A new study has found that the AIDS virus, HIV, can dodge treatment by staying dormant inside progenitor blood cells that develop into immune cells.
These cellular reservoirs are resistant to the immune response of the host and the highly active antiretroviral therapy (HAART) administered to patients.
This means that the virus can attack white blood cells known as CD4+ T lymphocytes - which help the body to mount an immune response - after treatment.
"There are so many instances where patients stopped taking drugs and the virus rebounded. There is no way to get people off the drugs," Nature quoted lead author Kathleen Collins, a cellular biologist at the University of Michigan in Ann Arbor, as saying.
HAART can stop the spread of the virus by preventing the viral genome from integrating into new cells.
The combination of drugs used don't attack the cell that makes copies of the virus, but they can prevent the newly made virus from infecting more cells.
The researchers show that HIV can target haematopoietic progenitor cells (HPCs) from the bone marrow before they give rise to white blood cells.
They took these cells from HAART-treated patients who have shown no detectable traces of the virus for at least six months.
When the researchers forced the cells to differentiate into white blood cells in the lab, they found that the HIV genome in these cells in about 40 percent of the donors.
They also took cells from the bone marrow of healthy people and showed that the virus killed some cells, but in other cells the virus integrated into the cells' chromosomes and did not reproduce.
The cells grew as if they were uninfected by the virus.
"If the drugs are on-board, the virus is still being made but it is not doing much harm because it can't spread," says Collins.
In the absence of the drugs, the infection renews, and patients need to return to the therapy and stay on it.
"If you shorten the therapy even to two years, that would dramatically change things," she said, by bringing down costs and allowing the therapy to be used more widely in poor countries.
The study has been published in Nature Medicine.