Pseudovirus rVSVΔG-ZEBOV-GP Infects Neurons in Retina and CNS, Causing Apoptosis and Neurodegeneration in Neonatal Mice

Abstract

Zaire Ebola virus (ZEBOV) survivors experience visual and CNS sequelae that suggests the ZEBOV glycoprotein can mediate neurotropism. Replication-competent rVSVΔG-ZEBOV-GP vaccine candidate is generally well tolerated; however, its potential neurotropism requires careful study. Here, we show that a single inoculation of rVSVΔG-ZEBOV-GP virus in neonatal C57BL/6 mice results in transient viremia, neurological symptoms, high viral titers in eyes and brains, and death. rVSVΔG-ZEBOV-GP infects the inner layers of the retina, causing severe retinitis. In the cerebellum, rVSVΔG-ZEBOV-GP infects neurons in the granular and Purkinje layers, resulting in progressive foci of apoptosis and neurodegeneration. The susceptibility to infection is not due to impaired type I IFN responses, although MDA5^-/-, IFNβ^-/-, and IFNAR1^-/- mice have accelerated mortality. However, boosting interferon levels by co-administering poly(I:C) reduces viral titers in CNS and improves survival. Although these data should not be directly extrapolated to humans, they challenge the hypothesis that VSV-based vaccines are non-neurotropic.

Keywords: Ebola glycoprotein; Ebola vaccine; Ebola virus; VSV; VSV vaccine; ebolavirus; filovirus; innate immunity; neurotropism; neurovirulence; pseudotyped virus.

Figure 1.. Inoculation of rVSVΔG-ZEBOV-GP Causes Lethal Infection with Eye and Brain Involvement in Neonatal C57BL/6 Mice

(A) Adult IFNAR1^−/− (inverted blue triangles, n = 7), adult WT (green squares, n = 7), P7 WT (yellow triangles, n = 8), and WT P3 (red circles, n = 31) mice were challenged with 1,000 TCID₅₀ of rVSVΔG-ZEBOV-GP and monitored for survival. (B) P3 WT mice were challenged with saline (non-infected, green squares, n = 8), VSV (Indiana strain, black diamonds, n = 22), rVSVΔG-ZEBOV-GPΔMUCIN (yellow triangles, n = 13), or rVSVΔG-RESTON-GP (inverted blue triangles, n = 16) and monitored for survival and weight change (data shown as means ± SEMs). P3 rVSVΔG-ZEBOV-GP infection survival and weight change is shown for comparison (red circles). Note that P3 mice that received rVSVΔG-RESTON-GP survived the length of the study (3 months). (C) P3 and P7 mice were treated with poly(I:C) (50 μg/mouse). Expression of ccl5, cxcl10, irf7, and isg15 in peripheral blood was measured after 24 h. Data shown as means ± SDs. Statistical differences were tested using an unpaired two-tailed t test (ns, not statistically significant). (D) Quantification of virus levels using TCID₅₀ analysis was performed in non-infected and rVSVΔG-ZEBOV-GP-infected mice at 3, 6, and 9 DPI (n = 7/group) in the blood, liver, eye, and brain. Data represent means ± SDs. Data represent 2–5 independent experiments.

Figure 2.. rVSVΔG-ZEBOV-GP Infects the Retina and Optic Nerve Head

Non-infected and rVSVΔG-ZEBOV-GP-infected eyes were collected at 3, 6, and 9 DPI. (A) Sections of the eye were stained with H&E to assess pathology. Scale bars, 50 μm. (B) Eye sections (retina and optic nerve head) were stained with antibodies to ZEBOV GP (red), GFAP (magenta), neurofilament (green), or DAPI (blue). Yellow arrows indicate bipolar cells; green arrows indicate horizontal cells; and white arrows denote virus within the optic nerve. Data represent 2 independent experiments, with n = 6 (non-infected) and n = 12 (infected) mice per group. Scale bars, 50 μm.

Figure 3.. Neurotropic rVSVΔG-ZEBOV-GP Infection in the Brain

C57BL/6 mice (P3) were inoculated with rVSVΔG-ZEBOV-GP, and brains were collected for IHC at 3, 6, and 9 DPI. (A) Sagittal sections of non-infected (n = 7) and infected (n = 9) mice were stained with antibodies to Ebola GP (red) and NeuN (green); scale bars, 1 mm. Boxed areas of infected midbrain and cerebellum regions are shown below; scale bars, 200 μm. (B) Confocal microscopy was performed on 6 DPI brain sections (n = 3) stained with DAPI (blue), Ebola GP (red), and either neurofilament (green) or GAD67 (green); scale bars, 50 μm. Right: merged images (red + blue + green). White arrows indicate infected cells. (C) Non-infected (n = 4) and rVSVΔG-ZEBOV-GP-infected (n = 5) sections were stained with antibodies to NeuN (blue), apoptosis marker TUNEL (green), and ZEBOV GP (red) at 9 DPI. Right: merged images. Scale bars, 500 μm. (D) Non-infected (n = 4) and rVSVΔG-ZEBOV-GP-infected (n = 9) sections were stained with Fluoro-Jade C (green), a marker for neurodegeneration, and DAPI (blue); scale bars, 500 μm. Data represent 2–4 independent experiments.

Figure 4.. Interferon Responses Affect Susceptibility to rVSVΔG-ZEBOV-GP Infection

(A) Disease progression in P3 C57BL/6 mice (red circles) challenged with rVSVΔG-ZEBOV-GP virus was compared with age-matched innate immunodeficient IFNAR1^−/− (blue diamonds, n = 12), IFNb^−/− (empty inverted triangles, n = 12), and MDA5^−/− (yellow triangles, n = 28) mice, as well as adaptive immunodeficient RAG1^−/− mice (black hexagons, n = 12). Infected P3 C57BL/6 mice were co-treated with poly(I:C) (50 μg, green squares, n = 8). (B) Gene expression of ccl5, cxcl10, irf7, and isg15 was measured in the peripheral blood of P3 and P7 neonatal mice inoculated with rVSVΔG-ZEBOV-GP ± poly(I:C) co-treatment. Data shown as means ± SDs. Statistical differences were tested using one-way ANOVA with post hoc Tukey’s multiple comparison test (ns, not statistically significant; *p < 0.05, **p < 0.01, and ***p < 0.001). (C) P3 C57BL/6 mice inoculated with rVSVΔG-ZEBOV-GP alone (red) or together with 50 μg poly(I:C) (green). Viral loads in the eye and brain were determined by TCID₅₀ at 3 (n = 5), 6 (n = 5), 9 (n = 5), and 23 DPI (n = 8). Data represent 2–5 independent experiments.