A new study shows that HIV can still live and grow in the body even after disappearing from the blood following aggressive antiretroviral therapy.

The research, published Wednesday in the journal Nature, involved looking at samples of cells from the lymph nodes of three patients who appeared to have cleared the virus. The researchers found that cells in the  lymph node tissue can still produce new virus and infect new target cells.

The groundbreaking idea of reservoirs of HIV made headlines in July 2014 when a Mississippi girl born with HIV, who was believed to be cured after early treatment, tested positive for the virus after stopping therapy. In her case, doctors think the infection reemerged from a viral reservoir containing cells in a resting state that were not proliferating.

The latest study appears to show a different type of "sanctuary," as the researchers called it, harboring cells with low levels of HIV replication that move into the blood. Researchers used a mathematical model to track the amount of virus and the amount of infected cells as they grew and circulated through the body.

This suggests that virus growth could occur in a place where drug concentrations are very low.

"These findings are important as it is critical for the field of HIV cure research to know whether new infectious cycles are indeed continuing in patients on seemingly effective treatment," Deborah Persaud, a professor of infectious diseases at Johns Hopkins University School of Medicine, told The Washington Post.

But Persaud, who was not involved in the lymph node research but was part of a team caring for a baby who had received a treatment similar to the Mississippi child, said the study is limited by its small sample size and the fact that the analyses were done during the first six months of combination treatment "when the infected pools are still very dynamic." She said a similar study on patients with longer-term treatment may offer more clues about what is occurring.

Co-author Steven Wolinsky, chief of infectious diseases at Northwestern University Feinberg School of Medicine, said in a statement that the research indicates that “the challenge is to deliver drugs at clinically effective concentrations to where the virus continues to replicate within the patient.”

Angela McLean, a professor of mathematical biology at Oxford University, who supervised the mathematical modeling, added that the study “really changes how we think about what is happening in treated patients."

"It helps explain why some strategies that tried to clear the reservoir have failed," she said.

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