The role of cell tropism for the pathogenesis of influenza in humans

Immunological considerations for developing antibody therapeutics for Influenza A

Influenza infection can give rise to serious illness leading to complications and hospitalization of patients. The efficacy of current standard of care is very limited and provides little relief for patients hospitalized with serious flu. Human monoclonal antibodies (mAb) against influenza are being developed as new treatment options for this patient population. When developing antibody therapeutics, it is important to consider all possible immunologic effects of the antibodies on viral infection and disease progression including those other than the postulated therapeutic mechanisms. An area of concern is the potential of antibody-dependent enhancement (ADE) of illness. ADE of viral infections has been extensively described for Dengue virus (DENV) but not for influenza. Recently, preliminary results from clinical viral challenge studies of anti-HA-stalk mAbs suggested the possibility of enhanced viral shedding, raising concerns for ADE when utilizing mAbs as therapeutic intervention for influenza although viral shedding was not enhanced in the clinical viral challenge of anti-M2 mAb TCN-032. We herein discuss the known mechanisms of ADE and their relevance to developing mAbs such as anti-HA and anti-M2 for influenza disease.

The olfactory nerve: a shortcut for influenza and other viral diseases into the central nervous system

The olfactory nerve consists mainly of olfactory receptor neurons and directly connects the nasal cavity with the central nervous system (CNS). Each olfactory receptor neuron projects a dendrite into the nasal cavity on the apical side, and on the basal side extends its axon through the cribriform plate into the olfactory bulb of the brain. Viruses that can use the olfactory nerve as a shortcut into the CNS include influenza A virus, herpesviruses, poliovirus, paramyxoviruses, vesicular stomatitis virus, rabies virus, parainfluenza virus, adenoviruses, Japanese encephalitis virus, West Nile virus, chikungunya virus, La Crosse virus, mouse hepatitis virus, and bunyaviruses. However, mechanisms of transport via the olfactory nerve and subsequent spread through the CNS are poorly understood. Proposed mechanisms are either infection of olfactory receptor neurons themselves or diffusion through channels formed by olfactory ensheathing cells. Subsequent virus spread through the CNS could occur by multiple mechanisms, including trans-synaptic transport and microfusion. Viral infection of the CNS can lead to damage from infection of nerve cells per se, from the immune response, or from a combination of both. Clinical consequences range from nervous dysfunction in the absence of histopathological changes to severe meningoencephalitis and neurodegenerative disease.

Importin-α7 is required for enhanced influenza A virus replication in the alveolar epithelium and severe lung damage in mice

Influenza A viruses recruit components of the nuclear import pathway to enter the host cell nucleus and promote viral replication. Here, we analyzed the role of the nuclear import factor importin-α7 in H1N1 influenza virus pulmonary tropism by using various ex vivo imaging techniques (magnetic resonance imaging, confocal laser scanning microscopy, and correlative light-electron microscopy). We infected importin-α7 gene-deficient (α7(-/-)) mice with a recombinant H1N1 influenza virus and compared the in vivo viral kinetics with those in wild-type (WT) mice. In WT mice, influenza virus replication in the bronchial and alveolar epithelium already occurred a few days after infection. Accordingly, extensive mononuclear infiltration and alveolar destruction were present in the lungs of infected WT mice, followed by 100% lethality. Conversely, in α7(-/-) mice, virus replication was restricted mostly to the bronchial epithelium with marginal alveolar infection, resulting in significantly reduced lung damage and enhanced animal survival. To investigate the host immune response during alveolar virus replication, we studied the role of primary macrophages in virus propagation and clearance. The ability of macrophages to support or clear the virus infection, as well as the host cellular immune responses, did not significantly differ between WT and α7(-/-) mice. However, cytokine and chemokine responses were generally elevated in WT mice, likely reflective of increased viral replication in the lung. In summary, these data show that a cellular factor, importin-α7, is required for enhanced virus replication in the alveolar epithelium, resulting in elevated cytokine and chemokine levels, extensive mononuclear infiltration, and thus, severe pneumonia and enhanced virulence in mice. Importance: Influenza A viruses are respiratory pathogens that may cause pneumonia in humans. Viral infection and replication in the alveoli of the respiratory tract are believed to be crucial for the development of the acute respiratory distress syndrome associated with fatal outcomes in influenza virus-infected patients. Here, we report the requirement of a cellular factor, importin-α7, for efficient virus replication in the alveolar epithelium of mice. Using complementary ex vivo imaging approaches, we show that influenza virus replication is restricted to the bronchial epithelium, followed by enhanced survival in importin-α7-deficient mice. In contrast, the presence of this gene results in enhanced virus replication in the alveoli, elevated cytokine and chemokine responses, mononuclear infiltration, alveolar destruction, and 100% lethality in wild-type mice. Taken together, our results show that importin-α7 is particularly required for virus replication in the alveolar epithelium in association with severe pneumonia and death in mice.

Decrease of virus receptors during highly pathogenic H5N1 virus infection in humans and other mammals

Highly pathogenic avian influenza H5N1 virus causes a severe, often fatal, pneumonia in humans. The tropism and pathogenesis of highly pathogenic avian influenza H5N1 virus can partly be explained by the presence of H5N1 virus receptors in the human alveoli, which are the site of inflammation during pneumonia. Although studies on the distribution of influenza virus receptors in normal respiratory tract tissues have provided significant insights into the cell tropism and pathogenesis of influenza viruses, the distribution of influenza virus receptors have not been studied during influenza virus infection. Therefore, we studied the distribution of H5N1 virus receptors, by virus and lectin histochemistry, during highly pathogenic avian influenza H5N1 virus infection in alveolar tissues of humans, macaques, ferrets, and cats. In all species, we observed a decrease of H5N1 virus receptors in influenza virus-infected and neighboring cells. The observed decrease of H5N1 virus receptors was associated with the presence of MxA, a known marker for interferon activity. Taken together, our data suggest that the decrease of H5N1 virus receptors might be part of a defense mechanism that limits viral replication in the lower respiratory tract.

European H16N3 gull influenza virus attaches to the human respiratory tract and eye

We explored the attachment of an H16N3 influenza virus to human, mallard, and gull tissues using virus histochemistry applied to tissue microarrays and employing human and mallard viruses as references. Of the viruses tested, the H16N3 gull virus most readily attached to the human respiratory tract and eye. These results underscore the need to assess the potential for gull influenza viruses to replicate in human tissues and further investigate the role of gulls in influenza virus ecology.

Tropism and infectivity of influenza virus, including highly pathogenic avian H5N1 virus, in ferret tracheal differentiated primary epithelial cell cultures

Tropism and adaptation of influenza viruses to new hosts is partly dependent on the distribution of the sialic acid (SA) receptors to which the viral hemagglutinin (HA) binds. Ferrets have been established as a valuable in vivo model of influenza virus pathogenesis and transmission because of similarities to humans in the distribution of HA receptors and in clinical signs of infection. In this study, we developed a ferret tracheal differentiated primary epithelial cell culture model that consisted of a layered epithelium structure with ciliated and nonciliated cells on its apical surface. We found that human-like (α2,6-linked) receptors predominated on ciliated cells, whereas avian-like (α2,3-linked) receptors, which were less abundant, were presented on nonciliated cells. When we compared the tropism and infectivity of three human (H1 and H3) and two avian (H1 and H5) influenza viruses, we observed that the human influenza viruses primarily infected ciliated cells and replicated efficiently, whereas a highly pathogenic avian H5N1 virus (A/Vietnam/1203/2004) replicated efficiently within nonciliated cells despite a low initial infection rate. Furthermore, compared to other influenza viruses tested, VN/1203 virus replicated more efficiently in cells isolated from the lower trachea and at a higher temperature (37°C) compared to a lower temperature (33°C). VN/1203 virus infection also induced higher levels of immune mediator genes and cell death, and virus was recovered from the basolateral side of the cell monolayer. This ferret tracheal differentiated primary epithelial cell culture system provides a valuable in vitro model for studying cellular tropism, infectivity, and the pathogenesis of influenza viruses.