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. 2017 Feb 12;7(4):912-925.
doi: 10.7150/thno.16615. eCollection 2017.

Visualization of a neurotropic flavivirus infection in mouse reveals unique viscerotropism controlled by host type I interferon signaling

Affiliations

Visualization of a neurotropic flavivirus infection in mouse reveals unique viscerotropism controlled by host type I interferon signaling

Xiao-Feng Li et al. Theranostics. .

Abstract

Flavivirus includes a large group of human pathogens with medical importance. Especially, neurotropic flaviviruses capable of invading central and peripheral nervous system, e.g. Japanese encephalitis virus (JEV) and Zika virus (ZIKV), are highly pathogenic to human and constitute major global health problems. However, the dynamic dissemination and pathogenesis of neurotropic flavivirus infections remain largely unknown. Here, using JEV as a model, we rationally designed and constructed a recombinant reporter virus that stably expressed Renilla luciferase (Rluc). The resulting JEV reporter virus (named Rluc-JEV) and parental JEV exhibited similar replication and infection characteristics, and the magnitude of Rluc activity correlated well with progeny viral production in vitro and in vivo. By using in vivo bioluminescence imaging (BLI) technology, we dissected the replication and dissemination dynamics of JEV infection in mice upon different inoculation routes. Interestingly, besides replicating in mouse brain, Rluc-JEV predominantly invaded the abdominal organs in mice with typical viscerotropism. Further tests in mice deficient in type I interferon (IFN) receptors demonstrated robust and prolonged viral replication in the intestine, spleen, liver, kidney and other abdominal organs. Combined with histopathological and immunohistochemical results, the host type I IFN signaling was evidenced as the major barrier to the viscerotropism and pathogenicity of this neurotropic flavivirus. Additionally, the Rluc-JEV platform was readily adapted for efficacy assay of known antiviral compounds and a live JE vaccine. Collectively, our study revealed abdominal organs as important targets of JEV infection in mice and profiled the unique viscerotropism trait controlled by the host type I IFN signaling. This in vivo visualization technology described here provides a powerful tool for testing antiviral agents and vaccine candidates for flaviviral infection.

Keywords: Bioluminescence imaging; Flavivirus; Interferon signaling.; Japanese encephalitis virus; Mouse.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interest exists.

Figures

Figure 1
Figure 1
Construction and characterization of Rluc-JEV. (A) Strategy for the construction of the infectious cDNA clone of Rluc-JEV. The Rluc gene preceded by N-terminal 34 amino acids of capsid protein (C34) of JEV is inserted at 5'UTR-C junction of JEV genome. Rluc gene fused with the FDMV-2A sequence (designated as “Rluc-2A”) is shown in red box. (B) IFA analysis of viral protein expression in BHK-21 cells transfected with in vitro transcribed genome-length RNAs of the parental JEV and Rluc-JEV. (C) Plaque morphology of JEV and Rluc-JEV in BHK-21 cells. (D) Growth curve of Rluc-JEV and the parental JEV in BHK-21 and C6/36 cells. Cells were infected with viruses at an MOI of 1, and viral titer in the culture supernatant was determined by plaque assay on BHK-21 cells. (E) Luciferase activity of Rluc-JEV in BHK-21 cells and correlation of viral titer to Rluc activity. BHK-21 cells were infected Rluc-JEV at an MOI of 0.01. Viral titers in the culture supernatant and luciferase activity in the cells at indicated time points were determined by plaque assay and luciferase assay, respectively. (F) Luciferase signals derived from virus-infected cells at MOI of 0.01, 0.1 or 1.
Figure 2
Figure 2
Genetic stability of Rluc-JEV in BHK-21 cells. (A) The Rluc-JEV virus (P0) was blindly passaged on BHK-21 cells for five rounds (P1-P5). Luciferase activity of each passaged viruses was detected as described in Materials and methods. Error bars represent the standard deviation of triplicate measurement. *, P<0.05; ***, P<0.001; ****, P<0.0001. (B) Detection of the Rluc gene during blind passage. Viral RNA was extracted from the supernatants of BHK-21 cells infected with each passaged virus, and RT-PCR assay was performed to amplify the region from 5'UTR to prM which includes the Rluc gene. (C) Individual Rluc-JEV viruses were isolated by six rounds of plaque purification, designated as P6 viruses. P6 viruses were then serially passaged on BHK-21 cells for another three rounds (P7-P9). Luciferase activity in the cells infected with P7-P9 viruses was detected. Error bars represent the standard deviation of triplicate measurement. (D) Detection of the Rluc gene of P7-P9 viruses by RT-PCR using the same primers in (B). The expected band, corresponding to 1.6 kb, indicates the presence of the Rluc gene in the passaged plaque-purified reporter viruses.
Figure 3
Figure 3
Noninvasive imaging to monitor JEV replication in mouse. (A) BLI of JEV infection in BALB/c mice. Groups of 3 to 4-week-old BALB/c mice were inoculated i.p. with 107 PFU of Rluc-JEV. Imaging was performed for 90 s at the indicated times. Representative images from two mice are shown. (B) Photon flux was quantified from ROI analysis of the dorsal and ventral side. The data are representative of at least three independent experiments, and error bars indicate the standard deviation.
Figure 4
Figure 4
Correlation between fluorescence signal and viral replication. (A) IFNAR-/- mice were infected i.c. with 102.5 PFU of Rluc-JEV and subjected to imaging at 24 and 72 h.p.i.. BLI was performed for 90 s, and fluorescence signals were quantified using Living Image software 3.0. (B) Viral RNA and viral titers in excised brains were quantified by real time RT-PCR and plaque assay, respectively. The photon flux determined as described in the legend to Fig. 3 was plotted against the RNA copies or viral titer (PFU/g).
Figure 5
Figure 5
Type I IFN plays a critical role in control of viral dissemination in mice. (A) BLI of Rluc-JEV-infected WT or IFNAR-/- mice. The mice were inoculated via the footpad with 106 PFU of Rluc-JEV. Imaging was performed for 90 s at the indicated times. Representative images from three or two mice are shown. (B) The photon flux was quantified from ROI analysis of the dorsal and ventral side. The data are representative of three independent experiments, and error bars indicate the standard deviation. *, P<0.05; ****, P<0.0001. (C) WT or IFNAR-/- mice were inoculated via the footpad with 106 PFU of Rluc-JEV. At 72 h.p.i., the infected mice was subjected to BLI. Immediately after being imaged, the animals were sacrificed and parenchymal organs including brain(B), liver (L1), kidney (K), spleen (S), intestine (I), heat (H) and lung(L2) were isolated for in vitro BLI.
Figure 6
Figure 6
Susceptibility of WT or IFNAR-/- mice to parental JEV and Rluc-JEV. (A) Survival curve of mice infected with viruses. WT or IFNAR-/- mice (n=6 or 7) were injected via the footpad with of 107 PFU of Rluc-JEV or the parental JEV, and monitored daily for 15 days to assess morbidity and mortality. Rluc-JEV-WT, WT mice infected with Rluc-JEV; JEV-WT, WT mice infected with parental JEV; Rluc-JEV-IFNAR-/-, IFNAR-/- mice infected with Rluc-JEV; JEV-IFNAR-/-, IFNAR-/- mice infected with parental JEV. The data are representative of at least three independent experiments. (B) Viral load in tissues of WT mice infected with viruses. Groups of WT mice (n=2) were injected via the footpad with of 106 PFU of Rluc-JEV or the parental JEV. At 72 h.p.i., brains and intestines were collected, homogenized, and subjected to virus titration by qRT-PCR. The dotted line indicates the limit of detection. The data are representative of at least three independent experiments.
Figure 7
Figure 7
Histology and immunohistochemistry of tissue sections from mice infected with Rluc-JEV. WT or IFNAR-/- mice were inoculated via the footpad with 107 PFU of Rluc-JEV. At 5 days p.i., parenchymal organs including brain, liver, kidney, spleen and intestine were collected for histology (A) and immunohistochemistry (B).
Figure 8
Figure 8
Application of Rluc-JEV for antiviral and vaccine efficacy evaluation. (A) Antiviral assay using Ruc-JEV with known antiviral agents. BHK-21 cells were infected with Rluc-JEV at an MOI of 0.01 and incubated with various concentrations of Ribavirin or NITD008. The Rluc activity was measured by Multimode Microplate Reader at 48 h.p.i.. One representative experiment of three is shown. (B) Graphic depiction of assessment of JE vaccine efficacy by using BLI. (C) and (D) BLI of the immunized BALB/c mice or IFNAR-/- mice challenged with Rluc-JEV. Group of BALB/c (C) or IFNAR-/- mice (D) were immunized s.c. with 105 PFU of live JE vaccine SA14-14-2. On 21 days p.i., the immunized mice were challenged with 107 PFU of Rluc-JEV by the i.p. route. BLI of the challenged mice was performed at indicated time points. (E) Survival curve of the immunized IFNAR-/- mice challenged with Rluc-JEV described in (D).

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