In the three decades since HIV was officially recognized, much progress has been made in developing drugs that allow  HIV-positive patients to live longer and healthier lives. Some people have such a low viral count in their bodies that the virus appears to be nearly gone. But if they stop taking HIV medications, the virus will rapidly reappear and reproduce. New evidence may help explain where the virus is hiding and why it is able to replicate so quickly.

In early March, researchers at the University of Michigan discovered HIV can hide in a specific type of cell in bone marrow, known as hematopoietic progenitor cells (HPCs). Commonly known as stem cells, they are responsible for creating all the blood cells in the body, such as those involved in the immune system.

It has been known for a couple years that HIV cells can hide in bone marrow, said Dr. Richard M. Novak, professor of medicine and infectious diseases at the University of Illinois at Chicago. What makes this research new, however, is the type of cells infected, he said.

“That’s a problem, because those are important cells,” Novak said. “If they’re infected, that means they’re just producing cells that are already infected with virus.”

HPCs, originally thought immune to the virus, were found able to harbor the disease without instigating an immune system reaction.

These latency periods are one of the reasons science is unable to eradicate HIV, said Dr. Daniel Berger, medical director of Northstar Healthcare in Chicago and clinical associate professor at the University of Illinois at Chicago. Reservoirs—areas that contain virus cells that are not replicating—are difficult to eradicate because organs  can become what he called “sanctuary sites” for years.  All current HIV drugs are antiviral and affect the replication stage, Berger explained. If the virus isn’t replicating, the antiviral drugs are ineffective.

“The fact they’re now getting in stem cells is an important piece of information that may help us look for ways of combating this problem,” he said.

It’s  also important, and difficult, to catchthe virus before reservoirs have a chance to form, Novak said. This involves diagnosing people soon after infection and starting medication as soon as possible.

While Novak advocates the study of prevention over eradication, he said it is important to keep healthy cells free from the virus because they could produce more healthy cells.

“It’s a good place to start,” he said. “We should protect those cells because they’re the source of all other cells.”

Karl Salzwedel, program officer at the Division of AIDS at the National Institutes of Health, called the research the latest “key finding” in the disease. While most of the patients had high levels of the virus in their bodies, which  may explain the presence of HIV in bone marrow, this was not the case for everyone.  One patient had no detectable amount of virus for two years.  However, the virus was found in her bone marrow.

“It’s the first indication that patients harbor HIV,” Salzwedel said. “[The] virus is remaining in the bone marrow and can grow out.”

It is not as easy as wiping out all bone marrow cells and replacing them with new, healthy ones from a donor, he said. Even if an HIV-positive person could be cured through a bone marrow transplant, he or she would need to stay on medication to keep the body from rejecting donor marrow.

“You’re kind of trading off,” Salzwedel said. “If you’ve been on HIV drugs the whole time, after the transplant you have to be on immunosuppressant drugs the rest of your life. [It’s] not the most attractive way to treat patients.”

The hope is to eventually look into gene therapy to eradicate the disease, he said.

“[There are] obstacles in being able to clear HIV from an individual’s body completely,” Berger said. “But on the plus side we have antiviral medications that are able to reduce the virus in the blood, which reduces infection rates and keeps people healthy. People can have a normal life expectancy if they get on medications at an early time … the earlier the treatment the better.”