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Evidence that class I major histocompatibility complex-restricted cytotoxic T lymphocytes (CTL) are involved in immunity to malaria has highlighted the potential importance of these cells in protection against intracellular parasites. Parasite-specific CTL are a prominent feature of the immune response of cattle to Theileria parva, a related apicomplexan parasite. The relationship between the appearance of these cells in the blood of immune cattle under challenge and the clearance of infection suggests that they are involved in the control of infection, but direct evidence is lacking that CTL can mediate protection. We have made a quantitative kinetic study of CTL responses in lymph originating from infected lymph nodes in a number of immune cattle under challenge with T. parva. Direct killing activity and the frequency of CTL precursors (CTLp) within responding cell populations were evaluated. A substantial increase in the proportion of CD8+ CTL was observed between days 8 and 11 after challenge. Frequencies of CTLp as high as 1:32 were observed and activity was essentially confined to the large blasting cell fraction. The analogous response in peripheral blood was of lower magnitude and delayed by 1-2 days. The high frequency of CTLp in efferent lymph permitted the adoptive transfer of this activity between immune and naive monozygotic twin calves. In separate experiments, naive calves lethally infected with T. parva were protected by inoculation of up to 10(10) responding CD8+ T cells derived from their immune twins. Elimination of CD8+ T cells within the inoculum abrogated this effect. These findings provide direct evidence that CD8+ T cells can control T. parva infections in immune cattle.

The specificity of bovine CD4+ T-cell responses to Trypanosoma brucei variant surface glycoprotein (VSG) has been examined by using a panel of seven T-cell clones and nested deletions of the ILTat 1.3 VSG gene expressed in Escherichia coli. All clones recognized the polymorphic N-terminal domain of the antigen, and the recognition sites of five of the clones were resolved to three areas with lengths of 14, 18, and 21 amino acids. Comparison of these regions with corresponding areas of other VSG molecules, including those derived from the same trypanosomal serodeme, has shown that the sites are not conserved. In the light of recent observations that VSG-specific T-cell responses are induced in mice infected with T. brucei, these results confirm with the belief that immune pressure from T cells may contribute to the generation of antigenic diversity on the surface of African trypanosomes.