Combinational antigen recognition is the most logical way to improve the safety of cancer therapy. CAR T cells therapy, combined with synthetic biology, protein engineering, and bioinformatics, can perform advanced computations to enhance tumor targeting specificity.
Immunotherapy, including biologic and cellular approaches, is a critical area of focus for cancer therapeutics. Of the methodologies used to ‘‘upgrade’’ a patient’s immune cells, one of the most promising has been chimeric antigen receptor (CAR) T cell therapy, wherein T cells are engineered to express synthetic antigen-specific receptors.
A CAR consists of a ligand-binding domain fused to signaling domains from T cell receptor and costimulatory receptors. Antigen binding by the CAR triggers the signaling domain, leading to T cell activation and cell killing.
Several CAR-T therapies have been approved by the FDA to treat different types of blood cancers such as acute lymphoblastic leukemia, large B cell lymphoma, and mantle cell lymphoma.
The solution to the issue of cell targeting specificity is obvious yet elusive. It is a common knowledge that most human cell types, including cancer cells, are best characterized by a combination of features, rather than just one ‘‘magic bullet’’ biomarker. Therefore, an agent that can sense and logically respond to multiple features of the cancer is the most sensible approach to develop an efficacious and safe therapy.
Using multiple synthetic receptors, have been developed CAR systems or synthetic Notch receptor (synNotch) that fully activate the T cell
only when two different antigens are presents on the cancer cells (AND gate).
Also, an inhibitory CAR (iCAR) using the intracellular domain of the PD-1 receptor has been developed. The iCAR, when combined with a conventional CAR, can form a 2-Input A AND NOT B gate. Inhibitory CARs could have a paradigm-shifting impact in cancer drug discovery because
they would enable the use of ‘‘missing proteins’’ as a target, which is not
possible by conventional therapeutics.
Lajoie, Boyken, and Salter et al. demonstrates an alternative AND gate and NOT gate CAR designs. Instead of using multiple receptors that integrate the signal intracellularly downstream of the receptor activation, Lajoie et al.’s system involves one receptor and performs the logic operation extracellularly through computationally designed protein logic circuits. Using their previously designed Latching Orthogonal Cage–Key pRotein (LOCKR) switches, they improved the system to be colocalization dependent (Co-LOCKR).
The resulting co-LOCKR AND gate circuit consists of a ‘‘cage’’ and ‘‘key’’ protein, each attached to an antigen-binding domain. The cage protein also contains a peptide that can bind to CAR T cells. The peptide, however, is sequestered by a latch domain on the cage protein. The key protein binds to the cage protein, causing a conformation change and exposing the peptide for binding to and activating theCAR.
The cage and key proteins are designed to not interact in solution. However, colocalized to the cell surface by antigen-binding domains favors cage-key
complex formation. They also demonstrated A AND NOT B CAR by adopting
a decoy design. The decoy binds to the key protein, thus preventing it from activating the cage.
The Co-LOCKR design displays a remarkable logic computation capability
with human immune cells. It also seems to have low off-target killing and robust activation in vitro. However, improvement is needed to translate this groundbreaking technology, especially for the NOT logic circuit.
Cancer has proven to be a formidable foe that is extraordinarily complex and remarkably relentless. We need therapeutic agents that can match the sophistication of cancer cells. CAR T cell developments, including the work by Lajoie et al. and Dannenfeser et al., are beginning to impart the intelligence and persistence into therapeutic agents required to level the playing field against cancer.While the synthetic receptor circuit designs may seem complicated, they are developed in direct response to the complex biology of many cancers