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Infection and Immunity, April 2007, p. 1626-1634, Vol. 75, No. 4
0019-9567/07/$08.00+0     doi:10.1128/IAI.01862-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Bradyzoite-Specific Surface Antigen SRS9 Plays a Role in Maintaining Toxoplasma gondii Persistence in the Brain and in Host Control of Parasite Replication in the Intestine{triangledown}

Seon-Kyeong Kim, Ariela Karasov,{dagger} and John C. Boothroyd*

Department of Microbiology and Immunology, Stanford University School of Medicine, 299 Campus Dr., Stanford, California 94305

Received 26 November 2006/ Returned for modification 5 January 2007/ Accepted 21 January 2007

Toxoplasma gondii is a ubiquitous parasite that persists for the life of a healthy mammalian host. A latent, chronic infection can reactivate upon immunosuppression and cause life-threatening diseases, such as encephalitis. A key to the pathogenesis is the parasite's interconversion between the tachyzoite (in acute infection) and bradyzoite (in chronic infection) stages. This developmental switch is marked by differential expression of numerous, closely related surface proteins belonging to the SRS (SAG1-related sequence) superfamily. To probe the functions of bradyzoite-specific SRSs, we created a bioluminescent strain lacking the expression of SRS9, one of the most abundant SRSs of the bradyzoite stage. Imaging of mice intraperitoneally infected with tachyzoites revealed that during an acute infection, wild-type and {Delta}srs9 strains replicated at similar rates, disseminated systemically following similar kinetics, and initially yielded similar brain cyst numbers. However, during a chronic infection, {Delta}srs9 cyst loads substantially decreased compared to those of the wild type, suggesting that SRS9 plays a role in maintaining parasite persistence in the brain. In oral infection with bradyzoite cysts, the {Delta}srs9 strain showed oral infectivity and dissemination patterns indistinguishable from those of the wild type. When chronically infected mice were treated with the immunosuppressant dexamethasone, however, the {Delta}srs9 strain reactivated in the intestinal tissue after only 8 to 9 days, versus 2 weeks for the wild-type strain. Thus, SRS9 appears to play an important role in both persistence in the brain and reactivation in the intestine. Possible mechanisms for this are discussed.

* Corresponding author. Mailing address: Department of Microbiology and Immunology, Stanford University School of Medicine, 299 Campus Dr., Stanford, CA 94305. Phone: (650) 723-7983. Fax: (650) 723-6853. E-mail: jboothr{at}stanford.edu.

{triangledown} Published ahead of print on 29 January 2007.

Editor: W. A. Petri, Jr.

{dagger} Present address: University of Wisconsin School of Medicine and Public Health, 750 Highland Ave., Madison, WI 53705-2221.

Infection and Immunity, April 2007, p. 1626-1634, Vol. 75, No. 4
0019-9567/07/$08.00+0     doi:10.1128/IAI.01862-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.



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