Vaccine Development Strategies to SARS-CoV-2
Vaccine development is protracted and risky. On average, vaccines take more than 10 years to move from the preclinical phase to approval, and the probability of market entry for any given candidate has been just 6%. With 140 COVID-19 vaccines currently in development, 13 of which are already in human trials, it is extremely likely then that multiple candidates will eventually be approved, but the question is, how long will this take? Four basic vaccine strategies are being employed across these 140. Each approach has its strengths and weaknesses to activate the immune response to attack the SARS-CoV-2 virus when presented. These approaches are:
- injection of whole virus (inactivated or attenuated)
- viral vectors (non-replicating or weakened)
- nucleic acids (DNA or mRNA)
- protein (with adjuvants or virus-like particles).
SHOTGUN APPROACH TO VACCINE DEVELOPMENT
Because of the urgent nature of our situation with the current COVID-19 pandemic, global medical science has adopted a shotgun approach to the quick identification of a method for producing a safe and effective vaccine against the disease. Of the current 140 vaccine development projects ongoing worldwide, over 70% are from industrial or private firms, most with a depth of experience in vaccines. Many of the approaches are tried and true, having been used in familiar and successful vaccines. Others involve immunogenic strategies that have never previously succeeded in receiving regulatory approval.
Salient characteristics and some of the underlying biology for the various approaches are discussed in the full blog post.
MarinBio is a biotech services company with a long and distinguished track record of success. MarinBio maintains a strong focus on providing clients with cell based pre-clinical and clinical bioassay data and drug release potency assays for client pharmaceutical, biotechnology, diagnostic, and medical device companies, as well as providing laboratory assistance for legal firms.
STRATEGIES FOR PROVOKING AN IMMUNE RESPONSE TO SARS-CoV-2
1. WHOLE VIRUS
There are two approaches being tried for using the SARS-CoV-2 virus itself as a vaccine. One way is to inactivate the virus via chemical treatment, e.g., with formaldehyde, such that the virus is no longer capable of producing an infection but still provokes an immune response from the body. This is a “killed” virus. Successful examples of this approach include vaccines for polio, hepatitis A, influenza (injected), and rabies.
An alternative method for using viruses as vaccines entails inducing mutations that weaken the virus to make it less infectious, i.e., less able to replicate inside cells. Such a virus is still alive but weakened (attenuated). Examples of attenuated virus vaccines are those against measles, mumps, chicken pox, and influenza (intranasal delivery). The traditional method of attenuation, cell culture adaptation, involves adapting the virus to grow in specialized cells grown in the lab rather than its normal host cells. However, Codagenix (US) is directly editing the RNA genome of SARS-CoV-2 to accomplish attenuation in its whole-virus vaccine.
Sophisticated cell culture is a specialty of MarinBio. This expertise forms the basis of another forte of the company – the creation of custom cell-based assays. Contact MarinBio for a free consultation on how our experience and expertise can assist your projects.
2. VIRAL VECTORS
In a second category of using viruses as vaccines, a virus that is not SARS-CoV-2 is genetically engineered to not only be less infectious but also to act as a vector by coding for a SARS-CoV-2 protein (usually the spike protein). There are also two approaches within this category. In the first, a non-replicating viral vector, e.g., adenovirus is used. Adenoviruses are commonly used to deliver gene therapy but have never received approval as vaccines.
In the other approach, an attenuated measles virus that is still capable of some replication is used to deliver SARS-CoV-2
genes. Due to even limited replication, immune responses can be much stronger here than with non-replicating viral vectors, although pre-existing immunity to the measles virus can have a counter effect. The recently approved Ebola virus vaccine uses an attenuated measles virus as a vector.
3. NUCLEIC ACIDS
Much excitement has been generated concerning vaccines consisting of nucleic acids (DNA or RNA) encoding viral genes because of their many potential advantages. For one, it is much safer (and easier) to handle only the genetic material rather than the whole virus. Another advantage is that designing antigens at the nucleic acid level promotes rapid production and testing of antigens for quicker transition of development candidates into clinical trials. There are also indications that proteins translated from DNA/RNA within human cells can provoke more powerful immune responses than traditional methods. And if such a vaccine is approved for use globally, mega-quantities of this type can likely be produced more quickly than conventional vaccines. While nucleic acid vaccines have never been approved for use in humans, DNA vaccines are being used for canine melanoma and against West Nile virus in horses, and trials of mRNA vaccines are being conducted in humans for the viruses that cause Zika and influenza.
For DNA vaccines, circular (plasmid) DNA makes its way to the nucleus after being delivered into cells. There it is transcribed into mRNA that then translocates to the cytoplasm for access to the cellular translational apparatus. RNA vaccines transport linear RNA across cell membranes into the cytoplasm where it moves directly to ribosomes for translation and then to the endoplasmic reticulum and Golgi apparatus for post-translational modifications. In either case, the nucleic acids typically code for the spike protein of SARS-CoV-2 expressed on the transfected cells whereupon the immune system becomes primed to mount an attack should it encounter the actual virus.
Two of the top nucleic acid COVID-19 vaccine prospects are delivered using transfection technologies that will be familiar to many life scientists. In the case of Inovio’s (US) DNA vaccine, a proprietary electroporation device is applied to the skin after injection to assist entry of the plasmid into cells. Moderna’s (US) RNA vaccine, on the other hand, uses lipid nanoparticles to transport the highly polar RNA molecules across the cell’s lipid bilayer.
MarinBio is proficient in performing nucleic acid transfections using biochemical, biological, or biophysical methods. These transfections can result in either transient or stable cell modifications, depending on the requirements of the project.
Nucleic acid vaccines use cells in the human body to produce immunogenic viral proteins that trigger a protective response. However, another approach being employed by over two dozen of the worldwide COVID-19 vaccine candidates is the production of recombinant proteins, employing different versions of SARS-CoV-2 protein or parts of those proteins for direct injection into humans. Typically, researchers are using the spike protein or its receptor binding domain (RBD), the portion that recognizes and binds to the human angiotensin-converting enzyme 2 (ACE2) cellular receptor for entry into the cell to replicate. A common tactic with viral protein injection is the addition of an adjuvant to boost the immune response. Adjuvants are compounds chemically unrelated to the viral antigen that act by putting the immune system in a state of heightened alert to enhance its reaction to the viral protein. Some of the vaccines for shingles and hepatitis B are in this category.
MarinBio’s professionals have years of experience with the production, purification, and characterization of recombinant proteins. Our scientists are experienced in a wide variety of expression systems, including but not limited to the use of bacterial, yeast, baculovirus-insect, and mammalian cell lines for producing recombinant proteins. Our scientists have extensive experience in purifying these proteins.
Another protein-based approach is to create virus-like particles (VLPs). These are virus shells densely decorated with multiple copies of a viral protein subunit, but they are empty of genetic material. In this way they mimic the native structure of SARS-CoV-2 but cannot replicate. As with injected viral proteins, VLPs are not infectious, but VLPs can provide the immune system a more accurate sense of how the actual virus will present itself. VLPs are trickier to manufacture but can provide a stronger immune response. The human papilloma virus (HPV) vaccine is an example of a successful, approved VLP vaccine.
An interesting aspect of the immune response to VLPs is that it appears to include neutralizing antibody titers that are maintained consistently for years. Because immunity conferred via antibodies produced by memory B cells involves those B cells waking and expanding when the remembered antigen is again encountered, the consistent production of antibodies after VLP vaccination is thought to indicate that VLPs may lead instead to the production of another class of B cells known as long-lived plasma cells (LLPCs). These LLPCs continuously produce antigen-specific antibodies from their locations in the bone marrow. A Nobel prize won in 1996 included research that found dense, highly repetitive proteins on the outside of viruses, similar to the design of VLPs, trigger the strongest immune responses. The mechanism is still unknown, but may involve the density of antigens enabling the cross-linking of multiple B cell surface receptors.
MarinBio has extensive experience in developing and validating cell based neutralizing antibody immunoassays. Call MarinBio scientists with your next project 415-883-8000 or contact us via our website marinbio.com.