Non-replicating vaccines are based on recombinant viral vectors that are made replication non-competent, meaning that these vectors are sufficient to induce host immune responses but cannot replicate inside host cells.
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In addition, RNA-based vaccines that involve mRNA sequences for coding pathogen-specific proteins (antigens) inside host cells can be generated using non-replicating mRNAs.
What are non-replicating vaccines?
In non-replicating vaccines, immunogens that are used to induce pathogen-specific host immune responses include killed pathogens, purified or synthetic pathogen structures, or recombinant pathogen products as antigens.
There are several types of non-replicating vaccines, including live-attenuated vaccines, inactivated vaccines, subunit, and conjugated vaccines, and RNA vaccines.
In live-attenuated vaccines, pathogens that infect humans are passaged serially in vivo (animal embryos) or in vitro (cell culture). For example, in measles vaccines, live measles virus undergoes a series of passages through chick embryos; and with every passage, the virus becomes more potent in replicating in chick cells and loses its replicability in human cells.
Although the replication ability of live-attenuated pathogens is mostly restricted in human cells, there is still a rare risk of a certain level of replication, which can make the pathogen more virulent through genetic mutations.
In inactivated vaccines, the target pathogen is killed or inactivated using heat or chemical (formalin) treatments. The treatments make the pathogen replication-defective; however, the pathogen remains intact and can be recognized by the human immune system to trigger desired responses. Because of the complete loss of replication ability, this type of vaccine is sometimes safer than live-attenuated vaccines.
However, to get the desired protection for a prolonged period, booster doses of inactivated vaccines may be required.
In subunit vaccines, a specific product of the target pathogen is used to induce immune responses. In general, pathogen products that are used in subunit vaccines include purified or recombinant proteins inactivated bacterial toxins, or polysaccharides present on the surface capsule.
In addition, a genetically engineered transgene coding for a vaccine antigen can be inserted into another virus or bacterium, which are used as vectors to transfer the vaccine antigen into the host cell and trigger pathogen-specific immune responses. These types of vaccines are called recombinant vaccines.
Conjugate vaccines are made by conjugating a polysaccharide residue from the bacterial surface capsule with a carrier protein, such as diphtheria toxoid protein. Mechanistically, the bacterial portion used in the vaccines does not generate robust immune responses alone; however, the combination of a bacterial portion and a carrier protein induces a powerful, combined immune response.
In RNA vaccines, mRNA is used as a template to endogenously synthesize the pathogenic protein (antigen) of interest. Non-replicating mRNA vaccines contain 3’ untranslated region (UTR), the target antigen sequence, and 5’ UTR. Such vaccines do not include additional protein sequences to induce self-amplification of the inserted mRNA sequence.
What are non-replicating viral vectors?
Several viruses, including adenovirus, adeno-associated virus, measles virus, and human parainfluenza virus, are widely used as viral vectors. Viral vectors that are genetically modified to make replication-defective are called non-replicating vectors. Eventually, the virus gains an attenuated state wherein it can still be able to trigger the desired human immune responses, but cannot replicate in human cells.
Adenoviruses are frequently used as viral vectors to produce both replication-competent and replication-defective vaccines. In humans, 57 adenovirus serotypes have been identified, which are further divided into 7 subgroups (A – G). the majority of adenovirus-based vaccines are prepared using adenovirus serotype 5 (Ad5).
Adenovirus-based vaccines are capable of inducing both antibody-mediated and T cell-mediated immune responses; however, the intensity of response depends on the virus serotype used to produce the vaccine.
To generate replication-defective vectors, two early genes of adenovirus, E1A, and E1B, are replaced by the transgene (inserted antigen of target pathogen) expression cassette, which prevents the virus from replicating inside the host cell. Also, host cell-mediated elimination of adenovirus-infected cells and instability of the transgenes are prevented by genetically depleting E3 and E4 genes of adenovirus vectors, respectively.
Immune response to non-replicating vaccines
In the case of replication-defective vaccines, each viral particle used as a vector is capable of infecting only a single cell and can utilize the transgene it carries only once to induce host cell immune responses.
Although more than a billion copies of viral vectors are used in a single dose of vaccine, the inability to replicate can significantly reduce the pathogen-specific immune response. To achieve a strong and sustained immune response, more vaccine doses are required, and thus, it takes longer to get the desired protection.
However, non-replicating vaccines are very safe as there is a very low risk of vaccine antigen-induced disease onset. Moreover, non-replicating adenovirus-based vaccines have been shown to induce strong CD8+ T cell-mediated as well as antibody-mediated immune responses.