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Monkey Transplant Study Rules Out Possible HIV Cure Mechanism

Macaque monkeys that received transplants of their own stem cells after undergoing intensive radiation that killed off their existing SHIV-infected immune cells experienced viral rebound soon after stopping antiretroviral drugs, indicating that pre-transplant "conditioning" was not solely responsible for the only known case of a person cured of HIV.

A number of different approaches are being explored in research towards a cure for HIV. As described in the September 25 edition of PLoS Pathogens, Maud Mavigner, Guido Silvestri,and colleagues from Emory University conducted a study of stem cell transplants in rhesus macaque monkeys in an effort to learn more about what factors contributed to curing Timothy Brown, also known as the Berlin Patient.

Brown has remained free of detectable HIV for 7 years without antiretroviral therapy (ART) after receiving bone marrow transplants to treat leukemia from a donor with an uncommon genetic mutation (CCR5-delta-32) that makes T-cells resistant to HIV infection. Bone marrow contains hematopoietic stem cells that give rise to all immune cells, including the CD4 T-cells targeted by HIV.

In Brown's case, it is unclear whether his apparent cure is attributable to the donor's genetic mutation, the conditioning process that uses chemotherapy or radiation to ablate or kill off cancerous immune cells and make room for the donor cells, or a graft-vs-host reaction in which the transplanted immune cells attack the recipient's pre-existing cells.

Based on Brown's proof-of-concept, researchers have studied other HIV positive people undergoing bone marrow transplants for cancer. As described at this year's Retrovirus Conference, Timothy Henrich and colleagues from Brigham and Women’s Hospital followed 2 bone marrow transplant patients in Boston who received reduced-intensity conditioning that did not fully destroy their existing immune cells, allowing them to stay on ART throughout the process. Unlike Brown, the Boston patients received stem cells from normal donors without the CCR5-delta-32 mutation.

After extensive testing over 3-4 years showed undetectable plasma HIV RNA, no HIV DNA in peripheral blood mononuclear cells, and no evidence of virus in their lymph nodes or gut tissue, the Boston patients underwent carefully monitored treatment interruptions to see if the virus would return. Unfortunately, both men did experience viral rebound, at 4 and 8 months after stopping ART.

Hoping to learn more about viral persistence in the setting of hematopoietic stem cell transplantation, the Emory team looked at macaque monkeys infected with a hybrid simian/human immunodeficiency virus known as SHIV.

Initially, 3 macaques had stem cell samples collected prior to SHIV exposure. The monkeys were infected with SHIV, started on ART, and reached undetectable viral load (SHIV RNA <100 copies/mL). After several weeks on ART, they underwent high-intensity total body irradiation that killed off 94%-99% of their circulating CD4 cells. The monkeys then received re-infusions of their own virus-free stored stem cells. Another 3 SHIV-infected monkeys on ART did not receive transplants and served as controls.

At 40 to 75 days after stem cell transplantation and successful engraftment, the macaques were taken off ART. Plasma SHIV levels rebounded rapidly in 2 out of 3 monkeys, indicating that a viral reservoir still remained despite irradiation. As expected, viral rebound also occurred in all 3 control monkeys after ART interruption. In fact, SHIV DNA levels in various tissues after stopping ART were the same in the transplant recipients as in the non-transplant controls. The third transplanted monkey still had undetectable plasma SHIV RNA and SHIV DNA in peripheral blood cells at 2 weeks after ART interruption, but had to be euthanized due to kidney failure. However, an autopsy revealed detectable SHIV DNA in circulating CD4 cells, spleen, and lymph nodes, though not in gut or tonsil tissue.

Based on these findings, the researchers concluded, "the massive reset of the hematopoietic compartment was not sufficient to eliminate the total-body virus reservoir in the setting of short term ART." However, they added, the study "supports the hypothesis that myeloablative [bone marrow destroying] total body irradiation can cause a significant decrease in the viral reservoir in blood cells, even though it was not sufficient to eliminate all reservoirs."

These results suggest that Brown's cure is not attributable to intensive pre-transplant conditioning alone. The donor's CCR5 mutation could have played a role, but that was not a factor for the Boston patients, who managed to control HIV for months after their bone marrow transplants, though it did eventually reappear.

This leaves the graft-vs-host reaction as a possible explanation -- potentially along with other as yet unknown factors. Graft-vs-host disease was a common factor for Brown and the Boston patients, but not in the monkey study, as the animals received their own stem cells and thus did not have incoming foreign immune cells to attack any pre-existing SHIV-infected cells that might have survived the radiation.

10/3/14

Reference

M Mavigner, B Watkins, B Lawson, et al. Persistence of Virus Reservoirs in ART-Treated SHIV-Infected Rhesus Macaques after Autologous Hematopoietic Stem Cell Transplant. PLoS Pathogens 10(9):e1004406. September 25, 2014.

Other Source

Public library of Science. Stem Cell Transplant Does Not Cure SHIV/AIDS after Irradiation of Infected Rhesus Macaques. Press release. September 25, 2014.