Engineering and Design of Chimeric Antigen Receptors

Abstract

T cells engineered with chimeric antigen receptors (CARs) have emerged as a potent new class of therapeutics for cancer, based on their remarkable potency in blood cancers. Since the first clinical reports of their efficacy emerged 7 years ago, investigators have focused on the mechanisms and properties that make CARs effective or toxic, and their effects on T cell biology. Novel CAR designs coupled with improvements in gene transfer technology, incorporating advances in gene editing, have the potential to increase access to engineered cell therapies, as well as improve their potency in solid tumors.

Keywords: CAR T cells; cancer; chimeric antigen receptors; immunotherapy.

Figure 1

Two Families of Hinges IgG-based hinges, derived from IgG1, IgG2, or IgG4, provide flexibility to the scFv to target membrane-proximal epitopes. However, binding of the CH2 domain to Fc receptors in myeloid cells can impair functionality. Modification or deletion of the CH2 domain can restore CAR T cell functionality. CARs with shorter hinges, including an IgG-derived hinge lacking the CH2-CH3 regions, or hinges derived from native CD28 and CD8 hinges, can be used to target membrane-distal epitopes.

Figure 2

The Next Generation CARs: New Strategies to Avoid Tumor Escape and Mitigate Toxicities Schematic representation of novel receptors. (A–C) Multitargeted CAR configurations. Dual CARs co-express two different CARs in one cell (A). Tandem CARs contain two different scFvs in a single CAR molecule that can either be stacked (B) in series or as a looped structure (C). Dual and tandem CARs function as OR-gate circuits: CAR T cells are activated following recognition of antigen A OR antigen B. (D and E) Combinatorial CARs combine two constructs: one bears the CD3z signaling motif and the other bears the costimulatory signaling domain (D). Synthetic Notch (syn-Notch) receptors induce the transcription of a CAR after antigen recognition of their cognate antigen (E). Combinatorial and syn-Notch CARs function as AND-gate circuits: CAR T cells are fully activated only when antigen A AND antigen B are recognized. (F) On-switch CAR T cells are inactive until specific activating agents are added, assembling a fully functional receptor. (G) Inhibitory CARs (iCAR) inhibit T cell activation following antigen recognition in normal cells. (H) Universal or switchable CAR T cells remain inactive until antibody-based molecules targeting a tumor antigen are supplied to reconstitute a fully active CAR construct.