02/27c Host-Viral Dynamics

Question 1

What are the two types of virus-cell interactions?

Answer 1

Cytolytic

Non-cytolytic

Question 2

What are the characteristics of lytic viruses? Name three

Answer 2

Generally non-enveloped (just protein and nucleic acid)
Tend to be more environmentally stable
Primary means of clearance is usually antibody neutralization

Question 3

How do lytic viruses spread?

Answer 3

By rupturing the host cell

Question 4

How does the addition of neutralizing antibodies prevent the spread of a lytic virus?

Answer 4

By binding the virus itself, OR by binding to the cell surface receptor that the virus uses to enter cells - both would terminate the infection

Question 5

What are the general characteristics of non-lytic viruses?

Answer 5

Most are enveloped
Bud from host cell membranes
Spread through intracellular bridges, and are thus protected from neutralizing antibodies
Primary means of clearance are cell-mediated host defenses and cytotoxicity

Question 6

What effect do neutralizing antibodies have on non-lytic viruses?

Answer 6

Bind to and prevent the spread of free virus that buds from the cell
Does NOT prevent the spread of virus to adjacent cells

Question 7

Aside from lytic and non-lytic spread, what types of interactions do viruses have with cells?

Answer 7

May be polar, i.e. only released from one surface (apical or basal)
May integrate their genomes with host genomes
Some viruses go into latency
May lead to recognizable changes in the host cell membrane

Question 8

What is interferon? How does it halt a viral infection?

Answer 8

Protein produced by virus-infected cells
Protect UNINFECTED cells from infection until the immune system is able to clear the virus
Also up-regulate natural killer cell activity

Question 9

What is tropism? How does it work?

Answer 9

Tropism is the preference of a virus to infect a certain cell type
Viruses express Virus Attachment Proteins (VAPs), and can only bind to and infect cells which express the receptor for their particular VAP

Question 10

What is ADCC? What is its function in clearing viral infections?

Answer 10

Antibody-Dependent Cell-Mediated Cytotoxicity
Viral proteins that are inserted onto the surface of infected host cells are recognized by antibodies
Binding of antibodies to these viral proteins tags the infected cells for destruction by the immune system
This stops the spread of virus to new cells

Question 11

What is antibody-facilitated entry?

Answer 11

Binding of non-neutralizing antibodies to a virus (e.g. dengue fever), which then allows uptake of the virus by macrophages
Macrophages are able to take up antibody-bound virus because they express Fc receptor

Question 12

What does antibody-facilitated entry mean in the course of subsequent infections of dengue fever?

Answer 12

Subsequent infections (often by a different serotype) will often be much more severe than the primary infection

Question 13

What other ways can viruses interact with the host immune system?

Answer 13

Interfering with immune presentation (HIV, adenovirus)

Directly infecting immune cells (HIV, EBV, measles)

Question 14

What are some host characteristics that can greatly affect the course of viral infections?

Answer 14

Age
Nutrition
Immunodeficiencies (acquired and genetic)
HLA types

Question 15

What are two viruses that present very differently, depending on the age of the host? Why?

Answer 15

Varicella - less severe in children, due to the fact that the virus is carried and disseminated by memory T cells, of which there are more in adults
Hepatitis A - asymptomatic in infants (unsure of why)

Question 16

What is an example of a virus that is very severe and often fatal in malnourished patients?

Answer 16

Measles

Question 17

What are the three ways that viruses enter the body?

Answer 17

Mucosal surfaces (live cells, exposed to the outside world) - most viruses
GI tract - mostly non-enveloped viruses (enveloped viruses cannot survive gastric acid), particularly CMV
Innoculation or vertical transmission

Question 18

How does your body control the spread of a virus?

Answer 18

Mechanical means (skin)
Antibodies (ADCC)
Interferon
Cytotoxic T lymphocytes
Natural killer cells

Question 19

What are the three types of interferon? What types of cells produce them?

Answer 19

Alpha and beta - produced by almost all infected cells

Gamma - produced primarily by macrophage and lymphocytes; more specific, but less effectively anti-viral

Question 20

What are cytotoxic T lymphocytes?

Answer 20

Specifically recognize and destroy virus-infected cells

Question 21

How does edema limit the spread of a viral infection? Against what types of viruses is it most effective?

Answer 21

Edema forces cells apart
Separation of cells reduces the availability of inter-cellular bridges, which prevents the spread of non-lytic enveloped viruses

Question 22

When does viral titer peak? What also peaks at this time?

Answer 22

Around 3-5 days after infection

IFN levels and NK cell activity also peaks with virus titer

Question 23

When do antibodies and cytotoxic T lymphocytes become detectable? How long do they stick around?

Answer 23

Around 4-6 days after infection
CTLs last for weeks or months
Antibodies can last a lifetime