VIROLOGY PART iiiVIRAL – HOST INTERACTIONS

Dr.Riyaz Sheriff

Viral – host interactions

• Cellular level

• Individual level

• Community level

@ Cellular level

• Cellular changes seen in tissue culture plate may not be seen in clinical infection

Cell death Malignant

transformationCellular

proliferationNo effect

• Non structural proteins shut down the host protein & DNA synthesis

• Large amount of Viral components change the cellular architecture of

host cell

• Toxic effects

• Change in permeability of Host cell AUTOLYSIS

• Formation of syncytium d/t fusion of adjacent cell membranes

• Viral antigens will confer newer properties on host cell (cell

adsorption, oncogenesis)

• Changes in the host chromosome ( Measles, Mumps, Adenovirus, CMV

& Varicella)

• Inclusion bodies

Inclusion bodies

• Structures with distinct size, shape , location and staining properties

• Can be seen under light microscope after staining

• May be seen in cytoplasm or nucleus

• Generally acidophilic in nature – Pink in colour on staining with Giemsa or

Eosin methylene blue stains

• Some may be basophilic as well

Inclusion bodies

• Helps in diagnosis

– Negri bodies – intra cytoplasmic inclusion – Rabies

– Guarnieri bodies – vaccinia

– Bollinger bodies – fowl pox

– Molluscum bodies – molluscum contagiosum

– Cowdry type A – Herpes virus , Yellow fever virus

– Cowdry type B – Adeno virus, Polio virus

• Inclusion bodies my be an aggregate of virions or collection of

viral antigens or the degenerative changes produced by viral

infection.

Negri bodies – Rabies –Intracytoplasmic

Guarnieri bodies – vaccinia

Bollinger bodies – fowl pox

Molluscum bodies – molluscum contagiosum

Cowdry type A

Hepatocyte with a large intranuclear inclusion body. Surrounded by a clear halo

Cowdry type b

Pathogenesis of viral infection

• Inapparent

• Apparent

– Acute

– Subacute

– Chronic

• Latent

• Persistent tolerant infections : Congenital / Neonatal infections

• Oncogenic virus

• HIV

Route of entry • Respiratory tract (Most common)

– Multiply locally Blood/Lymph Extensive multiplication Disease.

– Small pox, chicken pox

– Influenza , Rhinovirus (stay in respiratory tract itself)

• Alimentary tract

– All enveloped viruses are destroyed by bile

– Rhinovirus is inactivated by gastric juice

– Enterovirus, adenovirus, reo virus, hepatitis virus

– Some multiply in GIT and transported to target organs (eg.Polio virus)

• Genital tract

• Conjunctiva

• Skin

• Vertical transmission – Mother to baby

Route of entry • Skin

– Produce few local lesions

– Papilloma, vaccinia, cowpox & molluscum contagiosum.

– Viruses can enter through break in skin

– Abrasions – Papiloma virus

– Insect bites – Arbovirus

– Animal bites – Rabies

– Injections – Hepatitis

• Genital tract

– Human immuno deficiency virus

Route of entry • Conjunctiva

– Local disease – Adenovirus

– Systemic disease – Measles

• Vertical transmission – Mother to baby

– May occur at any stage till birth

– Usally leads to fetal death and abortion

– Maldevelopment – Rubella and Cytomegalovirus

– Many tumor virus spread via this route

Spread of virus in the body

• Studied by Fenner using mouse pox as a model

Mouse pox virus

Enters through skin

Local Multiplication

Skin & Lymphatics

Lymph Nodes Blood (Primary Viremia)

Spleen , Liver (Central foci )

Extensive multiplication

Spills into blood (Secondary

Viremia)

Clinical symptoms

Virus reaches

target organ

Multiplication CLINICAL

DISEASE

Incubation period

• Time taken for virus to spread from site of entry to

the organs of viral multiplication and causation of

disease.

• Localized diseases : Shorter incubation period

• Systemic diseases : Longer incubation period

• Incubation period shorter when directly introduced

into blood stream

Host response Depends on

1. Virulence of the infecting strain

2. Resistance of the host» Immunological

• Humoral

• Cell mediated immunity

» Non specific

• Interferon production

• Body temperature

• Age

• Malnutrition

• Viral infection liberates

– Surface antigens

– Internal antigens

– Non structural antigens

• Early proteins

– Humoral immunity

• IgG

• IgM

• IgA Mucosal surface

Blood and Tissue

Immunoglobulins on viruses

• Prevents attachment of virus to cell

• Enhances viral degradation

• Prevents release of virions from infected cell

Immunoglobulin + Complement

Surface damage of enveloped virions

Cytolysis of virus infected cells

Role of antibodies • Role of antibodies in viral infection is limited

• Antibody to internal antigen Non neutralizing

• Antibody to surface antigen varying neutralization

• Some antibodies can paradoxically increase infectivity

• May contribute to Pathogenicity

• Antibody may result in

– Complement dependent cell injury

– Immune complex type tissue injury

Role of cmi

• Plays a major role in viral infection

• Helps in recovery from viral infection

• Can cause tissue damage as well

• Deficient CMI increase in Herpes, Pox, Measles

• Most often an infection provides long lasting immunity

Non immunological responses

Macrophages phagocytose virus in blood.

Body temperature : > 39°C inhibits most virus

Exception – Herpes simplex – fever blisters

Hormones : Corticosteroids enhance viral infection.

Due to depression of immune system & inhibition of interferon synthesis

Malnutrition

Age

Interferons • Family of host coded proteins

• No direct action on virus

• Acts on host cells to make them refractory o viral infection.

• On exposure to interferon cells produce TIP (Translation Inhibition Protein)

inhibits translation of viral mRNA

• Does not affect translation of host mRNA

• Interferons are species specific.

• RNA viruses are better inducers of interferon production

• Temperature of > 40°C induces interferon secretion

• Steroids and increased O2 tension decrease interferon synthesis

• Synthesis starts in about 30mins of induction and reaches peak by 6-12 hours.

Interferons • α leucocytes

• ß fibroblasts

• Ɣ T-Lymphocytes

• Inactivated by proteolytic enzymes

• Resist 56°C – 60°C for 30-60mins

• α & ß are resistant at pH range of 2-10

• Ɣ is labile at pH of 2

• Non toxic

• Poorly antigenic

• Cannot be estimated by routine serological methods

Use of Interferons

• Ideal candidate for prophylaxis and treatment

– Non toxic

– Non antigenic

– Diffuses freely in the body

– Wide spectrum of antiviral activity

• Drawback species specific

• Current use URTI, Warts, Herpetic keratitis &

anticancer agent in lymphomas

Biological effects of interferons

• Anti-Viral : resistance to infection

• Anti –Microbial: Resistance to intracellular infections (Toxoplasma,

Chlamydia, Malaria)

• Cellular effects : Inhibition of cell growth and proliferation, Increased

expression of MHC antigens on cell surface

• Immunoregulatory :

– Increases activity of Natural killer (NK) calls and T cells.

– Activates cell destruction activity of macrophage

– Moderates antibody formation

– Activates suppressor T cells

– Suppresses DTH

Lab diagnosis • Microscopy :

Demonstration of virus by electron microscopy

Demonstration of inclusion bodies

Fluorescent antibody techniques

• Demonstration of viral antigen:Possible when viral antigens are abundant in the lesion

Precipitation in gel

Immunofluorescence

CIE

RIA

ELISA

PCR

Lab diagnosis • Isolation of virus :

• Need proper transport media at appropriate temperature

• Processing to remove bacterial contaminants

• Inoculation in eggs

• Animal inoculation

• Tissue culture

• Virus isolation has to be correlated with clinical history

Identified by Neutralization tests

Lab diagnosis • Serological diagnosis :

• Rise in titre of antibodies during course of disease

• Examine paired sera

– Acute

– Convalescent (10-14 days later )

• When IgM alone is tested Single sample is enough

– Neutralization test

– Complement fixation

– ELISA

– Hemagglutination Inhibition

Immunoprophylaxis• Infection / vaccine Prolonged and effective immunity

• Live vaccines more effective than killed vaccines

• Successful Live vaccines

– Small pox vaccine

– Yellow fever vaccine

– Polio Vaccine ( Sabin)

• Killed vaccines prepared by inactivating viruses using Heat, Phenol,

Formalin or BPL

• Subunit vaccines

– Hepatitis B

Live attenuated vaccines

Advantages

• Single dose

• Administered by route of natural infection

• Induce immunoglobulins

• Induce CMI

• Long lasting immunity

• Economical

• Apt for mass immunizations

Disadvantages

• Remote chance of reactivation of virus

• Cannot be used in immunocompramised

• Existence of other viruses may result in lessened immune response

• Needs proper cold chain

Killed vaccines • Safe

• Stable

• Can be given as combined

vaccines

• Multiple doses

• Does not induce local

immunity or cell mediated

immunity

Chemoprophylaxis • Challenge

Viruses are strictly intracellular , use host mechanisms for replication.

Hence therapy would destroy host cell as well

• Answer

Selective inhibition of viral enzymes

– Attachment

– Transcription of viral nucleic acid

– Translation

– Replication

– Viral assembly

– Release

Antiviral agents • 1960 : First antiviral – Pox virus : Small pox eradicated by

vaccination

• 1962 : Idoxyuridine : Herpes Keratitis

• 1970 : Acyclovir : effective in Herpes

• Nucleoside analogues – Deoxyuridines

• Idoxyuridine Herpetic keraitis

• Trifluridine Less toxic

• Bromovinyldeoxyuridine Herpes & Varicella

– Adenine arabinoside• Herpetic keratitis

• Herpes simplex

• VZV

– Acyclovir herpes

– GanciclovirCytomegalovirus

– Zidovudine HIV : inhibits viral reverse transcriptase

• Protease inhibitors

– Saquinavir

– Ritonovir

– Indinavir

• Others

– Amantidine – blocks cell penetration by influenza A but not

by B&C

Antiviral agents