Phenotypic and genotypic alterations associated with the attenuation of a Theileria annulata vaccine cell line from Turkey.

Attenuated vaccines, produced by prolonged in vitro culture of the macroschizont stage of the life-cycle, are the main method of controlling Theileria annulata infections. Little is known about the mechanism(s) of attenuation. Here we present data from a Turkish cell line demonstrating that attenuation is associated with reduced ability to differentiate into microschizonts and a reduction in matrix metalloproteinase activity. We also show that attenuation results in a change in the structure of the parasite population. Using the technique of differential mRNA display, we demonstrate that gene expression profiles differ between non-attenuated and attenuated macroschizont infected leucocytes. One differentially expressed gene is of parasite origin. These data are discussed in the context of a multifactorial model for virulence.