T cells are an important immune cell subset, capable of seeking out infected or cancerous cells and destroying them. Chimeric antigen receptor T cells (CAR-Ts) are modified T cells that use antibody-like domains to bind to their target and activate the T cell, killing the target cell. CAR-Ts have been developed as a successful therapy for B cell malignancies targeting the CD19 antigen. However, development of CAR-T therapies towards other cancer and viral antigens has been more difficult. HIV-directed CAR-Ts have been studied for almost 30 years with little success, largely due to low antigen access while patients are on traditional antiretroviral therapy (ART) which keeps viral burden at low or undetectable levels. However, the Kiem Group (Clinical Research Division) wanted to better understand whether CAR-Ts directed toward the HIV envelope could be successful at control viral burden. Their results were recently published in Blood.
First, the authors wanted to improve upon the current protocol for producing CAR-T cells. While traditional methods isolate T cells using the CD3 antigen, which captures both flavors of CD4 and CD8 T cells, the authors found more success isolating CD4 and CD8 T cells directly. Additionally, they found that expanding T cells with irradiated feeder cells, rather than traditional bead-based expansion, lead to more robust expansion and maintenance of central memory phenotype, important for the function of CAR-Ts once infused. With this enhanced production protocol in place, the authors used a non-human primate (NHP) model, infected with simian-HIV (SHIV), a robust chimeric model of HIV, to test their CAR-Ts. As T cells are a main cell subset able to be infected by HIV, the authors also modified their CAR-Ts so they would not express CCR5, a critical co-receptor for viral entry. However, this genomic editing was suboptimal, with less than 40% becoming CCR5 null, leaving room for further improvement in the future. Regardless, the authors transduced the T cells with their CAR construct directed at SHIV envelope (Env) to allow them to recognize SHIV-infected cells. Furthermore, the authors tested their production protocol with T cells from animals infected with SHIV, in order to demonstrate that their protocol could be used in both SHIV- and SHIV+ T cells, an important translational step as any human CAR-T production would be done from HIV+ individuals.
NHP were infected with SHIV for 12-13 weeks before being placed on ART for 59-70 weeks then infused with CAR-Ts. Because antigen burden is low once on ART, the authors tested a boosting strategy where they infused Env-expressing irradiated cells 19 days after CAR-T infusion. Twelve days post boost, animals were taken off ART to determine if CAR-Ts could suppress viral rebound. CAR-T expansion was seen in all animals after ART was stopped, indicating CAR-Ts were encountering antigen and expanding. However, in a few animals (Group 1), CAR-Ts expanded after the boost but before ART was stopped. SHIV rebound was significantly lower in animals that received CAR-Ts, and one animal in Group 1 had non-detectible viral load for 89 days post ART interruption. Group 1 controlled viral load for a longer period than Group 2, and the authors found this correlated with higher T cell activation markers and lower inhibitory markers like PD-1 on Group 1 CAR-Ts. In order to determine whether antigen escape contributed to lack of viral suppression in Group 2, the authors looked for mutations that may have allowed escape but did not find any in Group 2 animals. They hypothesized that CAR-T exhaustion could be the reason for lower control and treated Group 2 with the PD-1 inhibitor, nivolumab, at 227 days post CAR-T infusion. PD-1 inhibition led to increase in CAR-T numbers and transient viral control, indicating T cell exhaustion was at least partially the reason for lack of viral control in Group 2 animals. These studies show promise for translating HIV-directed CAR-Ts to the clinic, with a clinically feasible production protocol and a strategy to boost antigen expression in patients on ART.
This study was supported by the National Institutes of Health.
UW/Fred Hutch Cancer Consortium members Keith Jerome and Hans-Peter Kiem contributed to this work.
Rust B, Kean L, Colonna L, Brandenstein K, Poole N, Obenza W, Enstrom M, Maldini C, Ellis G, Fennessey C, Huang M, Keele B, Jerome K, Riley J, Kiem H, Peterson C. 2020. Robust Expansion of HIV CAR T Cells Following Antigen Boosting in ART-Suppressed Nonhuman Primates. Blood. doi: 10.1182/blood.2020006372. Online ahead of print.