Molecular Pathways: Breaking the Epithelial Cancer Barrier for Chimeric Antigen Receptor and T-cell Receptor Gene Therapy

Abstract

Adoptive transfer of T cells genetically engineered to express a tumor-targeting chimeric antigen receptor (CAR) or T-cell receptor (TCR) can mediate cancer regression in some patients. CARs are synthetic single-chain proteins that use antibody domains to target cell surface antigens. TCRs are natural heterodimeric proteins that can target intracellular antigens through recognition of peptides bound to human leukocyte antigens. CARs have shown promise in B-cell malignancies and TCRs in melanoma, but neither approach has achieved clear success in an epithelial cancer. Treatment of epithelial cancers may be particularly challenging because of a paucity of target antigens expressed by carcinomas and not by important healthy tissues. In addition, epithelial cancers may be protected by inhibitory ligands and soluble factors in the tumor microenvironment. One strategy to overcome these negative regulators is to modulate expression of T-cell genes to enhance intrinsic T-cell function. Programmable nucleases, which can suppress inhibitory genes, and inducible gene expression systems, which can enhance stimulatory genes, are entering clinical testing. Other work is delineating whether control of genes for immune checkpoint receptors (e.g.,PDCD1, CTLA4) and cytokine and TCR signaling regulators (e.g.,CBLB, CISH, IL12, IL15) can increase the antitumor activity of therapeutic T cells.

Figure 1. TCRs and CARs recognize tumor cells though distinct mechanisms

Genetically engineered TCRs complex with endogenous CD3 units to form structures with recognition and signaling capability. They signal with a physiological CD3 signal, and costimulation requires engagement of a separate coreceptor. TCRs recognize a peptide-HLA complex on the cell surface. The peptide in this complex is derived from a protein that is processed by intracellular machinery. Thus, TCRs can recognize intracellular antigens, but they also require antigen processing and presentation by the tumor cell, and their antigen recognition is HLA-restricted. CARs are synthetic proteins that do not exist in nature. They are composed of an antibody-derived recognition unit linked to a signaling domain from the CD3 molecule as well as a costimulatory domain from a costimulatory receptor. CARs recognize cell surface but not intracellular antigens. They do not require antigen processing or presentation by the tumor, and their target recognition is not HLA-restricted.

Figure 2. Emerging technologies and potential target genes for modified expression in therapeutic T cells

Genome editing with programmable nucleases (light blue) such as CRISPR-Cas9, TALENs, ZFNs, and meganucleases may decrease the presence of specific inhibitory genes in T cells. Inhibitory RNAs (magenta) can decrease the expression of negative regulators of T cell anti-tumor function. Target genes for these approaches might include inhibitory cell surface receptors (green), such as PD-1 or CTLA-4, or antagonists of TCR or cytokine signals such as CISH, CBL-B, or SHP-1 (dark blue).