Infectious Prions in Pre-Clinical Deer and Transmission of Chronic Wasting Disease Solely by Environmental Exposure

Key to understanding the epidemiology and pathogenesis of prion diseases, including chronic wasting disease (CWD) of cervids, is determining the mode of transmission from one individual to another. We have previously reported that saliva and blood from CWD-infected deer contain sufficient infectious prions to transmit disease upon passage into naïve deer. Here we again use bioassays in deer to show that blood and saliva of pre-symptomatic deer contain infectious prions capable of infecting naïve deer and that naïve deer exposed only to environmental fomites from the suites of CWD-infected deer acquired CWD infection after a period of 15 months post initial exposure. These results help to further explain the basis for the facile transmission of CWD, highlight the complexities associated with CWD transmission among cervids in their natural environment, emphasize the potential utility of blood-based testing to detect pre-clinical CWD infection, and could augur similar transmission dynamics in other prion infections

B Cells and Platelets Harbor Prion Infectivity in the Blood of Deer Infected with Chronic Wasting Disease▿ †

Substantial evidence for prion transmission via blood transfusion exists for many transmissible spongiform encephalopathy (TSE) diseases. Determining which cell phenotype(s) is responsible for trafficking infectivity has important implications for our understanding of the dissemination of prions, as well as their detection and elimination from blood products. We used bioassay studies of native white-tailed deer and transgenic cervidized mice to determine (i) if chronic wasting disease (CWD) blood infectivity is associated with the cellular versus the cell-free/plasma fraction of blood and (ii) in particular if B-cell (MAb 2-104+), platelet (CD41/61+), or CD14+ monocyte blood cell phenotypes harbor infectious prions. All four deer transfused with the blood mononuclear cell fraction from CWD+ donor deer became PrPCWD positive by 19 months postinoculation, whereas none of the four deer inoculated with cell-free plasma from the same source developed prion infection. All four of the deer injected with B cells and three of four deer receiving platelets from CWD+ donor deer became PrPCWD positive in as little as 6 months postinoculation, whereas none of the four deer receiving blood CD14+ monocytes developed evidence of CWD infection (immunohistochemistry and Western blot analysis) after 19 months of observation. Results of the Tg(CerPrP) mouse bioassays mirrored those of the native cervid host. These results indicate that CWD blood infectivity is cell associated and suggest a significant role for B cells and platelets in trafficking CWD infectivity in vivo and support earlier tissue-based studies associating putative follicular B cells with PrPCWD. Localization of CWD infectivity with leukocyte subpopulations may aid in enhancing the sensitivity of blood-based diagnostic assays for CWD and other TSEs

CWD bioassay inocula sources.

a<p>Water, feed buckets and bedding from CWD+ deer suites.</p>b<p>Colorado State University deer number 112 (PO brain inoculated deer from previous study<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0005916#pone.0005916-Mathiason1" target="_blank">[27]</a>).</p>c<p>Colorado State University deer number 121 (IC brain inoculated deer from previous study <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0005916#pone.0005916-Mathiason1" target="_blank">[27]</a>).</p>d<p>Wisconsin Department of Natural Resources, Madison, WI, USA.</p>e<p>Colorado State Veterinary Diagnostic Laboratory, Fort Collins, CO, USA.</p>f<p>Transitioning from pre-clinical to clinical.</p>g<p>Contains brain tissue from deer of pre-clinical and clinical status at terminal field collection.</p

CWD bioassay inoculation cohorts.

a<p>Water, feed buckets and bedding from CWD+ deer suites.</p

Western blot results of deer exposed to body secretions and excreta from CWD+ deer.

<p>Western blot demonstration of the typical PK digestion band shift (28–35 kD) associated with prion infection in the brain (medulla oblongata at obex) of deer receiving blood, brain, saliva or environmental exposure, but not urine and feces from CWD-infected donors. Lanes 1–4 represent CWD+/− deer controls (10% brain homogenate of medulla at obex) without and with PK digestion at 25 µg/ml. Lanes 5–12, 10% brain homogenate of blood, brain, urine/feces or saliva inoculated deer without and with PK digestion at 25 µg/ml. Lanes 13–15, brain homogenate from deer environmentally exposed to CWD+ fomites without and with PK digestion at 25 and 50 µg/ml.</p

PrP^CWD detection by longitudinal tonsil biopsy and terminal necropsy.

<p>PrP^CWD detection by longitudinal tonsil biopsy and terminal necropsy.</p

Immunohistochemistry results of deer exposed to body secretions and excreta from CWD+ deer.

<p>PrP^CWD demonstrated by immunohistochemistry in tonsil, brain (medulla oblongata at obex), and retropharyngeal lymph node of deer receiving saliva, blood, or environmental exposure from CWD-infected donors. CWD immunohistochemistry is shown in the medulla at obex (a–j) and either tonsil or retropharyngeal lymph node (k–t). Arrows indicate PrP^CWD staining (red) within brain and lymphoid follicles. Arrow with * indicates lymphoid follicle negative for PrP^CWD.</p

Summary of naïve deer exposed to inoculum from CWD+ deer—combined with our previous published findings

<p><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0005916#pone.0005916-Mathiason1" target="_blank">[27]</a>.</p