Ultrasound-controlled CAR-T cells to fight solid tumors

Reading time: 5 minutes

Anthony Tao

Introduction

For cancer to successfully grow, one of the major obstacles it must face is the immune system. Cells of the immune system, such as T cells, have natural and effective mechanisms that allow them to monitor the body and attack any cancer cell that might arise. For decades, researchers have investigated the therapeutic potential of bolstering these anti-cancer properties of immune cells. From these investigations, novel medicines have been developed that enhance immune cells and effectively eradicate certain cancers. These include antibodies that can be used to drive immune responses (e.g. nivolumab) as well as chimeric antigen receptor T cells, or CAR-T cells.

The role of T cells in cancer

To understand CAR-T cells, we should first understand the role of T cells in cancer. Because of the volatile nature of cancer genes, cancer cells often produce proteins, or antigens, that are unfamiliar to the immune system. When encountering these unfamiliar antigens ‒ known as neo-antigens ‒ specific immune cells will alert and activate certain T cells. These T cells can both directly kill the cancer as well as coordinate larger anti-cancer responses alongside other cells. However, not all T cells get activated because most are not equipped to recognize a particular cancer neo-antigen. Our bodies contain hundreds of millions of T cells, each of which expresses a unique protein receptor, or T cell receptor (TCR), that is specifically tuned for a particular antigen. Only a very small percentage of these cells will recognize and respond to a given cancer neo-antigen (Figure 1).

Figure 1: Anti-cancer T cells and CAR-T cells. (Left) In a physiological anti-cancer T cell response, a small percentage of T cells with the appropriate T cell receptor (TCR) are activated to attack tumor cells (black cells). (Right) CAR-T cells are engineered to artificially recognize and attack the tumor using chimeric antigen receptors (CAR).

What are CAR-T cells?

The idea behind CAR-T cells is simple: engineer a large number of T cells that  express the necessary cancer-targeting TCR,  that is, the chimeric antigen receptor, or CAR (Figure 1). Though original studies adopting this approach date back to 1989, CAR-T cells only recently saw major clinical success in the 2010s with B cell leukemias and lymphomas. Since then, many studies and trials have shown exciting progress in other cancers and even autoimmune diseases.

However, CAR-T cells are not without their shortfalls. For instance, excessive activation of CAR-T cells can lead to toxic effects, with some reported cases showing CAR-T cell overactivity resulting in lethal multi-organ failure. These can further be exacerbated by the fact that CAR-T cells can mistakenly attack normal cells, a phenomenon known as off-target effect. 

To address these shortcomings, a group of researchers at UCLA proposed an interesting approach: using focused ultrasound (FUS) to only activate CAR-T cells within a tumor (Figure 2). Restricting CAR-T cell activity in space and time would theoretically limit its off-target and toxic effects.

EchoBack CAR-T cells

FUS is a non-invasive method that can heat up a focused region of space in the body, such as a tumor. Unfortunately, CAR-T cells are not inherently sensitive to FUS. The researchers hypothesized that if they could reengineer CAR-T cells to become activatable by heat, they would be able to use FUS to regulate CAR-T responses with spatial and temporal precision. To do so, they focused on a genetic region called the promoter (Figure 2), which drives the expression of the CAR protein that is necessary for anti-cancer activity in CAR-T cells.

Figure 2: FUS-inducible EchoBack CAR-T cells. (Left) Normal promoters drive expression of the CAR gene in T cells indiscriminately. However, the EchoBack promoter silences the CAR gene until application of heat from a focused ultrasound (FUS). (Right) Application of FUS heats up the tumor, thereby activating local EchoBack CAR-T cells in a controlled manner.

The researchers took a promoter that naturally responds to heat, improved its sensitivity and strength, and introduced it into CAR-T cells. They dubbed these re-engineered CAR-T cells EchoBack CAR-T cells. Compared to normal CAR-T cells, these EchoBack cells were only activated when heated up to a temperature of 42^oC (or 107.6^oF), suggesting that FUS could be used for targeted CAR-T activation in the body.

Next, the team sought to determine the efficacy of the EchoBack cells in mouse models of cancer. They studied 2 tumors: glioblastoma, a tumor of the brain, and prostate cancer. Injecting the EchoBack cells into these cancer models had little effect on cancer growth. However, when the FUS signal was targeted to the tumor sites, the size of the tumors decreased significantly. The survival of these cancer-bearing mice was also substantially extended. Furthermore, in the glioblastoma model, the EchoBack CAR-T cells led to even stronger anti-cancer responses compared to normal CAR-T therapy. These findings strongly support the potential benefit of EchoBack T cells with FUS targeting.

Finally, the researchers wanted to know whether these EchoBack CAR-T cells were less prone to off-targeting. To address this question, they turned to a unique 2-tumor mouse model. In this model, 2 tumor cell types are injected into mice: one that expresses the cancer neo-antigen at high levels and one that expresses it at low levels, thereby mimicking normal cells. When typical CAR-T cells were injected into these mice, they attacked both the “cancer” and “normal” cells due to their lack of specificity. In contrast, the EchoBack CAR-T cells only targeted the “cancer” cells and largely ignored the “normal” cells. This proof-of-concept experiment suggests improved specificity using the EchoBack CAR-T cells and potentially reduced immune toxicity.

In summary, the development of EchoBack CAR-T cells represents a novel and transformative step in cancer immunotherapy. The system effectively allows FUS to be used like a remote controller, dictating when and where anti-cancer T cells should direct their focus. The findings in solid tumor models such as glioblastoma and prostate cancer are especially exciting since solid tumors have been infamously resistant to CAR-T therapy. Future efforts to validate the safety of this technology in a clinical setting, especially with regards to evaluating the side effects of the therapy, will be critical for establishing this method as a viable approach toward cancer treatment.

Header Image Source: Created by Author with Midjourney

Figures 1 and 2: Created by Author with BioRender

Edited by Mariella Careaga

References

1. Nivolumab – NCI. https://www.cancer.gov/about-cancer/treatment/drugs/nivolumab (2015).
2. Liu, L. et al. Engineering sonogenetic EchoBack-CAR T cells. Cell 188, 2621-2636.e20 (2025).
3. Epperly, R., Giordani, V. M., Mikkilineni, L. & Shah, N. N. Early and Late Toxicities of CAR T-Cells. Hematol Oncol Clin North Am 37, 1169–1188 (2023).
4. Mitra, A. et al. From bench to bedside: the history and progress of CAR T cell therapy. Front Immunol 14, 1188049 (2023).