PARASITE STRAIN SPECIFICITY OF PRECURSOR CYTOTOXIC T-CELLS IN INDIVIDUAL ANIMALS CORRELATES WITH CROSS-PROTECTION IN CATTLE CHALLENGED WITHTHEILERIA-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 cattle against infection with T. parva has been lacking. Animals immunized with one stock of T . parva show variations in the degree of protection against heterologo us challenge and also in the parasite strain specificity of their CTL responses. The present study investigated the relationship of strain s pecificity of CTL responses and cross-protection in an effort to verif y the role of CTL in protection. The parasite strain specificity of th e CTL responses generated in 23 cattle immunized with either of two im munologically distinct parasite populations was examined, and the susc eptibility of individual cattle to challenge with the heterologous par asite population was determined. The frequency of stock-specific or cr oss-reactive CTL precursor cells (CTLp) in individual animals was meas ured by a limiting-dilution microassay. A proportion of animals immuni zed with either parasite exhibited cross-reactive CTLp, whereas CTLp d etected in the remaining animals were specific for the homologous para site. On challenge with the heterologous stock, those animals with cro ss-reactive CTLp were solidly protected while those with strain-specif ic CTLp showed moderate to severe reactions, although many of them rec overed, The finding of a close association between strain specificity of the CTL response and protection against challenge provides strong e vidence that CTL are important in mediating immunity;