Prominent and Persistent Extraneural Infection in Human PrP Transgenic Mice Infected with Variant CJD
Posted Apr 09 2009 7:13pm
Prominent and Persistent Extraneural Infection in Human PrP Transgenic Mice Infected with Variant CJD
Vincent Béringue1*, Annick Le Dur1, Philippe Tixador1, Fabienne Reine1, Laurence Lepourry2, Armand Perret-Liaudet3, Stéphane Haïk4,5, Jean-Luc Vilotte2, Michel Fontés6, Hubert Laude1*
1 Institut Scientifique de Recherche Agronomique (INRA), UR892, Virologie Immunologie Moléculaires, Jouy-en-Josas, France, 2 Institut Scientifique de Recherche Agronomique (INRA), UR339, Génétique Biochimique et Cytogénétique, Jouy-en-Josas, France, 3 Service de Neurobiologie, Centre de Biologie et Pathologie Est (CBPE), Groupement Hospitalier Est des Hôpitaux de Lyon, Bron, France, 4 INSERM, Equipe Avenir, Maladies à Prions chez l'Homme, Paris, France, 5 Assistance Publique - Hôpitaux de Paris (AP-HP), Laboratoire de Neuropathologie R. Escourolle, Groupe Hospitalier Pitié- Salpétrière, Paris, France, 6 INSERM UMR 491-IPHM, Faculté de médecine de la Timone, Marseille, France
The evolution of the variant Creutzfeldt-Jakob disease (vCJD) epidemic is hazardous to predict due to uncertainty in ascertaining the prevalence of infection and because the disease might remain asymptomatic or produce an alternate, sporadic-like phenotype.
Transgenic mice were produced that overexpress human prion protein with methionine at codon 129, the only allele found so far in vCJD-affected patients. These mice were infected with prions derived from variant and sporadic CJD (sCJD) cases by intracerebral or intraperitoneal route, and transmission efficiency and strain phenotype were analyzed in brain and spleen. We showed that i) the main features of vCJD infection in humans, including a prominent involvement of the lymphoid tissues compared to that in sCJD infection were faithfully reproduced in such mice; ii) transmission of vCJD agent by intracerebral route could lead to the propagation of either vCJD or sCJD-like prion in the brain, whereas vCJD prion was invariably propagated in the spleen, iii) after peripheral exposure, inefficient neuroinvasion was observed, resulting in an asymptomatic infection with life-long persistence of vCJD prion in the spleen at stable and elevated levels.
Our findings emphasize the possibility that human-to-human transmission of vCJD might produce alternative neuropathogical phenotypes and that lymphoid tissue examination of CJD cases classified as sporadic might reveal an infection by vCJD-type prions. They also provide evidence for the strong propensity of this agent to establish long-lasting, subclinical vCJD infection of lymphoreticular tissues, thus amplifying the risk for iatrogenic transmission.
Citation: Béringue V, Le Dur A, Tixador P, Reine F, Lepourry L, et al. (2008) Prominent and Persistent Extraneural Infection in Human PrP Transgenic Mice Infected with Variant CJD. PLoS ONE 3(1): e1419. doi:10.1371/journal.pone.0001419
Academic Editor: Adam Ratner, Columbia University, United States of America
Received: September 20, 2007; Accepted: December 17, 2007; Published: January 9, 2008
Funding: This work was supported by INRA, Institut de Veille Sanitaire (InVS) and the Ministry of Research, France. The sponsors of this study had no role in study conduct, collection analysis, interpretation of the data, writing of the report or approval of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
In this study we used tg650 mice, a newly developed transgenic line expressing human PrPC, to investigate some aspects of the pathogenesis of vCJD infection. As main findings, we demonstrate that prion strain divergence can occur upon transmission of human, primary vCJD to such mice, and that peripheral challenge leads to an asymptomatic, life-long infection of the lymphoid compartment. A feature of tg650 mice is that following primary intracerebral vCJD challenge they developed a neurological disease with typically 100% attack rate, unlike for previously established PrP129Met, including overexpressing lines , . The mean survival time – typically around 500 days in homozygous mice - did not change notably on subpassaging, implying that vCJD agent might clinically infect the tg650 mice with little or no transmission barrier. This discrepant result may reflect the use of different constructs and genetic backgrounds (Text S1), and the transgene expression levels, although the latter does not seem to greatly differ as far as the tg650+/− and tg45 mice  are concerned.
A surprising result of these studies is the alternate pattern of disease that was induced by one of the inoculated vCJD cases, a WHO reference case here designated vCJD no. 4. Indeed, while vCJD strain features were faithfully propagated in the majority of tg650 mice, almost half of the vCJD 4-inoculated mice were found to propagate a prion replicating faster than vCJD agent, and exhibiting sCJD-like PrPres and neuropathological features. Although strain divergence upon transmission of BSE/vCJD agent to mice was reported to occur in earlier studies , , it was unprecedented within a context of homotypic transmission, i.e. full matching between the donor and receiver PrP sequences. To address the issue of a possible contamination, we performed independent transmission experiments, involving separate inoculum batches of the incriminated case, which all produced consistent results. Therefore, we consider the data inconsistent with contamination of the VCJD no. 4 material by a sCJD infectious source within our laboratory. An alternate possibility, i.e. a cross-contamination of the source material, was judged highly improbable owing to the procedures applied during the collect of the specimen and the preparation of the homogenates ( and P. Minor, personal communication). On the other hand, our observation intriguingly parallels the phenotypic disjunction observed upon transmission of BSE agent to human PrP129Met mice (tg35 line ). Together, these findings lend support to the hypothesis that a minor strain component might be created upon cattle-to-human transmission of BSE agent and could emerge upon subsequent human-to–human transmission. It is also worth mentioning that, while the probability to detect such a variant through mouse bioassay would be expected to depend on the amount - and possibly the regions - of brain tissue taken to establish the source material, the vCJD-4 homogenate was prepared using a larger amount of tissue from the same brain than for the other homogenates analyzed in this study (i.e. 100 mg instead of 1 mg of frontal cortex ).
The above finding has obvious implications in terms of public health as it raises the concern that some humans iatrogenically infected by vCJD agent may develop a clinical disease that would not be recognized as of vCJD origin , . Strikingly however, all vCJD-4-inoculated mice, notwithstanding the strain phenotype divergence propagated bona fide vCJD agent in their spleen, based on the PrPres pattern and the disease phenotype produced by secondary transmission to tg650 mice. This result is of direct relevance to the diagnosis of variant and sporadic CJD. Indeed, looking for peripheral lymphoreticular deposition of abnormal PrP on cases diagnosed as sporadic CJD might reveal a vCJD infection resulting from human-to-human, or cattle-to-human transmission. In this respect, it would be of interest to examine whether BSE-inoculated tg35 mice showing discordant PrPres signatures , or vCJD-challenged PrP129Val transgenic mice producing ‘type 5’ prion in their brain  do accumulate PrPvCJD in their spleens. In any case, our findings provide clear evidence that, as a consequence of strain-related tropism disparities, the same mouse can propagate different prions in different tissues following a single infection event.
Another salient finding emerging from this study was the remarkable ability of vCJD agent to establish asymptomatic infection despite sustained, life-long propagation in extraneural tissues. When challenged peripherally, tg650 mice remained asymptomatic over the whole observation period, and did not accumulate PrPres at detectable levels in their brain before 750 days pi, near the life end-stage. In the spleen of these mice however, PrPres accumulation reached its maximum at an early stage of infection, and remained at stable and substantial levels until death. Plateauing of prion infection in the spleen is consistent with earlier observations, and has been suggested to reflect an exhaustion of target cells (for review ) Importantly, the spleen tissue was highly infectious as it killed 100% of intracerebrally challenged mice within the minimal mean incubation time (~500 days). Altogether these data support the view that the sustained multiplication of the vCJD prion in lymphoid tissues was not accompanied by an efficient neuroinvasion in tg650 mice. Such an extremely delayed neuroinvasion appears to be rare in TSE rodent models, and to our knowledge was only reported for the mouse-adapted strain 87V on IM mice infected intraperitoneally with diluted inoculum . Clearly, while early accumulation of prions in lymphoid tissues may be essential for efficient neuroinvasion , efficient lymphoinvasion does not inevitably lead to rapid neuroinvasion. This finding strengthens the notion that humans infected by vCJD from a human source – including individuals of the MM genotype – might remain clinically asymptomatic for a very prolonged period of time while harboring relatively high levels of prion infectivity in their lymphoid tissues from an early stage of infection on, thereby amplifying the risk of iatrogenic transmission. It also supports the view that the large-scale survey of lymphoreticular tissues  may lead to a reliable assessment of the actual prevalence of vCJD infection in the UK population.
Finally, the human PrP transgenic model described in this study may help to further our understanding of peripheral vCJD pathogenesis, for instance in trying to identify factors that might enhance neuroinvasion efficiency, or modulate the shedding of prion infectivity from the lymphoreticular to the blood compartment. Moreover, preliminary results indicate that the search for abnormal PrP in the spleen of such mice culled at time intervals post infection ,  could allow the detection of low levels of vCJD infectivity within a reasonably short time scale.