Types of virus-cell interactions
The outcome of the infectious process at the cellular level can vary. The classification is based on the final result:
- Productive type. The infection culminates in the assembly and release of new virions. If the cell dies in the process, it is called a cytocidal effect (typical of non-enveloped viruses). If the virus buds off and the cell survives, it is a non-cytocidal action (enveloped viruses).
- Abortive type. The life cycle is interrupted at one of the stages, and new viral particles are not formed.
- Integrative type (virogeny). The viral genetic material integrates into the host cell chromosome. A provirus is formed, which is copied along with the host DNA during cell division.
- Plasmid-like type. Circular viral nucleic acids exist autonomously in the cytoplasm, similar to bacterial plasmids.
Stages of the reproductive cycle
The reproduction process proceeds in five consecutive stages:
- Adsorption. The virion attaches to the cell. Non-specific ionic attraction works first, followed by highly specific binding of viral ligands to specific cellular receptors (their density can reach 10–100 thousand per cell).
- Penetration and "uncoating" (deproteinization). The virus enters the cell. Non-enveloped viruses use viropexis (receptor-mediated endocytosis) or form a pore in the membrane. Enveloped viruses fuse with the cell membrane or endosomal membrane using special fusion proteins. Then the virus sheds its protein coats, releasing the nucleic acid (or nucleocapsid/core).
- Synthesis. The disjunctive mode of reproduction is implemented: virus components are synthesized in different parts of the cell, dissociated in time and space.
- Assembly. New genomes and structural proteins (capsid, supercapsid, genomic) combine into mature virions.
- Release. The new generation of viruses leaves the cell.
Protein and nucleic acid synthesis
Viruses force the cell to synthesize two groups of proteins: non-structural (replication and transcription enzymes, regulatory proteins) and structural (components of the virion itself). The key difference between virus groups lies in how they obtain messenger RNA (mRNA) required for protein translation:
- DNA viruses. The classical scheme: DNA $\rightarrow$ mRNA $\rightarrow$ protein. Most replicate in the nucleus using cellular polymerases (the exception being poxviruses, which use their own enzymes in the cytoplasm).
- Positive-sense (+RNA) viruses. The genomic RNA itself serves as ready mRNA. Transcription is not needed; translation proceeds immediately: +RNA $\rightarrow$ protein.
- Negative-sense (-RNA) viruses. Their RNA is not recognized by ribosomes. First, mRNA is synthesized using their own RNA-dependent RNA polymerase: -RNA $\rightarrow$ mRNA $\rightarrow$ protein.
- Retroviruses. They possess a diploid genome and the enzyme reverse transcriptase. The scheme: RNA $\rightarrow$ cDNA (single-stranded) $\rightarrow$ double-stranded DNA $\rightarrow$ integration into the host genome $\rightarrow$ mRNA $\rightarrow$ protein.
Briefly: answers to topical questions
What causes can lead to an abortive type of virus-cell interaction?
The abortive type of virus-cell interaction occurs due to viral factors, cellular factors, or environmental conditions. The main causes include infection of a susceptible cell with a defective virus or defective virion, as well as infection of a genetically resistant cell with a standard, complete virus. Additionally, the cause can be the infection of a susceptible cell with a standard virus under non-permissive conditions, such as elevated body temperature, pH shift, or exposure to antiviral drugs.
Which specific cellular receptors do viruses use for adsorption?
For adsorption, viruses use various specific cellular receptors to which they have a pronounced affinity. Influenza virus binds to sialic acid present in glycoproteins and glycolipids of the respiratory epithelium. Rabies virus exhibits tropism for acetylcholine receptors of neural tissue. Human immunodeficiency virus interacts with CD4 receptors and the chemokine coreceptors CCR5 and CXCR4. Other pathogens use ICAM-1 adhesion receptors, glycosides, integrins, or glycoprotein receptors.
What are the mechanisms of egress for enveloped viruses?
Enveloped viruses leave the host cell via budding, during which the cell can remain viable for a long time. During release, the viral nucleocapsid or core interacts with modified membrane regions and becomes "dressed" in a lipoprotein envelope. The localization of budding depends on the virus family: it can occur through the plasma membrane, through smooth membranes of the Golgi apparatus with subsequent exocytosis, through endoplasmic reticulum membranes, or through the nuclear membrane with subsequent vesicular transport.
Class and exam questions
What is the difference between the productive and abortive types of interaction?
The productive type ends with the formation of new virions (with cell death or survival). In the abortive cycle, the process is cut short, and no new viral particles are formed.
What is the disjunctive mode of reproduction?
This is a dissociated reproduction pathway. The synthesis of viral proteins and nucleic acids occurs in different parts of the infected host cell, and they assemble together only at the final stage.
How do positive-sense RNA viruses synthesize their proteins?
Their genomic RNA directly performs the function of messenger RNA (mRNA). It is immediately recognized by the host cell ribosomes, so their transcription stage is bypassed.
What else to study on this topic
- Pathogenesis of viral infection at the cellular level
- Features of caveolae-mediated endocytosis and macropinocytosis
- Specificity of viral receptors using HIV and influenza virus as examples
- Intracellular transport and localization of replication
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