This article was reviewed by Dr. Scott Covey, one of our subject matter experts.
An additional thank you to Professor Mark Howarth, who also took the time to review this article.
Introduction:
The paper we’re demystifying today can be found here, if you want to follow along[1].
On March 11, 2020, the World Health Organization declared COVID-19 a pandemic [2]. Since then, the pandemic has claimed over three million lives. Now, though, the end is finally in sight. COVID19 vaccines, including those developed by Pfizer, Moderna, and AstraZeneca, have been approved for public use and are being used across the world. The United States has administered at least one vaccine dose to 51.3% of its population, 59.2% of the population of the United Kingdom has been given at least one dose, and Canada has reached 58.5% of its population with at least one dose[3]. With this rapid rollout of vaccines, it seems the return to work, school, and social gatherings can slowly begin. The quick and efficient development of these vaccines was a scientific triumph- one truly for the history books. Now a new question: How do we stop this from happening again?
SARS-CoV-2 (the virus that causes COVID19), SARS-CoV (from the 2003 epidemic), and MERS are cousins. They’re all zoonotic betacoronaviruses that possibly evolved first in bats before eventually making their way to human hosts. As discussed in Demitri’s video sidebar, these SARS-like coronaviruses and their jumps from animals to humans are quite common. Given that any one of these coronaviruses could cause a future pandemic, we need to develop ways to protect people against them. A ‘pan-coronavirus’ vaccine would protect immunized people against many coronaviruses at once- stopping future pandemics before they happen. Here’s the fun part: how do we make one? Researchers from the California Institute of Technology and Oxford University may have the answer. Using technology similar to what we’ve discussed in this article, these researchers designed mosaic nanoparticle vaccines. These nanoparticle vaccines display RBDs from a variety of sarbecoronaviruses and may provide a broader range of protection. But how does this technology work?
The Technology:
Many of the existing COVID19 vaccines target the spike trimer, or S protein, of SARS-CoV-2. As you may recall from our sidebar about coronavirus infections, the S protein contains the receptor-binding domain (RBD) and the fusion domain. The S trimer and the RBD are also the main targets of neutralizing antibodies. This makes the RBD and the S trimer antigens. Past research has shown that when multiple antigens are presented to the immune system in a regular and tightly packed orientation, this multivalent presentation can lead to longer-lasting immunity[4]. Nanoparticle vaccines take advantage of this, presenting multiple antigens in a multivalent pattern like that seen on an actual virus.
The nanoparticles vaccines in today’s paper were made possible by the SpyTag/SpyCatcher system. For more information on how this system works and why it’s so powerful for vaccine development, we recommend that you watch this video produced by SpyBiotech. Here’s a recap: protein nanoparticle vaccines are made by attaching antigens to the outside of a virus-like particle (VLP). Virus-like particles are structures that look like a virus but don’t contain any virus genetic material[5]. One of the main limitations of making vaccines this way is that it’s difficult to attach the antigens to the VLP without either damaging the VLP components or having the antigens attach in a non-symmetrical pattern. The SpyTag/SpyCatcher system is a type of molecular superglue that can link antigens to the outside of VLPs with a very strong bond and with high levels of control over how and where the antigens are positioned. We recommend you watch the video!
The Nanoparticles
Some of the researchers involved in this project have previously used the SpyTag/SpyCatcher system to assemble a nanoparticle vaccine for SARS-CoV-2. Their vaccine caused a strong immune response in mice. In today’s study, this approach was expanded. The researchers started by preparing nanoparticles that were studded with RBDs from a variety of human and animal coronaviruses. Towards this, they identified different categories of RBDs across a variety of coronaviruses including SARS-CoV-2, SARS, several bat coronaviruses, and pangolin coronaviruses. Of these RBDs, they picked eight to include in their vaccine design. Three different vaccine nanoparticles were made: mosaic-4a, mosaic-4b, and mosaic-8. These vaccine candidates were then compared with the previously designed nanoparticles displaying the SARS-CoV-2 RBD only (homotypic mi3).
Here’s a handy table of the different nanoparticles and which RBDs are included in each:
| Nanoparticle | RBDs included |
| Mosaic-4a | SARS-CoV-2, RaTG13, SHC014, Rs4081 |
| Mosaic-4b | Pang17, RmYN02, Rf1, W1V1 |
| Mosaic-8 | All 8 (SARS-CoV-2, RaTG13, SHC014, Rs4081, Pang17, RmYN02, Rf1, W1V1) |
| Homotypic mi3 | SARS-CoV-2 only |
Four RBDs (SARS, Yun11, BM-4831, BtKY72) were not included in any of the nanoparticles. This was so that soluble (not nanoparticle bound) versions of these RBDs could be used to test whether the vaccines could protect against coronaviruses with RBDs that are similar, but not identical to the ones on the vaccine.
The Experiment
After the vaccines were assembled and quality tested, animal trials were carried out. For this, mice were immunized with one of:
- Soluble (not nanoparticle bound) SARS-CoV-2 S protein
- The previously made nanoparticles with only SARS-CoV-2 RBDs (homotypic mi3)
- Mosaic-4a
- Mosaic-4b
- Mosaic-8
The immune response was measured after both prime (first shot) and boost (second dose) using ELISAs against either the SARS-CoV-2 RBD or any one of a panel of RBDs. For a refresher on ELISAs, check out this sidebar. The researchers also carried out neutralization assays using pseudoviruses. We discussed pseudoviruses in this article, but here’s a quick recap: a pseudovirus is a particular virus particle that’s been modified to display the envelope proteins (in this case, the RBD) of a different virus that the researchers are trying to study. In a neutralization assay, the goal is to figure out whether the antibodies resulting from immunization can block virus particles (or pseudovirus particles) from entering cells[6]. This is a good indication of immunological protection. In today’s experiment, the researchers prepared pseudoviruses displaying the various coronavirus RBDs and used those to test how good the antibodies were at neutralizing particular targets. From these experiments, the researchers found that the level of antibody response calculated using ELISA matched up with the amount of neutralization seen in their neutralization assays.
The researchers found that immunizing mice with soluble SARS-CoV-2 S protein (not bound to a VLP) didn’t result in any significant antibody response. In contrast, serum from mice immunized with any of the nanoparticle vaccines contained antibodies that were capable of binding to all tested RBDs, including SARS-CoV-2 and other zoonotic RBDs. The antibodies were also capable of neutralizing the related coronaviruses, preventing them from entering cells. Immunization with any one of the nanoparticle vaccines resulted in equally strong antibody responses against SARS-CoV-2, indicating that the mosaic nanoparticles are just as effective at producing an antibody response against COVID19 as their previously developed homotypic SARS-CoV-2 vaccine. Serum from mice immunized with the nanoparticle vaccines also contained antibodies capable of binding to and neutralizing pseudoviruses with RBDs that weren’t present on the nanoparticle. This is promising because it suggests that the similarity between these RBDs may be enough for nanoparticle vaccines to protect against a broad spectrum of sarbecoronaviruses.
Later experiments in mice showed that after immunization with the mosaic vaccines, the resulting antibodies were better at binding and neutralizing particular RBDs than antibodies from mice immunized with the original, non-mosaic SARS-CoV-2 vaccine (mi3). The mosaic vaccines also resulted in overall higher antibody levels against mismatched RBDs (RBDs not included in the nanoparticle itself). All these findings suggest that the mosaic vaccines protect against a wider range of coronaviruses than the homotypic, non-mosaic nanoparticle vaccines. Even more excitingly, the antibodies produced were also capable of binding to mutant versions of the SARS-CoV-2 RBD.
Since being infected by SARS-CoV-2 also results in some level of natural immunity, the researchers wanted to see how immunity following immunization with a mosaic vaccine compares with immunity following SARS-CoV-2 infection. To investigate this, they repeated their earlier ELISA experiments using antibodies extracted from the plasma of human donors who had been infected with COVID19. They found that these donor plasmas contained antibodies that could bind well to SARS-CoV-2 RBDs, as expected. However, the donor antibodies bound much more weakly to the other RBDs. Also, while the donor plasma antibodies could neutralize SARS-CoV-2 pseudoviruses, they weren’t very good at neutralizing other pseudoviruses. This agreed with past research- being infected with SARS-CoV-2 gives you some protection against future COVID19 infection, but not against other coronaviruses. On the other hand, immunization with a mosaic nanoparticle may provide a broad range of strong protection.
Conclusions
This paper showed that mosaic nanoparticles displaying RBDs from a variety of coronaviruses may be an effective way of protecting against future pandemics. Immunization with these mosaic nanoparticles seems to result in antibodies capable of binding many different coronavirus RBDs and neutralizing a wide variety of coronaviruses. These nanoparticle vaccines provide a broader immunity than natural SARS-CoV-2 infection. Overall, this is an incredible study that highlights how scientific research is working to defend us now and keep us safe in the future.
The data we discussed today was collected in animal models. More research and experiments, as well as large-scale clinical trials, are needed before these vaccines are ready for humans. There’s a long road ahead before we have a pan-coronavirus vaccine, but this research is the first big step. Sometime soon, we may be able to stop future pandemics before they happen- and that’s a pretty big win for science.
References
1. Cohen, A.A., Gnanapragasam, P.N.P., Lee, Y.E., Hoffman, P.R., Ou, S., Kakutani, L.M., Keeffe, J.R., Wu, H.-J., Howarth, M., West, A.P., et al. (2021). Mosaic nanoparticles elicit cross-reactive immune responses to zoonotic coronaviruses in mice. Science 371, 735–741.
2. Listings of WHO’s response to COVID-19 Available at: https://www.who.int/news/item/29-06-2020-covidtimeline [Accessed May 16, 2021].
3. Coronavirus (COVID-19) Vaccinations – Statistics and Research Our World in Data. Available at: https://ourworldindata.org/covid-vaccinations [Accessed May 16, 2021].
4. Cai, H., Zhang, R., Orwenyo, J., Giddens, J., Yang, Q., LaBranche, C.C., Montefiori, D.C., and Wang, L.-X. (2018). Multivalent Antigen Presentation Enhances the Immunogenicity of a Synthetic Three-Component HIV-1 V3 Glycopeptide Vaccine. ACS Cent Sci 4, 582–589.
5. Roldão, A., Mellado, M.C.M., Castilho, L.R., Carrondo, M.J.T., and Alves, P.M. (2010). Virus-like particles in vaccine development. Expert Rev Vaccines 9, 1149–1176.
6. Gauger, P.C., and Vincent, A.L. (2020). Serum Virus Neutralization Assay for Detection and Quantitation of Serum Neutralizing Antibodies to Influenza A Virus in Swine. Methods Mol Biol 2123, 321–333.
