Synthetic Immunotherapy: Programming Immune Cells with Novel and Sophisticated Logic Capabilities

The development of chimeric antigen receptor (CAR) T cells began as a means toward specific yet modular therapies against cancer. Recent advancements in several CAR T cell therapies show the promise of cellular immunotherapy in cancer treatment. CAR T cell therapy is still immature, however, and improvements are needed to fully realize its curative potential. The approved CAR T cells are designed with simple logic capabilities; an antigen sensor that, when bound to the target antigen, triggers costimulation domains and native T cell activation. This single-type sensor and native activation design, although capable, also has severe limitations. Reliance on a single-type sensor leads to unwanted toxicity toward antigen-expressing normal tissues, and unmodulated activation leads to unwanted cytokine toxicity. Synthetic biology (SB) offers a powerful solution to these limitations: modular receptors with customizable sensors and output behaviors that enable higher Boolean logic. SB T cells already have shown incredible capabilities, such as multiple-antigen discrimination and improved persistence. In light of these results, cellular immunotherapy may already be branching into a new subfield that we term here as "synthetic immunotherapy." Here we review the current logic capabilities of CAR T cells, the resulting limitations, and the engineering undertaken to address these issues. We then discuss several tools of SB and show how SB CAR T cells pave the way for synthetic immunotherapy.