The 'Berlin patient' is an HIV-positive man who developed acute myeloid leukaemia, received successful treatment and subsequently experienced a relapse in 2007 that required a transplant of stem cells.

Doctors chose stem cells from an individual who had an unusual genetic profile: a mutation inherited from both parents that resulted in CD4 cells that lacked the CCR5 receptor. This mutation, called CCR5 delta 32 homozygosity, is present in less than 1% of Caucasians in northern and western Europe, and is associated with a reduced risk of becoming infected with HIV.

This is because all new infecting viruses need to use the CCR5 receptor on CD4 cells when infecting an immune system cell of the CD4 type.

Later in the course of HIV infection another type of virus emerges that can use the CXCR4 receptor instead.

Before the stem cell transplant the patient received chemotherapy treatment that destroyed most immune cells and total body irradiation, and also received immunosuppressive drugs to prevent rejection of the stem cells.

Antiretroviral therapy was halted on the day of the transplant, and the patient had to receive a second stem cell transplant 13 days after the first one, due to a further relapse of leukaemia.

The patient continued to receive immunosuppressive treatment to prevent rejection for 38 months, and at 5, 24 and 29 months post-transplant colon biopsies were taken to investigate possible graft-versus-host disease in the intestine. At each investigation additional samples were taken to check for signs of HIV infection in the abundant immune cells of the gut wall.

During the 38 month follow-up period the donor CD4 cells repopulated the mucosal immune system of the gut, to such an extent that the frequency of CD4 cells was almost twice as high as in HIV-negative healthy controls, and this phenomenon was also seen in a control group of ten HIV-negative individuals who received stem cell transfers.

The repopulation of CD4 cells was accompanied by the complete disappearance of host CD4 cells, and after two years the patient had the CD4 count of a healthy adult of the same age.

One of the challenges for any approach to curing HIV infection is long-lived immune system cells, which need to be cleared before a patient can be cured. In the case of the Berlin patient CCR5-bearing macrophages could not be detected after 38 months, suggesting that chemotherapy had destroyed these longer-lived cells, and that they had also been replaced by donor cells.

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