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Infection and Immunity, December 2000, p. 6865-6870, Vol. 68, No. 12
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Characterization of Allelic Variation in the Babesia bovis Merozoite Surface Antigen 1 (MSA-1) Locus and Identification of a Cross-Reactive Inhibition-Sensitive MSA-1 Epitope

Carlos E. Suarez,* Monica Florin-Christensen,dagger Stephen A. Hines, Guy H. Palmer, Wendy C. Brown, and Terry F. McElwain

Program in Vector-Borne Diseases, Animal Disease Research Unit, Agricultural Research Service, U.S. Department of Agriculture, and Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington 99164

Received 17 July 2000/Returned for modification 4 September 2000/Accepted 16 September 2000

The Babesia bovis merozoite surface antigen 1 (MSA-1), a member of the variable merozoite surface antigen (VMSA) family, is an immunodominant glycoprotein which elicits antibodies that inhibit erythrocyte invasion. While antigenic polymorphism is a general feature of vmsa genes, the molecular basis and extent of msa-1 sequence polymorphism have not been well characterized. In this study we defined the msa-1 locus in the biologically cloned Mexico Mo7 strain of B. bovis and identified the sequence differences between MSA-1 antigenically dissimilar strains. We then determined whether sequences conserved between distinct msa-1 alleles would induce cross-reactive CD4+ T lymphocytes or inhibitory antibodies. The msa-1 locus in Mo7 contains a single msa-1 gene flanked by transcribed genes with no sequence homology to members of the VMSA gene family. Argentina B. bovis strains R1A and S2P have msa-1 genes with amino acid sequences that are 98.8% identical to each other, and antibodies against S2P MSA-1 cross-react with native R1A MSA-1. In contrast, identity between the Argentina and Mexico Mo7 msa-1 alleles is only 52%, with no continuous stretch of identity longer than 16 amino acids. Despite limited sequence conservation, antibodies against R1A MSA-1 were able to inhibit invasion of erythrocytes by Mo7 merozoites. The results indicate that inhibition-sensitive epitopes are conserved despite significant sequence divergence between Mexico and Argentina strain alleles and support a conserved functional role for polymorphic MSA-1 in erythrocyte invasion.

* Corresponding author. Mailing address: Animal Disease Research Unit, United States Department of Agriculture, Agricultural Research Service, Animal Disease Biotechnology Facility, Pullman, WA 99164-7030. Phone: (509) 335-6341. Fax: (509) 335-8328. E-mail: ces{at}vetmed.wsu.edu.

dagger Permanent address: INEUCI, CONICET, Buenos Aires, Argentina.

Infection and Immunity, December 2000, p. 6865-6870, Vol. 68, No. 12
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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