High-pressure freezing in the study of animal pathogens.

High-pressure freezing is applicable to both morphological and immunocytochemical studies. We are investigating the morphogenesis of foot-and-mouth disease virus and African swine fever virus by the use of high-pressure freezing of infected cells. Foot-and-mouth disease virus particles are not detected in sections of conventionally immersion-fixed infected cells, but when the cells are prepared by high-pressure freezing, newly formed virions are readily seen throughout the cell. We report two methods for high-pressure freezing of virally infected cells: first, two sapphire discs frozen 'face to face' with a narrow spacer to prevent cell damage and, second, a fibrous filter substrate that can be easily cut into discs to fit into the freezing planchettes. Cells readily adhere to the fibres in vitro, and the complete disc can be rapidly transferred to the planchettes for freezing. Immunolabelling studies of the microneme proteins of the parasite Eimeria tenella indicate that high-pressure freezing followed by freeze-substitution in acetone with uranyl acetate allows high-sensitivity immunolabelling for these proteins.