Influenza A virus infection and innate immunity in an established murine alveolar-like macrophage model
MetadataShow full item record
Influenza is both a seasonal and occasional pandemic respiratory infection in humans, with the most vulnerable of society at the highest risk. With vaccine efficiency dropping and resistance against treatments, better understanding of the host-viral interactions is required to find novel therapy targets. Much of the current understanding for the host-viral interactions comes from epithelial cells or imperfect macrophage models. Alveolar macrophages (AMs) are crucial in the first stages of infection, but due to limited availability, true functional experiments have been impossible. By using the novel AM-like macrophage, Max Plank Institute (MPI) cells, which are available in practically unlimited numbers, this research hopes to expand the current understanding with in-depth functional assays on IAV-macrophage entry mechanisms, immune sensing and signalling, and the resulting effector molecules such as cytokines, chemokines and type I IFNs. MPI macrophages are susceptible to IAV infection but this results abortive replication, following established understandings. However, MPI macrophages are much more sensitive to influenza infection than non-AM models, resulting in a greater pro-inflammatory cytokine, chemokine, and type I IFN responses. This sensitivity 8 highlighted the differences between both strains and individual isolates, with specific binding, infectivity and resulting immune responses all highly variable in different cell types. The effective cleavage of the viral hemagglutinin was key to successful infection, with TPCK-treated trypsin in cell culture much less efficient than the egg counterpart. Influenza propagation techniques that result in this less efficient cleavage limit the impact of downstream functional immune assays. In MPI macrophages reduced pro-inflammatory cytokines and type-I IFN production was observed as a result of this difference in cleavage, however chemokine remained primarily unaffected. The entry mechanisms of IAV in macrophages are much more complex than their epithelial counterparts. While in both cell types sialic acid is the primary attachment receptor for the viruses, in macrophages co-receptor binding by a variety of receptors is utilised to internalise influenza efficiently. Receptors such as the C-type lectins MMR and MGL are utilised to different extents in different macrophage models. However, in MPI macrophages the scavenger receptor MARCO seems to be the main co-receptor, involved in viral uptake but not in direct viral binding to the cell surface. Viral uptake in MPI cells showed that both macropinocytosis and clathrin mediated endocytosis are required for optimal infection, unlike epithelial counterparts where the two mechanisms work in a redundancy partnership. Following from this the current model of IAV endosome escape is the same in MPI macrophages as epithelial cells. With endosomal acidification facilitating HA fusion to the endosome to allow viral RNP escape. Upon detection of IAV by the macrophage three distinct immune response phases occur, two ‘early’ before RNA replication and one ‘late’ after replication. These phases 9 line up with the binding, endosomal, and RNA replication phases of the viral infection cycle, resulting in the production of key effector molecules at each point. These two ‘early’ responses occur independently of viral RNA replication resulting in MAPK and NF-κB activation. The resulting pro inflammatory cytokines produced from these early responses aid the early influx of innate and adaptive immune cells to the infection site. On the other hand, the ‘late’ response is dependent on viral RNA replication, utilising TBK-1 and IRF-3 signalling to induce an antiviral state and viral killing. This late response also seems to play a role in resolving the ‘early’ responses to prevent the cytokine storm associated with severe uncontrolled response. The complexity and varied response to influenza in MPI macrophages, show the benefits of using relevant cell models, which are available in sufficient numbers for functional immune studies. Understanding influenza and AM interactions is paramount for future therapies against a disease with rising health burdens.