Reduction of Prion Infectivity in Packed Red Cells by Separation of Whole Blood and Washing of the Red Cell Fraction.

Although prion diseases are rare in humans, the established link between a new variant form of CJD (vCJD) and the consumption of cattle meat contaminated by BSE have raised concerns about a possible outbreak of a large epidemic in the human population. Over the past few years, BSE has become a significant health concern in several countries, and it now seems apparent that vCJD can also be iatrogenically transmitted from human to human by blood transfusion. Exacerbating this state of affairs is the lack of a reliable test to identify individuals incubating the disease during the long and silent period from the onset of infection to the appearance of clinical symptoms. The purpose of this research study was to evaluate the effectiveness of separation of whole blood and washing of the red cell fraction for the removal of infectious scrapie prion protein (PrP^Sc) from red blood cell (RBC) suspensions. Samples of human, whole blood were spiked with 5 × 10⁶ LD50 263K PrP^Sc. Analysis of the treated sample supernatants by Western blot revealed that approximately >88% of the PrP^Sc was removed with the initial plasma expression and the equivalent of 6% was detected in a saline wash (300 mL; 0.9% saline). The final sample of RBCs revealed no detectable levels of PrP^Sc by Western blots. Further analysis of the treated RBCs using the PMCA assay indicated detectable amounts of PrP^Sc only after 2 consecutive amplification rounds. Semi-quantitative analysis of PMCA amplification enabled us to estimate that the treated RBCs contained less than 1 × 10⁴ LD50 PrP^Sc. This corresponded to removal levels exceeding ≥99% of spiked material in whole blood. These in vitro estimations were confirmed by in vivo infectivity studies in a hamster model of disease transmission.

Results from in vivo studies displayed significant differences in the incubation periods of the spiked blood inoculated hamsters (100.1 ± 1.7) versus washed RBCs (135.8 ± 6.7). Moreover, a substantial difference in the attack rate (6/15: 40% in washed RBC, versus 13/13: 100% in spiked blood) further indicated a substantial removal of infectious prions. Comparison of this data with results of animals inoculated with different dilutions of infectious material, indicated a >99.94% reduction of infectivity. Washed, packed human red cells produced by this procedure were able to be stored in standard additive solutions (AS-3) for 42 days while still meeting all in vitro blood bank standards for acceptable red cell quality.

Conclusion This data suggests that separation of plasma coupled with a simple, low volume wash of red cells may represent an efficient method to remove prions from red blood cell fractions, thus reducing possible infectivity of these products.