Immunological correlates of protection afforded by PHV02 live, attenuated recombinant vesicular stomatitis virus vector vaccine against Nipah virus disease

Abstract

Introduction: Immune correlates of protection afforded by PHV02, a recombinant vesicular stomatitis (rVSV) vector vaccine against Nipah virus (NiV) disease, were investigated in the African green monkey (AGM) model. Neutralizing antibody to NiV has been proposed as the principal mediator of protection against future NiV infection.

Methods: Two approaches were used to determine the correlation between neutralizing antibody levels and outcomes following a severe (1,000 median lethal doses) intranasal/intratracheal (IN/IT) challenge with NiV (Bangladesh): (1) reduction in vaccine dose given 28 days before challenge and (2) challenge during the early phase of the antibody response to the vaccine.

Results: Reduction in vaccine dose to very low levels led to primary vaccine failure rather than a sub-protective level of antibody. All AGMs vaccinated with the nominal clinical dose (2 × 107 pfu) at 21, 14, or 7 days before challenge survived. AGMs vaccinated at 21 days before challenge had neutralizing antibodies (geometric mean titer, 71.3). AGMs vaccinated at 7 or 14 days before challenge had either undetectable or low neutralizing antibody titers pre-challenge but had a rapid rise in titers after challenge that abrogated the NiV infection. A simple logistic regression model of the combined studies was used, in which the sole explanatory variable was pre-challenge neutralizing antibody titers. For a pre-challenge titer of 1:5, the predicted survival probability is 100%. The majority of animals with pre-challenge neutralizing titer of ≥1:20 were protected against pulmonary infiltrates on thoracic radiograms, and a majority of those with titers ≥1:40 were protected against clinical signs of illness and against a ≥fourfold antibody increase following challenge (indicating sterile immunity). Controls receiving rVSV-Ebola vaccine rapidly succumbed to NiV challenge, eliminating the innate immunity stimulated by the rVSV vector as a contributor to survival in monkeys challenged as early as 7 days after vaccination.

Discussion and conclusion: It was concluded that PHV02 vaccine elicited a rapid onset of protection and that any detectable level of neutralizing antibody was a functional immune correlate of survival.

Keywords: Nipah virus; immune correlate; neutralizing antibody; recombinant VSV; vaccine.

Conflict of interest statement

TM is employed by Crozet Biopharma LLC. RN and JF are employed by Public Health Vaccines Inc. WR is an independent consultant. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2023 Monath, Nichols, Feldmann, Griffin, Haddock, Callison, Meade-White, Okumura, Lovaglio, Hanley, Clancy, Shaia, Rida and Fusco.

Figures

Figure 1

Experimental design of two studies in African green monkeys (AGMs) designed to test the protective efficacy of PHV02 and to define the immunological correlates of protection. D, Day.

Figure 2

(A) Survival ratios, African green monkeys (AGMs) (N= 4 per group) vaccinated with graded doses of PHV02 vaccine 28 days before intranasal/intratracheal (IN/IT) challenge with 2 x 105 TCID50 of Nipah (Bangladesh). Controls (n=4) received rVSV-EBOV. (B) Survival ratios, AGMs (N= 6 per group) vaccinated with PHV02 2 x 107 pfu 21, 14 or 7 days before IN/IT challenge with 1 x 105 TCID50 of Nipah (Bangladesh). Controls (n=2) received rVSV-EBOV 14 or 7 days before challenge.

Figure 3

Pre-challenge (Day -1) Nipah virus neutralizing antibody titers, AGMs vaccinated with graded doses of PHV02 or with rVSV-EBOV (Controls) 28, 21, 14, or 7 days before IN/IT challenge (Day +1) with 2 x 105 TCID50 of Nipah (Bangladesh). Animals in all dose groups in Study 1 are combined since there were no statistical differences between groups, (see text and Supplemental Figure 1). Day -1 neutralizing antibody titers are displayed for animals in study 2 that were vaccinated 21, 14 or 7 days before challenge. Individual animal titers and geometric mean (horizontal bar) and geometric mean standard deviation (GSD) are shown. The two animals in study 1 vaccinated with the low dose on day -28 that did not seroconvert and died are shown in open circles (o).

Figure 4

Nipah virus neutralizing antibody titers, AGMs vaccinated with graded doses of PH VO2 or with rVSV-EBOV(Controls) 28 days before IN/IT challenge (Day +1) with 2 x 105 TCID50 of Nipah (Bangladesh), Study 1. Individual animal titers and geometric mean (horizontal bar) are shown by day with respect to challenge. The two monkeys in the low dose group that failed to develop neutralizing antibodies (designated X) died 7 days after challenge.

Figure 5

Logistic regression analysis to assess the relationship between pre-challenge (Day -1) log2 neutralizing antibody titer (NT) and survival in 38 African green monkeys (Studies 1 and 2 combined). (A) Simple logistic regression model with the sole explanatory variable pre-challenge log2 transformed NT. For a pre-challenge titer of 1:5, the predicted survival probability is 100%, but the confidence interval cannot be estimated. (B) Random noise (±1.0 log2) added to assess the impact of measurement error on predicted survival. For a pre-challenge titer of 1:5, the predicted survival probability (95% CI) is 73.2% (47.7, 89.1%).

Figure 6

Linear regression analysis, maximum clinical scores by pre-challenge (Day -2) neutralizing antibody titers (NT) (A). Study 1 Maximum clinical score by neutralizing titer pre-challenge (Day -1), animals vaccinated IM with graded doses of PHV02 28 days before IN/IT challenge with Nipah (Bangladesh). Regression analysis without (left panel) and with random noise [± 1.0log2) added to the pre-challenge NTs to assess the impact of measurement error] (right panel) (B). Study 2 Maximum clinical score by neutralizing titer pre-challenge (Day -1), animals vaccinated IM with PHV02 (2x107 pfu) 21, 14 or 7 days before or with rVSV-EBOV 14 or 7 days before IN/IT challenge with Nipah (Bangladesh) without (left panel) and with random noise (right panel). Individual data points are color coded by days from vaccination at the time of challenge. Note, in these plots in which noise has been added to pre-challenge titers unmask data points which are identical for different AGMs. For example, 3 control AGMs in Study 2 had the same maximum clinical score of 35. The estimated linear regression lines are superimposed on each plot.

Figure 7

Geometric mean (±GSD) Nipah virus neutralizing antibody titers following challenge (Day +1) with 2 x 105 TCID50 of Nipah (Bangladesh). AGMs were vaccinated with graded doses of PHV02 or with rVSV-EBOV (Controls) 21, 14, or 7 days before IN/IT challenge.