The Yin and Yang of transplant conditioning for an HIV cure

Science Spotlight

The Yin and Yang of transplant conditioning for an HIV cure

from the Kiem lab (Clinical Research Division)

March 20, 2017
The figure summarizes the model for enhanced viral rebound in transplanted animals.

Model for enhanced viral rebound in transplanted animals. Following myeloablative conditioning and autologous transplantation in SHIV+, antiretroviral therapy (ART)-suppressed animals, peripheral blood cell counts are significantly impacted, and gut damage is induced (red boxes). Cytopenia and gut damage trigger an intense but incomplete regeneration of the hematopoietic system, including decreased anti-SHIV antibody titers in peripheral blood (orange boxes). Following ART withdrawal, these factors combine to facilitate enhanced viral rebound in transplanted animals, relative to untransplanted SHIV+ controls.

Figure provided by Dr. Christopher Peterson.

The case of Timothy Brown, also known as the Berlin patient, has been a stepping-stone in the HIV field. To date, Timothy Brown is the only HIV infected patient who has been cured. Diagnosed in 1995, he was treated with antiretroviral therapy (ART) for more than ten years before being diagnosed with Acute Myeloid Leukemia (AML). As a treatment for his cancer, he underwent two hematopoietic cell transplants from an unrelated donor carrier of a homozygous CCR5delta32 mutation in the HIV co-receptor, protecting the transplanted cells from HIV infection. Since then and despite ART withdrawal, Timothy Brown has been HIV-free for 10 years (see recent Fred Hutch news article here).

However, the probability of finding a compatible donor with the same homozygous CCR5delta32 mutation is very low. For this reason, researchers have been trying to reproduce this success by genetically modifying hematopoietic (blood) cells to disrupt the CCR5 co-receptor. The goal is for HIV-infected patients in need of a transplant to be treated with autologous (patient's own cells), CCR5-deleted cells which should be protected against HIV infection. Prior to the transplant, the patient is administered a conditioning regimen to eliminate the patient’s own hematopoietic cells (for instance AML cells) and to facilitate the engraftment of the newly transplanted cells. However, it is unknown whether the conditioning regimen could also have an impact on HIV.  

This question was addressed by Drs. Christopher Peterson and Hans-Peter Kiem (Clinical Research Division), in a recent study published in the Journal of Clinical Investigation Insight, along with colleagues from the Fred Hutch (Vaccine and Infectious Disease Division), University of Washington, and Case Western Reserve University. The authors investigated the effects of total body irradiation (TBI) conditioning regimen in SHIV-infected and ART-treated nonhuman primates. TBI is known to facilitate engraftment of hematopoietic stem cells and eliminate potential residual cancer cells. The hypothesis was that TBI being highly lymphotoxic would also help to eliminate HIV reservoir cells and thus potentially reduce the viral burden.

SHIV is a chimeric virus containing the HIV envelope gene and the simian immunodeficiency virus (SIV) viral genome, allowing the study of the human HIV virus in a simian model. The animals were infected with SHIV for six months before ART treatment that efficiently controlled virus replication. Seven months later, autologous hematopoietic transplant was performed in combination with TBI. After seven more months ART treatment was stopped and a quick rebound in virus replication followed in both transplanted and non-transplanted animals. Surprisingly, the amount of SHIV was persistently higher in the transplanted animals in the peripheral blood and in tissues known to harbor the virus. In these viral reservoir tissues, HIV infected cells hide out-of-reach of the ART drugs, and remain undetectable by the immune system. Hence, hematopoietic cell transplant with unprotected cells in infected animals is associated with a rise in virus replication following treatment interruption.

To understand the origin of this higher virus replication, the authors showed that the transplantation led to a depletion of T and B lymphocytes in the peripheral blood. This reduced the number of HIV-infected cells but also created a favorable environment for viral replication. Lymphocytes were later replenished, but presented a dysfunctional phenotype. Additionally, expression of several markers of gut integrity and inflammation were significantly modified post-transplant. An inflamed and damaged mucosal barrier could facilitate reservoir persistence in these tissues. The size of the latent viral reservoirs in the peripheral blood in ART-treated animals, measured by two independent assays quantifying ex vivo virus reactivation, did not correlate with the higher viral load after ART was withdrawn. “We think that as long as patients remain on ART, the size of their viral reservoirs will not change,” said Dr. Peterson. “However, when ART is removed, and even a few infected cells have survived the transplant procedure, the immune system is unable to respond, and they replicate out of control. What we’re doing now is measuring the viral reservoir in tissues, which we can compare to the data we already have in peripheral blood samples. From there, we can ask where this replication begins, and design strategies to control it.”

These data are one step closer toward understanding the intricate mechanisms that will allow the patients to persistently control or eradicate HIV. Dr. Peterson concluded, “the most exciting finding for us is that the TBI that we tested, which can make patients very sick, is not as important from the standpoint of clearing infected cells. This sort of conditioning regimen is very important for cancer patients, but isn’t reasonable for HIV+ patients on ART, who are otherwise healthy. Our results tell us that we can focus on less toxic conditioning regimens, without giving up the ability to reduce the size of viral reservoirs. In terms of an HIV cure, the fact that the conditioning regimen wasn’t crucial further argues that providing virus-protected cells, for example using gene editing to knock out the CCR5 locus and block viral entry, is hugely important”.

 

Funding for this study was provided by the National Institute of Allergy and Infectious Diseases (NIH), National Heart, Lung, and Blood Institute (NIH), and by the University of Washington / Fred Hutch Center for AIDS Research (CFAR).

 

Peterson CW, Benne C, Polacino P, Kaur J, McAllister CE, Filali-Mouhim A, Obenza W, Pecor TA, Huang ML, Baldessari A, Murnane RD, Woolfrey AE, Jerome KR, Hu SL, Klatt NR, DeRosa S, Sekaly RP, Kiem HP. 2017. Loss of immune homeostasis dictates SHIV rebound after stem-cell transplantation. Journal of Clinical Investigation Insight, 2(4):e91230.