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Infection and Immunity, June 2004, p. 3604-3608, Vol. 72, No. 6
0019-9567/04/$08.00+0 DOI: 10.1128/IAI.72.6.3604-3608.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
,
Laurent Rénia,2,
Naresh Singh,3 Bharath Balu,3 William Jarra,1 Tatiana Voza,4 Osamu Kaneko,5 Peter Blair,3 Motomi Torii,5 Irène Landau,4 and John H. Adams3*
Division of Parasitology, National Institute for Medical Research, London, United Kingdom,1 Département d'Immunologie, Institut Cochin, INSERM U567, CNRS 8104, Université René Descartes, Hôpital Cochin,2 Muséum National d'Histoire Naturelle, Parasitologie Comparée et Modèles Expérimentaux, Associés à l'NSERM U567, Paris, France,4 Department of Biological Sciences, University of Notre Dame, Notre Dame, Indiana,3 Department of Molecular Parasitology, Ehime University School of Medicine, Shigenobu-cho, Ehime, Japan5
Received 23 December 2003/ Returned for modification 27 January 2004/ Accepted 1 March 2004
| ABSTRACT |
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| INTRODUCTION |
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The circumsporozoite protein (CSP) and to a lesser extent the thrombospondin-related anonymous protein (TRAP, or sporozoite surface protein 2 [SSP2]) have been the focus of most research on the sporozoite stages. CSP and TRAP are major sporozoite proteins that are functionally important for sporozoite development in the mosquito stage and are widely considered as important targets for vaccine development. However, both have obstacles for development as vaccines against the pre-erythrocytic stages of development. Polymorphism in the critical T-cell epitopes recognized by helper T cells and cytotoxic lymphocytes of CSP compromise its potential efficacy as a vaccine. While even though TRAP is essential for invasion of hepatocytes, antibodies against this transmembrane protein were shown not to inhibit sporozoite development into the exoerythrocytic stages. Therefore, it is important to identify additional sporozoite antigens that are potential targets for development as part of a multivalent pre-erythrocytic vaccine.
MAEBL was identified in Plasmodium yoelii and P. falciparum blood-stage parasites as a minor type 1 membrane protein with erythrocyte binding activity expressed in the apical organelles and on the surface of invasive merozoites, but it was later identified as an abundant protein expressed in sporozoites (4, 6, 8-11, 17). MAEBL is a paralogue of the products from the ebl family, similar except that its two extracellular ligand domains have identity to apical membrane antigen 1 and not the consensus Duffy binding-like ligand domains of other ebl products (10). The ebl family of erythrocyte binding proteins includes some of the best-characterized malarial ligands, such as the Plasmodium vivax Duffy binding protein and the P. falciparum erythrocyte binding protein EBA-175 (1). Exon structure, including the conserved position of splicing junctions within codons at the exon boundaries, is an important characteristic of ebl genes, and it is conserved with maebl (2). However, maebl evolved independently of ebl and ama1 in the ancestral Plasmodium genome prior to speciation (15). Interestingly, it appears that MAEBL may have an essential function in the midgut sporozoite invasion of the salivary glands but not in the merozoite invasion of erythrocytes (11 and unpublished data). Although morphologically similar to midgut sporozoites, salivary gland sporozoites are much more infectious for the mammalian host, have a unique gliding motility on a solid substrate, and can induce a strong protective immunity. These phenotypic differences appear to correspond with the expression of new proteins once sporozoites invade the salivary glands (14). Since MAEBL is differentially expressed in midgut and salivary gland sporozoites, we were interested to determine what form of MAEBL is expressed in infectious salivary gland sporozoites and whether MAEBL has an important role in sporozoite development in hepatocytes. We find that MAEBL is expressed in a different molecular form in salivary gland sporozoites compared to midgut sporozoites. Furthermore, we find that antibodies against MAEBL inhibit sporozoite development in hepatocytes, suggesting that MAEBL has an important role in the sporozoite infection of hepatocytes.
| MATERIALS AND METHODS |
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Anti-MAEBL sera. Antiserum against the P. falciparum MAEBL CT1 was prepared by using a linear keyhole lympet hemocyanin-conjugated peptide (CHWFKNDHRKVV) injected in rabbits with complete Freund's adjuvant by intraperitoneal injection, followed by incomplete Freund's adjuvant at 2, 4, and 10 weeks. Rabbit antisera were produced against the deduced C terminus of P. yoelii yoelii MAEBL as previously described (18). Polyclonal antisera to the cysteine-rich regions of P. yoelii yoelii and P. falciparum MAEBL were prepared prior to the glutathione transferase fusion proteins of each domain (4, 18).
Western immunoblotting and indirect immunofluorescence assay. Approximately 105 sporozoites per lane were resolved by electrophoresis on 4 to 12% Bis-Tris NuPAGE Gels (Invitrogen) in 1x morpholineethanesulfonic acid sample buffer (Invitrogen) under reducing conditions. Specific proteins transferred onto Nitrocellulose Extra blotting membrane (Sartorius AG) were detected by polyclonal sera raised against the different regions of MAEBL and horseradish peroxidase-linked secondary antibodies (Bio-Rad) and by enhanced chemiluminescence (Pierce). The dilutions of the primary sera were 1:500 to 1:800. Serum reactivity was tested by an immunofluorescence assay using methanol-fixed midgut or salivary gland sporozoites, previously air dried on microscope slides, or malaria liver-stage parasites as previously described (20).
Inhibition of sporozoite development. Rabbit antisera (at a 1:100 dilution) were tested in triplicate for inhibition of sporozoite penetration and development in primary hepatocyte cultures as previously described (13). Sporozoites were added to cultures with immune serum (prepared against M1, M2, and C-Cys) or with preimmune serum as a negative control at a concentration in serum of 1:100. The cultures were incubated for 3 h and washed, and fresh medium was added. After 24 h the medium was changed once and incubated another 24 h. The schizonts in each culture well were stained with anti-PfHSP70 antibodies to count hepatic schizonts (20). Inhibition was determined by counting the number of hepatic schizonts in the test cultures relative the number in cultures with preimmune serum.
| RESULTS AND DISCUSSION |
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The creation of a different combination of binding domains due to posttranslational processing represents a possible mechanism by which a malaria parasite is able to use a single gene locus to create multiple ligands capable of recognizing different receptors. The expression of distinct molecular forms of MAEBL in different stages of parasite development is analogous to the tissue-specific expression of proteins in more complex organisms. Little is known about the posttranslational processing of malaria proteins in Plasmodium species, although many proteins are known to undergo extensive, functionally important processing. The expression of the full-length membrane form of MAEBL is consistent with other data that sporozoites undergo a transformation after invading the salivary gland as preparation to infect the vertebrate host. The inhibition of sporozoite development into the exoerythrocytic form identifies MAEBL as another target in a multivalent pre-erythrocytic-stage vaccine.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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P. Preiser and L. Rénia contributed equally to this work. ![]()
Present address: Nanyang Technological University, School of Biological Sciences, Singapore 637616, Republic of Singapore. ![]()
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