Construction of recombinant BmNPVs
In this study, we used the coding sequences for CprME and the prME polypeptide (GenBank: KU050695, Genewiz, New Jersey, USA). The anchor region of the capsid coding sequences for CprME was deleted for increased expression level and avoiding halt viral particle formation (Nasar et al. 2020). A linker sequence (GGGGSGGGGS) and PA-tag sequence (EGGVAMPGAEDDVV) were fused in the C-terminus and amplified by polymerase chain reaction (PCR) using a template (the synthetic gene described above). A set of primers (3CprME-F and 3CprME-R-EcoRI, Table 1) was used as a template for the DENV-3CprME coding sequence. The DENV-3prME primer set (3prME-F and 3prME-R-EcoRI, Table 1) was used to isolate the DENV-3prME coding sequence. The PCR protocol was as follows: initial denaturation at 98°C for 10 s; 35 cycles of 98°C for 10 s, 55°C for 5 s, and 72°C for 20 s; 72°C for 3 min for the final extension. A thermal cycler (TaKaRa, Kyoto, Japan) was used to carry out the PCR reaction. Each construct was ligated into pFastbac1 (Thermo Fisher Scientific K. K., Tokyo, Japan), and the resulting vector was introduced into Escherichia coli BmDH10bac CP- Chi- (Motohashi et al. 2005). The recombinant products, which included the BmNPV/3CprME and BmNPV/3prME bacmids, were extracted from white colonies, respectively. Each recombinant BmNPV bacmid was mixed with chitosan (Sigma–Aldrich, Tokyo, Japan) and injected into fifth instar silkworm larvae (Ehime Sansyu, Ehime, Japan). The hemolymph was collected from the larvae at 6–7 d post-injection (dpi) and mixed with a 1 mM solution of 1-phenyl-2-thiourea (Motohashi et al. 2005). The aliquots of hemolymph were kept at −80°C before use.
Table 1 Used primers
Name
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5′–3′
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3CprME-F
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TAA TGG ATC CAT GAA TAA CCA GCG CAA GAA
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3CprME-R-EcoRI
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TAA TGA ATT CTC AGA CTA CGT CGT CTT CCG C
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3prME-F
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TAA TGG ATC CAT GTT TCA TCT CAC TTC CCG TGA TGG C
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3prME-R-EcoRI
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TAA TGA ATT CTC AGA CTA CGT CGT CTT CCG CAC
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pFastBac1-F
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TAT TCC GGA TTA TTC ATA CC
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pFastBac1-R
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ACA AAT GTG GTA TGG CTG ATT
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pUC/M13-F
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CCC AGT CAC GAC GTT GTA AAA CG
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pUC/M13-R
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AGC GGA TAA CAA TTT CAC ACA GG
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Underlines indicate restriction enzyme cleavage sites.
Expression and purification of 3CprME and 3prME polypeptides in silkworm larvae
Fifth instar silkworm larvae (Ehime Sansyu) were injected with hemolymph that was diluted 100-fold in phosphate-buffered saline (PBS, 137 mM NaCl, 2.7 mM KCl, 8 mM Na2HPO4, and 2 mM KH2PO4, pH 7.4), and raised on an artificial diet (Silkmate S2, Nosan Co., Yokohama, Japan). The legs of the larvae were cut to collect the hemolymph. The fat bodies were collected by dissecting the larvae; 1 ml of Tris-buffered saline containing 0.1% Triton X-100 (TBST) was added to every 0.1 g of fat body and sonicated for a total of 5 min at 20-s intervals, followed by a 10-s break (Vibra Cell VC 130PB, Sonics & Materials Inc., Newtown, USA). After sonication, the fat body suspension was centrifuged (Kubota 3700, Tokyo, Japan) for 10 min at 12,000 × g, 4°C. The soluble fraction of the silkworm fat body suspension was mixed with 200-µL beads tagged with anti-PA antibody (FUJIFILM Wako Pure Chemical, Osaka, Japan) at 4°C for 2 h. The mixed beads were collected and washed five times with four times of bead volumes of TBS buffer (20 mM Tris-HCl and 150 mM NaCl). The elution was performed with a 0.1 M glycine-HCl solution (pH 3.0), and five fractions were collected to recover the PA-tagged target proteins. Amicon Ultra centrifugal filters (Merck Japan, Tokyo, Japan) were used to concentrate the eluation by ultrafiltration. The concentrations of the eluate were measured using a BCA protein assay kit (Thermo Fisher Scientific K. K.).
The 3CprME and 3prME constructs were also expressed in Bm5 cells and silkworm pupae (Ehime Sansyu). Bm5 cells were provided by Prof. K. S. Boo (Insect Pathology Laboratory, School of Agricultural Biotechnology, Seoul National University, Seoul, South Korea). Sf-9 and Bm5 cells were maintained at 27°C in Sf-900II serum-free medium (Thermo Fisher Scientific K.K.) supplemented with 1% fetal bovine serum (Thermo Fisher Scientific K.K.) and Antibiotic-Antimycotic solution (Thermo Fisher Scientific K.K.).
Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and western blot
SDS-PAGE using 10% or 12% gels was used to separate the proteins. Western blotting was subsequently performed by blotting the separated proteins onto an Immobilon-P polyvinylidene fluoride membrane (Merck Japan) using the Mini Trans-Blot Electrophoretic Transfer Cell (Bio-Rad, Hercules, CA, USA). After blotting, the membrane was blocked in 5% nonfat milk (FUJIFILM Wako Pure Chemical) in TBST (pH 7.6) and incubated in a rat anti-PA tag antibody (1:10,000; FUJIFILM Wako Pure Chemical). Other primary antibodies used included the anti-DENV E antibody (1:3000; GeneTex, Irvine, CA, USA) or the mouse anti-DENV prM antibody (1:3000; GeneTex). After incubating membranes with the primary antibodies and washing three times with TBST, the membranes were incubated for 1 h in horseradish peroxidase (HRP)-conjugated anti-rat IgG antibody (1:10,0000; FUJIFILM Wako Pure Chemical). Immobilon Western Chemiluminescent HRP substrate (Merck Japan) was used for the detection of protein bands. Membranes were imaged using a Fluor-S MAX Multi-Imager (Bio-Rad).
Transmission electron microscopy (TEM) and immunoelectron microscopy (IEM)
TEM and IEM were carried out as previously described (Utomo et al. 2019) with minor modifications. The purified antigen sample was added to the Cu-Grid transmission electron microscope (Nisshin EM Co., Ltd., Tokyo) and incubated for 30 s at room temperature, washed with 30 μL of PBS, and incubated for 30 s. This procedure was repeated three times. For IEM, 30 μL of 2% v/v bovine serum albumin (BSA) was used for blocking after the distilled antigen sample was added, and the sample was subsequently washed three times with PBS. The Cu-Grid was washed sequentially. Samples were incubated with Dengue virus anti-envelope rabbit polyclonal antibody (1:30; FUJIFILM Wako Pure Chemical) and goat anti-rabbit IgG conjugated to gold nanoparticles (1:50; FUJIFILM Wako Pure Chemical) for the first and secondary antibodies, respectively. The Cu-Grid was treated with 2% phosphotungstic acid, and the samples were analyzed using the TEM apparatus.
Heparin-binding assay
The heparin-binding assay was carried out as previously described (Utomo et al. 2019), with minor modifications. Biotin-labeled heparin (6 ng/ml; Sigma–Aldrich Japan) and heparin (1.8 ng) were immobilized onto avidin-coated microplate wells (blocking-less type) (Sumitomo Bakelite, Tokyo, Japan) washed three times with PBS. We used 2 µg BSA for negative control. Purified proteins (0.5, 1, 5, and 10 μg/ml) were added into wells, incubated at room temperature for 1 h, and subsequently washed with phosphate-buffered saline containing 0.1% Tween 20 (PBST). After serial washing, the rat anti-PA tag antibody (1:1000; FUJIFILM Wako Pure Chemical) and HRP-conjugated anti-rat IgG antibody (1:1000; FUJIFILM Wako Pure Chemical) were used as the primary and secondary antibodies, respectively. For detection, 100 µL of substrate (0.1 mg/ml 3.3′,5.5′-tetramethylbenzidine [TMB] in 100 mM sodium acetate [CH3COONa], pH 6.0) were added to each well with 0.2% (v/v) of 30% hydrogen peroxide. We added 50 μL of 1 N H2SO4 to each well to stop the reaction. Absorbance was measured at 450 nm.
VLP antigenicity by enzyme-linked immunosorbent assay (ELISA)
Direct ELISA was used to detect an interaction between antigens, 3CprME, 3prME, the mock silkworm fat body (negative control), and sera. Two types of sera were used: mouse sera immunized with DENV tetravalent DNA vaccine (mice-Ab) (Putri D.H., personal communication, June 2017), and sera from dengue patients (human-Ab) [NS1(+), RT-PCR (+)]. The dengue patient sera were collected during a dengue community study that occurred from March 2010 until December 2011. Ethical approval was given by the Research Ethical Committee of the Faculty of Medicine, Universitas Indonesia, No. 71/PT02.FK/ETIK/2009. RT-PCR positively confirmed the human-Ab from dengue patient sera based on the Lanciotti method (Lanciotti et al. 1992). Both of the sera originated from stocks from the Department of Microbiology, Faculty of Medicine, Universitas Indonesia.
For each diluted sample, 100 mL of sample (20 ng/ml) in coating buffer (0.05 M Carbonate-bicarbonate, pH 9.6), was applied to a 96-well ELISA microplate, followed by incubation at 4°C overnight. After incubation, the coating solution was discarded, and a 100 mL blocking solution (5% skim milk in PBS) was added into each well and incubated for 1 h at 37°C. The plates were then washed serially with PBST, followed by the addition of 100 mL of mouse-Ab or human-Ab in PBS (1:5000). Plates were then incubated at 37°C for 1 h and washed three times with wash buffer. Next, 100 mL of goat anti-mouse or anti-human IgG-HRP-conjugated antibody (1:5000) was added to each well. Plates were incubated at 37°C for 1 h and washed sequentially. TMB substrate (50 mL) was applied and incubated for 10 min. We added 50 mL 1 M H2SO4 to stop the reaction. The absorbance was read at 450 nm.
Immunization of mice
A total of 12 BALB/c mice, (4–6 weeks old) were divided into four groups: i) negative control (immunized with PBS), ii) immunized with 3CprME, iii) immunized with 3prME, and iv) immunized with Alhydrogel as an adjuvant. All mice were housed in a temperature-controlled, light-cycled room. Each mouse was immunized three times via intraperitoneal injection with 50 mg purified 3CprME and 3prME proteins with Alhydrogel adjuvant within a two-week interval. Blood samples were collected via the tail vein after 0, 16, and 30 d. Sera were isolated and stored at −80°C until further analysis. All animal procedures were conducted in compliance with the established guidelines from the Animal Laboratory of Center of Pharmaceutical and Medical Technology, Agency for Technology Assessment and Application (BPPT), Indonesia. Animal experimental protocols were reviewed and approved by the Research Ethical Committee for the Faculty of Medicine, Universitas Indonesia, No. KET-476/UN2.F1/ETIK/PPM.00.02/2019.