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

The relationship of strain specificity of class I major histocompatibility complex-restricted parasite-specific cytotoxic T lymphocytes (CTL) responses and cross-protection was investigated 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 2 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.