Why did the HIV cure work in the Berlin patient?

Why did the HIV cure work in the Berlin patient?

Nobody is really sure.

Three things happened during the Berlin patient's treatment.

First, chemotherapy killed off most of the cells infected with HIV. By itself, this would not be enough to cure HIV.

Second, the donor cells repopulated the patient's immune system. The new cells attacked and killed the patient's remaining white blood cells -- a process Zaia calls a "graft-versus-leukemia" response. This process likely killed off many of the remaining cells carrying HIV.

Third, the donor cells were resistant to HIV infection. As HIV emerged from resting cells, the virus helped kill off the old, susceptible cell. When the new donor cells expanded to take their place, the HIV had no place to go and withered away.

But none of these things fully explains what happened. One puzzle is that the stem cells used to repopulate the patient's immune system were HIV resistant -- but not HIV proof.

The cells lacked the most common doorway, CCR5, that HIV needs to infect cells. But people with long-term HIV infection usually carry HIV capable of using another doorway called CXCR4. And tests showed that the Berlin patient's blood carried HIV like this. Moreover, tests also showed that the donor cells were susceptible to infection via the CXCR4 pathway.

Even so, the Berlin patient mysteriously remains HIV free.