Module 4: Pandemics PDF

Module 4: Pandemics Malaria Malaria epidemiology and PfEMP1 Learning Outcomes Understand the global epidemiology of malaria infections Describe the life cycle and disease pathogenesis of the malaria parasite Explain the role PfEMP1/var genes in malarial infection Pandemics in a post-COVID era Pandem...

Module 4: Pandemics Malaria Malaria epidemiology and PfEMP1 Learning Outcomes Understand the global epidemiology of malaria infections Describe the life cycle and disease pathogenesis of the malaria parasite Explain the role PfEMP1/var genes in malarial infection Pandemics in a post-COVID era Pandemics are increasing in frequency; many near misses since HIV Why did COVID become the 'big one' - for one thing, it's a ‘goldilocks’ virus (low fatality rate but still fatal hence poor response) Pandemics of the past can persist for decades, centuries or even millennia Malaria (caused by Plasmodium falciparum) the first known zoonotic pandemic Life Expectancy Improvement in life expectancy is happening all over the world However covid has had a major impact on life expectancy Population Growth Rate Population growth rate is declining As communities and families become more confident that their children are gonna live, they will have less kids Fertility rate decreases as confidence that kids are going to leave increases Key Drivers of this Human and Economic Success Saving lives - especially reducing the impact of infectious diseases Innovations in health have underpinned the success Providing wide access to those innovations is crucial - ‘leave no one behind’ is a major lesson from COVID-19 Some major positives from the COVID-19 pandemic, especially in innovations like mRNA vaccines and home-based diagnostics Beating Malaria in the Tropics Beating malaria in the tropics is especially hard When its warmer and wetter, the intensity of mosquitoes is so much greater Countries in the tropics also make up the low and middle income part of the world - they are poor because of malaria Malaria is a Big Obstacle to Further Human Development Still close the the number 1 infections cause of child mortality Access to available interventions remains poor Drug and insecticide resistance rife Poor predictors of severe malaria No widely available vaccine and need new drugs Barriers to addressing malaria are both of a technical and non-technical nature Malaria: Some History Ancient problem Most lethal infectious organism in history Still a major problem Discovery in the blood ~1880 by Charles Laveran (Nobel Prize in 1907) Discovery of transmission by mosquitoes in 1897 by Ronald Ross (Nobel Prize in 1902) Taxonomic Classification Phylum; Apicomplexa ○ Apicomplexa invade cells; apical complex used to get in Genus; Plasmodium 4 species infect humans; P. falciparum, P. vivax, P. ovale, P. malariae (recently a 5 th , P. knowlesi) Many other species infect animals Incidence of Malaria Life Cycle begins with the Anopheles Mosquito and Ends in the Blood Three stages: 1) Mosquito stage - sexual reproduction ○ Mosquito injects apical form of parasite called sporozoite into skin ○ Parasite migrates and gets into blood vessel ○ Find its way to the liver 2) Liver stage - asexual reproduction ○ Replication of parasites in the liver ○ Transform into merozoite 3) Blood stage - asexual reproduction, major amplification stage ○ Merozoite infects RBCs ○ Haploid replication ○ Merozoites burst out and reinfect more RBCs Important aspects of lifecycle and spread Sexual stage in the female Anopheles mosquito (1-2 weeks) Injected sporozoites enter hepatocytes via the skin (~30 mins) Asexual liver stage (1-2 weeks) Asexual blood stage cycle is relatively synchronous and takes 2-3 days Disease occurs a week to a month after infection Gametocytes form in the blood and are taken up by a feeding mosquito Fertilization occurs in the midgut of the mosquito Mosquito does not just an inert vector that picks up parasites in the blood meal and take them over to another person to infect them - they are the definitive host Virulence and immune escape: The story of a single parasite protein, PfEMP1 Var genes (variant antigen genes) encode PfEMP1 Gene: Var gene Protein encoded: PfEMP1 Disease occurs only in the blood-stage Fever, chills, anaemia Most deaths (95%) are caused by P. falciparum ○ Cerebral malaria, coma ○ Severe anaemia ○ Placental malaria P. vivax also results in significant morbidity but low mortality ○ Relapsing malaria (hypnozoites in liver) ○ Latent form in liver that reactivates periodically Cytoadherence/sequestration: infected cell sticks to vascular endothelial wall of blood vessel, taking it out of circulation to avoid splenic clearance - parasite does this to avoid the spleen ○ PfEMP1 is an adhesin that sticks the parasites to the wall Cytoadherence and clinical outcomes Parasite put together hemozoin crystals of iron ○ Parasite digests haemoglobin and crystallises iron into metal parts Surface protein on different blood vessels are different Parasite encodes 60 different PfEMP1 Waves of parasitemia as parasite switches PfEMP1 it expresses PfEMP1 anchored on knob structure Can end up with PfEMP1 that binds to a receptor in tissue that causes a lot of damage ○ PfEMP 1 acts as an adhesin to help parasites stay out of circulation Var Genes Encode PfEMPs 14 chromosomes Each chromosome has genes encoding PfEMP1 Most of the var genes are at the end of the chromosomes Leads to some event where one is expressed and others are not How are most var genes ‘silenced’ while allowing just one to be expressed? If DNA is physically wrapped up in a ball, transcription factors can’t get access to it Chromosome ends are clustered in heterochromatin at the nuclear periphery and are in a ‘silent’ heterochromatin state ○ Periphery of the nucleus is quiet spot that transcription factors already have trouble accessing How is the one expressed? ○ Epigenetic control of transcription ○ Histone modification at the nuclear periphery generally has genes inaccessible to transcription factors ○ A particularly localisation is associated with a new histone modification that permits a transcription Introduction of plasmid to parasite with a drug resistant gene (pink) allows selection for parasites with this plasmid ○ Plasmid contains an element that is found on ends of all chromosomes (rep 20 repeat found on ends on chromosome) ○ Rep 20 does the crosslinking between parasite ends ○ When rep20 clustering was disrupted, binding of chromosome ends that are in alignment together were disrupted Mimicking what happens in a human placenta by panning the parasites on a dish that is coated chondroitin sulphate A (selecting parasites in the dish that are expressing Chr12) Keeping drug pressure on results in Chr12 being expressed in a location with the drug resistant gene ○ Privilege site where drug resistant gene is expressed and Chr12 gene is selected by CSA ○ Both these things should be expressed in just on special site When var2CSA is off and there is no selection pressure, the silent gene and active gene are separate (green and red dots are separate) When both are on together, the genes are physically together (green and red dots together) Indicates that there is a single location that is a privileged site ○ To be an expressed var gene it has to move into that location Var gene Expression may be more complex Var gene switching seems to involve switching through an intermediate, where at a low level it expresses lots of them Expressing a wide number at a low level so that it will be under immune radar Results in recrudescence - parasite persistence ○ When immunity is active, the form of the parasite that is surviving for a long time may be the low level intermediates Toward New Drug Targets for Malaria Plasmodium parasites export 100’s of proteins into the RBC cytosol: crucial virulence and nutrient uptake roles As parasite enters RBC it puts another membrane around it (parasitophorous vacuolar membrane, PVM) Parasite encodes proteins that are synthesised and transported to parasite cytosol and some to the surface Cell doesn’t have characteristics for adherence, waste disposal, nutrient uptake so the parasite has to provide for that What allows parasite protein export into the RBC cytosol & beyond? Somewhere at the N terminus of these proteins was information that allowed that green fluorescent protein to get through Just having the N terminus and not a reasonable amount of downstream sequence then the GFP didn’t get exported Exported Proteins Contain a Conserved ‘PEXEL’ Motif Lining up the N terminal sequences of proteins that get exported results in a signal sequence that can be seen - ER entry sequence Downstream of the ER entry sequence, there is a 3 amino acid signature that all exported proteins have ○ R X L X (E,Q or D) ○ Arginine, any amino acid, leucine, any amino acid, E Q or D ○ PEXEL motif A knockout screen of 51 exported proteins (46 PEXEL-containing) revealed proteins many virulence and survival roles ~25% of exported genes are essential to growth ○ Could not be grown in petri dish ○ Essential metabolically to the parasite The PEXEL protein-export translocon is a nexus through which many if not all these functions are connected Proteins can form channels to take up nutrients or get rid of waste Proteins that help refold other proteins Proteins involved in cytoadherence Proteins involved in malleable Protein export is a major point of vulnerability: how do these proteins cross the parasitophorous vacuole membrane? The PEXEL is a cleavage site that is recognised in the ER by and enzyme known as Plasmepsin V ○ Recognised at the time of synthesis ○ By a parasite protein called plasmepsin V To be exported, cleavage of plasmepsin V was necessary Licensing cleavage in the ER that licences proteins to go down a particular vesicular pathway A putative Plasmodium translocon of PEXEL proteins: PTEX Contains channel (protein called EXP2), molecular machine (triple plus ATPase called HSP101) and a range of accessory proteins (PTEX88, PTEX150 and Trx2) Key criteria that was looked for in a translocon: ○ Plasmodium specific & in the correct location ○ Essential to blood-stages ○ Energy source, an un-folding mechanism ○ Binds transiting cargo PEXEL proteins ○ ○ ○ ○ Genetic “knockdown” of PTEX components kills parasites Utilised conditional knockout Gene knockouts can’t be made against something that's essential Add something to the petri dish (glucosamine) Modify three prime end of transcripts that we are interested in knocking down Three prime end has ribozyme that is susceptible to glucosamine Toward blocking PTEX – a new anti-malarial drug strategy PTEX inhibitors are an approach to block many essential proteins/processes via the one target Recent 3D (cryEM) structure of PTEX is a big step forward for drug development Malaria Vaccines Malaria Diagnosis; the power of RDTs Most diagnostic tests detect blood-stage parasites - malaria symptoms manifest during blood-stage of parasite life cycle Critical for identifying and treating patients, surveillance and monitoring drug resistance. WHO recommends T3 - Test, Treat and Track pfHRP2 An abundant 30-kDa histidine rich water-soluble protein released into circulation by P. falciparum throughout its life cycle pfHRP2 concentrations reflect the total parasite biomass including sequestered parasite and are associated with disease severity and mortality Measurement of the presence/absence of pfHRP2 is the basis of a number of readily available malaria RDTs (most P. falciparum RDTs) pHRP2 is located subtelomerically on chromosome 8 Emergence of HRP2 gene deletions; unusual Darwinian evolution HRP2 deletions reported in multiple countries, most common in Central and South America Use of HRP2-RDTs appear to be selecting for parasites with HRP2 deleted genes Results in false negatives and eventually poor clinical management Missed diagnosis promotes parasite survival and transmission (of HRP2 negative parasites) 3 Types of Vaccine Pre-erythrocytic (eg, RTS,S, whole parasite) Transmission blocking (neutralise parasite in mosquito) Blood-stage (anti-merozoite) RTS,S Vaccine Recombinant protein vaccine based on the surface protein of the sporozoite (circumsporozoite antigen) ○ Circumsporozoite antigen placed into recombinant expression system ○ Expressed with hepatitis B surface antigen providing protection against malaria and hepatitis B Overall VE against clinical malaria 26% for 3 doses and 39% for 4 doses, declines rapidly and lower in the younger age groups Booster essential for efficacy against severe malaria but poorly boosted antibodies RTS,S vaccine – what next? A need for improved vaccine efficacy, immune responses and longevity – Optimize adjuvants, dosing regimens and vaccine platforms and overcome strain- specificity/immune escape Address host factors (prior exposure, nutrition, genetics) Improve current vaccine design e.g R21 is similar to RTS,S but has a different adjuvant and better/optimal antigen (CSP) presentation Target key epitopes (important CSP epitopes/domains missing in RTS,S) and functional antibody responses (complement, Fc receptors) Combine RTS,S with other antigens e.g. CSP + other blood-stage antigens (RH5) Combining vaccine with other interventions e.g. chemoprophylaxis, ITNs, IRS Toward a blood-stage malaria vaccine Apicomplexa parasites (e.g. toxoplasma, sporozoite) invading a hepatocyte or a merozoite infecting a RBC go in a basic similar way ○ Get the apical end against the membrane of the cell they are going to invade ○ Eject out ligands that bind receptors on their host cell ○ Goes in via a motor molecular machine inside parasite However different plasmodium species prefer different RBCs (e.g. young vs middle age vs old) ○ Shows that there are alternate ways to enter a RBC Parasite doesn’t have ligands on its surface the whole time and just secretes them at the right time to prevent antibodies from building up - just in time hypothesis The rationale for an anti-merozoite vaccine has long been recognised Rationale for a vaccine against these ligands are against a certain class of proteins on the surface but more so the apical organelles proteins (ligands that come out of them) but, empirical approaches haven’t worked and there was little functional or immunological knowledge to help inform vaccine design Why has it been so tough to make a blood-stage vaccine? One reason it’s difficult is because it’s rapid ○ Just in time hypothesis ○ Whole process of invasion is very fast Phase 1: where parasite is rolling around is only 10 seconds Phase 2: is about 20 seconds from the commitment to enter when the apical end has formed a tight junction, ligand is coming out, motor is switched on and starts to pull itself in Phase 3: RBC becomes a different shape and then recover due to parasite injecting Ca2+ which causes dehydration process; maybe modifying host protein cytoskeleton to make it easier to invade And because there are lots of targets The RBC Surface is Highly Polymorphic RBC is highly polymorphic Malaria has played a major role in selecting for polymorphisms in RBC Example: duffy and vivax interaction Overlaying real-time imaging with tools to block receptor-ligand interactions revealed 4 clear steps in the 10 second pre-invasion step 4 targets ○ MSP1 Complex (maybe interacts with band 3 and GlyA and tests cells to see which cell to invade) ○ Alternate ligands (rolling around on surface step) Erythrocyte binding antigens (EBA)/ erythrocyte binding ligands (EBL) Reticulocyte binding proteins ○ Rh5 complex/CyRPA complex (setting up port so parasite can inject Ca2+ in) ○ RON complex (parasite injects its own receptor) Two Broad Approaches to Blood-Stage Vaccines 1. The ‘obligatory’ steps - especially the “Rh5 complex Rh5-basogen complex AMA1-RON complex 2. The ‘alternate ligands’ Selective pressure can alter receptor-ligand usage E.g. the W2mef strain which uses EBA175 to bind to sialic acid on GlyA can become sialic acid independent Deleting the EBA175 ligand causes it to switch from EBA175 to Rh4 Alternate invasion pathways allow parasites to invade via a diverse range of RBC receptors and to avoid immunity Invasion pathway use and inhibitory antibodies Serum samples tested for inhibition of invasion W2mef: Sialic Acid dependent W2mef∆175: Sialic Acid Independent 80 samples from children and adults Variation in ligand usage is a mechanism of immune evasion EBA and Rh proteins are important targets of inhibitory antibodies Blood-stage vaccine summary Understanding function and immunity of merozoite antigens has provided a rational approaches to vaccine development Some essential steps – the “Rh5 complex” is especially promising Alternate ligands are also an option; they exist to accommodate RBC polymorphism AND to avoid immunity Vaccine must cover all major pathways HIV Learning Objectives Briefly describe the epidemiology of HIV in terms of risk factors and prevalence in different regions Discuss HIV virology with respect to genome, strain diversity and lifecycle Explain how HIV enters host cells and how this may change over the natural history of long-term viral infection Explain how innate host anti-viral factors function to prevent HIV infection, and how HIV counteracts the action of these factors to achieve infection Describe the host immune response to HIV infection and explain how HIV evades this host immune response Describe the natural history of HIV infection with respect to the effect on the host immune system, viral replication and the progression of symptoms and illness in patients Describe the effects of HIV infection on the host immune system and explain how these are caused by the viral infection Briefly describe the mechanism of action of the different classes of antiviral drugs for HIV, and explain the rationale behind combined antiretroviral therapy (ART) Discuss the limitations of ART as a treatment for HIV-infected patients Describe the medical and behavioural interventions currently in use to prevent the transmission of HIV in the population, including a brief description of how they work and the effectiveness of each Describe the current research approaches to a HIV vaccines and their limitations Explain how approaches to a HIV cure aim to overcome the known barriers to a cure Epidemiology, Virology and Immunology Global Burden of HIV In 2021: ○ 38.4 million people living with HIV ○ 1.5 million people newly infected ○ 650 thousand HIV related deaths Burden of disease is vastly different across the world: ○ 25. million in Africa ○ 3.8 million in America ○ 2.8 million in Europe ○ 430 thousand in eastern mediterranean ○ 1.9 million in Western PAcific Risk Factors for HIV Infection Globally in 2021 Global epidemic is different depending on where you live therefore tailored strategies are needed locally Risk of Infection for Key Populations in 2021 People who inject drugs have 35 times greater risk of acquiring HIV than adults who do not inject drugs Female sex workers have 30 times greater risk of acquiring HIV than adult women (15-49) in the general population Gay men and other men who have sex with with men have 28 times greater risk of acquiring HIV than adult men (15-49) in the general population Transgender women have 14 times greater risk of acquiring HIV than adult women (15-49) in the general population Decline in HIV Incidence and Mortality over time Antiretroviral therapy (ART) is highly effective and global uptake high ART can also stop HIV transmission as it makes an infectious person no longer infectious Reduces viral load to undetectable levels HIV Testing and Treatment Cascade in 2019 95-95-95 targets by 2030: 95% of people living with HIV know their status 95% of people living with HIV who know their status are receiving treatment 95% of people on treatment have suppressed viral loads Significant disparity in ART access by region HIV notification rate per 100,000 in Australia Numbers of new HIV infections/ HIV notification rate for 100,000 people in Australia is significantly declining and most notably in the last two years HIV in Australia HIV Notifications by Exposure Category Proportion of infections diagnosed as newly acquired Likely place of HIV acquisition Reduction in the likelihood of acquiring HIV overseas HIV Diagnosis and Care Cascade Pre-Exposure Prophylaxis (PREP) Cascade for HIV Negative Men Number of people living with HIV in Australia: an aging cohort HIV is a (complex) retrovirus Retroviruses of the Lentiviridae family Non-primate retroviruses ○ CAEV/Visna Caprine arthritis encephalitis / Visna Virus ○ EIAV Equine Infectious anaemia virus ○ BIV Bovine Immunodeficiency virus ○ FIV Feline Immunodeficiency virus Primate retroviruses African Green Monkey SIVagm Sooty Mangabey SIVsm (precursor of HIV-2) Macaque SIVmac Mandrills SIVmnd Sykes monkeys SIVsyk Chimpanze SIVCPZ (precursor of HIV-1) Lentiviruses: Properties Family: Retrovirus Major human virus: HIV-1, HIV-2 Size: 80-130 nm Capsid symmetry: Icosahedral Envelope: Yes Genome: Diploid linear 10kb + sense ssRNA; Genome replicated: Nucleus Virus assembly: Cytoplasm - plasma membrane Common features: Slow disease Diseases: AIDS; neurologic; arthritis; pneumonia HIV Genome Regulatory proteins: tat, rev, vpr, vpu, vif, nef High degree of variability exists for gag and env proteins Retroviruses: structure Envelope protein ○ Allows for cell attachment to the target cell ○ Two components: ○ Gp120: allows for cell attachment ○ Gp41: allows for viral fusion Gag protein ○ Three components: ○ P17: lines the virus under lipid bilayer membrane ○ P24: capsid protein protects ss RNA ○ P7: nucleocapsid proteins associated with ssRNA Polymerase: ○ P66/51: reverse transcriptase which allows copying from RNA to DNA ○ P32: integrase ○ P11: protease Global Distribution of HIV Clades Clades are like different variants Phylogenetic tree shows the similarities of the viral sequence between different strains of HIV and SIV Branches of the tree and size of branches reflect how similar or different those viruses are There is also diversity amongst HIV strains HIV Life Cycle 1) Gp120 binds to CD4 receptor on target cell 2) Binding is followed by a binding to a second co-receptor/chemokine co-receptor (CCR5 or CXCR4) resulting in conformational change of the gp120 protein, exposing gp41 allowing it to fuse with the target cell membrane 3) Following fusion, HIV RNA enters the cell 4) Cytoplasm undergoes reverse transcription (RNA to DNA) 5) Proviral DNA/ pre integration complex enters the nucleus through a nuclear pore and undergoes a process of integration 6) Once DNA is integrated, it becomes part of the host DNA for life 7) Integrated virus undergoes transcription of a range of different viral RNAs which are translated to make key viral proteins 8) RNA and viral proteins are assembled in the cytoplasm and then bud from the surface of the membrane taking some proteins from the cell surface 9) The virion undergoes maturation and forms a new HIV virion HIV replication: key features Rapid Error prone reverse transcriptase leads to rapid evolution of multiple quasispecies 10 billion particles produced per day Impact on host cells ○ CD4+ T-cells Activated CD4+ – death Resting CD4+ – latent ○ Monocyte/macrophages Long lived slow release of virus Host Cell Entry needs CD4 and a chemokine co-receptor The Main HIV Chemokine Receptors are CCR5 and CXCR4 Two different co-receptors that HIV uses in addition to CD4; CCR5 or CXCR4 Viruses that bind a new CCR5 are called R5 viruses (most common) Viruses that bind a new CXCR4 are called X4 viruses Viruses that can use either CCR5 or CXCR4 are called dual tropic viruses Virus populations containing a mixture of R5 tropic, X4 tropic and/or dual tropic HIV are called mixed tropic (D/M) Natural Resistance to HIV: CCR5 mutations The delta 32 mutation leads to a deletion of 32bp and no expression of CCR5 Made individuals with this mutation resistant to R5 viruses Innate antiviral cellular factors and HIV proteins Two Major Forms of HIV Infected Cells HIV predominantly infects activated CD4 T cells leading to productive infection In latent infection, virus integrates into resting CD4 T cell but doesn’t complete viral replication cycle It’s not binary, there’s a spectrum of activity of transcription Early Events in HIV Transmission HIV infects cervicovaginal epithelium if there’s a microbreak in the epithelium and/or infects endocervix by binding to a dendritic cell Cell becomes infected and the virus is then transferred into its local lymph node where there’s local expansion Here virus meets activated CD4+ T cells and can also infect resting CD4+ T cells Virus disseminated to lymphatic tissue and within 1-2 weeks virus is visible in the plasma Peak plasma virus levels occur and this is associated with CD4 immune memory loss and a very strong immune response to the virus Occurs through generation of antibodies through adaptive immunity of HIV specific CD8 T cells and HIV specific CD4 T cells designed to eliminate virus or eliminate infected cells Cell-Mediated Immunity to HIV Foreign antigen/virus is taken up by APC Foreign antigen is broken up into peptides which are presented in MHC II Peptide in the context of MHC II is recognised by a specific T cell receptor on a CD4 T cell, generating antigen specific immunity Immune Response to HIV Virus becomes detectable in plasma within 2 to 3 weeks of infection Starts to decline due to running out of targets to infect, individual makes immune response Generation of antibodies at around 6 weeks following infection Evasion of the immune response by HIV Sequence variation ○ Lack of recognition (both CTL and antibody) ○ Antagonism (original sequence alters function of cytotoxic T cell) Altered antigen presentation ○ Down regulation of MHC class I molecules by Tat, Vpu and Nef Loss of effector cells ○ Clonal exhaustion (proliferation of antigen specific T cells wears out) ○ Loss of CD4 T cell help ○ Replicative senescence (adapted immune system reaches end of lifespan) Latency ○ particularly in resting T-cells, macrophages and astrocytes Privileged sites of viral replication ○ Brain, testis, gastrointestinal tract ○ Blood brain barrier and blood testis barrier block the migration of antigen specific T cells Natural History of HIV Infection Natural history of HIV disease: acute and chronic depletion of CD4 T-cells Massive Depletion of CD4+ T cells from the GI Tract in Acute Infection Far more depletion of T cells in tissue compared to blood Large yellow deposits of tissue called peyer’s patches which are full of CD4 T Cells Individual with HIV infection, can’t see peyer’s patches as HIV has destroyed CD4 T cells in gut Chronology of CD4 T cell loss and disease Immunology: immune defects in people living with HIV CD34+ progenitor cells in the bone marrow generate double negative T cells Double negative T cells migrate to thymus where they become mature to become double positive CD4 T cells Undergo a selection process to become single CD4+ and single CD8+ T cells which are not autoreactive T cells then move to periphery and they undergo homeostasis and cell division and generate naive T cells Can activate and proliferate to form effector T cells which revert to resting T cell state or die establishing long term CD4 and CD8 memory Thymic function and recent thymic emigrants Recent thymic emigrants move into blood then move into secondary lymphoid organs Range of different markers that determine a recent thymic emigrant ○ In HIV it is the expression of CD31 or TREC or PTK7 Causes of CD4+ T-cell decline Increased destruction ○ Direct infection Lose more T cells in GIT than blood Incomplete reverse transcription in naïve T-cells ○ Indirect effects Syncitium formation (infected cell expresses gp120 on surface and binds CD 4 T Cells from uninfected cells, inducing large clusters of cells, eliminating uninfected CD 4 T cells) Apoptosis Immune activation Lymph node fibrosis Impaired production ○ Thymus ○ CD34+ progenitor cells HLA type important for immune response Why is CD4 T-cell depletion variable? Viral factors ○ CXCR4 virus accelerated T-cell loss (due to X4 being far more widely expressed on T cells than CCR5 which is only expressed on activated T cells) ○ Nef deleted virus limits T- cells loss (Nef down regulated MHC class I, allowing virus to escape from immune response) ○ Co-infection with other viruses eg., CMV, GBV-C Host factors ○ Immune response HLA type ○ Genetic factors - CCR5 D32 heterozygote has slower disease progression ○ Age - Impaired thymic function in very young and very old ○ Sex - Women have greater CD4 decline for the same viral load HIV-induced immunopathology: Depletion and/or dysfunction of other cells HIV causes chronic immune activation Mucosal depletion of CD4 T-cells ○ Increased microbial translocation ○ Activation of TLR4 by bacterial products (LPS) Activation of innate immune response (pDCs) ○ HIV RNA is a TLR7/8 ligand ○ Increased plasma IFN-a Cytomegalovirus (CMV)-specific response ○ Expansion of CMV-specific activated CD4+ and CD8+ T-cells Loss of T regulatory cells Elite controllers: