Cytopathic effect

Cytopathic effect or cytopathogenic effect (abbreviated CPE) refers to structural changes in host cells that are caused by viral invasion. The infecting virus causes lysis of the host cell or when the cell dies without lysis due to an inability to reproduce. Both of these effects occur due to CPEs. If a virus causes these morphological changes in the host cell, it is said to be cytopathogenic. Common examples of CPE include rounding of the infected cell, fusion with adjacent cells to form syncytia, and the appearance of nuclear or cytoplasmic inclusion bodies. Cytopathic effect or cytopathogenic effect (abbreviated CPE) refers to structural changes in host cells that are caused by viral invasion. The infecting virus causes lysis of the host cell or when the cell dies without lysis due to an inability to reproduce. Both of these effects occur due to CPEs. If a virus causes these morphological changes in the host cell, it is said to be cytopathogenic. Common examples of CPE include rounding of the infected cell, fusion with adjacent cells to form syncytia, and the appearance of nuclear or cytoplasmic inclusion bodies. CPEs and other changes in cell morphology are only a few of the many effects by cytocidal viruses. When a cytocidal virus infects a permissive cell, the viruses kill the host cell through changes in cell morphology, in cell physiology, and the biosynthetic events that follow. These changes are necessary for efficient virus replication but at the expense of the host cell. CPEs are important aspects of a viral infection in diagnostics. Many CPEs can be seen in unfixed, unstained cells under the low power of an optical microscope, with the condenser down and the iris diaphragm partly closed. However, with some CPEs, namely inclusion bodies, the cells must be fixed and stained then viewed under light microscopy. Some viruses' CPEs are characteristic and therefore can be an important tool for virologists in diagnosing an infected animal or human. The rate of CPE appearance is also an important characteristic that virologists may use to identify virus type. If CPE appears after 4 to 5 days in vitro at low multiplicity of infection, then the virus is considered slow. If the CPE appears after 1 to 2 days in vitro at low multiplicity of infection, then the virus is thought to be rapid. Inoculations always occur at low multiplicity of infection because at high multiplicity of infection, all CPEs occur rapidly. Typically, the first sign of viral infections is the rounding of cells. Inclusion bodies often then appear in the cell nucleus and/or cytoplasm of the host cell. The inclusion bodies can first be identified by light microscopy in patient blood smears or stained sections of infected tissues. However, to fully characterize their composition, electron microscopy must be performed. Inclusion bodies may either be accumulation of virus replication byproducts or altered host cell organelles or structures. Some viral infections cause a strange CPE, the formation of syncytia. Syncytia are large cytoplasmic masses that contain many nuclei. They are typically produced by fusion of infected cells. This mechanism is useful to the virus as it allows the virus to spread from infected to uninfected cells. Viral infections may have clinically relevant phenotypical CPEs. For example, with the hepatitis C virus (HCV), liver steatosis is characteristic enough of the virus that it may be used to help identify the genotype, the genetic composition of the virus. HCV genotype 3 patients are significantly more likely to develop liver steatosis than those with genotype 1. Also, CPEs may be used during research to determine the efficacy of a new drug. An assay has been developed that screens the dengue virus's CPEs in order to assess cell viability. Due to the host cell specificity of CPEs, researchers can also use them to test any discrepancies in an experiment. For many viral infections, different host cell strains may have a characteristic response. Currently, there are many concerns within the research community about the validity and purity of cell strains. Contamination has risen within and among laboratories. CPEs can be used to test the purity of a certain cell line. For example, HeLa CCL-2 is a common cell line used in a wide variety of research areas. To test the purity of the HeLa cells, CPEs were observed that occurred after inoculation with Coxsackievirus B3. These CPEs included morphology changes and cell morbidity rates. Carson et al. determined that the discrepancy is due to the heterogeneous nature of the commercial HeLa cells as compared to the homogeneous nature of HeLa cells that have been propagated for generations in a lab. Total destruction of the host cell monolayer is the most severe type of CPE. To observe this process, cells are seeded on a glass surface and a confluent monolayer of host cell is formed. Then, the viral infection is introduced. All cells in the monolayer shrink rapidly, become dense in a process known as pyknosis, and detach from the glass within three days. This form of CPE is typically seen with enteroviruses. Subtotal destruction of the host cell monolayer is less severe than total destruction. Similarly to total destruction, this CPE is observed by seeding a confluent monolayer of host cell on a glass surface then introducing a viral infection. Subtotal destruction characteristically shows detachment of some but not all the cells in the monolayer. It is commonly observed with some togaviruses, some picornaviruses, and some types of paramyxoviruses.