Retroviridae and Prions

Question 1

What are the characteristics of Retroviridae?

Answer 1

Retroviridae are complex, enveloped viruses.

The capsids contain some proteins.

Contain two copies of single stranded RNA per virus

Uses reverse transcriptase to make a DNA intermediate for replication

The DS DNA intermediate circularizes & integrates into a host chromosome. The host chromosome contains 500 to 1000 highly prefered sites.

Question 2

True or false: Reverse transcriptase is very error prone.

Answer 2

True; Reverse transcriptase does not have an editing function.

Question 3

What cell types do Human T-cell Lymphotropic Viruses (HTLV) I and II infect?

Answer 3

HTLV I & II infect helper T cells

These are usually nondefective, exogenous viruses that can cause cell proliferation

HTLV-I infection can lead to adult T cell leukemia / lymphoma (ATL) in humans

Question 4

How is HTLV-I transmitted?

Answer 4

HTLV-I is transmitted by sexual contact and exposure to infected blood and blood products.

Transmitted vertically by breastfeeding

HTLV-I accounts for 50 to 70% of all lymphoid leukemias in adulthood in endemic areas such as Japan

American Red Cross tests donated blood for HTLV-I and II

Question 5

What are the characteristics of Lentiviridae?

Answer 5

Lentiviridaw are slow-growing viruses

Human Immunodefciency Virus (HIV)

They are cytopathic, non-transforming viruses that cause death in infected cells

Cells in which the virus is latent may be activated to produce virus gene product and viral particals

Cause persistent & debilitating disease in animals proceeded by long incubation periods

These viruses rapidly mutate via antigenic drift and have a very mutable envelope gene

Question 6

What are the characteristics for HIV?

Answer 6

HIV infects helper and inducer T cells

HIV becomes latent in unactivated CD4+ helper T cells & can be reactivated when the immune system is activated

HIV-1 is spread through sexual contact and exposure to blood and blood products

Perinatal transmission is also a mode of transmission

AIDS was first described in 1981 but virus isolated in 1983.

Earliest known samples from West-Central Africa from 1959

Question 7

How is it postulated that HIV originated?

Answer 7

HIV-1 and 2 are thought to have originated from the Simian Immunodeficiency Virus (SIV)

Question 8

When was HIV-2 discovered and what SIV strain is it similar to?

Answer 8

HIV-2 was found in 1986

It is similar to the SIV virus in Sooty mangabeys in West Africa

Disease is endemic in West Africa

Close contact between Sooty Mangabeys and humans likely resulted in transmission

At least six independent transmissions of virus may have occured

HIV-2 causes AIDS but is less virulent than HIV-1

HIV-2 differs from HIV-1 by several genes

Longer clinical latency and lower mortality than HIV-1

Genomes of SIV_SM and HIV-2 are closely related

Question 9

What primate species did HIV-1 evolve from?

Answer 9

It is thought that HIV-1 evolved from Pan t. troglodytes

Came from SIV_CPZ

The natural range of this chimp coincides with the areas of HIV-1 endemnicity

Successive cross-species transmissions likely occured between chimps and the monkeys that they prey upon

Question 10

How is the HIV viron structured?

Answer 10

The enveloped virion has two identical + sense RNA strands

Has an RNA dependent DNA polymerase (Reverse transcriptase)

Integrase

Two tRNAs based paired to the genome within the protein core

Nucleocapsid is surrounded by proteins and a lipid bilayer envelope

Spikes embeded into the envelope are glycoproteins gp120 and gp41

Question 11

What are gp120 and gp41 used for?

Answer 11

HIV gp120 binds to CD4 and CCR5 or CXCR4 co-receptor on surface of host cell

Once this occurs, gp41 undergoes a conformational change and starts the process of fusing the HIV virion with the host cell membrane

Question 12

What cell type is CXCR4 co-receptor found on?

Answer 12

T cells

Question 13

What cell types are CCR5 co-receptor found on?

Answer 13

Macrophags and some T cells

Question 14

What is significant about gp120 in relationship to HIV infection?

Answer 14

gp120 is extensively glycosolated and its antigenicity and receptor-specificity can drift during the course of a chronic HIV infection

Question 15

What strain of HIV are macrophages that express CCR5 on their cell surface in danger of being infected by?

Answer 15

M Tropic HIV

Macrophages are persistently infected and are probably major reservoirs of HIV

Distribute HIV all over the body

Initially, M-Tropic HIV strains dominate but later, T-Tropic strains that have an affinity for T cells that express CXCR4 dominate

Question 16

What is host cell tropism?

Answer 16

This means that HIV has an affinity for one or the other receptor type

Some HIV strains are dual tropic however

Question 17

What kind of protein is gp41?

Answer 17

Membrane fusion protein

It inserts itself into the host cell membrane and catalyzes membrane fusion by HIV

The nucleocapsid is then able to enter the host cell

Question 18

What does CD4 normally bind to?

Answer 18

MHC Class II

MHC Class II mediates T cell responses

Question 19

What are CXCR4 and CCR5?

Answer 19

These are chemokine receptors that are present on immune cells.

Question 20

What do the HIV proteins, SU (gp120) and TM (gp41) do?

Answer 20

SU(gp120) and TM (gp41) bind to CD4 and co-receptors

Question 21

What are the genes of HIV and what do they do?

Answer 21

Three major genes that encode polyproteins for structural and enzymatic proteins.

These proteins are cleaved by protease to become active

GAG - Group-specific antigen, capsid prteins

POL - Polymerase (Reverse transcriptase), integrase, protease

Env - Envelope glycoproteins, SU (gp120) and TM (gp41)

Genes that encode accessory proteins

Question 22

What are prions?

Answer 22

Prions are infectious agents composed of a misfolded protein.

Usually found in central nervous system

Usually starts as a result of the introduction of an abnormal protein from another human, cow, or sheep.

Can also initiate due to a genetic propensity for the aberrant conformation.

They can also arise by chance.

Question 23

What are prions insensitive and sensitive to?

Answer 23

Prions are resistant to UV, ionizing radiation, formaldehyde, proteases, and other DNA-damaging agents

Rions are sensitive to protein denaturing agents, phenol, urea, guanidine, HCL, and NaOH

Question 24

True or False; Prions elicit an inflammator and/or antibody response.

Answer 24

False; no evidence that prions stimulate any immune resposne

Question 25

True or false: Prions can have a latency period that lasts for years.

Answer 25

True

Question 26

What is the proposed molecular disease process of prions?

Answer 26

PrPsc, protease resistant prion, causes a switch from PrPc to the PrPsc type protein

This altered protein cannot be degraded by normal cellular processes and the abnormal protein starts to accumulate and form plaques in cytoplasm, extracellular areas and thebrain

Can also be caused by a mutation in the gene that encoded PrP, a random event, or by ingesting PrPsc protein from another source (Kuru or Mad Cow)

Question 27

What are the prions that cause human disease?

Answer 27

Kuru - Eating brain from deceased individuals

Gerstmann-Straussler Scheinker Syndrome - Mutation of P102L and other genes. Autosomal dominant.

Creutzfeldt-Jakob Disease - Found in Jews, libyans. Not completley known how it is spread. Autosomal dominant.

Fatal Familial Insomnia - Severe atrophy of anterior ventral and mediodorsal thalamic nuclei. Autosomal dominant.

Varient CJD - Most likely derived from Bovine Spongiform Encephalopathy

Question 28

What are the prions that cause diseases in animals?

Answer 28

Scrapie - This is a disease that is seen in sheep and was first recognized in the mid 18th century

Bovine spongiform encephalopathy (BSE) - Progressive neurological disorder in cattle resulting in tremors, staggering, and death

Chronic Wasting Disease (CWD) - Seen in deer and elk. Results in weight loss and neurologic symptoms

Question 29

What are the characteristics of Creutzfeldt-Jakob Disease?

Answer 29

Mean age of onset is 63 yrs.

This is a genetic disease that arises due to a mutation in PrP

There has been transmission in Japan due to grafts of dura matter. This case had an incubation period of about 7 years.

CJD can arise without genetic mutation (3.1 / million) and can occur due to random protein misfolding

Question 30

How do you disinfect instruments used on a person suspected of having CJD or vCJD?

Answer 30

Autoclave for one hour at 134^o C with 1N NaOH

Use 2N NaOH if can’t be autoclaved

Only one case of human vCJD in U.S. and was confirmed to have originated in the U.K.

American Red Cross (2001) decided to defer donations from anyone who has spent 3 months in U.K.or 5 or more years in Europe since 1980-1996.

Two documented cases of vCJD originating from blood transfusions in U.K.