Human Antibodies againstPlasmodium falciparumLiver-Stage Antigen 3 Cross-React withPlasmodium yoeliiPreerythrocytic-Stage Epitopes and Inhibit Sporozoite Invasion In Vitro and In Vivo

The development of a malaria preerythrocytic vaccine has been greatly influenced by the observation that sterile immunity could be experimentally induced in humans by immunization with Plasmodium falciparum radiation-attenuated sporozoites (9). The critical role of liver-stage trophozoites (reviewed in reference 16) led researchers to initiate a detailed study of the antigens expressed in P. falciparum preerythrocytic stages (22). To date, scientists have identified a series of new molecules expressed at sporozoite and/or liver stage (14). Liver-stage antigen 3 (LSA3), expressed on both the sporozoite surface and in the liver forms, was found to be of particular interest. LSA3 was identified by differential screening of immune responses from protected versus nonprotected volunteers (11). A number of dominant B- and T-cell epitopes to which a high prevalence of responses is detected in individuals exposed to malaria or in LSA3-immunized animals were identified (1, 2). The vaccine potential of this molecule has been recently demonstrated in the chimpanzee model, an animal susceptible and fully receptive to P. falciparum preerythrocytic stages and whose immune system is closest to that of humans. In this model, protection against successive challenges with P. falciparum sporozoites was obtained (11). However, the use of this primate for research purposes is severely hampered by cost and ethical constraints. Several homologues of P. falciparum antigens have been identified, through a variety of immunological and molecular cross-reactivity assays, in other Plasmodium species and particularly in those infecting rodents (5, 10, 12, 13, 27, 28). The identification of structurally and functionally conserved homologues of P. falciparum proteins in rodent malaria species might help to better understand the role of these molecules in the P. falciparum parasite life cycle. This is particularly true for preerythrocytic stages, where the relative ease of obtaining sporozoites and mouse primary hepatocytes would allow more extensive investigations to be carried out. In the process of identification of LSA3, a preerythrocytic subset of P. falciparum genomic fragments (22) was used to affinity purify antibodies (Abs) from sera of individuals with hyperimmunity to malaria. These Abs were consequently checked for immunofluorescence antibody test (IFAT) cross-reactivity with rodent malaria sporozoites, and in this assay Abs against all clones coding for various fragments of LSA3 proved to be strongly reactive with Plasmodium yoelii sporozoites but not with those from Plasmodium berghei. The present study was undertaken to investigate the extent of antigenic cross-reactivity between the LSA3 molecule and its putative homologue in P. yoelii and to determine whether specific human Abs are of biological significance in the mouse model. We show that such cross-reactivity exists for human anti-LSA3 Abs, which encompass four distinct LSA3 epitopes, including both nonrepeated and repeated domains, and extend to both sporozoite and liver stages of P. yoelii. A similar result was obtained using Abs from chimpanzees immunized with LSA3. Moreover, P. yoelii, but not P. berghei, sporozoites were able to induce dose-dependent proliferation of T cells collected from one of these animals. Finally, we have established that these homologous epitopes are targets for immune responses which have a biological consequence, since LSA3-specific human Abs inhibit the invasion of P. yoelii sporozoites into mouse hepatocytes both in vitro and in vivo.