Neutralizing HIV Tat-specific antibodies might be involved in protection
By Andreas von Bubnoff
When talk turns to the kind of neutralizing antibodies that can prevent HIV infection, it always revolves around one and just one HIV protein: the Envelope (Env) protein that forms the viral spike. That’s because Env is thought to be the only protein HIV carries on its surface that is exposed to antibody targeting.
But a recent study led by Ruth Ruprecht at Harvard Medical School suggests that antibodies to the HIV protein Tat, which jumpstarts the expression of HIV genes and is secreted by infected cells, might also be involved in protection (J. Virol. 2013, doi: 10.1128/JVI.02888-12).
The researchers studied the antibody responses of rhesus macaques they had immunized by intramuscular injection of a vaccine that contained the SIV proteins Gag and Pol, and HIV-1 Env and Tat. They then challenged the animals with an SIV/HIV hybrid virus (SHIV) that contained these same proteins, except for Env which had been derived from a different HIV strain. After five low-dose rectal challenges, 16 of 17 unvaccinated animals got infected, while four of the 12 vaccinated animals remained virus free.
In a previous study of the same animals, they had reported that the protected animals differed from the ones that were not protected in that they had higher titers of neutralizing antibodies as well as cellular immune responses to Gag and Tat (PLoS One 6, e22010, 2011).
In the new study, Ruprecht and colleagues used an unbiased approach to see how the antibody repertoires in vaccinated animals that were protected differed from vaccinated monkeys that were infected: They mixed the serum of one protected animal with a library of phages that carried billions of different random peptides on their surface. They then discarded phages that bound to antibodies from an unprotected vaccinated animal. By repeating these steps, they isolated phages that only bound to the antigen binding sites of antibodies in the serum of the protected, but not the unprotected, animal.
As expected, they found that some of these phages carried Env-related peptides. But they also found peptides that were identical to parts of HIV Tat that are targeted by antibodies that neutralize Tat activity; what’s more, they found high levels of antibodies that bind to these peptides in all but one of the protected vaccinated animals, but not in the unprotected vaccinated animals. This suggests that neutralizing antibodies to Tat were in part responsible for the protection of these macaques.
One possible mechanism of protection, Ruprecht says, comes from a study published late last year that was led by Barbara Ensoli at the National AIDS Center in Rome, Italy (PLoS One 7, e48781, 2012). Ensoli and colleagues reported that HIV particles carry Tat molecules on their Env spikes. These Env-bound Tat molecules enable HIV to use an unusual receptor called integrin to infect dendritic cells, which can then transmit the virus to its main target cells, the CD4+ T cells. What’s more, when Tat binds Env, it appears to shield Env from neutralization by Env-specific antibodies. Tat antibodies, the researchers found, could prevent this by keeping Tat from binding to Env, and could neutralize HIV by binding to the Tat-Env complex on the virus.
For Ruprecht, the message for vaccine development is clear: Include Tat in a vaccine, she says, adding that the vaccine she used to immunize the macaques also induced cellular immune responses to Tat. “You get two for one,” she says.
Michael Murphey-Corb at the University of Pittsburgh, who heard Ruprecht present her study a few months ago, agrees that it makes sense to include Tat in a vaccine. She is especially impressed with the phage technology Ruprecht and colleagues used in the study. “It’s unbiased and that’s what I like about it,” she says.
But Robert Gallo—who with Ensoli discovered in 1990 that Tat is released from infected cells—says Ensoli’s recent results don’t convince him that Tat is important in preventing infection in a vaccine. “How can you prevent infection by targeting a protein that the virus doesn’t carry with it [as a bona fide surface protein]?” asks Gallo. “You only get Tat after the cell was infected.”
Gallo, who is at the Institute of Human Virology of the University of Maryland School of Medicine, says he found that Tat can have toxic effects on cells, and that he has unpublished results he presented at meetings that a candidate vaccine that’s based on modified Env protein is effective in monkeys, but loses that effectiveness once Tat is added. “When we add Tat, we take away all protection,” he says, “because it activates T cells that are targets for infection and you get more animals infected easier.”
This isn’t the first time Gallo’s observations differ from Ensoli’s. More than ten years ago, Gallo and colleagues found that in rhesus macaques, vaccination with a form of Tat that was modified to make it less toxic only led to a modest reduction in virus titers after challenge (Proc. Natl. Acad. Sci. 97, 3515, 2000). That was in contrast to a 1999 study by Ensoli and colleagues, who reported that vaccination with Tat protein could protect cynomolgus macaques from SHIV infection in some cases and reduce the virus to undetectable levels in others (Nat. Med. 5, 643, 1999).