III. ABSTRACTS AND POSTERS / ORAL PRESENTATIONS
6. Mice deficient in interferon regulatory factor 4 (IRF4) are more susceptible to infection with mouse-adapted influenza A/Aichi/2/68
3.1.4. Generation of transgenic and knock-out mice as animal models for
3.1.4.3. Animal models in Influenza Virus Infection
There are several mammalian models of influenza: ferrets, monkeys, pigs, horses and mice with each model having its particular advantages and disadvantages (Sweet, Macartney et al. 1981, Smith and Sweet 1988, Sweet, Jakeman et al. 1988, Cottey, Rowe et al. 2001).
Ferrets can be naturally infected with human influenza isolates, develop a fever and exhibit infection primarily of nasal turbinates with lesser amounts of infection of the trachea, bronchi and lungs.
Much understanding of the pathogenesis and host defense against influenza was defined in mice and only later confirmed in humans (Small 1990, Bender and Small 1992). Mice make a suitable animal model for the study of influenza virus infection, even though the ferret closely resembles the anatomy of the human airway.
Following an intranasal inoculation of influenza virus, mice develop an illness that closely resembles the infection in humans. It is a progressive upper and lower respiratory tract disease with virtually identical histopathology. Advantages of using the mouse model are that they are small, easily handled, relatively short-lived and have a well characterized immunological response to influenza. The severity of influenza infection in the mouse can be influenced by the virulence of the virus, the immune status of the host and the method of infection. Most strains of influenza virus can be used to infect mice although prior adaptation of the virus by multiple passages through mouse lung allows for selection of viral mutants that are able to replicate efficiently in the murine respiratory tract (Mount and Belz 2010).
In mice, influenza infection induces a strong Th1 response where the protective immunity is mediated by Th1-like responses. CD8+ T cell responses in humans peak at about 14 days post infection and levels of influenza-specific CTLs correlate with a
90 reduction in the duration and level of virus replication. Memory CD8+ T cells may play a role in improving the disease severity and facilitating recovery upon reinfection. Several distinct phases are recognized in which the first phase is mediated by memory T cells that reside in the lung airways (Woodland and Randall 2004).
These memory cells are able to respond to an infection when the viral load is at its initial stages. The second stage is mediated by memory T cells that are recruited to the airways within few days of the infection which is then followed by the third stage where antigen-driven expansion of memory T cells occurs in the secondary lymphoid organs. Memory cells proliferate for several days in the lymphoid organs only to be recruited to the airways after 5 days post-infection (Woodland and Randall 2004).
An example of the use of the mouse model in influenza is the observation that CD8+ T cells are the primary cell population needed for clearance and recovery from influenza infection. Nude mice have a deteriorated or absent thymus, resulting in an inhibited immune system due to a greatly reduced number of T cells fail to recover from an influenza infection unless adoptively transferred with viable CD8+ T cells from normal donor animals (Wells, Ennis et al. 1981). Adoptive transferred cloned murine CTL lines against influenza virus were able to provide protection to mice lethally infected by influenza virus (Lukacher, Braciale et al. 1984). CD8+ T cells- mediated response where found to be more efficient that CD4+ T cells in recovery from an influenza infection where β2m-/--mice cleared a viral challenge at a slower pace compared to normal mice (Eichelberger, Allan et al. 1991, Bender, Croghan et al.
1992). Graham et al. (1994) generated Th1 and Th2 clones from influenza virus- specific CD4+ T cell clones from influenza-primed BALB/c mice. Th1 clones were found to be cytolytic and protective against lethal challenge with virus in vivo. Th2 clones were non-cytolytic, non-protective and exacerbates pulmonary pathology.
91 They concluded that virus-specific Th2 CD4+ T may not play a primary role in virus clearance and recovery. It may even lead to immune mediated potentiation of injury (Graham, Braciale et al. 1994).
3.1.5. Antiviral innate signaling pathways activated by interferons
A key aspect of the antiviral innate immune response is the synthesis and secretion of the Type I IFNs. The IFNs are a family of multifunctional secreted proteins that share a significant amino acid homology with each other and exhibit similar biological effects on target cells such as being involved in antiviral defense, cell growth regulations and immune activation (Pestka, Langer et al. 1987, Platanias 1995, Pfeffer, Dinarello et al. 1998, Stark, Kerr et al. 1998). The IFNs are classified into two distinct types. Type I IFNs, are produced in response to virus infection, and are the product of the IFN-α multigene family which is predominantly synthesized by leukocytes and IFN-β gene which is synthesized by most cell types in particular fibroblasts (Goodbourn, Didcock et al. 2000). Type II IFNs consist of the product of the IFN-γ gene and is synthesized in response to the recognition of infected cells by activated T cells and NK cells (Vilcek and Sen 1996). While the two types of IFNs may share amino acid homology (Platanias and Fish 1999), they do not share any obvious structural homology (Goodbourn, Didcock et al. 2000).
Type I IFNs bind to the Type I IFN receptor, which is a multichain structure on normal and malignant hematopoietic cells (Uze, Lutfalla et al. 1995), and is made up of two distinct components, IFNAR1 and IFNAR2. The two receptors are capable of transducing signals and mediate the biological effects of interferons (Colamonici, Domanski et al. 1994). Induction of IFN-α and –β genes are mediated by the pattern recognition receptors (PRRs) which includes TLRs (Takaoka and Yanai 2006). The
92 IFNs are crucial in bridging the two aspects of host defense which is the innate and adaptive immune systems.