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BERLIN — To date, mRNA vaccines have had their largest global presence in combating the COVID-19 pandemic. Intensive research is underway on many other potential applications for this vaccine technology, which suggests a promising future. Martina Prelog, MD, a pediatric and adolescent medicine specialist at the University Hospital of Würzburg in Germany, reported on the principles, research status, and perspectives for these vaccines at the 25th Travel and Health Forum of the Center for Travel Medicine in Berlin.
To understand the future, the immunologist first examined the past. "The induction of cellular and humoral immune responses by externally injected mRNA was discovered in the 1990s," she said.
Instability Challenge
Significant hurdles in mRNA vaccinations included the instability of mRNA and the immune system's ability to identify foreign mRNA as a threat and destroy mRNA fragments. "The breakthrough toward vaccination came through Dr Katalin Karikó, who, along with Dr Drew Weissman, both of the University of Pennsylvania School of Medicine, discovered in 2005 that modifications of mRNA (replacing the nucleoside uridine with pseudouridine) enable better stability of mRNA, reduced immunogenicity, and higher translational capacity at the ribosomes," said Prelog.
With this discovery, the two researchers paved the way for the development of mRNA vaccines against COVID-19 and other diseases. They were awarded the Nobel Prize in medicine for their discovery last year.
Improved Scalability
"Since 2009, mRNA vaccines have been studied as a treatment option for cancer," said Prelog. "Since 2012, they have been studied for the influenza virus and respiratory syncytial virus [RSV]." Consequently, several mRNA vaccines are currently in development or in approval studies. "The mRNA technology offers the advantage of quickly and flexibly responding to new variants of pathogens and the ability to scale up production when there is high demand for a particular vaccine."
Different forms and designations of mRNA vaccines are used, depending on the application and desired effect, said Prelog.
In nucleoside-modified mRNA vaccines, modifications in the mRNA sequence enable the mRNA to remain in the body longer and to induce protein synthesis more effectively.
Lipid nanoparticle (LNP)–encapsulated mRNA vaccines protect the coding mRNA sequences against degradation by the body's enzymes and facilitate the uptake of mRNA into cells, where it then triggers the production of the desired protein. In addition, LNPs are involved in cell stimulation and support the self-adjuvant effect of mRNA vaccines, thus eliminating the need for adjuvants.
Self-amplifying mRNA vaccines include a special mRNA that replicates itself in the cell and contains a sequence for RNA replicase, in addition to the coding sequence for the protein. This composition enables increased production of the target protein without the need for a high amount of external mRNA administration. Such vaccines could trigger a longer and stronger immune response because the immune system has more time to interact with the protein.
Cancer Immunotherapy
Prelog also discussed personalized vaccines for cancer immunotherapy. Personalized mRNA vaccines are tailored to the patient's genetic characteristics and antigens. They could be used in cancer immunotherapy to activate the immune system selectively against tumor cells.
Multivalent mRNA vaccines contain mRNA that codes for multiple antigens rather than just one protein to generate an immune response. These vaccines could be particularly useful in fighting pathogens with variable or changing surface structures or in eliciting protection against multiple pathogens simultaneously.
The technology of mRNA-encoded antibodies involves introducing mRNA into the cell, which creates light and heavy chains of antibodies. This step leads to the formation of antibodies targeted against toxins (eg, diphtheria and tetanus), animal venoms, infectious agents, or tumor cells.
Genetic Engineering
Prelog also reviewed genetic engineering techniques. In regenerative therapy or protein replacement therapy, skin fibroblasts or other cells are transfected with mRNA to enable conversion into induced pluripotent stem cells. This approach avoids the risk for DNA integration into the genome and associated mutation risks.
Another approach is making post-transcriptional modifications through RNA interference. For example, RNA structures can be used to inhibit the translation of disease-causing proteins. This technique is currently being tested against HIV and tumors such as melanoma.
In addition, mRNA technologies can be combined with CRISPR/Cas9 technology ("gene scissors") to influence the creation of gene products even more precisely. The advantage of this technique is that mRNA is only transiently expressed, thus preventing unwanted side effects. Furthermore, mRNA is translated directly in the cytoplasm, leading to a faster initiation of gene editing.
Of the numerous ongoing clinical mRNA vaccine studies, around 70% focus on infections, about 12% on cancer, and the rest on autoimmune diseases and neurodegenerative disorders, said Prelog.
Research in Infections
Research in the fields of infectious diseases and oncology is the most advanced: mRNA vaccines against influenza and RSV are already in advanced clinical trials, Prelog told Medscape Medical News.
"Conventional influenza vaccines contain immunogenic surface molecules against hemagglutinin and neuraminidase in various combinations of influenza strains A and B and are produced in egg or cell cultures," she said. "This is a time-consuming manufacturing process that takes months and, particularly with the egg-based process, bears the risk of changing the vaccine strain."
"Additionally, influenza viruses undergo antigenic shift and drift through recombination, thus requiring annual adjustments to the vaccines. Thus, these influenza vaccines often lose accuracy in targeting circulating seasonal influenza strains."
Several mRNA vaccines being tested contain not only coding sequences against hemagglutinin and neuraminidase but also for structural proteins of influenza viruses. "These are more conserved and mutate less easily, meaning they could serve as the basis for universal pandemic influenza vaccines," said Prelog.
An advantage of mRNA vaccines, she added, is the strong cellular immune response that they elicit. This response is intended to provide additional protection alongside specific antibodies. An mRNA vaccine with coding sequences for the pre-fusion protein of RSV is in phase 3 trials for approval for vaccination in patients aged 60 years and older. It shows high effectiveness even in older patients and those with comorbidities.
Elaborate Purification Process
Bacterial origin plasmid DNA is used to produce mRNA vaccines. The mRNA vaccines for COVID-19 raised concerns that production-related DNA residues could pose a safety risk and cause autoimmune diseases.
These vaccines "typically undergo a very elaborate purification process," said Prelog. "This involves enzymatic digestion with DNase to fragment and deplete plasmid DNA, followed by purification using chromatography columns, so that no safety-relevant DNA fragments should remain afterward."
Thus, the Paul-Ehrlich-Institut also pointed out the very small, fragmented plasmid DNA residues of bacterial origin in mRNA COVID-19 vaccines pose no risk, unlike residual DNA from animal cell culture might pose in other vaccines.
Prevention and Therapy
In addition to the numerous advantages of mRNA vaccines (such as rapid adaptability to new or mutated pathogens, scalability, rapid production capability, self-adjuvant effect, strong induction of cellular immune responses, and safety), there are also challenges in RNA technology as a preventive and therapeutic measure, according to Prelog.
"Stability and storability, as well as the costs of new vaccine developments, play a role, as do the long-term effects regarding the persistence of antibody and cellular responses," she said. The COVID-19 mRNA vaccines, for example, showed a well-maintained cellular immune response despite a tendency toward a rapid decline in humoral immune response.
"The experience with COVID-19 mRNA vaccines and the new vaccine developments based on mRNA technology give hope for an efficient and safe preventive and therapeutic use, particularly in the fields of infectious diseases and oncology," Prelog concluded.
This story was translated from the Medscape German edition using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.
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