Engineering the T cell receptor for fun and profit: Uncovering complex biology, interrogating the immune system, and targeting disease

Abstract

T cell receptors (TCRs) orchestrate cellular immunity by recognizing peptide antigens bound and presented by major histocompatibility complex (MHC) proteins. Due to the TCR's central role in immunity and tight connection with human health, there has been significant interest in modulating TCR properties through protein engineering methods. Complicating these efforts is the complexity and vast diversity of TCR-peptide/MHC interfaces, the interdependency between TCR affinity, specificity, and cross-reactivity, and the sophisticated relationships between TCR binding properties and T cell function, many aspects of which are not well understood. Here we review TCR engineering, starting with a brief historical overview followed by discussions of more recent developments, including new efforts and opportunities to engineer TCR affinity, modulate specificity, and develop novel TCR-based constructs.

Figure 1.

Structural overview of TCR-peptide/MHC complexes. a) Representative structure of a TCR-peptide/MHC complex (in this case, the HCV1406 TCR bound to the HCV NS3 epitope presented by HLA-A2 [69]). The TCR α and β chains are indicated. The HLA-A2 heavy chain is in grey, the peptide in yellow, and the β₂-microglobulin subunit in light blue. b) View through the TCR onto the peptide/MHC complex showing the positions of the six CDR loops. The green and brown circles show the centers of mass of the HCV1406 TCR’s α and β chain variable domain. Although TCR binding geometries (including docking angles, positions, and CDR loop architecture) vary [70], most TCRs that have been structurally characterized adopt geometries resembling that shown in panels a and b. c) The peptide typically represents about one-third of the surface contacted by a TCR, with the MHC protein contributing the remainder. The image shows the number of times peptide and class I MHC amino acids are contacted by a TCR, using a database of 95 structures of TCRs bound to class I peptide/MHC complexes and a 4 Å cutoff for interatomic contacts. A representative nonameric peptide is shown. The residues of the MHC are represented by a mesh surface and the peptide is represented by a solid surface. Image generated by UCSF Chimera [71].

Figure 2.

Diagram illustrating how increasing TCR affinity towards a target antigen can introduce new off-target reactivities. TCR affinity enhancement towards the target antigen (red circle), shown as a 100-fold enhancement in K_D, brings affinity into a range of maximum T cell potency, as indicated by the white/green curve. The resulting TCR might be considered an ideal candidate for targeting the red peptide. However, TCR affinity is also enhanced towards an off-target peptide (blue circle), shown as a 4-fold enhancement in K_D, perhaps due to structural/chemical similarity between the target and off-target peptides. Although TCR affinity for the off-target blue peptide is improved only a small amount compared to the target peptide, the 4-fold enhancement is still sufficient to move TCR affinity towards the off-target peptide from a regime where it is biologically ignored into a regime where it is functionally recognized. Given the vast number of potential target peptides, in a real scenario, there could be many such blue peptides. Figure adapted from ref. [18] with permission of the authors. Note that the positions of the various lines are approximate, and factors other than TCR affinity impact the quantity and quality of the T cell response as discussed in the main text.