Despite its success against liquid tumours, CAR T-cell therapy often has no impact on solid tumours. A new study, published in Nature Biomedical Engineering, has genetically modified CAR T-cells to overcome this problem. Their new, thermally controlled cells have the potential to significantly improve cancer immunotherapy.
CAR T-cell therapy
In recent years, CAR T-cell therapy has revolutionised cancer treatment. It is a form of immunotherapy that genetically engineers a patient’s T-cells to produce a chimeric antigen receptor (CAR). The CAR is able to recognise a specific surface protein on cancer cells, allowing CAR T-cells to attack cancer cells.
CAR T-cell therapy is remarkably effective for patients with tumours that are circulating in blood, such as individuals with leukaemia. However, the therapy has not yet been successful against solid tumours. One of the main reasons for this is that the tumour microenvironment of solid tumours immunosuppresses CAR T-cells. Attempts are being made to improve the antitumour activity of CAR T-cells against solid tumours. However, the current methods are not specific and exhibit toxicity in healthy tissue.
Therefore, enhancing and specifying the antitumour activity of CAR T-cells against solid tumours will allow more patients to benefit from this form of immunotherapy.
Heat controlled CAR T-cells
The lab group behind this study previously explored remotely thermal-controlled cell therapies, but never in a clinical setting. In this study, they built on their previous work by modifying T-cells and testing them on tumours in mice.
To engineer T-cells with the ability to respond to heat, the researchers relied on previous knowledge of how T-cells respond to hyperthermia. The response is mediated by the binding of the transcription factor heat shock factor 1 (HSF1) to heat-shock elements (HSEs) in DNA. This binding then enables the upregulation of genes located downstream of HSEs.
Therefore, the team cloned HSE motifs into T-cells. The motifs were cloned at locations upstream of genes encoding CARs. This allowed the cancer targeting ability of T-cells to be ‘turned on’ when heated. Crucially, these modified T-cells maintained their ability to proliferate, migrate and kill target cells following heat treatments of up to 30 minutes long.
The addition of a thermal switch was not the only upgrade the study made to CAR T-cells. They also cloned a single-chain IL-15 superagonist (IL-15 SA) and bispecific T cell engagers (BiTEs) into the cells, again under the control of the heat switch.
IL-15 SA is a potent stimulant of CD8 T-cells and natural killer (NK) cells, both of which are key players in the immune response to cancer cells. Meanwhile, BiTEs are monoclonal antibodies that direct cytotoxic activity against cancer cells. The engineered T-cells were able to produce physiologically active levels of both IL-15 SA and BiTEs after heat treatments.
Heating tumours led to their destruction
Next, the team tested their T-cells in vivo. To do this, they injected the modified, thermally controlled T-cells into mice with breast cancer tumours.
Laser pulses were shone from outside the mouse body onto the tumour location. Gold nanorods delivered directly to the tumour were then used to transform the light waves into heat, raising the temperature to 40-42°C.
Heating the tumour directly allowed for the specific activation of T-cells in the immediate tumour location. Amazingly, the modified CAR T-cells were not only activated but had significantly enhanced antitumour activity. Well-established tumours in all mice greatly decreased in size, although some later relapsed. In three out of six mice, the tumours were completely eliminated, and they still had no detectable residual disease 45 days after treatment. This finding implies that in some cases, this method is capable of preventing relapse, which is critical for long-term survival of cancer patients.
The engineered CAR T-cells developed in this study demonstrated that solid tumours can be selectively and efficiently targeted using this form of immunotherapy. The findings show that IL-15 SA and BiTE proteins greatly increased the killing of cancerous cells, due to their incredibly potent nature.
“These cancer-fighting proteins are really good at stimulating CAR T-cells, but they are too toxic to be used outside of tumours,” co-author Gabe Kwong said. “They are too toxic to be delivered systemically. But with our approach we can localise these proteins safely. We get all the benefits without the drawbacks.”
Future studies will be needed to investigate further tailoring of T-cells and alternative methods of delivering heat to the tumour site, to see if this method can be translated into human treatments.
“We’ll use focused ultrasound, which is completely non-invasive and can target any site in the body,” said Kwong. “One of the limitations with laser is that it doesn’t penetrate very far in the body. So, if you have a deep-seated malignant tumour, that would be a problem. We want to eliminate problems.”