CHARACTERIZATION OF THE GENE ENCODING THE POLYMORPHIC IMMUNODOMINANT MOLECULE, A NEUTRALIZING ANTIGEN OF THEILERIA-PARVA

Theileria parva, a tick-transmitted protozoan parasite related to Plas modium spp., causes the disease East Coast fever, an acute and usually fatal lymphoproliferative disorder of cattle in Africa. Previous stud ies using sera from cattle that have survived infection identified a p olymorphic immunodominant molecule (PIM) that is expressed by both the infective sporozoite stage of the parasite and the intracellular schi zont. Here we show that mAb specific for the PIM Ag can inhibit sporoz oite invasion of lymphocytes in vitro. A cDNA clone encoding the PIM A g of the T. parva (Muguga) stock was obtained by using these mAb in a novel eukaryotic expression cloning system that allows isolation of cD NA encoding cytoplasmic or surface Ags. To establish the molecular bas is of the polymorphism of PIM, the cDNA of the PIM Ag from a buffalo-d erived T. parva stock was isolated and its sequence was compared with that of the cattle-derived Muguga PIM. The two cDNAs showed considerab le identity in both the 5' and 3' regions, but there was substantial s equence divergence in the central regions. Several types of repeated s equences were identified in the variant regions. In the Muguga form of the molecule, there were five tandem repeats of the tetrapeptide, QPE P, that were shown, by transfection of a deleted version of the PIM ge ne, not to react with several anti-PIM mAbs. By isolating and sequenci ng the genomic version of the gene, we identified two small introns in the 3' region of the gene. Finally, we showed that polyclonal rat Abs against recombinant PIM neutralize sporozoite infectivity in vitro, s uggesting that the PIM Ag should be evaluated for its capacity to immu nize cattle against East Coast Fever.