Research on the development of new animal models is important to test drug and vaccine candidates in the context of the ongoing coronavirus disease 2019 (COVID-19) pandemic. While first-generation vaccines are being rolled out in many countries around the world, with a resulting decrease in infections and deaths due to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), some hitches are becoming obvious as well.
A new preprint recently released on the bioRxiv* server describes the immunogenicity and protective potential of a novel adenovirus SARS-CoV-2 vaccine candidate against a form of the virus adapted to cause severe disease in mice. The findings show that this provides a useful mouse model for the study of new vaccines, in addition to demonstrating the protective efficacy of the current rhesus adenovirus vaccine candidate RhAd52.
Previous animal models for the study of SARS-CoV-2 infection have included transgenic mice that express the human angiotensin-converting enzyme 2 (hACE2), hamsters and non-human primates. These models allow viral replication and some degree of clinical disease.
The mouse-adapted virus (MA10) was developed to allow laboratory mice to be infected with the virus and develop key features of human COVID-19 – notably, viral replication in the respiratory tract and significant weight loss. Using this model, small molecules have been tested for their antiviral potential, as well as prototype monoclonal antibodies and vaccine candidates.
Strong immune response to single dose
In this study, the scientists explored immune responses to the use of the RhAd52-vectored SARS-CoV-2 spike protein. Antibodies to this virus are found at much lower prevalence in humans compared to the Ad26 vector used in the Oxford AstraZeneca vaccine.
The scientists used various forms of the spike protein, including only the S1 domain, the receptor-binding domain (RBD), or truncated portions of the spike without the cytoplasmic tail, for instance. Stabilized prefusion spike conformations containing a pair of prolines (S2P) were used, as were furin cleavage site mutants.
Mice immunized with a priming dose of the RhAd52 vaccines developed specific antibodies in all cases by week 8. Neutralizing responses were weakest with one dose of the S1 insert-containing vaccine, with some mice failing to show detectable neutralizing responses.
Pseudovirus infection was neutralized by several of these vaccine candidates, again the weakest performance being with that based on the S1 domain.
Ten-fold rise in antibody titer with two doses
After the booster dose, at week 8, anti-spike antibodies increased by ten-fold within two weeks irrespective of the vaccine antigen. Thus, a prime-boost vaccine regimen was proved to be effective in inducing immunity with this vaccine candidate.
When several types of vaccine regimens were compared, all immunized animals showed strong binding antibody responses to the spike and the RBD at week 12 from the first dose.
Again, the prime-boost regimen showed median antibody titers about one-log higher than with a single dose, but the lowest response was with the S1 antigen. High neutralizing responses were also found for all vaccines except the S1-based vaccine.
Reduction of viral load in lung tissue
Immunized mice were protected against the MA10 virus when administered intranasally. While controls showed dramatic weight loss of about 15% (median), the immunized mice did not, their weight remaining stable with all antigens and both dose regimens (single- or two-dose).
In control mice, infectious virus particles were found in the tissues of the respiratory tract at six-log higher concentrations, compared to the failure to recover such particles in mice immunized with the RhAd52-vectored full-length spike in its native or S2P form, or truncated spike antigens.
In terms of plaque formation, the former group had a median titer of 3.1 x 107 plaque-forming units (PFU)/lung compared to <100/lung in the latter.
The most effective was the RhAd52.S2P vaccine, which completely protected all mice, with no detectable viral load in the lung tissue. This confirms earlier findings with the S2P antigen in non-human primates, and so this is now undergoing clinical trials.
The S1 and RBD antigen-containing vaccines resulted in the presence of the infectious virus, but at lower titers, with the median being 3.9 x 105 and 5.0 x 103 PFU/lung, respectively. Thus, these offered only partial protection, as did single doses of the full-length spike- or truncated spike-containing vaccines.
Viral loads continue to decline from day 2 onwards (until day 7). Except for some mice which were immunized with two doses of the RhAd52.S and RhAd52.S1 vaccines, virus loads were undetectable at day 4.
Together, these data suggest that all vaccines led to a reduction in respiratory tract tissue viral loads at the typical peak of infection as well as significantly decreased the persistence of virus in the lungs,” writes the team.
The higher the peak antibody titers, the lower the PFU titer in the lungs, indicating that an efficient vaccine-induced immune response successfully neutralizes the virus in the respiratory tract.
What are the implications?
The findings of this study indicate the promise of the RhAd52 vector to elicit a strong binding and neutralizing antibody response in mice, protecting them against infection with SARS-CoV-2. This vector virus has low seroprevalence in humans, which suggests its potential for the development of effective COVID-19 vaccines, either as primary second-generation vaccines or as boosters.
Secondly, the MA10 mouse-adapted SARS-CoV-2 produced severe disease in mice and was also successfully replicated in the respiratory tract.
Mice are ideal small animal models, being available on a large scale, docile and with a short lifecycle. The successful establishment of severe disease following MA10 infection makes it a suitable model for human COVID-19, which is sorely needed since nonhuman primate models develop only mild disease.
This model has earlier been used to test antivirals, but this suggests its utility in studying the humoral immune response to vaccine prototypes. The use of a range of antigens shows how important it is to choose the right regimen and antigen to achieve complete suppression of viral replication.
These data support the development of RhAd52 vaccines and the use of the MA10 challenge virus to screen novel vaccine candidates and to study the immunologic mechanisms that underscore protection from SARS-CoV-2 challenge in wild-type mice,” concludes the team.
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.