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Clinical and Vaccine Immunology, May 2009, p. 775-778, Vol. 16, No. 5
1071-412X/09/$08.00+0 doi:10.1128/CVI.00432-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
Potential Use of the Hemagglutinin-Neuraminidase Glycoprotein of Newcastle Disease Virus Expressed in Rachiplusia nu Larvae as an Immunogen for Chickens
Silvina Chimeno Zoth,1,2
Evangelina Gómez,1,2
Juan Manuel Carballeda,1
Oscar Taboga,1,2
Elisa Carrillo,1,2 and
Analía Berinstein1,2*
Instituto de Biotecnología, CICVyA, INTA, Castelar, Cc 25 B1712WAA, Buenos Aires, Argentina,1
Consejo Nacional de Investigaciones Científicas y Tecnológicas (CONICET), Rivadavia 1917 C1033AAJ, Ciudad de Buenos Aires, Argentina2
Received 20 November 2008/
Returned for modification 11 December 2008/
Accepted 6 March 2009

ABSTRACT
The hemagglutinin-neuraminidase glycoprotein of Newcastle disease
virus (NDV) was obtained as a recombinant antigen in
Rachiplusia nu larvae. When it was used as an immunogen in chickens, a solid
immune response, including neutralizing antibodies, was detected,
demonstrating the potential use of this simple and economic
strategy in the design of recombinant anti-NDV vaccines.

TEXT
Insect cell-based baculovirus expression systems have been extensively
exploited for the large-scale production of recombinant proteins
(
12,
14,
19). The main disadvantage of this system is its high
cost, which may become prohibitive if large volumes are required.
Insect larvae infected with recombinant baculoviruses can be
an extremely useful alternative due to the high levels of protein
expression achieved, the lower costs for large-scale production,
and a higher efficiency of certain types of posttranslational
modifications (
19). In the last 10 years, larvae have been successfully
used to produce different recombinant proteins (
3,
5,
7,
13,
16,
21,
22).
Newcastle disease virus (NDV), an avian paramyxovirus, is the causative agent of an economically important and highly contagious disease of poultry. Several reports describe the use of different expression systems to obtain recombinant immunogens based on hemagglutinin-neuraminidase (HN) or F glycoproteins of NDV with various degrees of success (2, 9, 10, 17, 24). For instance, Kamiya and coworkers demonstrated that complete protection was achieved when either HN or F protein of a virulent strain or HN protein from an avirulent strain was used as an immunogen (9).
The aim of the present work was to obtain a recombinant immunogen against NDV by infection of lepidopteran larvae with a recombinant baculovirus for HN glycoprotein.
The HN coding region was amplified from cDNA synthesized from genomic NDV RNA (La Sota strain, provided by the National Service for Animal Health [SENASA]) isolated using TRIzol (Life Technologies, Grand Island, NY), following the manufacturer's instructions. Amplification of the HN coding region was done using oligonucleotides 5'-AAAGGATCCGCTAGCACACCTAGCGAT-3' and 5'-AGGCTCTAGATTGCCAGACCTGGC-3', which amplify a 1,585-bp fragment, excluding the first 150 bp coding for 50 amino acids of the protein predicted as the signal peptide and the transmembrane domain (6). Oligonucleotides included BamHI and XbaI restriction sites for cloning into pBacPAK8 (Clontech, Mountain View, CA) transfer plasmid (underlined). The PCR product was cloned into the transfer plasmid, and recombinant plasmids were isolated following classical protocols (15). Sf9 insect cells were cotransfected with the recombinant vector and linearized Autographa californica multiple nucleopolyhedrosis virus (AcNPV) DNA, following the BaculoGold kit instructions (Pharmingen, San Diego, CA). Recombinant baculoviruses were purified (19), and selected clones were amplified by three passages in Sf9 cells. Finally, viral stock (AcNPV/HN) was titrated and HN expression was characterized by Western blot assay. Briefly, monolayers of Sf9 cells were infected with the viral inoculum at a multiplicity of infection of 5. After 72 h, cells were collected, washed with phosphate-buffered saline (PBS), pH 6.2, and resuspended in cracking buffer. Cellular extracts were analyzed by 12% sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the presence of the recombinant protein was confirmed by Western blotting, following standard procedures (8). Figure 1A shows a double band slightly lower than 72 kDa, in agreement with the fact that the recombinant protein did not include the first 50 amino acids of HN, and because of that, its molecular mass is lower than that of the one corresponding to HN glycoprotein from purified NDV (data not shown). When HN expression was evaluated in extracts of infected Rachiplusia nu larvae (7, 13), a similar pattern was detected (Fig. 1A). Considering that HN glycosylation has a relevant role in its antigenicity and immunogenicity (20), we evaluated the ability of our system to render a correctly glycosylated protein. Sf9 cells were infected with AcNPV/HN (multiplicity of infection, 5). After the adsorption period, the viral inoculum was removed and replaced by fresh medium containing 5 µg/ml tunicamycin (Sigma-Aldrich, St. Louis, MO). After 72 h, cells were collected and processed for Western blot analysis. Results of this treatment demonstrated that the double band pattern corresponded to the glycosylated and nonglycosylated forms of the recombinant HN protein (Fig. 1B). To confirm our observation, both cellular and larval extracts were treated with PNGase F following the manufacturer's indications (New England BioLabs). PNGase F is an endoglycosidase that cleaves oligosaccharides from N-linked glycoproteins. A reduction in the apparent molecular size of the protein was observed in both systems, indicating that N glycosylation occurs in vitro and in vivo (Fig. 1B).
To evaluate the immunizing ability of the recombinant protein
obtained, 15-day-old specific-pathogen-free White Leghorn chickens
(Rosenbusch, Argentina) were divided into groups of five animals
and intramuscularly immunized on days 0 and 14. Each dose consisted
of 0.2 ml of the extract formulated in incomplete Freund's adjuvant
(Sigma-Aldrich, St. Louis, MO), containing approximately 2 µg
of recombinant antigen as estimated by comparison with a bovine
serum albumin standard curve stained with Coomassie brilliant
blue. The positive-control group consisted of chickens receiving
two doses of infected Sf9 cells (
9). Feed and water were provided
ad libitum. Blood samples were collected weekly from a wing
vein, allowed to clot, incubated at 4°C, and centrifuged.
Twenty-eight days after the first vaccination, animals were
sacrificed and intestine and ceca were dissected. The organs
were immersed in PBS, cut in small pieces, and washed intensively.
Tissue samples were incubated at 41°C and 5% CO
2 in RPMI
1640 medium (Gibco, Carlsbad, CA) and 10% fetal calf serum.
Two days later, supernatants were collected and stored at –20°C
until used. An indirect enzyme-linked immunosorbent assay (ELISA)
was used to measure chicken immunoglobulin G (IgG) against NDV.
Briefly, 96-well plates (Maxisorp Nunc) were coated with purified
La Sota NDV (
1) in 0.1 M bicarbonate buffer, pH 9.6, overnight
at 4°C. After blocking with 4% skim milk in PBS-T-ENS (0.05%
Tween 20, 0.5% equine normal serum), the plates were subsequently
incubated with samples and goat anti-chicken IgG antibodies
coupled to horseradish peroxidase (Bethyl Laboratories, Inc.)
and ABTS [2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonic acid)
diammonium salt]-H
2O
2 in citric acid buffer, pH 5. The analysis
showed that HN recombinant immunogen was able to elicit an antibody
response detectable from day 14 postinoculation, exhibiting
significant differences (
P < 0.05) from negative-control
groups on days 21 and 28 postvaccination (Fig.
2). The kinetics
of antibodies induced by HN antigen from insect larvae was essentially
similar to that of antibodies induced by the antigen from infected
cells (Fig.
2). To evaluate neutralizing activity, pools of
serum samples were inactivated for 30 min at 56°C and serially
fourfold diluted in 199 medium (Gibco, Carlsbad, CA). Dilutions
were incubated in the presence of 100 PFU of NDV strain Beaudette
(provided by SENASA) for 1 h at room temperature and 30 min
at 4°C. Then, each mix was added to chicken embryo fibroblast
primary cell culture. Cells were incubated for 1 h at 37°C
before being overlaid with minimum essential medium (Gibco,
Carlsbad, CA) containing 2% fetal bovine serum, 25 mM HEPES,
10% antibiotic-antimycotic, and 0.8% low-gelling-temperature
agarose. The plates were incubated for 4 days at 37°C with
5% CO
2 before being fixed with 10% Formol and stained with crystal
violet. To evaluate hemagglutination inhibition (HI) activity,
a conventional microtiter method was used (
11). The immunization
with infected larva extracts elicited the production of neutralizing
antibodies that could be detected from day 21 postvaccination,
showing the ability of this immunogen to elicit a solid neutralizing
immune response (Table
1) . Viral neutralization assay results
were, overall, in concordance with the ELISA results. Similar
results were obtained when the HI assay was performed, but in
this case, activity was detected from day 14 postvaccination
(Table
1). When samples from day 28 were individually analyzed,
we observed that there were statistical differences between
R. nu and Sf9 groups infected with AcNPV/HN, the cell culture
antigen being the one which gave better neutralizing titers.
However, both immunogens induced a solid neutralizing immune
response in chickens (Table
2), and the titers obtained were
significantly higher than the ones obtained for the negative-control
groups. Previous reports showed a high correlation between the
presence of neutralizing antibodies to NDV and protective immunity
and demonstrated that the presence of specific antibodies determined
by the viral neutralization test was imperative for protection
(
23). Also, an association between neutralizing antibody responses
to HN protein and protection against NDV was reported with recombinant
vaccinia virus (
18). Moreover, Kamiya et al., (
9) reported such
a correlation using recombinant baculoviruses expressing HN
and F proteins. For these reasons, we assumed that the solid
neutralizing response induced by the recombinant immunogen described
in this report constitutes an encouraging sign of the protection
that could be conferred by it.
View this table:
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TABLE 2. Viral neutralization activity individually evaluated in serum samples collected at 28 days postinoculationa
|
In order to evaluate the local production of specific IgG in
guts and ceca, supernatants of tissue fragment cultures were
analyzed by ELISA. No antibody response was observed in samples
of culture supernatants taken at time zero (data not shown),
allowing a consistent analysis of the in situ antibody production.
A solid anti-NDV specific IgG antibody response (
P < 0.05)
was detected in the supernatants of cultured intestine and cecum
fragments of chickens immunized with both cell extracts and
larval extracts expressing the HN glycoprotein (Fig.
3), demonstrating
local intestinal mucosal immune responses. These results resemble
the ones obtained when conventional vaccines are used (
4). These
antibodies may have a role in the protection of mucosa, where
NDV begins its infection cycle.
This study shows the efficacy of infected larval extracts as
an NDV recombinant immunogen and shows that the extracts constitute
an easier and less expensive approach for the production of
recombinant antigens, since production in
R. nu larvae was nearly
400 times more economical than the expression of the same protein
in Sf9 cells (
22). In our hands, a single infected larva yields
quantities of recombinant HN protein similar to those yielded
by 1.5
x 10
6 infected Sf9 cells (data not shown). The larval
expression system may be a very useful tool in the veterinary
industry for the inexpensive production of effective recombinant
immunogens.

ACKNOWLEDGMENTS
This work was supported by grant PICTO 08-12919 from the National
Agency of Scientific and Technological Promotion, Argentina,
and grant PIP 5522 from Consejo Nacional de Investigaciones
Científicas y Tecnológicas (CONICET), Argentina.
We are grateful to María Gabriela López for her technical assistance in the maintenance of R. nu larvae, to Osvaldo Zabal and Teresa Morán for their technical assistance during tissue culture experiments, and to Silvio Díaz for the animal care.

FOOTNOTES
* Corresponding author. Mailing address: Instituto de Biotecnología, CICVyA, INTA, Castelar, Cc 25 B1712WAA, Buenos Aires, Argentina. Phone: 54-11-4621-1278. Fax: 54-11-4621-0199. E-mail:
aberinstein{at}cnia.inta.gov.ar 
Published ahead of print on 18 March 2009. 

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Clinical and Vaccine Immunology, May 2009, p. 775-778, Vol. 16, No. 5
1071-412X/09/$08.00+0 doi:10.1128/CVI.00432-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.