1. Viral Replication

2. 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.

3. Stages of viral replication
1) Attachment (adsorption).
2) Penetration.(viropexis).
3) Uncoating.
4) Expression of viral genome & synthesis of viral components.
5) Assembly (morophogenesis) & maturation.
6) Release.

4. 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.

6. 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
there is transfer of the entire viral particle across the cell membrane by endocytosis ( endosome).
3- Fusion of the virion envelope with host cell plasma membrane
there is fusion between the plasma membrane & the envelope releasing the nucleocapsid inside the cell cytoplasm.

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

9. 4. Expression of viral genome & 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 rhabdoviruses.
4- Virion contains reverse transcriptase in which viral RNA code for complementary DNA, like retroviruses (HIV).
The intracellular sites of viral replication
The viral protein is synthesized in cytoplasm on ribosomes (composed of viral specific mRNA & host cell ribosome).
DNA viruses, with one exception, replicate in the nucleus.
Most RNA viruses undergo their entire replicative cycle in the cytoplasm.

12. 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.

14. Schematic diagram of the whole steps in viral replication.

15. Pathogenesis of viral disease

16. 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.

17. 1.Entry into the Host Example: Human papillomavirus, rabies virus.
Skin Dead cells cannot support virus replication 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: Human papillomavirus, 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: hepatitis A, poliovirus, rotavirus.
Genital tract Relatively less hostile than the above HIV, human papilloma virus, hepatitis B, HSV2.
Conjunctiva an exposed site and relatively unprotected HSV1.
Blood stream (Hepatitis B, hepatitis C, hepatitis D, HIV, CMV ).

18. 2- Primary Replication
Site of entry may be the site of replication as well, here virus produce disease at the site of entry & have no systemic spread.
Example of localized infection
Respiratory tract »» influenza viruses.
Gastrointestinal tract »» rotaviruses.
Epidermis »» Papillomaviruses.
Systemic infections
Here at the site of entry of the virus there is primary replication, followed by spread to the bloood to reach the main target where secondary replication take place.
Enteroviruses, enter through the intestinal epithelium then through lymphoid tissues, reach to the blood & then to C.N.S.
Herpesviruses, enter through the oropharynx or genitourinary tract then through the local nerve ending, reach to the C.N.S.

19. 3- Viral spread
Apart from direct cell-cell contact, there are 2 main mechanisms for viral spread
1- blood stream The presence of virus in the blood is called viremia.
Virus may get into the blood stream by
direct inoculation e.g. Arthropod vectors,
blood transfusion
I.V. drug abuse
spread from site of primary replication.
The virus may travel in the blood
Free in the plasma (Togaviruses, Enteroviruses),
or in association with: platelets (HSV), lymphocytes (EBV, CMV), monocytes (Lentiviruses).
2- nervous system In 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 .

20. 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 Poliovirus (gut epithelium - neurons in spinal cord & brain) .

21. 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.

22. 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.

23. 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.