Modeling Unique Patients in Humanized Mice: Toward a Curative Strategy for HIV

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After two decades of highly effective suppressive antiviral therapies, only one person may have been cured of his chronic HIV-1 infection. A common genetic variant in Caucasians, CCR5Δ32, confers resistance to CCR5-tropic HIV-1 entry, and homozygous individuals are protected from acquiring this infection. This unique, HIV-infected individual, referred to as the Berlin Patient, underwent bone marrow transplant for leukemia and received CCR5Δ32/Δ32 donor bone marrow. Over time, HIV-1 resistant donor-derived cells replaced his immune system and the HIV virus became undetectable, even in the absence of antiretroviral therapy. Whether this individual retains a reservoir of dormant HIV virus is actively debated, but, functionally, he appears to have been cured and no longer requires life-long antiviral therapy.¹ Importantly, the inability of HIV to infect cells harboring the CCR5Δ32/Δ32 mutation seems to be critical to this process, since allogeneic hematopoietic stem cell transplantation of CCR5-positive donors resulted in rapid rebound of HIV.²

Many questions as to how the Berlin Patient lost his HIV infection remain unanswered. One of the major challenges of studying HIV in animal models is its limited tropism to human cells and the presence of several restriction factors in mouse cells. In a recent paper published in Molecular Therapy Methods & Clinical Development, Khamaikawin et al.³ designed a model system that will become useful to study the bone marrow transplantation strategy as a functional cure for established HIV infection. These authors used human immune system (HIS) mice to model the treatment course of the Berlin Patient. HIS mice are created by transplanting human CD34⁺ hematopoietic stem cells into severely immunodeficient mice.⁴ Over time (typically 3 months) the absence of murine T and B cells allows for the development of human lymphocytes. These mice enable investigations into numerous human lymphocyte biological processes but have found particular use as models for HIV infection in laboratory animals.⁵ In the current report by Khamaikawin et al.,³ they first established stable HIV infection in HIS mice. This was followed by bone marrow ablation and a secondary transplantation. The second transplantation used a mixture of hematopoietic stem cells that were resistant or susceptible to HIV. To this end, cells were transduced with short hairpin RNA against CCR5 and the HIV-1 long terminal repeat, which conferred resistance to HIV-1⁶ or irrelevant controls. Over time, HIV-resistant CD4⁺ cells from the second transplantation overtook the majority of the CD4⁺ cell compartment in HIV-infected but not uninfected mice, suggesting a selective advantage.

These findings open many avenues of future investigations. In the current study, the authors used an allogeneic secondary stem cell transplantation, similar to the Berlin Patient. As the authors point out, this will limit the practical implementation across a wide spectrum of patients given the potentially severe complications associated with allogeneic stem cell transplantation. Will HIV-resistant autologous hematopoietic stem cells over time reconstitute the CD4⁺ compartments as efficiently as allogeneic transplants, lowering the potential for complications? And will more modern gene editing technologies improve the efficiency of CCR5 disruption? Ultimately, however, despite many approaches directed at completely eliminating HIV, the established pool of HIV-resistant CD4⁺ T cells may result in a selective advantage and prevent longitudinal CD4⁺ T cell loss, which is the primary reason for the development of acquired immunodeficiency syndrome (AIDS). Even in the absence of a complete cure, this may reduce the need for antiretroviral therapy.

One major caveat with HIS mice is the limited functionality of the human lymphocytes. This may explain the persistence of HIV viremia in the current study. Further manipulations, such as supplementation of human growth factors or co-transplantation of human liver, might broaden the humanization of these models.7, 8 Yet, future modifications in these models will be required to study the role of various anti-HIV immune functions. This will then also open possibilities to disrupt various other host factors essential for HIV replication so as to study their effect on the development and maintenance of CD4⁺ cell functions in vivo. Furthermore, even though HIV infection of humanized mice has been well established, it remains unclear whether HIV “latency,” epigenetically silenced proviral HIV DNA in long-lived cells described in HIV-infected patients, is accurately recapitulated in humanized mice. Since this HIV reservoir is not fully characterized in humans, the unique original Berlin Patient prevents the conclusion that all HIV reservoir cells are sensitive to radioablation. As such, the effectiveness of this approach remains to be confirmed.

Nevertheless, the report by Khamaikawin et al.³ represents a substantial step forward because it establishes the HIS mouse model as a tool to advance our understanding of how bone marrow transplantation can lead to a functional cure for established HIV infection and demonstrates that humanized mice can form a key pre-clinical tool for evaluating different therapeutic strategies.