Chimpanzee adenovirus- and MVA-vectored respiratory syncytial virus vaccine is safe and immunogenic in adults

Abstract

Respiratory syncytial virus (RSV) causes respiratory infection in annual epidemics, with infants and the elderly at particular risk of developing severe disease and death. However, despite its importance, no vaccine exists. The chimpanzee adenovirus, PanAd3-RSV, and modified vaccinia virus Ankara, MVA-RSV, are replication-defective viral vectors encoding the RSV fusion (F), nucleocapsid (N), and matrix (M2-1) proteins for the induction of humoral and cellular responses. We performed an open-label, dose escalation, phase 1 clinical trial in 42 healthy adults in which four different combinations of prime/boost vaccinations were investigated for safety and immunogenicity, including both intramuscular (IM) and intranasal (IN) administration of the adenovirus-vectored vaccine. The vaccines were safe and well tolerated, with the most common reported adverse events being mild injection site reactions. No vaccine-related serious adverse events occurred. RSV neutralizing antibody titers rose in response to IM prime with PanAd3-RSV and after IM boost for individuals primed by the IN route. Circulating anti-F immunoglobulin G (IgG) and IgA antibody-secreting cells (ASCs) were observed after the IM prime and IM boost. RSV-specific T cell responses were increased after the IM PanAd3-RSV prime and were most efficiently boosted by IM MVA-RSV. Interferon-γ (IFN-γ) secretion after boost was from both CD4(+) and CD8(+) T cells, without detectable T helper cell 2 (TH2) cytokines that have been previously associated with immune pathogenesis following exposure to RSV after the formalin-inactivated RSV vaccine. In conclusion, PanAd3-RSV and MVA-RSV are safe and immunogenic in healthy adults. These vaccine candidates warrant further clinical evaluation of efficacy to assess their potential to reduce the burden of RSV disease.

Trial registration: ClinicalTrials.gov NCT01805921.

Figure 1. Frequency of the maximum severity solicited adverse event, oral temperature and size of local injection site reactions within one week of vaccination

The number of volunteers is represented across the x-axis without distinction between low-dose and target-dose recipients; n=10 or 11 for events after prime and n=10 for events after boost due to withdrawals. Volunteers reported subjective symptoms as none, mild (does not interfere with routine activities), moderate (interferes with routine activities) and severe (unable to perform routine activities). Redness, swelling and induration at the site of injection used the maximal recorded diameter of any reaction for objective severity grading. Redness and induration were graded as none (0-2 mm), mild (3-50 mm), moderate (51-100 mm) and severe (≥101mm). Swelling graded as none (no visible reaction), mild (1-20 mm), moderate (21-50 mm) and severe (≥51mm). Fever was graded as none (≤37.6°C), mild (37.6.0-38.0°C), moderate (38.1-39.0°C) and severe (≥39.1°C). Overall 5587/5593 (99.9%) of expected data points for solicited adverse events within one week after vaccination were collected for analysis. The only missing data was for temperature recordings. Sore throat reactions were not a solicited symptom although occurred as an unsolicited event in 5/21 IN primed volunteers.

Figure 2. The RSV neutralising antibody in response to vaccination

Data for all volunteers at both doses of vaccine, summarised as the geometric mean titre with 95% confidence intervals for each study group (Group 1,Group 2, Group 3, Group 4). Responses after prime (P) are grouped by the route of PanAd3-RSV administration (IM or IN). At week 4 the IM PanAd3-RSV boost (B) was administered to group 2. At week 8, IM MVA-RSV and IM PanAd3-RSV boost (B) vaccines were given to the remaining volunteers. The results of individual volunteers are presented in the supplementary material (sFigure 3). The antibody titre 4 weeks following IM PanAd3-RSV prime was significantly elevated from baseline (p < 0.001, paired t-test) but not following the IN route (p = 0.816, paired t-test). For volunteers who received IM prime, the titres 4 weeks after boost were not statistically significant from pre-boost titres (group 1 between week 8 and week 12, p = 0.152; group 2 between week 4 and 8, p = 0.872; paired t-tests). Final measures of serum neutralising antibody titres were statistically indistinguishable from baseline in all groups (group 1 p=0.316, group 2 p=0.416, group 3 p=0.587, group 4 p=0.152; paired t-tests).

Figure 3. Ex-vivo B-cell (antibody secreting cell, ASC) response to vaccination

Fresh PBMCs were collected for analysis at baseline and one-week after prime and one-week after boost vaccinations and subjected to a dual-colour ELISpot. The responses are represented by scatter plot of ASC spots per million PBMCs after HSA background subtraction. (A) The anti-F specific IgG ASC response (B) the anti-F specific IgA ASC response to vaccination. The greatest ASC responses were detected after administration of the first IM vaccine. Overall 13/50 (26%) of plates for anti-F IgG and 10/50 (20%) of plates for anti-F IgA were rejected due to contamination or laboratory error. A total of 90/214 (73%) and 95/124 (77%) of data points were available for the analysis of anti-F IgG and anti-F IgA ASC responses respectively. Study groups; Group 1,Group 2, Group 3, Group 4. Combined groups; by route of PanAd3-RSV prime administration IM and IN.

Figure 4. The >ex-vivo T-cell IFNγ response to vaccination

Fresh PBMCs were collected for ex-vivo IFNγ ELISpot analysis at baseline, two weeks after prime, before boost and one week after boost. Cells were stimulated overnight by peptide pools Fa, Fb, M and N being representative of the vaccine antigens. (Panel A) The results for each group presented by scatter plot of the summed response for each volunteer [(Fa+Fb+M+N) − (4xDMSO)]. The red line denotes the geometric mean. (Panel B) Individual responses to the separate peptide pools linked between before vaccination and after prime (P) and boost (B). Empty circles denote volunteers who received the lower dose (n=2 per group). Overall 13/68 (19%) of plates failed due to contamination or laboratory error resulting in the loss of 30/163 (18%) of samples. There was a disproportionate loss of group 2 pre-boost samples. A further 5 peptide responses from 3 volunteers were rejected with a triplicate variance greater than 10. Study groups; Group 1,Group 2, Group 3, Group 4. Combined groups; by route of PanAd3-RSV prime administration IM and IN.

Figure 5. CD4+ and CD8+ IFNγ responses at baseline and one-week post boost by ICS

Empty circles are low-dose vaccine recipients (n=2 per group). Within each group the baseline response (left) is matched with the response one week after boost (right, at week 5 for group 2 and week 9 for the other groups). Overall the responses to Fa and Fb peptide pools were greater, with similar responses to N and fewer responses to M. The overall CD4+ and CD8+ responses were greatest following MVA-RSV boost compared to baseline. Study groups; Group 1,Group 2, Group 3, Group 4. The frequency of responses is presented in supplementary material (sFigure 8).

Figure 6. Vector neutralising antibody (anti-PanAd3) titres before prime and before boost vaccination

Anti-PanAd3 titres were measured for the 40 volunteers who completed the trial. No pre-screening of anti-PanAd3 titres was performed before enrolment and study group allocation. (A) Scatter plot of the anti-PanAd3 titre from before prime (baseline) and before boost vaccine. The lower limit of detection for the assay was a titre of 18, and titres ≤18 were arbitrarily assigned a titre of 9. (B) Fold change in anti-PanAd3 neutralising antibody after IM and IN PanAd3-RSV prime. The red bar denotes the geometric mean. Study groups; Group 1,Group 2, Group 3, Group 4. Combined groups; by route of PanAd3-RSV prime administration IM and IN.