HIV is a cunning virus. It integrates itself into the genes of a host cell and after treatment with anti-retroviral therapy (ART) is able to evade detection by hiding out in hidden reservoirs. ART can only suppress the virus, so even if the virus can no longer be detected in someone’s blood it may still be replicating, albeit much more slowly, or in stealth mode.

Measuring potential replication and the size and location of dormant reservoirs is problematic when the virus is suppressed. But a new study in PLOS Pathogens suggests more about where the virus might be found and how it might be targeted.

Using 40 mice that were bio-engineered to have human immune systems – known as bone marrow/liver/thymus (BLT) mice – scientists were able to study the virus’ reaction to ART drugs. They found that when ART was working, the HIV was able to remain in the mice’s immune cells in a number of tissues in the body including in bone marrow, the liver and lungs.

Getting this information from human patients undergoing ART is impossible because it’s highly invasive. But evidence collected from mice is already shedding light on the ability of an immunotoxin to specifically home in on the HIV virus.

The researchers used an antibody called 3B3 and a bacterial toxin called PE38 to selectively target and kill HIV. 3B3 is able to recognise and attach itself to a viral protein called Env which is expressed by cells that host the virus. Once the target is located, the toxin is able to enter the cells and kill the virus.

The study reports that half of the mice that received a two-week dose of 3B3-PE38 in addition to ART had a six-fold drop in virus levels in cells and tissues.

“Our study demonstrates the effectiveness of one possible kill strategy,” said Victor Garcia, co-author of the study. “The results show a clear reduction in the numbers of infected cells in all the tissues tested.”

“We measured the levels of HIV in tissues like the lungs, liver and spleen using standard molecular techniques. This is difficult to do in humans because it would involve invasive procedures to obtain samples.”

One way of testing if the HIV virus is still replicating is its presence in RNA. If it is detected in DNA this indicates that the virus has become dormant. As the immunotoxin in the study targets HIV proteins expressed by infected cells, it has not been shown to affect the sleeping virus in HIV reservoirs.

This falls short of a “kick and kill” approach, that would involve provoking a dormant HIV virus into multiplying, therefore making it more visible and open to attack with drugs. But it does suggest a potential new way of targeting persistent HIV that goes a significant step further than ART alone.

HIV doesn’t infect regular mice but will do in “humanised” BLT mice and using them could also help other researchers to more efficiently test new ways of attacking HIV before trials in primates or humans.

“BLT mice are chimeric animals that are created by implanting immunodeficient mice with a small piece of human thymic and liver tissue followed by a transplant with bone marrow stem cells,” Garcia said. “This in turn renders these animals susceptible to infection by HIV and other human specific pathogens.”

John Frater, an HIV specialist from the University of Oxford, said the paper was an interesting proof-of-concept and proved the effectiveness of using mice for this kind of research.

The issue of ongoing viral replication during ART had been the subject of some debate, he said. “That viral RNA can be detected in tissues despite ART is important and revisits the thorny issue of whether there is ongoing viral replication [for patients] on ART.”

He added: “The immunotoxin approach demonstrates a possible antiviral approach, but – and the authors acknowledge this in their discussion – it is not really talking to the ‘kill’ part of ‘kick and kill’ as the study doesn’t target the latent HIV reservoir.”

“The conclusions at this stage are that in this mouse model, ART can be potentially improved with the addition of an immunotoxin. How this translates to human treatment is not known. Also, whether this is a route for a cure strategy requires a number of other studies before any conclusions can be reached.”