The current COVID-19 pandemic is wreaking havoc on global health and economics. Widespread administration of an effective vaccine against the causative virus, SARS-CoV-2, would alleviate many of the challenges caused by the pandemic. Dozens of vaccines against COVID-19 are actively in development.
There are various types of vaccines, but each one is designed to work against a specific component of the target pathogen. Some vaccines, including DNA- and mRNA-based ones, utilize genetic-based technology. At this time, mRNA vaccines are frontrunners in addressing the COVID-19 pandemic. Moderna Therapeutics’ mRNA-1273 and BioNTech/Pfizer’s BNT162b2 lead the pack, but there is limited peer-reviewed and published preclinical data demonstrating their efficacy and safety. Understanding the effects of mRNA vaccines in preclinical models is important to informing clinical trials and developing successful human vaccines.
A paper set to be formally published this month (October 2020) in Immunity (Laczkó et. al), will release the first peer-reviewed published data on an mRNA vaccine for COVID-19 in a rodent preclinical model. The purpose of this paper was to investigate the immunological responses to a modified mRNA vaccine designed against SARS-CoV-2.
Laczkó et. al studied the effects of a single 30ug injection of SARS-CoV-2 modified mRNA-LNP vaccines in BALB/c mice. The vaccines included the modified mRNA nucleotide 1-methylpseudouridine-5’ triphosphate instead of uridine triphosphate, and they encoded portions of the spike glycoprotein of SARS-CoV-2, which is the protein the virus uses to enter human cells. Two versions of the vaccine were tested; one encoded the wild-type spike protein with a stabilizing deletion (Δfurin), and the second encoded only the receptor binding domain of the spike protein. The researchers used TriLink Biotechnologies’ modified mRNA nucleotides and CleanCap® technology to prepare their mRNA vaccines. CleanCap is a co-transcriptional mRNA capping solution that yields a Cap 1 structure, reducing immunogenicity of the capped mRNA. These nucleotide and cap mRNA modifications were made to improve vaccine protein expression and immunogenicity, as both of these factors significantly contribute to overall vaccine tolerability and efficacy. The modified mRNA was contained in a lipid nanoparticle (LNP) for delivery.
After a single injection, the scientists utilized techniques including flow cytometry and ELISAs to examine T cell and B cell responses in the spleen, lung, and bone marrow. They also determined the presence of long-lived plasma and memory B cells, which help mount an immune response upon exposure to the pathogen. From their single-injection experimental setup, Laczkó et. al showed robust CD4+ and CD8+ T cell responses, as well as robust B cell and long-lived plasma cell responses. The T cell response was evident in the spleen, lung, and lung parenchyma. Importantly, the researchers observed a Th1-type T cell response, rather than the Th2 response thought to contribute to dangerous antibody-dependent enhancement of replication (ADE). In order to investigate the possibility that their vaccines induced ADE, the researchers looked for ADE responses in cells in vitro, but found no evidence.
By conducting preclinical studies with modified mRNA vaccines, this paper informs ongoing clinical trials testing COVID-19 mRNA vaccine options. Data from Laczkó et. al suggest that a single dose of modified mRNA-LNP vaccine may be sufficient for an initial protection period, although this has yet to be seen in humans. Overall, this paper demonstrates that a modified, mRNA-based COVID-19 vaccine is successful at mounting the desired immunological responses in the appropriate tissues in a preclinical rodent model.
Although it is an important piece in the COVID-19 scientific puzzle, this paper has limits and leaves some questions unanswered. First, these experiments were conducted with a limited time course, so longer term information regarding a single vaccine dose and what the ideal dosing schedule is (timing and number of doses) remains to be understood. Another important determination that has yet to be made is what the minimal effective dose is, and extrapolating that to a potential human minimal effective dose.
The most significant limitation of this study was its inability to challenge its model with COVID-19 to test the vaccine’s protective effects. Unfortunately, the model rodents used in this study, BALB/c mice, are not sensitive to COVID-19 infection, and so are an irrelevant system in addressing vaccine protectiveness. Effective models were not available when this work began, but now include the generation of a transgenic human ACE2-expressing mouse (the receptor through which COVID-19 enters cells), or an intranasal delivery of human ACE2 to wild type mice prior to exposure to SARS-CoV-2. Protective studies with these models are ongoing, and the scientific community looks forward to reading the results.
Two current human vaccine candidates are most relevant to this study. The first is Moderna Therapeutics’ mRNA-1273 vaccine, which is also a modified mRNA-LNP. mRNA-1273 encodes the S-2P antigen, including the SARS-CoV-2 spike glycoprotein that this paper studied. Similar to the Δfurin vaccine used here, Moderna’s vaccine includes two proline substitutions to stabilize the vaccine protein in the pre-fusion structure. Both the Δfurin and mRNA-1273 vaccines include the transmembrane domain of the spike glycoprotein.
The initial Phase 1 clinical trial of mRNA-1273 included dosages of 25, 100, or 250ug of mRNA-LNP given in two injections 28 days apart. Just as in the Laczkó et. al preclinical data, Moderna reported robust Th1 T cell responses. However, Moderna did not see any neutralizing antibody responses after a single dose, but saw 100% response after a second dose. This suggests that with their vaccine in humans, two doses will likely be necessary for viral protection. Notably, mRNA-1273 also showed dose-dependent Th1 T cell responses and neutralizing antibodies in non-human primates, mimicking the immunological responses seen so far in humans.
The second modified mRNA COVID-19 vaccine relevant to this paper is being produced by Pfizer/BioNTech. The lead Pfizer vaccine candidate, BNT162b2, encodes the receptor binding domain of the spike glycoprotein of SARS-CoV-2. Phase 1/2 clinical trial data of another similar lead candidate, BNT162b1, described a dosing schedule of two doses of 10 or 30ug 21 days apart, or a single 100ug dose. Similar to data shown in the Moderna and Lackzó paper, the Pfizer vaccines showed dose-dependent immune responses and adverse effects. A pre-print publication demonstrated that BNT162b2 generated Th1 T cell responses and neutralizing antibodies in preclinical mouse and non-human primate models. As of September 2020, both BNT162b2 and mRNA-1273 are currently in Phase 3 clinical trials.
Generally, this paper demonstrates the promise and potential of a modified mRNA-LNP vaccine design against SARS-CoV-2 in a preclinical rodent model. It is important to keep in mind that the durability of response to these mRNA-LNP vaccines for COVID-19 is still being investigated. Overall, the data show that there may be a bright light at the end of the pandemic tunnel. COVID-19 presents an opportunity for the first mRNA vaccine to be approved in the United States, and it comes at a critical time in our epidemiological history.