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Evaluation of the Analytical Sensitivity of Tests used for BSE Surveillance and ConfirmationJohn Gray, Sandor Dudas and Stefanie Czub
CAHLN Meeting, June 7th, 2010
2
Presentation Outline
Principles of BSE surveillance in Canada
Disease associated prion protein characteristics utilized for BSE diagnosis
Reason to determine BSE test analytical sensitivity
Results from the analytical sensitivity determination
Conclusions and Future Direction
3
BSE Surveillance in Canada
Targeted surveillancecattle > 30 months: dead, down, diseased
or distressed (4-D animals)
cattle with clinical signs of BSE : nervous or aggressive behaviour abnormal posture lack of co-ordination difficulty rising
In 2009, Canada tested >34,000 cattle in 8 laboratories with nationally & internationally approved surveillance tests
4
BSE Test Principles
BSE identified by misfolded form of host encoded cellular prion protein Normal: PrPC Diseased: PrPD
Conversion results in changes in biochemical properties of the protein Relative resistance to protease digestion Propensity to aggregate (hydrophobicity) Specific binding to certain polyionic compounds
Properties utilized for PrPD isolation and detection/ BSE diagnosis in rapid (surveillance) & confirmatory tests
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Relative Protease Resistance
Protease Digestion
BSE Infected Animal Healthy Animal
PrPcPrPc
Digested cellular PrPC
Primarily intact PrPD
Digested cellular PrP
PrPc PrPc
PrPD
PrPD
Prionics PriostripPrionics WesternBioRad TeSeE ELISABioRad TeSeE CWB
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Conditioning
BSE Infected Animal Healthy Animal
PrPcPrPc
Ligand binds PrPD Wash away PrPC
PrPDPrPc PrPc
PrPD Specific Ligand Capture
PrPBSE
ligand
Wash away PrPC
PrPD
PrPD Idexx ELISA
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PrPD
Hydrophobic Aggregation
PrPc
BSE Infected Animal Healthy Animal
PrPc
PrPc
PrPc
PrPc
PrPc
Buffer conditioning
Ultracentrifugation to pellet aggregatesMild PK digestion
PrPc
PrPc
PrPc
PrPc
PrPc
PrPc
PrPcPrPD
PrPD
PrPD
PrPD
PrPD
PrPD PrPD
SAF Immuno-blot
8
Determination of Analytical Sensitivity Why?
1. To understand performance & performance characteristics of each test
2. To better interpret results that approach the test sensitivity limit (grey zone results!)
3. To compare surveillance & confirmatory tests with each other
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Determination of Analytical Sensitivity
How?
Endpoint dilution of homogenate from a experimentally infected cow into negative bovine brain pool
To:
Define penultimate dilution of homogenate in which analyte is no longer detectable
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Experimental Design
Negative brain
BSE + brain
PrionicsPriostrip
BioRad ELISA
Prionics WB
Idexx ELISA
BioRadcWB
SAF Immunoblot
Homogenizeto 50% pool
in ddH2O
Dilute to tissue and buffer
concentration
SerialDilutions
Prism Graph Pad Analysis
Tissue
Readin
g
Tissue
Readin
g
Tissue
Readin
g
Tissue
Readin
g
Tissue
Readin
g
Tissue
Readin
g
Testing 5 X
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Results
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Prionics-Check Priostrip
(log10)
Homogenize > Digest > Bead binding > Immunochromatographic detection
13
Prionics-Check Western Blot
(log10)
Homogenize > Digest > Gel Separation > Transfer > Immunodetection
14
BioRad TeSeE ELISA
(log10)
Homogenize > Digest > PPT > Antibody based sandwich ELISA
15
Idexx Herd Chek ELISA
(log10)
Homogenize > Conditioning > Ligand based sandwich ELISA
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BioRad TeSeE Confirmatory Western Blot
(log10)
Homogenize > Digest > PPT > Gel Separation > Transfer > Immunodetection
17
Scrapie Associated Fibril (SAF) Immuno-blot
(log10)
Homogenize > PPT > Mild digest > Gradient centifugation > Gel separation > Transfer > Immunodetection
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Kit defined cut off value detection limit
(log10)
19
Tissue ratio where performance curve crosses mean negative value
(log10)
20
Conclusion:
Each platform has a unique performance curve & response to declining amounts of PrPD
Detection limit of all surveillance tests within 2 logs of the most sensitive surveillance test
As expected, the confirmatory tests show an enhanced sensitivity
SAF Immuno-blot (BSE NRL confirmatory test) performed with the best analytical sensitivity
21
Future Directions
Classical BSE vs Atypical BSEDifferent molecular characteristics with
impact on analytical sensitivity ?
New approaches to BSE diagnosis / confirmationCaptureAmplificationBiomarkers
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Acknowledgments: BSE NRL
Renee Clark
Rheana Flitton
John Gray
Roberta Quaghebeur
Mark Snodgrass
Jianmin Yang
Keri Colwell
Yuan Mu Fang
Nancy Herman
Tammy Pickles
Catherine Graham
Stefanie Czub
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24
BioRad TeSeE ELISA
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Prionics Check Priostrip
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Prionics Check Western Blot