Retroviruses II

Question 1

HIV and AIDS - Consist of a characteristic decline in _____ _____

Answer 1

CD4 T-cells

Question 2

Transmission of HIV infection

Answer 2

Inoculation in blood - Transfusion of blood/blood products
- Needle sharing
- Open wound exposure
Sexual Transmission - Vaginal and anal intercourse
Perinatal transmission - Breast milk
- Intrauterine transmission

Question 3

HIV is a _______ retrovirus

Answer 3

complex

Question 4

Accessory Proteins (6)

Answer 4

Vif, Vpr, Vpu, Nef, Tat, Rev - required for replication or pathogenesis

Question 5

Regulatory Proteins and their functions

Answer 5

Tat: Transactivator of transcription - absolutely required for transcription
Rev: Regulator of virion expression - allows structural gene expression by promoting transport of unspliced RNA from nucleus to cytoplasm

Question 6

Restriction Factors

Answer 6

Viral proteins that overcome cellular defenses or ‘restriction’

Question 7

Vif

Answer 7

Virion infectivity factor - causes a cellular antiviral protein (a deoxycytidine deaminase) to be degraded: it otherwise is incorporated into new virions where block RT in the next cell by inducing massive mutations in viral dsDNA

Question 8

Vpu

Answer 8

Promotes virion release from cells by inhibiting a host protein “tetherin” which otherwise blocks release of virus from the cell: works on other enveloped viruses

Question 9

For HIV ___ is the initial receptor present on immune cells

Answer 9

CD4

Question 10

Cells that can bind HIV

Answer 10

1) CD4 T-helper cells (the main population)
2) Dendritic cells - Not productively infected - assist in virus dissemination
3) Macrophages - reservoir of virus production
4) Microglia - brain infection, contribute to AIDS dementia

Question 11

HIV tropisms for Macrophage and T cells and the co-receptors

M-Tropic:

Answer 11

These infect primary T-cells and macrophages, but not T-cell lines - responsible for initial infection and transmission, and predominate in asymptomatic persons

Question 12

HIV tropisms for Macrophage and T cells and the co-receptors

T-Tropic:

Answer 12

Infect primary T-cells and T-cell lines, but not macrophages

- Associated with disease progression, arise at AIDS stage of infection

Question 13

CCR5

Answer 13

The co-receptor for M-tropic HIV (R5 tropic)

• The receptor for chemokines RANTES, MIP-1α and MIP-1β
• These chemokines can specifically inhibit M-tropic HIV by occupying the receptor

Question 14

CXCR4

Answer 14

The co-receptor for T-tropic HIV (X4 tropic)

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

Question 15

Basis for strain tropisms:

Answer 15

envelope sequence of different HIV types have preference for different co-receptors

Question 16

Which HIV tropism is the source of person to person transmission?

Answer 16

M-tropic virus

Question 17

How do some individuals remain sero-negative despite high-risk behavior and presumably repeated viral exposure?

Answer 17

Explanation is a 32bp-deletion in CCR5 gene (∆32) that causes non-functional CCR5

• WT:WT - get infected and progress to disease normally
• WT:∆32 - get infected but progress to disease more slowly
• ∆32:∆32 - highly resistant to infection - People are essentially normal despite lack of CCR5 gene expression

Question 18

CCR5 antagonist drug

Answer 18

Selzentry (maraviroc)

Question 19

The fusion process

Answer 19

• Envelope initially contacts CD4 and induces a conformation change in envelope to expose the co-receptor binding site
• Then the gp41 “fusion domains” are exposed and that fusion domain enters the cell membrane
• Co-receptor engagement triggers a ‘snapback’ of the N and C-terminal helical regions of gp41 (yellow and red cylinders), which brings the membranes together and fuses them

Question 20

T20 “C” peptides

Answer 20

Antiviral (Fuzeon) - can bind the N-term helical region and block the snapback

Question 21

HIV pathogenesis and Immunity

Answer 21

1) Initial HIV infection - usually at mucosal surfaces
2) Spread to lymph nodes - DC cells can bind and carry HIV to the nodes, where T cells reside and are infected
3) Virus infects T cells, replicates to high levels, and spills into circulation
4) During asymptomatic phase, FDC traps virus, keeps viremia low, but nodes are major site of replication (1 billion/day)

Question 22

Disease mechanisms of HIV

Direct killing of CD4 T cells by HIV (3 methods):

Answer 22

1) Massive virus production leads to membrane leakage and death
2) Synctia (fused cells) induced by fusion of infected cell with uninfected cells - cells eventually die (bystander effect)
3) Apoptosis induced by infection, some evidence that cells undergo apoptosis even if infection is unproductive

Question 23

Disease mechanisms of HIV

Indirect effects on infected CD4 cells:

Answer 23

• Immune response kills infected cells, important for clearing initial viremia
• Soluble gp120 may bind uninfected cells, now susceptible to ADCC (antibody dependent cell-mediated cytotoxicity)

Question 24

Disease mechanisms of HIV

Impairment of immune system function:

Answer 24

• CD4 T-cell function altered and loss of CD4 T cell help leads to severely compromised immune system
• Infected macrophages are dysfunctional

Question 25

Acute infection and seroconversion (2 steps)

Answer 25

1) Initial burst of virus production coincides with decreased CD4 T-cells
2) Early, vigorous CTL, subsequent humoral response, with FDC help, clears viremia - high level virus production persists in lymph nodes

Question 26

Asymptomatic Phase

Answer 26

• 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

Symptomatic AIDS phase:

Answer 27

Late in infection
CD4 cells depleted
Immune system begins to fail
Viremia increases
Patients susceptible to many opportunistic infections

Question 28

Laboratory Diagnosis of HIV

Answer 28

Serology (cannot detect newly infected until 4-6 wks post infection)
- Ab ELISA
- Ag ELISA
- Western Blott
RNA RT-PCR - detect virus in blood
Real time RT-PCR - quantitate virus in blood (very sensitive)

Question 29

Presently available Drugs (5 types)

Answer 29

1) RT inhibitor: Nucleoside/nonnucleoside analogs - Drug resistant strains rapidly appear
2) Protease inhibitors: Extremely effective, reduce viral load by 30-100X alone
3) Fusion inhibitor (T-20) - available but expensive
4) Entry inhibitors - CCR5 co receptor antagonist
5) Integrase inhibitor

Question 30

HAART

Answer 30

Highly Active Anti-Retroviral Therapy

• Triple therapy of different inhibitors: virtually eliminate virus production in some individuals for many years; virus undetectable in plasma, increased CD4 cell counts
• Long-term patients experience toxicity

Question 31

Infected memory T cells: can detect virus from those on HAART for > ____ years

Answer 31

5

Question 32

Enthusiasm for HAART and ‘functional cure’ tempered because:

Answer 32

1) not all patients respond to HAART
2) drug regimen is difficult to follow
3) toxic effects seen in long term HAART users
4) Inaccessible pool of virus

Question 33

New attachment inhibitors

Answer 33

New CXCR4 and dual receptor inhibitors and anti-CCR5 antibody
RT inhibitors to common drug resistant RT viruses
New integrase inhibitors
Maturation inhibitors that work on gag and gag-pol proteins