Prions that cause scrapie, bovine spongiform encephalopathy (BSE) and Creutzfeldt-Jakob disease do not initiate an immune response, probably because they do not contain nucleic acids. So immune prophylaxis to these diseases is difficult to achieve. But in 20 July Lancet Shneh Sethi and colleagues from Ludwig-Maximilians-Universität München, Germany, show that CpG oligodeoxynucleotides — which stimulate innate immunity — can offer postexposure prophylaxis against prion disease in mice (Lancet 2002, 360:229-230).

Sethi et al. inoculated healthy mice with brain homogenates from mice infected with the scrapie prion. Then, they treated these animals with injections of CpG oligodeoxynucleotides and observed 38% longer survival times than after treatment with saline. Survival times were even longer after repeated application of CpG.

"The most likely explanation is the stimulation of TLR9-expressing cells of the innate immune system such as macrophages, monocytes and dendritic cells. CpG oligodeoxynucleotides have not been shown to have adverse...

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