Systematic assembly of a full-length infectious clone of human coronavirus NL63

Manipulation of the porcine epidemic diarrhea virus genome using targeted RNA recombination

Porcine epidemic diarrhea virus (PEDV) causes severe economic losses in the swine industry in China and other Asian countries. Infection usually leads to an acute, often lethal diarrhea in piglets. Despite the impact of the disease, no system is yet available to manipulate the viral genome which has severely hampered research on this virus until today. We have established a reverse genetics system for PEDV based on targeted RNA recombination that allows the modification of the 3′-end of the viral genome, which encodes the structural proteins and the ORF3 protein. Using this system, we deleted the ORF3 gene entirely from the viral genome and showed that the ORF3 protein is not essential for replication of the virus in vitro. In addition, we inserted heterologous genes (i.e. the GFP and Renilla luciferase genes) at two positions in the viral genome, either as an extra expression cassette or as a replacement for the ORF3 gene. We demonstrated the expression of both GFP and Renilla luciferase as well as the application of these viruses by establishing a convenient and rapid virus neutralization assay. The new PEDV reverse genetics system will enable functional studies of the structural proteins and the accessory ORF3 protein and will allow the rational design and development of next generation PEDV vaccines.

A network integration approach to predict conserved regulators related to pathogenicity of influenza and SARS-CoV respiratory viruses

Respiratory infections stemming from influenza viruses and the Severe Acute Respiratory Syndrome corona virus (SARS-CoV) represent a serious public health threat as emerging pandemics. Despite efforts to identify the critical interactions of these viruses with host machinery, the key regulatory events that lead to disease pathology remain poorly targeted with therapeutics. Here we implement an integrated network interrogation approach, in which proteome and transcriptome datasets from infection of both viruses in human lung epithelial cells are utilized to predict regulatory genes involved in the host response. We take advantage of a novel “crowd-based” approach to identify and combine ranking metrics that isolate genes/proteins likely related to the pathogenicity of SARS-CoV and influenza virus. Subsequently, a multivariate regression model is used to compare predicted lung epithelial regulatory influences with data derived from other respiratory virus infection models. We predicted a small set of regulatory factors with conserved behavior for consideration as important components of viral pathogenesis that might also serve as therapeutic targets for intervention. Our results demonstrate the utility of integrating diverse ‘omic datasets to predict and prioritize regulatory features conserved across multiple pathogen infection models.

Isolation and genetic characterization of human coronavirus NL63 in primary human renal proximal tubular epithelial cells obtained from a commercial supplier, and confirmation of its replication in two different types of human primary kidney cells

Background

Cryopreserved primary human renal proximal tubule epithelial cells (RPTEC) were obtained from a commercial supplier for studies of Simian virus 40 (SV40). Within twelve hrs after cell cultures were initiated, cytoplasmic vacuoles appeared in many of the RPTEC. The RPTEC henceforth deteriorated rapidly. Since SV40 induces the formation of cytoplasmic vacuoles, this batch of RPTEC was rejected for the SV40 study. Nevertheless, we sought the likely cause(s) of the deterioration of the RPTEC as part of our technology development efforts.

Methods

Adventitious viruses in the RPTEC were isolated and/or detected and identified by isolation in various indicator cell lines, observation of cytopathology, an immunoflurorescence assay, electron microscopy, PCR, and sequencing.

Results

Cytomegalovirus (CMV) was detected in some RPTEC by cytology, an immunofluorescence assay, and PCR. Human Herpesvirus 6B was detected by PCR of DNA extracted from the RPTEC, but was not isolated. Human coronavirus NL63 was isolated and identified by RT-PCR and sequencing, and its replication in a fresh batch of RPTEC and another type of primary human kidney cells was confirmed.

Conclusions

At least 3 different adventitious viruses were present in the batch of contaminated RPTEC. Whereas we are unable to determine whether the original RPTEC were pre-infected prior to their separation from other kidney cells, or had gotten contaminated with HCoV-NL63 from an ill laboratory worker during their preparation for commercial sale, our findings are a reminder that human-derived biologicals should always be considered as potential sources of infectious agents. Importantly, HCoV-NL63 replicates to high titers in some primary human kidney cells.

Infectious Bronchitis Virus as a Vector for the Expression of Heterologous Genes

The avian coronavirus infectious bronchitis virus (IBV) is the causative agent of the respiratory disease infectious bronchitis of domestic fowl, and is controlled by routine vaccination. To explore the potential use of IBV as a vaccine vector a reverse genetics system was utilised to generate infectious recombinant IBVs (rIBVs) expressing the reporter genes enhanced green fluorescent protein (eGFP) or humanised Renilla luciferase (hRluc). Infectious rIBVs were obtained following the replacement of Gene 5 or the intergenic region (IR) with eGFP or hRluc, or the replacement of ORFs 3a and 3b with hRluc. The replacement of Gene 5 with an IBV codon-optimised version of the hRluc gene also resulted in successful rescue of infectious rIBV. Reporter gene expression was confirmed by fluorescence microscopy, or luciferase activity assays, for all successfully rescued rIBVs following infection of primary chick kidney (CK) cells. The genetic stability of rIBVs was analysed by serial passage on CK cells. Recombinant IBV stability varied depending on the genome region being replaced, with the reporter genes maintained up to at least passage 8 (P8) following replacement of Gene 5, P7 for replacement of the IR and P5 for replacement of ORFs 3a and 3b. Codon-optimisation of the hRluc gene, when replacing Gene 5, resulted in an increase in genome stability, with hRluc expression stable up to P10 compared to P8 for standard hRluc. Repeated passaging of rIBVs expressing hRluc at an MOI of 0.01 demonstrated an increase in stability, with hRluc expression stable up to at least P12 following the replacement of Gene 5. This study has demonstrated that heterologous genes can be incorporated into, and expressed from a range of IBV genome locations and that replacement of accessory Gene 5 offers a promising target for realising the potential of IBV as a vaccine vector for other avian pathogens.

Isolation and characterization of current human coronavirus strains in primary human epithelial cell cultures reveal differences in target cell tropism

The human airway epithelium (HAE) represents the entry port of many human respiratory viruses, including human coronaviruses (HCoVs). Nowadays, four HCoVs, HCoV-229E, HCoV-OC43, HCoV-HKU1, and HCoV-NL63, are known to be circulating worldwide, causing upper and lower respiratory tract infections in nonhospitalized and hospitalized children. Studies of the fundamental aspects of these HCoV infections at the primary entry port, such as cell tropism, are seriously hampered by the lack of a universal culture system or suitable animal models. To expand the knowledge on fundamental virus-host interactions for all four HCoVs at the site of primary infection, we used pseudostratified HAE cell cultures to isolate and characterize representative clinical HCoV strains directly from nasopharyngeal material. Ten contemporary isolates were obtained, representing HCoV-229E (n = 1), HCoV-NL63 (n = 1), HCoV-HKU1 (n = 4), and HCoV-OC43 (n = 4). For each strain, we analyzed the replication kinetics and progeny virus release on HAE cell cultures derived from different donors. Surprisingly, by visualizing HCoV infection by confocal microscopy, we observed that HCoV-229E employs a target cell tropism for nonciliated cells, whereas HCoV-OC43, HCoV-HKU1, and HCoV-NL63 all infect ciliated cells. Collectively, the data demonstrate that HAE cell cultures, which morphologically and functionally resemble human airways in vivo, represent a robust universal culture system for isolating and comparing all contemporary HCoV strains.

Release of severe acute respiratory syndrome coronavirus nuclear import block enhances host transcription in human lung cells

The severe acute respiratory syndrome coronavirus accessory protein ORF6 antagonizes interferon signaling by blocking karyopherin-mediated nuclear import processes. Viral nuclear import antagonists, expressed by several highly pathogenic RNA viruses, likely mediate pleiotropic effects on host gene expression, presumably interfering with transcription factors, cytokines, hormones, and/or signaling cascades that occur in response to infection. By bioinformatic and systems biology approaches, we evaluated the impact of nuclear import antagonism on host expression networks by using human lung epithelial cells infected with either wild-type virus or a mutant that does not express ORF6 protein. Microarray analysis revealed significant changes in differential gene expression, with approximately twice as many upregulated genes in the mutant virus samples by 48 h postinfection, despite identical viral titers. Our data demonstrated that ORF6 protein expression attenuates the activity of numerous karyopherin-dependent host transcription factors (VDR, CREB1, SMAD4, p53, EpasI, and Oct3/4) that are critical for establishing antiviral responses and regulating key host responses during virus infection. Results were confirmed by proteomic and chromatin immunoprecipitation assay analyses and in parallel microarray studies using infected primary human airway epithelial cell cultures. The data strongly support the hypothesis that viral antagonists of nuclear import actively manipulate host responses in specific hierarchical patterns, contributing to the viral pathogenic potential in vivo. Importantly, these studies and modeling approaches not only provide templates for evaluating virus antagonism of nuclear import processes but also can reveal candidate cellular genes and pathways that may significantly influence disease outcomes following severe acute respiratory syndrome coronavirus infection in vivo.

Evidence supporting a zoonotic origin of human coronavirus strain NL63

The relationship between bats and coronaviruses (CoVs) has received considerable attention since the severe acute respiratory syndrome (SARS)-like CoV was identified in the Chinese horseshoe bat (Rhinolophidae) in 2005. Since then, several bats throughout the world have been shown to shed CoV sequences, and presumably CoVs, in the feces; however, no bat CoVs have been isolated from nature. Moreover, there are very few bat cell lines or reagents available for investigating CoV replication in bat cells or for isolating bat CoVs adapted to specific bat species. Here, we show by molecular clock analysis that alphacoronavirus (α-CoV) sequences derived from the North American tricolored bat (Perimyotis subflavus) are predicted to share common ancestry with human CoV (HCoV)-NL63, with the most recent common ancestor between these viruses occurring approximately 563 to 822 years ago. Further, we developed immortalized bat cell lines from the lungs of this bat species to determine if these cells were capable of supporting infection with HCoVs. While SARS-CoV, mouse-adapted SARS-CoV (MA15), and chimeric SARS-CoVs bearing the spike genes of early human strains replicated inefficiently, HCoV-NL63 replicated for multiple passages in the immortalized lung cells from this bat species. These observations support the hypothesis that human CoVs are capable of establishing zoonotic-reverse zoonotic transmission cycles that may allow some CoVs to readily circulate and exchange genetic material between strains found in bats and other mammals, including humans.

Coronavirus pathogenesis

Coronaviruses infect many species of animals including humans, causing acute and chronic diseases. This review focuses primarily on the pathogenesis of murine coronavirus mouse hepatitis virus (MHV) and severe acute respiratory coronavirus (SARS-CoV). MHV is a collection of strains, which provide models systems for the study of viral tropism and pathogenesis in several organs systems, including the central nervous system, the liver, and the lung, and has been cited as providing one of the few animal models for the study of chronic demyelinating diseases such as multiple sclerosis. SARS-CoV emerged in the human population in China in 2002, causing a worldwide epidemic with severe morbidity and high mortality rates, particularly in older individuals. We review the pathogenesis of both viruses and the several reverse genetics systems that made much of these studies possible. We also review the functions of coronavirus proteins, structural, enzymatic, and accessory, with an emphasis on roles in pathogenesis. Structural proteins in addition to their roles in virion structure and morphogenesis also contribute significantly to viral spread in vivo and in antagonizing host cell responses. Nonstructural proteins include the small accessory proteins that are not at all conserved between MHV and SARS-CoV and the 16 conserved proteins encoded in the replicase locus, many of which have enzymatic activities in RNA metabolism or protein processing in addition to functions in antagonizing host response.

Human coronavirus NL63: a clinically important virus?

Respiratory tract infection is a leading cause of morbidity and mortality worldwide, especially among young children. Human coronaviruses (HCoVs) have only recently been shown to cause both lower and upper respiratory tract infections. To date, five coronaviruses (HCoV-229E, HCoV-OC43, SARS-CoV, HCoV-NL63 and HCoV HKU-1) that infect humans have been identified, four of which (HCoV-229E, HCoV-OC43, HCoV-NL63 and HCoV-HKU-1) circulate continuously in the human population. Human coronavirus NL63 (HCoV-NL63) was first isolated from the aspirate from a 7-month-old baby in early 2004. Infection with HCoV-NL63 has since been shown to be a common worldwide occurrence and has been associated with many clinical symptoms and diagnoses, including severe lower respiratory tract infection, croup and bronchiolitis. HCoV-NL63 causes disease in children, the elderly and the immunocompromised, and has been detected in 1.0-9.3% of respiratory tract infections in children. In this article, the current knowledge of human coronavirus HCoV-NL63, with special reference to the clinical features, prevalence and seasonal incidence, and coinfection with other respiratory viruses, will be discussed.

Coronaviruses: propagation, quantification, storage, and construction of recombinant mouse hepatitis virus

The focus of this protocol is mouse hepatitis virus (MHV), with occasional references to other coronaviruses. Many of these protocols can be easily adapted to other coronaviruses. Protocols for propagating MHV in DBT and 17CL-1 cells; the storage and titration of viral stocks; purification of MHV on sucrose gradients; and the generation of recombinant viruses by a cDNA assembly method and by targeted recombination will be presented. Protocols are also included for the propagation of DBT, 17CL-1, and L2 cells used for growing and titrating MHV, and for the growth of BHK-R cells and FCWF cells. The latter two cell lines are used for regenerating infectious MHV by an in vitro cDNA assembly protocol and by a targeted recombination protocol, respectively, allowing reverse genetic manipulation of these viruses. An additional protocol for the maintenance of the large plasmids used for generating recombinant MHVs will also be presented.

Coronavirus pathogenesis

Coronaviruses infect many species of animals including humans, causing acute and chronic diseases. This review focuses primarily on the pathogenesis of murine coronavirus mouse hepatitis virus (MHV) and severe acute respiratory coronavirus (SARS-CoV). MHV is a collection of strains, which provide models systems for the study of viral tropism and pathogenesis in several organs systems, including the central nervous system, the liver, and the lung, and has been cited as providing one of the few animal models for the study of chronic demyelinating diseases such as multiple sclerosis. SARS-CoV emerged in the human population in China in 2002, causing a worldwide epidemic with severe morbidity and high mortality rates, particularly in older individuals. We review the pathogenesis of both viruses and the several reverse genetics systems that made much of these studies possible. We also review the functions of coronavirus proteins, structural, enzymatic, and accessory, with an emphasis on roles in pathogenesis. Structural proteins in addition to their roles in virion structure and morphogenesis also contribute significantly to viral spread in vivo and in antagonizing host cell responses. Nonstructural proteins include the small accessory proteins that are not at all conserved between MHV and SARS-CoV and the 16 conserved proteins encoded in the replicase locus, many of which have enzymatic activities in RNA metabolism or protein processing in addition to functions in antagonizing host response.

Culturing the unculturable: human coronavirus HKU1 infects, replicates, and produces progeny virions in human ciliated airway epithelial cell cultures

Culturing newly identified human lung pathogens from clinical sample isolates can represent a daunting task, with problems ranging from low levels of pathogens to the presence of growth suppressive factors in the specimens, compounded by the lack of a suitable tissue culture system. However, it is critical to develop suitable in vitro platforms to isolate and characterize the replication kinetics and pathogenesis of recently identified human pathogens. HCoV-HKU1, a human coronavirus identified in a clinical sample from a patient with severe pneumonia, has been a major challenge for successful propagation on all immortalized cells tested to date. To determine if HCoV-HKU1 could replicate in in vitro models of human ciliated airway epithelial cell cultures (HAE) that recapitulate the morphology, biochemistry, and physiology of the human airway epithelium, the apical surfaces of HAE were inoculated with a clinical sample of HCoV-HKU1 (Cean1 strain). High virus yields were found for several days postinoculation and electron micrograph, Northern blot, and immunofluorescence data confirmed that HCoV-HKU1 replicated efficiently within ciliated cells, demonstrating that this cell type is infected by all human coronaviruses identified to date. Antiserum directed against human leukocyte antigen C (HLA-C) failed to attenuate HCoV-HKU1 infection and replication in HAE, suggesting that HLA-C is not required for HCoV-HKU1 infection of the human ciliated airway epithelium. We propose that the HAE model provides a ready platform for molecular studies and characterization of HCoV-HKU1 and in general serves as a robust technology for the recovery, amplification, adaptation, and characterization of novel coronaviruses and other respiratory viruses from clinical material.

A mechanism of virus-induced demyelination

Myelin forms an insulating sheath surrounding axons in the central and peripheral nervous systems and is essential for rapid propagation of neuronal action potentials. Demyelination is an acquired disorder in which normally formed myelin degenerates, exposing axons to the extracellular environment. The result is dysfunction of normal neuron-to-neuron communication and in many cases, varying degrees of axonal degeneration. Numerous central nervous system demyelinating disorders exist, including multiple sclerosis. Although demyelination is the major manifestation of most of the demyelinating diseases, recent studies have clearly documented concomitant axonal loss to varying degrees resulting in long-term disability. Axonal injury may occur secondary to myelin damage (outside-in model) or myelin damage may occur secondary to axonal injury (inside-out model). Viral induced demyelination models, has provided unique imminent into the cellular mechanisms of myelin destruction. They illustrate mechanisms of viral persistence, including latent infections, virus reactivation and viral-induced tissue damage. These studies have also provided excellent paradigms to study the interactions between the immune system and the central nervous system (CNS). In this review we will discuss potential cellular and molecular mechanism of central nervous system axonal loss and demyelination in a viral induced mouse model of multiple sclerosis.

Understanding Human Coronavirus HCoV-NL63

Even though coronavirus infection of humans is not normally associated with severe diseases, the identification of the coronavirus responsible for the outbreak of severe acute respiratory syndrome showed that highly pathogenic coronaviruses can enter the human population. Shortly thereafter, in Holland in 2004, another novel human coronavirus (HCoV-NL63) was isolated from a seven-month old infant suffering from respiratory symptoms. This virus has subsequently been identified in various countries, indicating a worldwide distribution. HCoV-NL63 has been shown to infect mainly children and the immunocommpromised, who presented with either mild upper respiratory symptoms (cough, fever and rhinorrhoea) or more serious lower respiratory tract involvement such as bronchiolitis and croup, which was observed mainly in younger children. In fact, HCoV-NL63 is the aetiological agent for up to 10% of all respiratory diseases. This review summarizes recent findings of human coronavirus HCoV-NL63 infections, including isolation and identification, phylogeny and taxonomy, genome structure and transcriptional regulation, transmission and pathogenesis, and detection and diagnosis.

Genome engineering

For more than 50 years, those engineering genetic material have pursued increasingly challenging targets. During that time, the tools and resources available to the genetic engineer have grown to encompass new extremes of both scale and precision, opening up new opportunities in genome engineering. Today, our capacity to generate larger de novo assemblies of DNA is increasing at a rapid pace (with concomitant decreases in manufacturing cost). We are also witnessing potent demonstrations of the power of merging randomness and selection with engineering approaches targeting large numbers of specific sites within genomes. These developments promise genetic engineering with unprecedented levels of design originality and offer new avenues to expand both our understanding of the biological world and the diversity of applications for societal benefit.

Human coronavirus NL63 open reading frame 3 encodes a virion-incorporated N-glycosylated membrane protein

BACKGROUND

Human pathogenic coronavirus NL63 (hCoV-NL63) is a group 1 (alpha) coronavirus commonly associated with respiratory tract infections. In addition to known non-structural and structural proteins all coronaviruses have one or more accessory proteins whose functions are mostly unknown. Our study focuses on hCoV-NL63 open reading frame 3 (ORF 3) which is a highly conserved accessory protein among coronaviruses.

RESULTS

In-silico analysis of the 225 amino acid sequence of hCoV-NL63 ORF 3 predicted a triple membrane-spanning protein. Expression in infected CaCo-2 and LLC-MK2 cells was confirmed by immunofluorescence and Western blot analysis. The protein was detected within the endoplasmatic reticulum/Golgi intermediate compartment (ERGIC) where coronavirus assembly and budding takes place. Subcellular localization studies using recombinant ORF 3 protein transfected in Huh-7 cells revealed occurrence in ERGIC, Golgi- and lysosomal compartments. By fluorescence microscopy of differently tagged envelope (E), membrane (M) and nucleocapsid (N) proteins it was shown that ORF 3 protein colocalizes extensively with E and M within the ERGIC. Using N-terminally FLAG-tagged ORF 3 protein and an antiserum specific to the C-terminus we verified the proposed topology of an extracellular N-terminus and a cytosolic C-terminus. By in-vitro translation analysis and subsequent endoglycosidase H digestion we showed that ORF 3 protein is N-glycosylated at the N-terminus. Analysis of purified viral particles revealed that ORF 3 protein is incorporated into virions and is therefore an additional structural protein.

CONCLUSIONS

This study is the first extensive expression analysis of a group 1 hCoV-ORF 3 protein. We give evidence that ORF 3 protein is a structural N-glycosylated and virion-incorporated protein.

Seroconversion to HCoV-NL63 in Rhesus Macaques

HCoV-NL63 is a recently identified respiratory virus. Its pathogenesis has not been fully unraveled because an animal model is currently lacking. Here we examined whether rhesus macaques encounter HCoV-NL63 infections during life, by examining the levels of antibodies to HCoV-NL63 in time. The animals were followed for 7 up till 19 years, and in three animals we observed a steep rise in antibodies during follow up, indicative of a natural infection with HCoV-NL63.

Differential downregulation of ACE2 by the spike proteins of severe acute respiratory syndrome coronavirus and human coronavirus NL63

The human coronaviruses (CoVs) severe acute respiratory syndrome (SARS)-CoV and NL63 employ angiotensin-converting enzyme 2 (ACE2) for cell entry. It was shown that recombinant SARS-CoV spike protein (SARS-S) downregulates ACE2 expression and thereby promotes lung injury. Whether NL63-S exerts a similar activity is yet unknown. We found that recombinant SARS-S bound to ACE2 and induced ACE2 shedding with higher efficiency than NL63-S. Shedding most likely accounted for the previously observed ACE2 downregulation but was dispensable for viral replication. Finally, SARS-CoV but not NL63 replicated efficiently in ACE2-positive Vero cells and reduced ACE2 expression, indicating robust receptor interference in the context of SARS-CoV but not NL63 infection.

Coronaviruses post-SARS: update on replication and pathogenesis

Coronaviruses are positive strand RNA viruses that cause disease in humans, and domestic and companion animals. They are most notorious for causing severe acute respiratory syndrome (SARS) outbreaks in 2002–2003. All coronaviruses follow the same basic strategy of replication.

All coronaviruses encode 15 or 16 replicase related proteins, 4 or 5 structural proteins and 1–8 group-specific or accessory proteins. Many of the replicase proteins are assembled into replication machinery in double-membrane vesicles (DMVs) and on a reticular network of membranes that are derived from the endoplasmic reticulum.

Coronaviruses are readily transmitted across species. This phenomenon was illustrated when the SARS-coronavirus crossed species from bats to intermediate hosts, such as palm civets, and then to humans. It also explains the large number of species, including humans, that are infected with viruses closely related to bovine coronavirus.

In many coronavirus infections, disease severity increases during virus clearance, suggesting that the host immune response is both protective and pathogenic. Furthermore, inhibition of specific aspects of the immune response results in less severe disease and less tissue destruction, without diminishing the kinetics of virus clearance.

Like all successful viruses, coronaviruses have evolved both passive and active mechanisms to evade the interferon response. Replication in DMVs may contribute to passive evasion of the innate immune response by making double-stranded RNA inaccessible to cellular sensors.

Coronaviruses gained prominence during the SARS outbreaks of 2002–2003, but there are many different coronaviruses that infect humans and animals. Perlman and Netland describe the biology of the coronaviruses, including their replication, host immune response and interspecies transmission.

Although coronaviruses were first identified nearly 60 years ago, they only received notoriety in 2003 when one of their members was identified as the aetiological agent of severe acute respiratory syndrome. Previously these viruses were known to be important agents of respiratory and enteric infections of domestic and companion animals and to cause approximately 15% of all cases of the common cold. This Review focuses on recent advances in our understanding of the mechanisms of coronavirus replication, interactions with the host immune response and disease pathogenesis. It also highlights the recent identification of numerous novel coronaviruses and the propensity of this virus family to cross species barriers.

Plaque assay for human coronavirus NL63 using human colon carcinoma cells

Background

Coronaviruses cause a broad range of diseases in animals and humans. Human coronavirus (hCoV) NL63 is associated with up to 10% of common colds. Viral plaque assays enable the characterization of virus infectivity and allow for purifying virus stock solutions. They are essential for drug screening. Hitherto used cell cultures for hCoV-NL63 show low levels of virus replication and weak and diffuse cytopathogenic effects. It has not yet been possible to establish practicable plaque assays for this important human pathogen.

Results

12 different cell cultures were tested for susceptibility to hCoV-NL63 infection. Human colon carcinoma cells (CaCo-2) replicated virus more than 100 fold more efficiently than commonly used African green monkey kidney cells (LLC-MK2). CaCo-2 cells showed cytopathogenic effects 4 days post infection. Avicel, agarose and carboxymethyl-cellulose overlays proved suitable for plaque assays. Best results were achieved with Avicel, which produced large and clear plaques from the 4^th day of infection. The utility of plaque assays with agrose overlay was demonstrated for purifying virus, thereby increasing viral infectivity by 1 log 10 PFU/mL.

Conclusion

CaCo-2 cells support hCoV-NL63 better than LLC-MK2 cells and enable cytopathogenic plaque assays. Avicel overlay is favourable for plaque quantification, and agarose overlay is preferred for plaque purification. HCoV-NL63 virus stock of increased infectivity will be beneficial in antiviral screening, animal modelling of disease, and other experimental tasks.