Virtually all of the anti-HIV approaches now available or in development are effectively designed to prevent the infection of new human cells by reducing the levels of HIV virus particles in the body. Some researchers, such as Professor Jay Levy, argue that it may be more effective to develop treatments that attack human cells that have already been infected by the virus.

Levy points out that infected cells, particularly in the lymph nodes and other tissues, are responsible for the steady production of new virus. Some infected cells survive for very long periods, churning out new HIV viruses. Current anti-HIV drugs target these newly produced viruses, reducing the number of new cells they infect, but seem to have little effect on the cells that are already infected. In other words, the drugs are attacking new viruses as they come off the production line, but doing little to stop the machinery that is manufacturing them in the first place.

Levy also argues that HIV-infected cells produce abnormal levels of cytokines or emit HIV proteins that disrupt the immune system. Moreover, it is probable that HIV is usually transmitted from person to person by the transfer of HIV-infected cells, which can survive longer in someone else's body, rather than by the transfer of free-standing virus particles.

New treatment approaches should investigate ways of boosting the parts of the immune system that attack infected cells, such as natural killer cells and CD8 T-cells, according to Levy.

Caspase and protein transduction therapy

One approach to killing HIV-infected cells is to use genetic engineering to produce a protein that is a combination of a human enzyme and the HIV protein Tat. The human enzyme, called caspase-3, is a type of protease that can trigger the cell to commit suicide or 'apoptosis'.

Since Tat is very efficient at crossing the human cell's membrane, the caspase-3-Tat hybrid is taken up by human cells. Once inside the cell, HIV's protease enzyme cuts up the caspase-3-Tat construct to release caspase-3. Once released, it triggers the cell to commit suicide.

Test tube experiments have shown that this is a highly specific and efficient method of killing of HIV-infected cells, and causes no harm to uninfected human cells. It may be possible to administer such a treatment to humans by injection at irregular intervals, and it is unlikely to cause adverse effects. There is also little potential for resistance, because like hydroxycarbamide, protein transduction therapy acts on the cell rather than the virus, removing any direct selection pressure that might encourage the emergence of drug resistant virus. The final attraction of this method is that it would result in a relatively cheap form of therapy, easily administered and less complex to monitor than current therapy.

Cyclin dependent kinase inhibitors

Since HIV infection affects the structure of normal T-cells, infected cells could be more vulnerable than uninfected cells to inhibitors of specific cellular enzymes. One example is cyclin dependent kinase (CDK) inhibitors, more than 30 of which have been tested against chronically infected and uninfected T-cells in the test tube.

Several CDK inhibitors, including two purine analogs, one pyrazine derivative, and one paullone derivative, were ten- to 80-fold more toxic to infected than uninfected cells, and six compounds showed moderate selectivity of between four- and tenfold. Some compounds that were not effective alone showed selectivity when used in combination, suggesting that simultaneous inhibition of several different CDK may be required for selective killing of HIV-infected cells. The most potent compounds showed anti-HIV activity in primary T-cells and macrophages. Studies of CDK inhibitors and related molecules are continuing (Hesselgesser 2004).

Immunotoxins

An immunotoxin is a substance which damages the immune system. Man-made immunotoxins may be made up of toxic chemicals attached to immune proteins such as antibodies.

Scientists are investigating whether immunotoxins can be used to target inactive HIV-infected cells. These can be recognised by the presence of specific receptors on the cell surface called CD45RO. These 'memory cells' are a major latent viral reservoir in HIV-infected people. The presence of viral reservoirs hidden within human cells prevents viral eradication using antiretroviral therapy.

An anti-CD45RO immunotoxin can reduce the number of HIV-infected CD4 T-cells obtained from HIV-infected individuals with detectable HIV in their blood. It can target both those cells which are actively producing HIV and those which are latently infected and thus not producing HIV. Anti-CD45RO immunotoxin can also reduce the number of HIV-infected CD4 CD45RO T-cells when no detectable virus is present, according to a laboratory study, but it only has a modest effect on other types of immune memory cells (Saavedra-Lozano 2004).

Although preliminary, these results suggest that it may be possible to kill inactive HIV-infected CD4 T-cells in people taking antiretroviral therapy using the anti-CD45RO immunotoxin without seriously compromising other memory immune responses.

The anticonvulsant drug valproic acid (Depakote) could also be used in future clinical trials seeking to flush out HIV from viral reservoirs (Margolis 2004).

The clinical implications of reducing or removing this viral reservoir are unknown.

Surgical removal of viral reservoirs

Two research groups have reported that the scale of HIV infection of lymph nodes in early and asymptomatic HIV infection is far more limited than hitherto believed, raising the possibility that removal of the viral reservoirs provided by selected lymph nodes in asymptomatic individuals may allow longer treatment interruptions.

Both groups used a technique called positron emission tomography (PET) scanning to identify lymphoid tissue that was packed full of activated lymphocytes. These white blood cells are drawn to lymph nodes that are focal points of HIV infection. PET scanning is able to identify activated lymphocytes by tracking levels of radiolabelled glucose, which is preferentially taken up by activated lymphocytes.

Lymph nodes are the main reservoir of HIV infection, and PET scanning has revealed that people with early or non-progressive HIV infection have little sign of HIV activity in lymph nodes below the chest. Outside the lymph nodes, PET scanning could not detect substantive levels of lymphocyte activation, indicating that the vast majority of virus activity in the body is concentrated in the lymph nodes and lymphoid tissue such as the spleen, and that it is concentrated in the lymph nodes in the neck and upper torso. Consequently, both research groups suggest that surgical removal of lymph nodes may be an option.

The findings also raise interesting questions about the targeting of HIV therapy, particularly the possibility of delivering drugs much more intensively to lymphoid tissue in preference to other parts of the body. For example, one research group have suggested that selection of R5-tropic virus during earlier HIV infection reflects not only a selective advantage in the presence of relatively intact immunity, but an environmental advantage in the setting of peripheral lymph nodes. They have postulated that targeting of the lymph nodes may result in greater reduction in viral burden in less advanced HIV disease. This more specific targeting of the burden of activated cells might be coupled with strategies to activate latently infected cells, such as the combination of immunotoxin and interleukin-7 stimulation.

The ultimate aim of these approaches would be to reduce the viral burden so that any treatment interruption would result in a smaller viral rebound. The next experiments in this direction will need to look at whether there is a correspondence between the degree of lymph node activation, viral burden in the lymph nodes on treatment and the degree of viral rebound after treatment is interrupted. If a correlation is shown, the next step would be to test whether targeting the lymph nodes, either by surgery or treatment, would reduce the extent of rebound when treatment was stopped.

References

Hesselgesser J et al. Selective Killing of HIV Infected Cells by Small Molecule Cyclin Dependent Kinase Inhibitors. Eleventh Conference on Retroviruses and Opportunistic Infections, San Francisco, abstract 546, 2004.

Iyengar S et al. Anatomical loci of HIV-associated immune activation and association with viremia. Lancet 362: 945-950, 2003.

Levy JA et al. HIV research: a need to focus on the right target. Lancet 345: 1619-1620, 1995.

Margolis DM et al. Coaxing HIV-1 from Resting CD4+ T Cells: Valproic Acid Induces Latent Viral Expression. Eleventh Conference on Retroviruses and Opportunistic Infections, San Francisco, abstract 427c, 2004.

Saavedra-Lozano J et al. An anti-CD45RO immunotoxin kills HIV-latently infected cells from individuals on HAART with little effect on CD8 memory. Proc Natl Acad Sci 101: 2494-2499, 2004.

Scharko AM et al. Whole body positron emission tomography in patients with HIV-1 infection. Lancet 362: 959-961, 2003.

Vocero-Abkani AM et al. Killing HIV-1 infected cells by transduction with an HIV protease-activated caspase-3 protein. Nat Med 5: 29-33, 1999.