The majority of blood PrP(c) is found in the platelet and plasma compartments.

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Assays for the agent of Creutzfeldt-Jakob disease (CJD) include measurement of infectivity in different animal systems, such as wild-type or transgenic mice, and detection of PrP Sc by different methods and formats. The various assays could be best calibrated against each other by use of uniform readily available materials, and samples of four human brains, two from sporadic CJD patients, one from a variant CJD patient and one from a non-CJD patient, have been prepared as 10 % homogenates dispensed in 2000 vials each for this purpose. Results of in vitro methods, particularly immunoblot assays, were compared in the first collaborative study described here. While dilution end-points varied, the minimum detectable volume was surprisingly uniform for most assays and differences in technical procedure, other than the sample volume tested, had no detectable systematic effect. The two specimens from sporadic CJD cases contained both type 1 and type 2 prion proteins in approximately equal proportions.

Scaggregate is the cause of prion diseases. We developed a novel enzyme-linked immunosorbent assay (ELISA) that is specific for PrP aggregate by screening 30 anti-PrP monoclonal antibodies (MAbs) for their ability to react with recombinant mouse, ovine, bovine, or human PrP dimers. One MAb that reacts with all four recombinant PrP dimers also reacts with PrP Sc aggregates in ME7-, 139A-, or 22L-infected mouse brains. The PrP Sc aggregate is proteinase K resistant, has a mass of 2,000 kDa or more, and is present at a time when no protease-resistant PrP is detectable. This simple and sensitive assay provides the basis for the development of a diagnostic test for prion diseases in other species. Finally, the principle of the aggregate-specific ELISA we have developed may be applicable to other diseases caused by abnormal protein aggregation, such as Alzheimer's disease or Parkinson's disease.

A simple diagnostic test for the detection of bovine spongiform encephalopathy (BSE), based on a commercially available time-resolved fluorescence immunoassay (DELFIA) for the measurement of the normal and disease-associated isoforms of prion protein (PrP), is described. The isoforms are sequentially extracted from homogenized bovine brain tissue using two concentrations of guanidine hydrochloride. This procedure initially extracts a soluble isoform and subsequently a less soluble disease-associated aggregated isoform. Following quantification of the two fractions, the percentage of the insoluble prion becomes a measurable parameter, independent of protein concentration, clearly identifying normal from infected animals displaying clinical signs of BSE. The mean percentages of insoluble PrP in brain tissue from 60 BSE-confirmed-positive cattle and 100 cattle that had never been exposed to the disease were 52.6% (SD = 22.8) and 3.9% (SD = 1.5), respectively. The assay is sensitive, with a detection limit of less than 50 pg PrP, and is robust and precise (CVs < 10%) over the appropriate working range.

We report here a novel noncompetitive immunoassay applicable to the measurement of small-molecular-mass compounds and typified by the direct measurement of estradiol in serum. Two types of anti-idiotypic antibody recognize different epitopes within the hypervariable region of the specific primary antibody (e.g., anti-estradiol). The first anti-idiotype (betatype) recognizes an epitope at the unoccupied binding site, which is masked in the presence of the analyte (e.g., estradiol). The second (alphatype) recognizes an epitope close to the binding site and is unaffected by the presence or absence of the analyte, but is sterically hindered from binding to the primary antibody in the presence of the betatype. The use of these matched antibodies (primary antibody, alphatype, and betatype) has enabled the development of a method for determining antibody occupancy that is not based on a conventional two-site assay. An excess amount of purified monoclonal antibody against estradiol is immobilized onto the walls of microtiter wells. After the capture of analyte, the unoccupied antibody sites are blocked by the addition of an excess amount of betatype. Subsequently, analyte occupancy is determined by the addition of excess europium-labeled alphatype, incubation, washing, and time-resolved fluorometry. The method demonstrates good sensitivity, precision, and comparability with alternative competitive immunoassays.

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