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HIV has reappeared in the blood of two Boston patients who scientists had hoped had been cured of their infections.

This disappointing development, reported by The Boston Globe's Kay Lazar, is yet another cautionary tale of how researchers can never afford to underestimate the human immunodeficiency virus's ability to hide out in patients' bodies and overcome their most ingenious efforts to eliminate it.

The Boston patients have stirred considerable hope among HIV researchers since mid-2012, when scientists cautiously raised the possibility that bone marrow transplants had eliminated the virus from their systems.

The only person known to be cured of HIV so far is 47-year-old American Timothy Brown, widely known as the Berlin patient because he received a bone marrow transplant in Germany six years ago. A Mississippi toddler treated with antiviral drugs within 36 hours of birth might be deemed the second cure if she remains free of HIV.

But the Boston patients, who have not been named, would have represented something new and encouraging.

The Berlin patient received bone marrow from a donor who was genetically resistant to HIV infection. But the Boston patients' marrow donors had no such rare advantage, raising the hope that anti-HIV drugs might be enough to eliminate the virus — at least when combined with a bone marrow transplant.

Scientists also thought an immune reaction called graft-versus-host disease, or GVH, might have contributed to the apparent disappearance of HIV in the Boston patients' systems. In GVH, the transplanted marrow cells attack and kill the patient's native bone marrow — possibly helping to get rid of any cells where HIV might lurk.

But alas, those hopes were recently dashed. Researchers announced the possible Boston cures at an AIDS meeting in Kuala Lumpur in July, when their patients had been off anti-HIV drugs for seven and 15 weeks. (Their bone marrow transplants were three and five years earlier, for Hodgkin's disease, but they had kept taking the antiretroviral drugs until earlier this year.)

But in August, the virus reappeared in one man's blood. And in November, HIV re-emerged in the second patient. Both patients have gone back on antiretroviral medications, presumably for the rest of their lives. The drugs have proven effective in suppressing the virus in patients' systems, but not in eliminating it.

The Boston researchers decided to disclose the news at an AIDS meeting in Florida, because scientists in other institutions had already begun planning experiments to replicate the hoped-for cures in other bone marrow transplant patients infected with HIV.

Disappointing as the Boston experiment is, researchers say it has advanced the cause of HIV cure research. Just last Monday, President Obama announced a $100 million project to push that cause forward — a reflection of optimism among AIDS specialists after years of rarely uttering what one prominent researcher calls "the c-word."

Researchers say even negative results such as those from the Boston experiment contain important clues about what will be required to cure HIV infections.

"This is certainly telling us a lot about persistence, what we need to do, and how low we need to drop the levels of HIV reservoirs in order to allow patients to achieve remission," Dr. Katherine Luzuriaga of the University of Massachusetts told The Globe.

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