A new paper from the UK vaccine research group, currently available on the bioRxiv* preprint server, reveals that a single dose of ChAdOx1 nCoV-19 (AZD1222) can induce antigen-specific antibody and T cells responses against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and a booster shot additionally enhances antibody production with an increase in neutralizing titers – particularly in pigs.
A tremendous effort is currently underway across to address the ongoing coronavirus disease (COVID-19) pandemic caused by SARS-CoV-2. Approximately nine million people were infected as of June 23, 2020, as the virus transmits very readily via respiratory droplets that people sneeze and cough.
Novel Coronavirus SARS-CoV-2 Colorized scanning electron micrograph of an apoptotic cell (blue) heavily infected with SARS-COV-2 virus particles (yellow), isolated from a patient sample. Image captured at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. Credit: NIAID
A race to be the first in coronavirus vaccine hunt
Several different vaccine platform technologies were employed to date for generating candidate vaccines. Several of them used replication-deficient adenoviral vector technology, which expressed the SARS-CoV-2 spike protein (S-protein).
Clinical development of COVID-19 vaccine candidate – formerly known as ChAdOx1 nCoV-19 (and now as AZD1222) – was initiated in April 2020 and used a replication-deficient simian adenoviral vector expressing the full-length SARS-CoV-2 S-protein. Very quickly, the research progressed to non-human primate studies with a single immunization protocol.
And although no exact correlates of protection have been defined for COVID-19, recent publications suggest that neutralizing antibody titers may correspond with adequate protection in animal challenge models.
In this new study, the research group from The Pirbright Institute, the Jenner Institute of the University of Oxford, University of Surrey, Rutherford Appleton Laboratory Harwell Oxford and Public Health England decided to test the immunogenicity of either one or two doses of AZD1222 vaccine in mice and pigs in order to inform clinical development additionally.
'Prime-only' and 'prime-boost' animals
For the purposes of this study, the researchers utilized both small and large animal models to evaluate the immunogenicity of inoculating either one or two doses of a COVID-19 vaccine candidate AZD1222.
More specifically, 'prime-boost' vaccinated inbred and outbred mice were immunized on 0 and 28 days post-vaccination, whereas 'prime-only' mice received a single dose of AZD1222 on day 28. Serum and spleens were harvested from all mice three weeks after the final dose.
Furthermore, 'prime-only' and 'prime-boost' Large White-Landrace-Hampshire cross-bred pigs were immunized on day 0, while prime-boost pigs received a second immunization on day 28. Blood samples were taken from all pigs on a weekly basis.
SARS-CoV-2 specific antibody titers against S protein were determined in serum with standardized enzyme-linked immunosorbent assay (ELISA) by using recombinant soluble trimeric S (FL-S) and receptor-binding domain (RBD) proteins.
More neutralizing antibodies in pigs after booster shot
The analysis of SARS-CoV-2 S-protein-specific spleen cell responses in mice was done by IFNγ enzyme-linked immunospot (ELISpot) assay, which revealed no statistically significant difference between the 'prime-only' and 'prime-boost' vaccination regimens.
Conversely, the analysis of peripheral blood mononuclear cells (PBMC) in pigs showed responses on day 42 (i.e., two weeks after boost) that were significantly increased in the 'prime-boost' pigs when compared to 'prime-only' animals.
More specifically, when sera were assayed for neutralizing antibodies with the use of pseudovirus-based virus neutralization test (pVNT), it was found that antibody titers in 'prime-boost' pig sera were significantly higher in comparison to earlier time points and the 'prime-only' group. Such a difference between vaccine groups was not observed for mice.
"A single dose of ChAdOx1 nCoV-19 induces antibody responses, but we demonstrate here that antibody responses are significantly enhanced after a homologous boost in one mouse strain and to a greater extent in pigs", further explain study authors.
Finally, AZD1222 immunization prompted robust CD4+ and CD8+ T cell responses in both pigs and mice; however, no significant differences were observed in cytokine responses between 'prime-only' and 'prime-boost' mice.
Appraising synergistic effects to prioritize vaccine candidates
Data on mice generated from this study hints that the immunogenicity profile was at the upper end of a dose-response curve, which may have hindered our ability to ascertain differences between prime-only or prime-boost regimens by saturating the immune response.
This is why the introduction of a pig model is rather important, as the innate heterogeneity of an outbred large animal model is indeed more representative of immune responses in humans. Also, this study showed the importance of a robust T-cell response.
"It is likely that a combination of neutralizing antibodies and antigen-specific T cells would act in synergy to prevent and control infection, as we have recently shown in the context of influenza vaccination," say study authors.
"Whilst human immunogenicity and clinical read-outs are a critically meaningful endpoint, studies in small animals and pigs will help prioritize candidates to be tested in humans", they conclude.
In any case, further clinical studies are necessary to appraise immunogenicity profiles after prime-boost vaccination, as well as the impact on clinical efficacy and permanence of the immune response.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
- Graham, S.P. et al. (2020). Evaluation of the immunogenicity of prime-boost vaccination with the replication-deficient viral vectored COVID-19 vaccine candidate ChAdOx1 nCoV-19. bioRxiv. https://doi.org/10.1101/2020.06.20.159715.