TCR Therapy: How Does It Work?
An introduction to genetic modulation of immune cells by TCR therapy
What Is TCR Therapy?
T-cell receptor (TCR) therapy is a gene immunotherapy technique in which the patient’s T cells are genetically engineered to express a specific TCR that targets a particular antigen of interest in order to treat diseases1–3
- TCRs are protein complexes on the surface of T cells that recognize fragments of antigen as peptides bound to major histocompatibility complex (MHC) molecules found on cancer cells or other diseased cells3,4
- Upon recognition of the target peptide, modified T cells expressing specific TCRs become activated to attack the target cell4
Because TCR therapy is MHC restricted and depends on presentation by MHC molecules to recognize targets and activate T-cell function, the mechanisms by which TCRs recognize antigens are very different from those for chimeric antigen receptor (CAR) T-cell therapy3:
Uses heterodimers to recognize polypeptide fragments (including intracellular antigen fragments) presented by MHC molecules on the surface of diseased cells
Uses antibody fragments that bind to speciﬁc antigens already found on the surface of diseased cells in an MHC-independent manner
TCR Core Structure and Function
A TCR is a transmembrane heterodimer found on the surface of T lymphocytes and is composed of two subunits: α and β (both present in the majority of T cells)3,5
Within each of these subunits, there are complementarity-determining regions that govern the antigen to which the TCR binds6
Interaction between the TCR on the T cell and the peptide fragments presented by MHC molecules on the target cell causes stimulation of T-cell function2,4
This results in the phosphorylation of cluster of differentiation (CD)3 subunits (CD3γε, δε, and ζζ dimers) and initiates a protein kinase signaling cascade leading to an immune response2,7
Image adapted from Anguela XM, High KA. 20198.
CD, cluster of differentiation; α, alpha; β, beta; δ, delta; ε, epsilon; γ, gamma; ζ, zeta.
In addition, initiation of TCR signaling may require co-receptors (e.g. CD4, CD8)9
- These co-receptors act as cellular adhesion molecules that bind to their respective MHC molecules and stabilize the interaction between the T cell and the peptide–MHC9,10
Engineering T Cells for TCR Therapy
Engineering T cells for TCR therapy involves direct modification of the TCR to allow for binding to a specific antigen presented by MHC molecules on target diseased cells3
TCR therapy enhances both the specificity and affinity of T cells for diseased cells3
The construction of T cells for TCR therapy requires the identification of specific targets, which can be accomplished by the following steps3:
- The polypeptides presented by diseased cells are identified
- The polypeptides presented in normal tissues are screened out
- TCR library (e.g. TCRs that are displayed on phage, yeast, or mammalian cell systems) is established to screen for TCRs with high affinity and speciﬁcity
- A preclinical safety test is then performed to ensure minimal off-target effects and cross-reactivity
Advantages of TCR Therapy
Advantages of TCR therapy include3,8,12–14:
Functions through well-understood T-cell signaling pathways
Can target intracellular proteins
TCR-engineered T cells can recognize intracellular antigens not expressed on the surface of tumor or diseased cells as long as they are presented by MHCs.
This ability to target intracellular antigens is being investigated as a potential treatment strategy for solid tumors
High antigen sensitivity
Activation can be triggered by lower numbers of peptide–MHC antigens than CAR T-cell therapy
Based on a naturally expressed protein with low immunogenicity
Limitations of TCR Therapy
Limitations of TCR therapy include1,3,8,14,15:
MHC-dependent antigen recognition
Cannot be used to recognize antigens in cancer or other diseased cells in which the expression of the MHC has been downregulated or lost
Inability to target non-protein antigens
Reduces the pool of potential target antigens
Difficulty in the selection of appropriate targets
Both healthy and diseased cells may express the same target antigen.
In addition, tumor heterogeneity can make it challenging to identify appropriate target antigens in diseased cells
Due to the targeting of antigens that may be expressed in healthy tissues and cells
Due to the complex manufacturing and associated cost
Potential Approaches to Improve the Efficacy of TCR Therapy
Several methods have been proposed to improve the efficacy of TCR therapy. Some examples include15,16:
Strategies to enable enhanced targeting of diseased cells vs healthy cells with the same antigen
- Intratumoral delivery of TCR-engineered T cells enables high concentrations and bioavailability of T cells to be reached locally, while the actual systemic exposure of normal body cells to these infused T cells remains low
Strategies to improve T-cell trafficking to diseased tissues
Addition of genes coding for receptors against chemokines expressed by the diseased cells
- Helps to direct the T cells toward chemokines expressed by diseased cells
Strategies to increase the expression and functional activity of engineered TCRs
Use of genome editing technologies to selectively knock out preexisting TCRs on recipient cells
- Endogenous TCRs can compete with engineered TCRs for surface expression and can result in mixed dimer formation. Knockout of endogenous TCRs can limit TCR competition and mispairing with engineered TCR
Clinical Applications of Engineered TCR Therapy
TCR therapy has emerged as a potential treatment for both hematologic and solid tumors
Current experience of TCR therapy with hematologic malignancies remains limited3,15
- Various clinical trials (Phase 1 and 2) are investigating the use of TCR therapy for the treatment of malignant myeloid tumors3,15
- The most encouraging clinical outcomes have been observed for multiple myeloma3
- Identification of appropriate epitopes remains unclear3
Because TCRs can target antigens that are intracellular, numerous clinical trials (Phase 1 and 2) are also investigating the potential of TCR therapy to treat solid tumors3
- TCR therapy has been assessed in several types of solid tumors, including renal cancer, colorectal cancer, human papillomavirus-associated cancers, and melanoma3,15
- Difficulties related to T-cell trafficking and the immunosuppressive microenvironment of solid tumors are some of the limitations that need to be addressed to improve the efficacy of TCR therapy to treat solid tumors3,15
Clinical applications in the context of infectious diseases remain less advanced, with a few Phase 1 or 2 clinical trials currently ongoing for the treatment of human immunodeficiency virus and hepatitis viral infections1
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- Ramachandran P, et al. Gamma-delta T-cell Lymphoma: An overview. 2019. Available at: https://www.intechopen.com/books/peripheral-t-cell-lymphomas/gamma-delta-t-cell-lymphoma-an-overview. Accessed December 11, 2020.
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- Anguela XM, High KA. Annu Rev Med 2019;70:273–288.
- Alberts B, et al. T cells and MHC proteins. In: Molecular Biology of the Cell. 4th edn. New York, NY: Garland Science, 2002.
- Artyomov MN, et al. Proc Natl Acad Sci USA 2010;107(39):16916–16921.
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