Prion Protein Adsorption to Soil in a Competitive Matrix Is Slow and Reduced

Abstract

It is likely that the soil environment serves as a stable reservoir of infectious CWD and scrapie prions, as well as a potential reservoir of BSE. Prion adsorption to soil could play an important role in prion mobility, proteolysis, and infectivity. We modified previously published methods to quantify adsorbed prions via direct detection and studied prion adsorption to soil and soil minerals as a function of time through 60 days. Prion-infected brain homogenate was used as a complex, relevant prion source. We determined that maximum PrP adsorption requires days or weeks, depending on the soil or mineral, and is two to five orders of magnitude lower than previous studies using purified PrP Sc or recPrP. Because PrP adsorption to soil is slow and reduced in tissue homogenate, the possibility of prion transport in soil environments cannot be excluded and requires further investigation. Our results indicate that binding to soil may protect prions from degradation, consistent with prions' longevity in the environment. Adsorption of PrP to sterilized soil did not differ significantly from adsorption to unsterilized soil, which suggests that active biological processes do not significantly affect prion adsorption or degradation in the soil environment.

Replication Efficiency of Soil-Bound Prions Varies with Soil Type

“…Sorption of HY TME to SCL soil reduces prion replication in vitro. Sorption of HY PrP Sc to silty clay loam (SCL) soil, in a brain homogenate matrix, requires a minimum of 24 h to achieve maximum PrP adsorption (33). To investigate the effect of unbound PrP Sc on PMCA, we incubated SCL soil with HY TME-infected brain homogenate for either 2 or 24 h, collected supernatants and washes postincubation, and evaluated these using PMCA for the presence of PrP Sc .…”

“…Gamma-irradiated fine white sand (Fisher Scientific, Pittsburgh, PA), Dickinson sandy loam (SL) soil (a Typic Hapludoll), Rinda silty clay loam (SCL) soil (a Vertic Epiaqualf), sodium bentonite clay (CETCO, Arlington Heights, IL), silicon dioxide powder (Sigma Aldrich, St. Louis, MO), and humic acid (HA)-coated silica gel particles (SiO 2 -HA) were used. The physical and chemical properties of each of these materials have been described previously (33,35). To obtain soil-bound prions, 10% brain homogenate (aged or unaged) was combined with soil in 1ϫ DPBS and gently rotated at 24 rpm (Mini Labroller; Labnet, Edison, NJ) at 22°C.…”

“…To obtain soil-bound prions, 10% brain homogenate (aged or unaged) was combined with soil in 1ϫ DPBS and gently rotated at 24 rpm (Mini Labroller; Labnet, Edison, NJ) at 22°C. Incubation time, soil, buffer, and homogenate-to-soil ratios were selected based on previously published results and are detailed in Table 1 (32,33).…”

“…Scrapie PrP Sc has been detected on environmental surfaces on a farm where the disease was endemic (23). Prions are known to sorb (i.e., bind) to a wide range of soils and soil minerals, resist desorption, and remain infectious (20,31,33). The potential for CWD or scrapie transmission by exposure to contaminated soil exists since cervids and other ruminants are known to ingest and inhale soil (3,7), which are known routes of prion infection (6,18,19,22,37).…”

“…Adsorption of PrP to soil and soil minerals is influenced by the prion strain, species, PrP form (full length versus N-terminally truncated), soil type, and equilibration period (21,23,32,33). The effect of these variables on the ability of soil-bound prions to convert PrP c to PrP Sc and initiate infection is poorly understood.…”