Synthetic Chimeric Antigen Receptors (CARs) Rapidly Induce Exhaustion and Augmented Glycolytic Metabolism In Human T Cells and Implicate Persistent CD28 Signaling As a Driver Of Exhaustion In Human T Cells
Authors: Long Adrienne H. , Orentas Rimas J. , Mackall Crystal L. 
Published: October 2019

Abstract

Introduction
Chimeric antigen receptors (CARs) provide a promising new approach for the adoptive immunotherapy of cancer. Though impressive antitumor activity has been observed with some CAR T cells, other CAR T cells demonstrate poor antitumor efficacy in vivo despite high cytolytic capacity in vitro due to poor expansion and persistence. Whether exhaustion of CAR T cells mirrors exhaustion that occurs naturally in chronically stimulated human T cells has not yet been studied. Here, we report that expression of select CD28 containing CARs in normal human T cells rapidly induces an exhausted state characterized by high PD-1 expression, poor persistence and poor antitumor efficacy, whereas other CARs do not induce this phenotype.

Results
Human T cells were expanded with anti-CD3/CD28 beads, and then transduced with a second-generation (CD28-CD3ζ) disialoganglioside 2 (GD2) specific CAR or a second-generation (CD28-CD3ζ) CD19 specific CAR. By day 7 of in vitro expansion, GD2 CAR T cells developed a metabolism more highly dependent on glycolysis compared to CD19 CAR T cells or untransduced controls. Neither CAR population was exposed to antigen during this expansion period. Using a Seahorse Extracellular Flux Analyzer, the ratio of glycolysis to oxidative phosphorylation rates (ECAR:OCR ratio) of GD2 CAR T cells was found to be double that of CD19 CAR T cells or controls on day 7.
The highly glycolytic metabolism of GD2 CAR T cells was associated with an exhausted phenotype. GD2 CAR T cells expressed higher levels of PD-1, TIM-3 and LAG-3, and transcription repressor BLIMP-1, compared to CD19 CAR T cells or untransduced controls. Additionally, GD2 CAR T cells were poor cytokine producers, generating <10x lower levels of IL2, TNFα and IFNγ than CD19 CAR T cells upon in vitro co-incubation with a GD2+CD19+ osteosarcoma line (143B-CD19), despite maintaining comparable in vitro cytolytic ability. GD2 CAR T cells showed poor in vitro expansion and increased rates of apoptosis compared to controls. GD2 CAR T cells also did not persist and did not mediate antitumor effects against GD2+CD19+ tumors in a murine xenograft model in vivo, whereas CD19 CAR T cells completely eradicated CD19+ tumors and persisted in both the spleen and tumor compartments.
To rule out the possibility that diminished cytokine production and in vivo efficacy was related to antigen specific effects, T cells were co-transduced with both the GD2 and CD19 CARs. Though single-transduced CD19 CAR T cells show no signs of an altered metabolism or exhaustion and have strong antitumor efficacy, CD19 CAR T cells co-transduced with the GD2 CAR demonstrate an exhausted phenotype and diminished antitumor efficacy similar to that of single-transduced GD2 CAR T cells. Thus, expression of the GD2 CAR confers a dominant exhausted phenotype in T cells, and prevents otherwise efficacious CARs from mediating strong antitumor effects.
We hypothesized that chronic signaling of CD3ζ and CD28 via the GD2 CAR results in exhaustion. Interestingly, however, we did not identify GD2 expression in the culture system. Point mutations in the CAR antigen-binding site, though abrogating GD2 binding, did not prevent the development of exhaustion. Thus, we postulate that constitutive receptor signaling may occur via interactions between the framework regions of the CAR receptors. Importantly however, substitution of 4-1BB for the CD28 domain in the GD2 CAR substantially diminished PD-1 expression, one of the hallmark features of exhausted T cells.

Conclusions
We report that expression of a CD28 containing GD2 CAR induces both an altered metabolism and an exhausted state in human T cells, resulting in poor in vivo persistence and antitumor efficacy. We hypothesize that tonic signaling through the GD2 CAR induces this phenotype and have identified the CD28 domain as an important component contributing to this phenotype. Rapid induction of exhaustion mediated via a synthetic receptor provides a novel model system to identify mechanistic factors required for this phenotype in human T cells. Work is currently underway to molecularly define the basis for the exhaustion of GD2 CAR T cells and to probe a potential role for altered T cell metabolism as a contributor to T cell exhaustion in human T cells.

Disclosures:
No relevant conflicts of interest to declare.