By Leoma Bere
A new and improved method of cell therapy has shown promising early results in the treatment of glioblastoma, the most aggressive type of brain tumour. Just a few days after treatment the patients’ tumours started to shrink. The protagonists of this tale: the patients’ own immune cells, specifically their T-cells. Certain types of T-cells called cytotoxic T-cells are armed with unique receptors which allow killing when bound to targets on the surface of cells infected with viruses or bacteria. Scientists have redirected T-cells to kill cancer cellsin the same way they kill infected cells by generating Chimeric Antigen Receptor (CAR) T-cells (Raskov et al., 2021). ‘Chimeric’ comes from the Greek chimera, meaning ‘she-goat’, referring to goat-lion-snake-dragon monster in Greek mythology. To generate CAR T-cells the cancer-recognising part of an antibody was fused to the activation machinery of the T-cell (Figure 1). It combined the engineerable and customisable targeting of antibodies with the killing abilities of T-cells.
Figure 1. Structure of Chimeric antigen receptors (CAR). The tumour-binding region of an antibody is attached to the activation unit of a T-cell, enabling the T-cell to kill when the tumour target is bound
A team of researchers from Mass General Cancer Center lead by Dr Marcela Maus used CAR T-cells successfully in glioblastoma (Choi Bryan D. et al., 2024). A first-in-human clinical trial was conducted on a small cohort of patients with a certain type of glioblastoma; the gene EGFR had been mutated to EGFRvIII and was expressed on most tumour cells. EGFRvIII would become the target of the CAR T-cells. All three patients enrolled onto the study showed immediate tumour shrinkage following treatment with the CAR T-cells. Eventually the tumours regrew as CAR T-cell numbers declined over time. However, these were exciting initial results as cell therapies had shown poor results in glioblastoma until now.
To generate the cancer-targeting CAR T-cells, the patients’ T-cells were extracted from blood and introduced to a genetically modified lentivirus in the lab. Lentiviruses are used as carriers of genetic code and are a common tool in molecular biology. This simply exploits the natural lifecycle of lentiviral infection, whereby the virus infects the cell and unloads its genetic information into it (Figure 2a). The virus’s DNA inserts itself into the host cell’s DNA and is processed by the host cell. By reading the integrated DNA, the host cell ends up producing all the building blocks needed to produce an army of lentiviruses. Exploiting this process, scientists can give the lentivirus the code we want the host cell to process (Labbé, Vessillier and Rafiq, 2021).
Figure 2 Lentiviral infection is used as a tool to generate CAR T-cells against glioblastoma. Diagram showing parallels between normal lentiviral infection and when used as a method to genetically CAR T-cells against glioblastoma. (a) Lentivirus infects the host cells and injects its genetic code into the cell. The genetic code is integrated into the host cell DNA and is processed producing proteins which form functional lentiviruses. (b) Lentiviruses are modified in the lab to carry code to produce a chimeric antigen receptor (CAR) against EGFRvIII. Viral infection of T-cells leads to integration of the modified genetic code into the T-cell’s DNA. The T-cell reads the code and produces the anti-EGFRvIII CAR. The CAR is displayed on the T-cell’s surface and triggers killing when bound to EGFRvIII on glioblastoma cells.
Dr Maus and her team used this method of lentiviral infection to equip patient T-cells with glioblastoma-targeting machinery and then infused the CAR T-cells back into the patient to clear the tumour (Figure 2b). They gave the lentiviruses and consequently the T-cells the genetic code to make the part of an antibody which can bind EGFRvIII. This was the protein overexpressed by this cohort of glioblastoma patients. The T-cells read this code and produced something which could now bind glioblastoma cells via EGFRvIII. Additionally, they gave the T-cells the code to produce the activation machinery T-cells need to kill. By fusing it to the EGFRvIII binding region, the CAR T-cells now got triggered to kill when EGFRvIII was bound.
Using the same approach, they generated another antibody region which bound EGFR, but this time linked it to a different antibody region which bound CD3, a protein unique to T-cells. This is called a T-cell-engaging antibody molecule (TEAM). The anti-EGFR and anti-CD3 fragments were conjoined, acting as connecting adaptors between cancer cells and T-cells. They were secreted upon CAR T-cell activation and bound to cancer cells (Figure 3). This made it easier for surrounding bystander T-cells to detect and kill glioblastoma. Furthermore, the TEAM adaptors could turn regulatory T-cells, which dampen immune responses, into cytotoxic T-cells killing the cancer cells instead.
Figure 3. CAR T-cells produce TEAMs to increase glioblastoma killing. Schematic showing CAR T-cells killing and release of Tell-engaging antibody molecule (TEAM) adaptors when bond and activated by glioblastoma cells. TEAM adaptors bind to EGFR on glioblastoma cells and CD3 on bystander T-cells also leading to tumour killing.
All patients showed significant tumour shrinking in the first week after CAR T-cell treatment. One patient showed no signs of tumour growth and almost achieved complete remission following the first treatment with CAR T-cells. The other patients showed signs of growth after the initial shrinking, suggesting that repeated treatments with CAR T-cell therapy may be necessary to maintain active levels of tumour killing. Overall, CAR TEAM T-cells were tolerated well with manageable side effects. This therapy was an innovative fusion of gene therapy to modify lentiviruses to carry glioblastoma-targeting code, and cell therapy, where T-cells were equipped with glioblastoma detection and killing machinery. The CAR TEAM T-cells embody years of collaborative research between scientists across all fields of biology. Although the CAR TEAM T-cells in this study still require work to achieve long-term remission, this trial is encouraging for the use of cell therapies in glioblastoma, a cancer which desperately needs teamwork and novel approaches to overcome its complex challenges.
References
Choi Bryan D. et al. (2024) ‘Intraventricular CARv3-TEAM-E T Cells in Recurrent Glioblastoma’, New England Journal of Medicine, 390(14), pp. 1290–1298. Available at: https://doi.org/10.1056/NEJMoa2314390.
Labbé, R.P., Vessillier, S. and Rafiq, Q.A. (2021) ‘Lentiviral Vectors for T Cell Engineering: Clinical Applications, Bioprocessing and Future Perspectives’, Viruses, 13(8), p. 1528. Available at: https://doi.org/10.3390/v13081528.
Raskov, H. et al. (2021) ‘Cytotoxic CD8+ T cells in cancer and cancer immunotherapy’, British Journal of Cancer, 124(2), pp. 359–367.
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