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Pathogenesis and Immunology
The influenza virus is notoriously known for its unique ability to cause recurrent epidemics and global pandemics during which acute febrile respiratory illness occurs explosively in all age groups. Two qualities of influenza account for much of the epidemiological spread of the virus. First, is the ability to emerge and circulate in avian or porcine reservoirs by either genetic reassortment or direct transmission and subsequently spread to humans at irregular intervals. Second, is the fast and unpredictable antigenic change of important immune targets once the virus has become established in a human.
A highly contagious virus causing extensive morbidity and major case fatality rates is an archetypal anxiety. Influenza has the potential to create such a scenario. The influenza virus, as a pathogenic agent for humans, has been circulating in the human population since at least the sixteenth century (Cox & Kawaoka 1998) leading to recurrent epidemics of febrile respiratory disease every 1 to 3 years. In addition, each century has seen some pandemics rapidly progressing to involve all parts of the world due to emergence of a novel virus to which the overall population holds no immunity. The characteristics of pandemics include occurrence outside the usual season, extremely rapid transmission with concurrent outbreaks throughout the globe, and high attack rates in all age groups with high mortality rates even in healthy young adults. Given the growing world population and international travel and tourism, impending pandemic influenza outbreaks gain the potential to spread even more rapidly. In order to understand the background of this global epidemic threat more thoroughly, this chapter aims to describe both the pathogenesis of the disease and the contest between the virus and the immune system.
The pathogenicity and virulence of the influenza virus is determined by several interacting factors:
Viral entry: How does the virion enter the host?
The predominant way in which influenza is transmitted is from person to person by aerosols and droplets. Influenza then enters the host through the respiratory tract. In a human lung there are about 300 million terminal sacs, called alveoli, that function in gaseous exchange between inspired air and the blood. The total absorptive area of the human lungs ranges from 80-120 m2. The resting ventilation rate in humans is about 6 liters of air per minute, which introduces large numbers of foreign particles and aerosolized droplets potentially containing virus into the lungs. Deposition of foreign particles depends on their size: inhalation of very small particles does not result in absorption through the alveoli or bronchial system. Small droplets with a diameter of approximately 1 to 4 µm precipitate in the small airways. Much larger particles are either not able to enter the respiratory system or are deposited in the upper respiratory tract (Figure 1A).
Numerous host defense mechanisms including mechanical barriers block respiratory tract infection. The respiratory tract is covered with a mucociliary layer consisting of ciliated cells, mucus-secreting cells and glands (Figure 1 B). Foreign particles in the nasal cavity or upper respiratory tract are trapped in mucus, carried back to the throat, and swallowed. From the lower respiratory tract foreign particles are brought up by the ciliary action of epithelial cells. In the alveoli that lack cilia or mucus, macrophages are responsible for destroying particles (Figure 1).
Figure 1. Sites of influenza entry in the respiratory tract. (A) The anatomical and functional structures of the human airways are shown. Influenza first infects the upper airway and the ciliated cells in the bronchus and bronchioli. Resulting clinical syndromes include tracheitis, bronchitis, bronchiolitis, and bronchopneumonia. The adaptive immune response is initiated in lymph nodes along the airways. (B) The respiratory epithelia is especially equipped to defend from incoming pathogens by a layer of mucus (bronchus), ciliated cells (bronchus and bronchioli), and alveolar macrophages (alveoli).
Binding to the host cells
The main targets of the influenza virus are the columnar epithelial cells of the respiratory tract. These cells may be susceptible to infection if the viral receptor is present and functional. Thus, viral receptors are determinants of tropism. However, this simplified model is often insufficient to explain viral tropism since the receptor distribution in the host is generally more widespread than the observed virus tropism.
In influenza infection, the receptor binding site of viral hemagglutinin (HA) is required for binding to galactose bound sialic acid on the surface of host cells (Weis 1988). Certain areas of the binding site of HA are highly conserved between subtypes of the influenza virus (Daniels 1984). Hosts may prevent the attachment by several mechanisms: (1) specific immune response and secretion of specific IgA antibodies, (2) unspecific mechanisms, such as mucociliary clearance or production of mucoproteins that able to bind to viral hemagglutinin, and (3) genetic diversification of the host receptor (sialic acid), which is highly conserved in the same species, but differs between avian and human receptors (Matrosovich 2000). As a result, the avian virus needs to undergo mutations at the receptor binding site of hemagglutinin to cross the interspecies barrier between birds and humans. In pigs, polymorphisms of sialic acid species and linkage to galactose of both humans and birds are co-expressed in the tissue. Therefore, co-infection with avian and human influenza can occur in pigs and allow genetic reassortment of antigenic properties of both species in the co-infected cells. Recently, it has been shown that certain avian influenza viruses in human and birds are able to bind to different target cells (Matrosovich 2004). This could explain the observation of several cases since the end of the 1990s with transmission of avian influenza directly from poultry to humans. H5N1 and some other subtypes of influenza A virus are able to bind to receptors in the human eye (Olofson 2005).
As essential as the binding of the influenza virus is its cleavage from the binding site at the host cell. Cleavage is the functional role of viral neuraminidase (Chen 1998). The virulence of the influenza virus depends on the compatibility of neuraminidase with hemagglutinin. A virulent virus which has undergone mutations in the hemagglutinin needs compensatory mutations in the neuraminidase to maintain its virulence (Baigent & McCauley 2003, Hulse 2004). As a consequence, viral fitness and virulence were found to be reduced in influenza viruses resistant to neuraminidase inhibitors (Yen 2005).
Once the cell membrane and the virus have been closely juxtaposed by virus-receptor interaction, the complex is endocytosed. Importing H+ ions into the late endocytic vesicles as a physiologic event then acidifies the interior. Upon acidification, the viral HA undergoes a conformational rearrangement that produces a fusiogenic protein. The loop region of the HA becomes a coiled coil eventually bringing the viral and endosomal membranes closer so that fusion can occur. To allow release of viral RNA into the cytoplasm, the H+ ions in the acidic endosome are pumped into the virion interior by the M2 ion channel. As a result, viral RNA dissociates from M1 by disrupting the low pH-sensitive interaction between the M1 and ribonuclein complex after fusion of the viral and endosomal membranes. The viral RNA is then imported in an ATP-dependent manner into the nucleus for transcription and translation (Flint 2004).
Figure 2: Replication cycle of influenza A virus. Binding and entry of the virus, fusion with endosomal membrane and release of viral RNA, replication within the nucleus, synthesis of structural and envelope proteins, budding and release of virions capable of infecting neighboring epithelial cells (Modified from Cox & Kawaoka 1997)
Where does the primary replication occur?
Cellular proteases are often required to cleave viral proteins to form the mature infectious virus particle. Thus, additional factors to entry receptors can determine the site of viral replication. In humans, the replication of the influenza virus is generally restricted to the epithelial cells of the upper and lower respiratory tract. This is because of the limited expression of serine protease, tryptase Clara, secreted by nonciliated Clara cells of the bronchial epithelia. The purified enzyme cleaves the polypeptide HA chain precursor HA0 of extracellular particles and activates HA in virions rendering them infectious. Some highly virulent avian influenza strains, however, contain genetic insertions at the cleavage site of HA leading to processing by ubiquitous proteases. This may cause altered tropism and additional sites of replication in animals and humans (Gamblin 2004). Tissue tropism of avian influenza (H5N1) in humans is not well defined. In one case, viral RNA was detected in lung, intestine, and spleen by a reverse transcription polymerase chain reaction but positive-stranded viral RNA, indicating virus replication, was confined exclusively to the lung and intestine (Uiprasertkul 2005). Thus, H5N1 viral replication in humans may be restricted to the respiratory and intestinal tract in contrast to disseminated infections documented in other mammals and birds.
How does the infection spread in the host?
Once influenza has efficiently infected respiratory epithelial cells, replication occurs within hours and numerous virions are produced. Infectious particles are preferentially released from the apical plasma membrane of epithelial cells into the airways by a process called budding. This favors the swift spread of the virus within the lungs due to the rapid infection of neighboring cells.
Alterations in the HA cleavage site by naturally occurring mutants can dramatically influence the tropism and pathogenicity of influenza. As a result, it can be recognized by other cellular proteases. This would explain why many of the individuals infected with avian influenza (H5N1) in Hong Kong had gastrointestinal, hepatic, and renal, as well as respiratory symptoms and why viruses from these patients were neurovirulent in mice (Park 2002). Whether these symptoms result from hematogenic spread or reflect non-pulmonal means of viral entry into the host remains unclear. However, mutation in NA may also, in part, explain the pantropic nature of influenza. For example, the laboratory-derived WSN/33 strain of influenza, a variant of the first human influenza virus ever isolated, unlike most human influenza strains, can replicate in vitro in the absence of added trypsin. In this virus an in-frame deletion that removes the glycolization site at residue 46 of NA allows neuraminidase to bind and sequester plasminogen. This leads to higher local concentrations of this ubiquitous protease precursor and thus to increased cleavage of the HA. These findings suggest a means by which influenza A viruses, and perhaps other viruses as well, could become highly pathogenic in humans. (Goto & Kawaoka 1998). Interestingly, studies with the genetically reconstructed 1918 Spanish influenza pandemic virus (H1N1) revealed additional mechanisms of NA-mediated HA cleavability that may be relevant to the replication and virulence of that virus (Tumpey 2005).
Finally, animal studies have revealed that the site of inoculation can determine the pathway of spread of the influenza virus in the host. For example, the neutrotropic NWS strain disseminates to the brain by hematogenous spread when given intraperitoneally but reaches the central nervous system via the sensory neurons when the virus inoculum is placed in the nose (Flint 2004). The latter has been demonstrated with the Hong Kong H5N1 virus as well (Park 2002).
What is the initial host response?
Although a frequent disease, the specific inflammatory patterns or regulation of immune response and the pathogenesis of cytopathic effects in human influenza is incompletely understood. Most evidence comes from animal studies, where avian influenza is a disseminated disease. The pathophysiology of such models, however, may profoundly differ from that in humans.
Cytokines and fever
A central question is how an infection essentially localized to the respiratory tract can produce such severe constitutional symptoms. As in many other infectious diseases, it is the unspecific and adaptive immune response that contributes substantially to the clinical signs and symptoms in influenza and finally to the control of infection. These immune mechanisms can lead to both localized as well as systemic effects. Cytokines, rapidly produced after infection by epithelial and immune cells of the respiratory mucosa, are local hormones that activate cells, especially within the immune system. Chemokines are a subset of cytokines that act as chemoattractants for cells of the immune system. For example, influenza infection induces in human plasmacytoid and myeloid dendritic cells a chemokine secretion program which allows for a coordinated attraction of the different immune effectors (Piqueras 2005, Schmitz 2005). The most important cytokines serve as endogenous pyrogens and are involved in the pathogenesis of fever: IL-1α/β, TNF α/β, IL-6, interferon (IFN) α/γ, IL-8, and macrophage inflammatory protein (MIP)-1α.
Most of these cytokines have been detected in nasopharyngeal washes of humans who have been experimentally or naturally infected with influenza (Brydon 2005). It is proposed that these cytokines, produced locally or systemically following interaction of exogenous pyrogens (e.g. influenza) with phagocytes, reach the central nervous system. There is a small area in the hypothalamus, called the Organum vasculosum laminae terminalis, which has a reduced blood-brain-barrier and allows the passage of pyrogens. At this site, in a dose-dependent manner, they induce the production of prostaglandins and especially prostaglandin E2. These mediators increase the thermostatic set point and trigger complex thermoregulatory mechanisms to increase body temperature. The fact that none of the cytokines mentioned above correlated with the severity of disease in influenza infection, argues in favor of their pleiotropy and cross-talk amongst signaling pathways.
The relevance of cytokines may also differ between influenza strains or individuals. Influenza infections with the Hong Kong H5N1 strain from 1997 have been proposed to potently induce pro-inflammatory cytokines (particularly TNFα) by NS gene products (Cheung 2002, Lipatov 2005, Chan 2005). Studies aimed to identify other virion components that induce cytokine release revealed that double-stranded (ds) RNA, either from lungs of infected mice or synthetically derived from influenza, were pyrogenic when injected into the CNS-ventricle of mice. Such dsRNA is released from infected cells when they die and thus may stimulate cytokine production. Recent studies indicate that dsRNA-sensing Toll-like receptor (TLR) 3 is expressed on pulmonary epithelial cells and that TLR3 contributes directly to the immune response of respiratory epithelial cells (Guillot 2005, Akira & Takeda 2004). Interestingly, in humans the initiation of an innate immune response against influenza appears to be at least as dependent on sensing single stranded RNA via TLR 8 than on detecting dsDNA by TLR 3. Virus particles can also be pyrogenic, as virosomes depleted of RNA but including viral lipid, hemagglutinin, and neuraminidase may induce fever. Individual virion components were, however, not pyrogenic probably explaining why whole virus vaccines can produce influenza-like symptoms while subunit vaccines do not (Brydon 2005).
Hyperreactivity of the bronchial system (Utell 1980, Little 1978), obstruction predominantly of small airways (Hall 1976) and impaired diffusion capacity (Horner 1973) is common in influenza infection. Hyperreactivity and broncho-obstruction may persist for a prolonged period, especially in allergic disease (Kondo & Abe 1991), and might be a result of a pro-inflammatory cytokine profile which interferes with the ability to induce tolerance to aerosolized allergens (Tsitoura 2000).
In human influenza infection severe alveolar inflammation, presenting as primary viral pneumonia, is rare. It usually presents with extended inflammation in lower und upper respiratory tract with loss of ciliated cells and leads to hyperemic or hemorrhagic hyaline membranes and infiltrates of neutrophils and mononuclear cells (Yeldandi & Colby 1994).
In contrast to primary viral pneumonia, bacterial superinfection is common in human influenza and causes serious morbidity and mortality predominantly in elderly adults. Several factors have been identified, which could explain the increased risk for bacterial infection of the respiratory tract, including damage of columnar epithelial cells with disruption of the epithelial cell barrier (Mori 1995), decreased mucociliary clearance (Levandovsi 1985), enhancement of bacterial adherence (McCullers 2002), and functional alteration of neutrophils (Abramson 1986, Cassidy 1988).
Human influenza leads to complex cytopathic effects, predominantly at the columnar epithelial cells in the respiratory tract, that result in acute disease of lung and airways. Infection and viral replication of the influenza virus in the respiratory tract leads to cell damage induced by downregulation of host cell protein synthesis (Katze 1986, Sanz-Esquerro 1995) and apoptosis (Wiley 2001a). The latter, also called programmed cell death, is a series of defined cellular events that eventually results in the efficient removal of the cell and its contents. Apoptosis can be triggered by different mechanisms and is characterized by several morphological changes, including cytoskeleton disruption, condensation of cytoplasm and chromatin, loss of mitochondrial function, DNA fragmentation, and ultimately the formation of small membrane bound particles known as apoptotic bodies, which are cleared by phagocytic cells such as macrophages and dendritic cells.
The influenza virus-induced apoptosis is mediated by both Fas-mediated mechanisms and Fas-independent signals, such as the formation of FADD/caspase-8 complex by protein kinase R (PKR), which initiates a caspase cascade. PKR is a key regulatory component in many apoptotic pathways and is induced by IFN and activated by dsDNA (Brydon 2005). As a third pathway to apoptosis, influenza activates transforming growth factor (TGF)-β via viral neuraminidase. NA can activate latent TGF-β on the cell surface by facilitating cleavage of TGF-β into its active form. TGF-β initiates a signaling cascade leading to the activation of the c-Jun N-terminal kinase (JNK) or stress activated protein kinase (SAPK), resulting in the activation of transcription factors and upregulation of pro-apoptotic gene expression. This pathway, together with the effects on the mitochondrial membrane stability of a small protein, encoded by an alternative +1 reading frame in the PB1 protein (Chen 2001), has been implicated in the apoptosis of lymphocytes and could explain the lymphopenia observed during acute infection.
Lung tissue injury following infection with the influenza virus has been associated with cellular oxidative stress, generation of reactive oxygen species (ROS), and the induction of nitric oxide synthetase-2, which leads to the formation of toxic reactive nitrogen intermediates. Anti-oxidants, however, had little effect on apoptosis in bronchiolar cell lines in vitro.
Symptoms of H5N1 infections
Avian influenza is an infectious disease of birds caused by type A strains of the influenza virus. To date, all outbreaks of the highly pathogenic form have been caused by influenza A viruses of subtypes H5 and H7. It is currently unknown whether avian influenza in humans (H5N1) has the same cytopathic effects as mentioned above. Only a few studies in severe or fatal cases have been performed. However, asymptomatic or mild symptomatic disease is possible (Buxton Bridges 2000, Katz 1999) and its incidence may be underestimated.
The most common initial symptoms of H5N1 influenza in humans were high fever, and, in those patients referred to a hospital, pneumonia, pharyngitis, intestinal symptoms, conjunctivitis, and acute encephalitis (Yuen 1998, Tran 2004, Yuen & Wong 2005). Adult patients with initial signs of pneumonia often progressed to an ARDS-like disease. In fatal cases of H5N1-influenza, reactive hemophagocytic syndrome has been described as a prominent feature. Beyond pulmonary disease with organizing diffuse alveolar damage and interstitial fibrosis, extrapulmonary involvement has been described as extensive hepatic central lobular necrosis, acute renal tubular necrosis and lymphoid depletion (To 2001), although there was no virus found on isolation, reverse transcription polymerase chain reaction and immunostaining respectively. Soluble interleukin-2 receptor, interleukin-6 and interferon-gamma were increased. In addition, tumor necrosis factor-alpha mRNA was seen in lung tissue in other cases with H5N1 influenza in humans (Uiprasertkul 2005).
In comparison to human H1N1 viruses (Hayden 1998), the Hong Kong H5N1 strain from 1997 has been proposed to potently induce pro-inflammatory cytokines including IL-10, IFNβ, RANTES, IL-6 and particularly TNFα by NS gene products (Cheung 2002, Lipatov 2005, Chan 2005). The authors of these studies postulated that in a fatal human infection with the avian H5N1 subtype, initial virus replication in the respiratory tract triggers hypercytokinemia complicated by a reactive hemophagocytic syndrome, which might be a different pathogenesis of influenza A H5N1 infection from that of usual human subtypes (To 2001). Bacterial superinfection has not been found in fatal cases of H5N1 avian influenza (To 2001). This observation might be a bias of the early fatal outcome of these most severe cases, which hypothetically did not allow for the development of superinfection.
How is influenza transmitted to others?
Respiratory transmission depends on the production of virus-containing airborne particles and aerosols. Aerosols are produced during speaking and normal breathing. Shedding from the nasal cavity requires sneezing and is much more effective if the infection produces a nasal secretion. A sneeze produces up to 20,000 droplets in contrast to several hundred expelled by coughing. The largest droplets fall to the ground within a few meters. The remaining droplets travel a distance dependent on their size. Droplets measuring 1-4 µm in diameter may remain suspended for a long time and reach the lower respiratory tract. Experimental transmission of influenza in volunteers showed that bronchial inhalation of small droplets is superior in comparison to inoculation of large droplets into the upper respiratory tract or conjunctiva (Alford 1966, Little 1979, Bridges 2003). If the virus replicates early during the course of infection in the lower respiratory tract, this would result in smaller droplets with higher viral load and higher infectivity, because specific immunosurveillance is still not established. Transmission of H5N1 from animal to human may occur in a different way by direct (and indirect) contact to infected poultry.
High attack rates are necessary to result in an epidemic outbreak of influenza A. Therefore winter epidemics in Europe and North America may be explained by closer contacts and stay in less ventilated rooms. Influenza virus is well adapted: for unknown reasons its ability to survive is best in lower relative humidity and at lower environmental temperatures (Hemmes 1960). Avian influenza (H5N1) might be less adapted to droplet transmission: the incubation period is longer (Chotpitayasunondh 2005), theoretically resulting in less simultaneous onset in many persons during an epidemic. Intestinal replication and symptoms precede respiratory manifestations by up to one week (Apisarnthanarak 2004), allowing onset of specific immune response before spread by infectious droplets can evolve. As a consequence, nasopharyngeal replication in avian influenza is less than in human influenza (Peiris 2004) but viral replication is prolonged (Beigel 2005). Until now transmission of H5N1 between humans has been rare (Buxton Bridges 2000, Ungchusak 2005) and rather inefficient. In conclusion, avian influenza virus (H5N1) presumably requires several passages to enable human-to-human transmission and to finally reach an infectivity rate which is effective enough to generate an epidemic or pandemic.
Influenza causes an acute infection of the host and initiates a cascade of immune reactions activating almost all parts of the immune defense system. Most of the initial innate response, including cytokine release (IFNα/β), influx of neutrophil granulocytes or natural killer cells (Mandelboim 2001, Achdount 2003), and cell activation, is responsible for the acute onset of the clinical symptoms (see above). Innate immunity is an essential prerequisite for the adaptive immune response, firstly, to limit the initial viral replication and antigen load, and secondly, because the antigen-specific lymphocytes of the adaptive immune response are activated by co-stimulatory molecules that are induced on cells of the innate immune system during their interaction with viruses (Figure 3). Influenza viruses, however, encode in the non-structural protein 1 (NS1) mechanisms to evade and antagonize the IFN α/β response. NS1 is likely to sequester viral dsRNA which prevents recognition of this dangerous molecule by cellular sensors which would otherwise trigger IFN α/β release (Garcia-Sastre 1998, Garcia-Sastre 2005).
The adaptive immune response requires some days to be effective but then helps to contain the viral spread, to eradicate the virus, and finally to establish a memory response resulting in a long-lived resistance to re-infection with homologous virus. Cross-protection within a subtype of influenza has only rarely been observed and infections essentially induce no protection across subtypes or between types A and B (Treanor 2005). Influenza infection induces both systemic and local antibody (humoral immunity), as well as cytotoxic T cell responses (cellular immunity), each of which is important in recovery from acute infection and resistance to reinfection.
Figure 3. The humoral and cell-mediated immune response to influenza virus infection. The humoral branch of the immune system comprises B-lymphocytes (left), which after interaction with influenza differentiate into antibody-secreting plasma cells. The cellular response (right) starts with antigen presentation via MHC I (black) and II (blue) molecules by dendritic cells, which then leads to activation, proliferation and differentiation of antigen-specific T cells (CD4 or CD8). These cells gain effector cell function to either help directly, release cytokines, or mediate cytotoxicity following recognition of antigen (Adapted from Flint 2004). Not shown is the formation of a cellular memory immune response and the various forms of innate immunity induced by influenza.
The humoral immune response
Antibodies (e.g. IgG, IgA) are produced by plasma cells which are the final stage of B cell development, requiring that the B cells have recognized antigen and been stimulated by CD4 T cells and T cell-derived cytokines (Figure 3). Unlike T cells, B cells can recognize antigen in its native form. The antigen specificity arises from random rearrangements of genes coding for the hypervariable region of immunoglobulins in the cells, whilst still in the bone marrow. The naïve B cells then enter the circulation and travel via the blood stream and lymphatics through tissue and lymphoid organs. In the lymph nodes, naïve B cells recognize cognate antigen by their surface antibodies, become activated, switch from IgM to IgG production (class-switch), increase their immunoglobulin specificity and affinity, and differentiate into plasma cells or memory B cells as the cell continues to divide in the presence of cytokines. While IgA is transported across the mucosal epithelium of the upper airway, where it serves to neutralize and clear viral infection, IgG is primarily responsible for the protection of the lower respiratory tract (Palladino 1995, Renegar 2004).
Influenza infection results in the systemic production of antibody to both influenza glycoproteins HA and NA, as well as M and NP proteins. For example, HA-specific immunoglobulins, including IgM, IgA and IgG, appear within 2 weeks of virus inoculation. The development of anti-NA parallels that of hemagglutinin-inhibiting antibodies. The peak in antibody titers are seen between 4-7 weeks after infection, and are followed by a steady decline. Antibodies remain detectable for years after infection even without re-exposure. The anti-HA antibody protects against both disease and infection with homologous virus, and the induction of neutralizing antibodies is one of the main goals of immunization with vaccines. Serum HA-inhibiting titers of 1:40 or greater, or serum neutralizing titers of 1:8 or greater, are supposed to protect against infection. Higher levels of antibody are required for complete protection in older individuals (Treanor 2005).
In contrast to anti-HA antibody, anti-NA antibody does not neutralize virus infectivity, but instead reduces the efficient release of virus from infected cells (Johansson 1989). This is because neuraminidase cleaves the cellular-receptor sialic acid residues to which the newly formed particles are attached. Anti-NA antibody can protect against the disease and results in decreased virus shedding and severity of symptoms. Similar effects have been proposed for antibodies against M2 protein of influenza A, although in general, antibodies against internal antigens are non-neutralizing, disappear more rapidly and do not appear to play a role in protective immunity.
The mucosal immune response against influenza, as measured in nasal secretions, is characterized by the presence of IgA and IgG1 against HA. The mucosal anti-HA IgG levels correlate well with the respective serum levels, indicating passive diffusion from the systemic compartment, whereas IgA is produced locally. Studies suggest that resistance to reinfection is predominantly mediated by locally produced HA-specific IgA, although IgG might be relevant as well (Renegar 2004). Either mucosal or systemic antibody alone can be protective if present in sufficient concentrations, and optimal protection occurs when both serum and nasal antibodies are present (Treanor 2005). Antibodies act in immunity against influenza by neutralization of the virus or lysis of infected cells via complement or antibody-dependent cellular toxicity.
Hosts that survive an acute virus infection and clear the virus are in general immune to infections by the same virus. Nevertheless, acute infections caused by influenza virus occur repeatedly, despite active immune clearance. This is because influenza displays a structural plasticity as it can tolerate many amino acid substitutions in its structural proteins without losing its infectivity. As an example, the sialic acid receptor-binding molecule HA, responsible for entry of the virus into the target cell, is also a main target for neutralizing antibodies and cytotoxic T lymphocytes, which exhibit a continuous immunological pressure. This immune selection or diversity, which arises from copying errors, results in slight variations of HA over time that permit the virus to evade human immune responses (antigenic drift). These changes are the reason for the annual epidemic spread of influenza andrequire new vaccines to be formulated before each annual epidemic. In contrast, antigenic shift is a major change in the surface protein of a virion, as genes encoding completely new surface proteins arise after recombination or reassortment of genomes or genome segments. Antigenic drift is possible every time a genome replicates. In contrast, antigenic shift can only occur under certain circumstances, is relatively rare and the likely reason for pandemics.
The cellular immune response
Dendritic cells have been shown to play a central role in initiating and driving T lymphocyte responses. They are a sparsely distributed, migratory group of bone-marrow derived leukocytes that are specialized for the uptake, transport, processing and presentation of antigens to T cells (Figure 3). The basic paradigm is that lung-resident dendritic cells acquire antigen from the invading pathogen, become activated, and subsequently travel to the local draining lymph nodes (Legge & Braciale 2003). The antigenic sample is processed and fixed on the dendritic cell surface as peptides which are presented by major histocompatibility complex (MHC) molecules (Silver 1992). In the lymph nodes, the now mature dendritic cells efficiently trigger an immune response by any T cell with a receptor that is specific for the foreign-peptide-MHC complex on the dendritic cell surface (Shortman & Liu 2002). Endogenous antigens from the viral infection of dendritic cells are processed and presented to CD8 T cells on MHC I molecules. Exogenous antigens are presented via MHC II molecules to CD4 T lymphocytes. Alternatively, dendritic cells may present antigens they have acquired by uptake from infected cells, or transfer antigen to neighboring dendritic cells in the lymph node which then initiate a CD8 T cell response by a process called cross-presentation (Belz 2004, Heath 2004, Wilson 2006). The newly activated T cells acquire effector cell functions and migrate to the site of infection in the lung where they mediate their antiviral activities (Figure 3).
Following recovery from an infection, a state of immunological memory ensues in which the individual is better able to control a subsequent infection with the same pathogen (Ahmed & Gray 1996). Memory is maintained by antigen-specific T cells that persist at increased frequencies, have reduced requirements for co-stimulatory signals in comparison to naïve T cells, and respond quickly to antigenic re-stimulation (Woodland & Scott 2005). There is also evidence in favor of a site specific accumulation of influenza-specific CD8 memory T cells in human lungs for the immediate immunological protection against pulmonary re-infection (de Bree 2005, Wiley 2001b). During influenza infection, both CD4 and CD8 memory T cell subsets respond to, and mediate control of an influenza virus re-infection, which is in contrast to the primary infection where viral clearance depends on CD8 T lymphocytes (Woodland 2003).
Another important feature, in for example influenza infection, is that CD4 T lymphocytes help B lymphocytes to generate anti-HA and anti-NA antibodies (Figure 3). The epitopes in HA recognized by the CD4 T helper cells are distinct from those recognized by antibodies. T helper (Th) cells may also promote the generation of virus-specific CD8 cytotoxic T lymphocytes. Th cells can be further subdivided into at least Th1 and Th2 cells, based on the type of cytokines they produce. In mice, influenza infection induces a strong Th1 response, but Th2 cytokines (IL-4, IL-5, IL-6, IL-10) have also been found in the lungs of infected animals. Some evidence indicates that protective immunity is mediated by Th1-like responses. In influenza infection, CD8 cytotoxic T lymphocytes (CTL) recognize epitopes from HA or internal proteins M, NP, or PB2 presented on MHC class I molecules (Treanor 2005). Depending on their antigen specificity, CTLs may be subtype-specific or, in case they recognize internal antigens, broadly cross-reactive with influenza A. Animal experiments using adoptive transfer of CTLs revealed their proliferation and migration pattern during infection (Lawrence & Braciale 2004, Lawrence 2005) and their potential in mediating recovery from influenza infection. They are, however, not absolutely required for the control of influenza.
T lymphocyte responses in humans peak at about day 14 post infection and levels of influenza-specific CTLs correlate with a reduction in the duration and level of virus replication in adults. Memory CD8 T cells may play a role in ameliorating the severity of disease and facilitating recovery upon reinfection. Recent studies in animals suggest that the recall response in lungs is comprised of several distinct phases that are temporally and anatomically separated. The first phase is mediated by memory T cells that are resident in the lung airways (Woodland & Radall 2004). Importantly, these cells are able to respond to the first signs of infection when the viral load is still very low. While unable to proliferate in response to infection due to the constraints of the airway environment, they can produce cytokines that may limit viral replication and spread in the epithelium. The second phase of the response is mediated by memory T cells that are rapidly recruited to the airways in the first few days of the response. The third stage is the antigen-driven expansion of memory T cells that occurs in the secondary lymphoid organs. These memory cells proliferate for several days in the lymphoid organs and are only recruited to the lung airways after about 5 days of infection (Woodland & Randall 2004). Whether these complex models generated from animal experiments apply to the situation in humans is unclear. It will be essential, however, to better understand the types of immune response and the generation and maintenance of an effective memory T cell response during influenza infection in order to improve future vaccine strategies.
We have seen how influenza virus infection leads to the acute development of a febrile respiratory illness. The pathogenesis is characterized by the rapid replication and distribution of the virus within the lungs, causing local and systemic inflammation and cytokine release. These events, together with the adaptive immune response, help to reduce the viral burden, to eliminate the virus, and to trigger disease recovery. The humoral and cellular immune responses, provoked by infection or vaccination, provide individuals and populations with long-lasting protective immunity against related viral strains. Influenza, however, can undermine this infection- or vaccine-derived immunity by means of antigenic shift and drift, resulting in epidemic and pandemic outbreaks. Technical improvements, including genetic and functional studies, will help to gain a deeper insight into the pathogenesis of historic and currently circulating virulent influenza strains. This knowledge and an advanced understanding about the viral immune defense mechanisms in the human lung will hopefully facilitate the development of better treatment options and more effective vaccines to be distributed worldwide against present and future influenza virus variants.