Human immunodeficiency virus (HIV)-infected individuals are typically treated with daily doses of antiretroviral therapy (ART), which dramatically limits viral burden and prevents virus-associated morbidity but does not cure the disease. Allogeneic hematopoietic stem cell transplantation (HSCT) has yielded rare cures for HIV, including the well-publicized Berlin and London patients. The curative effects of these treatments likely occurred thanks to due to mutations present in the donor cells, rare in the population, that rendered the cells resistant to infection by HIV. Efficient engraftment of donor cells that, through graft-versus-host-mediated responses, eliminated latently infected patient cells contributed to the curative outcome. Unfortunately, allogeneic HSCT would likely not be suitable for most HIV patients due to dangerous side effects, including graft-versus-host-disease (GVHD), the rarity of HIV-resistant mutations in potential donors and high cost of the procedure. One attractive strategy for circumventing these limitations is using autologous engineered adoptive cell therapy, involving the use of chimeric antigen receptors (CARs) introduced into the patient’s own cells to specifically target HIV-infected cells for elimination. Members of the Kiem Laboratory, including former graduate student Isaac Barber-Axthelm, and staff scientist Christopher Peterson, in the Fred Hutch Clinical Research Division, tested the engraftment and persistence of CAR-expressing cells derived from engineered, adoptively transferred hematopoietic stem cells (HSCs) in a preclinical HIV model. Their work, recently published in JCI Insight, reveals that HSC-derived CAR cells engraft in tissue-associated HIV reservoirs and persist in these locations over long periods of time.
CAR-based therapies have typically been utilized in T cells, important immune mediators capable of recognizing and eliminating infected and transformed cellular targets. Traditionally, T cells are isolated from patients’ peripheral blood, genetically modified to express CARs, and expanded in culture before re-infusion back into the patients. Rather than directly modifying the T cells themselves, the Kiem lab has taken the approach of modifying HSCs, which are long-lived precursors to all hematopoietic lineages, including T cells. “Reprogramming hematopoietic stem cells (HSCs) with chimeric antigen receptor (CAR) molecules offers a few advantages over traditional CAR T cell approaches,” explained Dr. Peterson. “The first is that HSCs should persist lifelong in the body, whereas CAR T cells have been shown to have a relatively short time period during which they function, after being infused into a patient. For HIV as well as many cancers, which may lie dormant for years after treatment, we think that an HSC-CAR strategy will be helpful. We think of these cells as long-lived sentinels against recurrent disease. The second advantage is that HSC-derived CAR T cells could have more freedom to traffic to important tissues. This was the major focus of this work, quantifying where HSC-derived CAR cells reside, beyond the bloodstream.”
Intending to generate long-lived immunity against HIV, the researchers utilized a CAR containing the extracellular portion of the CD4 receptor, which targets the HIV envelope protein expressed on the surface of infected cells. To manifest lasting benefits, CAR-expressing effector cells must not only recognize and kill infected cells but also traffic to HIV tissue reservoir sites and persist long-term. In a previous study, the group demonstrated that CAR-expressing HSCs transplanted into non-human primates could indeed mediate viral control in a hybrid simian/human immunodeficiency virus (SHIV) model. However, the ability of HSC-derived CAR cells to traffic to and persist within HIV reservoir tissues remained unclear. In the present study, they examined tissues collected at the study endpoint –nearly two years after HSC transplantation and several months after withdrawal of suppressive ART– to assess engraftment and persistence at reservoir sites after SHIV viral rebound.
To measure CAR cell localization, the Kiem group first established a novel immunohistochemistry (IHC)-based detection method for staining their CAR-expressing cells in histological sections. They applied this assay to several known potential HIV reservoirs, including lymphoid tissues, the central nervous system (CNS), and the gastrointestinal tract (GIT). Encouragingly, they were able to detect CAR cells in each of the tissues, indicating that these cells were indeed able to traffic to and persist long-term in HIV reservoirs. Significantly, this confirmed that CAR cells were capable of engrafting in the brain, a notoriously difficult to access reservoir compartment. Next, they utilized multiplexed IHC approaches to assess the immune lineages represented by CAR cells at these sites. Surprisingly, while the majority of phenotypically quantifiable CAR cells in the GIT were T cells, the immune subset of greatest therapeutic interest in this setting, they observed that the majority of CAR cells within lymphoid tissues were B cells – immune cells known for their antibody-producing capabilities and not generally associated with CAR-mediated responses. They observed the presence of other CAR-expressing immune cell subsets with potential functionality against HIV, including myeloid cells and natural killer cells, demonstrating multilineage engraftment of HSC-derived cells at HIV-relevant sites
This work reveals that HSC-derived CAR cells can achieve long-term, multilineage engraftment in HIV tissue reservoirs and represent a promising avenue for future anti-HIV therapeutic development. Moving forward, the group is interested in leveraging the unexpectedly high representation of HSC-derived B cells in reservoir tissues. “One of the surprising observations we made is that CAR-modified, HSC-derived B cells were a major component of the CAR repertoire in tissues, especially lymph nodes. Because CAR molecules are specifically designed to augment T cell function, we don’t believe that a CAR B cell has any function,” explained Dr. Peterson. “However, the fact that these HSC-derived B cells are so prevalent and traffic so efficiently to tissues opens a new avenue for us: modification of these cells with antibody molecules, instead of CARs.” Indeed, members of the Kiem lab have already published preliminary findings in this area. “Looking forward, we’re interested in using CRISPR-Cas9 gene editing to target antibody molecules specifically to B cells’ antibody production loci within the genome […using a] similar approach to what was published by Dr. Justin Taylor in VIDD [Vaccine and Infectious Disease Division, Fred Hutch].”
This work was funded by the Foundation for AIDS Research, the National Institutes of Health, the California Institute of Regenerative Medicine, the James B. Pendleton Charitable Trust, the McCarthy Family Foundation, and the National Cancer Institute.
UW/Fred Hutch Cancer Consortium member Hans-Peter Kiem contributed to this work.
Barber-Axthelm IM, Barber-Axthelm V, Sze KY, Zhen A, Suryawanshi GW, Chen IS, Zack JA, Kitchen SG, Kiem HP, Peterson CW. Stem cell-derived CAR T cells traffic to HIV reservoirs in macaques. JCI Insight. 2021 Jan 11;6(1):e141502. doi: 10.1172/jci.insight.141502. PMID: 33427210; PMCID: PMC7821595.