AuCoin: Retroviruses and HIV Flashcards
Describe a retrovirus
spherical
enveloped
two identical copies of + strand RNA genome
capsid + envelope (acquired from the plasma membrane) with viral glycoproteins
What do retroviruses encode?
an RNA-dependent DNA polymerase (reverse transcriptase)
When the DNA copy of the viral genome is produced in a retrovirus, what happens?
It is integrated into the host chromosome to become a cellular gene, or a provirus
the first retrovirus to be isolated it produced solid tumors in chickens
Rous sarcoma virus
Cancer causing retroviruses are called (blank)
RNA tumor viruses
oncornaviruses
How do many retroviruses alter cellular growth?
by expressing oncogenes
This was the first retrovirus to be found to be associated with human disease
Human T-lymphocyte virus
When did AIDS first become a “problem?”
in the late 1970s, early 1980s
When is a retrovirus considered an oncovirus?
when it immortalizes or transforms its target tissue
These viruses are associated with neurologic and immunosuppressive diseases
lentiviruses (HIV)
There are three subfamilies of retroviruses. What are they?
- oncovirus (HTLV)
- lentivirus (HIV)
- Spurmavirus
What are some components contained in the retrovirus virion?
10-50 copies of reverse transcriptase and integrase enzymes
two cellular transfer RNA (tRNAs)
What are the tRNAs in the retrovirus virion used for?
they are the primer for the reverse transcriptase
All retroviruses include 3 major genes that encode polyproteins. What are they?
gag - structural protein
pol - polymerase
env - envelope
What is contained at each end of the retrovirus genome?
long terminal repeat sequences which contain promoters and enhancers to bind cellular transcription factors
What do complex viruses, like HTLV and HIV encode?
virulence-enhancing proteins that require more complex trx processing
How are viral glycoproteins produced?
proteolytic cleavage of the polyprotein encoded by the env gene
Which glycoprotein is cleaved to form the two glycoproteins that form the spikes on the virion surface?
gp160 —> gp41 and gp120
What does gp120 on the surface of the virus do?
it binds to cell surface receptors, which initially determines which cells/tissue the virus will enter
What is one factor that impedes the clearance of HIV?
gp120 on its surface is very glycosylated and its antigenicity and receptor specificity can drift during the course of infection
What does gp41 on the surface of HIV do?
promotes cell fusion
How does HIV replication begin?
gp120 binds to CD4 on T cells or CCR5 on macrophages, and other T cells)
Which other receptor on T cells can gp120 bind to during chronic infection?
CXCR4
What happens when gp120 binds its co-receptor?
the viral envelope and the cell plasma membrane come close together and gp41 can promote membrane fusion
What happens once the viral cell and host cell undergo fusion?
the + strand RNA is released into the cytoplasm and reverse transcriptase synthesizes a complementary - strand DNA
Besides transcribing DNA from RNA, what else does reverse transcriptase do?
acts as a ribonuclease H and degrades the + strand RNA and resynthesizes the + strand DNA to form the cDNA
This is a major target for antivirals
reverse transcriptase
The (blank) chemokine receptor is used upon initial infection of an individual, and after mutation of the env gene, the (blank) receptor is also used
CCR5; CXCR4
What is one problem with reverse transcriptase?
it is very error prone and has a high mutation rate which allows for the evolution of new strains of the virus and may help promote immune evasion
Viral cDNA is delivered to the nucleus and is integrated into host chromosome by this enzyme
Integrated viral DNA is called…
integrase; provirus
What sequences in the viral genome are replicated and allow for integration of the viral genome
long-terminal repeats
**contain enhancer/promoter regions for regulation of trx
This enzyme transcribes the provirus into full length RNA which is processed into gag, gag-pol, or env
host RNA pol II
HIV replication is regulated by many “accessory” gene products. What do the following genes regulate?
rev tat nef vpr vif vpu
rev: RNA splicing and export into cytoplasm
tat: transactivation of viral and cellular genes
nef: decreases CD4, activates T cells
vpr: transports cDNA to nucleus, arrests cell growth
vif: virus infectivity
vpu: facilitates virion assembly and release
Where are the viral glycoproteins translated? Then what happens?
on the RER; then they are glycosylated in the ER and delivered to the golgi
What promotes budding of the virion from the plasma membrane?
two copies of the genome + cellular tRNA molecules
So, think of the video posted by Dr. AuCoin. How does HIV viral infection occur?
HIV virion with its envelope has gp120 on its surface and binds to target CD4 T cells (CD4 and CCR5 are the co-receptors necessary for this binding). The host cell and the virus are brought in close proximity so that fusion can occur thanks to gp41. Then, once inside the host cell, ss viral RNA uses reverse transcriptase to form ss viral DNA. The viral DNA uses RT to produce a second copy. Then, this viral dsDNA is taken to the nucleus through a pore and is integrated into the host cell chromosome when integrase enzyme makes a “nick” in the host DNA. Next, DNA polymerase comes along and transcribes the DNA into proteins that are necessary for the virus to survive, such as envelope glycoproteins, etc. When polyproteins are generated, proteases cleave them into functional proteins.
What does HIV do to cells expressing large amounts of CD4?
synctia formation (multiple T cells brought together) –> cell lysis
What does HIV do to CD4 T cell numbers? CD8 T cell numbers? Macrophage function?
They all decrease :(
**CD4 T cells are essential for activation of CD8 T cells
What are some consequences of HIV infection?
immunodeficiency (loss of CD4s)
loss of B cell control
loss of DTH reactions –> severe systemic opportunistic infections
What does HIV infect during sexual transmission?
mucosal surfaces
Deficiency in this co-receptor may confer resistance to HIV infection?
CCR5
What cells serve as major reservoirs and means of HIV transmission?
macrophages
DCs
memory T cells
bone marrow stem cells
Development of the symptoms of AIDS correlates with increased release of (blank) into the blood, and a decrease in (blank)
virus; CD4 T cells
What protein promotes progression of HIV to AIDS? What happens to individuals who have natural nef gene mutants?
Nef protein; no progression to AIDS
What happens during the acute phase of HIV infection? What happens during the latent period? What happens late in the disease?
large burst of virus production; virus levels drop in the blood, but replication continues in the lymph nodes; virus levels in the blood increase while CD4 T cells significantly decrease - CD8 T cells also decrease and the patient becomes immunosuppressed
What is the time line of HIV progression to AIDS?
initial infection can last up to 12 weeks, then it goes latent for ~7yrs, then it will manifest again and cause weight loss, fevers, fatigue, and malaise, then rapidly progresses to death
What is the time line of HIV progression to AIDS?
incapacitated, because CD4 T cells are unavailable or non-functional; many opportunistic infections (fungi and intracellular bacteria)
What cells in the brain does HIV infect? What does this do to the brain? What does this manifest as?
microglia
macrophages (glial cells);
causes release of neurotoxic substances that cause an inflammatory response and neuronal death in the brain;
AIDS related dementia
**individual is at risk for opportunistic brain infection
What is the most dominant malignancy to develop in patients with AIDS?
Kaposi’s sarcoma
This is a MAJOR sign of AIDS infection
pneumocystis pneumonia (caused by pneumocystis jirovecii)
What CD4 T cell count is indicative of the onset of AIDS? What about full-blown AIDS?
<200 cells/microliter
About how many copies of the HIV virus are produced when significant disease occurs?
75,000
What is HIV wasting syndrome?
weight loss and diarrhea for more than one month
Patients with AIDS-related dementia experience symptoms similar to what disease?
Alzheimer disease
How must HIV be transmitted? What can happen to the virus before development of identifiable symptoms?
in the blood stream; it can be shed
Where is the HIV virus present in the body? In what population is it most prevalent in?
blood, semen, vaginal secretions;
IV drug users, sexually active people with many partners, prostitutes born of HIV mothers, used to be found in transplant recipients before functional screening
How can you control AIDS?
antiviral drugs limit the progression of the disease;
safe, monogamous sex;
sterile needle injection;
screening programs before transfusion/transplants
AIDS is most prevalent in this constituent
male homosexuals
Can serology tests be used to check for AIDS? What is one problem with this method?
yes, because of the chronic nature of the disease; will not work for recently infected individuals
So how would you identify recent HIV infection in an individual?
Detect:
viral RNA in blood
p24 viral antigen
RT enzyme
How is anti-HIV therapy currently administered?
as a cocktail of several antiviral drugs (HAART)
What can multidrug therapy effectively do for HIV patients?
reduces levels of virus in the blood to nearly ZERO, reduces morbidity/mortality
At what T cell count would you begin to administer HAART?
<350 cells/microliter
if HIV is detected after a needle prick
**or if viral loads are higher than 100,000
What is the current status of vaccines against HIV?
several trials are being conducted - glycoprotein vaccines to gp120 and gp160 only work against certain strains; still a lot of work to be done
What was the first successful anti-HIV therapy?
AZT
What do nuceloside analogs do for the prevention/control of AIDS?
inhibit viral polymerases by incorporating a terminal nucleoside
What are some points in HIV replication that can be inhibited by drugs?
reverse transcriptase
protease (required in the late stage of HIV replication)
fusion (entry of HIV into cell)
List some clinical signs of HIV?
sweats, weight loss, oral thrush, cervical lymph nodes swollen, low CD4 T cell count**
List some clinical signs of HTCV?
old age Caribbean islands enlarged liver and spleen skin rashes high Ca++
What is the main determinant in the pathogenesis and disease caused by HIV?
the virus TROPISM for CD4 T cells and myeloid cells (monocytes, macrophages, DC)
