Cells: Research status and challenges of TCR-T cell therapy

Adoptive T cell metastasis (ACT) is one of
the most promising immunotherapies in the treatment of tumors.
There are currently four mature ACT technologies, including autologous tumor-infiltrating lymphocyte (TIL) therapy, antigen-specific endogenous T cell therapy (ETC), T-cell receptor engineered T cell therapy (TCR-T), and chimeric antigen receptor T cell therapy (CAR-T).

TIL and ETC therapies rely on isolating and expanding T cells from tumors or peripheral blood, respectively, while TCR-T and CAR-T therapies utilize genetic modifications of T lymphocytes to give them tumor antigen specificity
.

TCR-T cell therapies initially targeted common tumor-associated peptide targets, and with recent technological developments, targeting neoantigens from tumor somatic cell mutations has become possible, representing a highly personalized treatment
.
TCR-T therapy has been used in many preclinical studies and clinical trials around the world to test
the clinical efficacy of solid tumors.
However, the efficacy of TCR-T in the treatment of solid tumors still faces many challenges, including low TCR affinity, targeted toxicity, and loss
of target antigens that cause tumor escape.

At present, some new technologies and tools are being applied to TCR-T to help improve the efficacy and safety of TCR-T therapy, and TCR-T cell therapy is showing great potential
for anti-tumor therapy.

Comparison of CAR-T and TCR-T

TCR molecules belong to a superfamily of immunoglobulins and consist of two covalently bound polymorphic subspecies, each antigen-specific, which is associated
with at least four different types of signal transduction chains.
In order to activate T cells, there must be an interaction
between the TCR and the major histocompatibility complex (MHC).

The strength of TCRs' interaction with pMHC (peptide-MHC) determines the fate of immature thymocytes and is essential
for the survival of naïve T cells.
Therefore, TCR-T immunotherapy technology activates the host's immune system
through effective interaction with MHC, especially class II molecules.
TCR-T cells can recognize tumor-specific antigens within cells, while CAR-T cells mainly recognize specific antigens on the tumor surface
.
This makes TCR-T cells more effective
in tumor treatment.

CAR contains a single-chain antibody targeted by tumor antigens, a transmembrane domain, and an intracellular activation domain
of CD3ζ.
In this way, the engineered CAR is able to recognize specific tumor-associated antigens, and the CAR is able to bind untreated tumor surface antigens
without MHC treatment.

In contrast, TCR is a α/β isodimer
bound to the MHC antigen complex.
Compared with TCRs, CARs recognize tumor antigens with certain drawbacks, such as extratumoral toxicity
.
TCRs have several structural advantages over CARs in T cell-based therapies, such as more sub-(10:1) in their receptor structure, more tyrosine-based activating motifs (ITAMs) for immune receptors (10:3), less antigen-dependent (1:100), and more co-stimulatory receptors (CD3, CD4, CD28, etc.
).

TCRs with a low MHC affinity range (104-106 M-1) can effectively activate T cells, whereas CARs require a higher affinity range (106-109 M-1).

Thus, CAR-mediated cytotoxicity relies on higher densities of cell surface antigens
.
In addition, T cell/antigen interactions are initiated in the immune synaptic (IS) structure, where TCR presents a cyclic region with peripheral LFA-1 adhesion and CAR presents a diffuse LFA-1 distribution
without a cyclic region.
Therefore, TCR-IS is slower but longer
lasting signaling than CAR-IS.
At the same time, CAR-T cells exhibit faster killing function and migrate to the next tumor target (serial kill), which is in stark contrast
to TCR-T cells to prolong signaling and prolong killing time.

Clinical status of TCR-T cell therapy

As of August 9, 2021, a total of 175 studies using TCR-T therapies are ongoing on ClinicalTrials, of which 71 are specific TCRs for specific TAAs or neoantigens, and 32 studies have been completed
.
NY-ESO-1 is the most common targeted antigen and is expressed in a variety of cancers, including myeloma and melanoma
.
Other tumor testicular-associated antigens, such as the PRAME and MAGE proteins, as well as the melanoma differentiation antigens MART-1 and gp100, and more recent cancer drivers such as WT1, KRAS, and TP53, are also popular TCR-T targets
.

A total of 83 sponsors/collaborators initiated or participated in TCR-T cell therapy research, including the National Institutes of Health (NIH), government organizations, industry, and universities/academic institutions
.
Currently, the National Cancer Institute (NCI) supports 53 TCR-T programs, accounting for 20% of
all ongoing projects.

Of the 29 pharmaceutical companies developing TCR-T therapies, GlaxoSmithKline and Adaptimunime initiated the most clinical trials, with 11 and 7,
respectively.
Recently, a Phase 1 clinical trial (NCT02858310) of TCR-T cells against human papillomavirus (HPV)-16 E7 protein was reported in the treatment of metastatic human papillomavirus-associated epithelial carcinoma.

In this study, 6 of the 12 treated patients developed an objective clinical response, observing robust tumor regression
.
This is a landmark clinical trial of TCR-T cell therapy, proving that targeting viral antigens has a good clinical effect
in patients with virus-associated cancer.
Other viral antigens explored as TCR targets include HPV-E6 protein, antigens from Epstein-Barr virus (EBV), and human endogenous retroviral (HERV) targets such as HERV-E
.

TAAs-targeted MART-1 and NY-ESO-1 TCR-T therapies have also shown clinical efficacy
in advanced melanoma, myeloma, and non-small cell lung cancer.
The total effective rate (ORR) of completed TCR-T clinical trials is between
0~60%.
It is important to note that most of these TCR-T clinical trials enroll only a small number of patients (2 to 25), so ORR may be statistically inaccurate
.
Therefore, larger phase II and phase III clinical trials are needed to confirm the actual clinical efficacy of
these TCR-T therapies.

Challenges and potential solutions for TCR-T cell therapy

Although TCR-T-cell-based immunotherapies have shown some clinical efficacy in most patients treated, there are still many challenges
in many areas.
These challenges include: (1) immunotoxicity caused by targeting normal tissue; (2) insufficient or transient expression of TCR in engineered T cells; (3) T cell depletion and dysfunction; (4) tumor immune evasion, and (5) most cancer patients lack effective tumor-specific antigens as targets
.
Overcoming these challenges will be key
to greater clinical success in the future.

Discovery of new targets

Currently, peptide antigen targets for TCR-T and safe immunotherapy are very limited
.
Most of the targets currently in use are TAA, which, despite being upregulated in tumor tissue, remains expressed at low levels in normal tissue, which can lead to autoimmune toxicity
.
Therefore, neoantigens appear to be the safest targets for
TCR-T cancer treatment.
However, the main challenges in developing neoantigens in TCR-T clinics include: (1) neoantigen-forming mutations are largely individualized and vary between cancer patients, making it difficult to develop widely used immunotherapy products; (2) The expression of neoantigens in tumor tissues is often heterogeneous
.

Still, reports in recent years have highlighted the emergence of immunogenic neoantigens that are widely shared by tumor cells, including mutated KRAS and TP53
.
Many other studies have also demonstrated the immunogenicity
of shared neoantigens that can be used to generate potentially therapeutic tumor-specific TCRs.
With the development of next-generation sequencing technologies, especially single-cell DNA sequencing, transcriptome sequencing and proven in vitro validation methods, TCR-T immunotherapy targeting personalized neoantigens may become a popular cancer treatment
in the coming years.
In addition, emerging TAA classes, such as carcinoembryonic antigens, may also constitute viable targets for future TCR-T development
.

Maximize therapeutic TCR expression

The correct pairing of transgenic α and β strands is one of the major challenges hindering the development of
TCR-T cells.
Since each transduced T cell includes two endogenous TCR strands and two transformed TCR strands, heterodimers with unknown specificity can lead to potential autoimmune consequences
.
Another related issue is that inappropriate α/β-stranded TCR pairing will compete for CD3 complexes, thereby reducing surface expression and signal transduction
of therapeutic TCRs.

There are several ways to properly pair transduced TCR strands, including: (1) a constant region of partially murine-derived TCRs; (2) adding cysteine residues to facilitate the introduction of disulfide bonds into the TCR chain; (3) change the secondary structure of the endogenous TCR constant region; (4) adding a signaling domain to the intracellular fraction of the transduced TCR; (5) Introduce TCR-α/β strands into alternative effector cells or construct single-stranded TCRs
.

Methods to enhance therapeutic TCR expression include: (1) codon optimization of TCR-α and TCR-β stranded transgenes, and (2) modification of TCR-α/TCR-β vector configuration to optimize expression
.

Reduction of adverse events

Often, targeting non-tumor toxicity is a major key obstacle to TAA, and this risk has prompted researchers to study common neoantigens
more closely.
Currently, multiple oncogene hotspot mutations are being studied as potential TCR targets, such as phosphoinositol-3-kinase (PI3K), KRAS, and TP53
.
In addition, TCR-T cells genetically engineered with suicide genes are an important safety measure
employed.
Clearly, the development of reliably identified individualized, highly specific, and immunogenic tumor antigen targets is critical
to reducing adverse events associated with TCR-T cell therapy.

Graft-versus-host disease of allogeneic T cells

The use of allogeneic T cells is a very promising option to overcome manufacturing problems, patient-associated immune cell deficiencies, and treatment delays
.
In order to use allogeneic T cells, it is necessary to control graft-versus-host disease caused by transduced alloreactive lymphocytes and rejection
of engineered lymphocytes by the host immune system.

Deletion of endogenous TCR genes, HLA-I sites, or CD52 molecules is one of the strategies to avoid TCR-T transplant failure, which can be achieved through a variety of methods, such as gene editing or the use of siRNAs
.
In addition, pluripotent stem cell technology is also considered a potential solution
.

brief summary

TCR-T therapy is a very promising cancer immunotherapy method with advantages unmatched by
other T cell adoptive therapies.
However, there are still several key challenges in improving the anti-tumor efficacy of TCR-T immunotherapy, including how to safely increase the affinity of therapeutic TCR, how to identify shared tumor-specific antigens and TCRs in the patient population, and how to regulate TCR expression and achieve optimal function
.
The solution of these problems will help to fully realize the potential of TCR-T cell therapy and bring hope
to tumor patients to relieve their pain.

References:

1.
Evolution of CD8+ T Cell Receptor(TCR) Engineered Therapies for the Treatment of Cancer.
Cells.
2021 Sep; 10(9): 2379.