#RETHINK HIV

In the aftermath of World AIDS day on 1^st December 2015, whereby the National AIDS Trust (NAT) encouraged the public to rethink outdated stereotypes and challenge myths by adopting the ‘Be positive: Rethink HIV’ campaign, it seems opportune to ‘rethink’ or reflect on the current landscape of medical research in this area – is it really something to ‘be positive’ about?

A devastating pandemic that has killed in excess of 35 million people since its identification in 1984 [1], the human immunodeficiency virus (HIV) still affects more than 100,000 people living in the UK, and 34 million people worldwide [2].   Each year 6000 people are newly diagnosed in the UK alone (Fig 1 [3]). But daunting figures and statistics aside, scientific advances have been made in HIV treatment, understanding of the disease has vastly improved, and laws have been put in place to protect people living with HIV. In fact, highly active antiretroviral therapy (HAART; Fig 3) has substantially transformed HIV infection from an inevitably fatal condition into a chronic disease with a longer life expectancy [3]. But, with prolonged treatment comes other problems such as tolerability, side effects, adherence to medication and drug resistance. Accordingly, strategies to optimise the therapeutic response, prevent adverse drug reactions, and find drugs with a novel action of mechanism must now come to the forefront of this enduring war between HIV and medicine.

Figure 1 New HIV diagnoses, AIDS and deaths over time; 1999-2014. If diagnosed early, people now living with HIV can expect a near-normal life span. People diagnosed with HIV late have a ten-fold increased risk of death in the year following diagnosis compared to those diagnosed promptly. In 2014, 346 were diagnosed with AIDS for the first time and 613 people with HIV infection were reported to have died, most of whom were diagnosed late. [3]

An HIV infection causes a chronic viral infection that leads to the selective depletion of CD4⁺ T cells. This class of T cell plays active roles in the immune system, particularly in the acquired immune response. They recognise and bind to antigens on the surface of antigen presenting cells (APCs) via a T cell receptor-CD3 complex, triggering downstream signalling cascades that ultimately aim to rid the body of these antigens. However, a gradual loss of these T cells comprises immune competence and progresses to acquired immune deficiency syndrome (AIDS). As the virus is unable to reproduce alone, it exploits these CD4⁺ T cells as vectors for amplification. It makes sense then that understanding the life cycle of the virus is key to treatment: effective combined antiretroviral drug therapy can decrease viral load below detectable levels by targeting mechanisms employed by the virus during its life cycle (Fig 2).

Figure 2 Thin sections of the entry of the HIV-1 virus into a CD4 + cell by fusion. The life cycle begins with the HIV virus recognising CD4 on the surface of the CD4+ T cells. (a) The virus is initially attached to the cell membrane. Several virus-cell interconnections (double-headed arrow) are seen between the virus and the cell membrane. It is thought that binding via a CCR5 or CXCR4 coreceptor [4] allows for gp41-mediated fusion of the virus with the host cell. (b) The Contact surface increases. (c) The envelope of the virus fuses to the cell membrane (double-headed arrow). Some virions (arrow) are tagged with ferritin (arrowheads). Fusion enables the reverse transcription of the viral RNA into DNA. For the viral DNA to integrate into the host genome, hydroxyl groups are added during 3’ processing, which is catalysed by the enzyme integrase. This complex of protein and viral DNA then can enter the nucleus, where the strand transfer reaction integrates the viral DNA into the host genome [5]. Virus replication occurs when the DNA in transcribed into both genomic RNA and mRNA. The mRNA is translated into multiple Gag-Pol polyproteins, the precursors to the structural proteins and enzymes of the virus, which cause migration to the plasma membrane. This allows for budding of the immature virions from the CD4+ T cell before they can then undergo maturation to form infectious viral particles. Scale bars indicate 100nm [6]

So, why does HIV remain a worldwide health challenge? Well, in part, this can be attributed to the implementation of widespread treatment in developing countries being impeded by economic, political and cultural factors. The costly nature of HAART, combined with need for regular administration of the drugs, only fosters the HIV epidemic. Despite an increase in the number of HIV-infected patients receiving treatment and a fall in the number of new cases, there is still an imbalance between those new cases and those new patients gaining access to treatment [7]. In addition to obstacles accessing treatment, another contributing factor is the continued evolution of drug resistance. There are many different strains of HIV, grouped into two main types HIV-1 and HIV-2. An infected person can carry multiple subtypes of the virus at any one time. The World Health Organisation [8] estimated that as many as 17% of new HIV infections in developed countries are due to resistance of virus strains to one or more of the cocktail of antiretroviral drugs used in standard HAART (Fig 3). This therapy usually includes two different NRTIs (nucleoside analog reverse transcriptase inhibitors) and an NNRTI (non-nucleoside reverse transcriptase inhibitor) or a protease inhibitor. NRTIs, for example, permit the rapid development of resistance of HIV to the treatment when administered alone. Clinically proven in HIV-1 suffers, the use of allosteric inhibitors is an attractive alternative to solely using active site inhibitors. When used in combination with the active site inhibitor, an NRTI such as azidothymidine helps to reduce the evolution of resistance [9].

Figure 3 The HIV life cycle and antiretroviral drug intervention. Entry inhibitors interfere with viral entry into the host cell by inhibiting several key proteins that mediate the process of viron attachment, co-receptor binding and fusion [10]. Reverse transcriptase inhibitors include NRTIs, which are analogs of endogenous deoxyribonucleotides and high affinity for the viral reverse transcriptase. These are therefore incorporated into the viral DNA strand during synthesis and causes transcription termination as they lack the 3’-OH group necessary for phosphodiester bond formation in DNA strand elongation [11]. NNRTIs are compounds that bind to the allosteric site of the HIV-1 reverse transcriptase and interfere with its activity causing the selective block of HIV-1 transcription [9]. Integrase inhibitors bind cofactors needed for the interaction of integrase and the host DNA. This blocks the insertion of viral DNA into the host genome [12]. Protease inhibitors bind the active site of the viral protease with high affinity and as a result inhibits the cleavage of polypeptides necessary for viral maturation after budding from the host cell [13]. Maturation inhibitors, similarly to protease inhibitors stop the processing of the HIV-1 polypeptides but do this by binding to the polypeptides themselves [14].

Finally, one of the main challenges of completely eradicating HIV infection is the concept of HIV latency [15]. A latent virus is usually used to describe one that is not able to give rise to new viral particles. Contrary to this, HIV is thought to be able to hide in a persistent viral reservoir harboured within the host. This ‘reservoir’ contains integrated and replication-competent HIV DNA in the host genome, which is unaffected by HAART, and unable to be cleared by the host’s own immune defence [15]. As a consequence there is always a possibility for new rounds of infection even after achieving undetectable levels of viral load. Despite the concept of a ‘viral reservoir’ being accepted, understanding of it from a cellular and tissue level, and therefore how to quantify this in a patient, still requires some elucidation. The reservoir exists in the CD4⁺ T cell compartment [16], and therefore could be expected to be present in higher levels in those tissues with high levels of CD4⁺ cells. However, its distribution between tissues is not known, and factors such as viral subtypes, the age and gender of the patient, and medications being taken, could affect this. Furthermore the extent to which the viral DNA, when present, is able to replicate has not be determined but it is predicted to be as low as 2% [16]. Regardless, it only takes one cell containing viral DNA able to replicate to trigger a new round of infection after cessation of HAART. In order to reach a stage where a drug-free remission of HIV infection is possible, clearance of this persistent virus must be attained [17].

Blog written by Victoria Miller

Reference

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