A recent study published on the prepress server bioRxiv* in October 2020, reported the development of a new vaccine based on a simian adenoviral vector obtained from a group C gorilla isolate called GRAd32. This can help to speed up the development of vaccines.
Low seroprevalence for simian adenoviruses
Simian adenoviruses are isolated from non-human primates such as chimpanzees and gorillas and do not infect humans. Its prevalence is therefore low or non-existent among humans. They have been used to vector vaccine antigens in humans to test their immunogenicity to viruses and other pathogens, including Ebola, malaria, HIV and hepatitis C.
Viral vectors are not only safe and highly immunogenic, but also produce high-quality humoral and T-cell responses that remain active in the long term. The pandemic has further advanced the use of this platform, since one of the pioneering vaccines, now in phase 3 tests, is built on one of these vectors.
Simian adenoviral vector designed
The viral vector GRAd32 is a new simian adenovirus isolated from a captive gorilla. Like human adenovirus 5, it belongs to group C. When a pilot seroprevalence study was carried out, human adenoviral antibodies were present in 84%, with 68% having significant titers.
The current study used the GRAd32 vector, but with several deletions in the genome, in regions E1 and E3, while E4 is replaced by E4 ORF6 from human adenovirus 5. It also contains an insert, be it the complete SARS-CoV-2 – peak of length or its stabilized pre-fusion form, S-2P, which has epitopes recognized by neutralizing antibodies at the tip. In addition, this form increases the level of peak expression in the transduced cells.
The researchers generated four variants of the candidate vaccine, two based on the full-length peak (wild type or pre-fusion) and two on the S-2P protein. They also had only E1 and E3 exclusions or all of the exclusions listed above.
The E1 and E3 deletions ensured that the virus was defective in replication and had greater cloning capacity. The E4 substitution is designed to increase growth rates and viral yields in human cell cultures.
The researchers found that both of the vector’s skeletons remained stable over 10 amplification passages, but the replaced E4 type showed a small loss of infectivity. The S-2P antigens appeared to be trafficked to the cell surface more efficiently than the wild-type peak, although both appeared to be produced at equal levels. The S-2P surface bound more strongly to recombinant soluble ACE2, indicating that RBD is more accessible in the pre-fusion variant.
Phylogenetic analysis of GRAd32 and seroprevalence in human sera. A. Phylogenetic analysis using adenoviral polymerase sequences identifies GRAd32 as a Group C adenovirus. HAdV = human adenovirus, SAdV = simian adenovirus, GAdV = gorilla adenovirus B. Neutralizing antibody titers measured in sera collected from a cohort of 40 donors healthy humans. The data are expressed as the reciprocal of the serum dilution, resulting in a 50% inhibition of SEAP activity. The horizontal black dotted line indicates the cut of the test (title 18). The red dotted line indicates a Nab titer of 200, which is reported as potentially impacting on the immunogenicity of the vaccine. The solid red lines indicate the geometric mean. The table shows the absolute numbers and the percentage of sera with NAb titers for Ad5 or GRAd32 below the cut (<18), between 18 and 200 (<200) e acima de 200 (> 200).
Rapid induction of humoral and cellular immunity
The researchers selected GRAd32c-S-2P (GRAd32c contains E1, E3 and E4 deletions with E4 replaced by orf6) for further study due to its higher productivity and better ACE2 binding.
The researchers found that anti-S IgG titers increased rapidly after immunization and increased over time, mainly directed against RBD. Specific T cell responses were also induced, including those that secrete IFN-γ and directed against epitopes on both subunits of the peak protein.
The Th1 dominance of the cellular immune response was indicated because the IgG2a / IgG1 ratio was greater than 1. This is in contrast to the biased Th2 response obtained with a recombinant S vaccine with alum adjuvant.
One dose also produced neutralizing functional antibodies that prevented the cytopathic effect (CPE) and pseudovirus infection in cell lines. With a dose of the GRAd-COV2 viral vector vaccine in NHP and mice, a dominant Th1 T cell response was obtained, along with neutralizing antibodies to the ACE2 receptor and to SARS-CoV-2. This indicates its promise of further development.
Immunogenicity in NHPs
The comparative dose-response immunization study found that in 1×106 viral particles, all animals tested had a detectable immune response. IFN-γ secreting cells were detected in response, both in the spleen and in the lungs of immunized animals.
The researchers then tested the vaccine’s immunogenicity in NHPs to evaluate it in a model relevant to human immunity. They found that, prior to the vaccine, all animals had varying degrees of IgGs with cross-reactivity against the peak and RBD of this virus. Specific IgG titers increased after vaccination, to a maximum between the second and fourth weeks after immunization. After that, he stabilized.
Neutralizing antibodies were rapidly elicited and reached the peak, although at different times in different animals. The neutralization titers remained stable and, at week 10, their titers remained comparable or superior to those found in convalescent patients with COVID-19.
Implications and future directions
Given these findings, the researchers suggest that “a full-length wild-type S protein with proline-stabilizing mutations (S.PP) represented the best antigen in terms of immunogenicity and protective efficacy compared to several other forms of S antigen. “
This reflects the higher expression of the pre-fusion peak and the higher neutralizing antibody titer induced than the wild type antigen. A more extensive study with a viral challenge in NHP is currently underway and should confirm the present findings and assess the vaccine’s neutralizing efficacy.
The single dose regimen has many important advantages when it comes to mass vaccination. First, these virus vectors induced the expression of robust and durable antigen in the lymph nodes, but decreased innate immunity, unlike less efficient vectors.
Second, this approach allows for a rapid increase in manufacturing capabilities. Overall, these vaccines may allow adjustments using the right cell lines and GMP practices to meet the immense demand for a vaccine worldwide.
* Important news
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered conclusive, guide clinical practice / health-related behavior or treated as established information.