Infectious Potential and Stages of Infection
An infectious process is always the result of the interaction between viral and cellular genomes, leading to cellular damage. For an infection to take place, the virus goes through several strictly sequential stages.
The portal of entry is most commonly the mucous membranes. The pathogen must possess marked resistance to the aggressive macroorganism environment. For example, enteric viruses can withstand extremely acidic pH values in the stomach, the detergent action of bile salts, and the destructive activity of proteolytic enzymes.
After overcoming barriers, the stage of tropism begins—the search for a specific target. Once inside, the virus encounters the phenomenon of permissivity: it must bypass intracellular defense systems to force the cell to deproteinize ("uncoat") the virion. The cycle concludes with dissemination (invasion of new cells), the development of pathological disorders, and transmission—passing the pathogen to a new host.
Attachment Mechanisms and Evasion Strategy (The "Canyon Model")
Contact with the cell begins with specific adsorption: viral attachment proteins (antireceptors) bind to cellular receptors. In non-enveloped viruses, these are embedded in the protein capsid; in enveloped viruses, they are in the lipoprotein envelope (supercapsid). Mutations in the primary structure of antireceptors can either deprive the pathogen of adsorption capacity or adapt it to a new host.
To protect these critically important sites from neutralizing antibodies during adsorption, a clever structural defense is used—the "canyon model". The essence of the mechanism is that the conservative binding site is hidden at the bottom of deep depressions ("canyons") on the virion surface. Large active centers of antibody molecules cannot physically penetrate deep inside. They only react with the hypervariable edges of the depression, which in no way prevents the pathogen from successfully binding to the required receptor at the bottom.
Penetration and the Role of Fusion Proteins
Adsorption alone does not guarantee cell entry. The entry of enveloped viruses (e.g., Paramyxoviridae) is ensured by specific fusion proteins. They disrupt the integrity of cell membranes by changing their permeability.
Two types of fusion are distinguished:
- Fusion from without: occurs almost immediately after infection at a high multiplicity of infection, mediated by proteins of the virion itself.
- Fusion from within: characteristic of late stages at a low multiplicity of infection, provided by newly synthesized viral proteins integrated into the membrane.
Post-translational modification—proteolytic cleavage of the precursor protein by enzymes—is necessary to activate the infectivity of fusion proteins. This process is analogous to the activation of bacterial protoxins. If proteolysis is blocked due to mutations, non-infectious virions are formed, and the infection becomes abortive.
In addition, fusion proteins cause the formation of syncytia (fusion of affected and healthy cells). The virus is transmitted via intercellular bridges without entering the extracellular space, making it inaccessible to virus-neutralizing antibodies.
Cellular Restrictions and Immune Evasion
The eukaryotic cell imposes metabolic restrictions on the virus for replication. Viral genome transcription is strictly regulated. Transactivator proteins play a key role here. They bind to regulatory regions of genes, provoking "explosive" production of viral particles. They can also activate genes of other viruses (during co-infection), cellular oncogenes, or trigger the transition of an infection from a latent to a lytic form.
For successful replication, the virus must suppress host immune surveillance mechanisms. Infectivity is determined by the ability to block cellular apoptosis (programmed cell death) and reduce the production of protective cytokines.
Pathogens evade immunity through:
- Camouflage (formation of an envelope from the host cell membrane).
- Variability of surface antigens.
- Secrecy (integration of the viral genome into the cellular genome).
- Direct attack (destruction of immunocompetent cells, such as T-lymphocytes).