Science - November 24, 2005

From pathogen to protector

Plant viruses can ruin harvests, but do nothing to humans. This is the standard line in virology textbooks. Wageningen researchers are working on a project that might lead to these textbooks being changed. They think that plant viruses can affect people, in a positive way. They protect them against human viruses.

Dr Daniella Kasteel, a plant virologist who got her PhD in Wageningen, was studying the poliovirus at RIVM (National Institute for Public Health and the Environment) at the beginning of this century. She suddenly realised that this dreaded pathogen resembled a virus she had worked on in Wageningen.

‘If you look at them under the electron microscope, you see no difference between the polio virus and the cow pea mosaic virus,’ says Kasteel. ‘They also behave the same in the cell. When they reproduce they make the same errors. When a poliovirus replicates it also makes lots of empty shells that contain no genetic material. When the plant virus infects cells, you see the same thing happen.’

If two different viruses bear such a strong resemblance to each other, would it be possible to protect humans against polio by exposing them to cow pea mosaic virus (CPMV)? This is the question that has occupied Kasteel ever since. Meanwhile she is back in Wageningen, and now heads a project at Plant Research International to examine this unexpected aspect of plant viruses.

‘Up to now we have always regarded plant viruses as harmful factors that we need to fight in order to protect our agriculture,’ says Kasteel. ‘But these same viruses might be able to protect humans against disease. We want to find out if this is the case.’

The idea is revolutionary. Scientists have always assumed that animal organisms do not react to plant viruses. ‘Every so often, though, someone suggested that perhaps reality is not as simple as that,’ says Kasteel. ‘We know that in the 1970s Russian researchers investigated whether another plant virus, the tobacco mosaic virus, offers protection against flu. The research indicated that this was the case. When the researchers exposed mice to an influenza virus, all of the mice died. When they first exposed the mice to the tobacco mosaic virus, sixty percent survived.’

Kasteel and her team got money from the Dutch ministry of agriculture in 2002 as part of the Strategic Expertise Research to carry out similar research, but in test tubes. Antibodies against plant viruses – especially those against CPMV – also appeared to react to human viruses. British researchers published results from a similar study in 2003 in the respected Virology journal. The British injected mice with antibodies against CPMV and discovered that the experimental animals were less susceptible to infection from the human measles virus.
‘We discovered a similar relation between CPMV and another human virus in our test-tube studies,’ continues Kasteel. ‘That might mean that plant viruses can have a broad protective effect.’

It is not yet clear which plant virus protects against which human virus. ‘Genetic similarity has nothing to do with it,’ says Kasteel. ‘Just because a plant virus belongs to a certain family, does not necessarily mean that it will offer protection against a human virus from the same family.’ What is more relevant is the three-dimensional structure of the viruses. The proteins on the outside of CPMV resembled the proteins on the inside of the human viruses.
It sounds almost too good to be true. If it is the case though, it means that pathogens that do not infect humans could help to strengthen the immune system against viruses that are dangerous for humans.

In 2005 Kasteel started a new project financed by the ministry of agriculture, under Kennisbasis (Knowledge Base), projects that are intended to provide the ministry of agriculture with fundamental scientific knowledge. The project should make it clear whether plant viruses can indeed protect humans against diseases.

Willem Koert