Immunodominant Epitopes inBabesia bovisRhoptry-Associated Protein 1 That Elicit Memory CD4+-T-Lymphocyte Responses inB. bovis-Immune Individuals Are Located in the Amino-Terminal Domain

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Abstract
Babesia bovisrhoptry-associated protein 1 (RAP-1), which confers partial protection againstB. bovischallenge, is recognized by antibodies and T lymphocytes from cattle that have recovered from infection and are immune to subsequent challenge. RAP-1 is a 60-kDa protein with an N-terminal (NT) region that contains four cysteine residues conserved among allBabesiaRAP-1 family members and a C-terminal (CT) region that contains multiple, degenerate, tandem 23-amino-acid (aa) repeats. To define the location of CD4+-T-cell epitopes for vaccine development using a recombinant protein or minigene construct, a series of truncated recombinant RAP-1 proteins and peptides were tested for stimulation of T-cell lines derived fromB. bovis-immune cattle. CD4+-T-cell lines from threeB. bovis-immune cattle with differentDRB3haplotypes responded to the NT region of RAP-1, whereas T cells from only one animal responded weakly to the CT region. T-cell lines from the three individuals recognized two to six NT-region peptides spanning aa 134 to 316 and representing at least four dominant epitopes. Using RAP-1-specific CD4+-T-cell clones, two NT-region epitopes, EYLVNKVLYMATMNYKT (aa 187 to 203) and EAPWYKRWIKKFR (aa 295 to 307), and one CT-region repeat epitope, FREAPQATKHFL, which is present twice at aa positions 391 to 402 and 414 to 425, were identified. Several peptides representing degenerate repeats of the agonist CT-region peptide FREAPQATKHFL neither stimulated responses of T-cell clones specific for this peptide nor inhibited responses to the agonist peptide. Upon stimulation with specific antigen, T-cell clones specific for NT or CT epitopes produced gamma interferon. The presence of T-helper-cell epitopes in the NT domain of RAP-1, which is highly conserved among otherwise antigenically different strains ofB. bovis, supports the inclusion of this region in vaccine constructs to be tested in cattle.

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