The Pathophysiology of Acquired Immune Deficiency
Syndrome by Human Immunodeficiency Virus
Brandon ToyMississippi CollegeGraduate Seminar
Virus Classification• Group VI Virus• All use virally encoded RNA-Dependent DNA Polymerase
• Reverse Transcriptase• Family: Retroviridae
• Enveloped virus• Contains single-stranded RNA • Stores nucleic acid in mRNA genome• Positive Sense RNA
• Genus: Lentivirus• Longer incubation period
• HIV-1 & HIV-2
Attachment
• Where does it attach?• Virus attaches to the immune system cell, the dendritic cell found
in the mucosal areas• Transported by the lymphatic system to lymph nodes where other
cells become infected• Replication occurs primarily in activated CD4+ T Lymphocytes and to
lesser extent in macrophages and dendritic cells
Attachment –Receptor Binding
• Receptor Bindings• The two major glycoproteins involved with HIV receptors are gp120
and gp41• gp120 Binds to CD4 receptor
• Conformational change occurs and binding occurs to CCR5 or CXCR4• Cytokine Receptor CCR5
• T cells and Monocytes• Most important co-receptor
• CXC Chemokine Receptor• Integral Membrane Protein• Bind and respond to chemokines• G Protein Receptor, 7-TMD• CXCR4 known as fusin• HIV predominantly T-Cells
Attachment
• Chemokine binding causes gp120 to move and gp41 to bind• gp41• Hydrophobic HR1 binds to membrane and HR2
“zips” into the groves of H1, pulling the virus closer, forming a large pore for the virus to enter• Extremely stable bond
Attachment• Viral nuclear capsid enters the host cell• 3 enzymes, 2 RNA strands are released
• Protease – Cleaves protein strands into their smaller, active form• Integrase – carries viral DNA into the nucleus and integrates dsDNA
into host DNA• Reverse Transcriptase – transcribes viral RNA into viral dsDNA
Replication/Transcription• Once the virus inside, the cell replicates virus• Reverse Transcription
• Causes the most mutations leading to resistance• Primary genes are p51 & p66• Finger, palm, thumb region protein structure
• Must be in closed form to transcribe
• Two Primary Active Sites• Polymerase active site transcribes RNA/DNA double helix from viral RNA• Ribonuclease H active site breaks down the RNA and DNA• Polymerase completes the double stranded replication forming double
stranded DNA
Integration• Preintegration Complex• Viral dsDNA imported into cell nucleus through the nuclear pore• Contains both viral and cellular proteins• Integrase and other cofactors used• Integrase cleaves dinucleotides from 3’ end and forms sticky ends
• Integrase carries into the host nucleus and sticks into host DNA• Host polymerase fils in complementary base pairs• 3’ OH group of 5’ phosphorous• Magnesium ions hydrolyze 3’ end exposing new OH groups• 5 bp apart, nicks DNA• Host ligase and other cellular enzymes repair the single gaps
• Two unpaired nucleotides at the 5’ ends of viral DNA removed
• Integrated DNA known as proviral DNA
T-Cell Ribosomal Transcription• Activation of the infected cell will cause transcription of pro-
viral DNA into viral mRNA• Ribosomes at the rough endoplasmic reticulum synthesize HIV
envelope proteins first which are directed to the host membrane
• The same ribosomes build the poly protein chains that contain a total of two RNA viral strands per virus
Integration
• Protease cleaves larger proteins into smaller core proteins• Protease cleaves the poly-protein strand and the interior
structures • Env• Pol• Gag
Genetics• Env genes encode the precursor 160 140 kDa glycoproteins
cleaved into gp160 and gp 140 respectively. • Nucleocapsid proteins formed• P24 capsid protein• P17 matrix protein• P7 nuclear protein
• Pol genes code for the viral enzymes (polymerase reverse transcriptase, integrase, protease)• Precursor protein cleaved into
• P31• P51• p66
• Gag genes code for the cone shaped viral capsid
Genetics• Other genes include• Tat – Influences function of genes distance away. Controls
transactivation of HIV protein• Rev – Regulates expression of virus protein• Vif – virus infectivity factor gene; Required for infectivity as cell-
free virus• Nef – Regulator that attenuates HIV replication• Vpr – R gene. Has an undetermined function
Genetic Classification of HIV• HIV-1• Vpu gene
• U gene required for efficient viral replication and release• Only found in HIV-1
• HIV-2• VpX gene has an undetermined function• Only found in HIV and SIV (Simian Immunodeficiency Virus)
Formation • Two viral RNA strands and the replication enzymes are
surrounded by the smaller core proteins. Final assembling around them forming the capsid• Once complete, the virus leaves• Obtains part of the host membrane for cell recognition
• Leaves as an immature virus • Matures after leaving cell
Initial Body Response• Acute Illness• Viral load increases and CD4 cells drop
• Can test for p24 antigen or HIV-RNA• Host develops HIV antibodies• A newborn less than 18 months old will have mother’s antibodies so any
test would be inaccurate• Seroconvert
Initial Immune Response• Returns to the viral set point• Several million virus particles per mL of blood will cause a drop in
CD4 cell count• CD8 killer T cells destroy the virus and HIV infected cells • May also produced b-chemokine that competitively inhibit HIV
attachment and down-regulate the HIV co-receptor and chemokine receptor proteins (Kinter et al. 1998; Saha et. al, 1998)
Clinical Latency• Latent period• Chronic phases can occur.
• Basically an asymptomatic HIV infection• Viral replication continues to occur during latent period, continual
replication still occurs• Period can last for several years
Constitutive Period• Towards the end of the latent period, viral load will increase. • Massive increase in viral load will cause a complete
elimination of CD4 cells • Will lose the ability to regenerate new T-cells
• Resources of the T-cells have been overtaken by the synthesis of HIV• Cell cannot develop its own required proteins
Acquired Immunodeficiency Syndrome
• CD4 T cell count below 200/uL• Average, healthy CD4 T-Cell Count between 500-1500/uL
• High Viral Load• Opportunistic Infections Due to Attenuated Immune System• Tuberculosis• AIDS Dementia Complex• AIDS Wasting Syndrome• Hepatitis A & B
• Pneumocystis Pneumonia• Cytomegalovirus
• Vision Loss• Cancer risks less than 500/uL
• Kaposils Sarcoma• Non-Hodgkins Lymphoma
Clinical Tests• ELISA & Simple/Rapid Tests
• Detects anti-HIV antibodies• Gp120 antibodies present in all infected patients
• Western Blot Assay• Nucleic Acid and Antigen Screen Tests• Viral Load Test
• Low viral load below 10,000 copies• High viral load more than 10,000 copies
Pharmacology• Fusion inhibitors• Nucleoside Reverse Transcriptase Inhibitors• Non-Nucleoside Reverse Transcriptase Inhibitors• Integrase Inhibitors• Protease Inhibitors
Fusion Inhibitors• Three gp120 subunits mediate receptor and co-receptor
attachment• Three gp41 subunits response for membrane fusion• Receptor antagonists prevent gp120 attachment to the
receptor and co-receptors• gp41 conformation can be inhibited• Enfurvitide
• Only approved fusion inhibitor• Peptide based• Competitively binds to gp41 and blocks the formation of the post-
fusion structure
• Several other gp41 & gp120 inhibitors are being developed
• Abbreviated NRTIs• Analogs of deoxyribonucleosides
• Lack a 3’ OH group• NRTIs must be metabolically converted by host cell kinases to
corresponding triphosphate form• NRTI-TPs inhibit reverse transcription by acting as chain-terminator• Clinically Approved NRTIs• ddI• TDF• ddC• 3TC• FTC• AZT• d4T
Nucleoside Reverse Transcriptase Inhibitors
• Abbreviated NNRTIs• Treat HIV-1 Infection• Do not require intracellular metabolism for activity• Hydrophobic Compounds• Interact with HIV-1 RT• Binds to site on p66 subunit of the HIV-1 RT p66/p51 heterodimer
• Also known as the binding pocket
Non-Nucleoside Reverse Trasncriptase Inhibitors
• Raltegravir• “β diketo acid (DKA) moiety with potent and selective inhibi- tory
activity against the strand transfer step of integration”• Known as strand-transfer inhibitors
• Can uncouple the two integrase reactions• Can block strand transfer without affecting 3’ processing
• Thought that strand-transfer inhibitors bind at the interface of the integrase-metal cofactor• Chelating the divalent metal, interfering with the binding of the
chromosomal target DNA
Integrase Inhibitors
• Peptide-like chemicals• Competitively inhibit the action of the enzyme aspartyl protease• Bind to the site where protein cutting occurs
• Proteolytic cleavage of the polypeptide precursors inhibited• Cannot form into mature enzymes and structural proteins• Gag & Pol proteins
Protease Inhibitors
Research/Development• RT leads to Mutation• Vaccine• Neutralizing Antibodies (Nabs)
• Preventing initial receptor engagement events for binding• Binding after attachment and inhibiting fusion process
• Nucleotide Reverse Transcriptase Inhibitor• Tenofovir (Tenofovir Disoproxil Fumarate)
• First nucleotide reverse transcriptase inhibitor• Nucleoside RTI must undergo three intracellular phosphorylation
steps• Nucleotide Reverse Transcriptase Inhibitors only require two steps• Could produce more rapid and complete conversion of drug to
pharmacologically active form
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