Pathogenesis & Control of Viral Diseases

Medical VirologyLecture 01Youjun Feng

Youjun Feng

Center for Infection & Immunity, Zhejiang University School of Medicine

fengyj@zju.edu.cn

Pathogenesis of viral disease

Principles of viral disease

Iceberg concept of virus infection

Cell responses

Lysis of cell

Inclusion body formationor

Cell transformationor

Cell dysfunction

Viral multiplicationwithout visible change or

incomplete viralmaturation

Exposure withoutattachment and/or

cell entry

Host responses

Death of organism

Classic andsever disease

Moderate severityMild illness

Infection withoutclinical illness(asymptomatic

infection)

Exposurewithoutinfection

Entry into the host

Sel-replication

Spread to another host

What does a pathogen have to do?

Pathogenesis of viral disease

Steps in viral pathogenesis

Stages in virus-host interaction

Entry into the host

Primary replication

Host immune responseSpread in the host

Cell and tissue tropism

Secondary replication

Cell injury and persistence

Entry into the host

Skin - dead cells, therefore cannot support virus replication. Most viruses which infect via the skin require a breach in the physical integrity of this effective barrier, e.g. cuts or abrasions. Many viruses employ vectors, e.g.ticks, mosquitoes or vampire bats to breach the barrier.Respiratory tract - In contrast to skin, the respiratory tract and all other mucosal surfaces possess sophisticated immune defencemechanisms, as well as non-specific inhibitory mechanisms(cilliated epithelium, mucussecretion, lower temperature)which viruses must overcome.

SARS-CoV

Flavivirus

Entry into the host

Gastrointestinal tract - a hostile environment; gastric acid, bile salts, etc.

Genitourinary tract - relatively less hostile than the above, but less frequently exposed to extraneous viruses

Conjunctiva - an exposed site and relatively unprotected

Aerosols - inhalation of droplets, e.g. Rhinoviruses, the 'Common Cold Virus' or Adenoviruses. Faecal-Oral - e.g. Astroviruses, Caliciviruses; these viruses cause acute gastroenteritis. Vector-borne - e.g. in Arthropods such as mosquitos, ticks, fleas: Arboviruses.Close personal contact - especially exchange of bodily fluids: Sex; Blood, e.g. Herpesviruses

Horizontal Transmission

DIRECT person-to-person spread

Vertical Transmission

Relies on PERSISTENCE of theagent to transfer infection fromparents to offspring

VS

Neonatal infection at birth e.g. gonorrhoea, AIDS

Infection in uteroe.g. syphilis, CMV,

Rubella (CRS), AIDS.

Germ line infectionvia ovum or sperm.

Transmission of viruses

Direct contactsAerosols

Sex Parental

Primary vs. secondary replication

Localized Infections:

Virus:

Rhinoviruses

Rotaviruses

Papillomaviruses

Virus:

Enteroviruses

Herpesviruses

Primary Replication:

U.R.T.

Intestinal epithelium

Epidermis

Systemic Infections:

Primary Replication:

Intestinal epithelium

Oropharynx or G.U.tract

Secondary Replication:

Lymphoid tissues, C.N.S.

Lymphoid cells, C.N.S.

Spread Throughout the Host

1- via the direct cell-cell contact

2- via the bloodstream

From primary to secondary replication

3- via the nervous system

The direct cell-cell contactCell/Tissue Tropism

• Tropism - the ability of a virus to replicate in particular cells ortissues

•

•

controlled partly by the route of infection

largely by the interaction of a virus attachment protein (V.A.P.) with aspecific receptor molecule on the surface of a cell,

• has considerable effect on pathogenesis

• Many V.A.P.'s and virus receptors are now known

Virus entryis highly specific for the host and for the cell to be infected

HumanImmunodeficiency

Virus (HIV)

Co-receptor:Chemokine

Receptors CCR-5/CXCR4

Main-receptor:

CD4 antigen

CD4+ T lymphocytes

Monocytes

Macrophages

•

•

•

The bloodstream

by direct inoculation - e.g. Arthropod vectors, blood transfusion or I.V.drug abuse.

The virus may travel free in the plasma (Togaviruses, Enteroviruses),in association with red cells (Orbiviruses), platelets (HSV),lymphocytes (EBV, CMV) or monocytes (Lentiviruses).

Primary viraemia usually proceeds and is necessary for spread to theblood stream, followed by more generalized, higher titre secondaryviraemia as the virus reaches other target tissues or replicates directlyin blood cells.

•

•

•

The nervous system

Spread to nervous system is preceded by primary viraemia

In some cases, spread occurs directly by contact with neurons at theprimary site of infection, in other cases via the bloodstream

Once in peripheral nerves, the virus can spread to the CNS by axonaltransport along neurons (classic - HSV). Viruses can cross synapticjunctions since these frequently contain virus receptors, allowing thevirus to jump from one cell to another

Secondary Replication

•

•

Occurs in systemic infections when a virus reaches other tissues inwhich it is capable of replication, e.g. Poliovirus (gut epithelium -neurons in brain & spinal cord) or Lentiviruses (macrophages - CNS +many other tissues).

If a virus can be prevented from reaching tissues where secondaryreplication can occur, generally no disease results.

Localized Infections:

Virus:

Rhinoviruses

Rotaviruses

Papillomaviruses

Virus:

Enteroviruses

Herpesviruses

Primary Replication:

U.R.T.

Intestinal epithelium

Epidermis

Systemic Infections:

Primary Replication:

Intestinal epithelium

Oropharynx or G.U.tract

Secondary Replication:

Lymphoid tissues, C.N.S.

Lymphoid cells, C.N.S.

Incubation periods of viral infections

Influenza

Common cold

Bronchiolitis,croup

Acute respiratory

1-2d

1-3d

3-5d

5-7d

Chickenpox

Mumps

Rubella

Mononucleosis

13-17d

16-20d

17-20d

30-50d

disease

Dengue

Herpes simplex

Enteroviruses

poliomyelitis

Measles

5-8d

5-8d

6-12d

5-20d

9-12d

Hepatitis A

Hepatitis B

Rabies

Papilloma

HIV

15-40d

50-150d

30-100d

50-150d

1-10y

Outcomes of viral infections

acute, self limiting

Locally restricted(z. B. Rhinitis, Enteritis)

Systemiclocal replication => Viremia(eg. Masern)

chronic

Latent, reactivation (Phaseswithout viremia)(e.g.: Herpesviruses)

Chronic persistent(permanent virus production)(e.g.: Hepatitis B and C)

Active progression(e.g.: HIV)

Patterns in acute and persistent infections

raute 7/2002

Chronic Infection

• Virus can be continuously detected; mildor no clinical symptoms may be evident.

Latent infectionThe virus persists in an occult, or cryptic,from most of the time. There will beintermittent flare-ups of clinical disease,Infectious virus can be recovered duringflare-ups. Latent virus infections typicallypersist for the entire life of the host.

Slow virus infection• A prolonged incubation period, lasting

months or years, daring which viruscontinues to multiply. Clinical symptomsare usually not evident during the longincubation period .

Overall fate of the cell

• The cell dies in cytocidal infectionsthis may be acute (when infection is brief andself-limiting) or chronic (drawn out, only a fewcells infected while the rest proliferate)-Cytocidal effect

• The cell lives in persistent infectionsthis may be productive or nonproductive(refers to whether or not virions are produced)or it may alternate between the two by way oflatency and reactivation - Steady stateinfection

Special cases

• Transformation-Integrated infection (Virusesand Tumor)

• Apoptosis

Types of Viral infections at the cellular level

Virus production

-

+

Fate of cell

No effect

Death

Type

Abortive

Cytolytic

Persistent

Productive

Latent

Transforming

DNA viruses

RNA viruses

+

-

-

+

Senescence

No effect

Immortalization

Immortalization

Mechanisms of viral cytopathogenesis

Inhibition of cellular protein

synthesis

Polioviruses, HSV,

poxviruses, togaviruses

Inhibition and degradation of

cellular DNA

herpesviruses

Alteration of cell membrane

Structure

Glycoprotein insertion

Syncytia formation

Disruption of cytoskeleton

permeability

All enveloped viruses

HSV, VZ virus, HIV

HSV, naked viruses

Togaviruses,

Herpesviruses

Inclusion bodies Rabies

Toxicity of Virion components Adenovirus fibers

Viral Immunopathogenesis

• Influenza-like symptoms (IFN, Lymphokins)

• DTH and inflammation (T-cell, PMNs)

• Immune-complex disease (AB, Complement)

• Hemorrhagic disease ( T-cell, AB, Complement)

• Post-infection cytolysis ( T-cells): Enveloped viruses

• Immuno-suppression: HIV; CMV; Measles virus and Influenza

Viral pathogenesisthe seven steps from entry to disease

HSVVZV

Cytomegalo-

virus

1 Replication at thesite of entry

2 Lymph node

3 Primary viremia

4 Replication sites

5 Secondary viremia

6 Replication sites

7 Transmission toother hosts

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Pathogenesis of viral disease

Host immune responses

The two columns of immunity

immunity

raute 5/2001

constitutional, skinconstitutional, blood

reactive, blood

defensinscomplement

interferons

raute 5/2001

Time tableof innate and specific immunity

complement

interferon a/b

NK cells

CD4 lymphocytes

CD8 lymphocytes

antibodies

days after infection

polymorphonuclear Monocytes NK cells

Early phase of immunity

Infectious agent

Epithelial barrier

Granulocytes (PMN)

drainingLymphnode

Interleukin/

CytokinBalance

Polarization of theT-cell response

?

Antigen-specific responses

Toll-like receptors

Surveillance systems for microbes

Surveillance systems for microbespathogen associated microbial patterns (PAMP)

pattern recognition receptors (PRR)

Toll-like receptor pathways

TLR4 has been reserved forgram-negative bacteria

Inhibition of Toll-like receptor pathways

Hengel, H., Koszinowski, U. H., Conzelmann, K. K., 2005

Prevention and treatment ofviral infection

Antiviral chemotherapy

Antiviral chemotherapeutic agents

• Antiviral drugs are available to treat only afew viral diseases.

• The reason for this is the fact that viralreplication is so intimately associated withthe host cell that any drug that interferessignificantly with viral replication, is likelyto be toxic to the host

Targets for chemotherapeutic agents

•••••

Attachment to host cellUncoating –(amantadine)Synthesis of viral mRNA-(interferon)Translation of mRNA-(interferon)Replication of viral RNA or DNA- (nucleosideanologues)

• Maturation of new virus proteins-(proteaseinhibitors)

• Budding , Release

Diseases for which effective therapy is available

• AIDS: Cocktail therapy (Zidovudine + Lamivudine + protease inhibitors)

•

•

•

•

•

Influenza: Amantadine

Herpes simplex virus: Acyclovir

Varicella-Zoster virus: Acyclovir

Cytomegalovirus : Gancyclovir, Foscarnet

Respiratory syncytial virus: Ribavirin

Nucleotide analogues

• Nucleotide analogues competes withnormal nucleotide for incorporationinto viral DNA or RNA.

Prevention and treatment ofviral infection

Interferons

human interferons

source

inductor

antiviral action

IFN-a

all cells

viral dsRNA

+++

IFN-b

all cells

viral dsRNA

+++

IFN-g

T-lymphos

antigen/mitogen

+

localization

number of subtypes

chromosome 9

22

chromosome 12

1

chromosome 9

1

Function Antiviralinfection

activation ofNK cell

Interferon-inducing dsRNA: 30 bp

anti-tumor regulation ofenhancement of CMI

immunity

Mechanism of action

• Release from an initially-infected cell occurs

• IFN binds to a specific cell surface receptor on another cell

• IFN induces the “antiviral state”: synthesis ofprotein kinase, 2’5’ oligoadenylate synthetase,and ribonuclease L

• Viral infection of the cell activates these enzymes

• Inhibition of viral and cellular protein synthesisoccurs

production ofa & b Interferon

interferon a and b: gene products to remember2’, 5’-oligo adenylate cyclase

RNA dependent protein kinase

Interferonoccupies receptors

on vicinal cells

30 bp dsRNA induces

raute 10/2002

interferon activates > 100 genes

2‘, 5‘-oligoadenylate synthetase

activation of anendonuclease

RNA dependentProtein kinase (PKR)

phosphorylation &inactivation of

arrest of

Inhibition of virus replication

Diseases currently treated withIFN-alpha and IFN-beta

• hepatitis C• hepatitis B• papilloma warts and early trials with cervical

carcinoma• Kaposi sarcoma of AIDS,• colon tumors• kidney tumors ( usually in combination with

other drugs).• Basal cell carcinoma• Breast cancer combined with tamoxifan.

Summary

1. To know the transmission routes of viral infection;

2. To understand the clinical patterns of viral infection and disease;

3. To grasp the pathogenesis of virus;

4. To know the properties and anti-viral activities of interferon

1.

Self control questions

Which one of the following substances is NOT released by activated helper T cells?A. Alpha interferon

B. Gamma interferon

C. Interleukin-2

D. Interleukin-4

2. Which one of the following statements concerning interferons is LEAST accurate?

A. Interferons are proteins influence host defenses in many ways, one of which is the induction of anantiviral state

B. Interferons are synthesized only by virus-infected cells

C. Interferons inhibit a broad range of viruses, not just the virus that induced the interferon

D. Synthesis of several host enzymes is induced by interferon in target cells

3. Each of the following statements concerning interferon is correct EXCEPT:

A. Interferon inhibits the growth of both DNA and RNA viruses

B. Interferon is induced by double-stranded RNA

C. Interferon made by cells of one species acts more effectively in the cells of that species than in the cellsof other species

D. Interferon acts by preventing viruses from entering the cell

4. Please describe the Sites of virus entry.

5. Please describe the mechanisms of virus transmission from person-person.

6. How does the virus spread throughout the host?

7. What determines the cell/tissue tropism for a virus?

8. Please describe the patterns in acute and persistent viral infections.

9. What are the mechanisms for the anti-viral activities of interferon?

10. Term explanation: Horizontal Transmission & Vertical Transmission;

Thank you!