Parasite strain specificity of precursor cytotoxic T cells in individual animals correlates with cross-protection in cattle challenged with Theileria parva

Class I major histocompatibility complex-restricted parasite-specific cytotoxic T lymphocytes (CTL) are known to be a major component of the bovine immune response to the protozoan parasite Theileria parva, but formal proof for their role in protection of came against infection with T. parva has been lacking. Animals immunized with one stock of T. parva show variations in the degree of protection against heterologous challenge and also in the parasite strain specificity of their CTL responses. The present study investigated the relationship of strain specificity of CTL responses and cross-protection in an effort to verify the role of CTL in protection. The parasite strain specificity of the CTL responses generated in 23 cattle immunized with either of two immunologically distinct parasite populations was examined, and the susceptibility of individual cattle to challenge with the heterologous parasite population was determined. The frequency of stock-specific or cross-reactive CTL precursor cells (CTLp) in individual animals was measured by a limiting-dilution microassay. A proportion of animals immunized with either parasite exhibited cross-reactive CTLp, whereas CTLp detected in the remaining animals were specific for the homologous parasite. On challenge with the heterologous stock, those animals with cross-reactive CTLp were solidly protected while those with strain-specific CTLp showed moderate to severe reactions, although many of them recovered. The finding of a close association between strain specificity of the CTL response and protection against challenge provides strong evidence that CTL are important in mediating immunity.