By LIZA McCLELLAN, M.D.

Researchers at Brigham and Women's Hospital in Boston have discovered that, following bone marrow transplants, two men no longer have detectable HIV in their blood cells.

The finding is significant because it suggests that by giving these patients transplants while they were on anti-retroviral therapy, they may have been cured of the AIDS-causing virus.

"We expected HIV to vanish from the patients' plasma, but it is surprising that we can't find any traces of HIV in their cells," said Dr. Timothy Henrich, one of the researchers studying the two men. "It suggests that under the cover of anti-retroviral therapy, the cells that repopulated the patient's immune system appear to be protected from becoming re-infected with HIV."

The findings were presented Thursday at the AIDS 2012 conference in Washington, D.C. The story shares similarities with that of Timothy Ray Brown, also known as "the Berlin patient," but there are important differences. While the cells used in Brown's transplant procedure were specifically chosen from a donor who had a genetic mutation that resisted HIV, these patients received transplants with normal cells. Also, the two patients whose cases were presented at the meeting are still taking anti-retroviral medications normally used to treat HIV-positive patients, while Brown is no longer taking these medications.

Further study will need to be done to prove that the two patients are truly cured.

"Studies over time including biopsies of lymphatic tissue would be required," said Dr. Michael Saag, an infectious disease expert from University of Alabama at Birmingham. He said only time will tell if these patients remain HIV-free.

While it appears from these cases, as well as that of the Berlin patient, that altering a patient's immune system may lead to a "cure" for HIV, bone marrow transplants are currently too costly and dangerous for all HIV patients to be able to undergo them.

Separately, scientists are trying to use gene therapy to alter patients' immune systems to free them of HIV. Most of the research in this field is very preliminary, but scientists at the Fred Hutchinson Cancer Research Center are trying to perform stem cell transplants with cells that have been genetically modified to be resistant to HIV, much like the cells that the Berlin patient received.

"We have not yet transplanted any patient as part of our study," said Dr. Hans-Peter Kiem of the Clinical Research Division at Fred Hutchinson Cancer Research Center and an attending transplant physician at Seattle Cancer Care Alliance. But Kiem and his research team have recently been awarded a research grant to further investigate stem cell transplantation as treatment as a means to find a cure for HIV.