A new study, released as a preprint on the bioRxiv* server, reports an optimized coronavirus disease 2019 (COVID-19) vaccine candidate, which, claim the researchers, may be less likely to produce reactions like fevers. It may also have increased protective potential against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the pathogen that causes COVID-19.

Reactogenicity of mRNA vaccines

The two earliest vaccines to receive emergency use authorization against the virus were both messenger ribonucleic acid (mRNA)-based formulations, encoding the full-length viral spike glycoprotein to elicit anti-spike neutralizing antibodies.

Among the chief undesirable effects of these vaccines is their reactogenicity, or the production of typical vaccine-related side effects like fever or soreness at the site of injection, due to immunological activation. In order to prevent this, the researchers behind the new paper optimized the mRNA of the vaccine antigen.

This vaccine candidate encodes the receptor-binding domain (RBD) that binds to human angiotensin-converting enzyme 2 (hACE2), the human host cell receptor, mediating virus entry into the host cell. The aim is to elicit a robust immunological reaction, both humoral and cellular, against the RBD that can prevent spike binding and thus avert infection.

Many human trials have also reported on the use of various vaccine types against SARS-CoV-2. The two mRNA vaccines in current use were authorized based on phase III data that showed comparable dose-activity ratios, in the form of neutralizing geometric mean titers (GMTs) similar to that of a panel of convalescent sera against the virus.

Interestingly, two versions of one of these vaccines, namely, BNT162b1 and BNT162b2 – encoding the RBD and the full-length spike, respectively – showed varying incidences of adverse reactions, more so in older adults. The latter was more tolerable than the former.

This may be due to a reduction in length of the RBD-encoding mRNA to one-fifth of that encoding the full-length spike, resulting in a five-fold increase in a molar ratio of the former at the same dose.

The endogenous adjuvant activity of RNA is well known, since the mRNA itself may activate Toll-like receptors or other cytoplasmic RNA sensors. Methylation, pseudouridine incorporation, or other modifications to the mRNA, may prevent the activation of innate immunity and introduce new immunostimulatory sites inadvertently.

Species-specific differences in immunogenicity

The current study uses an mRNA candidate encoding the RBD, encapsulated in a lipid nanoparticle (LNP) coating. The researchers tested the vaccine's immunogenicity in two mouse breeds, the BALB/c strain and the C57BL/6, both of which have shown B and T cell responses to this formulation.

The researchers found that in six-week-old mice of the BALB/c, but not C57BL/6, strain, two intramuscular injections of the vaccine-induced high antibody titers to the RBD. The frequency of both T follicular helper (Tfh) and germinal center (GC) B cells was much higher in the former strain, in keeping with the higher serum antibody levels.

The scientists also found that of eight peptide pools of 16 peptides each, each having a 20 amino acid sequence of the spike antigen, with ten overlapping amino acids, pools 3 and 4 induced IFN-γ production in splenic T cells from immunized C57BL/6 mice, but only pool 3 in BALB/c mice.

The latter responded to pools 3 or 4 with increased numbers of anti-RBD polyfunctional CD8 and CD4 T cells. However, only polyfunctional CD8 T cells were observed in immunized C57BL/6 mice.

The conclusion is that the vaccine candidate produces strong T and B cell responses in BALB/c mice, but not the second strain, suggesting a different level of immunogenicity in the second.

Purification enhances immunogenicity

Purification of the mRNA before encapsulation in LNP led to a marked reduction in type I interferons from human immune cells. This vaccine candidate produced higher anti-RBD B cell responses in both mouse strains than the non-purified LNP-mRNA-RBD formulation.

It also produced higher frequencies of the polyfunctional CD4 and CD8 T cells, with higher levels of type I cytokines, including IFN-γ, than the earlier formulation.

Confirmatory findings were obtained from non-human primate (NHP) studies, where the purified LNP-mRNA vaccine produced specific anti-RBD antibodies, and a neutralizing response, at higher levels in the conjunctiva, nose and trachea, as well as in the rectum.

This was associated with lower viral RNA and infectious virus titers in vaccinated macaques at days 1 and 7 post-infection with SARS-CoV-2, as well as the absence of fever and pneumonia. This demonstrates a high level of protection.

The lung tissue from vaccinated macaques also showed lower levels of pathology, and especially the development of bronchus-associated lymphoid tissue (BALT), associated with mucosal immunity in the respiratory tract. This might lead to lower viral shedding following viral challenge.

What are the implications?

The researchers suggest that optimization of the vaccine improves immunogenicity while reducing its reactogenicity.

Secondly, the high levels of protective IgG immunity may help induce neutralizing mucosal immunity rapidly in the upper airways.

Fine-tuning of the balance between endogenous adjuvant activity and antigen translation efficiency of LNP-mRNA, may provide a means towards better efficacy and safety and will also be crucial for the development of anti-SARS-CoV2 vaccines in the near future.”

*Important Notice

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.

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.