Influenza virus-induced lung injury: pathogenesis and implications for treatment

Susanne Herold; Christin Becker; Karen M. Ridge; G.R. Scott Budinger

doi:10.1183/09031936.00186214

Figures

Tables

Download figure
Open in new tab
Download powerpoint
FIGURE 1
Replication of influenza A viruses in the lung epithelium. Binding of haemagglutinin (HA), expressed on the surface of the influenza virion, with sialic acid residues linked to cell surface glycans induces binding and fusion of the virion with the plasma membrane of the target cell. The HA in human viruses interacts with sialic acid residues linked to surface glycans via an α-2,6 linkage, which is found in the upper and lower human airway epithelium and in alveolar type II cells. In contrast, the HA in avian viruses interacts with sialic acid residues in an α-2,3 linkage. The virus then enters the cell via endocytosis or micropinocytosis and is trafficked to the lysosome where acidification activates the proton selective matrix protein-2 viral channel (M2), inducing membrane fusion and dissociation of the viral ribonucleoprotein (RNP) core, which is then transported to the nucleus where viral RNA replication occurs. Progeny viral RNP cores are generated in the cytosol and, with the viral surface proteins, HA and neuraminidase (NA), and other viral proteins, are concentrated in and near lipid rafts at the plasma membrane. Budding of these plasma membrane regions forms complete viral progeny, which is linked to the plasma membrane by HA/sialic acid interactions. Cleavage of sialic acid residues by neuraminidase releases the viral progeny so they are then free to infect other cells, which can be prevented by NA inhibitors. An alveolar epithelial cell is shown as an example but the life cycle is similar in the airway epithelium.
Download figure
Open in new tab
Download powerpoint
FIGURE 2
Influenza infection results in the sequential activation of beneficial and detrimental host-immune pathways in the lung. a) The earliest responses are seen in the infected airway or alveolar epithelial cell (AEC). From left to right, the presence of intracellular viral RNA activates Toll-like receptors (TLRs), primarily TLR7 and TLR3, to induce pathways that culminate in activation of interferon regulatory factor (IRF)3 or IRF7, which increase the transcription of the type I interferon (IFN)-α/β. Activation of this pathway can also induce the transcription of pro-inflammatory cytokines and chemokines by activating nuclear-factor (NF)-κB. 5′ triphosphorylated double stranded RNA (5′-PPP dsRNA) released into the cytosol during influenza infection induces a conformational change in retinoic acid-inducible gene-I (RIG-I), which interacts with mitochondrial antiviral signalling protein (MAVS) allowing it to activate nucleotide-binding oligomerisation domain-containing protein (NOD)2. This also induces the transcription of type I IFNs via IRF3 and pro-inflammatory cytokines via NF-κB. The inflammasome proteins ASC (adapter protein apoptosis-associated speck-like protein containing a CARD), pro-interleukin (IL)-1β and pro-IL-18 are induced by NF-κB. In the presence of viral RNA, ASC interacts with NLR family, pyrin domain-containing (NLRP)3 and MAVS to induce activation of the NLRP3 inflammasome, which cleaves and activates caspase-1 to generate IL-1β and IL-18, thereby amplifying the inflammatory cascade. b) Infection of the airway or alveolar type II cells results in the release of damage-associated molecular pattern molecules (DAMPs) and pathogen-associated molecular pattern molecules (PAMPs), which are sensed by resident dendritic cells (DCs). DCs migrate to regional lymph nodes to activate cytotoxic (CD8⁺) and helper (CD4⁺) T-cells, as well as rare memory T-cells capable of inducing a specific antiviral response (not shown). At the same time, DAMPs, PAMPs, endocytosed viruses and perhaps influenza infection itself induce the release of type I IFNs and inflammatory cytokines from tissue-resident alveolar macrophages (TR-MΦ) and DCs (not shown). These cytokines/chemokines induce the recruitment of neutrophils and the recruitment and differentiation of peripheral blood monocytes into monocyte-derived alveolar macrophages (MD-MΦ). Both neutrophils and MD-MΦ amplify the inflammatory response. Damage to the underlying endothelium causes the loss of negative regulators of inflammatory cell recruitment and inflammation, including signalling through the sphingosine-1 phosphate receptor (SP1R) and the release of angiotensin via angiotensin converting enzyme (ACE), thereby amplifying the inflammatory response. c) The resulting release of type I IFNs and inflammatory cytokines and the action of cytotoxic T-cells is critical for viral

Tables

Figures

TABLE 1

Influenza A virus (IAV): a segmented antisense RNA virus encoding 11 proteins

Viral proteins	Location	Function	Comments	Novel antiviral therapies tested in clinical studies
Haemagglutinin	Viral envelope	Attachment to sialic acid residues on airway and alveolar epithelium, membrane fusion and viral entry	Alternate binding to α-2,3 or -2,6 linked sialic acid contributes to pathogenicity In the human airway, proteolytic cleavage and disease severity is connected to host proteases TMPRSS2 and HAT [25–27] The addition of N-linked glycosylations mask haemagglutinin as an antigenic epitope [28–31] but trigger recognition by C-type lectins [32–34]	DAS181 (cleavage of sialic acids), flufirvitide (peptide inhibitor binding to haemagglutinin)
Neuraminadase	Viral envelope	Enzymatic cleavage of terminal sialic acid residues from newly formed virions and release of progeny	After haemagglutinin, neuraminadase is the major antigenic epitope Currently approved therapies for IAV infection (osteltamivir, zanamivir, laninamivir and peramivir) target neuraminadase
Matrix protein-2	Viral envelope	Proton selective ion channel required for viral entry, assembly and budding	Matrix protein-2 has been suggested to promote apoptosis by inhibition of autophagy [35] and induces degradation of ENaC and CFTR [36, 37] and, therefore, impacts alveolar fluid clearance	AVI-7100 (interference with M segment gene expression)
Matrix protein-1	Viral matrix	Structural links between the membrane and ribonucleoprotein core
Polymerase proteins: PB1, PB2 and PA	Ribonucleoprotein core	RNA polymerase complex proteins	The polymerase subunits have been connected to viral pathogenicity and interact with >300 cellular proteins [38] Mutations can alter replication efficiency; e.g. a single amino acid substitution (E627K) induces higher pathogenicity in the 1918, 1957 and 1968 pandemic IAV strains [39, 40] and is found in circulating H5N1 and H7N9 variants	Favipiravir (nucleoside inhibitor targeting PB1)
Nucleocapsid protein	Ribonucleoprotein core	Packaging of the viral genome
Nonstructural protein-1	Expressed during replication, not part of the mature virion	Immune evasion	Critical for viral evasion of the host immune response It binds a complex of viral RNA, RIG-1 and TRIM25 to inhibit downstream antiviral signalling [41, 42] It also induces the downregulation of the IFN-α receptor [43] and upregulates inhibitors of JAK/STAT signalling (SOCS1 and SOCS3) [43, 44] During infection, non-structural protein-1 also blocks pro-apoptotic signalling by protein kinase R [45, 46] and prevents the activation of NF-κB [47]
Nonstructural protein-2	Expressed during replication, not part of the mature virion	Export of viral ribonucleoproteins from the nucleus during viral replication
PB1-F2^#	Expressed during replication	Induction of host cell apoptosis	PB1-F2 increases mitochondrial membrane permeability through interactions with mitochondrial membrane proteins ANT3 and VDAC, to enhance BAK/BAX-mediated cytochrome C release [48–50]

The mature virion comprises eight structural proteins; the other three are expressed during viral replication. Haemagglutinin and neuraminidase, required for virus binding and release, respectively, are present in the viral envelope. Together, haemagglutinin and neuraminidase determine the antigenic properties of the virus and are used to define different viral strains, e.g. the H1N1 strain responsible for the 2009 pandemic and the H7N9 strain responsible for a recent outbreak of avian influenza in Asia. Some of the viral proteins represent putative novel targets for antiviral therapy. TMPRSS2: transmembrane protease, serine 2; HAT: human airway trypsin-like protease; ENaC: epithelial sodium channels; CFTR: cystic fibrosis transmembrane conductance regulator; RIG-1: retinoic acid inducible gene-1; IFN: interferon; JAK: janus kinase; STAT: signal transducer and activator of transcription; NF-κB: nuclear factor-κB; VDAC: voltage-dependent anion channels. ^#: not expressed by all viruses.