Viral Replication

Requirements for viral replication:

1- Viruses multiply only in living cells, redirect host cell biosynthetic processes for the synthesis of viral components.

2- Host cell must provide:

         energy, 

         synthetic machinery,

 low molecular weight precursors for synthesis of viral proteins   &                  nucleic  acids. 

3- Viral nucleic acids carries genetic specificity code for all virus specific macromolecules. Therefore the virus must be able to produce usable mRNA.

The stages of viral replication include:

1) Attachment (adsorption).

2) Penetration (viropexis).

3) Uncoating.

4) Expression of viral genome &  synthesis of viral components.

5) Assembly (morophogenesis) & maturation.

6) Release.

   

1. Attachment (adsorption):

•         Interaction of virion with specific receptor on surface of host cell.

•         Host cell receptor differ for different viruses but generally they are glycoproteins located on host cell.                                          

•         Some cases virus bind to protein sequences (e.g. picornavirus), & other oligosaccharide ( orthomyxovirus & paramyxovirus).

•         The receptor binding reflect a configurational homologies between virus surface structure & cell surface components.

•         The presence & distribution of receptors are important determinant in cell tropism.  

Examples of viral receptors:                                                                                                   

•         Human immunodefeciency virus bind to CD4 receptor of the immune system.

•         Epistein-Barr virus bind to CD21 receptor on B cell.

2. Penetration to host cell ( viropexis) :

•         The process by which the virus particle taken up inside the infected cell called penetration (engulfment).

   

     The virus can enter the cell by several mechanisms :

    1- Direct penetration:

       only the viral genome transfer through the plasma cell membrane.

     2- Receptor mediated endocytosis:

    transfer of the entire viral particle across the cell membrane by endocytosis ( endosome).

    3- Fusion of the virion envelope with host cell plasma membrane:

    fusion between the plasma membrane & the envelope releasing the nucleocapsid inside the cell cytoplasm.

3. Uncoating

•         The removal of capsid & core proteins, liberating viral nucleic acid, is termed uncoating.

•         With exception of poxviruses, the process utilizes preexisting cellular proteases, poxvirus uncoating requires the synthesis of a new poxvirus-coded protease.

•         By this process virion disrupted & loss it is infectivity.

•          This phase of viral growth cycle is called the eclipse period; its duration varies depending on both the particular virus & the host cell.

•         This period is followed by rapid accumulation of infectious progeny virus particles.

4. Expression of viral genomes & synthesis of viral components.

•         Here specific mRNA transcribed from viral NA for successful expression & duplication of genetic information.

•         Then viruses use cell components to translate the mRNA.

•         Mechanism of transcription:

    1- Directly transcription of mRNA without intermediates like most DNA viruses ( adenoviruses, herpesviruses).

    2- Viral NA serve as mRNA  example in +ss RNA viruses (picornaviruses).

    3- Virion carry RNA polymerase to synthesis mRNA, RNA viruses of this type are called negative strand viruses( negative sense ), example rotaviruses.

    4- Virion contains reverse transcriptase in which viral RNA code for complementary DNA, like retroviruses (HIV).

The intracellular sites where events of viral replication take place:

Ø The viral protein is synthesized in cytoplasm on ribosomes ( composed of viral specific mRNA & host cell ribosome).

Ø Viral DNA is usually replicated in the nucleus.                         

Ø Viral RNA generally replicated in the cytoplasm.

5- Assembly, maturation & release:

    Newly synthesized viral genome & capsid assemble together to form progeny viruses.

    After formation of complete virions, the viral particles released from the cells.

    Mechanism of viral release from the host cell:

     1- Host cell lysis:

     referred to as the lytic cycle, which results in the death of the host cell, the non enveloped viruses released in this way.

    2- Budding through cytoplasmic membranes:                    

      Enveloped virus mature & released by budding process.

   The virus specific envelope glycoproteins are inserted into cellular membranes, & viral nucleocapsids bud through the membrane at these sites & acquire the envelope.

    Sometime viral maturation is inefficient process, excess amount of viral components accumulate & lead to the formation of inclusion bodies in the cells.

Schematic diagram of the whole steps in viral replication

Pathogenesis of viral disease

Steps in viral pathogenesis:

1- Viral entry into the host.

2- Primary viral replication.

3- Viral spread.

4- Cell and tissue tropism.

5- Cellular injury.

6- Viral shedding.

7- Host immune response.

1.Entry into the Host:

•               Skin:                                                      

Most viruses which infect via the skin require a breach in the physical integrity of this effective barrier ( cuts or abrasions) Many viruses employ vectors (ticks, mosquitoes or vampire bats to breach the  barrier).

     Example: Papillomaviruses, rabies virus.

•         Respiratory tract:The respiratory tract and all other mucosal surfaces possess immune defense mechanisms like secretary IgA  as well as non-specific inhibitory mechanisms (cilliated epithelium, mucus secretion, lower temperature) which viruses must overcome.                                                           Example: influenza virus, rhinoviruses.                       

•         Gastrointestinal tract: hostile environment because of gastric acid, bile salts. Example: enteroviruses, rotavirus.

•         Genitourinary tract: Relatively less hostile than the above.                                  HIV, HSV type 2. 

•         Conjunctiva: an exposed site and relatively unprotected.  HSV type 1. 

Virus may enter directly into the bloodstream by needles    ( Hepatitis B, HIV      

2- Primary Replication

•         Site of entry may be the site of replication as well, they produce disease at the portal of entry & have no systemic spread.

•         Example of localized infection:

          Respiratory tract like influenza viruses.              

          Gastrointestinal tract such as rotaviruses.                                           

          Epidermis like Papillomaviruses.                                                

•          Example of systemic Infections:                                                           

     Site of entry  primary replication, spread & secondary Replication:  

     Enteroviruses,  enter through the intestinal epithelium then to the  lymphoid tissues, & then to C.N.S.    

     Herpesviruses,  enter through the oropharynx or genitourinary tract then in local nerve ending, C.N.S.

3- Viral spread :

Apart from direct cell-cell contact, there are 2 main mechanisms for viral spread :

    1- Via the bloodstream :                                                                The presence of virus in the blood is called viremia.

Virus may get into the bloodstream by:

     direct inoculation e.g. Arthropod vectors,

     blood transfusion

     I.V. drug abuse

     spread from site of primary replication.                               

The virus may travel the blood:               

Free in the plasma (Togaviruses, Enteroviruses), or in association with: platelets (HSV),lymphocytes (EBV, CMV),monocytes (Lentiviruses).

     2- Via the nervous system: some cases neural spread is involved like:         rabies virus reach the brain to cause the disease,herpes simplex virus moves to the ganglia to initiate latent infection .

4- Cell & tissue tropism :

•         Tropism - the ability of a virus to replicate in specific cells or tissues.

•         This organ & cell specificities, reflect the presence of specific surface receptors for the virus.

•         Receptors it is the components of cell membrane with which the viral surface ( capsid or envelope) can specifically interact & initiate infection.

•         Example

    HIV bind to CD4 of T cells,  oliovirus (gut epithelium - neurons in spinal cord & brain) .     

5- Cellular injury:

•         Destruction of viral infected cells (viral replication, or immune response to virally infected cells) in the tissues are responsible for the development of clinical illness.

•         As a result of viral replication, cellular cytopathic effects develop & cell death.

•         Sometime cell is not damaged by the virus & result in persistent infection.

•         Some tissue like intestinal epithelium can rapidly regenerate & withstand extensive damage better than others such as brain.

•         Some time as a result of viral infection there is transformation of the infected cell,  which later on can lead to neoplasia, this kind of viruses is call oncogenic virus.

6- Viral shedding:

•         It represent the time at which an infected individual is infectious to contacts.

•         Viral discharge into the environment, to maintain a viral infection in population of hosts.

•         Shedding usually occur from the body surfaces involved in viral entry:

           Respiratory tract: influenza virus.

           Gastrointestinal tract: Rotaviruses.

           Genital tract: HSV-2.

•         Shedding occur at various stage of disease depending on particular agent involved.

7- Host immune response:

•         Has a major impact on the outcome of the viral infection.

•         The most prominent is the induction of interferons, which induced soon after viral infection.

•         Interferons inhibit viral replication, before specific humoral & cell mediated immunity begin.

•         Virus encoded proteins serve as good targets for immune response.

•         Virus infected cells lysed by cytotoxic T lymphocyts.

•         Humoral immunity protect the host against re-infection, neutralizing antibody block the initiation of viral infection.