HIV Epidemiology, Transmission, Structure & Course of Infection Epidemiology: 38 million people worldwide currently live with HIV Approx 2 million are newly infected annually 770,000 death from AIDs in 2018 In Western Europe, 2.2 million people live with HIV Approx 100,000 people living with HIV in the UK Subtype Epidemiology: Subtype C is the most common globally (mainly affecting African continent) Subtype B is the most common in the UK WHO Target: By 2020, the WHO wanted to achieve the 90/90/90 goal. 90% of people with HIV diagnosed 90% of those diagnosed with HIV on treatment 90% of those on treatment virally suppressed Positive vs Negative Sense RNA Viruses: Positive-sense viral RNA is similar to mRNA and thus can be immediately translated by the host cell. Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA polymerase before translation. HIV-1 Infection Process (Infection & Fusion): Dendritic cells in epithelial or mucosal tissues typically capture the virus following sexual transmission and transport it to lymph nodes to be presented to immune cells, where infection then quickly spreads. HIV is able to infect many types of immune cell (T helper cells, dendritic cells, macrophages, monocytes, etc) and thus displays viral tropism. GP120 is the main glycoprotein allowing entry into the T Helper cell. It binds to the CD4+ receptor on T cells leading to fusion of the viral and T cell membranes GP120 must then bind to a co-receptor on the T-cell surfacein order to begin fusion. The most common co-receptors are the CXCR4 receptor (found mainly on T cells) and the CCR5 receptor (found on T cells, macrophages, monocytes and dendritic cells). Typically, the CCR5 receptor is used. It should be noted that some people have homozygous mutations in their CCR5 receptors which means that they are immune to GP120 binding and thus resistant to HIV infection. This is the CCR5 Delta 32 mutation and those who are homozygous for this are resistant to HIV infection. It is interesting to note that this mutation is specific to European individuals and it is therefore theorised that the plagues of the middle ages may have played a part in bringing about this mutation. Around 10% of Europeans have this mutation. Following binding, GP120 undergoes a conformational change to allow the binding of GP41. Following GP120 binding and its conformational change, GP41 undergoes a structural change, exposing previously hidden hydrophobic gp41 fusion peptides that are inserted in the T Helper cell membrane allowing fusion to take place HIV-1 Infection Process (After Fusion): HIV is a single stranded positive sense RNA virus. It has two single strands of RNA, which are injected into the host cell following fusion alon with three enzymes: Reverse Transcriptase Integrase Protease Reverse transcriptase, injected with the ssRNA, then transcribes complementary double stranded proviral DNA This enzyme has two main sites: the polymerase (generates cDNA) and the RNase H site which degrades the RNA strand, allowing the polymerase to form the second strand of DNA. Reverse transcriptase makes 3-4 errors every time it replicates a HIV virion, thus makes every individual HIV virion unique. Thus, a collection of HIV cells is generally referred to as a 'quasi-species' as every virion behaves slightly differently but they are all related. The integrase enzyme then splices the provirus into the host's genome (DNA). It cleaves off the last two bases at either end of the newly generated HIV DNA It then causes a nick in the host cell chromosome, before then integrating the viral DNA into the host genome Whenever the immune cell then needs to replicate in response to an immune signal (e.g. infection), it also transcribes and translates HIV proteins, which are then spliced to be functional by the HIV protease that was originally injected into the cell. Transmission: Transmission is mainly by sexual intercourse Male to male transmission is the most common source Intravenous drug abuse is another common source Mother to child transmission can also occur Blood transfusions are a less common source but do occur Timeline of Infection: The initial acute phase generally lasts approximately 4 weeks, in which time the individual will experience a flu like illness (fever, night sweats, malaise). Seroconversion occurs during this time, which is the period during which the body produces detectable levels of the HIV virus. This usually occurs at around 4 weeks. At the point where the body has mounted a response to the virus and has suppressed it a bit, the virus enters the chronic phase (at around 12 weeks). During the chronic phase, an individual loses between 1-2 billion T cells per day. Despite this, during the chronic phase T cells usually remain >500 cells/mm3 During the chronic phase an individual can develop a mutation within the HIV cells forming the X4 HIV strain which targets only the CXCR4 co-receptor (thus only T cells). If this strain is present, the T cells are more rapidly destroyed. At 200-500 T cells/mm3, the body starts developing symptoms of chronic HIV infection: Swollen Lymph Nodes Oral Hairy Leukoplakia (white patch on tongue caused by Epstein Barr virus) - arises due to immunosuppression Oral candidiasis At <200 T cells/mm3, the immune system is severely compromised and an individual is considered to have AIDS Patients will start experiencing extreme infections, persistent fever, fatigue, weight loss and diarrhoea Such infections include pneumocystis pneumonia and candidiasis of the eosophagus Tumours such as Kaposi sarcoma (red lesions on skin) are also common manifestations of AIDS
Anti-Retroviral Therapy (ART) Antiretroviral therapy typically utilises three (or so) medications at once to successfully destroy all viruses within a population. As the HIV population is highly variable using a variety of drugs with different mechanisms allows the destruction of viruses that are only resistance to, say, one of the antiretroviral drugs. If only one drug were to be given, widespread resistance would soon occur. Reverse Transcriptase Inhibitors (NRTIs): NRTI stands for Nucleoside Reverse Transcriptase Inhibitor These medications work on the basis that nucleotides have a hydroxyl (-OH) group. What these medications do is add a base to the developing DNA strand on the reverse transcriptase polymerase arm which does not have a hydroxyl group, thus meaning that the chain is terminated and the virus is not replicated properly. An example of an NRTI is Zidovudine Resistance: There are, however, mutated forms of HIV virus which can actually get round this thus making the virus resistant to NRTIs. Reverse Transcriptase Inhibitors (NNRTIs): NNRTI stands for Non-Nucleoside Reverse Transcriptase Inhibitor These drugs act via allosteric inhibition, binding outside the active site causing a conformational change meaning that the enzyme cannot bind the RNA to its polymerase and thus cannot form complementary DNA An example of an NNRTI is Efavirenz Integrase Inhibitors: Typically, Integrase Strand Transfer Inhibitors (INSTIs) are used Studies show that the most effective of these inhibitors bind with the metal cofactors of the enzyme, Magnesium and Manganese. An example of an Integrase Inhibitor is Raltegravir Protease Inhibitors: These inhibitors directly block the protease enzymes by blocking their active site. This means there can be no cleavage of precursor proteins and thus particle maturation cannot occur. An example of a Protease Inhibitor is Lopinavir Maraviroc: This form of medication is an entry inhibitor. It can only be used when the virus only binds to the CCR5 co-receptor. Maraviroc directly blocks this co-receptor on T-cells thus preventing the virus from entering the cell. Determining co-receptor type of virus can be carried out by sequencing the HIV env gene. If there are any CXCR4 using viruses then Maraviroc cannot be used as this will only allow selection for those viruses utilising the CXCR4 co-receptor, thus rendering treatment useless. TAT (Trans-Activator of Transcription): Tat is a protein that is encoded for by the tat gene in HIV-1. Tat is a regulatory protein that drastically enhances the efficiency of viral transcription by activating Reverse Transcriptase Viral Suppression: On HAART, Viral Suppression is classed as <50 HIV-1 copies/ml in plasma Resistance to Treatment: If viral load hasn't dropped within 4 weeks on ART, resistance to medication is suspected. Most tests will not measure resistance in the virus unless it makes up >20% of the total virus population. This is a huge limitation that researchers are working on improving. Next Generation sequencing could be a way round this problem. Both NRTIs and NNRTIs only need 1 or 2 mutations in the entire HIV genome to cause resistance For example: Replacing A with G at position 181 causes a change in amino acid to Cytosine, which gives the virus immunity to both Nevirapine (NRTI) and Efavirenz (NNRTI). This mutation is documented as Y181C as a tyrosine (Y) has been replaced with a cytosine (C) at position 181 Resistance to HIV: The CCR5 Delta 32 mutation is a mutation in the CCR5 co-receptor whereby 32 amino acids are missing thus preventing HIV virions from binding to it. Those who are homozygous for this mutation are resistant to HIV infection. It is interesting to note that this mutation is specific to European individuals and it is therefore theorised that the plagues of the middle ages may have played a part in bringing about this mutation. Around 10% of Europeans have this mutation. Residual Viraemia: Following successful ART there is always residual viraemia. There are two theories for this: The first of which discusses ongoing viral replication whilst on HAART. This is unlikely as this would appear on diagnostic tests. The second, and more likely, hypothesis discusses reactivation of HIV viruses from proviruses that are integrated into the genomes of latent cells (e.g. memory T cells) which cannot be eradicated by HAART.
Theories for Curing HIV Infection: Shock and Kill: This works on the premise that once an individual is virally suppressed, there are still latent reservoirs of HIV proviruses around the body which exist in dormant cells and thus cannot be targeted. These cells exist in the lymphoid tissues, the central nervous system and, occasionally, in the tissues themselves. Latency reversing agents (LRAs) can be used to bring about viral transcription, thus causing the T cells to produce HIV viruses. The cell is now active again (latency has been reversed) and can thus be targeted by both the body's immune processes and ART, thus destroying all HIV infected cells within the body. There are a wide variety of latency reversing agents, but the most promising are Prostratin, Panbinostat and the class of histone deacetylase inhibitors Histone Deacetylation: Histone deacetylase enzymes are important mediators of cell latency. Thus, Histone Deacetylation Inhibitors have been discussed as potential LRAs for Shock and Kill HIV therapy An example of a histone deacetylase inhibitor is Vorinostat Block and Lock: This theory works on the basis that, in latent reservoirs, ART is used to block the replication and transmission of HIV from proviruses, before latency inducing agents are used to effectively lock these proviruses away, locking them in a 'deep latency' so that they are never able to re-emerge. Recent studies have found multiple drugs capable of performing this function, one of the main example being Danusertib. Genetic Modification: It has been discussed that the genetic targeting of CCR5 could be a potential curative strategy for HIV. By introducing the CCR5 Delta 32 mutation to an individual, HIV resistance could be provided. Could be done ex vivo using blood stem cells or in vivo using gene therapy vectors/nanoparticles CAR T Cell Therapy: This stands for Chimeric Antigen Receptor T Cell Therapy Using convertibleCAR T Cells (basically genetically modified Cytotoxic T Cells) it is possible to combine these with a multitude of broadly neutralising antibodies to target latent reservoirs. Broadly Neutralizing HIV-1 Antibodies (bNAbs) are neutralizing antibodies which neutralize multiple HIV-1 viral strains. bNAbs are unique in that they target conserved epitopes of the virus, meaning the virus may mutate, but the targeted epitopes will still exist. This is helpful as HIV is known to mutate very widely and rapidly. A small number of clinical trials for a eatment of this type have been tested, the most prominent utilising the 3BNC117 antibody (antibody specific to CD4bs) which has recently been subject to phase 2 clinical trials. This technology has been used against tonsil T Cells infected with HIV (a known reservoir) and was observed to specifically destroy infected T cells whilst ignoring uninfected T cells. This, therefore, is currently the most promising strategy for a HIV cure. Definitely know this one for exams. Despite its promise, it is very expensive to provide (around £300,000 per patient)