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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.
Antibodies against MAEBL Ligand Domains M1 and M2 Inhibit Sporozoite Development In Vitro
Peter Preiser ,1,
,
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
MAEBL is a type 1 membrane protein that is implicated in the
merozoite invasion of erythrocytes and sporozoite invasion of
mosquito salivary glands. This apical organelle protein is structurally
similar to the
ebl erythrocyte binding proteins, such as EBA-175,
except that the tandem ligand domains of MAEBL are similar to
part of the extracellular domain of apical membrane antigen
1 and not the Duffy binding-like domain. Although midgut and
salivary gland sporozoites are morphologically similar, salivary
gland sporozoites undergo a period of new gene expression after
infecting the salivary glands, display distinct phenotypic differences,
and are more infectious for the mammalian host. The objectives
of this project were to determine the molecular form of MAEBL
in the infectious salivary gland sporozoites and whether the
ligand has a role in the sporozoite development to exoerythrocytic
stages in hepatocytes. We determined that MAEBL is newly expressed
in salivary gland sporozoites and in a form distinct from what
is present in the midgut sporozoites or present in erythrocytic
stages. Both ligand domains (M1 and M2) were expressed as part
of a full-length membrane form of MAEBL in the salivary gland
sporozoites in contrast to the other stages that retain only
the M2 ligand domain as part of the membrane form of the protein.
Antisera developed against the cysteine-rich regions of the
extracellular portion of MAEBL inhibited sporozoite development
to exoerythrocytic forms in vitro. Together these data indicate
that MAEBL has a role in this third developmental stage in the
life cycle of the malaria parasite. Thus, MAEBL is another target
for pre-erythrocytic-stage vaccine development against malaria
parasites.

INTRODUCTION
Malaria is one of the most serious human diseases, causing several
million deaths and clinical illness in hundreds of millions
of people every year. Infection is spread from person to person
by the bite of a
Plasmodium-infected anopheline mosquito. Sporozoites
injected by the mosquito must infect and develop in a hepatocyte,
which can then initiate the blood-stage infection that causes
the severe pathogenesis of malaria. Molecules in the apical
organelles and on the surface of malaria sporozoites play a
role in sporozoite motility and target this infective stage
to the liver. The completion of the
Plasmodium falciparum genome
has provided the basis for the identification of many proteins
in sporozoites (
5,
12), but very few of these proteins are characterized
for their function in sporozoite infectivity and development
in the liver. Proteomic and transcript analyses have identified
a number of apical organelle and membrane-associated proteins
expressed both in sporozoites and merozoites, with many belonging
to molecular families conserved among the diverse species of
Plasmodium (
3,
5,
7,
19,
22-
24).
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
Parasites and DNA material.
The salivary glands and midgut were dissected from infected
Anopheles stephensi and gently ground in phosphate-buffered
saline to release
P. yoelii sporozoites. After the removal of
tissue fragments by centrifugation at 18
x g for 3 min, sporozoites
were collected from the supernatant by centrifugation at 5,000
x g for 3 min.
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
We used a panel of anti-MAEBL sera, prepared against the three
extracellular cysteine-rich domains (M1, M2, and C-CYS) and
the C terminus of the cytoplasmic domain (CT1) (Fig.
1) to determine
the MAEBL expression patterns in sporozoites. Serum reactivity
was tested by immunofluorescence on air-dried and methanol-fixed
midgut or salivary gland sporozoites, dissected from the salivary
glands and midgut of infected
A. stephensi mosquitoes. MAEBL
was shown to localize to internal and surface structures of
P. falciparum and
P. yoelii salivary gland sporozoites, confirming
the expression of MAEBL products in sporozoites (Fig.
2A). Importantly,
antisera to the M1 ligand domain and the cytoplasmic terminus
CT1 were reactive to salivary gland sporozoites (Fig.
2A and
data not shown), indicating that expression of the full-length
transmembrane form of MAEBL occurred in the infectious sporozoites.
Immunolocalization for most of the anti-MAEBL sera showed a
pattern at the apical end and on the surface of sporozoites
similar to that observed for CSP. This pattern for MAEBL is
also similar to that of TRAP, which only completely redistributes
to the sporozoite apical end after injection by the mosquito
(
16). The MAEBL immunofluorescence patterns appeared to be significantly
more intense in midgut sporozoites than in salivary gland sporozoites
(Fig.
2A). The anti-MAEBL sera were also tested on methanol-fixed
malaria liver-stage parasites as previously described (
20).
All anti-MAEBL but no control sera were reactive with
P. yoelii liver-stage parasites (Fig.
2B and data not shown).
We were particularly interested to determine the molecular form
of MAEBL present in the salivary gland sporozoites, since this
is the infective stage injected into the blood of a new host
and a possible target for immune attack. Unexpectedly, we detected
the full-length membrane form of MAEBL in the
P. yoelii salivary
gland sporozoite as a 240-kDa protein reactive with all anti-MAEBL
sera (anti-M1, anti-M2, anti-C-CYS, and anti-CT1) (Fig.
3A).
Further analysis of
P. yoelii compared MAEBL expression in midgut
sporozoites to its expression in salivary gland sporozoites
(Fig.
3B). There appeared to be significantly more protein product
reactive with the anti-MAEBL sera in the midgut sporozoites
than in the equivalent number of salivary gland sporozoites.
However, the intact 240-kDa full-length MAEBL product was not
detected in these midgut sporozoite preparations. Instead, two
major products were detected in salivary gland and midgut sporozoites
at 96 and 90 kDa, which appeared to represent amino and carboxyl
fragments of posttranslationally processed MAEBL (Fig.
1 and
3B). These data clearly show that the full-length membrane form
of MAEBL is newly expressed in sporozoites after they invade
the salivary glands, demonstrating that the salivary gland sporozoites
are actively synthesizing molecules for the next stage of invasion.
In addition, most of the processed forms of MAEBL in midgut
sporozoites were lost after invading the salivary glands, which
is consistent with its predicted important role in the invasion
of the salivary glands (
11).
The active expression of MAEBL in salivary gland sporozoites
and its presence on the surface of the sporozoite stages that
are invasive for the vertebrate host suggested that MAEBL has
a role in the invasion of hepatocytes. Therefore, rabbit antisera
(1:100 dilution) were tested for the inhibition of sporozoite
penetration and development in primary hepatocyte cultures as
previously described (
13). Antiserum to the M1 domain had the
highest level of inhibition, reducing the number of developing
liver-stage parasites by greater than 42% (Fig.
4). Inhibition
with antisera to both M1 and M2 domains suggests that both ligand
domains are functionally important for the sporozoite invasion
of hepatocytes.
Previous studies have indicated an essential role for MAEBL
for the sporozoite invasion of salivary glands (
11,
21). Consistent
with this role during the invasion of salivary glands, we demonstrate
that several MAEBL products were very abundant in midgut sporozoites
and then disappeared from sporozoites after infection of the
salivary glands. Once inside the salivary glands,
P. yoelii sporozoites expressed the full-length MAEBL, suggesting that
this form of MAEBL is important for the sporozoite invasion
of hepatocytes. The inhibition of
P. yoelii developing liver-stage
parasites by both anit-M1 and anti-M2 sera supports this conclusion.
It is interesting that infective salivary gland sporozoites
have both M1 and M2 ligand domains retained in a full-length
type 1 membrane form of MAEBL. This is a major functional difference
compared to erythrocytic stages, in which the M2 domain appears
to be the principal ligand and M1 is posttranslationally deleted
(
6,
18).
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
This work was supported by the National Institutes of Health
(grant R01 AI33656) and a Burroughs Wellcome Fund Travel Grant
(J.H.A.). Travel (P.R.P. and L.R.) was funded by The Royal Society's
Joint Project under European Science Exchange Programme and
by The British Council and French Ministry of Foreign Affairs
Alliance: Franco-British Partnership Programme. T.V. held a
fellowship from MRT. This work was supported in part by Grants-in-Aid
for Scientific Research 13576007 (M.T.) from the Ministry of
Education, Science, Sports and Culture, Japan.

FOOTNOTES
* Corresponding author. Mailing address: Departments of Biological Science, University of Notre Dame, P. O. Box 369, Notre Dame, IN 46556-0369. Phone: (574) 631-8676. Fax: (574) 631-7413. E-mail:
jadams3{at}nd.edu.

Editor: W. A. Petri, Jr.
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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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.
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