Pharmacologically regulated bioorthogonal stabilization domain for regulation of CAR T cells

Technical Summary: Pharmacologically Regulated Bioorthogonal Stabilization Domain for Regulation of CAR T Cells

Problem Statement
Precise and reversible control of protein function in living cells is a central challenge in synthetic biology and therapeutic cellular engineering. In Chimeric Antigen Receptor (CAR) T cell therapy, excessive or prolonged receptor activity can lead to severe toxicities, such as cytokine release syndrome and neurotoxicity. While strategies exist to regulate CAR T cells, current solutions have significant limitations:

• Transcriptional control: Systems regulating transgene expression at the transcriptional level suffer from slow response times, which are critical when managing acute adverse events.
• Suicide switches: Inducible "suicide" approaches allow for the elimination of CAR T cells but permanently ablate the therapeutic cells, resulting in a loss of therapeutic potential.
• Existing protein-level regulators: Current ligand-dependent stabilization or degradation systems often rely on non-human proteins (e.g., E. coli-derived ecDHFR or FKBP12), posing immunogenicity risks. Furthermore, the small molecules required to modulate these domains (e.g., trimethoprim, Shield-1) lack FDA approval, hindering clinical translation.

There is a critical need for bioorthogonal domains derived from human proteins that can be regulated by FDA-approved small molecules to enable rapid, reversible, and clinically compatible control of CAR T cell activity.

Methodology
The authors developed a strategy utilizing the ligand-binding domain (LBD) of the human estrogen receptor (ER) to create pharmacologically regulated CAR T cells. The study employed two distinct mechanisms: an "ON-switch" based on ligand-induced stabilization and an "OFF-switch" based on PROTAC-mediated degradation.

1. Construct Design:

   • Second-generation anti-CD19 and anti-HER2 CARs were engineered with 4-1BB and CD3ζ intracellular signaling domains.
   • These CARs were fused at the C-terminus to various ER variants: wild-type ERα, the mutant ERt2 (harboring G400V, M543A, and L544A substitutions to reduce endogenous estrogen affinity), and the ER50 degron.
   • A Yellow Fluorescent Protein (YFP) reporter was fused to the C-terminus for flow cytometric quantification.

2. ON-Switch Mechanism (Stabilization):

   • The hypothesis was that adding 4-hydroxytamoxifen (4-OHT), an FDA-approved selective estrogen receptor modulator, would stabilize the ER-fused CARs, preventing proteasomal degradation.
   • Experiments were conducted in Jurkat T cells and primary human T cells to assess surface expression, cytotoxicity, cytokine secretion (IL-2, IFNγ), and proliferation in response to 4-OHT.

3. OFF-Switch Mechanism (Degradation):

   • The authors utilized ARV-471, an FDA-approved PROTAC (Proteolysis-Targeting Chimera) that targets ERα and recruits the ubiquitin-proteasome system.
   • CARs fused to the wild-type ERα domain were treated with ARV-471 to induce targeted degradation of the CAR receptor.
   • Kinetic studies and reversibility tests (using competitive binding with 4-OHT) were performed to evaluate the speed and control of degradation.

Key Contributions and Results

• Identification of ERt2 as an Efficient Degron: Contrary to expectations that wild-type ERα would be stable, the study found that the ERt2 domain functions as an efficient degradation domain in the absence of ligand. In contrast, the ER50 variant showed lower induction and higher basal leakage. Consequently, ERt2 was selected for the ON-switch platform.
• Reversible ON-Switch Control:
  • Stabilization: Treatment with 4-OHT selectively stabilized CAR-ERt2 in a dose-dependent manner in both Jurkat and primary human T cells.
  • Functionality: CAR-ERt2 T cells exhibited ligand-dependent cytotoxicity against CD19+ (Raji) and HER2+ (MCF-7) target cells, comparable to conventional CARs. Antigen stimulation induced robust, ligand-dependent production of IL-2 and IFNγ.
  • Kinetics: CAR expression was detectable ~2 hours after ligand addition, reaching maximum levels at 24 hours. Upon 4-OHT washout, CAR degradation occurred with a half-life of approximately 4.1 hours.
  • Reversibility: The system demonstrated dynamic reversibility; target cell killing could be repeatedly toggled by the addition and removal of 4-OHT.
• PROTAC-Mediated OFF-Switch Control:
  • Degradation: Treatment with ARV-471 induced dose-dependent depletion of CAR-ERα surface expression. Half-maximal depletion occurred approximately 1.5 hours after ARV-471 addition.
  • Functional Suppression: Pre-treatment with ARV-471 resulted in dose-dependent inhibition of target cell killing, reduced cytokine secretion, and impaired antigen-driven T cell proliferation.
  • Reversibility via Competition: While ARV-471 washout alone did not restore CAR abundance, simultaneous addition of 4-OHT effectively blocked PROTAC-mediated degradation, demonstrating that the system's reversibility can be mediated by competitive ligand binding.
• Generalizability: The platform was successfully validated for both anti-CD19 and anti-HER2 CARs, indicating broad applicability across different target antigens.

Significance and Claims
The paper claims to present a clinically compatible strategy for the reversible pharmacological control of CAR T cell function using human-derived domains and FDA-approved small molecules (4-OHT and ARV-471).

• Clinical Relevance: By utilizing human ER domains and drugs already approved for breast cancer treatment, the system minimizes immunogenicity risks and leverages established pharmacokinetics and biodistribution profiles.
• Precision and Safety: The ability to rapidly toggle CAR activity (ON and OFF) offers a mechanism to balance therapeutic efficacy with patient safety, potentially mitigating severe toxicities without permanently eliminating the therapeutic cells.
• Platform Expansion: The results expand the bioorthogonal ON-OFF switch platform for post-translational control of therapeutic proteins. The authors suggest that 4-OHT could be used to activate CAR T cells, or potentially delivered via scaffold/caged systems for tumor-selective activation.
• Synergistic Potential: The authors note the possibility of combining these agents with CAR T cell therapy for synergistic tumor targeting, given that both 4-OHT and ARV-471 are approved for treating ER-positive breast cancer.

The study concludes that ER-based degron tagging provides a tunable, reversible, and clinically relevant method for regulating CAR T cell activity, though the authors note that in vivo evaluation of these effects remains a necessary future step.