HIV

Template:Taxobox begin Template:Taxobox image Template:Taxobox begin placement virus Template:Taxobox group vi entry Template:Taxobox familia entry Template:Taxobox genus entry Template:Taxobox species entry Template:Taxobox species entry Template:Taxobox end placement Template:Taxobox end The human immunodeficiency virus, commonly called HIV, is a retrovirus that primarily infects vital components of the human immune system such as CD4+ T cells, macrophages and dendritic cells. It also directly and indirectly destroys CD4+ T cells. As CD4+ T cells are required for the proper functioning of the immune system, when enough CD4+ cells have been destroyed by HIV, the immune system barely works, leading to AIDS. HIV also directly attacks certain human organs, such as the kidneys, the heart and the brain leading to acute renal failure, cardiomyopathy, dementia and encephalopathy. Many of the problems faced by people infected with HIV results from the failure of the immune system to protect them from certain opportunistic infections and cancers.

HIV is transmitted through penetrative (anal or vaginal) and oral sex; blood transfusion; the sharing of contaminated needles in health care settings and through drug injection; and, between mother and infant, during pregnancy, childbirth and breastfeeding.

AIDS is thought to have originated in sub-Saharan Africa during the twentieth century and it is now a global epidemic. At the end of 2004, UNAIDS estimated that nearly 40 million people were currently living with HIV ^[1]. The World Health Organization estimated that the AIDS epidemic had claimed more than 3 million people and that 5 million people had acquired HIV in the same year.

Introduction

In 1983, scientists in France led by Luc Montagnier, first discovered the virus that causes AIDS ^[2]. They called it lymphadenopathy-associated virus (LAV). A year later, Robert Gallo of the United States, confirmed the discovery of the virus, and they named it human T lymphotropic virus type III (HTLV-III) ^[3]. In 1986, both names were dropped in favour of the term human immunodeficiency virus (HIV) ^[4].

HIV is a member of the genus lentivirus ^[5], part of the family of retroviridae ^[6]. Lentiviruses have many common morphologies and biological properties. Many species are infected by lentiviruses, which are characteristically responsible for long duration illnesses associated with a long period of incubation ^[7]. Lentiviruses are transmitted as single-stranded negatively-sensed enveloped RNA viruses. Upon infection of the target-cell, the viral RNA genome is converted to double-stranded DNA by a virally encoded reverse transcriptase which is present in the virus particle. This viral DNA is then integrated into the cellular DNA for replication using cellular machinery. Once the virus enters the cell, two pathways are possible: either the virus becomes latent and the infected cell continues to function or the virus becomes active, replicates and a large number of virus particles are liberated which can infect other cells.

Two species of HIV infect humans: HIV-1 and HIV-2. HIV-1 is the more virulent and easily transmitted, and is the source of the majority of HIV infections throughout the world; HIV-2 is largely confined to west Africa ^[8]. Both species originated in west and central Africa, jumping from primates to humans in a process known as zoonosis. HIV-1 has evolved from a simian immunodeficiency virus (SIVcpz) found in the chimpanzee subspecies, Pan troglodyte troglodyte ^[9]. HIV-2 crossed species from a different strain of SIV, found in sooty mangabeys, an old world monkey of Guinea-Bissau ^[10].

The clinical course of HIV-1 infection

Figure 1. Graph showing HIV virus and CD4+ levels over the course of an untreated infection

Infection with HIV-1 is associated with a progressive loss of CD4+ T-cells. This rate of loss can be measured and is used to determine the stage of infection. The loss of CD4+ T-cells is linked with an increase in viral load. The clinical course of HIV-infection generally includes three stages: primary infection, clinical latency and AIDS (Figure 1). HIV plasma levels during all stages of infection range from just 50 to 11 million virions per ml ^[11].

Primary Infection

Primary, or acute infection is a period of rapid viral replication that immediately follows the individuals exposure to HIV. During primary HIV infection, most individuals (80 to 90 %) develop an acute syndrome characterised by flu-like symptoms of fever, malaise, lymphadenopathy, pharyngitis, headache, myalgia, and sometimes a rash ^[12]. Within an average of three weeks after transmission of HIV-1, a broad HIV-1 specific immune response occurs that includes seroconversion. Because of the nonspecific nature of these illnesses, it is often not recognized as a sign of HIV infection. Even if patients go to their doctors or a hospital, they will often be misdiagnosed as having one of the more common infectious diseases with the same symptoms. Since not all patients develop it, and since the same symptoms can be caused by many other common diseases, it cannot be used as an indicator of HIV infection. However, recognizing the syndrome is important because the patient is much more infectious during this period.

Clinical Latency

As a result of the strong immune defence, the number of viral particles in the blood stream declines and the patient enters clinical latency (Figure 1). Clinical latency is variable in length and can vary between two weeks and 20 years. During this phase HIV is active within lymphoid organs where large amounts of virus become trapped in the follicular dendritic cells (FDC) network early in HIV infection. The surrounding tissues that are rich in CD4+ T-cells also become infected, and viral particles accumulate both in infected cells and as free virus. Individuals who have entered into this phase are still infectious.

The declaration of AIDS

AIDS is the most severe manifestation of infection with HIV. Acute HIV infection progresses over time to clinical latent HIV infection and then to early symptomatic HIV infection and later, to AIDS, which is identified on the basis of certain infections. In 1990, the World Health Organization (WHO) grouped these infections and conditions together by introduced a staging system for patients infected with HIV-1. This was updated in September 2005. Most of these conditions are opportunistic infections that can be easily treated in healthy people.

Stage I: HIV disease is asymptomatic and not categorized as AIDS
Stage II: include minor mucocutaneous manifestations and recurrent upper respiratory tract infections
Stage III: includes unexplained chronic diarrhea for longer than a month, severe bacterial infections and pulmonary tuberculosis or
Stage IV includes toxoplasmosis of the brain, candidiasis of the esophagus, trachea, bronchi or lungs and Kaposi's sarcoma; these diseases are used as indicators of AIDS.

In the USA, the definition of AIDS is goverened by the Centers for Disease Control and Prevention (CDC). In 1993, the CDC expanded their definition of AIDS to include healthy HIV positive people with a CD4 positive T cell count of less than 200 per µl of blood. The majority of new AIDS cases in the United States are reported on the basis of a low T cell count in the presence of HIV infection.

Many factors such as host susceptibility and immune function ^[13]^[14]^[15], health care and co-infections ^[16]^[17]^[18], as well as factors relating to the viral strain ^[19]^[20] may affect the rate of progression to AIDS. Thus, following infection with HIV-1, the rate of clinical disease progression varies enormously between individuals.

HIV tropism

The term viral tropism refers to the cell type that the virus infects and replicates in. HIV can infect a variety of cells such as CD4+ helper T-cells and macrophages that express the CD4 molecule on its surface. HIV-1 entry to macrophages and T helper cells is mediated not only through interaction of the virion envelope glycoproteins (gp120) with the CD4 molecule on the target cells but also with its chemokine coreceptors. Macrophage (M-tropic) strains of HIV-1, or non-syncitia-inducing strains (NSI) use the beta-chemokine receptor CCR5 for entry and are thus able to replicate in macrophages and CD4+ T-cells. The normal ligands for this receptor, RANTES, macrophage inflammatory protein (MIP)-1-beta and MIP-1-alpha, are able to suppress HIV-1 infection in vitro. This CCR5 coreceptor is used by almost all primary HIV-1 isolates regardless of viral genetic subtype. Indeed, macrophages play a key role in several critical aspects of HIV disease. They appear to be the first cells infected by HIV and perhaps the very source of HIV production when CD4+ cells are markedly depleted in the patient. Macrophages and microglial cells are the cells infected by HIV in the central nervous system. In tonsils and adenoids of HIV-infected patients, macrophages fuse into multinucleated giant cells that produce copious amounts of virus. T-tropic isolates, or syncitia-inducing (SI) strains replicate in primary CD4+ T-cells as well as in macrophages and use the alpha-chemokine receptor, CXCR4, for entry. The alpha-chemokine, SDF-1, a ligand for CXCR4, suppresses replication of T-tropic HIV-1 isolates. It does this by down regulating the expression of CXCR4 on the surface of these cells. Viruses that use only the CCR5 receptor are termed R5, those that only use CXCR4 are termed X4, and those that use both, X4R5. However, the use of coreceptor alone does not explain viral tropism, as not all R5 viruses are able to use CCR5 on macrophages for a productive infection ^[21].

HIV can also infect dendritic cells ^[22].

Viral Structure

File:800px-HIV Viron.png

Diagram of the HIV virus

HIV is different in structure from previously described retroviruses. It is around 120 nm in diameter (120 billionths of a meter; around 60 times smaller than a red blood cell) and roughly spherical.

HIV-1 is composed of two copies of single-stranded RNA enclosed by a conical capsid comprising the viral protein p24, typical of lentiviruses (Figure 3). This is in turn surrounded by a plasma membrane of host-cell origin. The single-strand RNA is tightly bound to the nucleocapsid proteins, p7 and enzymes that are indispensable for the development of the virion, such as reverse transcriptase and integrase. The nucleocapsid (p7 and p6) associates with the genomic RNA (one molecule per hexamer) and protects the RNA from digestion by nucleases. A matrix composed of an association of the viral protein p17 surrounds the capsid, ensuring the integrity of the virion particle. Also enclosed within the virion particle are vif, vpr, nef, p7 and viral protease (Figure 2). The envelope is formed when the capsid buds from the host protein, taking some of the host-cell membrane with it. The envelope includes the glycoproteins gp120 and gp41, which are derived from the gp160 precursor; gp41 is a transmembrane protein that is covalently linked to gp120 ^[23].

Life cycle of HIV

Viral entry to the cell

The interaction between the gp120, coreceptor and CD4 provokes conformational changes in gp120 that exposes a previously buried portion of the transmembrane glycoprotein, gp41, and allows access of the V3 loop of gp120 to the coreceptor. gp41 causes the fusion of the viral envelope and the host-cell envelope, allowing the capsid to enter the target cell. The exact mechanism by which gp41 causes the fusion is still largely unknown ^[24]^[25].

Once HIV has bound to the CD4+ T-cell a viral protein known as gp41 penetrates the cell membrane and the HIV RNA and various enzymes including but not limited to reverse transcriptase, integrase and protease are injected into the cell.

Viral replication and transcription

Once the viral capsid has entered the cell, an enzyme called reverse transcriptase liberates the single-stranded (+)RNA from the attached viral proteins and copies it into a negatively sensed viral complementary DNA of 9 kb pairs (cDNA) (Figure 5). This process of reverse transcription is extremely error prone and it is during this step that mutations (such as drug resistance) are likely to arise. The reverse transcriptase then makes a complementary DNA strand to form a double-stranded viral DNA intermediate (vDNA). This new vDNA is then transported into the nucleus. The integration of the proviral DNA into the host genome is carried out by another viral enzyme called integrase. This is called the latent stage of HIV infection ^[26].

To actively produce virus, certain transcription factors need to be present in the cell. The most important is called NF-kB (NF Kappa B) and is present once the T cells becomes activated. This means that those cells most likely to be killed by HIV are in fact those currently fighting infection.

The production of the virus is regulated, like that of many viruses. Initially the integrated provirus is copied to mRNA which is then spliced into smaller chunks. These small chunks produce the regulatory proteins Tat (which encourages new virus production) and Rev. As Rev accumulates it gradually starts to inhibit mRNA splicing ^[27]. At this stage the structural proteins Gag and Env are produced from the full-length mRNA. Additionally the full-length RNA is actually the virus genome, so it binds to the Gag protein and is packaged into new virus particles.

Interestingly, HIV-1 and HIV-2 appear to package their RNA differently; HIV-1 will bind to any appropriate RNA whereas HIV-2 will preferentially bind to the mRNA which was used to create the Gag protein itself. This may mean that HIV-1 is better able to mutate (HIV-1 causes AIDS faster than HIV-2 and is the majority species of the virus).

Viral assembly and release

The final step of the viral cycle is the assembly of new HIV-1 virions, begins at the plasma membrane of the host cell. The Env polyprotein (gp160) goes through the endoplasmic reticulum and is transported to the Golgi complex where it is cleaved by protease and processed into the two HIV envelope glycoproteins gp41 and gp120. These are transported to the plasma membrane of the host cell where gp41 anchors the gp120 to the membrane of the infected cell. The Gag (p55) and Gag-Pol (p160) polyproteins also associate with the inner surface of the plasma membrane along with the HIV genomic RNA as the forming virion begins to bud from the host cell. Maturation either occurs in the forming bud or in the immature virion after it buds from the host cell. During maturation, HIV proteases (proteinases) cleave the polyproteins into individual functional HIV proteins and enzymes. The various structural components then assemble to produce a mature HIV virion ^[28]. This step can be inhibited by drugs. The virus is then able to infect another cell. There are two forms of the virus:

Immature form

When the virus leaves the cell it is not infectious and the inner part of the virus particle contains a spherical core (stains dark on electron micrographs). There are spikes on the outer membrane that are the Env proteins (gp120 and gp41) (Figure 6). Sometimes a virus can be seen during the process of budding, when it looks like a dark arc sitting under the cell membrane. The Env proteins link together in groups of three (trimers).

Mature form

Once the virus protease has cleaved the gag proteins, the core rearranges into a truncated cone (like a traffic cone sliced at an angle across the top). Some reports also show a small filament linking the core to the membrane. The envelope spikes are often much rarer on mature particles, because they are easily dislodged. It is the mature conical core that makes HIV easily identifiable.

Treatment

HIV infection is a chronic infectious disease that can be treated, but not yet cured. There are effective means of preventing complications and delaying, but not preventing, progression to AIDS. At the present time, not all persons infected with HIV have progressed to AIDS, but it is generally believed that the majority will. People with HIV infection need to receive education about the disease and treatment so that they can be active partners in decision making with their health care provider.

Transmission

HIV is transmitted through penetrative and oral sex whether vaginal or anal, blood transfusion, the sharing of contaminated needles through drug injection and in health care settings, and between mother and infant during pregnancy, childbirth and breastfeeding ^[29]. The use of physical barriers such as the latex condom is widely advocated to reduce the sexual transmission of HIV.

At the end of 2004 there were between 36 and 44 million people living with HIV, of whom 25 million were in sub-Saharan Africa. Global estimates for new HIV infection in 2004 were 4.3-6.4 million. ^[30].

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