Influenza viruses are human respiratory pathogens that cause seasonal epidemics and pandemics. The host restricts the virus infection by inducing immune responses aiming at virus clearance. The immune response has two arms. The innate immunity is the first line defense mechanism that is activated immediately after the recognition of the pathogen. The adaptive immunity, which consists of humoral and cell-mediated immunity, takes more time to develop. The epithelial cells of the respiratory tract and innate immune cells, such as macrophages and dendritic cells, are equipped with a plethora of receptors and signaling molecules that are designed for pathogen recognition. These receptors include Toll-like receptors (TLRs) and RIG-I-like receptors (RLRs). The pathogen recognition by these receptors leads to the activation of complex cellular signaling cascades that culminate in the production of cytokines, small proteins that mediate the communication between cells. In influenza infection, one important class of cytokines is interferons (IFNs) which induce the production of antiviral proteins that are able to inhibit virus infection. On the other hand, influenza viruses are capable of evading innate immune surveillance and there are differences between influenza virus types or strains in their immune evasion mechanisms.
In this thesis work influenza virus-induced IFN responses were studied in human macrophages and dendritic cells in vitro. Firstly, we showed that in macrophages influenza B virus infection induced a very early IFN-β and IFN-λ1 gene expression that coincided with the nuclear entry of the virus and the activation and nuclear import of IFN regulatory factor 3 (IRF3). This early activation did not take place in influenza A virus-infected cells. Furthermore, our study indicated that RIG-I receptor was essential for the early IFN gene expression. Secondly, we compared the cytokine responses induced by pandemic H1N1 influenza A virus to the ones induced by seasonal influenza A viruses in human macrophages and dendritic cells. We showed that the pandemic influenza A virus induced weak IFN responses but was highly sensitive to the antiviral actions of IFNs.
During the infection, different types of microbial structures are present and can be recognized by different cellular receptors. Another aim of this thesis was to elucidate the mechanism of receptor cooperation in inducing synergistic cytokine production. We confirmed the previous findings that TLR3 or TLR4 together with TLR7/8 induces synergistic interleukin (IL)-12 and IFN gene expression in human dendritic cells. We studied, which regulatory factors bound to IL-12 and IFN-λ1 gene promoters during a synergistic stimulation and which cell signaling pathways took part in the cytokine production. We conclude that at the transcriptional level, several different IRF proteins and cell signaling pathways cooperate in the synergistic IL-12 and IFN-λ gene expression. In addition, we propose that IFNs produced after stimulation of the TLR3 pathway induce the expression of TLR7 receptor and other cell signaling components that create a positive feedback loop that further augments the cytokine and IFN production during synergistic stimulation.
This thesis discusses the host-pathogen interactions in the human system and clarifies the cell signaling pathways leading to synergistic cytokine gene expression. Moreover, the early events in influenza B virus infection and IFN responses induced by pandemic H1N1 influenza A virus are described. More detailed knowledge of the human innate immune responses induced by host-pathogen interactions is needed for the development of effective vaccines and antiviral treatments against influenza virus.