The establishment of Ar. subalbatus laboratory strains
Armigeres subalbatus were collected from a campus (23°12′03.1′′N, 113°17′20.1′′E) located in Guangzhou City, Guangdong Province, China. Representative morphology and COI gene sequencing confirmed the identity of Ar. subalbatus (Additional file 1: Fig. S1, Additional file 2: Table S1) [20, 31]. All mosquitoes were maintained under standard insectary conditions of 27 ± 1 °C, 70–80% relative humidity, and a light: dark cycle of 16 h:8 h [17]. The larvae of Ar. subalbatus were reared in stainless steel trays (24 cm × 34 cm × 6 cm) and fed daily with turtle food (INCH-GOLD®). After pupation, the pupae were picked with a pipette into a 300-ml plastic cup containing 200 ml of dechlorinated water and then transferred into mosquito cages (25 cm × 25 cm × 35 cm), and adults of Ar. subalbatus were fed on 10% glucose water.
Cell line and virus
C6/36 cells were cultured in RPMI-1640 medium supplemented with 10% heat inactivated fetal bovine serum (FBS) and maintained at 28 ℃.
ZIKV (GenBank Acceptance No. KU820899.2) was provided by the Centers for Disease Control of Guangdong Province, China, which was isolated from a ZIKV patient in Zhejiang Province of China in 2016 and classified as the Asian lineage [32, 33]. The virus had been passaged once via intracranial inoculation of suckling C57 mice and twice in C6/36 cells. ZIKV were collected after 5–7 days of enrichment in C6/36 cells at 28 ℃ and stored at − 80 ℃.
DENV-2 (New Guinea C, GenBank: AF038403.1) was provided by the Key Laboratory of Tropical Disease Control of Sun Yat-sen University (Guangzhou, China), and had been passaged one generation in our lab. DENV-2 were collected after enrichment in C6/36 cells at 37 ℃ for 36–48 h until obvious cytopathic effects were observed and stored at − 80 ℃.
ZIKV and DENV-2 supernatant were harvested and the titers were ready to be quantified with plaque assay before being blood-fed to the mosquitoes.
Oral infection of mosquitoes
The ZIKV and DENV-2 supernatant with the viral titer of 106 pfu/ml were mixed with defibrinated sheep blood at a ratio of 2:1. Armigeres subalbatus had been passaged to the 12th generations in laboratory. Female adults aged 3–5 days post-emergence were anesthetized with CO2 and transferred into cages (60–70 females per pool). Female mosquitoes were starved for 20–24 h and allowed to feed on the blood meal maintained at 37 ℃ for 1 h with a Hemotek blood feeding system (Discovery Workshops, Lancashire, UK). Fully engorged females were selected and incubated at 28 ℃, 80% relative humidity, and a light: dark cycle of 16 h:8 h and fed on 10% glucose water for viral detection. The midgut, ovaries, and salivary glands were dissected to detect ZIKV at 4, 7, 10, 14 and 21 dpi or DENV-2 at 4, 7, 10 and 14 (24 females at each time point).
Horizontal transmission assays
After 10 days of the ZIKV-feeding blood meal, parts of the leg of females were used to detect ZIKV by RT-PCR. The ZIKV-positive females were selected and divided into four groups (10–11 females per group). These females continued to be reared for two days and then starved for 18–20 h. Each group was allowed to bite one 4-day-old suckling mice for 2 h. All the suckling mice were euthanized at 8 day post mosquitoes feeding and the brain of each individual mouse was dissected to detect the virus titer by RT-qPCR and plaque assay.
Urine infection
1 ml of ZIKV supernatant was mixed into 200 ml clean water containing 0.5 ml urine as artificial viral urine (single addition, final titers was 105 pfu/ml). The viral activity of ZIKV in artificial viral urine was detected by RT-qPCR at 0 h, 24 h and 48 h. Every 24 h until pupation, 1 ml of ZIKV and DENV-2 (final titers was 104 pfu/ml) supernatant was added into 200 ml clean water containing 0.5 ml urine, respectively as artificial virus urine (continuous addition).
The larvae of Ar. subalbatus (200 larvae per pool), in 33rd generation in laboratory were reared in the artificial viral urine at first, second and third instar, respectively. The larvae were reared to the fourth instar and then washed in clean water for 3 times (5 min per time) before ZIKV and DENV-2 detection. 8–12 days after the emergence of larvae reared in artificial virus urine (continuous addition, final titers was 105 pfu/ml), the midgut, ovaries and salivary glands of 426 females were dissected to detect ZIKV in adult stage.
The route of ZIKV infection
1 ml of ZIKV supernatant was mixed into 20 ml clean water containing 50 μl urine as artificial viral urine (single addition, final titers was 106 pfu/ml). Third instar larvae (160 larvae per pool) were reared in the artificial viral urine. 16 pools of the urine-infected larvae (10 larvae per pool) were collected at 1, 2, 3 and 4 dpi for ZIKV detection. Meanwhile, the midgut, anal papillae and carcass of 37 pools of larvae (10 larvae per pool) were dissected at 4 dpi for the route of ZIKV infection. The remaining of larvae were washed with clean water and allowed to remove part of anal papillae at 4 dpi to detected ZIKV by RT-PCR. ZIKV-positive larvae were fixed in 4% formaldehyde solution for Immunohistochemistry assay.
Saliva collection
ZIKV-positive female mosquitoes after 14 days of oral infection were anesthetized with CO2. The legs and wings of adults were removed with dissecting scissors, and the mosquito's proboscis was then inserted into a pipette tip containing 20 μl of FBS for 30 min to collect mosquito saliva. Afterward, the saliva was placed into a 1.5-ml RNase-free EP tubes containing 180 μl of Dulbecco's modified eagle medium (DMEM) and stored at −80 ℃.
ZIKV and DENV-2 detection and quantification
Total RNA of collected samples were extracted according to the manufacturer’s protocol with TRIzol reagent and dissolved in 20 µl of RNase-free water. cDNA synthesis was performed by using the GoScript Reverse Transcription System (Promega, Madison, WI, USA), reverse specific primer (ZIKV) [17] or random primer (DENV-2).
Reverse transcriptase-PCR (RT-PCR) was used to detect the viral genome in tissues of adults and larvae, following previous protocol [17]. Absolute quantitative real-time PCR (RT-qPCR) was used to quantify the viral copies of the positive sample following previous protocol [17, 18]. A standard curve for ZIKV detection was established by tenfold dilutions of the plasmid standard (5.73 × 103–5.73 × 107 copies/μl). A standard curve for DENV-2 detection was established by tenfold dilutions of the plasmid standard (1.83 × 103–1.83 × 107 copies/μl). RNA copies from each sample were quantified by comparing the cycle threshold value with the standard curve. These experiments were conducted according to standard procedures in a biosafety level 2 laboratory.
Plaque assay
BHK-21 cells were suspended in 10% FBS in DMEM and plated in 12-well plates. The cells were incubated in a cell culture incubator until 90% to 95% confluency was reached. The saliva of female mosquitoes and supernatants of the mouse brain were inoculated into the well individually with serial dilutions. After incubating cells and viruses in a cell incubator for 1 h, the cell culture medium was removed from the wells, and the cells were washed twice with phosphate-buffered saline (PBS). Monolayers of cells inoculated with the virus were covered with methylcellulose. Six days after incubation, the cells were fixed in a 4% paraformaldehyde solution for 1 h and then stained with a crystal violet solution for 30 min at room temperature. The wells were washed with tap water and dried. The number of plaques in the wells was easily observed and counted. The plaque forming units (pfu/ml) per ml were calculated as follows: pfu/ml = average number of plaques/[(dilution factor for wells) (volume of inoculum per plate)].
Immunohistochemistry assay
The entire body of ZIKV-positive larvae fixed in 4% formaldehyde solution was embedded in paraffin and serially cut for histologic examination. The slides were stained with rabbit monoclonal ZIKV NS1 protein antibody (Thermo Fisher) and anti-rabbit IgG secondary antibody (Abcam) [34], and the specimens were examined under a light microscope (Olympus, Japan). Larvae reared in artificial urine without ZIKV were used as the control.
Statistical analysis
All statistical analyses were performed with SPSS 20.0 (IBM, Chicago, IL, USA). Chi-square test was applied to compare the infection rate (IR) of the same tissue at different time points. The P value significance was corrected by Bonferroni adjustments. The RNA copy number of ZIKV was first logarithmically converted, and then One-way ANOVA was used to compare the amount of ZIKV in the same tissue at different time point. P < 0.05 was considered statistically significant.