HIV Flashcards
HIV-1
- Responsible for most cases of AIDS in the US
Antigenicically disntinct genotypes of HIV-1
- Groups M, O, N, P
- Group M is responsible for the global AIDS epidemic
Group M HIV genotype
- Based on sequence differences of env and gag genes
- results in antigenicity differences b/w gp120 and capsid proteins
- Subtype B predominates in US
Clades definition
- Varying subtypes of a gene
Essential genes (all retroviruses)
- gag (group antigens) gene
- pol (polymerase) gene
- env (envelope) gene
Gag gene
- Essential gene of retroviruses
- Encodes:
*matrix protein (p17)
*capsid protein (p24)
*nucleocapsid (p9)
Pol gene
- Essential gene of retroviruses
- Encodes:
*protease (p10)
*reverse transcriptase (p50)
*RNase (p15)
*integrase (p31)
Env gene
- Essential gene of retroviruses
- Encodes:
*surface subunit (gp120)
*transmembrane subunit (gp41)
HIV regulatory (accessory) genes
- tat
- rev
- nef
- vif
- vpu
- vpr
- These genes enhance the replication and infectivity to counter some host defense mechanisms
Tat gene
- HIV regulatory gene
- transactivates transcription of HIV genes
Rev gene
- HIV regulatory gene
- regulates RNA splicing and promotes export of mRNA to the cytoplasm
Nef gene
- HIV regulatory gene
- Reduces cell surface expression of CD4 and MHC class I; alters T cell signaling pathways; required to maintain high viral loads; essential for progression to AIDS
Vif gene
- HIV regulatory gene
- Promotes assembly; blocks a cellular antiviral protein that produces hypermutations during cDNA transcription
Vpu gene
- HIV regulatory gene
- Facilitates release of virus by countering a cellular protein that tethers virions to the infected cell; induces degradation of cell surface CD4
Vpr gene
- HIV regulatory gene
- Transports cDNA to the nucleus; induces cell cycle arrest; facilitates replication in myeloid cells
Long Terminal Repeats (LTR)
- On either end of the HIV genome
- Contains promoters and sequences used as binding sites by transcription factors
- Activated cells produce transcription factors that bind LTR and activate transcription of viral genes
- Activated T cells can be productively infected by HIV and generate progeny virions
- Resting T cells are not productively infected by HIV
*incomplete viral cDNA transcripts accumulate or
*proviral DNA is made but does not integrate into the host chromosome (pre-integration latency)
HIV replication outline
- Viral attachment and entry
- Genomic RNA reverse transcribed into cDNA
- cDNA enter the nucleus and integrates into host chromosomes
- Transcription/translation of genes from the proviral DNA template
- Assembly of a premature particle and budding through the plasma membrane
- Maturation into infectious virus
HIV Attachment and entry
- Host cell receptors:
*CD4; primary co-receptor to HIV gp120
*chemokine receptors (CCR-5, CXCR-4); primary co-receptor to HIV gp120
*alpha-4 beta-7 integrin (GALT homing receptor for activated T-cells)
*DC-SIGN (intercellular adhesion molecule on dendritic cells)
- Viral attachment
*gp120
- Entry
*virus-cell fusion at the plasma membrane
*receptor-mediated endocytosis- fusion at the endosomal membrane
*mediated by gp41 (activated only when both co-receptors (CD4, chemokine) are bound by HIV ligand (gp120)
Assembly and Maturation of HIV
- Assembly at the cell surface
*gag and gag-pol polyprotein precursors package viral RNA genome
- Budding through the cell membrane
*acquires lipid envelope w/gp120 and gp41
- Maturation into infectious virus
*HIV protease cleaves gag and gag-pol polyprotein into individual proteins- changes the viral particle into a fully infectious one
*protease inhibitors inhibit this processing so mature proteins are not produced
Classes of antiretrovirals
- Nucleoside reverse transcriptase inhibitors (NRTIs); nucleoside analogues
- Non-nucleoside reverse transcriptase inhibitors (NNRTI)
- Protease inhibitors
- Attachment/Entry inhibitors
*fusion inhibitors
*CCR5 inhibitor
- Integrase inhibitors
Nucleoside reverse transcriptase inhibitors
- Zidovudine (azidothymidine or AZT)
- Requires phosphorylation for activation
*cellular enzymes carry out all 3 phosphorylation steps
- The activated drug binds to and inhibits RT
- Incorporation into the DNA strand results in chain termination
- Resistance is due to mutations in the HIV RT
*encoded by the pol gene
Non-nucleoside reverse transcriptase inhibitors
- Nevirapine
- Does not require phosphorylation for activation
- Bind to RT at a site distinct from the active site, but inhibit its activity
- Inhibit HIV-1 RT, but not HIV-2 RT
- Resistance is due to mutations in RT, distinct from those responsible for resistance to NRTIs
Protease inhibitors
- Saquinavir
- Small molecules that bind in the enzymatic pocket of HIV protease
- Inhibition of the protease inhibits the maturation of infectious viral particles
- Resistance is due to mutations in the protease (pol gene)
*cross resistance b/w protease inhibitors is common
Fusion inhibitors
- Enfuvirtide
- 36 amino acid peptide that binds to gp41
- Blocks the conformational change that occurs after gp120 bind to CD4
- gp41 is unable to mediate fusion b/w the viral envelope and the host cell membrane
*entry is inhibited
- Resistance will occur if HIV mutates the binding site of enfuvirtide
CCR5 inhibitor
- Maraviroc
- Binds CCR5 and alters its conformation
- Inhibits HIV binding to CCR5
- NOTE: this antiretroviral binds to a host protein
- Mechanisms of resistance:
*HIV develops affinity for CXCR4 chemokine receptors
*pre-existing variants that use CXCR4 will be selected for
*HIV develops affinity for the drug-bound CCR5 conformation
Integrase inhibitors
- Raltegravir (Isentress and MK-0518)
- MOA:
*integrase binds to the ends of the DNA provirus and helps to form the “preintegration complex” (PIC)
*it cleaves two nucleotides from each 3’ end of the provirus and then binds to host cell’s DNA
*it catalyzes a covalent joining (strand transfer) of the viral DNA 3’ ends to the cell’s DNA- inhibited by raltegavir
*the gaps are filled, presumably w/a host enzyme
Points about antiretroviral therapy
- Antiretroviral drugs disrupt productive infection, not latent
- While antirretroviral therapy inhibits active viral replication, it provides selective pressure for drug resistance
- Latency/sporadic reactivation from viral reservoirs makes it unlikely that HIV will be completely cleared from an infected individual
*multidrug resistance will not develop during latency
HIV infection of CD4 T cells
- Productive infection in activated CD4 T cells
*loss of CD4 T cells via direct HIV-induced cell lysis or apoptosis or CD8- mediated cell killing (1/2 life about 0.5 days)
- Nonproductive infection of resting CD4 T cells
*pre-integration latency- short term HIV reservoir
*impaired reverse transcription results in accumulation of incomplete viral cDNA transcripts
*cDNA transcripts trigger an inflammatory form of cell suicide (pyroptosis)
- Post-integration latency
*proviral DNA integrates into host chromosomes
*T cells differentiate into memory cells-long term HIV reservoir
Viral Reservoirs
- Short term reservoirs
*pre-integration latency in resting CD4 T cells
*extracellular virus particles trapped on follicular dendritic cells
*productive, persistent infection of monocyte-macrophage lineage cells
*latent infection of monocyte-macrophage lineage cells
- Long term reservoir
*post-integration latency in memory CD4+ T cells
Early stages of HIV infection
- R5 viruses
*binds CCR-5 chemokine receptors
*responsible for majority of viral transmission
*nonsyncytia-inducing strains (NSI)
*macrophage tropic- previous designation
- R5 viruses persist throughout the infection
- Quasispecies appear after viremia peaks
*closely related viral genomes; have some variability
*result of point mutations within the viral genome
Late stages of HIV infection
- X4 viruses
*binds CXCR-4 chemokine receptors
*occurs in about 50% of patients
*syncytia-inducing strains (SI); more cytopathic
*involves acquiring point mutations in the env gene
- Some viruses use both co-receptors (R5X4)
*occurs in later stage patients
Course of HIV infection general overview chart
Clinical Latency
- Immune response limits the productive infection, but doesn’t eliminate it
*productive infection still occurs (lymph nodes)
*steady state viral load until T cells are depleted; viral set pint
- Persistent, low level productive infection
*macrophages; dendritic cells etc.
- Latent infection
*memory T cell
*possibly monocyte lineage cells including hematopoietic stem cells
*can reactivate => productive infection
- Dont have AIDS at clinical latency; you are HIV+; you dont have AIDS until your immune system has been compromised to the point where you are developing opportunistic infections (<200 CD4 T cells in a mircoliter)
NOTE: clinical latency in an HIV+ individual is not the same as viral latency at the cellular level
Acute Infection
- Typically 2-4 wks after exposure
- Mononucleosis-like syndrome
*T cell proliferation to try to clear virus
*viral levels in blood drop, but virus is not completely cleared
*virus persists in lymph nodes
*microglial cells in the CNS
- Typically assoc. w/R5 virus
- ~80% mucosal transmission- HIV infections are due to a single virus (founder virus)
- HIV bind to dendritic cells; taken to lymph nodes
*binding mediated by DC-SIGN
*presented to CD4 T cells by dendritic cells
*HIV internalized by DCs; spread to CD4 T cells
- Loss of CD4 T cells within mucosal-assoc. lymphoid tissue (especially GALT)
*extensive depletion of CD4+ CCR5+ memory T cells in GALT
+low % of CD4 T cells in GALT become productively infected and are directly killed by HIV-mediated cytolysis/apoptosis
+many resting CD4 T cells are killed indirectly by pyroptosis
+~80% of CD4 T cells in GALT are depleted in the 1st 3wks of HIV infection
*CD4 T cell counts return to ~normal levels in blood, but not in GALT
- Virus incompletely cleared by the immune response- viral set point
The importance of dendritic cells to the establishment of HIV infection
- HIV stored in endocytic vesicles following entry via DC-SIGN; released via exocytosis
- Productive infection of DCs; released via budding
- HIV bound to DC-SIGN on surface of DC
- Concentration of virions in the vicinity of CD4+ T cells
- DCs present HIV peptides through MHC class II molecules to the TCR, resulting in T cell activation
*activates HIV gene transcription
Loss of CD4 T cells
- Highest rate of CD4 T cell death occurs during primary infection
- HIV-induced cytolysis correlates w/CD4 levels on cells
*monocytes express less CD4 on their surface than T cells and are less easily killed (persisten, productive infection)
- X4 viruses are more cytopathic than R5 viruses
- Cell-mediated killing
*clears productively infected cells, not latently infected cells
Stages of HIV infection Graph
Immune response to HIV
ARC vs AIDS
- AIDS-related complex (ARC)
*lymphadenopathy, fever, weight loss, malaise
*opportunistic infections, diarrhea, fatigue
- AIDS
*CD4 T cell counts <200/microliters; viral load >75,000 copies/ml
*AIDS-defining illnesses
+HIV wasting syndrome
+Kaposi’s sarcoma
+opportunistic infections
+HIV-assoc. dementia (HAD)
