A polymorphic multigene family encoding an immunodominant protein from Babesia microti

Human babesiosis in the United States is caused predominantly by Babesia mi croti, a tick-transmitted blood parasite. Improved testing methods for the detection of infection with this parasite are needed, since asymptomatic B. microti infection represents a potential threat to the blood supply in are as where B, microti is endemic. We performed immunoscreening of an expressi on library of genomic DNA from a human isolate of B, microti (strain MN1). Among 17 unique immunoreactive clones, we identified 9 which represent a re lated family of genes with little sequence homology to other known sequence s but with an architecture resembling that of several surface proteins of P lasmodium. Within this family? a tandem array(1) of a degenerate sis-aminoa cid repeat (SEAGGP, SEAGWP, SGTGWP, SGTVGP) was found in various lengths be tween relatively well conserved segments at the N and C termini, In order t o examine within-clone variation, He developed a PCR protocol for direct re covery of a specific bmn1-6 homologue directly from 30 human blood isolates , 4 corresponding hamster isolates, and 5 geographically corresponding Pero myscus leucopus (white-footed mouse) isolates, Isolates from the hamsters h ad the same sequences as those found in the corresponding human blood, sugg esting that genetic variation of bmn1-6 does not occur during passage, Howe ver, clones from different patients were often substantially different from each other with regard to the number and location of the degenerate repeat s within the bmn1-6 homologue, Moreover, we found that strains that were cl osely related geographically were also closely related at the sequence leve l; nine patients, all from Nantucket Island, Mass., harbored clones that we re indistinguishable from each other but that were distinct from those foun d in other northeastern or upper midwestern strains, We conclude that consi derable genetic and antigenic diversity exists among isolates of B. microti from the United States and that geographic clustering of subtypes may exis t. The nature of the bmn1-6 gene family suggests a mechanism of antigenic v ariation in B. microti that may occur by recombination, differential expres sion, or a combination of both mechanisms.