Targeted Rejuvenation of Exhausted Chimeric Antigen Receptor T Cells Regresses Refractory Solid Tumors

UNLABELLED: Chimeric antigen receptor (CAR) T-cell therapies have proven to be effective in treating hematologic malignancies but demonstrate only marginal efficacy in eradicating solid tumors. Although several mechanisms can account for these differences, a major cause is thought to derive from CAR T-cell exhaustion, where chronic exposure to tumor antigen can activate feedback pathways that suppress CAR T-cell cytotoxicity. We describe here a strategy to reverse this CAR T-cell exhaustion using a universal anti-fluorescein CAR that concurrently serves as (i) a cancer recognition receptor that enables engagement of multiple cancer cell clones upon addition of a cocktail of bispecific fluorescein-linked tumor-targeting ligands, and (ii) a drug-internalizing receptor that mediates uptake of a CAR T-cell activator comprised of fluorescein linked to an immune stimulant. By attaching a Toll-like receptor 7 agonist (TLR7-1A) to fluorescein, we enable the anti-fluorescein CAR to bind and internalize TLR7-1A, leading to both downregulation of exhaustion markers (i.e., PD-1, TIM3, LAG3) and reactivation of exhausted CAR-T cells without causing the toxicities commonly associated with systemic administration of TLR7 agonists. The resulting rejuvenated CAR-T cells are observed to regress otherwise refractory solid tumors. Moreover, because no other immune cells are altered by this treatment, the data demonstrate that the exhaustion state of the CAR-T cells constitutes a major property that determines the efficacies of CAR T-cell therapies in solid tumors.

IMPLICATIONS: A novel strategy for rejuvenating exhausted CAR-T cells is described previously that promotes downregulation of exhaustion markers and renewed eradication of cancer cells in a tumor mass.