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BSE: Science & research - TSE Diagnostics - Rapid infectivity mouse and cell assays

While uncertainty remains about the nature of the TSE agent, it is necessary to confirm infectivity of any putative TSE identified through the detection of abnormal prion proteins.  The conventional method to detect TSE infectivity involves bioassay i.e. an assay where animals - cattle, sheep, mice, hamsters - are given potentially infected material and monitored for the development of disease characteristics.  It is often possible to define the strain and type of TSE disease by this method.  Since the incubation period for TSEs in mice can be up to 18 months, the results from bioassay experiments take a long time to appear even in small mammals like mice.  Research in this area is therefore, on developing rapid assays.

Research supported by Defra aims to develop assays capable of detecting TSE infectivity rapidly through:

It should be noted that there is some doubt about the absolute correlation of abnormal, disease-associated prion protein (PrPSc) with TSE infectivity and therefore research is being undertaken to investigate this further.

Mouse bioassays

Mouse bioassays have been used since the 1960’s as a way to detect TSE infectivity. Mouse bioassay can be used to determine:

  • Whether a sample contains TSE infectivity.
  • The level of infectivity in a sample (the titre).
  • The TSE strain present.
Inbred mouse lines

It was found that conventional inbred mouse lines (often referred to as ‘wild type’ mice) underestimate the concentration of agent in sheep or cattle tissues (because of what is known as the ‘species barrier’ effect). Therefore, research has been undertaken in both the UK and overseas to develop transgenic mouse lines that express sheep or cattle prion proteins and which may be more sensitive to TSEs than wild-type mice and have a shorter incubation period for the disease. They are used to model the behaviour of a TSE in its natural host.

Transgenic mouse lines

There are two different methods for constructing transgenic mouse lines. The first is to inject DNA encoding the prion protein gene of the desired species directly into the fertilised egg. This method often results in a high expression of the foreign transgene in the mouse.  The additional prion protein means that such mice are often very sensitive to challenge with a TSE agent.  For example, German research has shown that a transgenic mouse line that over-expresses cattle prion protein was 10,000 times more sensitive than wild type mice and 10 times more sensitive than cattle to BSE (Buschmann, A & Groschup, M (2005) J. Infect. Dis. 192(5) 934-42).

The second method involves replacing the native mouse prion protein gene with one of the desired species. This leads to a mouse with a foreign prion protein expressed at normal levels. Such mice are not usually particularly sensitive to disease but may be a better surrogate model for the host. 

Defra has funded construction of transgenic mice carrying sheep and cattle genes, including the production of:

  • Transgenic lines carrying bovine or kudu prion protein (as kudu are more sensitive to BSE than cattle) (project SE1753).
  • Transgenic lines carrying the AHQ and ARR alleles of the sheep prion protein (project SE1753).
  • Transgenic mice expressing human genes to compare the relative transmissibility of classical scrapie, atypical scrapie, sheep-passaged BSE, and cattle BSE to humans carrying the common human variants of the prion protein (projects SE1439, SE1441).

Cell culture assays

Since mouse bioassays take many months, if not years, to complete, a more rapid method is required to determine if TSE infectivity is present in a sample.  Defra has funded three projects to explore the adaptation of cell culture assays to analyse ruminant TSEs. The development of such assays is also in line with Defra’s policy to minimise the number of animals used in research and testing (projects SE2001, SE2002 and SE2003).

Links to other information

Department for Environment, Food and Rural Affairs