Virus, serum, plasmid, and cells
A total of 13 strains of IAV (Table 4) isolated from live poultry markets, including nine different subtypes of IAV (H1, H3, H4, H5, H6, H8, H7, H9, and H10), were used to prepare sera. All viruses were propagated in specific pathogen free (SPF) embryonated chicken eggs. Two kinds of sera against the H5 subtype of vaccine strains (2.3.4.4d and 2.3.2.1d) were purchased from YEBIO Company (China). The H5N1 avian influenza virus origin rescue plasmids (pHW-PB2, pHW-PB1, pHW-PA, pHW-HALo, pHW-NP, pHW-NA, pHW-M, and pHW-NS) were constructed by Shi et al [42]. Primary chicken embryo fibroblasts (CEF) were prepared from 9–10 d SPF chicken embryos and cultured in M199 (HyClone Laboratories, USA) containing 4% fetal bovine serum (FBS, Shuangru Biotech, China).
Serum preparation
Forty-five 21-day-old SPF chickens (Beijing Meria Vitong experimental animal technology co., Ltd, China) were housed in cages under biosafety conditions with ad libitum access to food and water, three chickens in each group were immunized with inactivated IAVs (106 EID50) with oil emulsion adjuvant or BSA-conjugated peptides (GL Biochem Ltd, China) with Freund's adjuvant (Sigma, USA) and boosted at a two–week interval. Chickens were inoculated with PBS as control group. Chicken were euthanatized by manual cervical dislocation at two weeks after the second vaccination and their sera were collected and identified by hemagglutination inhibition assay for whole virus immunized antisera (Table 4), and peptide chip for peptide immunized antisera.
Microarray experiment
According to the deduced amino acid sequence of the HA2 protein of the H5 subtype virus A/Mallard/Huadong/S/2005 (S, GenBank accession numbers: EU195389-EU195396), overlapping peptides (10 amino acids overlapped between two adjacent peptides) were synthesized by GL Biochem Ltd (China), except for the failure of the 11th peptide. To confirm key amino acids for the serum binding activity, the selected 14th peptide from the HA2 protein of the H5 subtype virus (KELGNGCFEFYHKCDNECME) was further cut into four overlapping peptides. To verify whether the selected peptide from the other subtype virus had similar serum binding activity, the 14th peptide from the HA2 protein of the H7 subtype virus (A/Chicken/Huadong/JD/17) was synthesized (Table 5). Synthetic peptides were sampled onto iPDMS (modified silica gel film) and workflow of the microarray was mainly based on the previous study [43]. Sera were diluted 1:50 (v/v) with serum dilution buffer (GuardianTM Peroxidase Conjugate Stabilizer/Diluent, Thermo Fisher Scientific, USA) and a 200 μL dilution was added in each well of the chip. The chip was incubated on a shaker for 30 min (500 r/min, 37 °C) and subsequently washed three times with TBST (20 mM Tris-base, pH 6.8, 137 mM NaCl, 0.1% Tween 20). Following incubation with 100 μL of 1:10000 diluted HRP (horseradish peroxidase)-labeled goat anti-chicken IgY for an additional 30 min and washing three times with TBST, 15 μL chemiluminescent substrate (SuperSignal West Pico PLUS Chemiluminescent Substrate, Thermo Fisher, USA) was added to each well of the chip. Chemiluminescent signals were captured by a CCD (charge coupled device) camera (LAS4000 imaging system, GE Healthcare Life Sciences, USA) and saved as an image in TIFF format. Thereafter, chemiluminescence intensity of each peptide point and background was converted into the signal-to-noise ratio (SNR,SNR = (signal strength–background intensity)/background intensity) using GenePix Pro 6.0 software. SNR ≥2 were identified as seropositive [44].
Site-direct mutagenesis and virus rescue
Site-direct mutagenesis of selected amino acid residues on the S strain HA2 protein was performed by the Mut Express II Fast Mutagenesis Kit V2 (Vazyme Biotech, China). Modified HA genes were inserted into the pHW2000 vector [45] and confirmed by sequencing (BGI Company, China). Recombinant viruses were rescued via an 8-plasmid reverse genetics technology as described previously [46]. HEC293T and M90 cells were plated at a ratio of approximately 1.5:1 in six-well plates and cultured in Dulbecco's Modified Eagle Medium (DMEM) medium (HyClone Laboratories, USA) containing 10% FBS. The modified HA plasmid combined with seven S strain rescue plasmids were transfected using the PolyJetTM transfection reagent (SignaGen Laboratories, USA). At 48–72 h post-transfection, the cells and supernatant were collected and inoculated into chicken embryo allantoic cavity (7-day-old SPF chicken embryo, 0.25 μL each) to propagate recombinant viruses. The Median Tissue Culture Infectious Dose (TCID50) of rescued viruses in CEF cells were determined according to Wagner (2000) and calculated by the Reed–Muench method [47]. To determine growth curve, CEF were infected with each virus at an MOI of 0.01 in M199 for 1 h. The infected cells were washed with PBS and then serum-free M199 was added. Cells were incubated at 37 ˚C under 5% CO2. The virus titers in the supernatant were monitored periodically by determination of TCID50 in CEF cells.
Western-blot analysis
Chicken embryo fibroblasts was inoculated with viruses (MOI = 0.01) and incubated for 1 h at 37 °C. Cells were washed twice with phosphate buffered saline (PBS, pH 7.2). Thereafter, M199 medium containing 1% FBS was added and incubated for 12 h. Cells were washed once with pre-cooled PBS (4 °C) and lysed with 200 μL of RIPA Lysis Buffer (strong) (CWBIO, Beijing, China) individually on ice for 15–20 min. Supernatants were collected by centrifugation at 12000 r/min for 10 min at 4 °C and mixed with protein loading buffer (Beyotime Biotechnology, China). Following boiling at 100 °C for 6–8 min, samples were subjected to 12% SDS-PAGE, and transferred to a PVDF membrane. The membrane which was first blocked in TBST containing 5% non-fat powdered milk at 25 °C for 1 h was incubated with the primary antibody against the 14th peptide (diluted to 1:1000 with TBST), and then incubated with the secondary antibody (Goat Anti-Chicken IgY (H + L) HRP, Abcam, USA, diluted 1:5000 with TBST). Meanwhile, protein bands of β-actin were incubated successively with the primary antibody (Anti-β-actin monoclonal antibody, Sigma Company, USA) and secondary antibody (HRP labelled anti-mouse IgG goat polyclonal antibody, Abcam, USA). Protein bands were developed using a chemiluminescence imaging analysis system (Tanon 5200, Tanon Biotech, China).
Bioinformatics analysis
HA gene sequences of IAVs available from the GISAID (https://platform.gisaid.org/epi3/frontend) and GenBank influenza database (https://www.ncbi.nlm.nih.gov/genomes/FLU/Database/nph-select.cgi#mainform) were aligned by MEGA 7.0 and then analyzed by WebLogo3 (http://weblogo.threeplusone.com/). HA molecule was analyzed by using Protein Data Bank (PDB), SWISS-MODEL system and the PyMOL System (https://pymol.org/2/). Taking PDB file (PDB ID 4JUK, 6NTF, 6PCX) as a template, the amino acid sequence of target virus was modeled by Alignment Mode on SWISS-MODEL. The PDB file of HA protein of target virus was further modified with PyMOL. The epitope was also analyzed via the Immune Epitope Database (IEDB).