Establishing an immune correlate of protection for Nipah virus in nonhuman primates

Abstract

The limited but recurrent outbreaks of the zoonotic Nipah virus (NiV) infection in humans, its high fatality rate, and the potential virus transmission from human to human make NiV a concerning threat with pandemic potential. There are no licensed vaccines to prevent infection and disease. A recombinant Hendra virus soluble G glycoprotein vaccine (HeV-sG-V) candidate was recently tested in a Phase I clinical trial. Because NiV outbreaks are sporadic, and with a few cases, licensing will likely require an alternate regulatory licensing pathway. Therefore, determining a reliable vaccine correlate of protection (CoP) will be critical. We assessed the immune responses elicited by HeV-sG-V in African Green monkeys and its relationship with protection from a NiV challenge. Data revealed values of specific binding and neutralizing antibody titers that predicted survival and allowed us to establish a mechanistic CoP for NiV Bangladesh and Malaysia strains.

Fig. 1. Survival curves, body weights, and body temperatures after NiV_B challenge.

African Green monkeys were vaccinated with different doses of Hendra virus soluble glycoprotein vaccine (HeV-sG-V) or vehicle alone (alum) and challenged with a Nipah virus Bangladesh strain (NiV_B) 28 days after vaccination (Study Day 28). The animals were monitored for clinical signs of infection for 28 days (Study Day 56). A Kaplan-Meier survival curves. B Body weight changes. C Body temperature changes after the NiV_B challenge. The results combine the data obtained during the two iterations of the study. (*) Indicates a significant difference (**p < 0.01) compared to the control group (alum only). Error bars represent SE.

Fig. 2. Viral RNA and virus particles in plasma after NiV_B challenge.

Plasma samples were collected from African Green monkeys vaccinated with Hendra virus soluble glycoprotein vaccine (HeV-sG-V) and challenged 28 days (Study Day 28) after vaccination with a Nipah virus Bangladesh strain (NiV_B). Samples were collected on Study Days 28, 32, 35, 38, 43, 49, and 56 (corresponding to 0, 4, 7, 10, 15, 21, and 28 days post-challenge) or when animals reached the humane endpoint. A Viral RNA average levels per group. B Individual animal values at each sampling date. Viral RNA was detected in animals that succumbed to infection (reached the humane endpoint) except for animal NiVCoP30-2 from the 30 mcg group that only had transient viremia but survived the virus challenge. C Replicating virus particles in plasma. The virus was detected as early as 4 days post-challenge in some infected animals. D Individual animal virus titer in plasma. Only animals that succumbed to infection had detectable virus titer (plaque forming units or PFU) as determined by plaque assay. Of notice, not all the animals that succumbed to infection had detectable infectious virus particles in plasma. Error bars represent SE.

Fig. 3. Anti-NiV antibody response at Study Day 21 after HeV-sG-V vaccination.

Serum samples were collected from African Green monkeys vaccinated with different doses of Hendra virus soluble glycoprotein vaccine (HeV-sG-V) or vehicle alone (alum). Samples were collected on Study Days 0 (vaccination day), 14, and 21. Values of total IgG binding antibody titers against Nipah virus (NiV) glycoprotein Bangladesh strain (A) and Malaysia strain (B) from samples collected on Study Day 21 are shown. A trend for dose-dependance response is observed among the groups. All the animals from the control group had no detectable antibody titers (limit of quantification of LOQ). Anti-NiV neutralizing antibodies were measured on Study Day 21 against the Bangladesh strain (C) and the Malaysia strain (D). The animals with an FRNT-50 within the gray square succumbed to the virus challenge. Error bars represent SE. (Green circle): Animal NiVCoP30-2 from the 30 mcg group survived the NiV_B challenge with transient, mild clinical signs of infection.

Fig. 4. Anti-NiV antibody response after HeV-sG-V vaccination presented per animal independent of the vaccine dose received.

To better understand the role of the anti-NiV binding (A) and neutralizing (B) antibody titers in survival, the results from the immunogenicity studies are presented here with results from individual animals presented from highest to the lowest for samples obtained at Study Day 21 (21 days after vaccination). A Results indicate that animals with anti-Nipah virus Bangladesh strain (NiV_B) binding antibody titers below 10³ all succumbed to the NiV_B challenge independently of the vaccine dose received. B All the animals with anti-NiV_B neutralizing antibody titers response at Study Day 21 below an FRNT50 of 60, succumbed to infection. For both types of antibody responses (A, B), animal NiVCoP30-2 that received 30 mcg of vaccine (HeV-sG-V) had an antibody response below these thresholds and survived NiV_B challenge with transient, mild clinical signs of infection (green bar).