Features of the immune response during infection and prospects for the vaccines creation

Abstract

The influenza virus belongs to the family Orthomyxoviridae and is a major cause of respiratory infections in humans. Each year, influenza viruses cause, according to experts, 3-5 million severe course of the disease and 250 000-500 000 deaths. Influenza A viruses are divided into serotypes based on their surface glycoproteins - known currently 17 subtypes of HA and NA subtypes ten. Upon infection with an influenza virus, both innate and adaptive immune responses are inducing. In recent years the annual seasonal epidemics were causing strains of the virus A (H1N1 and H3N2) and virus B. This may be due to their ability to be unrecognizable virus specific antibodies due to antigenic drift (Figure 1). Seasonal flu vaccine, to be effective, must be updated almost annually, according to new epidemic strains. In this work will discuss various strategies used by influenza  viruses to evade innate immune responses and recognition by components of the humoral and cellular immune response, which consequently may result in reduced clearing of the virus and virus-infected cells.The primary targets for influenza viruses are the epithelial cells that line the respiratory tract and which initiate an antiviral immune response upon detection of the virus. The first line of defense is formed by the innate immune system, which is quick but lacks specificity and memory. Innate immunity is formed by physical barriers and innate cellular immune responses. Here, we outline several of the innate defense mechanisms directed against influenza infections. During homeostasis, alveolar macrophages exhibit a relatively quiescent state, producing only low levels of cytokines, and suppress the induction of innate and adaptive immunity. Activated macrophages enhance their pro-inflammatory cytokine response, including IL-6 and TNF-α. Alveolar macrophages have a direct role in limiting viral spread by phagocytosis of apoptotic infected cells and by phagocyte-mediated opsonophagocytosis of influenza virus particles. They are also involved in regulating the adaptive immune response. The second line of defense against influenza is the adaptive immune response. This highly specific response is relatively slow upon first encounter with a pathogen. The adaptive immune response consists of humoral (virus-specific antibodies) and cellular (virus-specific CD4+ and CD8+ T cells) immunity.  Influenza virus infection induces the production of influenza virus-specific antibodies by B cells. Antibodies directed to the viral HA and NA correlate with protective immunity.Immune pressure on influenza viruses forces them to adopt strategies to evade immunity.Various mechanisms contribute to immune evasion of influenza viruses from the humoral immune response. Due to the lack of proofreading activity, the transcription of viral RNA by the viral RNA polymerase is error prone and results in mis-incorporation of nucleotides. Under the selective pressure of antibodies that are present in the human population, induced after influenza virus infections and/or vaccination, variants are positively selected from the quasi species that have accumulated amino acid substitutions in the antigenic sites of HA that are recognized by virus-neutralizing antibodies. This phenomenon is known as antigenic drift. Introduction of influenza viruses of a novel antigenically distinct subtype into the human population is known as antigenic shift and may cause a pandemic outbreak, since neutralizing antibodies against the new virus strain are absent in the population at large.Introduction of antigenically distinct viruses can occur after zoonotic transmission. However, in most cases, pandemics were caused by viruses that had exchanged gene segments between human and avian or swine influenza viruses.

Currently used seasonal influenza vaccines are predominantly inactivated vaccine preparations. Development of vaccines that induce broad range of antibodies and preferably long heterosubtypic CTL response is desirable. Yes viral proteins such as the NP and M1 highly conservative, they are likely targets for the induction of cross-reactive T cells. This requires effective delivery of viral proteins in the cytosol. Several vaccine candidates cytosolic delivery is currently being investigated, including DNA vaccines, recombinant viral vectors, Iscoms and virosomal. They have already passed clinical trials. In addition, the induction of cross-reactive antibodies has attracted attention in recent years, antibodies directed against conserved regions of molecules on the stem are of particular interest. Unlike subtype specific antibodies induced against the main round of the AB, these stem HA-specific antibodies capable of neutralizing a broad activity against a large number of subtypes of influenza virus.  In addition, ectodomen M2 highly conserved protein and antibodies induced against this region are able to create protection against infection.  Thus, vaccines that induce both humoral and cell-mediated immune responses aimed at conserved areas of virus, in addition to the strain-specific antibodies can afford. Protective immunity to many different flu viruses, including new variants and subtypes drift. Knowing the complexity of the interaction between the immune system and variable pathogen such as influenza virus, has increased significantly in recent years. This can help reduce both morbidity and mortality, through the creation of effective seasonal vaccine, but there are still gaps in understanding, providing opportunities for improvement and developing a more widely-protecting vaccines.    Induction STL-reactions with the same epitope can be a way to create fully protecting vaccines. Current research also focuses on the responses of cross antibodies directed to a more conservative regions of the surface proteins. Together, these will create new ways to confront the changing nature of influenza virus, and subsequently be able to protect even the emergence of new pandemic strains.