Early peripheral immunological events dictate chronic wasting disease
Date
2013
Authors
Michel, Brady, author
Zabel, Mark, advisor
Hoover, Ed, committee member
Dow, Steve, committee member
Ross, Eric, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Chronic wasting disease (CWD) is an emerging prion disease of captive and free-ranging cervid populations that, like scrapie, has been shown to involve the immune system, which most likely contributes to their relatively proficient horizontal and environmental transmission. While CWD prions probably interact with the innate immune system immediately following peripheral exposure, little is known about this initial encounter. In the first chapter of this dissertation we examined initial events in lymphotropic and intranodal prion trafficking by tracking highly enriched, fluorescent CWD prions from infection sites to draining lymph nodes. We observed biphasic lymphotropic transport of prions from the initial entry site upon peripheral prion inoculation. CWD prions rapidly reached draining lymph nodes in a cell autonomous manner within two hours of intraperitoneal administration. Monocytes and dendritic cells (DCs) showed a strong dependence on Complement for optimal prion delivery to lymph nodes hours later in a second wave of prion trafficking. B cells comprised the majority of prion-bearing cells in the mediastinal lymph node by six hours. As most B cells are mainly located in the follicles, acquisition of prions by these cells most likely occurred through interaction with resident DCs, subcapsulary sinus macrophages, or directly from the follicular conduit system. These data highlight a novel mechanism of cell autonomous prion transport, and a vital role for B cells in intranodal prion trafficking. Upon entry into the draining lymph nodes, prion accumulation and replication on follicular dendrtic cells (FDCs) is greatly facilitated by the complement system. Complete elimination of CD21/35 significantly delays splenic prion accumulation and terminal prion disease in mice inoculated intraperitoneally with mouse-adapted scrapie prions. In the second chapter of this thesis we show that mice overexpressing the cervid prion protein and susceptible to CWD (Tg(cerPrP)5037 mice) but lack CD21/35 expression completely resist clinical CWD upon peripheral infection. Ablation of complement receptors CD21/35 greatly diminished splenic prion accumulation and replication throughout the course of disease, similar to CD21/35 deficient murine PrP mice infected with mouse scrapie. Mice with deficiencies in CD21/35 showed a reduction in severity of neuropathology and deposition of misfolded, protease-resistant PrP associated with CWD. Prion infection resulted in translocation of CD21/35 to lipid rafts in B cells, and FDC expression of CD21/35 mediated a strong germinal center response that may be conducive to prion amplification. Complement component C3 is a central protein in the complement system whose activation is essential for the elimination of infectious pathogens. C3 is the most abundant complement protein, being found in the blood at physiological concentrations of 1 mg/ml. Among the complement proteins, C3 is perhaps the most adaptable and multifunctional protein identified to date, having evolved structural characteristics that allow it to associate with over 25 different proteins. Previous experiments suggest a vital role of C3 in scrapie prion pathogenesis. In the last chapter of my thesis we showed that lack of C3 expression by 5037 mice either transiently or genetically leads to delays in prion pathogenesis. C3 impacts disease progression in the early stages of disease by slowing the kinetic rate of accumulation and/or replication of PrPRES. This slower kinetic increase in PrPRES correlates with an increase in survival time in mice deficient in C3. This delay in disease is in sharp contrast to the complete rescue we saw in CWD infected Tg 5037;CD21-/- mice. This suggests a role for CD21/35 in peripheral prion pathogenesis independent of their endogenous ligands. Taken together we show that the innate immune system dictates the course of CWD. We have discovered novel immune cells, trafficking pathways, and complement components important in CWD pathogenesis. These data not only highlight the key role of the innate immune system in CWD, but also provide a strong foundation for future immunological studies of prion diseases.