Empirical and Rational Design of T Cell Receptor-Based Immunotherapies

Abstract

The use of T cells reactive with intracellular tumor-associated or tumor-specific antigens has been a promising strategy for cancer immunotherapies in the past three decades, but the approach has been constrained by a limited understanding of the T cell receptor's (TCR) complex functions and specificities. Newer TCR and T cell-based approaches are in development, including engineered adoptive T cells with enhanced TCR affinities, TCR mimic antibodies, and T cell-redirecting bispecific agents. These new therapeutic modalities are exciting opportunities by which TCR recognition can be further exploited for therapeutic benefit. In this review we summarize the development of TCR-based therapeutic strategies and focus on balancing efficacy and potency versus specificity, and hence, possible toxicity, of these powerful therapeutic modalities.

Keywords: T cell receptor; T cell receptor mimic monoclonal antibody; T cell receptor-T cell; bispecific T cell engager; cross-reactivity; immune mobilizing monoclonal T cell receptors against cancer; peptide- major histocompatibility complexes; tumor infiltrating lymphocytes.

Figure 1

TCR-based therapeutics recognize peptide/MHC antigens (red and pink) on cells by utilizing either TCRs (light blue) or TCRm antigen-binding domains (green). Left: Soluble ImmTAC molecules bind peptide/MHC on cancer cells via alpha/beta TCR heterodimer similar to membrane-bound TCR and redirect the T cells by engaging extracellular CD3-epsilon (purple) via an anti-CD3 scFv. Right: TCRm mAb recognize peptide/MHC complex via its variable region (green) and to engage effector cells such as NK cells and macrophages to elicit Fc-receptor (orange) mediated ADCC or ADCP. TCRm CAR and bispecific mAb leverage TCRm-derived scFv to harness T cell effector function via engagement with intracellular CD3-zeta (blue) or extracellular CD3-epsilon (purple), respectively.

Figure 2

Methods to determine targets and off targets of TCR-based therapeutics. (A) Peptide Scans: Peptide scanning techniques determine which residues are important and in which positions. Alanine scans and x-scans hold all positions in the native peptide (blue) except one (yellow); one position is switched to an alanine for alanine scans or to all other amino acids for X-scans. CPL scans hold one position constant (yellow) and all other positions (blue) can be any combination of amino acids. Peptides are then pulsed onto target cells to assay for TCR reactivity. (B) Yeast Display: Yeast display libraries genetically encode a peptide linked to an MHC. Multiple rounds of selection with soluble TCRs select for peptide-MHC complexes that are recognized by the TCR. (C) Multimers: Multimers can identify T cells that bind specific peptide-MHC complexes. To identify reactive peptide-MHC complexes in a pooled setting multimers are DNA barcoded prior to incubation with and binding to T cells. (D) T scan and SABR: The T Scan method genetically encodes for longer peptides that go through endogenous processing and presentation. Target cells that present peptide-MHC complexes targeted by T cells fluoresce through a granzyme reporter system and are subsequently sorted by FACS. For the SABR method, TCR expressing cells bind to target cells expressing the SABR receptor. After TCR binding, this receptor, which consists of an MHC linked to a CD3z and CD28 domain, signals to an NFAT report system causing target cells to fluoresce. Target cells can then be sorted by FACS. (E) PresentER: The PresentER system genetically encodes for peptides to be presented in MHC complexes on TAP-deficient cells. Target peptides are identified through coculture depletion assays with T cells.