Boosting CAR-T cell endurance to take on solid tumors

Chimeric antigen receptor (CAR) T cell therapies have transformed cancer care for various types of blood cancer. These therapies – that involve engineering a patient’s own cells to directly detect, attack, and kill cancerous cells – have so far not been very successful in treating solid tumors. The major reason is that CAR-T cells quickly become exhausted in solid tumor microenvironments. 

T cell exhaustion is a process in which T cells lose their ability to function properly after chronic stimulation, like someone trying to run a marathon who slows down after 15 miles because she feels tired and weak. It is a common phenomenon in cancer where incessant encounters with cancerous cells lead T cells to stop responding.

Exhausted T cells, including exhausted CAR-T cells, express inhibitory receptors that restrict their effector functions. One strategy to overcome T cell exhaustion in cancer is blocking these inhibitory receptors. This type of therapy, called immune checkpoint inhibition, provides a boost to cells to keep fighting. It is like giving our tiring marathon runner a sugary electrolyte drink to keep her going. Importantly, immune checkpoint inhibition works best to reinvigorate cells that are in the process of becoming exhausted – called precursor exhausted T cells, or T_PEX – not cells that have already fully turned off their function. Giving a sports drink to a runner who has already given up doesn’t do much to encourage her to reach her distance goal. 

“Checkpoint inhibitors that target the PD-1/PD-L1 pathway can revive natural T cell responses and lead to impressive outcomes in some patients. When these drugs are combined with CAR-T cells, however, the benefit has been surprisingly limited,” said Dr. Shivani Srivastava in the Fred Hutch Human Biology Division. In a recent study spearheaded by graduate student Andrew Snyder, the Srivastava lab investigated why engineered CAR-T cells don’t respond to immune checkpoint inhibition in the same way as native T cells.

The study, published in Science Immunology, found that “CAR-T cells don’t engage the same supportive niches as endogenous T cells, which limits their ability to persist and respond to checkpoint blockade,” indicated Dr. Srivastava. The T_PEX  cells that best respond to immune checkpoint inhibition typically exist in lymphoid tissues, where interactions with other immune cells help them maintain their capacity to fight cancerous cells. CAR-T cells in solid tumors, unlike native T cells, do not traffic to lymphoid tissues or interact with the other immune cells in the same way, hampering their ability to adopt a T_PEX  phenotype. Instead, they quickly become terminally exhausted. Our marathoner gives up sooner when sprinting alone in the middle of the forest than when jogging on the streets of a city with crowds of people cheering her on.

The team evaluated a strategy to prevent CAR-T cell exhaustion. They developed CAR-T cells that overexpress a protein called cJun, which is known to inhibit the expression of genes involved in T cell exhaustion. In a mouse model of aggressive lung cancer, the cJun-expressing CAR-T cells outperformed traditional CAR-T cells, showing improved infiltration and function within tumors. However, cJun overexpression alone wasn’t enough – the CAR-T cells ultimately still exhausted and weren’t able to control tumors much better than traditional CAR-T cells.

Interestingly, overexpressing cJun in CAR-T cells did allow them to form a T_PEX-like subset within tumors. The most dramatic result of cJun overexpression was revealed when the CAR-T cells were used in combination with a PD-1 immune checkpoint inhibitor. There was a 10-fold increase in CAR-T cell presence within tumors compared to treatment with traditional CAR-T cells. Mice that received the cJun CAR-T and immune checkpoint combination treatment had marked improvement in tumor control and significantly improved survival. Testing the same combination therapy in a mouse model of breast cancer yielded similarly encouraging results.

“By boosting expression of the transcription factor cJun, we were able to help CAR-T cells maintain a more stem-like state and dramatically improve their response to PD-1 axis blockade,” commented Dr. Srivastava. This study showed that CAR-T cell constructs can be modified to promote specific CAR-T cell phenotypes within tumors. Moving forward, the team plans to further investigate how CAR-T cells become exhausted so quickly. Dr. Srivastava hopes that this could the guide the development of strategies “to enhance CAR-T function without the need for checkpoint inhibitors.”

Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium Members Drs. Cyrus Ghajar, Scott Furlan, Evan Newell, and Shivani Srivastava contributed to this research.

The spotlighted research was funded by the National Institutes of Health, American Lung Association, V Foundation for Cancer Research, American Cancer Society, Department of Defense, Lyell Immunopharma, and Institute of Translational Health Sciences.

Snyder AJ, Garrison SM, Kluesner MG, Nutt WS, Shasha C, Ho T, Marsh SA, Linde M, Wu F, Meyer L, Wilhelm AR, Ortiz-Espinosa S, Zepeda V, Bingham E, Malik H, Mak SR, Gad E, Bhise SS, Fan E, Sarvothama M, Wang X, Potluri S, Long A, Elz A, Ghajar CM, Furlan SN, Newell EW, Srivastava S. 2026. Modulating AP-1 enables CAR T cells to establish an intratumoral stemlike reservoir and overcomes resistance to PD-1 blockade. Science Immunology. doi: 10.1126/sciimmunol.adw7685