New Approach to CAR T-Cell Therapy for Glioblastoma Aims to Circumvent Antigen Escape
Key findings
- Chimeric antigen receptor (CAR) T-cell therapy has limited effectiveness against glioblastoma, partly because of antigen escape when CAR T-cells are directed against a single antigen
- A new approach relies on a CAR specific for EGFRvIII, a glioblastoma-specific tumor antigen, and a bispecific T-cell engager (BiTE) against EGFR, which is expressed in both glioblastoma and normal tissues
- In mice, CART-EGFRvIII.BiTE-EGFR was effective against both EGFRvIII-expressing and EGFRvIII-negative tumors
- CART-EGFRvIII.BiTE-EGFR showed antitumor activity against a patient-derived glioma neurosphere that contained both amplified EGFR and EGFRvIII
- The CART.BiTE approach may expand the use of CAR T-cell therapy for other types of solid tumors
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Therapy with chimeric antigen receptor (CAR) T-cells targeting CD19 has revolutionized the treatment of certain hematologic malignancies, but it has not been uniformly successful against glioblastoma. The inconsistency of response is attributable in large part to heterogeneous tumor antigen expression, as antigen escape can occur when CAR T-cells are directed at a single target.
Marcela V. Maus, MD, PhD, director of the Cellular Immunotherapy Program at the Massachusetts General Hospital Cancer Center led the team with lead author Bryan Choi, MD, PhD, resident in the Neurosurgery Department, and collaborated with Bob S. Carter, MD, PhD, chief of the Neurosurgery Department, and William T. Curry, MD, co-director of Mass General Neuroscience, and other colleagues who have been studying CAR T-cells that target the epidermal growth factor receptor variant III (EGFRvIII). This mutation is expressed solely in tumors; it is absent from all normal tissues.
Previous research by Dr. Maus has shown that a single infusion of EGFRvIII CAR T-cells (CART-EGFRvIII) led to a reduction of EGFRvIII-expressing glioma cells. However, viable tumor remained in all patients and expressed high levels of wild-type EGFR. That latter antigen is amplified in more than 80% of glioblastomas, but it is also present in many normal cells, so targeting it can cause significant toxicity.
To boost the effectiveness of CAR T-cell therapy for glioblastoma, the research team has designed a way to target both EGFRvIII and wild-type EGFR. They gene-modified CART-EGFRvIII so that it secretes a bispecific T-cell engager (BiTE), an antibody that directs EGFRvIII CAR T-cells to tumor cells and recruits bystander T-cells against wild-type EGFR. Their report is published in Nature Biotechnology.
Efficacy Against EGFRvIII-Expressing Tumors in Mice
The researchers generated two CART.BiTE constructs, one with a BiTE designed against wild-type EGFR and the other designed against CD19, and transferred them into human T-cells. The CD19 construct served as both a negative control and as a proof of concept for generalizing research findings across antigens. The BiTEs were produced at low concentrations.
To test the constructs, the researchers implanted glioma cells (10% positive for EGFRvIII, 90% positive for wild-type EGFR) in the brains of immune-deficient mice. Two days later, they injected CART-EGFRvIII.BiTE-EGFR or CART-EGFRvIII.BiTE-CD19 cells intraventricularly, to maximize the local effects. Other mice received untransduced T-cells as a control.
CART-EGFRvIII.BiTE-EGFR resulted in complete and durable responses in all mice. The results with CART-EGFRvIII.BiTE-CD19 were intermediate between those of CART-EGFRvIII.BiTE-EGFR and those of untransduced cells.
Efficacy Against Glioma Models and Patient-derived Neurospheres
In vitro, CART-EGFRvIII.BiTE-EGFR, but not CART-EGFRvIII.BiTE-CD19, activated T-cells against EGFRvIII-negative, EGFR-positive glioma cell lines. CART-EGFRvIII.BiTE-EGFR was even more cytotoxic than the positive control, CART-EGFR.
EGFRvIII expression fluctuates in human tumors, so the researchers also studied 11 patient-derived glioma neurospheres. Only one of them proved to contain both amplified EGFR and EGFRvIII. The team found that CART-EGFRvIII.BiTE-EGFR, and to a lesser extent CART-EGFRvIII.BiTE-CD19, had antitumor activity against that neurosphere.
Efficacy Against EGFRvIII-Negative Tumors in Mice
Next, the researchers implanted EGFRvIII-negative glioma cells into mouse brains, then injected CART.BiTE cells intraventricularly. Other mice received untransduced T-cells.
Following injection of CART-EGFRvIII.BiTE-EGFR cells, the animals showed durable regression of established glioma. Conversely, mice treated with cells expressing CART-EGFRvIII.BiTE-CD19 or conventional CART-EGFRvIII demonstrated progressive tumor burden comparable to that of the controls.
Safety in Mice
Dermatologic reactions are a major complication of several FDA-approved therapies that target EGFR. To evaluate the dermatologic safety of their new treatment approach, the researchers collaborated with Curtis L. Cetrulo, MD, from the the Division of Plastic and Reconstructive Surgery and Shawn Demehri, MD, PhD, from the Department of Dermatology to graft human skin onto the backs of mice, then intravenously administered CART-EGFRvIII.BiTE-EGFR, CART-EGFRvIII.BiTE-CD19 or CART-EGFR.
Mice treated with CART-EGFR exhibited signs consistent with cutaneous graft-versus-host disease. Such signs were absent in the mice treated with CART-EGFRvIII.BiTE-EGFR or CART-EGFRvIII.BiTE-CD19.
The Future of CART.BiTE
The researchers predict that CART.BiTE therapy might be feasible for targeting other antigens, such as HER2, that cannot be targeted directly by CAR T-cells or BiTEs administered at effective systemic doses because of unacceptable toxicity.
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