The HIV-1 virus has evolved to exploit the human immune system and counteract all immune defenses mounted against it. Many different vaccination approaches and therapy treatments have been attempted to control, eliminate, or prevent HIV-1 infection, however, these attempts have not proved successful for virus elimination from the body.
One approach that has been used to combat HIV-1 infection is the use of ART, or antiretroviral therapy, and this approach has proven to be effective at reducing viral levels in the blood. ART suppresses HIV-1 viremia to undetectable levels within 12-48 weeks in a majority of patients. This therapy is a “cocktail” or combination therapy of three or more drugs, including 2 nucleoside-based reverse transcriptase inhibitors (prevents production of the virus genome by inhibiting RNAàDNA transcription), and one or more of the following drugs: non-nucleoside reverse transcriptase inhibitors, membrane fusion inhibitors, viral protease inhibitors, or integrase inhibitors. Each of these drugs targets a specific part of the viral lifecycle, making it unable to replicate and infect other cells. Because each drug is mutually exclusive and targets a different area of the lifecycle, it is unlikely that a virus will develop resistance to all three drugs. Therefore, because the virus does not produce escape mutants from the therapy, this approach is effective at reducing viral levels in the blood (ART information) (more on ART) However, by the time ART is put into effect, the virus has already established residence in T cells in the body, and can continue to infect other cells undetected by the drug by avoiding exposure in the blood via direct cell-to-cell infection. Another problem with the use of ART for HIV-1 treatment is that it must be administered indefinitely, and if it is discontinued, there is a rapid rebound of viremia, or viral infection of the blood. Taking ART indefinitely is undesired due to negative side effects (common ART side effects) and resistance to the drugs that develops overtime due to mutation.
Another approach to combat HIV-1 infection has been the use of monoclonal neutralizing anti-HIV-1 antibodies, however, in the past, these antibodies have proven to be ineffective at controlling infection of humanized mice (hu-mice) (Hu-mice as the murine model for the analysis of human hematolymphoid differentiation and function). However, as of late, more potent antibodies have been uncovered that have decreased viremia in hu-mice and have longer half lives. Antibodies that target gp120 glycoprotein in the HIV viral envelope have been studied as a potential means of neutralizing HIV infection. Gp120 binds to CD4 on T cells, enabling initial viral-cell contact before membrane fusion and insertion of the viral genome (more on gp120).
In a recent study conducted by Horwitz et al (2013) (published in the Proceedings of the National Academy of Sciences of the USA: HIV-1 Suppression and durable control by combining single broadling neutralizing antibodies (bNAbs) and antiretroviral drugs in humanized mice), three antibodies targeting different epitopes of gp120 were used in combination with ART and the effects were analyzed. The three antibdies were 3BNC117 (a potent CD4 binding site antibody with a long half life), PG16 (which recognizes the V1/V2 loop region), and 10-1074 (which targets the base of the V3 stem).
First, HIV-1 infected mice were treated with all three antibodies for 6 weeks, 2-3 weeks after infection, and viral loads were monitored by quantifying RT (reverse transcriptase). They also measured the total HIV-1 DNA in human lymphoid cells. All mice showed rapid and continuous viral load suppression, as well as decreased cellular HIV-1 DNA. It was concluded that combined immunotherapy with all three antibodies was sufficient to control plasma HIV-1 RNA levels and cellular HIV-1 DNA levels in hu-mice.
Next, they tested whether or not these highly potent bNAbs could prolong the suppression of HIV-1 in mice treated with ART, and whether immunotherapy could prevent viral rebound after stopping ART. Mice were treated for 3 weeks with ART, then they were given a bNAb trimixture, and continued the immunotherapy for 4 weeks after ART termination. Mice that were given the bNAb therapy did not have viral rebound after ending ART, unlike mice treated with ART alone, who experienced rapid viral rebound upon stopping treatment. To see if the viremic control after stopping ART was due to the antibodies, authors obtained gp120 sequences from mice, which demonstrated that although some clones had developed resistance to one of the three antibodies, no gp120 clones showed resistance to all three antibodies! Therefore, it was concluded that the potent bNAb trimix protected against rebound after ART was discontinued.
The research team next tried to determine whether a single bNAb could prevent against viral rebound when viral load was initially suppressed with ART. They administered ART, added one of the bNAbs being studied, and after 3-4 weeks, terminated ART while continuing monoclonal Ab therapy alone. The infection remained controlled in 50-86% of mice, whereas it rebounded quickly in those without any monoclonal therapy. Next, they sought to determine whether the viral escape from monoclonal Abs was due to specific mutations in the Env gene of HIV-1, the gene that encodes the gp120 protein. The authors cloned and sequenced gp120 from HIV-infected mice that had rebounded the drug after or during immunotherapy, and found that sequences from mice that rebounded during therapy had specific mutations that made them antibody-resistant, whereas mice that rebounded after immunotherapy had no antibody-resistant mutations, and remained sensitive to neutralization by antibodies.
This study shows that bNAbs may alleviate some of the burdens associated with lifelong treatment of ART because of their longer half-lives, ability to eliminate infected cells, and ability to control viremia after ART termination. Future studies may look into the potential side effects of long term administration of bNAb immunotherapy, as well as the discovery of other bNAbs that could prove successful in controlling HIV-1 infection. The trimixture-immunotherapy combined with ART should be tested in clinical, human trials to see if it has an effect on the patient progression to AIDS or lifespan of HIV-1 infected individuals.
Horwitz J.A., Halper-Stromberg A., Mouquet H., Gitlin A.D., Tretiakova A., Eisenreich T.R., Malbec M., Gravemann S., Billerbeck E., Dorner M., Büning H., Schwartz O., Knops E., Kaiser R., Seaman M.S., Wilson J.M., Rice C.M., Ploss A., Bjorkman P.J., Klein F., Nussenzweig M.C. (2013) HIV-1 suppression and durable control by combining single broadly neutralizing antibodies and antiretroviral drugs in humanized mice. Proc Natl Acad Sci USA 110:16538-16543.