AIDS Flashcards
what kind of virus is HIV (class)?
retrovirus
also a human T-cell lymphotropic retrovirus
is HIV enveloped?
yes
contains glycoproteins including gp120 and gp41
does HIV virion contain reverse transcriptase?
yes
what is the genome structure of HIV?
two identical molecules of single-stranded positive RNA (diploid)
what type of cells does HIV infect? what happens to these cells once they’re infected?
infects helper (CD4) T lymphocytes
kills these cells
can also infect other cells that have CD4 proteins on their surfaces, such as macrophages and monocytes
what is the consequence of HIV on the immune system?
since it kills helper T cells, infected individuals lose cell-mediated immunity, resulting in a high probability of developing opportunistic infections and certain cancers including Kaposi’s sarcoma and lymphoma
what is the incubation period of HIV?
very long - it’s a slow virus
which genes does the HIV genome encode for?
the three typical retrovirus genes: gag, pol, env
six regulatory genes:
- tat and rev required for replication
- nef, vif, vpr, and vpu = accessory genes - not required for replication
what does the gag gene encode? what is its importance medically?
encodes internal core proteins - including p24
antigen in initial serological test that determines whether patient has antibody to HIV
what does the pol gene encode?
virion reverse transcriptase
integrase
protease
what does the virion reverse transcriptase in HIV do?
synthesizes DNA by using the genome RNA as a template
also has ribonuclease H activity - degrades RNA when it’s in the form of an RNA-DNA hybrid molecule = essential step in synthesis of double-stranded proviral DNA
what does HIV integrase protein do?
integrates viral DNA into the cellular DNA
what does the viral protease in HIV do?
cleaves various viral precursors
what does the env gene in HIV encode? what does this protein do?
gp160 protein
precursor glycoprotein that is cleaved to form the two surface glycoproteins = gp120 and gp41
what are clades and how are they determined? what factors vary between clades?
subclasses of HIV
classified based on differences in the base sequence of the gp120 gene
seem to vary geographically and also by transmission
what HIV clade is most common in north america? what type of cells does it preferentially infect? how is it most easily spread?
- b most common in north america
- preferentially infects mononuclear cells
- readily passed during anal sex
where do HIV clade E virions preferentially infect? how are they most easily transmitted?
infect female genital tract cells
transmitted readily during vaginal sex
what enzymes are located in the HIV virion nucleocapsid? (3)
reverse transcriptase
integrase
protease
how does HIV affect cell-mediated immunity? what viral proteins are involved?
tat and nef proteins repress synthesis of MHC class I proteins - reduces ability of cytotoxic T cells to kill HIV infected cells
what does the protein encoded by the HIV rev gene do?
controls passage of late mRNA from nucleus into cytoplasm
what does the HIV protein vif do?
enhances infectivity by inhibiting action of APOBEC3G = enzymet hat causes hypermutaton in retroviral DNA
what is APOBEC3G and what viral gene affects it?
enzyme that causes hypermutation in retroviral DNA
deaminates cytosines in both mRNA and retroviral DNA - inactivates these molecules
vif inhibitis
what are the important antigens of HIV? (list)
gp120 and gp41
p24
what are gp120 and gp41?
type-specific envelope glycoproteins in HIV
what does gp120 do?
protrudes from surface of HIV
interacts with CD4 receptor and a chemokine receptor
what does gp41 do?
mediates fusion of viral envelope with the cell membrane at the time of infection
how many antigenic variants are there? what regions of the protein display the most antigenic variation?
gene encoding it mutates rapidly => many antigenic variants
most immunogenic region is V3 loop - varies antigenically to significant degree
why is production of an HIV vaccine difficult?
antibody against gp120 can neutralize the virus, but the gene for gp120 mutates so rapidly that antigenic variants are generated too quickly
where is p24 protein located? how many antigenic variations are there? how is it used clinically?
located in core
doesn’t seem to vary, so no variations
antibodies to it don’t neutralize DNA, but it’s used as a serological marker for HIV infection
what is the range of species HIV can infect?
mostly humans but can infect certain primates in the laboratory
what is the difference between HIV-1 and HIV-2? where is HIV-2 found?
most common is HIV-1, and that’s what we mean usually when talking about HIV
HIV-2 found in AIDS patients in west africa
proteins only 40% identical to those in HIV-1
still localized primarily to west africa
much less transmissible than HIV-1
what is simian immunodeficiency virus (SIV)? what species is it in? how does it compare with HIV?
isolated from monkeys with aids-like illness but antibodies in some african women cross-react with it
proteins compare with HIV-2 more than those of original HIV isolates
describe the steps of the HIV entry into cells
1: virion gp120 envelope protein binds CD4 protein on host cell
2: gp120 protein interacts with chemokine receptor on host cell
3: virion gp41 mediates fusion of viral envelope with cell membrane
4: viral core with nucleocapsid, RNA genome, and reverse transcriptase enters cytoplasm
how are chemokine receptors involved in HIV infection? which receptors are important for which strains of HIV?
receptors required for entry into CD4 positive cells - gp 120 protein interacts with them
CXCR4 and CCR5 proteins are examples
T-cell tropic strains bind CXCR4
macrophage-tropic strains bind to CCR5
how can genetic mutation in CCR5 affect development of AIDS? (what is CCR5?) what is a common mutation?
chemokine receptor that macrophage-tropic strains preferentially bind to
mutations in CCR5 protect from infection - homozygotes completely protected, heterozygotes have slow disease progression
most common mutation is delta-32 mutation - 32 base pairs deleted from gene
describe the steps in HIV replication.
- in cytoplasm, virion reverse transcriptase transcribes genome RNA into double-stranded DNA
2: DNA migrates to nucleus
3: DNA integrated into host cell DNA
4: viral mRNA transcribed from proviral DNA by host cell RNA polymerase
5: mRNA translated into several large polyproteins
6: immature virion with precursory polyproteins forms in cytoplasm
7: cleavage by viral protease occurs as immature virion buds from cell membrane
8: cleavage process results in mature, infectious virion
where in the host cell genome does HIV DNA integrate? what mediates this integration?
can integrate at multiple sites and multiple copies can integrate
mediated by integrase (virus-encoded endonuclease)
what proteins does the gag polyprotein make when cleaved?
main core protein - p24
matrix protein - p17
several smaller proteins
what proteins does the pol polyprotein make when cleaved?
reverse transcriptase
integrase
protease
which proteins cleave the HIV polyproteins?
gag and pol polyproteins cleaved by viral protease, env polyprotein cleaved by cellular protease
on which cellular proteins is HIV replication dependent?
CD4 and the chemokine receptors
actin and tubulin to move viral DNA to nucleus
cyclin T1 involved in complex that transcribes viral mRNA
some proteins also involved in budding process
how is HIV transmitted?
primarily by sexual contact and by transfer of infected blood
perinatal transmission can also occur - both across placenta and at birth and through breast milk
50% neonatal infections at time of delivery and rest split equally between transplacental and breast milk transmission
small amount of virus have been found in other fluids, inc. saliva and tears, but no evidence that this plays role in infection
what is transmitted when HIV is transmitted?
transfer of either HIV-infected cells or free HIV
generally follows pattern of transmission of hep B but HIV much less effectively transmitted
what factors increase risk of acquiring HIV?
having an STD, especially those with ulcerative lesions (syphilis, chancroid, herpes genitalis)
uncircumcised males have higher risk
how is HIV transmission through blood transfusion prevented?
blood banks test for p24 antigen
there is still a window period between when infection occurs and when can be detected in blood
what are HIV infection rates in the US?
at end of 2008, 1.1 million people infected
aprox. 50,000 people infected each year
600,000 have died of AIDS since 1981
how has the HIV transmission rate in the US changed since the epidemic began and why?
has gone down due to increased prevention efforts and improved treatments, which reduce the number of people with high titers of HIV virions
what are HIV infection rates worldwide? where are the rates highest?
33 million people infected
2/3 of these are in sub-saharan africa
africa, asia and latin america have highest rates of new infections
AIDS is 4th leading cause of death worldwide
what is the predominant mode of HIV infection in Africa?
heterosexual transmission
how high must the dose of HIV be to be infectious?
thought that it must be very high because there have been very few cases of infection due to needle-stick injuries and continuing exposure in health care personnel
where does HIV initially infect? how does it spread?
in the genital tract in the dendritic cells that line mucosa (Langerhans cells)
CD4 positive helper T cells then become infected
HIV first found in blood 4-11 days after infection
what subsets of cells does HIV target? what are these cells important in? what is the consequence of the infection of these cells?
targets subset of CD4-positive cells = Th17 cells
important mediator in mucosal immunity, esp in GI tract
produce IL-17, which attracts neutrophils to site of bacterial infection
killing them predisposes infected individuals to infections by bacteria in normal flora of colon
also infects brain monocytes and macrophages => multinucleated giant cells and significant central nervous system symptoms
how does HIV cause giant multi-nucleated cells? what is the consequence?
mediated by gp41
cells in syncytia that’s formed ultimately die
what kills HIV infected cells?
those that join a syncytia die can also be attacked by cytotoxic CD8 lymphocytes - depending on the ability of the viral tat and nef proteins to reduce MHC class I proteins HIV could also act as superantigen = indiscriminately activates many helper T cells
where is the main site of ongoing HIV infection?
lymphoid tissue - infection doesn’t have to have cytopathic effect and so virus can actively reproduce
how long is a person infected with HIV infected for? why?
infected for life
likely because of integration of viral DNA into host genome
what are elite controllers? what factors affect whether someone is one?
rare group of HIV-infected people who have no detectable HIV in their blood
CD4 counts are normal
mechanism unclear but doesn’t depend on gender, race or mode of transmission
evidence that certain HLA alleles are protective adn that inhibitor of cyclin-dependent kinase (p21) plays important role
why might some HIV infected individuals be long-term “nonprogressors”?
some viruses have mutation in Nef gene - allows cytotoxic T cells to retain their activity
some may produce large amounts of alpha-defensins = family of positively charged peptides with antibacterial and antiviral activity - interfere with HIV binding to CXCR4 receptor
what does HIV do to B cells?
polyclonal activation of B cells
results in high Ig levels
autoimmune diseases occur such as thrombocytopenia
what is the main immune response to HIV infection?
cytotoxic CD8-positive lymphocytes
respond to initial infection, can control it for many years even if mutants in HIV occur (since they can proliferate and control the new mutuant strain)
why does HIV develop into AIDS?
failure of cytotoxic T cells
lose their effectiveness because so many CD4 helper T cells have died => supply of lymphokines such as IL-2 required to activate cytotoxic T cells is no longer sufficient
mutations in what human genes can accelerate progression of AIDS?
mutations in any of the genes encoding class I MHC because these proteins can’t present HIV epitopes => cytotoxic T cells can’t kill infected cells
are there antibodies to HIV produced? are these effective?
yes, to various HIV proteins, such as p24, gp120, and gp41
neutralize virus poorly and so have little effect
what are the mechanisms HIV uses to evade the immune system?
1: integration of viral DNA into host cell DNA => persistent infection
2: high rate of mutation on the env gene
3: production of tat and nef proteins that downregulate class I MHC proteins
4: can infect and kill CD4 helper T cells
what are the stages of the clinical picture of HIV infection? (summary card)
1: early, acute stage: usually begins 2-4 weeks after infection - get mono-like picture including fever, lethargy, sore throat, generalized lymphadenopathy, maculopapular rash on trunk, arms, legs, leukopenia, but CD4 cell count usually normal
high-level viremia typically occurs - infection readily transmissible
resolves spontaneously in about 2 weeks, usually accompanied by lower level of viremia and rise in CD8+ T cells directed against HIV
2: middle, latent stage: long latent period, can be years - typically 7-11 years - patient asymptomatic during this period but lots of HIV still being produced in lymph node but is sequestered in the nodes (virus not in a latent state)
AIDS-related complex can occur during this period - persistent fevers, fatigue, weight loss, lymphadenopathy - often progresses to AIDS
3: late stage, AIDS: decline in number of CD4 cells to below 400/uL and increase in frequency and severity of opportunistic infections, patient immunocomprimised, severe neurological problems
when do antibodies to HIV typically appear?
10 to 14 days after infection
how long after infection will most patients have seroconverted?
3-4 weeks (seroconverted means development of specific detectable antibodies in blood) - can get false negative before that time
how should you test for HIV?
look for antibodies
if test is negative but infection is still suspected, use PCR-based assay for viral RNA cause can get false negatives with serological tests
what percentage of HIV infected patients are symptomatic following initial infection?
87% who become seropositive during the acute phase are symptomatic
what is the viral set point? when is it established? how does this affect progression of the disease?
after initial viremia represents amount of virus produced remains constant for years varies from person to person higher the set point, more likely the person is to progress to AIDS
how would you determine a patient’s set point?
assay for viral RNA in patient’s plasma - detects RNA in free virions in plasma
how is set point used clinically?
amount of RNA in patient’s blood used to determine whether drug regimine is effective, as an effective one would reduce viral load
predicts prognosis
how can the number of CD4 positive T cells that a patient has be used clinically?
used to determine whether person needs chemoprophylaxis against opportunistic organisms and determine whether patient needs anti-HIV therapy and response to this therapy
what’s the lower limit of CD4 count considered normal?
500 cells/mm3
people with this level or higher are usually assymptomatic
what are the common opportunistic infections associated with AIDS?
pneumocystis pneumonia and kaposi’s sarcoma but also viral infections such as herpes simplex, herpes zoster, and CMV and progressive multifocal leukoencephalopathy, funcal infections, cryptococcal meningitis, desseminated hestoplasmosis, protozoal infections, disseminated bacterial infections
what is the clinical presentation of a patient during the early/acute stage of HIV infection?
get mono-like picture including fever, lethargy, sore throat, generalized lymphadenopathy, maculopapular rash on trunk, arms, legs, leukopenia
but CD4 cell count usually normal
when does the early/acute stage of HIV infection usually occur?
usually begins 2-4 weeks after infection
is the HIV infection transmissible during the early stages of infection?
yes, high-level viremia typically occurs - infection readily transmissible
how long does the early/acute stage of HIV infection usually last? what are the signs that it’s ended?
resolves spontaneously in about 2 weeks, usually accompanied by lower level of viremia and rise in CD8+ T cells directed against HIV
how long does the latent/middle stage of HIV infection usually last?
long latent period, can be years - typically 7-11 years
what are the clinical symptoms of HIV during the middle/latent phase of HIV infection? is the virus still being produced?
patient asymptomatic during this period but lots of HIV still being produced in lymph node but is sequestered in the nodes (virus not actually in a latent state even though this is called the latent phase)
when does AIDS-related complex (ARC) occur and what is the clinical presentation?
can occur during latent/middle stage of infection - persistent fevers - fatigue - weight loss - lymphadenopathy often progresses to AIDS
what is the clinical presentation of a patient in the late stage of HIV infection?
AIDS: decline in number of CD4 cells to below 400/uL and increase in frequency and severity of opportunistic infections, patient immunocomprimised, severe neurological problems
how would you diagnose HIV? (laboratory diagnosis)
detection of antibodies to p24 protein of HIV in patient’s serum - using ELISA test
because this can create false positives, definitive test made by western blot - viral proteins would be detected
OraQuick = rapid screening for antibody - uses blood sample from finger prick - results in 20 minutes but positive results require western blot confirmation
can HIV be grown in culture?
yes, but it’s not readily available
when can false-negative tests for HIV occur? why do they occur?
occur during first month after infection - antibody tests may be negative
due to insufficient antibody being made early in infection to be detected by ELISA
can the diagnose of HIV be made by serological tests?
yes, but not in everyone and not for a while. most infected patients seroconvert in 10-14 days, and almost all have converted by 4 weeks, but not all, so you don’t know if the test is negative because the patient doesn’t have HIV or if it’s because he/she hasn’t seroconverted yet
what is the best test for HIV during the acute phase?
plasma HIV RNA assay (used to determine viral load)
useful because viremia is typically high at this stage
can also use p24 antigen test or viral culture
what are the goals of HIV treatment?
restore immunologic function => reduction of incidence of both opportunistic infections and certain malignancies and viral load => reduction of risk of transmission to others
why does treatment of HIV require multiple drugs?
high rate of mutation of HIV => drug resistance
what factors must be considered when deciding on drugs for HIV treatment?
whether its an initial infection or an established infection
number of CD4 cells
viral load
resistance pattern of the virus
whether patient is pregnant
whether patient is coinfected with HBV or HCV
what are the two drug regimens initially used to treat HIV?
1: two nucleoside reverse transcriptase inhibitors (NRTI) and a protease inhibitor
2: two NRTIs plus a nonnucleoside reverse transcriptase inhibitor (NNRTI)
both combinations known as HAART
what is HAART? what does it stand for?
combinations of drugs used to treat HIV
stands for “highly active antiretroviral therapy”
how effective is HAART?
effective in improving quality of life, reducing viral load, prolonging life
but does not cure chronic HIV infection
discontinuation almost always results in viremia and fall in CD4 count so must continue use
what are some nucleoside/nucleotide reverse transcriptase inhibitors (NRTI)? (list of drug names)
abacavir didanosine emtricitabine lamivudine stavudine zidovudine tenofovir (nucleotide - all others nucleoside)
how do NRTIs work? what is a limitation of these drugs?
don’t have 3’ hydroxyl group on ribose ring - chain terminating
inhibit HIV replication by interfering with proviral DNA synthesis by reverse transcriptase
cannot cure infected cell of already integrated copy of DNA
what are the main problems with NRTI?
1: emergence of resistance
2: adverse effects - eg suppression of bone marrow leading to anemia and neutropenia
what are some NNRTIs? (list of drug names)
delavirdine efavirenz etravirine nevirapine rilpivirine
efavirenz (sustiva) and nevirapine (viramune) most commonly used
how do NNRTIs work?
not base analogues (nothing else said…)
how are vertical transmissions of HIV prevented?
the NNRTI nevirapine and the NRTI zidovudine (ZDV)
what are some side effects of NNRTIs?
skin rashes and stevens-johnson syndrome
what are some protease inhibitors (list of drugs)?
amprenavir atazanavir darunavir fosamprenavir indinavir nelfinavir ritonavir saquinavir tipranavir combination of lopinavir and ritonavir
how do protease inhibitors work in the treatment of HIV?
in combination with nucleoside analogs, inhibit viral replication and increase CD4 cell counts
how would you treat a patient with HIV that is resistant to protease inhibitors?
resistance to one usually conveys resistance to all
but combining two (ritonavir and lopinavir) effective against both mutant and nonmutant strains
darunavir effective against many strains that are resistant to others
what is a side effect of protease inhibitors?
abnormal fat deposition in specific areas including back of neck = buffalo hump
abnormal fat deposits = type of lipodystrophy
what should be done for a neonate born to a mother infected with HIV?
give zidovudine (mother should have been treated during pregnancy too)
what are some entry inhibitors used in HIV treatment? (list of drugs)
enfuvirtide and maraviroc
what are fusion inhibitors? what is an example?
prevent fusion of viral envelope with the cell membrane
first was enfuvirtide (fuzeon)
how does enfuvirtide work?
binds to gp41 on HIV viral envelope
blocks entry of HIV into cell
what are the disadvantages of enfuvirtide treatment?
must be administered by injection
expensive
how does maraviroc (selzentry) work?
blocks binding of gp120 envelope protein to CCR-5 - prevents virion entry into cell
what type of drug is raltegravir (isentress) and how does it work? which patients is it used in?
integrase inhibitor
inhibits HIV encoded integrase
used in patients for whom other antiretroviral drugs haven’t worked in reducing the levels of HIV
what is immune reconstitution syndrome? in what patient population does it occur? what would be the clinical presentation?
in patients with HIV who are coinfected with other microbes (such as Hep B, C, and some bacterias) and are also treated with HAART
exacerbation of clinical symptoms occurs because antiretroviral drugs enhance inflammatory response
how can acquisition of HIV be prevented?
no vaccine
avoid exposure - condoms, don’t share needles, discard donated blood contaminated with HIV
give anti-retroviral therapy to infected mothers and neonates, hiv infected mothers shouldn’t breastfeed, use c-section
circucision
what types of post and pre-exposure prophylaxis can be used?
in post exposure (for example, after a needle stick) - give two drugs
can also give truvada for pre-exposure to men who have sex with men
does HIV contain a viral oncogene?
no
list of HIV virus life cycle steps:
attachment co-receptor binding fusion reverse transcription integration transcription translation cleavage of precursor proteins nucleocapsid assembly budding virion maturation
what determines HIV tropism?
the viral gp120 protein in the virion envelope
describe the steps in the genome replication process of HIV (be very specific, sort of summary card)
1: HIV-associated reverse transcriptase engages a host tRNA molecule as a primer and the HIV RNA genome as the template - uses these to make single-stranded DNA copy of the RNA genome
RNaseH activity of reverse transcriptase removes the RNA strand and reverse transcriptase uses ssDNA template to make linear double-stranded DNA copy of HIV genome
this DNA copy then goes to the host cell nucleus and is integrated into the host cell chromosome - mediated by HIV integrase enzyme
HIV genome transcribed by host cell RNA polymerase to make HIV mRNA
this is translated into proteins
how does activation of the immune system affect transcription of HIV genetic material?
activated CD4+ cells will activate the expression of HIV mRNA from the proviral integrated HIV genome => production of new HIV virions
