Helper-T-lymphocyte cells are white blood cells that stimulate cytotoxic T lymphocytes (CTL), which destroy foreign pathogens. These cells also help other lymphocytes respond to an antigen. The importance of helper-T-cells is demonstrated by the pandemic known as AIDS (acquired immunodeficiency syndrome). This disease is caused by HIV (human immunodeficiency virus), which can infect and lyse these cells, rendering the body susceptible to normally harmless infections.

In 1981, the first cases of AIDS were recognised shown by a crippled immune system. Luc Montagnier identified the cause two years latter as HIV. The main form' are HIV-1 and a less common HIV-2 which exist at this time, both are related to HTLV, which causes a rare leukaemia and is used in some HIV trials(l).

HIV

HIV itself does not kill the patient; it weakens the immune system allowing other infections to develop. At present there is no cure so the virus continues to spread on a worldwide scale. The United Nations Global Program on AIDS estimates that more than 30 million people are currently infected by HIV, with over 5 million infections in the past year(2).

HIV is a class VI retrovirus containing an RNA genome, this RNA is converted to DNA within the host cell via the enzyme reverse transcriptase. HIV comprises of a nucleoprotein core surrounded by a lipid-envelope containing the viral surface(gpl20) and transmembrane (gp41) env glycoproteins. Gpl20 contains viral determinants that bind to host-cell surface receptors. GP120 contains viral determinants that bind to host-cell surface receptors. Gp41 contains an N-terminal hydrophobic domain, which initiates the process of virus-cell membrane fusion, the transmembrane cytoplasmic tail domains anchor env in the lipid bilayer. The nucleoprotein core of the viron comprises of two copies of viral genomic RNA and associated tRNA, with the structural proteins gag and pol(2).

HIV recognises 2 cell surface receptors principally a cell surface protein called CD4+ found exclusively on helper-T-cells, it then needs to bind an accessory receptor P-chemokine. However there is evidence of another receptor involved as both CD4+ and CDS+ cells are infected, i.e. CXCR4(3). Once bound the viral protein p41 allows insertion of the amino-terminal head in to the host cell's membrane. During penetration the viri loses its glycoprotein envelope, the RNA genome is then released directly into the cytosol.

Treatment of HIV Infection

In a search for treatment there has been fairly successful combined therapy, however this only delays the disease and is confined to the western world when it is more needed in the third world. AIDS is a much greater issue in third world countries due to lack of knowledge on provention, combined with cultural issues to do with use of condoms. As such the disease is much more widespread in these countries than in the western world therefore the need in these countries is much greater than in the West. Irronically most development has been for complex and expensive drugs, far from the grasps of the poorer nations. Combined therapy consists of reverse transcriptase inhibitors and a protease inhibitor that stops the viral polypeptide being spliced (see Table 1). The main aim for combating HIV is a cheap effective vaccination program as historically this has been shown to drastically reduce disease e.g. irradiation of smallpox.

Table 1 (35)

Drugs most commonly prescribed

  • Protease inhibitors
  • Crixivan (Indinavir)
  • Fortovase and Invirase (Saquinavir)
  • Norvir (Ritonavir)
  • Viracept (Nelfinavir)

Non-nucleoside reverse transcriptase inhibitors

  • Rescriptor (Delavirdine)
  • Sustiva (Efavirenz)
  • Viramune (Nevirapine)

Nucleoside reverse transcriptase inhibitors

  • Videx (Didanosine, also known as DDI)
  • Epivir (Lamivudine, also known as 3TC)
  • Zerit (Stavudine, also known as d4T)
  • Hivid (Zalcitabine, also known as DDC)
  • Retrovir (Zidovudine, also known as AZT or ZDV)
  • Combivir (Lamivudine and Zidovudine)

There are two main immune systems, which work together to fight infections; these are the humoral and the cellular response. The first producing antibodies by the B-cell, the latter involving T cells including CTL. Initial vaccine's centred on antibodies to fight the infection. More recently it has been found that CTL response has a very important role, ideally both need to be combined.

What Stands in the Way?

Some of the barriers preventing the development of a successful vaccine are, firstly sequence variation. This variation is due to the lack of proofreading in reverse transcriptase combined with the high turnover of new particles i.e. 1010 new virons per day (4). This leads to a huge diversity, all of which cannot be combated at once. Another fundamental barrier is the lack of information regarding what type of immune response may protect against HIV. Past successful vaccines were dismissed by further mechanism research, once they proved effective. This leaves us with a poor understanding of the mechanisms involved in immune responses to viruses, as well as mucosal transmission, which is poorly understood.

Once the HIV genome has entered the cell it incorporates DNA in to the host's genome usually replicating straight away, but it can lie dormant i.e. ‘latent’. If the DNA is inactive then it is difficult to detect as there are no proteins to detect. Approximately <l % of infected cells are latently infected, even following antiretroviral therapy(5). This helps to explain how patients seem to go in to remission for several years before AIDS manifests. Additionally infection of CDS+ cells appears to be associated with this phenomenon as they are infected much latter(6). To compound these problems there is also the lack of financial resources, therefore the aim is a cheap vaccine and would eventually replace the expensive drugs. There are also calls to stop animal testing, this unfortunately would bring safe vaccine development to a near standstill.

Research

Evidence supporting antibodies role in vaccine protection includes their ability to prevent the cellular infection compared to CTL, which acts after infection. It has also been shown that antibodies can neutralise several HIV isolates(5,8). SIV trials in animals show an increase in antibodies following the original infection correlating with protection (9). The ability of IgG(antibody) from an infected monkey to delay progression in another when injected prior to an SIV infection(10). The ability of monoclonal antibodies to protect mice from two injections of HIV-1 isolates(11). Lastly the protection of a chimpanzee from HIV-1 isolates by an anti-gpl20 V3- specific monoclonal antibody (12).

Evidence supporting the CTL response in immunisation is shown by some experiments done in vivo. Firstly it was shown HIV-specific CTL develops before antibodies are detectable (13). Secondly HIV-1 specific CTL is able to inhibit viral replication in vitro(14) and therefore CTL also selects for the evolution of escape mutants(15). Further evidence comes from the fact that SIV-specific vaccination results in lower viral numbers when infected with SIV(16). Protection of macaques immunised with pathogenic SIVmac occurs in the absence of specific antibodies and is linked to an SIV-specific CTL response(17).

Approaches Taken for Immunisation Against HIV

Immunisation requires the body to recognise the infectious agent so the first problem in vaccination is delivering a part of HIV to the body successfully and without infection. There are several methods, some of which have been isolated and discussed below.

  • Recombinant subunit vaccines are subunit proteins/parts of the membrane of the virus being derived from gpl60 and gpl20. The advantage being relative safety, however viral proteins are not in their native conformation so are not as effective in protecting against actual infection of HIV.
  • Live recombinant vaccines mimic antigen presentation that occurs during- viral infection. These present the antigens in their natural form and give a more prolonged delivery of antigen. The genome of HIV is 'attenuated' i.e. remove necessary genes and then inserted in to a vector to deliver the HIV genome producing proteins in the host. Examples of vectors include poxvirus(IS), adenovirus(19), salmonella(20), poliovirus (21), mycobacteria (22), influenza (23) and listeria (24).
  • Whole inactivated HIV immunisations use HIV itself, which has been inactivated. The disadvantages are the proteins are not necessarily in their correct conformation after inactivation. Although there have been questions about safety, clinical trials using this method have reached phase III in humans and show signs of increased helper-T-cell responses (25).
  • Pseudovirons are replication-incompetent viruses produced in mammalian cells that contain all the viral proteins required for viron assembly, but do not contain the viral genome i.e. are non infectious. It has been shown these could be used since the proteins produced can self-assemble and be purified to produce the antigens(26). Testing of these particles have begun, in both infected and non-infected human volunteers.
  • Peptide based vaccines target specific epitopes that lie in conserved areas of the virus. However without more detailed information on the immune response mechanism these have a limited use. The most effective peptide-based vaccines use the V3 loop, however the loop has only had serious impact when combined with other drugs such as PPD (28). It was shown that this loop vaccine initially protected chimpanzees, but eventually developed AIDS anyway(29).
  • Live attenuated HIV historically gives the most effective immunity. However the concerns of safety are most vocal in this area since live HIV is used. In SIV testing attenuation by nef deletion has been the most effective approach so far(30). However to be safer more attenuation is required(31) which reduces resemblance to the viral particles and so is less effective. Another approach is to include a suicide gene into the virus making it susceptible to anti-viral agents (32).
  • DNA-based vaccines have been very successful in influenza vaccines. Attempts to generate a CTL response to a HIV envelope with a DNA vaccine have been successful in a mouse model (33). Recently more successful responses from antibody and T- cells(34) have been achieved in non-human primates. These vaccines seem to offer the greatest promise of a cure to date
  • Another potential method involves gene therapy, inserting genes into bone marrow of infected patients to be incorporated in helper-T-cells to produce RNA ribozymes. These are specific to the HIV genome and are able to cut it in nine precise sites, rendering it ineffective. If this insertion were permanent it would infer life long protection. Tests are hoped to start soon which will use inactivated HIV as a vector that naturally infects these cells(35). This may be the only long-lasting treatment due to the ability of HIV to lie dormant for years before manifesting itself again.

    Did we inherit a virus?

    Retroviruses can incorporate themselves in a cell's genome, so possibly be genetically inherited. Some experts suggest as much as 1 % of the human genome is composed of retroviruses. These normally lie dormant like HIV, but can be switched on by an unusual event. 70% of infected people have an ancient active retrovirus, HERV-K, compared with 3% of uninfected people. It has been suggested HIV switches on this gene product helping HIV evade effective treatment long enough for it to develop an escape mutant of that treatment(36).

    Summary

    HIV has presented the most formidable challenge from a virus to date. It is a highly versatile virus and can replicate at an impressive speed generating more diversity than the body's immune system can accommodate. The immune system also attacks itself whilst trying to irradiate the virus. It also appears that our own genome is working against us with ancient retroviruses lurking within our genome. So far vaccine knowledge has improved but is still far from producing a safe effective vaccine. It appears that the only way to combat the virus is to find regions of DNA that are highly conserved enough to still be present in 'escape mutants' and use these as markers to find the virus/infected cells. Or is it possible a different approach is required for effective treatment as shown by gene therapy. This however is too expensive for use in the third world were the diaease is so widespread, resulting in a greater need for treatment. As such, it would seem that the more responsible thing to do would be to continue work on a cheap vaccine which could be widely distributed; as opposed to a more expensive and possibly more effective vaccine, available only for those present in the wealthier western world.

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