Monkeys, DNA, and COVID— Oh My!
Background
What do monkeys, viruses, and DNA editing all have in common? You might answer something along the lines of mad scientist’s lab or world domination. However, a study published by Yu et al. uses all of these to make something with great potential for good — a vaccine against SARS-CoV-2. We’ll have to know a few background details about the virus and our immune system to understand this.
First off, viruses generally use a protein on their surface called a spike protein (or S) to hijack a receptor on the outside of a host cell, tricking the cell into bringing it inside. Once inside, it commandeers the cell’s machinery to make more copies of itself and its genome. The SARS-CoV-2 virus follows these same basic steps as well. One of the ways our bodies fight against viruses like this is by producing antibodies (or Ab’s). These immune proteins recognize virus surface proteins and grab on very tightly, serving as signals for other parts of the immune system or stopping the virus from docking by blocking the spike’s receptor binding site (these antibodies are called neutralizing antibodies, or NAb’s). Furthermore, a proper response to a viral infection involves both antibody production (humoral immunity) and killer T cell activation (cell-mediated immunity).
Normally, viral vaccines work by introducing purified protein from a virus, weakened viruses, or dead viruses — these trigger the immune response and signal the body to start making antibodies. DNA vaccines trigger antibody protection in a non-traditional way. Researchers make a DNA sequence that codes for a virus protein, then introduce them into the cytoplasm of host cells. The cells start making small amounts of virus protein from the new DNA, mimicking a normal infection without replication or presence of the virus. The immune system picks up on the foreign protein and makes antibodies specifically against the viral protein expressed. If the vaccine works, then the host now has protection against a virus that it has never encountered before. So how did this lab generate a vaccine against SARS-CoV-2?
Summary: Monkey Business
In this study, researchers investigated how variants of their DNA vaccine work in macaques, a monkey species biologically similar to humans. They separated the monkeys into six groups: one for each variant of the DNA vaccine, and a larger group to serve as a control. Each variant of the vaccine encoded some form of the SARS-CoV-2 S protein, from the intact protein (S), S proteins with a deleted cytoplasmic tail (S.dCT), or the S receptor binding domain (RBD). The researchers administered the vaccine over several doses, then evaluated its performance in several ways. Using ELISA assays, they found that two of their groups (S and S.dCT) produced NAbs in greater amounts than convalescent human or monkey groups. When they tested other immune cells using ELISPOT (like ELISA) and cytokine staining to determine the cell’s response, they found that each variant induced S-specific IFN-γ+ helper T cells and killer T cells —in easier language, the vaccine was inducing proper immune cell responses to viral infections, and these cells were specifically targeting the viral S protein.
All of these promising results would mean nothing if the monkeys’ responses didn’t actually protect them against the virus. To test this, the researchers challenged the monkeys’ immunity by giving them two doses of live SARS-CoV-2, then measured the amount of subgenomic mRNAs (sgmRNA) recovered. These fragments of the viral genome are made during viral replication to increase expression of critical proteins, and thus suggest how many viruses are replicating. The monkeys experienced only mild clinical symptoms, and each group showed substantial reduction in sgmRNA levels (Figure 4) — the S group experienced the largest reduction (1000 times less sgmRNA recovered!). To wrap up their study, the researchers found that NAb’s and T cells stuck around for the remainder of the study (2 weeks after challenge), suggesting that the protection is lasting.
While this is all very exciting and promising, there are a few things to keep in mind. First and foremost, no DNA or mRNA vaccine has ever been approved for clinical use by the FDA, giving this vaccine a slimmer chance to actually get into circulation. Second, there are a huge number of hurdles for any drug to overcome before the FDA even considers approving it, and most otherwise promising drugs and vaccines fail during this pipeline. Finally, the researchers noted that immune responses remained highly elevated for longer than expected after the challenge, suggesting that the vaccine protection does not eliminate all virus (“sterilizing protection”) but inhibits virus infection to the point where the virus is not detectable (“rapid virologic control”).
This may seem like splitting hairs, but diseases work slightly differently in humans and model animals. This difference may become more pronounced in human testing, or it may not matter at all — this added uncertainty is concerning. If nothing else, this paper demonstrates the ingenuity and efficiency of vaccine researchers — just months after the pandemic was announced, they had already developed a working model of a new type of vaccine against this novel virus.
Exciting News!
About a week after writing this initial review, Pfizer announced that its BNT162b2 vaccine could provide 90% protection against the virus in clinical trials! This vaccine is a modified mRNA vaccine, which works on the same principles as the DNA vaccine discussed in this paper. Pfizer actually has two variants of its vaccine in the pipeline, BNT162b1 and BNT 162b2, encoding the RBD domain and a full S protein, respectively. Interestingly, both the Pfizer trials and this paper suggest that variants encoding the full S protein outperform RBD variants: something that requires some further study.
RNA and DNA vaccines are good candidates for quick vaccine production, because huge amounts of these biomolecules can be synthesized very quickly, as opposed to the slow production of more traditional vaccine doses. Disappointingly, the website states that the Pfizer vaccine must be stored at -94F to remain stable, a temperature outside the range of freezers in most clinics. However, if infrastructure adapts quickly, more RNA/DNA vaccines would become more economically viable for pharmaceutical companies to produce, and we could have an explosion of new, cheaper vaccines on our hands.
About a week after editing in the paragraphs above, Moderna also announced an efficacy of nearly 95% for its mRNA vaccine, mRNA-1273. This candidate encodes the intact coronavirus spike protein, just like Pfizer’s BNT162b2 vaccine. Encouragingly, mRNA-1273 can remain in stable storage at just -4F, a temperature well within the range of even a home freezer. This would make the vaccine much easier to ship and store than the Pfizer vaccine, hopefully allowing for more effective rollout in the months to come.
Further Reading:
Safety and Immunogenicity of Two RNA-Based COVID-19 Vaccine Candidates (Pfizer Study)
