The human immunodeficiency virus
The Human immunodeficiency virus (HIV) is considered a worldwide pandemic by the World Health Organisation (WHO) with major social and economic consequences. Since its discovery in 1981, HIV has become a burden globally with an estimate of 25 million people being killed to date. (WHO Annual Epidemic HIV Estimates). In terms of epidemiology, the prevalence has increased in people who reside in the Sub Saharan Africa and according to current estimates, HIV is set to infect 23 million people in this region. (WHO, 2007)
HIV is transmitted via infected blood products, such as syringes, or by sexual contact. It also may be transmitted via breast milk from an infected mother. Therefore screening of blood products for HIV has largely eliminated transmission through blood transfusions or infected blood products in the developed countries. Other less likely methods of transmission include infection in health care settings where safety procedures and equipment such as gloves and safe disposal of waste product are not so well implemented.
HIV's method of progression is not completely understood, but the distinctive feature is the infection of vital cells in the human immune system. The virus infects macrophages, dendritic cells and helper T Cells, specifically the CD4+ T cells which play the most important role in the virus's progression to acquired immunodeficiency syndrome (AIDS). CD4+ is the prime receptor used by HIV to enter the cell and the 3 mechanisms used by HIV are: the direct viral killing of infected cells; second, increased rates of apoptosis in infected cells; and third, killing of infected CD4+ T cells by CD8+ cytotoxic lymphocytes that recognize infected cells. The constant decrease in the cell count causes the host to lose its cell mediated immunity (immunity by the activation of macrophages, natural killer cells and cytotoxic T cells) and become vulnerable to opportunistic infections. This occurs when CD4+ T cell counts become >200 cells/µg and is now known as AIDS .
The discovery and proper use of highly active antiretroviral therapies (HAART) changed the views of AIDS into a tolerable infection . It became a revolutionary therapy but shows a major flaw; as soon as the treatment is interrupted, viral replication is resumed within weeks leading scientists to believe that there must be a reservoir of virus, which is out of reach for the drugs [3, 4]. This could be because the virus stays in a latent stage.
The current treatment paradigm is based on the modulation of cytokines, namely Interleukins 2, 7, 15 and 21, through the use of recombination techniques [5-8]. These have shown to increase CD4+ counts. IL-2 shows by far the most promising immunomodulant approach and wide-spread research has been done to prove the efficacy and benefits of this cytokine. However despite these strategies, mortality and morbidity still remain high. Therefore current research is focusing on formulating new therapies for the treatment of HIV, such as use of recombinant IL-2 (rIL-2) with controlled antiretroviral therapies (cART) and highly active antiretroviral therapy (HAART) . This has shown promising results and will be discussed further.
Although many new immunotherapeutic agents and drugs are thought to have great potential, most are preclinical or at a very early clinical stage, with the exception of rIL-2. Results of the 2 large phase III clinical trials using rIL-2 with cART and HAART have been published in early 2009 and if these therapies show clinical benefits, licenses and grants will be awarded in the near future for regular use in hospital care.
The aim of this review is to introduce the basic information behind the HIV and the use of IL-2 as a therapeutic tool. First, some background information about the history and epidemiology of HIV will be discussed followed by some background information on the mode of infection and the basic structure of the virus. Current therapies and the role of IL-2 will be reviewed followed by the trials conducted with the use of IL-2 in HIV infected patients. The results of these trials will be explained at the end accompanied by some future work which will be clinically trialled in the near future.
The discovery of HIVwas initiated when unusual observations were discovered in certain groups of people in 1981. Reports of Pneumocystis carinii and Kaposi's sarcoma were shown to be increased in homosexual men and shortly after, in intravenous drug users. This pattern led researchers to believe a transmittable agent with unusual properties may be involved. These 2 diseases are usually found in immunosurpressed people with extremely low CD4+ T Cell count (AIDS) . These findings directed scientists to hypothesize that the aetiological agent responsible for this could be a retrovirus and the only known retrovirus at the time was the Human T lymphocyte virus (HTLV). Therefore scientists in the Montagniers group in Paris biopsied a lymph node from a patient with AIDS. The findings showed a retrovirus was indeed the cause for this but it was not HTLV but a new unknown virus. This new virus was later named the Human Immunodeficiency Virus. It is also important to note the oldest possible documented case of the then-unknown syndrome was in the Royal Infirmary of Manchester (MRI) in 1959 and presented itself in the British printer Davir Carr who travelled in the Navy but never to Africa.
The latest World Health organization (WHO) HIV epidemic update has estimated that a total of 33.2 million people in the world were living with HIV in 2007, with more than half residing in the Sub Saharan countries. The lowest estimates derive from North America and Europe (Fig 2). Since 1998, an annual estimate report has been published by WHO and shows a constant rise in HIV carriers (Fig 1). WHO also estimates a steady increase for future years until a proper cure is found. As development of new treatments continue to progress, HIV continues to evolve and show resistance to them. This causes the trend of the graph to keep increasing until a proper cure or vaccine is introduced.
Mode of Infection
Being a transmittable agent, HIV is spread easily in various ways; the most likely being sharing of blood products such as syringes and sexual contact. HIV acquired through sexual means is due to unprotected sexual secretions coming in contact with the genital, oral or rectal mucous membrane . This is why death by opportunistic diseases was first observed in homosexual men who did not use protection. Later, these diseases were seen in intravenous drug users and haemophilliacs who used syringes . This type of transmission has decreased considerably in the developed world due to screening of blood, improvement in sterilization techniques and better knowledge of diseases [10, 13]. The last major route of transmission is from mother to child during pregnancy, giving birth or during breast feeding. Chances of transmission by this method is as low as 25% with absence of treatment but by the use of antiretroviral drugs and a Cesarian section, the risk of transmission is reduced to only 1% .
HIV's structure is very peculiar as it has none of the structural properties which other retroviruses posses. All of the genetic material of the virus is contained in RNA and each virus has 2 single stranded RNAs, each bonded by a molecule of reverse transcriptase and integrase . The genome also contains the proteins p10 protease and p32 integrase and is surrounded by a nucleocaspid consisting of an inner layer called p24 and outer layer called p17. As the capsid buds from the host cell, a new lipid envelope is formed consisting of the viral proteins gp41 and gp120 . This virus-host attachment causes the virus to form a spiked envelope structure and is thought to be of immense importance in determining the effectiveness of the virus.
Infection by HIV is correlated to a steady decline in CD4+ T Cell count and increase in viral pools. The infection can be divided into 4 stages; the incubation phase, acute infection, latency stage and AIDS. The stage of infection can be determined by measuring the patients CD4+ T Cell count and HIV levels in blood.
The incubation phase is usually asymptomatic and lasts about 2 to 4 weeks followed by the acute infection stage.
In the acute phase, the main receptor by which the virus recognizes the host cell is the CD4+ receptor . The gp120 glycoprotein found on the virus's surface along with the chemokine CCR5 are responsible for the HIV entry in cells. This is accompanied by an activation of CD8 cytotoxic cell which target and kill HIV infected cells, in this case being the CD4+ T cells. Upon entering the cell, the virus sheds its nucleocapsid and the reverse transcriptase initiates transcription of the single stranded RNA into a double stranded DNA . At this point, viral replication increases which leads to an abundance of viral pools in blood close to a few million per mL as the HIV binds and infects the CD4+ T Cells . The viral double stranded DNA is integrated into the hosts cell DNA and this is now known as a provirus. This is the latency stage of the virus and can lie dormant for many years in this stage [10, 17].
In the latency stage, viral replication rate increases with about 30% of the viral reservoirs being eliminated everyday while up to 2x10⁹ CD 4 T cells are destroyed but most are replaced by the immune system . Accounting for all of these, it shows a slow but steady decline in CD4+ T cell counts . It is not entirely understood why the immune system cannot clear the virus completely but could be due to the numerous features the virus possesses such as:
- The very high replication rate making this the major reason why the virus is never cleared away completely.
- It can hide as a provirus making is undetectable for the immune system.
- It has an extremely high mutation rate therefore CD8 cytotoxic T cells and antibodies are made against the new mutated antigens have to keep up with the new mutated antigen.
The end of the latency stage is accompanied by the materialization of a variety of symptons which indicate the development to AIDS . At this point, the proviral DNA is transcribed using the host's synthetic machinery and these particles bud from the cell something killing the host cell and infecting other cells [10, 13, 19].
Symtoms and Complications
The incubation phase is usually asymptomatic and lasts about 2 to 4 weeks followed by the acute infection stage where symptoms such as fever, pharyngitis, rashes, lymphadenopathy (swollen lymph nodes), myalgia (sore muscles), weight loss, recurring yeast infections, inability to control muscles, paralysis, mental deterioration, chronic dry cough, fatigue and mouth and easophageal sores show.
Advanced HIV symptoms are wide ranged and often very unpredictable but can be divided into the following groups:
Neurological symptoms can start at an early stage with minor depressions, confusions and numbness but can lead to more serious diseases such as AIDS dementia complex (ADC), sensory polyneuropathy and aseptic meningitis [9, 18].
Mostly seen in the later stages but are sight threatening. The most common and severe is cytomegalovirus retinitis .
Many of the symptoms presented by HIV infection are due to other pathogens such as fungi or bacteria. These involve Pruritus, Aphthous ulcers, Herpes and Eosinophilic folliculities .
Usually presented in advanced HIV infection, these complications include; lymphopenia, anaemia, neutropenia, pancytopenia and agranulocytosis [9, 18].
HIV associated nephropathy (HIVAN) is quite rare but causes untreatable damage to the renal system. Nephritic syndrome occurs more often but is more treatable with dialysis [9, 18].
Lymphoid interstitial pneumonitis (LIP) though uncommon, can present itself in advance HIV patients. Epstein-Barr Virus is also known to be related to advance HIV patients .
Abnormalities include reduction of testosterone and irregular adrenal secretions .
Most cardiac diseases are actually related to toxicity from HAART treatment rather than the HIV itself. These include lipid dysregulation, cardiomyopathy and necrotic myocarditis [4, 14].
Antiretroviral drugs are primarily used for retroviruses and the clinical use of highly active antiretroviral therapies (HAART) has changed the views of HIV into a tolerable infection. HAART forms the administration of several antiretroviral drugs as a cocktail and early use of this therapy is believed to reduce risk of infection [1, 2]. Current HAART therapies include 2 nucleoside analogue reverse transcriptase inhibitors (NARTI) and either a protease inhibitor or a non-nucleoside reverse transcriptase inhibitor (NNRTI). Both NARTI and NNRTI inhibit the reverse transcriptase in HIV therefore reducing replication rates. Even though HAART is probably the best treatment available for HIV patients, several concerns have arisen in the near past due to the serious side effects the drugs have on the patient such as toxicity . Concerns about interruption in treatment causes virus multiplication to resume, leading scientists to think that there could be a reservoir of virus to which drugs have no access to [3, 20]. Failure of taking the treatment pill is another concern, as one miss can cause the virus to develop a resistance to it. The last concern is the high cost of the drug meaning developing countries which need it most, find it hard to access them.
No public vaccine or cure is currently available for HIV or AIDS. However, a new vaccine which is a combination of two previously failed vaccines have recently become a candidate for trials in early 2009 in Thailand .
Current research has found a role for the use of recombinant interleukins as treatment for HIV patients. The first set data from the two major trials on IL-2 have recently published their findings and will be discussed later in this essay.
Potential role of Interleukin-2 in HIV patients
Interleukins are a type of secreted proteins derived from the cytokine family. Even though these cytokines are secreted by many different cell types, most interleukins play a vital role in the immune system. One of the most important interleukins is interleukin-2 (IL-2). IL-2 is a major T cell growth factor and stimulates the proliferation and function of T lymphocytes (CD4+ T cells) and natural killer cells (NK Cells) . IL-2 is produced during an immune response during a pathogen invasion into the body. The pathogen activates the T Cell Receptors which stimulates the production and secretion of IL-2 [22, 23]. It was observed that the HIV impairs IL-2 production and thus lowering CD4+ T cell counts, leading to the proposition of the hypothesis that exogenous administration of IL-2 could help restore CD4+ T cell counts and the immune system [24-27]. This hypothesis became the foundation upon which the SILCAAT and ESPRIT trials were based on [25, 27].
ESPRIT and SILCAAT
Evaluation of Subcutaneous Proleukin in a Randomized International Trial or ESPRIT began in March 2000 and ended in November 2008. The trial was coordinated by International Network for Strategic Initiatives in Global HIV Trials (INSIGHT) and was a worldwide study. It tested the hypothesis whether using rIL-2 with HAART would reduce the rate of AIDS-related opportunistic diseases and mortality rate more than the HAART alone. The study involved 4112 HIV infected people from both sexes with ages between 18 to 25 years. Exactly half of these patients were injected with 7.5 MIUs of Proleukin (IL-2) two times a day for five consecutive days every eight weeks for at least half a year with HAART and the other half received only HAART. Assessment was done every four months for seven years by recording CD4+ T cell counts, viral load or amount of HIV in blood and signs of illnesses .
Subcutaneous, Recombinant, Human Interleukin-2 in HIV-Infected Patients with Low CD4+ Counts under Active Antiretroviral Therapy or the SILCAAT study began in April 1999 and ended in November 2008. This study followed 1695 HIV infected adults who were assigned in groups to receive 4.5 MIUs of Proleukin. The administration was exactly the same as in the ESPRIT but for an entire year instead of six months and the assessment was the same in both trials as well .
Results & Further Studies
Results from the two phase III clinical trials astounded everyone in early 2009. Even though results showed that all volunteers who received antiretrovirals along with IL-2 presented a sustained and marked increase in CD4+ T Cell counts as predicted, the increase in CD4+ T Cell counts did not contribute to any decrease risk of HIV-associated opportunistic diseases or death. Both volunteers, with and without IL-2 administration showed same probability of attaining any opportunistic diseases, confirming that IL-2 administration cannot revoke damages done by HIV to its host.
But the trials were not a total failure; the director of the National Institute of Allergy and Infectious Diseases (NIAID) Dr. Anthony S. Fauci noted ‘Although further analyses may help us better understand these findings, the two studies clearly demonstrated that the use of IL-2 did not improve health outcomes for HIV-infected people.'
The results of the trials baffled scientists around the world and no reasonable conclusion has been made as to why an increase in CD4+ T Cell counts did not translate into a decreased risk of attaining a HIV-associated opportunistic disease. Some theories have postulated but none have been proven. Dr. James D. Neaton suggests, “It could be that the types of CD4+ T cells induced by IL-2 play no role in protecting the HIV-infected patient, and therefore the administration of IL-2 has no benefit. A second possibility is that the CD4+ T cells are at least somewhat functional or that IL-2 has some modest benefit, but that the side effects of IL-2 may neutralize any possible benefit."
Further studies on other cytokines have begun, namely IL-7, IL-15 and IL-21. IL-7 is the vital cytokine in maintaining CD4+ and CD8+ T cell counts and in primate studies. Il-7 administration has shown increase in both CD4+ and CD8+ T cell pools . Administration were on alternative days for 14 days by small dose-escalation of rIL-7 in refractory cancer patients and both CD4+ and CD8+ T cell pools expanded in a dose-dependant manner .
IL-15 has also sparked curiosity and could have potential in becoming a therapeutic treatment for HIV. It has been shown that IL-15 is a potent inducer of CD4+CD235hi forkhead box and thus might have an effect on decreasing immune activation to secondary HIV [7, 29].
IL-21 is the most recent cytokine which became an interest to scientists. IL-21s activity shows increase in proliferation and cytotoxic activity of NK cells and some even postulate that it shows promising future in becoming a vaccine or an immunotherapeutic tool for HIV infected patients . IL-21 shows potential to be co-administered with IL-2 for clinical benefits although it is just a theory still
Although these cytokines show potential, all are in preclinical phase but IL-2. Results from the IL-2 trials were disappointing and unexpected, although definite and real answers have finally been found.
Apart from cytokines, vaccines have also become of great interest. Prophylactic vaccines have been tested for more than a decade. These vaccines aim to provide protection to individual who have not been infected by the HIV .
HIV's disastrous effect on the human population is still being witnessed by too many since its discovery. Due to the easy method of transmission and the special properties the virus posseses such as high mutation rates and being able to hide against administered drugs have made it one of the most difficult viruses to study and treat.
One of the major breakthroughs in HIV related treatments and therapies came by the proper use of HAART making the infection seem much more bearable. Even though HAART shows a radical change in patient health and life expectancy, it also comes with a price for the patient. The therapy is highly toxic, expensive and cannot be interrupted.
HIV primarily affects CD4+ T Cells thus weakening the immune system. It was observed that Interleukin-2 plays an important role in stimulating and proliferating CD4+ T cell counts thus leading scientists to postulate that administration of Interleukin-2 into a HIV infection patient could restore the immune system to its original state. This hypothesis became the bases of the worldwide trials ESPRIT and SILCAAT.
ESPRIT and SILCAAT both finished in November 2009 and although both could not find any type of cure or proper treatment, a definite answer about the utility, effects and role of IL-2 have been successfully reached and more importantly, they have opened doors for future potential research on cytokines. Even though most clinical trials on other cytokines will finish in more than a decade's time, the theories and hypothesis behind each trial seem to have given hope to many.