Cancer Therapy With TCR-Engineered T Cells: Current Strategies, Challenges, and Prospects

Abstract

To redirect T cells against tumor cells, T cells can be engineered ex vivo to express cancer-antigen specific T cell receptors (TCRs), generating products known as TCR-engineered T cells (TCR T). Unlike chimeric antigen receptors (CARs), TCRs recognize HLA-presented peptides derived from proteins of all cellular compartments. The use of TCR T cells for adoptive cellular therapies (ACT) has gained increased attention, especially as efforts to treat solid cancers with ACTs have intensified. In this review, we describe the differing mechanisms of T cell antigen recognition and signal transduction mediated through CARs and TCRs. We describe the classes of cancer antigens recognized by current TCR T therapies and discuss both classical and emerging pre-clinical strategies for antigen-specific TCR discovery, enhancement, and validation. Finally, we review the current landscape of clinical trials for TCR T therapy and discuss what these current results indicate for the development of future engineered TCR approaches.

Keywords: CAR; T cell receptor; TCR; TCR T; TCR-engineered T cells; adoptive cell therapy; chimeric antigen receptor.

Figure 1

Antigen recognition by CARs and TCRs. CARs recognize surface proteins typically through an antibody-derived scFv recognition domain. Antigen recognition leads to T cell activation via phosphorylation of ITAMs in a conjugated intracellular CD3ζ domain. In the case of later generation CARs, ligand binding also leads to additional stimulation of conjugated costimulatory receptors (e.g. CD28, 4-1BB). TCRs recognize HLA-presented peptides which may be derived from any cellular compartment. Antigen recognition by TCRs leads to T cell activation through phosphorylation of ITAMs in the associated CD3ϵ/γ/δ/ζ subunits. Depending on T cell subtype, T cell activation through either receptor type will trigger effector functions including proliferation, cytokine secretion, and target cell killing through directed secretion of perforin and granzyme.

Figure 2

TCR-recognized tumor antigens. Viral antigens result from viral oncogenes which are not present in normal cells. Neoantigens arise from somatic mutations not found in normal cells. Viral antigens and neoantigens are collectively referred to as tumor-specific antigens (TSAs). Cancer germline antigens (CGAs) are derived from proteins that are normally only expressed in germ cells such as testis which lack HLA class I expression. Overexpressed antigens arise from proteins highly overexpressed in cancer tissue as compared to normal tissue. Cancer differentiation antigens are expressed by cancer cells and their expression is otherwise limited to only the normal cells of the same tissue origin as the cancer. Overexpressed antigens and cancer differentiation antigens are collectively referred to as tumor-associated antigens (TAAs).

Figure 3

TCRs recognize antigens presented by specific HLA alleles. TCR antigens are predominantly presented by six HLA genes. These include genes for HLA class I (A, B, and C), and class II (DR, DP, and DQ). These HLA genes are highly polymorphic, with many allele variants in the human population. Humans inherit one set of each gene from each parent, and human cells can therefore express up to twelve different HLA presenting alleles. For a given TCR, the specific HLA allele that presents the cognate peptide is referred to as the ‘restricting HLA’ of the TCR.

Figure 4

TCR modifications to prevent mispairing and maximize surface expression. Illustration of mispairing between endogenous TCR and engineered TCR. Murinized TCRs replace the human TCR constant regions with those of a mouse TCR constant region. The addition of an extra disulfide bond in the TCR constant region through cysteine substitutions stabilizes interchain binding affinity of engineered TCR α/β chains while reducing their binding affinity with endogenous TCR α/β chains. Stability of the engineered TCR α chain can be increased through select hydrophobic substitutions in its transmembrane region. Domain swapped TCRs invert large or specific segments of the engineered TCR α/β constant regions, which reduces propensity of engineered TCRs to mispair. Single-chain TCRs (scTCR) encode TCR antigen recognition and signaling domains into a single chain. Three-domain and two-domain scTCRs differ by the inclusion or absence of the TCR β constant region, respectively. Genome engineering strategies utilize RNA interference or endonuclease technologies to reduce or ablate endogenous TCR expression.

Figure 5

Trends in TCR T trials initiated thus far. TCR T trials registered in clicaltrials.gov were assessed as of October 3^rd 2021. (A) The number of new TCR T trials initiated each year and the cumulative number of registered TCR T trials by year. (B) Clinical status of the 120 TCR T trials. (C) Classifications of 118 tumor antigen targets in 116 TCR T trials with specified target antigens. (D) Ten most common targets in TCR T trials. (E) Diseases targeted in TCR T trials. (F) Frequency of 111 target antigen-restricting HLAs in 100 TCR T trials that specified HLA restriction. (G) Locations where TCR T trials have been conducted by country. (H) Primary sponsors of the 80 TCR T trials conducted in the United States.