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Infection and Immunity, January 2003, p. 411-417, Vol. 71, No. 1
0019-9567/03/$08.00+0     DOI: 10.1128/IAI.71.1.411-417.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Roles of the Maltese Cross Form in the Development of Parasitemia and Protection against Babesia microti Infection in Mice

Naoaki Yokoyama, Sabine Bork, Mitsuhiro Nishisaka, Haruyuki Hirata, Tomohide Matsuo, Noboru Inoue, Xuenan Xuan, Hiroshi Suzuki, Chihiro Sugimoto, and Ikuo Igarashi^*

National Research Center for Protozoan Diseases, Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido 080-8555, Japan

Received 20 May 2002/ Returned for modification 29 June 2002/ Accepted 4 October 2002

Babesia microti, a hemoprotozoan parasite of rodents, is also important as a zoonotic agent of human babesiosis. The Maltese cross form, which consists of four masses in an erythrocyte, is characteristic of the developmental stage of B. microti. Monoclonal antibody (MAb) 2-1E, which specifically recognizes the Maltese cross form of B. microti, has been described previously. In the present study, we examined the roles of the Maltese cross form during the infectious course of B. microti in mice. The number of the Maltese cross form increased in the peripheral blood of infected mice prior to the peak of parasitemia. With confocal laser scanning microscopy, MAb 2-1E was found to be reactive with the ring form, with the parasites undergoing transformation to the Maltese cross form and subsequent division, and also with extracellular merozoites. Furthermore, the Maltese cross form-related antigen (MRA) gene was isolated from a B. microti cDNA library by immunoscreening with MAb 2-1E, and the nucleotide sequence was determined. Genomic analyses indicated that the MRA gene exists as a single-copy gene in B. microti. Immunization of mice with recombinant MRA induced significant protective immunity against B. microti infection. These findings indicate that the Maltese cross form plays important roles in both the development of parasitemia and the protective response against the infection.

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* Corresponding author. Mailing address: National Research Center for Protozoan Diseases, Obihiro University of Agriculture and Veterinary Medicine, Inada-cho, Obihiro, Hokkaido 080-8555, Japan. Phone: 81-155-49-5641. Fax: 81-155-49-5643. E-mail: igarcpmi{at}obihiro.ac.jp.

Editor: W. A. Petri, Jr.

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Infection and Immunity, January 2003, p. 411-417, Vol. 71, No. 1
0019-9567/03/$08.00+0     DOI: 10.1128/IAI.71.1.411-417.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

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