Mechanism of TCR-T Therapy
Structure and Signaling Initiation of T cell receptor (TCR)
T cells are essential components of the adaptive immune system, and the T cell receptor (TCR) is
the primary membrane receptor responsible for antigen recognition and T cell activation. The TCR
is a finely regulated transmembrane multi-subunit signaling complex. Antigen recognition is
mediated by the TCRαβ heterodimer, which specifically recognizes the peptide-MHC (pMHC)
complexes on the surface of antigen-presenting cells. However, the intracellular domain of TCRαβ
lacks signaling motifs.
Signal transduction is instead mediated by associated CD3 subunits (CD3γε, CD3δε, and ζ chains),
whose cytoplasmic regions are enriched in immunoreceptor tyrosine-based activation motifs
(ITAMs). Upon antigen binding, SRC family kinases such as LCK phosphorylate ITAMs, leading to
the recruitment of downstream signaling molecules, including ZAP-70, and initiation of signaling
cascades.
In addition to ITAMs, CD3 subunits contain multiple regulatory motifs (e.g., PRS, RK, BRS, and
endocytosis-related motifs), which contribute to signal amplification, membrane localization,
kinase recruitment, and receptor internalization. These features enable precise spatial and
temporal regulation of TCR signaling.
Structurally, the TCR–CD3 complex forms tightly packed transmembrane helical bundles with an
ordered architecture, while its cytoplasmic tails remain highly dynamic. This structural
flexibility underlies the conformational changes required for signal transduction.
Figure 1. TCR and its functional signal modes[1].
TCR activation involves multi-level conformational changes and spatial reorganization after
antigen binding. According to the "kinetic segregation model", TCR engagement with pMHC leads to
receptor clustering and formation of tight contact zones within the immune synapse. This process
excludes large phosphatases such as CD45 and CD148, thereby promoting sustained LCK-mediated
ITAM phosphorylation and signal amplification.
Concurrently, conformational rearrangements within the TCR–CD3 complex expose functional motifs
such as ITAM, PRS, and RK on CD3ζ and CD3ε, facilitating the recruitment of signaling proteins
such as ZAP-70, NCK, and PI3K.
Cryo-electron microscopy studies further suggest that resting TCR complexes adopt a
self-inhibited conformation stabilized by membrane cholesterol and cytoplasmic tail
interactions. Antigen binding or membrane changes release this inhibition, triggering
activation.
Additionally, CD4/CD8 co-receptors enhance LCK recruitment and stabilize pMHC binding, enabling
T cells to surpass activation thresholds even under conditions of low antigen affinity or
density.
TCR-T Therapy vs. CAR-T Therapy
TCR-T cell therapy involves genetically modifying autologous T cells to express tumor-specific
TCRs. Unlike CAR-T cells, which recognize surface antigens in an MHC-independent manner, TCR-T
cells recognize intracellular tumor-derived peptides presented by MHC molecules.
This MHC-dependent recognition allows TCR-T cells to target a broader range of antigens,
including cancer-testis antigens (e.g., NY-ESO-1, MAGE-A4) and neoantigens derived from somatic
mutations (e.g., KRAS G12D). Consequently, TCR-T therapy expands the scope of targetable tumor
antigens beyond surface molecules.
Figure 1. Different mechanisms of action of CAR-T cell and TCR-T cell therapies[2].
The manufacturing processes of TCR-T and CAR-T cells: T cells are broadly similar, involving T
cell collection, genetic modification, ex vivo expansion, and reinfusion following
lymphodepletion.
Table 1. Comparison of CAR-T and TCR-T cell therapies[2].
| Feature |
CAR-T Cell Therapy |
TCR-T Cell Therapy |
| Target antigens |
Surface antigens (eg, CD19, BCMA) |
Intracellular peptide antigens presented on MHC (eg, NY-ESO-1, MAGE-A4 |
| T-cell receptor structure |
CAR |
TCR |
| Major histocompatibility complex (MHC) dependency |
MHC-independent |
MHC-dependent |
| Approved therapies (mainly for
hematologic malignancies) |
Tisagenlecleucel
Axicabtagene ciloleucel
Brexucabtagene autoleucel
Idecabtagene vicleucel
Lisocabtagene maraleucel
Ciltacabtagene autoleucel |
Afamitresgene autoleucel |
Notably, in August 2024, the U.S. FDA granted accelerated approval to afamitresgene autoleucel,
a TCR-T cell therapy that targets the MAGE-A4 antigen presented by a specific human leukocyte
antigen (HLA) type, for unresectable or metastatic synovial sarcoma. The SPEARHEAD-1 clinical
trial reported an overall response rate of 39% in 44 patients, representing a significant
milestone for TCR-T therapy in solid tumors.
Challenges of TCR-T Therapy in Solid Tumors
Despite promising clinical activity, TCR-T therapy faces several critical challenges in
solid tumors. These can be broadly categorized into four areas: TCR mismatch, potential
toxicity, cytokine storm, and the
tumor microenvironment
(TME).
Figure 3. Challenges of TCR-T therapy in solid tumors[3].
TCR mismatch
Mismatch between exogenous and endogenous TCR chains is major challenge. Introduced TCR
chains may pair with endogenous chains to form mixed dimers with unpredictable specificity,
potentially impairing function and causing safety risks such as graft-versus-host disease
(GVHD).
Additionally, mixed dimers may compete for limited CD3 components, reducing the efficiency
of engineered TCR expression and increasing off-target effects.
Strategies to address this issue include:
• Disulfide Bond Engineering: Introducing cysteine residues into the TCR constant domains to
form supplementary disulfide bonds significantly enhances the pairing stability of exogenous
TCR chains.
• Murinization: Constructing hybrid TCRs with murine constant regions prevents cross-pairing
with endogenous TCRs, though this approach must be weighed against potential immunogenicity
concerns.
• Affinity Maturation: Utilizing yeast surface display to screen for high-affinity TCRs
inherently minimizes the risk of mispairing.
• Transmembrane Modification: Applying hydrophobic modifications to the transmembrane domain
further augments chain pairing and structural stability.
• Single-chain TCRs (scTCRs): Fusing the recognition and signaling domains into a single
polypeptide chain serves as a highly effective structural strategy to circumvent mispairing
entirely.
Potential Toxicity
TCR-T therapy is highly sensitive to antigen recognition, making it prone to both on-target
and off-target toxicity. On-target toxicity arises when target antigens are also expressed
in normal tissues, while off-target toxicity results from cross-reactivity with unrelated
peptides.
Clinical evidence highlights these risks. For example, high-affinity TCRs targeting MAGE-A3
caused fatal cardiotoxicity in patients despite the lack of myocardial MAGE-A3 expression,
underscoring the lethal consequences of off-target peptide cross-reactivity. Similarly,
targeting melanocyte antigens damages normal melanocytes, and carcinoembryonic antigen
(CEA)-targeted TCR-T therapy can induce severe colitis.
A key challenge lies in balancing TCR affinity: while higher affinity enhances anti-tumor
activity, it also increases the risk of cross-reactivity and T cell exhaustion.
Cytokine Storm
Cytokine storm (CS) is a severe systemic inflammatory response induced by cellular
immunotherapies, clinically manifesting as fever, tachycardia, hypotension, and dyspnea. Its
pathogenesis is primarily driven by the persistent elevation of pro-inflammatory cytokines,
including IFN-γ, IL-1, IL-6, TNF, and IL-18.
Although TCR-T therapy typically exhibits a lower CS incidence than CAR-T owing to its
physiological signaling, significant clinical risks persist. For instance, affinity-enhanced
TCRs targeting NY-ESO-1 or MAGE-A4 have still triggered early-onset CS and immune effector
cell-related neurotoxicity syndrome (ICANS), requiring tocilizumab intervention.
Therefore, decoupling systemic inflammation from potent anti-tumor efficacy remains a
critical challenge for optimizing the safety and clinical translation of TCR-T therapies.
Tumor Microenvironment
The tumor microenvironment presents a major barrier to TCR-T efficacy. It is characterized
by hypoxia, chronic inflammation, and immunosuppression. Hypoxic and acidic conditions
intrinsically limit T cell proliferation, cytokine secretion, and cytotoxicity.
Concurrently, metabolic abnormalities drive the accumulation of suppressive metabolites that
accelerate CD8+ T cell exhaustion. This suppression is further compounded by
tumor-associated macrophages (TAMs) and stromal cells, which highly express inhibitory
ligands like PD-L1, and the upregulation of molecules like CD39 on T cells. Together, these
factors synergize to induce T cell dysfunction and apoptosis, severely restricting T
cell-mediated anti-tumor efficacy.
Optimization and Combination Strategies for TCR-T Therapy
TCR Endogenous Engineering Optimization: Constructing a Highly Consistent Receptor
Expression System
CRISPR/Cas9-mediated knockout of endogenous TRAC and TRBC genes, combined with targeted
knock-in at the TRAC locus, enables exogenous
TCRs to be expressed under physiological regulatory control. This strategy ensures uniform
receptor expression, significantly reduces
TCR chain mispairing, and improves signaling consistency. Compared with conventional random
viral integration, it minimizes expression
heterogeneity and allows engineered T cells to more closely mimic the signaling behavior of
native T cells[4].
In addition, personalized neoantigen screening combined with CRISPR-edited TCR-T cell
reinfusion has demonstrated promising potential in the treatment of solid tumors[5].
Protein engineering approaches further enhance TCR structural stability. These include
introducing additional disulfide bonds in the constant region to promote αβ chain pairing,
replacing human constant regions with murine counterparts to reduce heterologous pairing,
and designing scTCRs to prevent inter-chain recombination. Collectively, these strategies
improve the consistency and controllability of TCR expression.
Figure 4. CRISPR/Cas9-targeted CAR gene integration into the TRAC locus[4].
TCR Affinity Optimization and Specificity Engineering: Establishing a Functional Recognition
Window
Technologies such as phage display, yeast display and structure-guided mutagenesis enable
precise optimization of TCR binding to peptide-MHC complexes while preserving antigen
specificity. Affinity maturation strategies follow the functional "golden window"
principle—optimizing TCR affinity to a moderately enhanced range (low nM level) to improve
anti-tumor activity while minimizing off-target recognition and T cell exhaustion associated
with excessive affinity[5].
Current TCR engineering strategies emphasize structural precision. Tumor-specific
recognition is enhanced through complementary determining region (CDR) redesign, while
overall receptor stability is improved via framework mutations. In parallel, structural
information of pMHC is increasingly used for reverse screening to eliminate potential
cross-reactivity[6].
T cell Functional Reprogramming: Construction of Armored TCR-T cells
"Armored TCR-T cells" are engineered to express additional functional modules that enable
sustained activity within immunosuppressive TME[7]. A core strategy involves the expression
of immunomodulatory cytokines: IL-12 enhances Th1 responses and cytotoxicity, IL-15 supports
memory T cell persistence, and IL-18 promotes IFN-γ production and long-term anti-tumor
effects. These modifications have shown enhanced anti-tumor activity and resistance to T
cell exhaustion in preclinical models.
Chemokinase receptor engineering further enhances tumor infiltration. For example,
expression of CCR2b or CXCR3 improves T cell migration and accumulation within tumor
tissues. Additionally, "receptor switch engineering" converts inhibitory signals into
activating ones by replacing inhibitory domains (e.g., PD-1) with co-stimulatory domains
such as CD28 or 4-1BB. This approach sustains T cell activation in suppressive environments,
and promotes the transition of TCR-T cells from passive response responders to actively
adaptive immune effectors[7].
Combination Immunotherapy Strategies: Mechanisms of Synergy
Combining TCR-T therapy with immune checkpoint inhibitors restores T cell proliferation and
cytotoxicity by blocking the PD-1/PD-L1 axis, resulting in improved tumor control compared
with monotherapy in multiple clinical studies[8].
Radiotherapy enhances antigen release and presentation by inducing immunogenic cell death,
thereby broadening the antigen repertoire and promoting epitope spreading. Chemotherapy
facilitates TCR-T cells engraftment through lymphodepletion, while also reducing
immunosuppressive cell populations and improving the TME. These combination strategies have
demonstrated feasibility and therapeutic benefit in clinical studies[5,8].
Next-Generation Cell Therapy Platform: Development of Allogeneic Universal TCR-T
Allogeneic "off-the-shelf" TCR-T are engineered through multiplex gene editing to improve
compatibility and persistence. This includes knockout of endogenous TCRs to prevent
graft-versus-host disease (GVHD), deletion of HLA-I molecules (e.g., β2-microglobulin) to
reduce host rejection, and incorporation of immune-evasion strategies to prolong in
vivo survival[7,9].
Encouraging progress has been achieved in clinical trials for solid tumors such as synovial
sarcoma, providing a foundation for the development of allogeneic TCR-T platforms[9]. With continued advances in gene editing and cell
manufacturing technologies, allogeneic TCR-T therapy is poised to shift cell therapy from a
personalized approach toward a standardized, scalable biopharmaceutical model.
Summary
TCR-T cell therapy expands the spectrum of targetable tumor antigens by recognizing
intracellular peptides presented by MHC molecules and is showing early clinical activity in
solid tumors. However, its efficacy is still limited by challenges including TCR mispairing,
safety concerns, off-target toxicity, and the immunosuppressive TME. Advances in TCR
engineering, functional reprogramming, combination strategies, and allogeneic platforms are
expected to improve therapeutic performance and accelerate its broader clinical application.
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Mechanism of TCR-T Therapy
Challenges of TCR-T Therapy in Solid Tumors
Optimization and Combination Strategies for TCR-T
Therapy
