Role of T cell receptor affinity in the efficacy and specificity of adoptive T cell therapies

Abstract

Over the last several years, there has been considerable progress in the treatment of cancer using gene modified adoptive T cell therapies. Two approaches have been used, one involving the introduction of a conventional αβ T cell receptor (TCR) against a pepMHC cancer antigen, and the second involving introduction of a chimeric antigen receptor (CAR) consisting of a single-chain antibody as an Fv fragment linked to transmembrane and signaling domains. In this review, we focus on one aspect of TCR-mediated adoptive T cell therapies, the impact of the affinity of the αβ TCR for the pepMHC cancer antigen on both efficacy and specificity. We discuss the advantages of higher-affinity TCRs in mediating potent activity of CD4 T cells. This is balanced with the potential disadvantage of higher-affinity TCRs in mediating greater self-reactivity against a wider range of structurally similar antigenic peptides, especially in synergy with the CD8 co-receptor. Both TCR affinity and target selection will influence potential safety issues. We suggest pre-clinical strategies that might be used to examine each TCR for possible on-target and off-target side effects due to self-reactivities, and to adjust TCR affinities accordingly.

Keywords: T cell cross-reactivity; T cell sensitivity; TCR affinity; adoptive T cell therapy; tumor-associated epitopes.

Figure 1

Relationship between T cell activities and TCR affinities for a class I pepMHC antigen, in either CD8 or CD4 T cells. Various TCRs whose affinity for their target class I pepMHC complexes have been measured are depicted on an affinity scale (K_d). The relative activity ranges for those receptors are listed for those TCRs expressed in CD8 (left) and CD4 (right) T cells. The activity boundaries are approximated from the best-known systems. Sensitivity at low TCR affinities is achieved due to TCR synergy with the CD8 co-receptor. This same principle can yield CD8-dependent, undesirable cross reactivities with structurally similar self-peptides.

Figure 2

T cell receptor affinity, specificity, and cross-reactivity in the 2C TCR system. (A) The m6α TCR engineered from the 2C TCR for increased affinity for QL9/L^d exhibited more sensitive reactivity with structurally related peptides with single-amino acid substitutions. Sensitization doses of various QL9 position 5 variant peptides for IL-2 production by CD8-negative TCR transfectants are shown. The log of the SD₅₀ value was plotted for each of the peptides used to stimulate 2C TCR (yellow bars) and m6α TCR (blue bars) transfectants [*Reproduced with permission from Ref. (56)]. (B) The 2C TCR reacts with the agonist SIY peptide/K^b complex and the putative positive-selecting peptide dEV8/K^b complex with K_d values of 30 and 80 μM, respectively. While the sequences share only two amino acids in common, they are structurally very similar [shown here aligned from their H2-K^b-bound structures, SIY in blue and dEV8 in red; PDB IDs 1G6R (92) and 2CKB (173)]. (C) Performing a protein BLAST search of the mouse proteome with the SIY peptide sequence string and an Expect value cut off of 5.0 yielded only two sequence-similar peptides. (D) Performing a proteome scan to find sequences similar to SIY, based in part on alanine scan data of the peptide epitope, and the tolerance for mutations at each position yielded 43 peptides considered to be similar (only 33 sequences predicted to bind with SYFPEITHI scores>16 are shown). Using this technique, the putative positive-selecting peptide, and the self-peptide that reacts with the higher affinity TCR m33 (71), called dEV8 was identified (shown in bold, highlighted in yellow).

Figure 3

Analysis of selected tumor epitopes for homologous sequences in the human and mouse proteomes. (A) MHC-binding prediction scores for a set of characterized T cell epitopes, including six HLA-A2-restricted human tumor-associated epitopes. (B) MHC-binding predictions for peptides identified in a MAGE-A3 [112–120] homology scan (no gaps) of the human and mouse proteomes. (C) MHC-binding predictions for peptides identified in a WT1 [126–134] homology scan of the human and mouse proteomes (including an allowed, single-amino acid gap). For (B,C), peptides were subjected to ANN and SMM prediction algorithms along with SYFPEITHI and BIMAS, and prediction to bind above the arbitrary thresholds described in the text in any of the algorithms was taken to indicate a potential binder. (D) A comparison of MAGE-A3 and WT1 proteome scan results. The total number of predicted binders identified in the human proteome and the percent of the binders identically found in the mouse proteome for both epitopes (searched without gaps) are highlighted in yellow.