In Vitro Models for Studying Entry, Tissue Tropism, and Therapeutic Approaches of Highly Pathogenic Coronaviruses

Coronaviruses (CoVs) are enveloped nonsegmented positive-sense RNA viruses belonging to the family Coronaviridae that contain the largest genome among RNA viruses. Their genome encodes 4 major structural proteins, and among them, the Spike (S) protein plays a crucial role in determining the viral tropism. It mediates viral attachment to the host cell, fusion to the membranes, and cell entry using cellular proteases as activators. Several in vitro models have been developed to study the CoVs entry, pathogenesis, and possible therapeutic approaches. This article is aimed at summarizing the current knowledge about the use of relevant methodologies and cell lines permissive for CoV life cycle studies. The synthesis of this information can be useful for setting up specific experimental procedures. We also discuss different strategies for inhibiting the binding of the S protein to the cell receptors and the fusion process which may offer opportunities for therapeutic intervention.

Lack of effects of viral sialoadenitis and depletion of epidermal growth factor on initiation of hepatic carcinogenesis in the rat

Sialodacryoadenitis (SDA) is a commonly-encountered coronaviral infection in laboratory rats that causes acute destruction of submandibular salivary glands. SDA results in depletion of salivary epidermal growth factor (EGF) and may thereby affect EGF-dependent cell growth processes. The purpose of this study was to determine the effects of SDA virus (SDAV) infection on the growth factor-dependent stages of experimental liver carcinogenesis. Rats were injected ip with the carcinogen diethylnitrosamine (DENA) at 1, 2, or 3 weeks following inoculation with SDAV. Uninfected control rats were treated only with DENA. The salivary glands of SDAV-inoculated and control rats were stained using the immunoperoxidase method for the detection of EGF. Residual submandibular salivary gland lesions and focal depletion of EGF were still evident in affected submandibular glands for up to 42 days after SDAV infection. Serum EGF concentrations measured at 9, 28, and 42 days following SDAV inoculation were reduced, but were not significantly different in comparison with non-inoculated, DENA-treated control rats. Initiated hepatocytes were detected 21 days after DENA treatment in formalin-fixed sections by an immunoperoxidase stain for the P isoenzyme of the enzyme glutathione S-transferase (GST-P). There was no significant difference in the number of foci of GST-P positive cells in a comparison of initiated cells in SDAV-inoculated and non-inoculated rats. Based on this model, concurrent infection with SDAV does not appear to have any significant effects on the initial stages of chemical hepatocarcinogenesis in the rat.

Relapsing subacute demyelinating encephalomyelitis in rats during the course of coronavirus JHM infection

Temperature-sensitive mutants of the murine coronavirus JHM induced a subacute demyelinating encephalomyelitis (SDE) in young rats. Neurological symptoms were associated with marked lesions of primary demyelination in the white matter of the central nervous system (CNS), and developing after an incubation time of several weeks to months. Many rats survived this infection and recovered completely from this CNS disease. Among 43 survivors of SDE, 9 rats developed a relapse 27-153 days after onset of the first attack. Neuropathological examination of these animals revealed areas of fresh demyelination together with old remyelinated lesions. Viral antigens were detectable in the neighbourhood of fresh lesions and in some cases infectious virus was re-isolated from rats revealing low antibody titers to JHM virus. These results demonstrate that mutants of JHM virus can induce a relapsing demyelinating disease process, associated with a persistent infection, which possesses some similarities to chronic experimental allergic encephalomyelitis.

JHM infections in rats as a model for acute and subacute demyelinating disease

An animal model with different central nervous system (CNS) disease processes associated with demyelination is described which provides a basis to analyse the pathogenetic mechanisms leading to these disorders. Intracerebral infection of rats with the murine coronavirus strain JHM can result in an acute encephalomyelitis with a short incubation period or in subacute to chronic encephalomyelitis occurring after prolonged incubation. The most prominent finding of the latter two diseases consists of typical demyelinated lesions distributed in selected areas of the CNS. The induction of high rates of animals with demyelination depends both on properties of the virus used for infection and host factors such as age and immune status. A high number of rats with demyelination was obtained by intracerebral inoculation of temperature sensitive mutants into suckling rats with maternal JHM antibodies.

Inborn resistance of mice to mouse hepatitis virus type 3 (MHV3): liver parenchymal cells express phenotype in culture

Primary monolayer cultures of hepatocytes isolated from adult resistant A/J or partially resistant A/Sn or C3H/HeJ mice exhibited resistance to MHV3 as the respective macrophages do: Compared to susceptible C57BL/6 hepatocyte cultures, cytopathic effect occurred later and was restricted to small foci, coinciding with areas specifically labelled by immunofluorescence. Production of infectious particles was delayed, titers being 100 to 1000 fold lower at the moment of maximal yields in susceptible cultures. Pretreatment with interferon could reduce the titers in susceptible cultures to a level as seen in resistant cultures not treated with interferon. Nevertheless, interferon was not responsible for the genetic resistance of hepatocytes: it reduced virus titers in susceptible and resistant cultures to the same extent and the addition of specific antibodies to interferon after infection did not augment susceptibility of resistant cultures. We assume that intrinsic resistance of liver parenchymal cells is an important facet of inborn resistance of mice in vivo.

Persistent infection with mouse hepatitis virus, JHM strain in DBT cell culture

After inoculation with JHM strain into DBT cell monolayers, a persistently infected DBT cell culture was established without producing typical cytopathic changes after about 15th passages. By immunofluorescence virus specific antigen was demonstrated in 10 to 15% DBT cells. This persistently infected culture (JHM-CC) was resistant to superinfection with parental JHM, but such resistance was not shown against vesicular stomatitis virus. JHM-CC virus produced small plaques on DBT cell monolayers. Temperature sensitive (TS) mutant, defective interfering (DI) particle or interferon was not detected in the JHM-CC. To intracerebral inoculation with JHM-CC virus, cortisone treated ICR mice survived without showing clinical signs, however, demyelinating lesions were produced in the brain and spinal cord of them.