CD4(-HELPER LYMPHOCYTE-RESPONSES AGAINST BABESIA-BIGEMINA RHOPTRY-ASSOCIATED PROTEIN-1() T)

A multigene family of 58- to 60-kDa proteins, which are designated rho ptry-associated protein 1 (RAP-1) and which come from the parasites Ba besia bigemina and Babesia bovis, is a target for vaccine development. The presence of multiple gene copies and conserved sequences and epit opes of RAP-1 implies that these proteins are functionally important f or the survival of these parasites. Furthermore, it was previously sho wn that B. bigemina RAP-1 induced partial protection against challenge infection, However, the lack of correlation between protective immuni ty to B. bigemina infection and antibody titers against a merozoite su rface-exposed, neutralization-sensitive epitope of B. bigemina RAP-1 i ndicated the potential importance of RAP-1-specific T helper (Th) cell s in the observed protection. To begin to understand the mechanism of RAP-1-induced protective immunity, RAP-1-specific T-cell responses wer e characterized in cattle. Vigorous and sustained proliferative respon ses of peripheral blood mononuclear cells from native RAP-1-immunized cattle were observed. The anamnestic response in immunized cattle was specific for B. bigemina RAP-1 and predominantly comprised CD4(+) T ce lls, which upon cloning expressed type 1 cytokine mRNA profiles and hi gh levels of gamma interferon protein. The T cells responded to both n ative and recombinant forms of RAP-1, indicating the potential to use recombinant protein or epitopes derived therefrom as a vaccine that co uld evoke specific recall responses after exposure to natural infectio n. The differential responses of peripheral blood mononuclear cells an d seven Th-cell clones derived from RAP-1-immunized cattle to differen t Central American strains of B. bigemina indicated the presence of at least one conserved and one variable Th-cell epitope. The lack of res ponse to B. bovis RAP-1 indicated that a strictly conserved 14-amino-a cid peptide shared by the two babesial species was not immunogenic for Th cells in these experiments. However, the Th cell epitope conserved among strains of B. bigemina may be a useful component of a RAP-1 sub unit vaccine.