Prion-Filtering Technique Aimed at Protecting Blood Supply

BALTIMORE, Dec. 22 -- An investigational approach to sifting infectious prions from donated blood could help quell fears, focused in Britain, about the possible spread by transfusion of variant Creutzfeldt-Jakob disease, researchers here reported.

In a proof-of-concept study, no animals exposed to blood treated to remove scrapie-causing prions developed the disease, whereas 15 of 99 animals exposed to untreated blood developed scrapie, reported Robert G. Rohwer, Ph.D., of the University of Maryland, and colleagues, in the Dec. 23 issue of The Lancet.

Scrapie is a transmissible spongiform encephalopathy similar to variant Creutzfeldt-Jakob disease (vCJD),

The blood treatment method involves removing white cells from the blood through leukoreduction, and then removing prions from the red-cell concentrate using a technique commonly employed for purifying proteins for drug research.

If proven in further studies using animal and human blood, it could be an a more effective alternative to the current practice in the U.S. and in Britain of screening blood donors for possible contacts with people who are known to have vCJD.

"Blood-associated transmissible spongiform encephalopathy infectivity presents unique challenges to risk management absent from other blood-borne pathogens," the investigators wrote. "The low concentration of infectivity, although difficult to detect, has nevertheless been sufficient for efficient transfusion transmission of these fatal diseases. People with certain genotypes could harbor the infection in a transmissible but undetectable form for decades."

In the Dec. 8 issue of The Lancet, British researchers reported a third case of vCJD in an at-risk group who were given tainted blood transfusions. The patient was the first to be identified before death as having the disease. The 32-year-old man developed the disease after a red-cell transfusion donated by a person later determined to have vCJD.

Two other cases -- identified after death and reported two years ago -- were previously identified among 66 people who got blood transfusions from donors who subsequently developed vCJD, according to Stephen Wroe, M.D., of the National Prion Clinic at the National Hospital for Neurology and Neurosurgery in London.

The American Red Cross asks all potential blood donors whether they have ever had a blood relative with Creutzfeldt-Jakob disease, and whether they had lived in the United Kingdom for at least three months since 1980, whether they had received a blood transfusion in England since 1980, or whether they had traveled or lived in Europe for a cumulative five years or more since 1980.

The questions are meant to tease out possible exposure to bovine spongiform encephalopathy or vCJD, but the incubation period for human prion diseases such as vCJD may be longer than 60 years, according to researchers who studied the closely related disease kuru in former cannibals in New Guinea.

That research suggests that an epidemic of variant CJD could crop up decades from now, with the aging of people who in the 1990s consumed beef from cows infected with bovine spongiform encephalopathy, also known as mad-cow disease, noted John Collinge, Ph.D., a professor of neurology at University College London, and colleagues in Australia, and Papua New Guinea, in the June 24 issue of The Lancet

In fact, the incubation period for vCJD could be even longer than that of kuru, because of a species-barrier effect that typically slows the rate of interspecies transmission.

In the current study, Dr. Rohwer and colleagues looked at the ability of affinity resin chromatography to remove infectious prion particles from the blood of hamsters infected with scrapie.

The technique involves the use of a specific ligand, selected from among millions of possible candidates, that has high binding affinity with both normal and abnormally folded proteins -- in this case, the prion that causes scrapie.

The authors looked at the ability of the affinity resin, labeled L13 and mass-produced version of the same resin, labeled L13A, to remove transmissible spongiform infectivity endogenously present in hamster blood.

They took 500 mL of scrapie-infected hamster whole blood and purged it of white cells using leukoreduction. They then passed the red-cell concentrate through columns of the affinity resins, with each sample getting two pass-throughs. They then tested the samples for infectivity by limiting dilution titration.

They found that leukoreduction alone removed 72% of input infectivity, but when they challenged a group of 99 hamsters with the leukoreduced but unfiltered red cell concentrate, 15 of the animals became infected.

In contrast, none of either 96 transfused with blood that had been passed through the L13 affinity resin, nor any of the 100 animals exposed to blood passed through the L13A resin, went on to develop the disease at the last follow-up of 540 days.

The limit of detection of the bioassay was 0.2 infectious doses per mL, and the overall reduction of the challenge infectivity was more than 1.22 log₁₀, the authors noted.

"The results showed removal of endogenous transmissible spongiform encephalopathy infectivity from leukoreduced whole blood by affinity ligands," they wrote. "The same resins adsorb normal and abnormal prion protein from human infections with variant, sporadic, and familial Creutzfeldt-Jakob disease, in the presence of blood components."

But assessing such technologies in the real world may be a good deal more difficult, given that there is currently no assay sensitive enough to reliably detect the disease in people with clinical vCJD, cautioned Marc L. Turner, M.D., Ph.D., a clinical director of the Scottish National Blood Transfusion Service and Chair of the vCJD Working Group of the United Kingdom Transfusion Services Specialist Advisory Committee on Transfusion Transmitted Infections.

"Until these technologies can be clinically and operationally assessed, the best protection against the uncertain risk of transfusion-associated prion disease remains in ensuring that blood products are used only if needed and that the uncertainties surrounding potential risks are communicated effectively to patients and to the public at large," Dr. Turner wrote in an accompanying editorial.