HIV Replication and Pathogenesis Flashcards
Where did HIV come from?
- HIV is a zoonotic disease
- humans and chimpanzees interact in central Africa
- humans first infected 1902-21 in Cameroon
- three independent introductions of SIV into humans occurred (O, M, N)
How is HIV spread today?
- routes of HIV transmission depend on cultural factors
- not linked to HIV transmission: causal contact, exposure to saliva or urine, blood-sucking insects
- the most common methods of transmission are unprotected sex with an infected partner and sharing needles with infected person
- almost eliminated as risk factor for HIV transmission are: transmission from infected mother to fetus, infection from blood products
Who is the highest risk of infection of HIV in the US?
- men who have sex with men
- injection drug users
- young people ages 13-24
- young black men
How is one exposed/infected with HIV?
- few virions breach the epithelium ad establish infection (small Ro <1)
- brief window to attempt prevention with drugs or vaccines
- virions infect tissue macrophages, dendritic cells
- infected cells produce virions and also migrate to lymph nodes where further infections occur
- virions spread from regional lymph nodes and gut lymphoid tissue (GALT) to other sites
- women are more likely to become infected then men
How is HIV replication?
- HIV is an RNA virus
- retroviridae
- lentivirus = slow to cause disease
- ssRNA (+) strand
- two copies in each virion (diploid)
- early infection: HIV binds to CCR5 coreceptor (macrophage)
- late infection: HIV binds CXCR4 co receptor on T cells and those can fuse together and cause synctium
What is the HIV Lifecycle overview?
1) Attachment and fusion
2) Uncoating
3) Reverse transcription
4) Migration of genome to nucleus
5) Integration into host chromosome
6) mRNA and genome synthesis
7) mRNA export
8) Viral protein synthesis
9) Genomic RNA export
10) Spliced mRNA synthesis
11) Viral membrane protein synthesis
12) Protein maturation
13) Protein accumulation at plasma membrane
14) Virion assembly at plasma membrane
15) Budding
16) virion maturation
How does HIV attach?
- HIV Env is composed of TM (transmembrane, gp41) and SU (surface, gp120) domains
- SU binds CD4 and chemokine receptors which cause conformational change in TM
- fusion peptide on TM inserts in target cell membrane and induces virus entry
- CCR usage shifts between CCR5 (macrophages) and CXCR4 ( T cells)
- presence of CCR determines susceptibility to HIV infection (human genetic variant delta 32)
How does HIV uncoat?
1) Attachment and membrane fusion
2) Uncoating and partial capsid disintegration
3) Reverse Transcription aof ssRNA genomes to DNA
4) migration of circlar genomes to the nucleus
- HIV uses dNTPs in the cytoplasm, which may be limiting in non-dividing cells. RT enzyme is error prone. The newly made circular DNA genomes are bound to HIV protein integrase which has a nuclear localization signal that directs it to the nucleus
How does HIV Reverse transcriptase work?
- tRNA bound to ssRNA is a primer
- templates switch during replication
How does HIV integrate its genome?
- HIV provirus integrates into the host chromosome at sites of active genes
- this event is permanent: the viral genome becomes part of that cell for the life of the cell. This creates a reservoir of latent virus throughout the body. Latency is the primary obstacle to eradicating HIV from a person
- integration marks the end of Phase 1 of HIV replication
- viral and cellular TFs bind to promoter in LTR and recruit RNA Pol II. Several types of mRNA are transcribed, spliced, and exported to the cytoplasm. This marks start of Phase 2 of HIV replication
What HIV latency?
- latently infected resting memory CD4+ T cells are the best characterized latent reservoir for HIV-1
- less than 1 cell per 1,000,000 resting CD4+ cells from patients on HAART harbor latent HIV-1 provirus
- sequence of latent proviruses does not evolve, which suggests no ongoing viral replication
- discontinuation of HAART allows viral relapse from latent reservoir
- patient successfully treated with HAART forl onger than 10 years exhibit no appreciable decrease in the size of the latent reservoir
- the persistence of latently infected memory CD4+ T lymphocytes precludes their elimination by HAART alone for the lifetime of the patient
- other drug insensitive reservoirs including brain, macrophages, and hematopoietic stem cells may also exist
- latency is likely established and maintained by numerous blocks at multiple steps in the HIV-1 replicative pathway, which potentially complicates eradication strategies
How is HIV mRNA transcribed and translated
- some mRNAs are translated on ribosomes that are free in the cytoplasm
- ribosomal frameshifting infrequently causes a read-through which translates the Gag-Pol precursor. This creates the right balance of Gag»_space;> Gag-Pol
- genomic mRNAs are full length transcripts
- short mRNAs encode Env
- Env mRNA is translated at RER. Env is an integral membrane protein that is cleaved by host proteases into the SU + TM domains
How are HIV proteins sorted
- Env matures in the Golgi: glycosylation, cleavage, and trimer formation
- secretory vesicles traffic Env to the plasma membrane
- all other proteins and genomic mRNAs accumulate at the plasma membrane and concentrate in lipid rafts. Some enzymes are included in the virion: RT, IN, PR
How do virions bud and mature?
- virions are formed when they bud from the plasma membrane. These immature virions are not infectious
- HIV protease cleaves Gag into its subunits (MA-CA-NC) within the virion, resulting in the capsid assuming its trapezoidal shape in the mature virion. These extracellular virions are infectious
What is HIV protease?
- Gag polyprotein has 3 domains: MA binds to the cytoplasmic tail of TM +SU, CA is structual and NC binds to the RNA genome
- the viral protease is packaged in the virion and must cleave Gag into its three subunits to form infectious particles
- protease processing results in a change from an indistinct core to the typical trapezoidal core of mature HIV-1
- antiretroviral drug therapy targets the protease and blocks infectivity
