The primary target of severe acute respiratory syndrome-associated coronavirus (SARS-CoV) is epithelial cells in the respiratory and intestinal tract. The cellular receptor for SARS-CoV, angiotensin-converting enzyme 2 (ACE2), has been shown to be localized on the apical plasma membrane of polarized respiratory epithelial cells and to mediate infection from the apical side of these cells. Here, these results were confirmed and extended by including a colon carcinoma cell line (Caco-2), a lung carcinoma cell line (Calu-3) and Vero E6 cells in our analysis. All three cell types expressed human ACE2 on the apical membrane domain and were infected via this route, as determined with vesicular stomatitis virus pseudotypes containing the S protein of SARS-CoV. In a histological analysis of the respiratory tract, ACE2 was detected in the trachea, main bronchus and alveoli, and occasionally also in the small bronchi. These data will help us to understand the pathogenesis of SARS-CoV infection.

Virus-host interactions in the respiratory epithelium during long term influenza virus infection are not well characterized. Therefore, we developed an air-liquid interface culture system for differentiated porcine respiratory epithelial cells to study the effect of virus-induced cellular damage. In our well-differentiated cells, α2,6-linked sialic acid is predominantly expressed on the apical surface and the basal cells mainly express α2,3-linked sialic acid. During the whole infection period, release of infectious virus was maintained at a high titre for more than seven days. The infected epithelial cells were subject to apoptosis resulting in the loss of ciliated cells together with a thinner thickness. Nevertheless, the airway epithelium maintained trans-epithelial electrical resistance and retained its barrier function. The loss of ciliated cells was compensated by the cells which contained the KRT5 basal cell marker but were not yet differentiated into ciliated cells. These specialized cells showed an increase of α2,3-linked sialic acid on the apical surface. In sum, our results help to explain the localized infection of the airway epithelium by influenza viruses. The impairment of mucociliary clearance in the epithelial cells provides an explanation why prior viral infection renders the host more susceptible to secondary co-infection by another pathogen.

Bats (Chiroptera) host major human pathogenic viruses including corona-, paramyxo, rhabdo- and filoviruses. We analyzed six different cell lines from either Yinpterochiroptera (including African flying foxes and a rhinolophid bat) or Yangochiroptera (genera Carollia and Tadarida) for susceptibility to infection by different enveloped RNA viruses. None of the cells were sensitive to infection by transmissible gastroenteritis virus (TGEV), a porcine coronavirus, or to infection mediated by the Spike (S) protein of SARS-coronavirus (SARS-CoV) incorporated into pseudotypes based on vesicular stomatitis virus (VSV). The resistance to infection was overcome if cells were transfected to express the respective cellular receptor, porcine aminopeptidase N for TGEV or angiotensin-converting enzyme 2 for SARS-CoV. VSV pseudotypes containing the S proteins of two bat SARS-related CoV (Bg08 and Rp3) were unable to infect any of the six tested bat cell lines. By contrast, viral pseudotypes containing the surface protein GP of Marburg virus from the family Filoviridae infected all six cell lines though at different efficiency. Notably, all cells were sensitive to infection by two paramyxoviruses (Sendai virus and bovine respiratory syncytial virus) and three influenza viruses from different subtypes. These results indicate that bat cells are more resistant to infection by coronaviruses than to infection by paramyxoviruses, filoviruses and influenza viruses. Furthermore, these results show a receptor-dependent restriction of the infection of bat cells by CoV. The implications for the isolation of coronaviruses from bats are discussed.

The importance of sialic acid for infection by avian Infectious bronchitis virus (IBV) has been analysed. Neuraminidase treatment rendered Vero, baby hamster kidney and primary chicken kidney cells resistant to infection by the IBV-Beaudette strain. Sialic acid-dependent infection was also observed with strain M41 of IBV, which infects primary chicken kidney cells but not cells from other species. In comparison with Influenza A virus and Sendai virus, IBV was most sensitive to pre-treatment of cells with neuraminidase. This finding suggests that IBV requires a greater amount of sialic acid on the cell surface to initiate an infection compared with the other two viruses. In previous studies, with respect to the haemagglutinating activity of IBV, it has been shown that the virus preferentially recognizes α2,3-linked sialic acid. In agreement with this finding, susceptibility to infection by IBV was connected to the expression of α2,3-linked sialic acid as indicated by the reactivity with the lectin Maackia amurensis agglutinin. Here, it is discussed that binding to sialic acid may be used by IBV for primary attachment to the cell surface; tighter binding and subsequent fusion between the viral and the cellular membrane may require interaction with a second receptor.

We detected a highly pathogenic avian influenza A(H5N8) virus in lung samples of 2 gray seals (Halichoerus grypus) stranded on the Baltic coast of Poland in 2016 and 2017. This virus, clade 2.3.4.4 B, was closely related to avian H5N8 viruses circulating in Europe at the time.