Gene Editing of CCR5 in Hematopoietic Stem Cells in a Nonhuman Primate Model of HIV/AIDS

Background: Hematopoietic stem cell (HSC) transplantation remains the only clinically observed path to functional cure of HIV infection. To better understand the mechanism of HSC-driven HIV control, and apply this therapy to a greater number of patients, we have developed a model of combination antiretroviral therapy (cART)-suppressed HIV infection in the pigtailed macaque, applicable to both gene therapy- and allogeneic transplant-based cure strategies. Following transplantation of HIV-resistant, autologous cells into conditioned animals, we are evaluating the extent to which protected cell progeny impede infection by SIV/HIV (SHIV) chimeric virus in vivo.

Methods: Animals are challenged with SHIV virus containing an HIV envelope, after which a 3-drug cART regimen is initiated. Autologous HSCs are engineered to resist infection through targeted disruption of the CCR5 genetic locus using Zinc Finger Nucleases (ZFNs). Engraftment, persistence, and SHIV response of these autologous stem cells, and stem cell-derived lymphoid and myeloid cells, are measured in vivo.

Results: SHIV infection in the pigtailed macaque model results in sustained viremia with consequent reduction in CD4⁺ T cells. Moreover, administration of three-drug cART leads to rapid and durable suppression of plasma viremia to <30 copies/mL plasma - suggesting that this model recapitulates key features of HIV infection and treatment in humans. CCR5 targeting experiments yield up to 60% gene disruption in CD34⁺ cells ex vivo, translating to approximately 5% disruption in vivo following transplant. Importantly, up to 10% of transplanted cells carry two disrupted alleles of CCR5; these cells should preferentially reconstitute CD4⁺ T-cell pools and other susceptible subsets following SHIV challenge. Consistent with this prediction, our preliminary data suggest that CCR5-deleted cells undergo positive selection following SHIV challenge in vivo.

Conclusions: Our pigtailed macaque model of HIV infection and cART represents a promising platform for modeling functional cure strategies. Here we show that CCR5 deletion does not impair HSC engraftment or differentiation, and that CCR5-deleted cells can undergo SHIV-dependent positive selection even when present at low levels. Our model enables the evaluation of novel therapeutic approaches in the clinically relevant context of cART controlled SHIV infection - a setting of particular importance to approaches aimed at addressing the viral reservoir.