Retroviruses II Flashcards
HIV and AIDS - Consist of a characteristic decline in _____ _____
CD4 T-cells
Transmission of HIV infection
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
HIV is a _______ retrovirus
complex
Accessory Proteins (6)
Vif, Vpr, Vpu, Nef, Tat, Rev - required for replication or pathogenesis
Regulatory Proteins and their functions
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
Restriction Factors
Viral proteins that overcome cellular defenses or ‘restriction’
Vif
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
Vpu
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
For HIV ___ is the initial receptor present on immune cells
CD4
Cells that can bind HIV
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
HIV tropisms for Macrophage and T cells and the co-receptors
M-Tropic:
These infect primary T-cells and macrophages, but not T-cell lines - responsible for initial infection and transmission, and predominate in asymptomatic persons
HIV tropisms for Macrophage and T cells and the co-receptors
T-Tropic:
Infect primary T-cells and T-cell lines, but not macrophages
- Associated with disease progression, arise at AIDS stage of infection
CCR5
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
CXCR4
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
Basis for strain tropisms:
envelope sequence of different HIV types have preference for different co-receptors
Which HIV tropism is the source of person to person transmission?
M-tropic virus
How do some individuals remain sero-negative despite high-risk behavior and presumably repeated viral exposure?
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
CCR5 antagonist drug
Selzentry (maraviroc)
The fusion process
- 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
T20 “C” peptides
Antiviral (Fuzeon) - can bind the N-term helical region and block the snapback
HIV pathogenesis and Immunity
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)
Disease mechanisms of HIV
Direct killing of CD4 T cells by HIV (3 methods):
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
Disease mechanisms of HIV
Indirect effects on infected CD4 cells:
- 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)
Disease mechanisms of HIV
Impairment of immune system function:
- CD4 T-cell function altered and loss of CD4 T cell help leads to severely compromised immune system
- Infected macrophages are dysfunctional
Acute infection and seroconversion (2 steps)
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
Asymptomatic Phase
- Strong immune response, but gradual decline in CD4 counts
- Progression measured by CD4 counts, CD4:CD8 ratio, “viral load” by measuring RNA by PCR
Symptomatic AIDS phase:
Late in infection CD4 cells depleted Immune system begins to fail Viremia increases Patients susceptible to many opportunistic infections
Laboratory Diagnosis of HIV
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)
Presently available Drugs (5 types)
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
HAART
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
Infected memory T cells: can detect virus from those on HAART for > ____ years
5
Enthusiasm for HAART and ‘functional cure’ tempered because:
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
New attachment inhibitors
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
