MOLECULAR VARIATION AND DIVERSITY IN CANDIDATE VACCINE ANTIGENS FROM BABESIA

Recombinant vaccines are being developed against a number of species o f protozoan parasites in the genus Babesia. Protozoan parasites are no torious for their diversity of strains and their ability to express fa milies of equivalent, but antigenically distinct, surface proteins. In order to reduce the likelihood of evasion of the immune response indu ced by a recombinant vaccine, ideal components should be essential pro teins encoded by single copy genes. The proteins should also have a li mited ability to tolerate polymorphism in amino acid sequence, especia lly in critical epitopes. While little is known about the function of the candidate protective antigens, there is now considerable informati on concerning the variation of a number of candidate vaccine antigens from several species of Babesia. Four of the well studied antigens are all members of multi-gene families. The members of the VMSA gene fami ly of Babesia bovis are also highly polymorphic in sequence. The membe rs of the Bv60/p58 family of rhoptry protein homologues exhibit more l imited polymorphism within a single species of Babesia. However, compa rison of the sequences of the equivalent proteins and the organisation of the corresponding genes from B. bovis, Babesia bigemina, Babesia c anis and Babesia ovis suggests that members of this family have the po tential to acquire and to tolerate substantial polymorphism in amino a cid sequence. The choice of protein, and particular region of the prot ein, suitable for incorporation in a recombinant vaccine may require e xtensive analysis of the genetic systems encoding the candidate antige ns.