a beneficial function that became pathogenic. If so, identifying that function, and how it was usurped, will be important for understanding the pathogenesis of transmissible spongiform encephalopathies.

TWiV 299: Rocky Mountain virology

On episode #299 of the science show This Week in VirologyVincent visits the Rocky Mountain Laboratories in Hamilton, Montana and speaks with Marshall Bloom, Sonja Best, and Byron Caughey about their work on tick-born flaviviruses, innate immunity, and prion diseases.

You can find TWiV #299 at

TWiV 181: ORFan poxviruses and nIRFing prions

On episode #181 of the science show This Week in Virology, Vincent, Rich, and Kathy discuss Cotia virus, a new poxvirus, Orf virus infections associated with handling goats and lamb, and the innate immune response to prions.

You can find TWiV #181 at