Nor do they take into account dependence of the cellular response on experimental conditions. For example, fetal bovine serum abrogates apoptosis in HeLa cells infected with poliovirus in the presence of guanidine hydrochloride [45] , suggesting that variability of host responses to viral infection may partly be due to differences in experimental settings.
The response to a given picornavirus is also host cell-specific. Theiler's murine encephalomyelitis virus, a cardiovirus, elicits necrosis in permissive cells but triggers apoptosis in restrictive ones [46]. Although productive poliovirus infection of HeLa leads, as already said, to necrosis [8] , infection of partially permissive promonocytic cells brings about apoptosis [47].
The difference in response to a virus is not necessarily related to the level of host permissiveness: poliovirus [48] and EMCV [49] may elicit apoptosis also in permissive cells. Another type of host-dependence is demonstrated in human rhabdomyosarcoma RD cells: inhibition of poliovirus reproduction in these permissive cells does not result in the necrosis-to-apoptosis switch as it does in HeLa cells [50].
Furthermore, the response to picornavirus of a given population is very rarely, if ever, homogeneous. Usually, it is possible only to identify the predominant reaction, and both apoptotic and necrotic cells are present in noticeable proportions in some cases 50 , Destruction of the infected cell may be useful for the virus, in particular, to help externalization of the progeny.
The innate immune system should also select between two possible choices: to keep the infected cell alive, even if this permits efficient viral reproduction, or to push cell sacrifice to limit viral growth and spread. Furthermore, different modes of sacrifice are available: to die peacefully, occluding as many virus particles as possible apoptosis or to permit the viral progeny to go out together with various cell components, emitting loud alarm signals necrosis.
Each of these options is biologically relevant. The choice depends on a multitude of factors, such as the genetic background of the interacting partners, the multiplicity of infection, physiological state of the host, and environmental conditions. The evolutionary pressure, however, appears to force the fighting parties viruses and hosts to negotiate for a peaceful coexistence.
Remarkably, predecessors of such dangerous human pathogens as the human immunodeficiency virus HIV or severe acute respiratory syndrome SARS virus are relatively harmless in their earlier hosts, non-human primates and bats, respectively 52 , The severe human diseases inflicted by these viruses do not result from the acquisition by them of new pathogenic factors.
Rather they appear to be due to miscalculated innate immunity mechanisms. Growing evidence from studies on both small RNA viruses and large DNA viruses strongly suggests that various types of CPE may represent manifestations of the virus-modulated host-encoded innate immunity program s.
In other words, virus-induced injuries, both necrotic and apoptotic, may largely reflect fighting between cellular defenses and viral counter-defenses, the efficiency of viral reproduction being not necessarily directly coupled to the level of cellular injury.
Further work is required for the validation of this viewpoint, assessment of its generality, elucidation of the underlying mechanisms, and utilization of its consequences for the treatment of viral diseases Box 3.
Current work of our group is supported by a grant from the Russian Foundation for Basic Research. I apologize for not citing many relevant papers owing to journal limits.
National Center for Biotechnology Information , U. Trends Microbiol. Published online Oct Vadim I. Agol 1, 2.
Agol 1 M. Author information Copyright and License information Disclaimer. All rights reserved. Elsevier hereby grants permission to make all its COVIDrelated research that is available on the COVID resource centre - including this research content - immediately available in PubMed Central and other publicly funded repositories, such as the WHO COVID database with rights for unrestricted research re-use and analyses in any form or by any means with acknowledgement of the original source.
This article has been cited by other articles in PMC. Abstract The capacity to injure infected cells is a widespread property of viruses.
Keywords: virus-induced apoptosis, virus-induced necrosis, picornaviruses. The problem The ability to injure and often to kill infected cells is a common, although not obligatory, property of viruses. Picornavirus reproduction and its outcomes Some major features of the structure and reproduction of picornaviruses are briefly summarized in Box 1.
Box 1 Picornaviruses, their genomes and life cycle. Open in a separate window. Figure I. Box 2 Major types of cellular death. Viral induction and prevention of apoptosis The ability of picornaviruses to activate cellular apoptotic pathways was first discovered in poliovirus [8] and then described for numerous representatives of this viral family.
Factors contributing to virus-induced necrosis Another variant of death of virus-infected cells is necrosis. Necrotic CPE: a host-encoded program modulated by viruses? Competition between death programs of virus-infected cells Different cellular antiviral defensive death programs compete with each other. Figure 1. Figure 2. The generality of this understanding of CPE The above arguments for the fight between cellular defenses and viral antidefenses as the major cause of CPE and for necrotic CPE as being largely controlled by a host-encoded program came from studies on picornaviruses, relatively small RNA viruses.
Unstable balance between the death programs The models presented in Figure 2 b and c are based on still poorly understood mechanisms of picornaviral CPE. Biological relevance of CPE Destruction of the infected cell may be useful for the virus, in particular, to help externalization of the progeny.
Concluding remarks Growing evidence from studies on both small RNA viruses and large DNA viruses strongly suggests that various types of CPE may represent manifestations of the virus-modulated host-encoded innate immunity program s. Box 3 Outstanding questions. Acknowledgment Current work of our group is supported by a grant from the Russian Foundation for Basic Research. References 1. Persistent infection. In: Ehrenfeld E. The Picornaviruses. ASM Press; Agol V. Viral security proteins: counteracting host defences.
Romanova L. Mikitas O. Gosselin A. Poliovirus-induced apoptosis is reduced in cells expressing a mutant CD selected during persistent poliovirus infection in neuroblastoma cells. Deszcz L. Apoptotic events induced by human rhinovirus infection. Wahid R. Dendritic cells and macrophages are productively infected by poliovirus. Tolskaya E. Apoptosis-inducing and apoptosis-preventing functions of poliovirus.
Willis S. Peng J. VP1 of foot-and-mouth disease virus induces apoptosis via the Akt signaling pathway. Jin H. Induction of immature dendritic cell apoptosis by foot and mouth disease virus is an integrin receptor mediated event before viral infection.
Gullberg M. A single coxsackievirus B2 capsid residue controls cytolysis and apoptosis in rhabdomyosarcoma cells. Blondel B. Apoptotic signaling cascades operating in poliovirus-infected cells. Koyama A. Suppression of apoptotic and necrotic cell death by poliovirus. Salako M. Coxsackievirus protein 2BC blocks host cell apoptosis by inhibiting caspase Neznanov N.
Dougherty J. Interference with cellular gene expression. Kuechler E. Picornavirus protease-mediated shutoff of host translation: direct cleavage of a cellular initiation factor. In: Semler B. Molecular Biology of Picornaviruses. Dasgupta A. Effects of picornavirus proteinases on host cell transcription. Svitkin Y. Rapamycin and wortmannin enhance replication of a defective encephalomyocarditis virus.
Host cell seeding density — It is critical to seed enough host cells to quickly grow and cover the surface of the well, which allows better visualization of the CPE when it occurs.
Virus concentration — It is important to perform a viral titer experiment prior to the CPE assay to determine the proper range of the virus concentrations in order to obtain the appropriate TCID50 results.
Throughput — Utilizing the plate-based image cytometry method, the assay throughput can be significantly increased from the standard 6 — well plates to 96 and well plates. Imaging — Image cytometry can rapidly scan whole wells of the entire plate to digitally capture bright field and fluorescent images for analysis.
Unlike the conventional microscopy method, which requires manual observation of the CPE, and lacks digital records. Analysis — Image analysis algorithms can automate the identification of CPE via the destruction of cell monolayer, reduction in host cell count, and morphological changes at an individual cell level. In contrast, traditional CPE assays require trained technicians to visually inspect and identify, which is tedious, time-consuming, and generate a high level of uncertainties.
Directly analyze cytopathic effect using the Celigo Image Cytometer The Celigo Image Cytometer is a sophisticated plate imager that can rapidly image the entire microplate in bright field and fluorescence. Measure the host cell monolayer using confluence application The pseudo-green color shows where cells are present and that area is quantified and compared between different viral treatments. Healthy cells left and infected cells right 2.
Count the total number of cells in each well The green outlines identify the counted cells in the well. Healthy cells left and infected cells right 3. Measure the morphological changes of the host cells The aspect ratios are measured from the counted cells to quantify the morphological changes. All cells in the monolayer rapidly shrink, become dense pyknosis , and detach from the glass within 72 hours. This CPE is typical of most enteroviruses. It consists of detachment death of some but not all of the cells in the monolayer which can be observed using the 20X objective.
Some togaviruses alphaviruses , some picornaviruses, and some of the paramyxoviruses may cause this type of CPE. Instead of causing generalized destruction of the cell monolayer, some viruses produce localized areas foci of infection.
Cells initially become enlarged, rounded, and refractile, then eventually detach from the growth surface leaving cleared areas surrounded by rounded up cells as the infection spreads concentrically. Focal degeneration is characteristic of the herpesviruses and poxviruses. For example, adenoviruses. Vacuolation is visible only after staining. Retroviruses, paramyxoviruses, and flaviviruses may cause vacuolization. It involves the fusion of the plasma membranes of four or more cells to produce an enlarged cell with four or more nuclei.
Some paramyxoviruses; and herpesviruses may produce syncytia. Syncytia are much easier to observe after staining. These are areas of altered staining in cells, which cannot be seen in live cell cultures. Depending on the causative virus, these inclusions may be single or multiple, large or small, round or irregularly shaped, intranuclear or intracytoplasmic, eosinophilic pink staining , or basophilic blue-purple staining.
Earlier, virologists used to rely on cytopathic effects for the identification of viruses but nowadays the reliance has decreased substantially, due to a raise in nucleic acid amplification tests such as polymerase chain reaction PCR which is faster and more specific.
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