Plasmid constructions and bacterial strains
The peptide (VP4N20) that corresponds to first 20 residues at the N-terminal of VP4 of EV71 (Bj08) was inserted to HBcAg (HBc-N149) loop region between amino acids 78 and 79. The fusion protein was named as HBc-N149-VP4N20. To construct the plasmid expressing the fusion protein, DNA fragment encoding HBc-N149-VP4N20 was synthesized and amplified using primers P1u (5'- CCGCTCGAGCACCACGGTGGTT-3') and P1d (5'- GGAATTCCATATGGATATTGATCCGTATAAAG-3'). The PCR products were double-digested by XhoI and NdeI and subsequently inserted into the vector pET22b(+) (Novagen, USA). DNA fragment encoding HBc-N149 was amplified by using the primers P1u, P2d (5′-TGGGCAGCAATCTGGAAGATCCGGCGAGCCGCGAACTG-3′), P2u (5′- ACCAGTTCGCGGCTCGCCGGATCTTCCAGATTGCTGCCCA-3′) and P1d by using HBc-N149-VP4N20-encoding gene as a template and further inserted into the vector pET22b (+). The accuracy of the constructs was confirmed by sequencing. E. coli strain BL21 (DE3) (BeiJing TIANGEN BIOTECH, China) were used for protein expression.
Expression and purification of recombinant proteins
Overnight cultures of BL21 (DE3) cells harboring the recombinant plasmids were diluted 1:400 in 1 L of LB broth containing 100 μg/ml ampicillin, and grown until reaching an OD600 of 0.4-0.6. Protein expression was then induced by 0.1 mM of isopropyl-β-d-thiogalactopyranoside (IPTG). After shaking at 37°C for 5 h, the bacteria were collected by centrifugation at 12,000 rpm for 10 min at 4°C, and the pellets were resuspended in 100 ml of balance buffer (pH 8.0, 50 mM Tris, 100 mM NaCl, 10 mM imidazole). For protein purification, the bacterial cells were lysed by ultrasonication, followed by centrifugation at 13,000 rpm for 15 min at 4°C to remove bacterial debris. The clear supernatant was applied to a Ni Sepharose column (GE Healthcare Life Sciences, USA) according to the manufacturer’s recommendations. The columns were washed with washing buffer (pH 8.0, 50 mM Tris–HCl, 100 mM NaCl, 50 mM imidazole,) and bound proteins were eluted with elution buffer (pH 8.0, 50 mM Tris–HCl, 100 mM NaCl, 200 mM imidazole). The peak fractions were collected and analyzed by SDS-PAGE. The purity of the samples was determined by densitometric scanning. The proteins were dialyzed to PBS buffer (pH7.4) for 8 hours at RT and examined by electron microscopy and used for immunology studies.
SDS-PAGE and Western blotting
Electrophoresis was performed in 12% SDS polyacrylamide gels and the recombinant proteins were detected by Western blotting using a monoclonal antibody (mAb) against the polyhistidine (His) tag in the C-terminal region of the fusion protein. Briefly, the transferred PVDF membrane was blocked with 2% (w/v) BSA in TBS for 1 h at 37°C, and washed thrice with TBS – 0.05% (v/v) Tween 20, then the membrane was incubated with a 1:5,000 dilution of anti-His tag (mouse mAb, CWBIO, Beijing, China) in a 0.2% BSA-TBS – 0.05% Tween 20 solution for 1 h at 37°C, and washed thrice with TBS – 0.05% Tween 20. Protein bands were probed with 1:2,000 dilution of HRP-conjugated goat anti-mouse IgG (CWBIO, Beijing, China) and washed thrice as described above. Chemiluminescence was applied as instructed by the manufacturer (Li-COR Odyssey, USA).
The formation of HBcAg VLPs and chimeric VLPs (HBc-N149-VP4N20) was analyzed by negative staining electron microscopy according a previously described method . Briefly, proteins were adsorbed to 230 mesh carbon-coated copper grids and incubated for 1 min. The grids were then washed once with PBS and stained for 45 s with 2% phosphotungstic acid. Specimens were evaluated using an electron microscope (H-7650, HITACHI, Japan).
Immunization of animals
Pathogen-free female BALB/c mice were purchased from Beijing HFK Bioscience Co. (Beijing, China). All animals were housed at pathogen-free conditions. Animal experiments were performed in accordance with current guidelines for the Care and Use of Laboratory Animals of Experimental Animal Center of Military Medical Sciences and approved by the center. For mice experiments, five female BALB/c mice (6–8 weeks) per group were vaccinated intramuscularly (i.m.) with recombinant proteins HBc-N149 (5 μg/mouse) or HBc-N149-VP4N20 (5 μg/mouse) at week 0. The second injection was performed at week 3. QuickAntibody™ from KBQ Biotechnology Co. (Beijing, China) was used as an adjuvant. Control group was immunized with PBS plus adjuvant. The immunized animals were bled at week 0, 2, 5, 8 for antibody detection.
Direct ELISA was used for detection of antibodies in the sera of immunized animals. The peptide VP4N20 was synthesized by Scilight-Peptide (Beijing, China) and conjugated with Bull Serum Albumin (BSA-VP4N20). The peptides were purified using high-pressure liquid chromatography. ELISA plates (96-well) were coated with 250 ng/well of BSA-VP4N20 in coating buffer (50 mM Na2CO3–NaHCO3, pH 9.6) overnight at 4°C. After washing with PBS-0.05% (v/v) Tween 20 thrice, the plates were blocked with 2% (w/v) BSA in PBS for 2 h at 37°C. Sera were tested at 2-fold serial dilutions starting at 1:100. The plates were incubated at 37°C for 1 h and washed thrice with PBS-0.05% Tween 20. HRP-conjugated goat anti-mouse IgG (CWBIO, Beijing, China) was then added into each well in a 1:2000 dilution, and incubated at 37°C for 1 h. The plates were washed thrice and developed with TMB solution (Tiangen Biotech, Beijing, China) in a dark room for 15 min, and the enzyme reaction was stopped by adding 2 M H2SO4. The absorbance at 450 nm was measured using a microplate reader (Bio-Rad, USA).
Enzyme-linked immunosorbent assay (ELISA) protocols were used for all the epitope mapping experiments. Peptides truncated at the carboxyl end or the amino terminus were purchased from Scilight-Peptide (Beijing, China). The peptides were conjugated with Bull Serum Albumin (BSA). The purity was higher than 90%.
In vitro neutralization assay
EV71 BJ08 (genogroup C4) and BrCr-TR (genogroup A), were propagated in RD cells. Virus titers were determined using RD cells by the microtitration method and expressed as the 50% tissue culture infective dose (TCID50) according to the Reed–Muench method. Two-fold serial dilutions of sera were prepared using Minimum Essential Medium (MEM,Gibco®) containing 2% FBS. The EV71 stock was diluted to a working concentration of 100 TCID50/50 μl. The neutralization assay was conducted using 96-well plates. In each well, 50 μl of diluted serum sample was mixed with 50 μl of EV71 at 100 TCID50, and incubated overnight at 37°C. Next, 100 μl of cell suspension containing 10,000 RD cells was added to wells containing the virus/antiserum mixtures and incubated at 37°C. After 7 days, the cells were observed to evaluate the appearance of cytopathic effects. Neutralization titer was defined as the highest serum dilution that could completely protect cells from developing cytopathic effects.
Mouse protection assay
To evaluate protective efficacy of the immunized sera against EV71 infection, in vivo infection experiments were performed. Briefly, 50 μl of sera or PBS were incubated with 10 LD50 of EV71 BrCr-TR (50 μl in sterile RD cell supernatant) at 37°C for 2 hour. Groups of one-day-old BALB/c suckling mice (n = 10 per group) were inoculated intraperitoneally (i.p.) with the virus-sera mixture or virus-PBS mixture. All mice were monitored daily for clinical symptoms and death for up to 16 days after inoculation.