9 - Retroviruses II: HIV and AIDs

Question 1

When and how was AIDs first recognized?

Answer 1

In 1981 as an increasing incidence of pneumocystis carnii pneumonia, kaposi’s sarcoma, and other opportunistic infections in homosexual men, heroin addicts, hemophiliacs, and haitians.

These are all normally benign, except in the immunocomp.

What that had in common was severe declines in CD4 T cells.

Question 2

Why did the death rate from AIDs for males under 40 yrs peak in ‘94 and ‘95 but then decline?

Answer 2

Declined with the development of the protease inhibitor and with 2 retroviral drugs: HAART: highly active anti-retroviral therapy, which has made HIV a chronic disease.

Question 3

What are the known transmission routes of HIV?

Answer 3

Inoculation in the blood, sexual transmission, and perinatal transmission.

These can occur from transfusions, needing sharing, sticks, vaginal/anal intercourse, intrauterine transmission, and through breast milk.

Question 4

Can HIV be transmitted through close personal contact? What does this mean for household members and healthcare workers?

Answer 4

NO.

They they will not get it unless they are in contact with the blood.

Question 5

What are the key to HIV’s pathogenesis?

Answer 5

It’s 6 accessory proteins in its genome in addition to the gag, pol, and env that exist in simple viruses.

Question 6

What regulatory proteins does HIV have that are crucial for replication?

Answer 6

Transactivator of transcription (Tat)

Regulator of virion expression (Rev): needed to export unspliced RNA from nucleus to cytoplasm

Question 7

What restriction factors does HIV use to overcome cellular defenses? What is the function or each?

Answer 7

Virion infectivity factor (Vif): causes cellular antiviral protein to be degraded (otherwise it would be incorporated into new virions and block RT)

Vpu: promotes virion release from cells by inhibiting a host protein called tetherin, which otherwise blocks release of virus from the cell. (Vpu degrades tetherin so the virus releases just fine).

Question 8

What are the major receptors for HIV? What four places are these found?

Answer 8

CD4 is initial receptor on immune cells:

1. CD4 T cells (depleted in AIDs)
2. Dendritic cells: binds HIV but are not productively infected, can assist in dissemination
3. mø: infected by not killed, short term reservoir of virus production
4. microglia in brain contribute to AIDs dementia

Question 9

Is CD4 binding sufficient for HIV to cause membrane fusion?

Answer 9

NO, a co-receptor is needed.

Question 10

What are the HIV tropisms for macrophage and T cells? What does each infect?

Answer 10

M-tropic: lab observation that these infect T cells and mø but not T cell lines. These cause initial infections and transmission and predominate in asymptomatic ppl.

T-tropic: infect primary T cells and T cell lines but not mø . Associated with disease progression and arise at AIDs stage.

Question 11

What co-receptor is used for M-tropic HIV? What does this receptor normally do?

Answer 11

CCR5

This is the receptor for chemokines RANTES, MIP-1a, and MIP1B - these specifically inhibit M-trophic HIV by occupying the receptor

Question 12

What is the co-receptor for T-tropic HIV? What does this receptor normally do?

Answer 12

CXCR4

Natural ligand is the cytokine stromal derived factor 1 (SDF-1) which can specifically block T-tropic HIV infection.

Question 13

What is the basis for strain tropisms? Which tropism is the most cause for concern?

Answer 13

Env sequence of different HIV types have preference for different co-receptors.

Most concern for M-tropic, which is the source of person to person transmission. (T-tropic only seen in those with late AIDs)

Question 14

Why do some individuals remain seronegative despite high-risk behavior and presumable repeated viral exposure?

Answer 14

For some, the reason is a 32bp-deletion in CCR5 gene causes non-functional CCR5

Question 15

What results from being hetero or homozygous for the 32bp deletion in the CCR5 gene?

Answer 15

Heterozygous: get infected but progress to disease more slowly (express about half as much as normal)

Homozygous: highly resistant to infection. People are essentially normal despite lack of CCR5 expression on surface of cells.

Question 16

What does the absence of a homozygous phenotype for the 32bp deletion in the CCR5 gene indicate?

Answer 16

Therapeutic feasibility of drugs to block CCR5 function; ie a CCR5 antagonist.

Question 17

What steps take place for fusion to occur?

Answer 17

1. Env contacts CD4 and induces a conformational change in Env to expose the co-receptor binding site
2. gp41 “fusion domains” are exposed and those enter the cell membrane

Question 18

What occurs during fusion once the gp41 fusion domains enter the cell membrane?

Answer 18

Co-receptor engagement triggers a “snapback” of the N and C terminal helical regions of gp41, which brings the membranes together and fuses them.

Question 19

How can the “snapback” process of membrane fusion be blocked?

Answer 19

T20 “C” peptides can bind to the N-term helical region and block the snap back.

This is an effective antiviral on the market.

Question 20

Where is the initial HIV infection and where does it spread?

Answer 20

Initial infection occurs at mucosal surfaces or by blood products and spreads to the lymph nodes.

DC cells can bind and carry HIV to the nodes where T cells reside and are infected.

Question 21

What happens once T cells are infected in the lymph nodes?

Answer 21

They replicate at high levels and spill into the circulation (viremia).

During asymptomatic phase,, follicular dendritic cells trap virus and keep viremia low but nodes are a major site of replication (especially GALT)

Question 22

What are the three disease mechanisms of HIV?

Answer 22

1. Direct killing of CD4 cells by HIV -massive virus production leads to membrane leakage and death
2. Syncytia (fused cells) induced by fusion of infected cells with uninfected cells (via env on infected cell interacting with CD4/CCR5 on uninfected cell)
3. Apoptosis induced from infection

Question 23

What indirect effects does HIV have on CD4 cells?

Answer 23

1. Immune response kills infected cells, important for clearing initial viremia
2. Soluble gp120 may bind uninfected cells, now susceptible to ADCC

Question 24

What immune system impairment is seen with HIV?

Answer 24

CD4 T cell function is altered, loss of CD4 T cell help, leads to severely compromised immune system.

Infected mø are dysfunction and cause abnormal immune function

Question 25

What occurs with acute HIV infection and seroconversion?

Answer 25

Initial burst of virus production coincides with decreased CD4 T cells

Early vigorous cytotoxic T cell and humoral response clears viremia and immune response “appears” to control infection.

High levels of virus producvtion persist in lymph nodes and GALT.

Question 26

What occurs in the asymptomatic stage of HIV? How is progression measured?

Answer 26

Continued strong immune response but gradual decline in CD4 counts.

Progression measured by CD4 counts, CD4:CD8 ratio, “viral load” by measuring RNA by PCR.

Question 27

What occurs during the symptomatic/AIDs phase of infection?

Answer 27

Late infection, CD4 cells depleted (<200/microL).

Immune system begins to fail, viremia increases, pts susceptible to many opportunistic infections.

Question 28

What opportunistic infections are AIDs pts very susceptible to?

Answer 28

Crypto
Toxo
Cytomagalovirus
HSV
Hepatitis 
Pneumocystis carinii
Candida
Mycobacterium
Salmonella 
Kaposi's sarcoma

Question 29

How is HIV diagonised using serology? What are the benefits to each?

Answer 29

Cannot detect newly infected, unreliable until about 4-6 wks post-infection.

Ab ELISA: initial screening detects Ab to virus: rapid, oral, inexpensive

Ag ELISA: detects p24 capsid antigen earlier than Ab (7-14 days)

Western blot: confirmation test, uses pt Ab to detect HIV protein

Question 30

What diagnostic method detects virus in the blood? What are the benefits to this method?

Answer 30

Real time PCR (RT-PCR) for NRA.

Very sensitive, detects virus before seroconversion and gauges viral load in asymptomatic pts with low titers (important for prognosis).

Question 31

What can therapeutic agents target for treating HIV? What does the progression of HIV tell you about treating it?

Answer 31

Any step in the replication cycle, esp virus specific proteins.

Since progression is related to viral load, you may not have to completely block replicated to remain healthy.

Question 32

Name some different types of inhibitors used to treat HIV infections (ie what parts of HIV replication can be targeted)? What is the best option thus far?

Answer 32

Reverse transcriptase inhibitor

Protease inhibitor

Integrase inhibitor

Fusion inhibitor

Entry inhibitor

HAART: highly active anti-retroviral therapy or ART for short. “virutally” eliminated virus production in some for many years.

Question 33

How much time does it take HAART to clear free virus and infected T cells? What about other compartment with longer half lives such as macrophages? Infected memory T cells? Resting t cells?

Answer 33

Free virus and infected T cells: ~2 mo

Other compartments and mø: ~1-2 yrs

Infected memory T cells: >5 yrs

Resting T cells: >75 yrs

Question 34

Why was the enthusiasm for HAART as a functional cure for HIV tempered?

Answer 34

Not all pts response to HAART

Drug regimen is difficult to follow

Toxic effects with long-term use

Inaccessible poorl of virus in memory T cells is a long-lived reservoir