Batterjee Medical College. Basic Virology: Introduction Viruses are not cells They are not capable of independent replication They synthesize neither

Batterjee Medical College


Basic Virology: Introduction

• Viruses are not cells

• They are not capable of independent replication

• They synthesize neither their own energy nor their

own proteins

• They are too small to be seen in light microscope.


Basic Virology: Introduction

Comparison of Viruses and Cells

Property Viruses Cells

Type of nucleic acid DNA or RNA but not both DNA and RNA

Proteins Few Many

Lipoprotein membrane Envelopeviruses present

in some

Cell membrane present in all

cells

Ribosomes Absent Present

Mitochondria Absent Present in eukaryotic cells

Enzymes None or few Many

Multiplication by binary

fission or mitosis

No Yes


Virus Size & Structure

• Viruses range in size from (~20 nm to ~300 nm) .

• Most viruses appear as spheres or rods in the

electron microscope.


• All viruses have protein coat called capsid that covers genome.

• Capsid is composed of repeating subunits called capsomers virus symmetric appearance used for classification.

• Some viruses, capsid is covered with lipoprotein envelope.


Capsid


• It composed of nucleic acid genome & capsid

proteins.


Nucleocapsid


• Viral nucleocapsids have : -Spherical (icosahedral) symmetry (enveloped or naked) -Helical symmetry (enveloped).


Nucleocapsid


• Viruses contain either DNA or RNA, but not both.

• These DNA & RNA genomes can be either single-stranded or double-stranded.

• Some RNA viruses (e.g. influenza virus & rotavirus), have segmented genome.

• All viruses have one copy of their genome (except retroviruses, which have two copies).

Viral Nucleic Acids


• It mediate attachment to host cell receptors (determines host & organ specificity of virus).

• Surface proteins are targets of antibody which "neutralizes" (inhibits) viral replication.

• Some viruses produce antigenic variants of their surface proteins evade host defenses.

Internal proteins :

They are DNA or RNA polymerases.

Viral Proteins

Viral surface proteins


It mediates interaction between viral nucleocapsid

proteins & envelope proteins.

Viral Proteins

• Antibody against one antigenic variant (serotype) will not neutralize different serotype. • Some viruses have one serotype; others have multiple serotypes.

Matrix protein


• Envelope consists of membrane that contains lipid

derived from host cell & proteins encoded by virus.

• Envelope is acquired as virus exits from cell in

process called budding.

Viral Envelope


Viral Envelope

• Enveloped viruses :

-Easily inactivated

-transmitted by direct contact

via blood & body fluids.

• Naked viruses

- Survive longer in

environment

- Transmitted by indirect

means e.g. fecal-oral route


•One virion infects cell & hundreds of progeny

virions are produced within hours.

•This is remarkable amplification & explains rapid

spread of virus from cell to cell.

The eclipse period

It is time when no virus particles are detected within

infected cell & occurs soon after cell is infected.

Viral Growth Curve


The steps in viral replication are as follows:

1.Attachment

2.Penetration and uncoating.

3.Transcription & Translation

4.Assembly

5.Release

Viral Replication


• Virus particles can only infect cells possessing

surface “receptors” specific to particular virus

species.

• Virus attached to cell with:

- Capsid (Naked viruses)

- Envelope proteins

(enveloped viruses)

Viral Replication

1-Attachment


Viral Replication

• Viruses penetrate into cell by means of pinocytosis

(viropexis).

• In enveloped viruses, envelope fuse with cell

membrane, releasing virus into cytoplasm.

2-Penetration

Adsorption of such an enveloped virus to two cells at the same time may result in cell fusion


Viral Replication

• It involves release of nucleic acid from capsid

• It is activated by cellular enzymes & contribution

from cell membranes (except smallpox virus)

3- Uncoating


• All DNA viruses genome is double-stranded

(except parvoviruses, single-stranded)

• They use host cell RNA polymerase to synthesize

viral mRNA.

• All DNA viruses replicate in nucleus (except

poxviruses in cytoplasm).

Viral Replication

4- Replication of nucleic acid

DNA viruses


Viral Replication

•Genome of all RNA viruses is single-stranded

(Except reoviruses, e.g., rotavirus double-

stranded).

• All RNA viruses replicate in cytoplasm, except

retroviruses, influenza virus, & hepatitis D virus,

which require intranuclear step in their replication.

4- Replication of nucleic acid

RNA viruses


•Viral proteins:

•Early proteins are typically enzymes used in

synthesis of viral components, whereas late

proteins are typically structural proteins of progeny

viruses.

•poliovirus & retroviruses, translate their mRNA into

precursor polyproteins, which must be cleaved by

proteases to produce functional proteins.

Viral Replication


Assembly and release:

•All enveloped viruses acquire their envelope by

budding through the external cell membrane as they

exit the cell,

•herpesviruses, acquire their envelope by budding

through nuclear membrane.

Viral Replication


Viral Replication


Lysogeny

•It is the process by which viral DNA becomes

integrated into host cell DNA, replication stops, and

no progeny virus is made.

• Later, if DNA is damaged by, for example, UV light,

viral DNA is excised from the host cell DNA and

progeny viruses are made.

•The integrated viral DNA is called a prophage.

Viral Replication


Lysogeny

Bacterial cells carrying a prophage can acquire new

traits, such as the ability to produce exotoxins such

as diphtheria toxin.

Transduction is the process by which viruses carry

genes from one cell to another.

Lysogenic conversion is the term used to indicate

that the cell has acquired a new trait as a result of

the integrated prophage.

Viral Replication


Viral Replication


Genetics

• Mutations in the viral genome can produce antigenic variants and drug-resistant variants.

• Mutations can also produce attenuated (weakened) variants that cannot cause disease but retain their antigenicity and are useful in vaccines.

• Temperature-sensitive mutants can replicate at a low (permissive) temperature but not at a high (restrictive) temperature.

• Temperature-sensitive mutants of influenza virus are used in vaccines against this disease.


Genetics

Reassortment (exchange) of segments of the genome RNA of influenza virus is important in the pathogenesis of the worldwide epidemics caused by this virus.

•Complementation occurs when one virus produces a protein that can be used by another virus.

•A medically important example is hepatitis D virus that uses the surface antigen of hepatitis B virus as its outer coat protein.


•Phenotypic mixing occurs when two different

viruses infect the same cell and progeny viruses

contain proteins of both parental viruses.

• This can endow the progeny viruses with the ability

to infect cells of species that ordinarily parental

virus could not.

Genetics


The Infected Patient•Viral infection in the person typically has four stages:- incubation period,-prodromal period- specific-illness period,- recovery period.

•The main portals of entry are the respiratory, gastrointestinal, and genital tracts, but through the skin, across the placenta, and via blood are important as well.

Pathogenesis


The Infected Patient•Transmission from mother to offspring is called vertical transmission; all other modes of transmission, e.g., fecal–oral, respiratory aerosol, and insect bite, are horizontal transmission.•Transmission can be from human to human or from animal to human. •Most serious viral infection are systemic, i.e., the virus travels from the portal of entry via the blood to various organs. •some are localized to portal of entry as common cold & involves only upper respiratory tract.

Pathogenesis


Pathogenesis

•The symptoms of viral diseases are usually caused

by death of the infected cells and a consequent loss

of function.

• For example, poliovirus kills neurons, resulting in

paralysis.

Pathogenesis


Pathogenesis

Immunopathogenesis

•It is the process by which the symptoms of viral

diseases are caused by the immune system rather

than by the killing of cells directly by the virus.

•One type of immunopathogenesis is the killing of

virus-infected cell by the attack of cytotoxic T cells

that recognize viral antigens on the cell surface


Pathogenesis

•Damage to the liver caused by hepatitis viruses

occurs by this mechanism.

•Another is the formation of virus–antibody

complexes that are deposited in tissues.

•Arthritis associated with parvovirus B19 or rubella

virus infection occurs by this mechanism.


Pathogenesis

•Viruses can evade host defenses by producing multiple antigens, thereby avoiding inactivation by antibodies.

•and by reducing the synthesis of class I MHC proteins, Thereby decreasing the ability of a cell to present viral antigens and blunting the ability of cytotoxic T cells to kill the virus-infected cells.

•Viruses produce receptors for immune mediators, as IL-1 and TNF, thereby preventing the ability of these mediators to activate antiviral processes


Carrier state refers to people who produce virus for long periods of time and can serve as a source of infection for others. The carrier state that is associated with hepatitis C virus infection is a medically important example.

Latent infections are those infections that are not producing virus at the present time but can be reactivated at a subsequent time. The latent infections that are associated with herpes simplex virus infection are a important example.

Persistent Viral Infections


•Slow virus infections refer to those diseases with a long incubation period, often measured in years.

•Some, such as progressive multifocal leukoencephalopathy, are caused by viruses, whereas others, such as Creutzfeldt-Jakob disease, are caused by prions.

•The brain is often the main site of these diseases.

Persistent Viral Infections


Host Defenses

•Host defenses against viruses fall into two major categories:

(1) Nonspecific, of which the most important are interferons and natural killer cells

(2) Specific, including both humoral and cell-mediated immunity.

• Interferons are an early, first-line defense, whereas humoral immunity and cell-mediated immunity are effective only later because it takes several days to induce the humoral and cell-mediated arms of the immune response


Nonspecific DefensesInterferons

• Interferons inhibit virus replication by blocking production of viral proteins, by degrading viral mRNA.

• They induce the synthesis of a ribonuclease that specifically cleaves viral mRNA but not cell mRNA.

• Double-stranded RNA viruses are the most potent inducers of interferons.

• Many viruses induce interferons, and many viruses are inhibited by interferons, i.e., neither the induction of interferons nor its action is specific.


•Interferons act by binding to a receptor on the cell surface that signals the cell to synthesize the ribonuclease and the other antiviral proteins.

•Interferons do not enter the cell and have no effect on extracellular viruses.

•Alpha & beta interferons have a stronger antiviral action than gamma interferon.

•Gamma interferon activates macrophages.

Nonspecific DefensesInterferons


• Natural killer (NK) cells are lymphocytes that destroy cells infected by many different viruses, i.e., they are nonspecific.

• NK cells do not have an antigen receptor on their surface.

• NK cells recognize and destroy cells that do not display class I MHC proteins on the surface.

• They kill cells by secreting perforins and granzymes.

Nonspecific Defenses


•Phagocytosis by macrophages and the clearance of

mucus by the cilia of the respiratory tract are also

important defenses.



• Increased corticosteroid levels suppress various

host defenses and predispose to severe viral

infections as disseminated herpesvirus infections.

• Malnutrition predisposes to severe measles

infections in developing countries.

• The very young and the very old have more severe

viral infections.



Specific Defenses

Active immunity to viral infection is mediated by both antibodies and cytotoxic T cells.It can be elicited either by exposure to the virus or by immunization with a viral vaccine.

Passive immunity consists of antibodies preformed in another person or animal.

The duration of active immunity is much longer than (years) that of passive immunity (weeks to a few months).


Passive immunity is effective immediately, whereas it takes active immunity 7 to 10 days in the primary response (or 3–5 days in the secondary response) to stimulate detectable amounts of antibody.

Herd immunity is the protection of an individual that results from immunity in many other members of the population (the herd) that interrupts transmission of the virus to the individual. Herd immunity can be achieved either by immunization or by natural infection of a sufficiently high percentage of the population.

Specific Defenses


Identification in Cell Culture

•The presence of a virus in a patient's specimen can

be detected by seeing a "cytopathic effect" (CPE) in

cell culture.

•CPE is not specific, i.e., many viruses cause it.

• A specific identification of the virus usually

involves an antibody-based test as fluorescent

antibody, complement fixation, or ELISA.

Laboratory Diagnosis


Microscopic IdentificationInclusion bodies•They are formed by aggregates of many virus particles, can be seen in either the nucleus or cytoplasm of infected cells.

•They are not specific. •Two important examples are the nuclear inclusions formed by certain herpesviruses and the cytoplasmic inclusions formed by rabies virus (Negri bodies).



Multinucleated giant cells

•They are formed by several viruses, notably certain herpesviruses, respiratory syncytial virus & measles virus. •Fluorescent antibody staining of cells obtained from the patient or of cells infected in culture can provide a rapid, specific diagnosis. •Electron microscopy is not often used in clinical diagnosis but is useful in the diagnosis of certain viruses, as Ebola virus, that have a characteristic appearance and are dangerous to grow in culture.



Serologic Procedures•The presence of IgM can be used to diagnose current infection. •The presence of IgG cannot be used to diagnose current infection because the antibody may be due to an infection in the past.•acute and convalescent serum sample should be analyzed. •An antibody titer that is fourfold or greater in the convalescent serum sample compared to the acute sample can be used to make a diagnosis.



Detection of Viral Antigens & Nucleic Acids

•The presence of viral proteins, such as p24 of HIV and hepatitis B surface antigen, is commonly used in diagnosis.

•The presence of viral DNA or RNA is increasingly becoming the "gold standard" in viral diagnosis.

•Labeled probes are highly specific and rapidly.•Small amounts of viral nucleic acids can be amplified using reverse transcriptase to produce amounts detectable by the probes e.g."viral load" assay of HIV RNA.


Documents

Batterjee Medical College. Basic Virology: Introduction Viruses are not cells They are not capable of independent replication They synthesize neither