2. Scope
◦ Definitions
◦ Epidemiology
◦ History
◦ Viral Properties & Life Cycle
◦ Immune System Overview
Objectives
◦ Understand morphology of HIV
◦ Understand process HIV infection and progression to AIDS
3. Definitions
HIV – Human Immunodeficiency Virus
Retrovirus /Lentivirus
Positive sense SS RNA virus
AIDS – Acquired Immunodeficiency syndrome
Severe immune suppression leading to OIs & Neoplasms due to
HIV infection
HIV + CD4+ <200/ml or HIV + AIDS defining OI
4. EPI:
Global
(2018)
37.9 million
36.2 million
18.8 million
17.4 million
1.7 million
1.7 million
160 000
1.6 million
–
–
–
–
770 000
670 000
100 000
Source: UNAIDS/WHO estimâtes
People living with HIV New Infections HIV-related deaths
6. Origin of HIV
HIV 1 – SIVcpz, HIV 2 - SIVsmm
Hunter/Bush meat??
Around 1920s in Congo.
Phylogenetic studies instrumental in tracking origin & evolution
HIV 1 - 4 distinct cross over from chimpanzees -Corresponds sub groups = M, N, O, P
7. History/Timeline
1900s – A form of SIV transmitted to humans from Chimps in the Congo (DRC)
1920s ––HIV was circulating in Kinshasa
1959 – 1st Known case (from preserved blood Samples)
1960s – HIV 2 tramsmitted to humans from Sooty Mangabey monkeys in Guinea-Bissau
1966- 1st case in Americas – Haiti
1969- Robert Rayford died of HIV **
1976 – Arvid Noe dies
1977 – Danish Physician Grethe Rask died of AIDS
1978 – Senhor Jose – Died of HIV 2
1980 – A cluster of Pnemocystis Penumonia cases reported to CDC
1982 – 121 people had died of HIV
8. Timeline….ctd
1982 – terry Higgins died of AIDS in UK
1982- Term AIDS coined
1983 – PCR developed
1984 – Retrovirus discovered by Luc Montegnor
& Robert Gallo
1986 – HIV name adopted
9. HIV 1 Diversity
M, N, O, P
M- 11 subtypes
Subtype C – southern Africa, Eastern Africa,
India, Nepal, and parts of China
Recombinants- Gives rise to new subtype
Most diversity – Central Africa – Supports
origin theory
Founder Effect – Eg White SA Homosexuals-
Type B
10. Morphology of HIV
Components
◦ 2 Identical Positive ssRNA strands
◦ Viral Enzymes – Integrase, Protease, RT
◦ Viral Capsid – outer Matrix P17 ,Major Capsid P24, Nuclear Protein (P7)
◦ Lipid Envelope
◦ Envelope Glycoproteins GP 160 GP 120, GP 41
Size : 120nm Diameter
Shape: Icosahedral Symmetry
Genes: gag, pol, env
13. CD4 Cells
Syn: T Helper, T lymphocytes
Central role in activation of cellular & Humoral immune systems
Express CD4 antigen (a GP)on their cell membrane
CD4 is a co-receptor of the T cell receptor (TCR) and assists the latter in communicating
with antigen-presenting cells.
Bind to APC’s MHC II antigens.. Highly specific
Causes activation of tyrosine kinase which leads to activation of molecular processes of
activated T cells
Eg Secretion of IL 2
14.
15. HIV Life Cycle
1. Fusion –GP 120/41 to CD4 & CXCR4 +CCR5
2. Entry – Into Host Cell
3. Reverse transcription- From RNA to DNA - RT
4. Integration- Into Host genome - Integrase
5. Replication – Using host cell resources
6. Assembly – Viral components move towards cell membrane,
7. Budding- pinching off cell membrane to form virion envelope, proteases cleaves
polyproteins forms mature virion
17. Step 1 & 2: Fusion & Entry
1.Attachment of GP 120 to CD4 on T Cells
2.Conformational change in GP120- allows attachment to Co-receptors CCR5/
CXCR4
3.GP 41 unfolds, attaching to cell membrane
4.GP41 then fold back on itself… pulling the viral membrane towards the cell
membrane
5.Then the two membranes fuse
6.Allows injection of Viral capsid into cell cytoplasm
18.
19. Step 3: Reverse Transcription
Viral capsid lysed by cytoplasmic enzymes freeing viral RNA and enzymes
Two catalytic domains –
Ribonuclease H domain moves along , reads and makes a DNA strand from the RNA which is then
destroyed.
Polymerase domain makes another complimentary DNA(cDNA) strand to form a double strand.
In quiescent T cells, HIV cDNA may remain in the cytoplasm in a linear episomal form
20. Step 4: Integration
In actively dividing T cells, Viral cDNA and Integrase migrates to the nucleus
Integrase slices cellular DNA allowing the viral DNA to attach into host genome forming provirus.
Once DNA is integrated… its permanent
May remain dormant until activated….Latency
Activation – Antigens /Cytokines upregulates several transcription factors, including
NF-κB, which moves from the cytosol into the nucleus.
NF-κB binds to regulatory sequences including genes for cytokines and other immune mediators,
promoting their transcription.
The long-terminal-repeat sequences that flank the HIV genome also contain NF-κB binding sites, so
binding of NF-κB activates viral gene expression
Activation by other pathogens or HIV itself leads to viral RNA production – A vicious cycle.
21. Step 5: Synthesis & Assembly
mRNA transcribed from the viral DNA in the host
gag, pol, and env genes, encode for precursor proteins
tat, rev, vif, nef, vpr, and vpu, genes - regulate the synthesis and assembly of
infectious viral particles and the pathogenicity of the virus
Protease cleave polypeptides to active viral proteins
Viral component aggregate together to form a virion particle
22. Step 6: Budding
• Viral particle moves towards the cell membrane
• Bulges through enveloping itself with a part of the cell membrane – forms viral lipid bilayer
membrane
• Process leads to death of T cell
23. Loss of T Cell Function
1. Direct
i. Membrane defect due to viral particle shedding
ii. Hijacked cellular protein synthesis
2. Indirect
i. Chronic activation due to HIV or OIs ----> Apoptosis
ii. HIV infection of cells in lymphoid organs (spleen, lymphnodes,
tonsils)
iii. Fusion of infected and uninfected cells, leading to formation of
syncytia (giant cells) – Cell death within hours
iv. Qualitative defects in T cell function eg Loss of memory CD4 cells
NB: During clinical latency – balance between destruction and replenishment.
25. HIV Latency
Normally.. Activated CD4+ cells die quickly after HIV infection
A few infected macrophages & Langerhan cells survive long enough and revert back to memory
state..G0
Hide in safe havens – LN,Spleen, BM, CNS
Persist as memory cells, with an altered pattern of gene expression enabling long-term survival
and rapid responses after re exposure to antigen.
Account for inability to cure HIV even with ART.
Ongoing Research –activation of latent infected CD4 cells, then cleared by Cytopathic T cells
26. HIV Invasion
Genital Mucosal route – Most common - 70% Esp. Squamous Non Keratinised epithelium
Mechanism of Mucosal Invasion? Mucosal Entrapment, Trancytosis
Mucosal tears greatly aids in access to target cells
Dendritic Cells – Initial targets in the mucosa …then present to CD4 cells in LNs
Dissemination through lymphatic Channels
Multiplication in LNs leads to destruction of T cells as new virions are formed.
New waves of invasion and destruction leads to progressive loss of CD4 cells.
29. Natural History of HIV InfectionLevels(SeparateScales)
CD4+ T cell
HIV viral loadCD8+ T cell
Neutralizing Antibodies
Years
AIDS and
DeathAcute Asymptomatic
(clinical latency)
4 – 8
weeks
Primary
infection
30. HIV disease progression
–Acute infection
Primary infection of cells in blood or mucosa
(HIV directly infects T cells and microphages
oris carried to those cells by dendritic cells)
Viral replication in the regional lymph nodes
leads to Exponential viral growth and
widespread dissemination
Development of anti-viral responses and
symptoms of acute infection occur
Decrease in plasma viral load and symptoms
of acute infection resolve
31. Immune Reaction to HIV
Humoral + cell mediated responses used in partial control of viral replication ..but… this leads to
destruction of infected CD4+ cells
HIV specific CTLs kill infected cells
Antibodies recognize HIV infected cells ..by surface ag …leads to Antibody dependent cell-
mediated cytotoxicity
Apoptosis of infected cells
32. HIV disease progression -- AIDS
• Acquired Immune Deficiency Syndrome:
• Catastrophic breakdown of host defenses, marked increase
in viremia and clinical disease.
• CD4+ cell count less than or equal to 200 per microliter
• Clinical Features:
• Opportunistic infection
• Neoplasms
• CNS involvement
34. References
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2587276/
Cold Spring Harb Perspect Med 2011;1:a007096
https://www.nps.org.au/australian-prescriber/articles/hiv-fusion-inhibitors-a-review
https://www.niaid.nih.gov/diseases-conditions/hiv-replication-cycle
Setting the stage – HIV host invasion Florian Hladik1,2,3 and M. Juliana
McElrath1,2,4,5
Editor's Notes
AIDS defining OI
TB
PJP
KS
CMV retinitis
Cervical Cancer
Lowest Wajir – 0.5
M Major
O other
N Neither M or o
P Pending Identification of more ecases
Phylogenetic Modelling, genetic studies
Haitian Connection
** Molecular biology studies at Tulane University in 1987.
Who discovered HIV… settled by Presidents of France & US
Terry Higgins Trust
PCR developed by Kary Mullis
From US to SA by homosexual spread
Subtype C = Most prevalent
Viral genes code for
Gag- cone shaped icosahedral capsid
Pol - viral enzymes
Env – surface prot gp 120 , 41
Lipid Bi layer- From Host CD4 cell Membrane
Scanning electron micrograph of HIV-1 (in green) budding from cultured lymphocyte.
Multiple round bumps on cell surface represent sites of assembly and budding of virions.
CD- Cluster of Differentiation
10b virions produced/ day
Importance = Each step can be targeted with drugs that block replication
Trojan Horse technique
Genetic Mutation of CCR5 – Immunity to HIV
Berlin Pt, London Pt
Relevance – Infusion Inhibitors enfuvirtide
Resistance to Infection- Δ32 mutation of CCR5 gene … Berlin Pt. Timothy Ray Brown…Recent London Pt
? Why is BMT not widely used?
VPU – present only in HIV 1 ===respon for higher virulence?
Death of T cell – Membrane failure due to budding viral particles
- Attacked by bodys immune system
progressive destruction of the architecture and cellular composition of lymphoid tissues.
Fusion – Role of GP 120 on cell surface of Infected cells.
Integrated DNA in the HOST nucleus = Nothing much you can do about it.
Direct Inoculation- Straight forward
Mucosal Route – Most significant, Best studied
CD4 cells – central coordinates and controls other lymphocytes
VMMC
4 main phases
Viral Latency Vs Clinical Latency
Markers Of Progression and response to ART
Acute Primary Infection
Once HIV enters the body, the virus infects a large number of CD4+ T cells and replicates rapidly. During this acute phase of infection, the blood has a high number of HIV copies (viral load) that spread throughout the body, seeding in various organs, particularly the lymphoid organs such as the thymus, spleen, and lymph nodes. During this phase, the virus may integrate and hide in the cell’s genetic material. Shielded from the immune system, the virus lies dormant for an extended period of time. In the acute phase of infection, up to 70 percent of HIV-infected people suffer flu-like symptoms.
The Immune System Strikes Back
Two to 4 weeks after exposure to the virus, the immune system fights back with killer T cells (CD8+ T cells) and B-cell-produced antibodies. At this point, HIV levels in the blood are dramatically reduced. At the same time, CD4+ T cell counts rebound, and for some people the number rises to its original level.
Candidiasis of bronchi, trachea, esophagus, or lungs
Invasive cervical cancer
Coccidioidomycosis
Cryptococcosis
Cryptosporidiosis, chronic intestinal (greater than 1 month's duration)
Cytomegalovirus disease (particularly CMV retinitis)
Encephalopathy, HIV-related
Herpes simplex: chronic ulcer(s) (greater than 1 month's duration); or bronchitis, pneumonitis, or esophagitis
Histoplasmosis
Isosporiasis, chronic intestinal (greater than 1 month's duration)
Kaposi's sarcomav
Lymphoma, multiple forms
Mycobacterium avium complex
Tuberculosis
Pneumocystis carinii pneumonia
Pneumonia, recurrent
Progressive multifocal leukoencephalopathy
Salmonella septicemia, recurrent
Toxoplasmosis of brain
Wasting syndrome due to HIV
The HIV set point is the viral load of a person infected with HIV, which stabilizes after a period of acute HIV infection.
The only effective way to lower the set point is through highly active antiretroviral therapy.
The set point is not completely fixed and constant, but can be quite dynamic with significant fluctuations.