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HIV regulatory protein Tat


The HIV regulatory protein Tat oversees not only production of new virus, but also the synthesis of chemicals that lure unsuspecting immune cells toward a viral ambush, say US-based scientists (Nature Med 2003, 9:191–197). Drawn by the chemicals to infected cells, healthy CD4 T cells and other targets are more likely to encounter newly-minted HIV particles and become infected—speeding the spread of virus.

`‘Efforts to develop drugs against the HIV Tat protein could enhance our therapeutic tools against AIDS’', says Anna Aldovini of Harvard Medical School, who led the research team. Scientists have known for a few years that Tat controls transcription of HIV genes. Its newfound role in recruiting immune cells suggests that disabling the protein would deliver a therapeutic double whammy by crippling both replication of the virus and its dispersion in the body.

Aldovini and her colleagues discovered the seductive influence of Tat by investigating 7070 genes in infected and uninfected human immature dendritic cells. These sentinels of the immune system inhabit mucosal tissue and are among the first cells infected by HIV.

Using high-density DNA microarrays, the team found that HIV infection upped the activity of 191 dendritic cell genes, of which 33 were controlled by Tat. Four of those genes govern production of chemicals known to influence HIV target cells: interferon inducible protein-10, human monokine induced by interferon-γ, and monocyte chemoattractant proteins MCP-2 and MCP-3. Amounts of these chemokines increased in cultures of dendritic cells after the scientists infected them with HIV or an adenovirus that made only Tat. However, none of the four chemicals appeared in cultures of dendritic cells that expressed only Nef, another HIV regulatory protein.

The researchers also found evidence of Tat-orchestrated chemical attraction in vivo lymph nodes of SIV-infected rhesus macaques contained higher levels of MCP-2 (the easiest of the four chemokines to stain in tissue cultures) compared to lymph nodes of HIV-free monkeys.

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Women and infants transmission of HIV

Even if a woman has received antiretroviral therapy (ART) during pregnancy, a high level of HIV lurking in her genital tract increases the likelihood that she will pass the virus to her baby, say researchers with the Women and Infants Transmission Study (J Infect Dis 2003:187, 375–384). They found that the risk of transmission during vaginal delivery or unplanned Cesarean section doubled with each 10-fold increase in viral DNA, or cell-associated HIV-1, found in genital tract secretions.

The research team compared samples washed from the genital tracts of 26 women who transmitted HIV to their babies and 52 women who did not. All but one of the mothers had undergone ART treatment. Subsequent analysis for viral RNA and DNA in the cellular material, and viral RNA in the separated liquid, found ‘‘a positive association between genital tract virus and vertical transmission for all the women’', says study leader Ruth Tuomala of Brigham and Women's Hospital in Boston. However, levels of genital tract HIV did not correlate with the amount of HIV measured in the blood of the women.

The findings point to the need for further studies of genital tract HIV, says Tuomala. If cell-associated virus, and thus cell-to-cell transmission, prove to play a key role in infant infection, the use of inexpensive microbicidal or anti-inflammatory agents during childbirth may curb mother-to-child transmission in developing countries.

In developed countries, where ART has cut vertical transmission to low levels, determining which antiretroviral drugs best control HIV in the genital tract might better protect newborns as well as prevent the appearance of resistant virus, says Tuomala. ‘‘Even with pretty good suppression of virus peripherally, there can be healthy and replicating virus in the genital tract’', she says. ‘‘We might find that even though we control peripheral virus with ART, virus in the genital tract becomes resistant. Then, there could be an upswing in vertical transmission.’'

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In vivo model for testing anti-HIV vaginal gels

With four anti-HIV vaginal gels scheduled to enter Phase III trials later this year, dozens of microbicides that more specifically target and disable HIV in vitro wait in the wings. A key problem, however, has been a suitable in vivo model for testing them. In February, a team of researchers in the US and Britain reported successfully demonstrating the effectiveness of one of the new crop of microbicides, human monoclonal antibody b12, using rhesus macaques and a potent simian–human immunodeficiency virus hybrid known as SHIV-162P4 (Nature Med 2003; 9:343–346).

The study is ‘proof of concept’ for both b12 and the model. ‘‘This is the first demonstration that a topically applied antibody that specifically targets HIV can prevent mucosal transmission’', says co-author Robin Shattock, an immunologist at St. Georges Hospital Medical School in London. ‘‘But b12 is only one in a wide armory of potential inhibitors that can go in this model and be evaluated.’'

The combination of progesterone-treated macaque monkey and SHIV-162P4 is a slightly new twist in in vivo testing. Hormone treatment renders a monkey more susceptible to infection by thinning its vaginal lining. Such monkeys were used to test the vaginal microbicidal gels currently being evaluated in humans. However, the researchers, led by John Moore of Cornell University, exposed the animals to a laboratory-made virus that was not widely available when the earlier microbicides were tested.

SHIV-162P4 is an SIV with an HIV gene spliced into it. The inserted gene hails from an R5 strain of HIV and produces the viral envelope protein, gp120.

Like the strains of HIV most often transmitted from person to person, R5 viruses target immune cells with both CCR5 and CD4 surface molecules. So thanks to its R5 lineage, the gp120 protein of SHIV-162P4 homes in on CCR5- and CD4-bearing immune cells, too.

The b12 antibody, which binds to gp120, prevents it from docking with CD4 receptors and entering cells even, as Shattock and his colleagues found, in vivo. However, the protection is both time- and dose-dependent. Only three out of 12 monkeys became infected after receiving 5 mg of b12 vaginally then being exposed to the virus up to 2 h later. Antibody doses of 0.2 mg and 1 mg proved less effective. And a macaque treated with 5 mg of b12 but exposed to the virus 6 h later also became infected.

Despite the apparent effectiveness of b12, the research team refrains from championing it. Other molecules that target the fusion and attachment process of HIV may prove better candidates, says Shattock. He points out that a major obstacle with b12 or other antibodies is producing them economically in the huge quantities needed worldwide for use in highly concentrated vaginal gels. One solution is harvesting them from genetically modified plants, such as corn. In January, the California-based biotech company Epicyte announced that it is growing the first greenhouse plant lines to yield HIV-blocking antibodies. The company plans to produce three human IgA antibodies, 2G12, 4E10 and 2F5, all of which bind HIV envelope proteins critical to infection.

Charlene Crabb

© 2003 Lippincott Williams & Wilkins, Inc.