Catch-bond engineering surpasses high-affinity maturation for T cell receptor therapies against solid tumors

Technical Summary: Catch-bond engineering surpasses high-affinity maturation for T cell receptor therapies against solid tumors

Problem Statement
The sensitivity of T cell receptors (TCRs) has traditionally been attributed to static binding affinity, a paradigm that has driven TCR engineering strategies for decades. High-affinity maturation remains the predominant approach to enhance TCR sensitivity, utilized in the only FDA-approved TCR-T cell therapy (Tecelra/afamitresgene autoleucel) and TCR-based T cell engagers (TCEs). However, this strategy is associated with severe off-target toxicities in clinical settings, and the highest affinity variants are not necessarily the most sensitive or effective. While recent studies suggest TCRs function as mechanosensors capable of forming "catch bonds" (interactions that strengthen under mechanical force), it remains uncertain whether engineering catch bonds offers a superior, safer alternative to affinity maturation across diverse therapeutic applications.

Methodology
The study utilized the FDA-approved, high-affinity-matured TCR4 (targeting MAGE-A4/HLA-A2) as a clinical benchmark to compare against novel catch-bond-engineered variants.

• Screening and Engineering: The authors performed a comprehensive amino acid scanning of the TCR complementarity-determining regions (CDRs) using all polar and charged residues. They identified "hotspots" where mutations enhanced T cell activation. Histidine scanning proved most effective, identifying ten hotspots.
• Variant Generation: Two strategies were employed: (1) mutating identified hotspots to various polar/charged amino acids to maximize signaling, and (2) combinatorial engineering of the top five histidine-identified hotspots (creating variants like 2mut, 5H, and 5mut).
• Biophysical Characterization:
  • Affinity: Surface plasmon resonance (SPR) measured 3D binding affinities.
  • Catch Bond Dynamics: Magnetic tweezers were used to measure force-dependent bond lifetimes (2D off-rates) at the single-molecule level.
  • Structural Analysis: AlphaFold3 predictions were refined using 500 ns molecular dynamics (MD) simulations. Steered molecular dynamics (SMD) simulations were conducted to observe the dynamic evolution of interfacial interactions under shear force.
• Functional Assays:
  • In Vitro: TCR-transduced SKW3 cells and primary human T cells were co-cultured with MAGE-A4+ tumor cells (A375) to assess activation (CD69, TNF, IFN-γ), proliferation, serial cytotoxicity, and exhaustion markers (PD-1, TIM-3).
  • In Vivo: Immunodeficient NOG mice bearing A375 tumors received adoptive transfer of TCR-T cells or were treated with bispecific T cell engagers (BiTEs) based on the TCR scaffold.
  • Safety: X-scan analysis and off-target peptide screening (top 100 predicted peptides) were conducted to assess cross-reactivity and alloreactivity.

Key Results

• Histidine as a Key Residue: Among all polar and charged amino acids, histidine was identified as the most effective residue for pinpointing engineering hotspots that enhance TCR sensitivity.
• Superiority of Catch Bond Engineering: The engineered 5mut TCR variant, which formed strong catch bonds, significantly outperformed the high-affinity-matured TCR4 in multiple metrics:
  • Potency: 5mut TCRs induced higher T cell activation and proliferation against A375 tumor cells.
  • Cytotoxicity: In serial killing assays, 5mut TCR-T cells maintained superior tumor cell elimination over three rounds, whereas TCR4 and wild-type (WT) T cells failed to suppress tumor proliferation after the first round.
  • In Vivo Efficacy: In mouse models, 5mut TCR-T cells significantly delayed tumor progression and prolonged survival compared to TCR4 TCR-T cells, without inducing weight loss or toxicity.
• Mechanism of Action:
  • Affinity vs. Catch Bonds: While TCR4 exhibited an 11-fold higher static affinity than WT, the 5mut variant had a lower affinity (173 nM) yet demonstrated superior functional potency. Crucially, TCR sensitivity correlated with the strength of catch bonds (peak bond lifetime under force), not static affinity.
  • Molecular Dynamics: SMD simulations revealed that 5mut mutations (M50Tα, F52Hα, Q100Hβ) facilitated a "force-induced reconfiguration" of the interface. Under shear, these variants formed new, sequential hydrogen bonds (e.g., F30α–E166, Y97β–T163) that resisted dissociation, creating a robust catch bond mechanism. In contrast, TCR4 relied on pre-existing interactions that did not transition as effectively under force.
  • Cellular Phenotype: 5mut TCRs promoted prolonged T cell–tumor cell interactions, enhanced immunological synapse formation (larger synapse area, CD2-looped configuration), and increased phosphorylation of ZAP70 and CD3ζ. Notably, 5mut T cells exhibited lower expression of exhaustion markers (PD-1, TIM-3) compared to TCR4, suggesting a more sustainable functional state.
• Safety: Both 5mut and TCR4 variants showed similar tolerance patterns in X-scan analysis and no cross-reactivity against the top 100 predicted off-target peptides or alloreactivity against MAGE-A4-negative/HLA-A2-negative cell lines.
• Extension to BiTEs: Bispecific T cell engagers (BiTEs) constructed on the TCR scaffold also formed catch bonds with pMHC. Catch-bond-engineered BiTEs demonstrated superior tumor inhibition and cytokine production compared to high-affinity-matured BiTEs, which failed to control tumor growth in vivo.

Significance and Claims
The paper claims to demonstrate that catch bond formation, rather than static affinity, is the primary determinant of TCR sensitivity and therapeutic efficacy. By shifting the engineering paradigm from static affinity optimization to the dynamic stabilization of bonds under mechanical force, the authors present a strategy that:

1. Surpasses Clinical Benchmarks: Catch-bond-engineered TCRs (specifically the 5mut variant) outperformed the only FDA-approved high-affinity TCR-T therapy in both in vitro and in vivo models against solid tumors.
2. Enhances Safety: This approach achieves high potency without the off-target toxicities or alloreactivity often associated with high-affinity maturation, likely due to the "mechano-checkpoint" nature of catch bonds which requires specific force-dependent reorganization for activation.
3. Preserves T Cell Function: The strategy promotes sustained T cell functionality and reduces exhaustion, leading to more durable therapeutic responses.
4. Broad Applicability: The findings extend beyond cellular therapies to protein-based modalities like BiTEs, suggesting that catch bond engineering is a versatile framework for next-generation TCR-based immunotherapies.

The study concludes that engineering force-induced catch bonds represents a conceptual shift in TCR biology, aligning therapeutic development with the intrinsic mechanobiological principles of T cell activation.