Plasmodium falciparum infection and exoerythrocytic development in mice with chimeric human livers John B. Sacci Jr. a, * , Uzma Alam a , Donna Douglas b , Jamie Lewis b , D Lorne J. Tyrrell c,d , Abdu F. Azad a , Norman M. Kneteman b,d a Department of Microbiology and Immunology, University of Maryland School of Medicine; 655 W. Baltimore Street, Baltimore, MD 21201 USA b Departments of Surgery, University of Alberta, Alta, Canada c Medical Microbiology and Immunology, University of Alberta, Canada d KMT Hepatech, Inc., Edmonton, Alberta, Alta, Canada Received 26 July 2005; received in revised form 22 September 2005; accepted 24 October 2005 Abstract The exoerythrocytic stage of Plasmodium falciparum has remained a difficult phase of the parasite life-cycle to study. The host and tissue specificity of the parasite requires the experimental infection of humans or non-human primates and subsequent surgical recovery of parasite- infected liver tissue to analyze this stage of the parasites development. This type of study is impossible in humans due to obvious ethical considerations and the cost and complexity in working with primate models has precluded their use for extensive studies of the exoerythrocytic stage. In this study we assessed, for the first time, the use of transgenic, chimeric mice containing functioning human hepatocytes as an alternative for modeling the in vivo interaction of P. falciparum parasites and human hepatocytes. Infection of these mice with P. falciparum sporozoites produced morphologically and antigenically mature liver stage schizonts containing merozoites capable of invading human red blood cells. Additionally, using microdissection, highly enriched P. falciparum liver stage parasites essentially free of hepatocyte contamination, were recovered for molecular studies. Our results establish a stable murine model for P. falciparum that will have a wide utility for assessing the biology of the parasite, potential anti-malarial chemotherapeutic agents and vaccine design. q 2005 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Plasmodium falciparum; Chimeric mouse; Liver stage; Malaria; Reverse transcriptase-PCR 1. Introduction Plasmodium falciparum has a complex life cycle involving intracellular and extracellular stages in both the human host and mosquito vector. Among the different developmental stages of P. falciparum, the least well characterized is the hepatic or exoerythrocytic (EE) phase. This stage encompasses the infection and multiplication of the parasite following entry into the hepatocyte. Because of the host specificity of P. falciparum, infection and parasite development can only be studied in human or primate hepatocytes. This has imposed a major limitation on the analysis of the in vivo host–parasite interaction that occurs during the transitional liver stage. Although the enzymatic isolation and culture of human hepatocytes has produced in vitro systems to study the EE stage (Mazier et al., 1985), the utility of these cultured forms might be limited because they may not accurately reflect in vivo development. The thorough characterization of antigens relevant to vaccine or drug development as well as the immunologic dissection of the liver stage will require an in vivo model to understand the complex interaction of the host and parasite. The continuing development of human chimeric small animal models has allowed researchers to study diseases that previously were not amenable to detailed investigation (Davis and Stanley Jr., 2003). We now report the utilization of a chimeric human liver mouse model, which had previously shown susceptibility to infection with Hepatitis C (Mercer et al., 2001) and Hepatitis B (Meulemann et al., 2005), to assess the EE stage of P. falciparum in an in vivo setting and the use of this model to recover highly enriched parasite RNA for molecular analysis. International Journal for Parasitology 36 (2006) 353–360 www.elsevier.com/locate/ijpara 0020-7519/$30.00 q 2005 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpara.2005.10.014 * Corresponding author. Tel.: C1 410 706 4071; fax: C1 410 706 0282. E-mail address: jsacc001@umaryland.edu (J.B. Sacci).