Understanding the basis of parasite strain-restricted immunity to Theileria parva.

Infection with Theileria parva is asymptomatic in African buffalo but results in severe disease in cattle. Currently, vaccination relies on infection and treatment, using a mixture of three parasite isolates to overcome the strain specificity of immunity. Genotypic analyses of field populations of T. parva indicate a panmictic population structure, reflecting frequent sexual recombination. Profound immunodominance of protective CD8 T cell responses, together with polymorphism of the target antigens and frequent genetic recombination, contribute to the strain-restricted immunity. The dominant CD8 target antigens are highly polymorphic, but the live vaccine appears to contain limited diversity. A model to explain the ability of the vaccine to confer immunity against highly diverse field parasite challenge is discussed. Parasites in cattle exhibit much more limited antigenic diversity than parasites in buffalo, consistent with other evidence that the cattle-maintained population represents a subset of T. parva recently adapted to cattle.