Humoral Immunogenicity and Efficacy of a Single Dose of ChAdOx1 MERS Vaccine Candidate in Dromedary Camels

Abstract

MERS-CoV seronegative and seropositive camels received a single intramuscular dose of ChAdOx1 MERS, a replication-deficient adenoviral vectored vaccine expressing MERS-CoV spike protein, with further groups receiving control vaccinations. Infectious camels with active naturally acquired MERS-CoV infection, were co-housed with the vaccinated camels at a ratio of 1:2 (infected:vaccinated); nasal discharge and virus titres were monitored for 14 days. Overall, the vaccination reduced virus shedding and nasal discharge (p = 0.0059 and p = 0.0274, respectively). Antibody responses in seropositive camels were enhancedby the vaccine; these camels had a higher average age than seronegative. Older seronegative camels responded more strongly to vaccination than younger animals; and neutralising antibodies were detected in nasal swabs. Further work is required to optimise vaccine regimens for younger seronegative camels.

Figure 1

Evaluation of ChAdOx1 MERS vaccine in dromedary camels. Schematic diagram representing the study of immunogenicity and efficacy of ChAdOx1 MERS vaccine candidate in dromedary camels. +: Seropositive; −: seronegative; C: control group; V: vaccinated group. d.p.i.: days post immunisation. d.p.b.: days post boost. ● and ♦ indicate camels that were boosted with PBS or the vaccine, respectively, at 45 d.p.i.

Figure 2

Antibody responses in dromedaries vaccinated with ChAdOx1 MERS. (A) Seronegative calves were either immunised with control injections (PBS or ChAdOx1 eGFP) in group −C (open bars), or with ChAdOx1 MERS in group −V (green bars). Two calves in group −V were boosted with a second dose of ChAdOx1 MERS at 45 d.p.i. Data in green bars prior to 84 d.p.i. are from the two boosted calves only; and data after 84 d.p.i. (after infection challenge) are from all five calves. (B) Seropositive camels were either immunised with control injections (PBS or ChAdOx1 eGFP) in group +C (open bars) or with ChAdOx1 MERS in group +V (red bars). Serum samples from different days (d.p.i. and d.p.b.) were evaluated for anti-S1 Ab endpoint titres. Infectious camels used as a natural challenge model (from 84 d.p.i.) were also evaluated (black bars) in (A,B). (C) The fold increase in Ab titres of groups +C and +V. (D) Analysis of nAbs in samples collected from 0, 28 and 365 d.p.i. (in A,B) using MERSpp NA. ns: No significant difference in antibody titres at day 0 in +C versus +V camels, by Kruskal-Wallis test with Dunn’s Multiple comparison test. +: Seropositive; −: seronegative; C: control group; V: vaccinated group. d.p.i.: days post immunisation. d.p.b.: days post boost.

Figure 3

Severity and abundance of nasal discharge post natural infection challenge. The twenty camels in all groups (−C, −V, +C, and +V) were challenged with ten naturally infectious camels; and nasal discharges were assessed for their abundance and severity for 14 d.p.c.; using arbitrary units of 0 (N: normal), 0.25 (R: recovered), 1 (+: mild), 2 (++: moderate), or 3 (+++: severe). (A) Representative photos taken from different camels in different days. (B) Areas under the curve (AUC) for the nasal discharge scores collected daily from 1 to 14 d.p.c. are plotted. The analysis of mixed model on AUC of nasal discharge scores and fixed factors (d.p.c., seropositivity, and vaccine) shows that the overall vaccine effect over time was significant, p = 0.0274.; also +V had statistically significantly lower scores as compared to +C, by Kruskal-Wallis test with Dunn’s Multiple comparison test. d.p.i.: days post immunisation. +: Seropositive; −: seronegative; (C) control group; V: vaccinated group.

Figure 4

Efficacy of ChAdOx1 MERS in dromedaries based on GE TCID₅₀/ml. Nasal swab samples from 1 to 14 days post challenge (d.p.c.) were assessed in absolute RT-qPCR quantification and the MERS-CoV viral genome equivalent (GE) to TCID₅₀/ml is reported. (A) The mean titres of MERS-CoV GE (TCID₅₀/ml) are reported from 1 to 14 days post challenge (d.p.c.) with SEM error bars for each group. (B) Areas under the curve (AUC) for the mean MERS-CoV GE (TCID₅₀/ml) titres measured from 1 to 14 d.p.c. are plotted for each group. +: Seropositive; −: seronegative; (C) control group; V: vaccinated group. The analysis of mixed model on the log-transformed MERS-CoV GE (TCID₅₀/ml) titres and fixed factors (d.p.c., seropositivity, and vaccine) shows that the vaccine was significant with p = 0.0059.

Figure 5

Antibody immune responses to ChAdOx1 MERS in dromedary calves at different ages. Seronegative dromedary calves, below 1 year old, were immunised with PBS (n = 2) or ChAdOx1-eGFP (n = 1) in group AG01 (black triangles) or with ChAdOx1 MERS in group AG02 (n = 3; blue squares). Older calves, 1–2 years old, were immunised with ChAdOx1 MERS in group AG03 (n = 3; red circles). (A) Anti-spike Ab titres were assessed in ELISA; and nAb titres were determined in MERS-CoV neutralisation assay (B) and in MERS pseudotyped virus neutralisation assay (MERSpp NA) (C). (D) Linear regression between the values of MERS-CoV neutralisation assay and MERSpp NA showed a significant relationship with R² = 0.96, p < 0.0001. (A) Nasal mucosal nAbs were determined by MERSpp NA in samples from 0 and 21 d.p.c.