Molecular genetic technologies of protection against pathogens
D.Y.Logunov, PhD, B.S.Naroditsky, MD, Prof., A.L.Ginzburg, Academician of the Russian Academy of Sciences, Vice President of the Russian Academy of Sciences,
N.F.Gamalei State Research Institute of Epidemiology and Microbiology of the Russian Academy of Medical Sciences
Remedium Magazine No. 2-2008
According to WHO, about 11 million (19%) of the 57 million people who died in 2002 died from infectious diseases [WHO, 2004]. Such an impressive scale of the impact of pathogens on the human population is explained by at least two reasons. The overpopulation of territories and the development of transport communications create favorable conditions for the rapid spread of particularly dangerous infections not only within one or several neighboring states, but also in geographically remote territories.
The world community is constantly waiting for the emergence of new epidemics or even pandemics (an example is the powerful public outcry associated with outbreaks of infections such as SARS, avian flu, anthrax, ebola, etc.), and therefore the development of modern means of protection against pathogens is one of its most important tasks.
One of the most reliable ways to protect the population from pathogens is vaccination. To date, vaccines have been created against 34 socially significant infections. The use of these vaccines in medical practice has led to a decrease in the incidence of diphtheria, measles, tetanus, tularemia, polio and caused the disappearance of such a dangerous infection as smallpox. However, despite the obvious success of vaccine prophylaxis, only 34 vaccines have been created, while effective preventive measures have not yet been created for the remaining more than 400 known human pathogens.
As is known, the effectiveness of vaccination is associated with the induction of a protective humoral and/or cellular immune response, which in turn is determined by the peculiarities of the structure of the antigens of the pathogens against which vaccination occurs, as well as the nature of the interaction of the microorganism with the innate immunity system. Currently, vaccines against pathogens that cause acute infections are being successfully used. No effective preventive vaccines have been created for latent or chronic diseases caused by pathogens such as HIV, human herpes virus, hepatitis C virus, Mycobacterium tuberculosis, etc., as well as for diseases caused by microorganisms characterized by intraspecific variability (serological variants, antigenic drift or shift, change of variants of specific antigens). It is obvious that in order to obtain preventive drugs against these infections, a detailed understanding of the biology and pathogenesis of each specific pathogen and the development of individual approaches to the creation of preventive and therapeutic drugs is necessary.
Advances in such fields of knowledge as molecular biology, immunology and microbiology have contributed to the development of new and promising areas of protection against pathogens. The following three areas deserve special attention: 1) genetically engineered preventive vaccines and therapeutic means of protection against pathogens, 2) immunomodulators capable of activating the immune system through the activation of Toll-like receptors, 3) means of passive immunization (humanized monoclonal antibodies).
Genetically engineered vaccines are divided into 2 types: subunit (protective antigens of various pathogens expressed in yeast or E.coli) and genetic. "Genetic" vaccines, in turn, are divided into DNA vaccines and vaccines based on viral and bacterial vectors (less often plant cells are used as an expression system). Unlike most traditional inactivated vaccines, "genetic" vaccines are capable of inducing a cellular and humoral immune response, and can also be used not only for preventive, but also for therapeutic purposes for the treatment of certain autoimmune diseases, allergic conditions, and malignant neoplasms.
The experience of researchers from various countries with candidate genetically engineered vaccines shows that the greatest protective effect is observed with a combined method of immunization (prime-boost immunization). The first stage is the priming of the immune response (induction of cellular immunity), the second stage is the boosting of the immune response (induction of the humoral immune response). At the same time, various combinations of prime-boost agents are possible (DNA vaccine - recombinant protein, DNA vaccine - recombinant adenovirus, recombinant adenovirus - recombinant protein, etc.). Activation of a strong immune response when using the prime-boost immunization system with candidate genetically engineered vaccines has been shown in relation to pathogens causing tuberculosis, herpes, malaria, AIDS, etc.
Currently, dozens of genetically engineered vaccines are in various phases of clinical trials in the USA and European countries. Two of them are in phase 3 clinical trials (vaccines against HIV and papilloma). Three genetically engineered vaccines (against hepatitis B virus, borreliosis and rabies virus) are used in medical practice for vaccination. Such a small number of approved and licensed genetically engineered vaccines is quite simple to explain. The history of the creation of genetically engineered vaccines has about 20 years. Given that the preclinical and clinical trials of new vaccines take more than 10 years, it is obvious that at the moment many genetically engineered vaccines are at the stage of testing or approval. In this regard, in the next 5-10 years, it is expected to introduce a number of licensed genetically engineered preventive vaccines into medical practice. As a fresh example, the recent approval by the FDA (Food and Drugs Administration) committee of a new genetically engineered vaccine against herpes virus type 2.
Despite the fact that many genetically engineered vaccines undergoing trials are undoubtedly promising candidate means of disease prevention, additional approaches to their improvement are currently emerging. These approaches are based on the results of fundamental research revealing the mechanisms of activation of innate immunity and its influence on the development of adaptive immune response. It has been established that a key role in the activation of innate immunity is played by Toll-like receptors localized on various immunocompetent cells and recognizing evolutionarily conservative pathogen-associated molecular structures (PAMS). After binding to PAMS, Toll-like receptors transmit an intracellular signal to activate the synthesis of cytokines and co-stimulating factors. Various Toll-like receptors activate specific combinations of cytokines and co-stimulating factors, which ultimately determines the type and effectiveness of the developing acquired immune response. Understanding the mechanisms of functioning of the innate immune system makes it possible to carry out a "rational design" of vaccines based on the combination of signals of activation of innate (PAMS) and acquired immunity (antigen). The possibility of activating the innate immunity system with various combinations of PAMS additionally opens the way for the creation of means of rapid nonspecific protection against unknown pathogens, including in the case of acts of bioterrorism. In the near future, in the research programs of Western companies, it is planned to introduce PAMS into the structure (conjugation with a vaccine or composition with it) of genetically engineered vaccines.
It is clear that vaccination, when it is possible, is the most effective way to protect against pathogens. However, in medical practice, there are often situations associated with unexpected outbreaks of infections, in which drugs are required to immediately block the spread of pathogens and their toxins in the body. Similar drugs may be required in cases of possible acts of bioterrorism. Pathogen-specific antibodies have the properties necessary for emergency protection against pathogens. Their use as a therapeutic means of protection against pathogens (passive immunization) has been known for a long time, but currently it is used only for vital indications, due to the fact that a specific blood serum (usually horses) is injected into the human body, which can lead to the development of serum sickness. A fundamental solution to the problem of creating effective and safe protective antibodies became possible after the development of technology for the production of recombinant humanized monoclonal antibodies. This technology is based on the methods of genetic engineering and nanobiotechnology. Using these methods, a number of bacterial producers of single-stranded humanized monoclonal antibodies have now been created. It has been shown that such humanized nanoantibodies are effective in the treatment of various types of tumors and in blocking the spread of pathogens in the human body. A large number of new humanized antibodies for the treatment of tumor and infectious diseases are expected to appear in the near future. Additionally, humanized antibodies are being developed - means of emergency protection against bioterrorism. In particular, a whole range of humanized antibodies against pathogens of particularly dangerous infections has been obtained in the world. Recently, humanized antibodies have been obtained that effectively block the development of infection caused by anthrax bacillus. These antibodies were taken into "service" by the German army.
In conclusion, it should be noted that the approaches developed to protect against pathogens (genetically engineered vaccines and humanized antibodies) are not absolutely universal. It would be a mistake to believe that the introduction of these technologies into medical practice will solve all the problems associated with bacterial and viral pathogens. At the same time, it is impossible to remain on the positions and approaches of traditional vaccination. It is already clear today that the use of advanced technologies makes it possible to solve specific and important problems related to the development of prophylactic agents against particularly dangerous (anthrax bacillus, rabies virus) and poorly cultivated pathogens (hapatitis B virus, herpes virus type 2). According to the data of ongoing clinical studies, it is assumed that within 3-5 years the production of the first drugs for human genetic immunization against rabies virus, papilloma virus is expected, in 6-8 years - vaccines against malaria and tuberculosis (recombinant BCG). In the next 1-2 years, preparations of humanized antibodies against such particularly dangerous human pathogens as tularemia, plague and brucellosis will be created.
Various strategies for obtaining genetically engineered vaccines are based on the use of cloning techniques of genes responsible for the synthesis of various pathogen proteins into plasmid vectors that ensure the implementation of the necessary genetic information in the producing microorganism (the target protein isolated from the producer is later used for vaccination) or in the vaccinated organism (less often plant-based cells).
Portal "Eternal youth" www.vechnayamolodost.ru21.11.2008