The first steps of human coronavirus NL63 (HCoV-NL63) infection were previously described. The virus binds to target cells by use of heparan sulfate proteoglycans and interacts with the ACE2 protein. Subsequent events, including virus internalization and trafficking, remain to be elucidated. In this study, we mapped the process of HCoV-NL63 entry into the LLC-Mk2 cell line and three-dimensional (3D) tracheobronchial tissue. Using a variety of techniques, we have shown that HCoV-NL63 virions require endocytosis for successful entry into the LLC-MK2 cells, and interaction between the virus and the ACE2 molecule triggers recruitment of clathrin. Subsequent vesicle scission by dynamin results in virus internalization, and the newly formed vesicle passes the actin cortex, which requires active cytoskeleton rearrangement. Finally, acidification of the endosomal microenvironment is required for successful fusion and release of the viral genome into the cytoplasm. For 3D tracheobronchial tissue cultures, we also observed that the virus enters the cell by clathrin-mediated endocytosis, but we obtained results suggesting that this pathway may be bypassed. Available data on coronavirus entry frequently originate from studies employing immortalized cell lines or undifferentiated cells. Here, using the most advanced 3D tissue culture system mimicking the epithelium of conductive airways, we systematically mapped HCoV-NL63 entry into susceptible cells. The data obtained allow for a better understanding of the infection process and may support development of novel treatment strategies.

To date, six human coronaviruses have been known, all of which are associated with respiratory infections in humans. With the exception of the highly pathogenic SARS and MERS coronaviruses, human coronaviruses (HCoV-NL63, HCoV-OC43, HCoV-229E, and HCoV-HKU1) circulate worldwide and typically cause the common cold. In most cases, infection with these viruses does not lead to severe disease, although acute infections in infants, the elderly, and immunocompromised patients may progress to severe disease requiring hospitalization. Importantly, no drugs against human coronaviruses exist, and only supportive therapy is available. Previously, we proposed the cationically modified chitosan, N-(2-hydroxypropyl)-3-trimethylammonium chitosan chloride (HTCC), and its hydrophobically-modified derivative (HM-HTCC) as potent inhibitors of the coronavirus HCoV-NL63. Here, we show that HTCC inhibits interaction of a virus with its receptor and thus blocks the entry. Further, we demonstrate that HTCC polymers with different degrees of substitution act as effective inhibitors of all low-pathogenic human coronaviruses.

Understanding the mechanisms of augmented bacterial pathogenicity in post-viral infections is the first step in the development of an effective therapy. This study assessed the effect of human coronavirus NL63 (HCoV-NL63) on the adherence of bacterial pathogens associated with respiratory tract illnesses. It was shown that HCoV-NL63 infection resulted in an increased adherence of Streptococcus pneumoniae to virus-infected cell lines and fully differentiated primary human airway epithelium cultures. The enhanced binding of bacteria correlated with an increased expression level of the platelet-activating factor receptor (PAF-R), but detailed evaluation of the bacterium-PAF-R interaction revealed a limited relevance of this process. INTRODUCTIONThe concept of excessive morbidity and mortality of bacterial infection occurring during or shortly after viral infection was first formulated for influenza virus in the early 19th century. Analysis of influenza pandemics showed that the incidence of bacterial pneumonia was increased and contributed substantially to mortality rates (Abrahams et al., 1919;Muir & Wilson, 1919;Stone & Swift, 1919;Wilson & Steer, 1919). Comparison of bacteriological and virological data from children hospitalized for respiratory disease shows a high degree of occurrence of viral and bacterial infections positively correlating with the severity of illness (Duttweiler et al., 2004;Kneyber et al., 2005;Randolph et al., 2004;Thorburn et al., 2006). Although the role of a preceding viral infection in development and severity of bacterial respiratory diseases is a clinically welldocumented phenomenon, the exact mechanism has not been elucidated fully.Initially, it was proposed that respiratory viruses facilitate bacterial colonization through physical damage of the respiratory tract epithelium, with exposed basement membrane components being responsible for increased bacterial adherence (Louria et al., 1959;Muir & Wilson, 1919;Wilson & Steer, 1919;Wolbach, 1919). Such a mechanism undoubtedly occurs for highly pathogenic viral species, but it does not explain the occurrence of increased severity of bacterial infection during and shortly after relatively mild viral infections. Analysis of published data suggests that interplay between viruses and bacteria is a complex process, where the final outcome depends heavily on multiple factors, including modulation of innate immune responses resulting in delayed clearance of bacteria, hypersensitization of infected cells leading to enhanced immune-mediated lung damage and modulation of bacterial adherence (Okamoto et al., 2004). The increase in bacterial adherence occurs due to exposure of novel binding sites for bacteria on the epithelial surface, either by expression of highly glycosylated viral proteins (McCullers & Bartmess, 2003;Peltola & McCullers, 2004) or by the alteration of a bacterial receptor expression pattern (McCullers & Rehg, 2002;Patel et al., 1995;Terajima et al., 1997).Bacterial pathogens predominantly involved in secondary infection of the r...

Human coronavirus (HCoV) NL63 was first described in 2004 and is associated with respiratory tract disease of varying severity. At the genetic and structural level, HCoV-NL63 is similar to other members of the Coronavirinae subfamily, especially human coronavirus 229E (HCoV-229E). Detailed analysis, however, reveals several unique features of the pathogen. The coronaviral nucleocapsid protein is abundantly present in infected cells. It is a multi-domain, multi-functional protein important for viral replication and a number of cellular processes. The aim of the present study was to characterize the HCoV-NL63 nucleocapsid protein. Biochemical analyses revealed that the protein shares characteristics with homologous proteins encoded in other coronaviral genomes, with the N-terminal domain responsible for nucleic acid binding and the C-terminal domain involved in protein oligomerization. Surprisingly, analysis of the subcellular localization of the N protein of HCoV-NL63 revealed that, differently than homologous proteins from other coronaviral species except for SARS-CoV, it is not present in the nucleus of infected or transfected cells. Furthermore, no significant alteration in cell cycle progression in cells expressing the protein was observed. This is in stark contrast with results obtained for other coronaviruses, except for the SARS-CoV.

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