The rVSV-EBOV vaccine provides limited cross-protection against Sudan virus in guinea pigs

Abstract

Recombinant vesicular stomatitis viruses (rVSVs) engineered to express heterologous viral glycoproteins have proven to be remarkably effective vaccines. Indeed, rVSV-EBOV, which expresses the Ebola virus (EBOV) glycoprotein, recently received clinical approval in the United States and Europe for its ability to prevent EBOV disease. Analogous rVSV vaccines expressing glycoproteins of different human-pathogenic filoviruses have also demonstrated efficacy in pre-clinical evaluations, yet these vaccines have not progressed far beyond research laboratories. In the wake of the most recent outbreak of Sudan virus (SUDV) in Uganda, the need for proven countermeasures was made even more acute. Here we demonstrate that an rVSV-based vaccine expressing the SUDV glycoprotein (rVSV-SUDV) generates a potent humoral immune response that protects guinea pigs from SUDV disease and death. Although the cross-protection generated by rVSV vaccines for different filoviruses is thought to be limited, we wondered whether rVSV-EBOV might also provide protection against SUDV, which is closely related to EBOV. Surprisingly, nearly 60% of guinea pigs that were vaccinated with rVSV-EBOV and challenged with SUDV survived, suggesting that rVSV-EBOV offers limited protection against SUDV, at least in the guinea pig model. These results were confirmed by a back-challenge experiment in which animals that had been vaccinated with rVSV-EBOV and survived EBOV challenge were inoculated with SUDV and survived. Whether these data are applicable to efficacy in humans is unknown, and they should therefore be interpreted cautiously. Nevertheless, this study confirms the potency of the rVSV-SUDV vaccine and highlights the potential for rVSV-EBOV to elicit a cross-protective immune response.

Fig. 1. rVSV-SUDV protects against SUDV challenge in guinea pigs.

Groups of six guinea pigs were vaccinated with rVSV-SUDV at a dose of either 2 × 10⁵ PFU or 2 × 10³ PFU. Control animals (n = 6) received an equivalent volume of saline. Twenty-eight days after vaccination, animals were challenged with 1000 LD₅₀ of GPA-SUDV. Animals were monitored for survival (a), weight change (b), and body temperature (c). The area shaded pink in (c) highlights temperatures above 39.5 °C. Blood samples were obtained from each animal on day 5 post-infection and either day 9 post-infection or at the terminal time point (T) if it occurred before day 9. Samples were assessed for levels of virus RNA via RT-qPCR, and data are presented as Log₁₀ genome equivalents (GEQ) per milliliter for each animal, with means and standard deviations indicated (d). A single animal (†) that was vaccinated with 2 × 10⁵ PFU rVSV-SUDV died during sampling on day 9; this animal did not exhibit signs of disease and is considered a survivor. Survival curves (a) were compared using the Log-Rank test with the Bonferroni correction for multiple comparisons; virus RNA levels (d) were compared using a two-way ANOVA with Tukey’s multiple comparison test. **p ≤ 0.01; ****p ≤ 0.0001.

Fig. 2. rVSV-SUDV elicits a robust humoral immune response.

Serum samples were obtained from all animals on days 7, 14, 21, and 28 after vaccination with rVSV-SUDV. Samples were assessed for levels of SUDV GP-specific IgG (a) or EBOV GP-specific IgG (b) via ELISA. Data are presented as Log₁₀ endpoint titers for each animal, with the geometric means and standard deviations indicated. The lower limit of detection is indicated with a red dashed line. Mean IgG (a, b) levels were compared using a two-way ANOVA with Tukey’s multiple comparison test. *p ≤ 0.05; ***p ≤ 0.001; ****p ≤ 0.0001.

Fig. 3. rVSV-EBOV protects against EBOV challenge in guinea pigs.

Guinea pigs were vaccinated with rVSV-EBOV (n = 6) or rVSV-LASV (n = 5) at a dose of 5 × 10⁶ TCID₅₀. Twenty-eight days after vaccination, animals were challenged with 1000 LD₅₀ of GPA-EBOV. Animals were monitored for survival (a), weight change (b), and body temperature (c). The area shaded pink in (c) highlights temperatures above 39.5 °C. Blood samples were obtained from each animal on day 5 post-infection and either day 9 post-infection or at the terminal time point (T) if it occurred before day 9. Samples were assessed for levels of virus RNA via RT-qPCR, and data are presented as Log₁₀ genome equivalents (GEQ) per milliliter for each animal, with means and standard deviations indicated (d). Survival curves (a) were compared using the Log-Rank test; mean virus RNA levels (d) were compared using a two-way ANOVA with Tukey’s multiple comparison test. **p ≤ 0.01; ****p ≤ 0.0001.

Fig. 4. rVSV-EBOV provides limited cross-protection against SUDV challenge in guinea pigs.

Guinea pigs were vaccinated with rVSV-EBOV (n = 14) or rVSV-LASV (n = 5) at a dose of 5 × 10⁶ TCID₅₀. Twenty-eight days after vaccination, animals were challenged with 1000 LD₅₀ of GPA-SUDV. Animals were monitored for survival (a), weight change (b), and body temperature (c). The area shaded pink in (c) highlights temperatures above 39.5 °C. Blood samples were obtained from each animal on day 5 post-infection and either day 9 post-infection or at the terminal time point (T) if it occurred before day 9. Samples were assessed for levels of virus RNA via RT-qPCR, and data are presented as Log₁₀ genome equivalents (GEQ) per milliliter for each animal, with means and standard deviations indicated (d). Data from animals that were vaccinated with rVSV-EBOV but did not survive challenge with GPA-SUDV are highlighted in light blue and indicated with an “x” on the symbol. Survival curves (a) were compared using the Log-Rank test; mean virus RNA levels (d) were compared using a two-way ANOVA with Tukey’s multiple comparison test. *p ≤ 0.05; **p ≤ 0.01.

Fig. 5. rVSV-EBOV elicits a cross-reactive humoral immune response.

Serum samples were obtained from all animals 28 days after vaccination with rVSV-EBOV and prior to challenge with GPA-SUDV or GPA-EBOV. Samples were assessed for levels of SUDV GP-specific IgG (a) or EBOV GP-specific IgG (b) via ELISA. Data are presented as Log₁₀ endpoint titers for each animal, with the geometric means and standard deviations indicated. The lower limit of detection is indicated with a red dashed line. Data from animals that were vaccinated with rVSV-EBOV but did not survive challenge with GPA-SUDV are highlighted in light blue and indicated with an “x” on the symbol. Mean IgG levels (a, b) were compared using an unpaired, two-tailed t test; all comparisons resulted in p values that were >0.05 (i.e., not significant).