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Viruses from the Coronaviridae, Togaviridae, and Hepeviridae families ​all contain genes that encode a conserved protein domain, called a macrodomain; however, the role of this domain during infection has remained enigmatic. The recent discovery that mammalian macrodomain proteins enzymatically remove ADP-ribose, a common post-translation modification, from proteins has led to an outburst of studies describing both the enzymatic activity and function of viral macrodomains. These new studies have defined these domains as de-ADP-ribosylating enzymes, which indicates that these viruses have evolved to counteract antiviral ADP-ribosylation, likely mediated by poly-ADP-ribose polymerases (PARPs). Here, we comprehensively review this rapidly expanding field, describing the structures and enzymatic activities of viral macrodomains, and discussing their roles in viral replication and pathogenesis., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

A novel betacoronavirus, human coronavirus (HCoV-EMC), has recently been detected in humans with severe respiratory disease. Further characterization of HCoV-EMC suggests that this virus is different from severe acute respiratory syndrome coronavirus (SARS-CoV) because it is able to replicate in multiple mammalian cell lines and it does not use angiotensin-converting enzyme 2 as a receptor to achieve infection. Additional research is urgently needed to better understand the pathogenicity and tissue tropism of this virus in humans. In their recent study published in mBio, Kindler et al. shed some light on these important topics (E. Kindler, H. R. Jónsdóttir, M. Muth, O. J. Hamming, R. Hartmann, R. Rodriguez, R. Geffers, R. A. Fouchier, C. Drosten, M. A. Müller, R. Dijkman, and V. Thiel, mBio 4[1]:e00611-12, 2013). These authors report the use of differentiated pseudostratified human primary airway epithelial cells, an in vitro model with high physiological relevance to the human airway epithelium, to characterize the cellular tropism of HCoV-EMC. More importantly, the authors demonstrate the potential use of type I and type III interferons (IFNs) to control viral infection.

The recent emergence of a novel human coronavirus (HCoV-EMC) in the Middle East raised considerable concerns, as it is associated with severe acute pneumonia, renal failure, and fatal outcome and thus resembles the clinical presentation of severe acute respiratory syndrome (SARS) observed in 2002 and 2003. Like SARS-CoV, HCoV-EMC is of zoonotic origin and closely related to bat coronaviruses. The human airway epithelium (HAE) represents the entry point and primary target tissue for respiratory viruses and is highly relevant for assessing the zoonotic potential of emerging respiratory viruses, such as HCoV-EMC. Here, we show that pseudostratified HAE cultures derived from different donors are highly permissive to HCoV-EMC infection, and by using reverse transcription (RT)-PCR and RNAseq data, we experimentally determined the identity of seven HCoV-EMC subgenomic mRNAs. Although the HAE cells were readily responsive to type I and type III interferon (IFN), we observed neither a pronounced inflammatory cytokine nor any detectable IFN responses following HCoV-EMC, SARS-CoV, or HCoV-229E infection, suggesting that innate immune evasion mechanisms and putative IFN antagonists of HCoV-EMC are operational in the new host. Importantly, however, we demonstrate that both type I and type III IFN can efficiently reduce HCoV-EMC replication in HAE cultures, providing a possible treatment option in cases of suspected HCoV-EMC infection. IMPORTANCE A novel human coronavirus, HCoV-EMC, has recently been described to be associated with severe respiratory tract infection and fatalities, similar to severe acute respiratory syndrome (SARS) observed during the 2002-2003 epidemic. Closely related coronaviruses replicate in bats, suggesting that, like SARS-CoV, HCoV-EMC is of zoonotic origin. Since the animal reservoir and circumstances of zoonotic transmission are yet elusive, it is critically important to assess potential species barriers of HCoV-EMC infection. An important first barrier against invading respiratory pathogens is the epithelium, representing the entry point and primary target tissue of respiratory viruses. We show that human bronchial epithelia are highly susceptible to HCoV-EMC infection. Furthermore, HCoV-EMC, like other coronaviruses, evades innate immune recognition, reflected by the lack of interferon and minimal inflammatory cytokine expression following infection. Importantly, type I and type III interferon treatment can efficiently reduce HCoV-EMC replication in the human airway epithelium, providing a possible avenue for treatment of emerging virus infections.

Three strains of human coronavirus (HCV) OC43 were compared for their ability to cause enteric infections and to induce interferon alpha (IFN alpha) using the Caco-2 human colon carcinoma cell line which exhibits spontaneous epithelial differentiation in vitro. MRC-5 cell culture grown stocks were prepared from: 1. CV Paris, a strain of OC43 recovered from an outbreak of necrotizing enterocolitis in newborns. 2. CV Mb, a neurotropic strain of OC43 which exhibits strict neuronal specificity in murine neuronal cell cultures. 3. CV Rd, a strain of OC43 which grows to a high titer in human rhabdomyosarcoma (RD) cells. Immunofluorescent staining for nucleocapsid antigen and plaque assay in MRC-5 cells was used to detect viral replication. BG-9 (human foreskin) cells challenged with vesicular stomatitis virus were used to detect IFN alpha production by human peripheral blood monocytes (PBMC) stimulated by virus infected Caco-2 cells. Caco-2 cells infected with virus at a multiplicity of infection of 0.5 yielded 10(4.6) and 10(4.4) plaque forming units/ml (pfu/ml) with CV Rd and CV Paris respectively, while CV Mb yielded only 10(3) pfu/ml. Caco-2 cells infected with CV Rd induced 64 IU/ml of IFN alpha in PBMC while these cells infected with CV Paris induced less than 2 IU/ml IFN alpha. In cells infected with CV Mb 4 IU/ml IFN alpha was detected. The results suggest that a lack of IFN alpha induction by CV Paris may be an indicator of its enteropathogenic potential.(ABSTRACT TRUNCATED AT 250 WORDS)

The in vitro adaptation of enteric coronaviruses is difficult. In our Laboratories, the multiplication of the COI bovine strain (F15 origin), on Vero cells is commonly practised. Since other laboratory tests (hemagglutination inhibition, cytopathic effect and immunofluorescence) are not totally reliable, electron microscopic studies present definite advantages in the interpretation of viral synthesis, as demonstrated at the 11th passage in our study. We were able to observe the following: --that the rate of infected and productive cells was important; --that the stages of viral morphogenesis were well established; --that the viral synthesis was prolific and regular and could be witnessed by important intracytoplasmic storage. The results of this study prove that the choice of our cell system is very satisfactory.

Bovine embryonic lung (BEL) cells, grown in 8-chamber cell-culture slides, were inoculated with 2.5, 25, and 250 TCID50 of neonatal calf diarrhea coronavirus (NCDCV)/chamber. The NCDCV inocula were prepared in serum-free, trypsin-free regular medium (RM), and in serum-free medium containing 1 microgram (TM1), 5 microgram (TM5), and 10 microgram (TM10) of trypsin/ml; the the cells were maintained under identical media. Cytopathic effect (CPE), hemadsorption of rat RBC, formation of syncytia, appearance of immunofluorescing cells, and release of infectious virus was followed from postinoculation hours 4 to 100. Maintaining the BEL cells in medium containing trypsin up to 10 microgram/ml did not affect them adversely. The BEL cells appeared resistant to infection with NCDCV in RM by CPE, syncytia, immunofluorescing cells, or infectious virus-release parameters. In TM1, TM5, and TM10, all parameters were positive for NCDCV replication. They manifested and developed in positive correlation with increasing trypsin concentration from 1 microgram to 10 microgram/ml. Release of infectious virions preceded all other parameters of virus replication. Replication of NCDCV was demonstrated earlier by hemadsorption and immunofluorescing cells than by syncytia or CPE. The TM5 and TM10 increased the susceptibility of BEL cells to NCDCV by more than a millionfold, as measured by the titer of released virus in these media (greater than 1-(6) TCID50/ml) as compared with those in RM (infectious virus not demonstrated).

Sialodacryoadenitis virus of rat readily propagated and induced marked cytopathic effect in a rat cell line, LBC cell culture, which provides a sensitive, practical assay system for viral infectivity and neutralizing antibody, and a satisfactory source of the virus.

Coronaviruses can establish persistent infections in the central nervous system of rodents, and these are associated with demyelinating encephalomyelitis. The effects of persistence on the virus are difficult to study in vivo but may have a crucial influence on the course of infection. We therefore produced a persistent infection in vitro using the neurotropic coronavirus JHM, in order to investigate the events underlying the establishment of such an infection and the adaptation of the virus to persistence. The persistent infection was maintained for over 115 passages and continued to release high levels of infectious virus. During the 18 months of culture the number of cells expressing virus antigen detected by indirect immune fluorescence decreased to 40%. Analysis showed that the carried virus contained a significant proportion of heterogeneous temperature-sensitive mutants. All virus clones isolated possessed the capacity to induce a more productive growth cycle, a less pronounced cytopathic effect and showed a much reduced neurovirulence when inoculated into newborn and weanling rats. Evidence for structural changes involving the surface peplomer protein (E2) was obtained using hybridoma antibodies, which neutralized the parental JHM virus but not the JHM-Pi virus. Defective interfering particles and interferon activities have been excluded as possible agents instrumental in the establishment and maintenance of the chronic infection, and we suggest that the emergence of virus variants of lowered virulence is central to these processes.

Three Beaudette strains of infectious bronchitis virus (IBV) [two chicken-embryo trachea-culture-adapted (E71CET10 and E71CEK10CET30) and one embryo-adapted (E72)] were propagated in 10-day-old embryonated chicken eggs (CE) to investigate growth and lethality patterns. The E72 and E71CEK10CET30 viruses were similar in growth pattern as to logarithmic phase and maximum virus production, but 95% of the CE mortality with E72 virus occurred between 20 and 32 hours whereas with E71CEK10CET30 60% mortality occurred between 36 and 84 hours. The yield of the E71CET10 virus was minimum, with little lethality.

Sialodacryoadenitis (SDA) is a naturally-occurring infection of the laboratory rat raused by the coronavirus, sialodacryoadenitis virus (SDAV). The study of SDAV has been limited because there is no widely available continuous cell line for the propagation of high titers of the virus. The purpose of this study, therefore, was to compare the ability of SDAV to replicate in the permanent cell lines, LBC, of rat origin, and the mouse cell lines. L-929 and L-2. Following 2 to 6 repeated passages of SDAV in LBC cells, the virus could be readily propagated in LBC and L-2 cells, but not in L-929 cells. Similarly, SDAV adapted to replicate directly in L-2 cells could be readily propagated in LBC, but not L-929 cells. In LBC and L-2 cells, cytopathic effect (CPE), viral antigen, viral particles, and virus infectivity could be demonstrated. Titers of up to 10(8.0) infectious viral particles/0.25 ml of culture fluid were obtained at 48 hours in L-2 cells. Titers in LBC cells were one to two logs lower. When susceptible rats were inoculated with eighth passage L-2 cell-adapted virus, they developed typical lesions of SDA. Virus could be recovered from infected tissues and propagated in L-2 cells on first passage. The ability to propagate SDAV to high titers in the widely available L-2 cell line should promote the study of this virus and facilitate its comparison with other murine coronaviruses.