TRENDS in Parasitology Vol.17 No.5 May 2001 http://parasites.trends.com 1471-4922/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S0169-4758(00)01862-7 219 Opinion Falciparum malaria infects several million individuals and kills nearly a million children in Africa each year 1 . This situation is expected to worsen because of the spread of drug-resistant parasites and insecticide-resistant vectors. Despite important advances, such as the circumsporozoite protein (CSP)-based vaccine called RTS,S (Ref. 2), the goal of a safe and broadly effective malaria vaccine remains unfulfilled. The parasite’s complex life-cycle offers several targets for intervention in the human host and the mosquito vector, and vaccines against sporozoite, intrahepatic, blood and sexual stages of the parasite are currently in development 3 . Targeting intrahepatic parasites for vaccine development has specific advantages, including the relatively low parasite burden at this stage of infection, the putative presentation of antigens by liver cell MHC class I molecules and an array of potential effector mechanisms against liver-stage antigens. Plasmodium falciparum liver-stage antigen-1 (LSA- 1) is expressed specifically in liver-stage parasites 4 , unlike other identified pre-erythrocytic antigens. Investigation of its immunological significance is restricted to human studies because no homologue in mouse or non-human primate malarias has been identified. In this article, we discuss the immunobiology of hepatic stage parasites, evidence relating immune responses against this stage to protection, and the rationale for development of liver- stage vaccines based on LSA-1. LSA-1 and the biology of liver-stage parasites Sporozoite invasion occurs by a series of incompletely characterized molecular interactions that involve sporozoite antigens such as CSP (Ref. 5) and thrombospondin-related anonymous protein (TRAP) 6,7 . CSP and TRAP are carried into the hepatocyte, but are detected in decreasing amounts as liver-stage development progresses 8,9 . By contrast, synthesis of LSA-1 begins soon after sporozoite invasion of the hepatocyte and increases throughout the liver cycle 8 . Consequently, LSA-1 might be processed and presented to the immune system in a way that differs from that of sporozoite antigens. LSA-1 appears as part of a flocculent mass within the parasitophorous vacuole of the hepatocyte and surrounds the developing merozoite 8,10 (Fig. 1). When the hepatocyte ruptures, merozoites are released in this flocculent mass 11 into the liver sinusoid where they invade red blood cells. LSA-1 is a 200 kDa protein 8 that contains a central region with 86 copies of a 17 amino acid (aa) repeat 8,10 , flanked by N- and C-terminal non-repeat regions containing B and T epitopes 8,12 (Fig. 2). Unlike many other vaccine candidates, the sequence of LSA-1 is generally conserved across strains 8,13 . This might suggest a crucial role during liver schizogony, perhaps protecting the merozoite surface. Protective immunity targeting the infected hepatocyte Although the homologue of LSA-1 has not been identified in rodent parasites, studies in mice have Pre-erythrocytic immunity to Plasmodium falciparum : the case for an LSA-1 vaccine Jonathan D. Kurtis, Michael R. Hollingdale, Adrian J.F. Luty, David E. Lanar, Urszula Krzych and Patrick E. Duffy A vaccine is urgently needed to stem the global resurgence of Plasmodium falciparum malaria. Vaccines targeting the erythrocytic stage are often viewed as an anti-disease strategy. By contrast, infection might be completely averted by a vaccine against the liver stage, a pre-erythrocytic stage during which the parasite multiplies 10 000-fold w ithin hepatocytes. Sterilizing immunity can be conferred by inoculating humans with irradiated pre-erythrocytic parasites, and a recombinant pre-erythrocytic vaccine partially protects humans from infection. Liver-stage antigen-1, one of a few proteins known to be expressed by liver-stage parasites, holds particular promise as a vaccine. Studies of naturally exposed populations have consistently related immune responses against this antigen to protection. Jonathan D. Kurtis David E. Lanar Urszula Krzych Patrick E. Duffy* Walter Reed Army Institute of Research, Dept of Immunology, Silver Spring, MD 20910, USA. *e-mail: patrick.duffy@ na.amedd.army.mil Michael R. Hollingdale Gates Malaria Programme, London School of Hygiene and Tropical Medicine, London, UK WC1E 7HT. Adrian J.F. Luty University of Tübingen, Dept of Parasitology, 72074 Tübingen, Germany. Fig. 1. Electron micrograph showing immunogold localization of LSA-1. Plasmodium falciparum merozoites developing in a chimpanzee hepatocyte (H) are identified by their nuclei (N) and rhoptries (R). The parasitophorous vacuole (PV), enclosed within the vacuolar membrane (PVM), is filled with flocculent material containing LSA-1. Micrograph courtesy of Jacques Meis, University of Nijmegen, The Netherlands. Scale bar = 0.5 μm.