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Transgenic white clover as oral vaccine against shipping fever
By Birgit Reinert

Canadian researchers have created white clover that expresses a gene from a bacterium that can cause respiratory disease in cattle. This is a step towards developing plants as edible vaccines against bovine pneumonic pasteurellosis, also known as shipping fever. The clover plants were able to produce bacterial antigens stimulating immunity to the pathogenic microorganism Mannheimia haemolytica.

White clover Trifolium repens.

The research team first identified a major virulence factor, leukotoxin (Lkt), in M. haemolytica. Next, the researchers created a derivative of Lkt (Lkt50) and fused it to a green fluorescent protein (GFP), which was used as a marker. Finally, the fusion construct (Lkt50-GFP) was introduced into white clover. As expected, the plants produced green fluorescence indicating that the fusion gene was expressed.

"An extract containing Lkt50-GFP from white clover was able to induce an immune response in rabbits," the researchers report in Infection and Immunity. This led to the production of antibodies that neutralized the authentic Lkt.

"This is the first demonstration of the expression of an M. haemolytica antigen in plants and paves the way for the development of transgenic plants expressing M. haemolytica antigens as an edible vaccine against bovine pneumonic pasteurellosis," the researchers write. The study was done by Reggie Y. C. Lo and colleagues of the University of Guelph, Canada.

Pneumonic pasteurellosis results from the stress of handling and transporting cattle. A major cause of sickness and death, the disease has a great economic impact on the feedlot cattle industry. As an alternative to conventional immunization methods such as intramuscular injection, plant-based oral vaccines are more easily administered and less stressful for the animals.

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Lee, R.W.H. et al. Towards development of an edible vaccine against bovine pneumonic pasteurellosis using transgenic white clover expressing a Mannheimia haemolytica A1 leukotoxin 50 fusion protein. Infect Immun 69, 5786-5793 (September 2001).

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