A Synthetic Influenza Virus Vaccine Induces a Cellular Immune Response That Correlates with Reduction in Symptomatology and Virus Shedding in a Randomized Phase Ib Live-Virus Challenge in Humans

Abstract

Current influenza vaccines elicit primarily antibody-based immunity. They require yearly revaccination and cannot be manufactured until the identification of the circulating viral strain(s). These issues remain to be addressed. Here we report a phase Ib trial of a vaccine candidate (FLU-v) eliciting cellular immunity. Thirty-two males seronegative for the challenge virus by hemagglutination inhibition assay participated in this single-center, randomized, double-blind study. Volunteers received one dose of either the adjuvant alone (placebo, n = 16) or FLU-v (500 μg) and the adjuvant (n = 16), both in saline. Twenty-one days later, FLU-v (n = 15) and placebo (n = 13) volunteers were challenged with influenza virus A/Wisconsin/67/2005 (H3N2) and monitored for 7 days. Safety, tolerability, and cellular responses were assessed pre- and postvaccination. Virus shedding and clinical signs were assessed postchallenge. FLU-v was safe and well tolerated. No difference in the prevaccination FLU-v-specific gamma interferon (IFN-γ) response was seen between groups (average ± the standard error of the mean [SEM] for the placebo and FLU-v, respectively, 1.4-fold ± 0.2-fold and 1.6-fold ± 0.5-fold higher than the negative-control value). Nineteen days postvaccination, the FLU-v group, but not the placebo group, developed FLU-v-specific IFN-γ responses (8.2-fold ± 3.9-fold versus 1.3-fold ± 0.1-fold higher than the negative-control value [average ± SEM] for FLU-v versus the placebo [P = 0.0005]). FLU-v-specific cellular responses also correlated with reductions in both viral titers (P = 0.01) and symptom scores (P = 0.02) postchallenge. Increased cellular immunity specific to FLU-v correlates with reductions in both symptom scores and virus loads. (This study has been registered at ClinicalTrials.gov under registration no. NCT01226758 and at hra.nhs.uk under EudraCT no. 2009-014716-35.).

Copyright © 2015, American Society for Microbiology. All Rights Reserved.

Figures

FIG 1

Consort profile. Trial profile and baseline demographic and clinical data for enrolled volunteers. Average indicates the arithmetic mean of the sample. IFN-γ values correspond to the cytokine production determined by ELISA following in vitro stimulation of PBMCs with FLU-v for 24 h, with each volunteer's sample tested in triplicate. BMI, body mass index.

FIG 2

FLU-v-specific cellular immune responses pre- and postvaccination. Values are fold increases in the in vitro IFN-γ response to FLU-v over that to the negative control (medium plus BSA). The IFN-γ responses to the negative control pre- and postvaccination in both groups are 99.2 ± 25.7 versus 80.6 ± 12.9 pg/ml (average ± SEM). The IFN-γ responses to the positive control (ConA) pre- and postvaccination in both groups are 311 ± 85 versus 378 ± 35 pg/ml (average ± SEM). The IFN-γ responses to FLU-v pre- and postvaccination in the placebo group are 153 ± 49 versus 111 ± 47 pg/ml (average ± SEM). The IFN-γ responses to FLU-v pre- and postvaccination in the FLU-v group are 122 ± 37 versus 251 ± 55 pg/ml (average ± SEM). The dotted line represents the threshold point for a positive IFN-γ response to FLU-v (i.e., a ≥2-fold increase over the negative-control value). Statistical significance is represented by the P value determined by Mann-Whitney analysis.

FIG 3

FLU-v-specific cellular immune responses postvaccination—CD8 depletion—determined by ELISPOT assay and flow cytometry. (A) Effect of CD8 depletion in the PBMC population postvaccination as analyzed by flow cytometry with FITC-conjugated anti-CD3 and PE-conjugated anti-CD8 antibodies. Untreated and CD8-depleted PBMCs are represented, respectively, in the left and right graphs. The location of the CD8 T-cell population is circled in black. The percentages of CD8 T cells (± SEM) left in untreated and depleted populations are shown within the circles. (B) Postvaccination (prechallenge) in vitro IFN-γ ELISPOT assay response to FLU-v in PBMC suspensions either depleted of CD8 T cells or left untreated. Statistical significance was established by Mann-Whitney analysis.

FIG 4

Correlations between cellular IFN-γ responses and illness severity. Correlations between cellular IFN-γ responses specific to FLU-v prechallenge with A/Wisconsin/67/2005 (H3N2) and measurements of influenza severity postchallenge, i.e., symptom scores (A) and viral shedding (B). The magnitude of an individual's FLU-v-specific IFN-γ responses is represented as the fold increase in the response over the negative-control value. The dotted line represents the threshold point for a positive IFN-γ response to FLU-v (i.e., a ≥2-fold increase over the negative-control value). An individual's total symptom score represents the total sum of the daily scores (days 1 to 7 postchallenge). An individual's total viral shedding represents the total sum of daily viral shedding measurements by TCID50 (days 1 to 5 postchallenge). All of the analyses were carried out with the Spearman rank correlation test.