I looked at the statistics of the blog and it turned out that I had already “ faked ” more than 200 posts. Many of them are on the topic of the day. But there are also less momentary, “general education” ones. Meanwhile, the audience of the blog is expanding and not all new readers have the time and desire to study the “complete collection of my LJ works.” And I thought that it makes sense to make a ” remake ” of some posts, topics that do not have a rigid timeline. And since the vaccine topic is now dominant, it makes sense to start with vaccine platforms. So, the “Adenovirus” platform.
Briefly about adenoviruses
Meet – in the figure below the adenovirus, real viral particles (scientifically – virions) and their model. They’re “photogenic” aren’t they? At least for my taste, adenoviruses are the most “cute” among the virions of various viruses in humans and animals. By the way, all adenoviruses look the same under an electron microscope.
Pay attention to the highly ordered, symmetrical shape of the virions (this is a perfect polyhedron with 20 triangular faces – an icosahedron). From each vertex of the polyhedron (there are 12 of them in total) a long process, similar to a drumstick, “sticks out” (it is called a ” fiber “). It is the fiber that is the “anchor” by which adenoviruses “moor” to the cell by binding to the receptor. Cell receptors are different for different adenoviruses. Inside the virion there is genomic DNA – in adenovirus it is linear, double-stranded . Some readers may wonder – why mention that the DNA of adenovirus is double-stranded ? Everyone knows that DNA is a “double helix” – it turns out like “butter”. The fact is that there are many oddities in the realm of viruses. In particular, DNA here is single-stranded (in parvoviruses ), and RNA is double-stranded (in rotaviruses ). I will not go into details.
In terms of the size of the virions and genome, “average” adenoviruses are not small, but not very large (genomic DNA length is 30-35 thousand base pairs). In the genome of adenoviruses (as, indeed, of other viruses), two types of proteins are encoded – structural (those that make up the virion) and non-structural (those that are necessary for the replication of the virus, but are not part of the virions). In order to turn an ordinary adenovirus into a vector for the delivery of certain genes, a genetically engineered “trick” is performed with it. Some genes for non-structural proteins are deleted. And in their place, you can insert an “insert” – a gene that a modified virus (called a vector) can deliver into cells. But how to get a “harvest” of modified virions if the genes of non-structural proteins necessary for viral replication are removed from them? This requires special cell cultures capable of compensating for the defect in the viral genome. This allows you to get a “harvest” of adenoviral virions carrying the insert. But in ordinary cells, these virions cannot reproduce. In appearance, “man-made” virions are no different from the natural adenovirus. They are able to moor on the same cells as the virus from which they are “made”, penetrate into cells and trigger the synthesis of viral proteins, both their own and the protein encoded by the insert . But such adenoviral vectors are not capable of forming a new “generation” of virions . Therefore, they are called non-replicating . The fact is that for reproduction inside ordinary cells, the virus needs those non-structural proteins, the genes of which have been removed from the vector. The immune system recognizes all viral proteins, including the protein encoded by the insert, and reacts to them with different types of immune responses . In a sense, the immunogenicity of such a vaccine (the response to the protein encoded by the insert) is a “side effect” of the immune response to the adenovirus.
Adenoviral vectors and gene therapy
The first adenoviral vectors for gene delivery were made long ago – in the late 1980s and early 1990s. Initially, non-replicating viral vectors were tried to be used for gene therapy for monogenic hereditary diseases (this is when a disease is caused by a defect in a single gene and an attempt is made to “transplant” a normal copy of this gene). These studies reached the level of clinical trials, but in 1999. a tragedy happened – a child died who received gene tepapia with a non-replicating adenoviral vector (with a “correct” copy of the gene that was defective in the child ). Unfortunately, the investigation left no doubt that the vector was to blame – very high doses of the virus were required for gene therapy, and the inflammatory response to such an overload turned out to be uncontrollable. Another problem has emerged related to antibodies against human adenoviruses (more on this below). In general, the gene therapy “career” of adenoviral vectors did not work out.
Adenoviral vectors as a platform for vaccines
In the field of vaccine development, non-replicating adenoviral vectors have found a “second wind”. Over the past 20 years, hundreds, if not thousands, of papers have been published describing experimental vaccines based on non – replicating adenoviral vectors.
Vaccines on adenovirus platforms have many advantages. They mimic natural infections and trigger all types of immune responses ( antibody , T-cell, congenital). These vaccines do not require an adjuvant (additive that increases immunogenicity). Non-injection options for administration are possible (aerosol, food capsules). There are no dangers associated with the “unauthorized residence” of the vaccine vector – a non – replicating adenoviral vector does not multiply in the cells of a vaccinated person and its DNA is not able to integrate into the viral genome (the latter is typical for widely used retro- and lenti -viral vectors).
The absence of the need for an adjuvant is due to the fact that vaccines on adenoviral vectors induce a local inflammatory response. It is usually short and controlled. But more serious side effects can develop (the higher the dose of the vaccine, the more often). The rapidity of the quasi- infectious process caused by the vaccine affects the intensity and especially the duration of the immune responses to the vaccine. To achieve the required level of immunity and to maintain it, ” boosts ” (repeated injections of the vaccine) are required . A side effect of repeated repeated vaccinations is the development of an immune response to the adenoviral components of the vector themselves. A vicious circle arises – a drop in the level of antibodies to a specific antigen (encoded by an insert) requires a boost , and each repeated immunization stimulates an antibody response (neutralizing antibodies) against the adenoviral antigens themselves. A common problem with adenoviral vectors (based on human adenoviruses) is that many adenoviruses (about 70 types) circulate in human populations, and many people have neutralizing antibodies against some of them. If a person who has such antibodies is injected with a vaccine based on human adenovirus, then part of the administered dose is neutralized. Because of this, the immunogenicity of the vaccine is significantly reduced. It so happened that the first non-replicating adenoviral vector was made on the basis of human adenovirus type 5 (Ad-5). And this type of adenoviral vector has become the most “popular” – the vast majority of research in this area has used this vector. Unfortunately, it turned out that antibodies to human adenovirus type 5 are present in many people (up to 50% of the population). The problem has been “highlighted” especially in the largest clinical trials of an HIV vaccine made on this platform. High hopes were pinned on this American experimental vaccine. The vaccine proved to be ineffective (in terms of protection against HIV). Moreover, it turned out that in those subjects who already had antibodies against adenovirus type 5 before vaccination, their sensitivity to HIV was even slightly increased. As an alternative to “vectors Ad-5”, similar vectors were made based on human adenovirus type 26 (Ad-26), antibodies to which are much less common. They are trying to use a more radical method – non-replicating adenoviral vectors have been made based on the viruses of the great apes – chimpanzees and gorillas. These viruses easily infect human cells, and humans do not have antibodies to them. The latter, however, needs clarification. The fact is that, at least among Africans in sub-Saharan countries, such antibodies are found. This has been known for a long time, but this issue has not yet been systematically studied.