Live vectors
Live recombinant vector vaccines are constructed by inserting HIV or SIV genes into genomes of live, infectious, but non-disease-causing forms of viruses or bacteria such as vaccinia virus or Bacille Calmette-Guerin (BCG). One can think of these viral and bacterial vectors as backpacks that shuttle the cargo of their own genes as well as “foreign” genes into cells.
Viral Vectors
Mechanism of Delivery of Antigens
- Viruses have evolved sophisticated structures and mechanisms to infect cells and hence they serve as efficient delivery vectors for the HIV genes.
- Scientists use viruses other than HIV as delivery vehicles or vectors to transport and express HIV genes. The modular nature of viral genomes allows scientists to insert foreign genes and sometimes replace the vector’s own genes with the desired HIV genes. Such a recombinant virus is safe and cannot cause HIV infection.
- HIV proteins generated from the recombinant genes, inside the cell, are either secreted or displayed on the cell surface and presented to the immune system in the same way that proteins from a virus-infected cell would be.
For a virus to be considered a delivery vehicle, it must have the capacity to accommodate large pieces of foreign genes, maintain genetic stability, be feasible for large-scale manufacturing, and most importantly, not cause disease or be toxic. Viruses used as vaccine vectors can be engineered to retain their replication capacity or be rendered non-replicating. In the replication-defective kind, one or more gene(s) that play a role in virus replication are removed or mutated, and the resultant virus will not reproduce itself as it can only infect one cell. Replicating viral vectors infect and reproduce in cells using the cell’s machinery. The new copies of the virus generated can subsequently infect other cells but because it has been manipulated it cannot cause disease. In other words, the virus has been attenuated.
Several replication-incompetent viral vectors have been tested in clinical trials for HIV vaccines, including adeno-associated virus, alphavirus, and measles virus. Adenovirus, vesicular stomatitis virus, herpes virus and poxvirus are available in both non-replicating and replicating forms. Except for early trials with vaccinia vectors, replicating viral vectors have been tested with varying levels of success in preclinical studies in animals. Some vaccine developers are focusing on the use of replicating vectors in clinical trials as they are more fruitful in animal studies in activating elements of innate immunity, the first responders to an invading organism, and more likely to induce stronger cellular and mucosal immune responses, and antibodies. These effects are achieved at much lower doses than the non-replicating vectors.
Viruses induce strong, long-lasting immune responses against the expressed proteins, including antibody and T cell responses in the blood, and many generate immune responses at mucosal surfaces, depending on their cell targets and sites of replication. Importantly, immune responses can be generated to the vector as well as to the incorporated immunogens. Ironically, immune responses to the vector components could limit the effectiveness of subsequent vaccinations using the same vector.
Viral vector are well into development having been used in Phase III trials.
Bacterial vectors
Mechanism of Delivery of Antigens
- Large fragments of HIV genes can be easily and stably inserted into the bacterial chromosome or be carried as extrachromosomal plasmid within the bacteria to the host.
- Bacteria enter cells via endocytosis or phagocytosis. Some bacteria, e.g. Listeria or Shigella, possess mechanisms to lyse the endocytic vesicles and deliver contents into the cytoplasm. While others such as Salmonella and BCG trapped in specialized membrane-bound compartments, known as phagosomes, are engineered to destruct these compartments and release the DNA into the cytoplasm.
- The released DNA is transcribed in the nucleus into messenger RNA, which is then translated into proteins.
- The protein antigens are processed and presented to T and B lymphocytes to elicit immune responses.
Bacterial vectors have many advantages; they are easy to manufacture, have low production and delivery costs, are easy to store and remains stable at room temperature, and can be administered orally. These features make bacterial vectors suitable for mass immunization programs in developing countries. In addition, BCG provides a distinct advantage as an antigen delivery vehicle due to the thick lipid cell-wall that has adjuvant properties, making it an attractive vector for the induction of both T and B cell responses. Unlike viral vectors, bacterial vectors have the additional safety feature of being readily treated with antibiotics. Many of these bacterial carriers such as Listeria, Salmonella, Shigella infect professional antigen presenting cells (APC), such as dendritic cells and macrophages, thus enabling the vaccine antigens to target the key cells involved in priming naive T cells. The type of immune responses required for protection dictate vector choice. For example, Listeria elicits strong antigen-specific Th1 driven CD8 T cell responses, making this carrier suitable for vaccination against viral infections in which cytotoxic T cells play a major role. In contrast, BCG and Salmonella induces a mixed Th1/Th2 pattern of responses, thus making this carrier suitable when inducing both CD4 and CD8 T cell responses is necessary.
Bacterial vectors have yet to deliver on their promise of efficient antigen delivery. Some of the reasons for the failure include:
- Mutations to render the bacteria (e.g. Shigella and Salmonella) harmless can result in highly weakened bacteria that die after a couple of replication cycles
- Shortened life-cycle may generate insufficient antigen levels for presentation to the immune system
- Genetic instability may cause the loss of foreign gene- plasmids from the bacterial vectors, that will affect the amount of antigen expressed, and hence reduce the antigen-specific immune responses. Plasmid stabilization strategies have been implemented to neutralize this risk.
Currently only a few bacterial vectors are under development in small and large animal models, and a few Phase I trials. Although bacterial vectors present an attractive approach, these vectors have not yet been widely tested in clinical trials. Lack of animal models and complex nature of bacteria hampers the development of bacterial vector systems.
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