Targeting solid tumor antigens with chimeric receptors: cancer biology meets synthetic immunology

Abstract

Chimeric antigen receptor (CAR) T cell therapy is a medical breakthrough in the treatment of B cell malignancies. There is intensive focus on developing solid tumor-targeted CAR-T cell therapies. Although clinically approved CAR-T cell therapies target B cell lineage antigens, solid tumor targets include neoantigens and tumor-associated antigens (TAAs) with diverse roles in tumor biology. Multiple early-stage clinical trials now report encouraging signs of efficacy for CAR-T cell therapies that target solid tumors. We review the landscape of solid tumor target antigens from the perspective of cancer biology and gene regulation, together with emerging clinical data for CAR-T cells targeting these antigens. We then discuss emerging synthetic biology strategies and their application in the clinical development of novel cellular immunotherapies.

Keywords: CAR-T cells; antigen targeting; cellular immunotherapy; solid tumor; synthetic biology.

Fig. 1 |.. Evolution of CAR engineering.

A. Chimeric antigen receptors (CARs) have a modular design consisting of an antigen-binding domain, a hinge, a transmembrane domain and an intracellular signaling domain; the “conventional” CAR portrayed in this figure is a second-generation CAR, which incorporates an additional intracellular costimulatory domain, typically from CD28 or 4–1BB, which improves the persistence, survival, and proliferation of activated CAR-T cells. B. “Switchable” CARs are developed in part to circumvent toxicities associated with CAR-T cell therapy. In SNIP and VIPER-CARs, protease domains are incorporated within the CAR and act as inducible ON or OFF switches controlled with the administration of an FDA-approved small molecule[143,145]. A “universal adapter” CAR splits the T cell signaling domains from the antigen recognition elements through the use of a switch molecule; either a leucine zipper, or a separate antigen-monoclonal antibody pair. C. Combinatorial tumor antigen sensing has been accomplished with synNotch receptors that can operate on IF/THEN logic by recognizing a specific priming antigen, which can be used to locally induce expression of a CAR [148,149]. LINK CARs replace traditional CD3ζ domains with cooperative intracellular proximal T cell signaling molecules, which were further engineered to be activated by separate antigens- therefore functioning as an AND-gate for T cell activation [153]. D. Finally, CAR T cells can produce additional transgenes or act as “cellular micropharmacies”- enabling tumor-localized delivery of therapeutic payloads. CAR-T cells can be engineered to secrete T cell engager molecule (TEAM)[176], a bispecific secreted protein that conjugates CD3 to a second tumor antigen.

Fig. 2 |.. CAR Targets for solid tumors

Unlike currently approved CAR-T therapies targeting highly specific lineage antigens; investigational CAR targets against solid tumors involve neoantigens and tumor-associated antigens, which often have specific functions in tumor biology. Mitogenic receptors are prototypical proto-oncogenes constitutively activated in cancer, and include the receptor tyrosine kinase HER2 as well as EGFR, one of the most recurrently mutated and amplified genes in cancer[–10,26,27,32,105,122]. Potential lineage-specific solid tumor antigens include isoform 2 of Claudin 18 (Claudin 18.2), a tight junction protein that is retained on gastric cancers[75,76]. Mesothelin is expressed in a wide variety of solid tumors; its signaling prevents apoptosis and promotes motility and invasion[,,–186]. Prostate-specific membrane antigen (PSMA) is most highly expressed in prostate cancer and non-prostate cancer neovasculature[97,99,103]. Ligands of negative co-regulatory immunoreceptors selectively expressed in tumors include B7-H3, which also has diverse roles in mitogenic signaling, metabolism, chemoresistance, and cell migration. Oncofetal antigens include the disialoganglioside GD2, an immunosuppressive glycosphingolipid that is highly abundant in various tumors[41,48,51,56,57,105]. In the cancer / testis antigen group is IL-13Rɑ2, a “decoy” receptor for type 2 profibrotic cytokine IL-13, which has identified roles in mitogenic signaling, invasion, and metastasis[68]. Finally, an example of tumor neoantigen is EGFRvIII, a constitutively active variant of EGFR with a potentially immunogenic truncated extracellular domain[120,121,124].