HIV/AIDS Flashcards
describe the process of reverse transcription
- A tRNA comes in acting as a primer. Attaches to the 5’ end and transcribes the LTR region.
- Then there is a transfer toward the 3’ end. Transcription occurs in a 5’ to 3’ direction for the new strand
- RNAse H degrades all of the original 3’ LTR region except for a piece at the u3. That acts as a primer at the 3’ end and transcribes the 3’LTR.
- Then another transfer to the 3’ direction of the newly synthesized strand. And then 5’ to 3’ transcription
Tat protein
transactivator, powerful stimulator of transcription from the HIV promotor
Tat - activator of HIV transcription.
Binds not to the HIV promoter but to tar (tat response element) of the transcript, which forms a stem-loop structure. tar-bound Tat binds cellular proteins which stimulate transcription. One is a protein kinase which phosphorylates the C-terminal domain of RNA polymerase, facilitating promoter clearance and rapid transcription.
Rev
Inhibits splicing of viral transcripts
Rev prevents splicing of HIV mRNA by binding to the Rev response element (RRE) in HIV transcripts and to exportins, proteins which transport molecules out of the nucleus. Early after infection, Rev levels are low and most viral transcripts are fully spliced, so that the small regulatory proteins are translated. As Rev accumulates, splicing is blocked, and unspliced and singly-spliced transcripts are able to leave the nucleus. These are translated into the gag, gag/pol, and env polyproteins.
nef
named for negative effect but actually stimulates HIV replication and may be responsible for damage to immune system.
People infected with defective nef are slow progressors and those with nef-deleted do not develop disease.
Nef down-regulates expression of CD4, MHC antigens, and cytokines, including IL-2 and IFN-γ. Nef induces macrophages to secrete soluble factors which up-regulate proteins on resting T cells, causing them to become susceptible to HIV infection.
Vif
vif Virion infectivity factor. Present in virion, required for infectivity. Causes degradation of a DNA cytosine deaminase APOBEC which produces mutagenic damage to reverse transcripts.*
Vif and DNA cytosine deaminase. A cellular defense against retroviruses is APOBEC, a deaminase that converts cytosine to uracil in viral DNA, creating mutations. The deaminase is incorporated into new virions and acts on the DNA strands produced by reverse transcription. Vif is incorporated into HIV virions and excludes the deaminase. [APOBEC = Apolipoprotein B-Editing Cytosine deaminase. One member of this protein family produces an intestine-specific form of Apo-B shorter than the liver form, by converting a cytosine in mRNA to uracil and creating a stop codon. Another causes hypermutation of Ig-gene variable regions in germinal centers.]
Vpu
vpu
Stimulates virus release from the cell surface by delivering CD4 to the proteosome, and reducing cell surface tetherin, a protein that binds virions to the cell surface and promotes their endocytosis.
Vpr
vpr Present in virion, may aid in transport of the viral core to nucleus; blocks cell division cycle in G2, when HIV transcription is more efficient than in G1. Degradation targets are presently unknown.
Acute HIV infection
Acute Infection typically follows exposure to HIV by 4-8 weeks. Symptoms are non-specific and may include (from most to least common) fever, lymphadenopathy, rash, pharyngitis, myalgia, diarrhea, headache, nausea, hepatosplenomegaly, peripheral- or CNS symptoms. Because of the nonspecific nature of the illness, the fraction of cases in which it occurs has been difficult to determine retrospectively, but it seems to occur in at least half of those infected. When the time of infection is known and the patient can be prospectively monitored, as with occupationally-exposed health care workers, nearly all have developed this syndrome.
In acute disease there is rapid viral replication and high-level viremia. Infectious virus and CA ( p24) antigen can be easily found in blood. After a few weeks, HIV-specific CTL appear and a strong cell-mediated immune response clears virus from the bloodstream. Anti-HIV antibodies may not appear (“seroconversion”) until 2-6 months after infection.
Gut lymphoid tissue is a major site of early HIV replication. It is almost completely destroyed. This may be important in later development of opportunistic infections, and an argument for starting treatment as early as possible. Massive early loss of T cells creates “holes” in the T-cell repertoire as clones of memory cells are eliminated by the virus. T cells with anti- HIV specificity may be especially susceptible to infection and killing. The HIV SU protein binds integrin α4β7, the receptor which causes memory T cells to ‘home’ to mucosal lymphoid tissue. This in turn up-regulates the membrane protein LFA-1, which promotes cell-cell association of T cells and may promote cell-to-cell spread of HIV.
Asymptomatic period of HIV
The ‘asymptomatic period’. Untreated, the median interval between seroconversion and development of AIDS is 10.5 years. During this period there are few overt symptoms of infection. Patients do not feel ill, and unless diagnosed, do not curtail activities that transmit the virus to others. Plasma viremia is typically low, due to production of anti-HIV antibodies which bind virus particles. Dendritic cells trap these immune complexes and remove virus from the circulation; however this may facilitate infection of dendritic cells and/or transfer of virus to T cells. Plasma virus load determines the speed of progression to symptomatic disease: the higher, the faster.
During the ‘asymptomatic period’ there is slow but steady alteration of the immune system. Early there is B-cell dysregulation, polyclonal B-cell proliferation, and proliferation of CD8+ CTL. Later, although the total concentration of circulating lymphocytes does not change, the ratio of CD4+ to CD8+ cells progressively declines. Within the TH population, TH2 cells increase and TH1 cells decrease. T-cell proliferation and CMI are compromised and may ultimately lead to failure of the anti-HIV CTL response. Response of T-cells to previously-encountered antigens also declines, perhaps because HIV preferentially infects memory T cells. Ultimately the immune system fails, and lymph nodes become totally depopulated of lymphocytes and dendritic cells.
Late Stage HIV disease
Late-Stage Symptomatic HIV Disease. Initial manifestations of immune-system failure include lymphadenopathy, fever, weight loss, persistent diarrhea, and Candida infection (the “AIDS-related” complex or ARC). AIDS, the last stage of HIV disease, is defined by rapid decline of CD4+ lymphocytes, rapid increase in plasma viremia, and development of opportunistic infections.
What marks the transition from asymptomatic period to AIDS?
Initially HIV uses CCR5. Late in infection, virus variants appear which use CXCR4 and are highly-cytopathic for T cells. (Mnemonic: X comes late in the alphabet.) Neurotropic viral variants also appear, and perhaps also viral populations adapted to other organs. [AIDS is frequently associated with dementia, in addition to the psychological and emotional problems it creates.]
How is HIV diagnosed?
Serology
The standard screening test is an ELISA; a positive ELISA is repeated and then confirmed by Western Blot. However anti-HIV antibodies appear only after a lag of up to several months, creating a “window of insensitivity”. PCR tests are very sensitive and can be useful if Western Blots are ambiguous, but their sensitivity renders them prone to false positive reactions, which compromises their use as initial tests. In recent studies some patients who were antibody-negative were positive by PCR. ELISA tests for p24 CA antigen are also available.
What are the uses of viral load and CD4 levels?
Viral load predicts how rapidly a patient’s disease is likely to progress
CD4 level indicates the patient’s current stage of disease.
Sp1
ubiquitous, positive factor for HIV. It interacts with TATA and Tat.
What is contained in the HIV LTR?
U3, R, U5
U3 contains NF-kB.
U3 contains the promoter
The R region contains TAR, the binding region of Tat allowing for positive feedback
What does tat do?
So RNA polymerase starts transcribing but it is slow and non productive. Tat binds to the stem loop of newly formed structure recruiting CycT and Cdk9. The kinase phosphorylates the RNA polymerase II which drives the transcription at really high levels.
