Balancing the immune system is a delicate act. If it’s too weak, infections and cancer can take hold; if it’s too strong, autoimmune diseases or transplant rejection can occur. Researchers have harnessed synthetic biology to engineer chimeric antigen receptor (CAR) T cells that precisely target and destroy cancer. However, these powerful cells can also attack healthy tissues. Meanwhile, treatments for autoimmune diseases and transplants rely on broad immunosuppressants, which suppress the entire immune system and leave patients vulnerable to infections.
To overcome these challenges, Wendell Lim, a systems biologist at the University of California, San Francisco, developed suppressive T cells designed to exert a localized, rather than systemic, effect. “We wanted…to create custom cells that trigger an anti-inflammatory or immunosuppressive response only where needed,” said Lim.
Inspired by regulatory T cells, the immune system’s natural brakes for inflammation, Lim designed a cellular platform using conventional CD4+ T cells, the same type used in CAR T cell therapy. In a recent study published in Science, his team engineered these cells to release anti-inflammatory compounds and absorb pro-inflammatory cytokines, dampening immune responses, only in the presence of cells with specific surface antigens.1 This modular system could lead to therapies that fine-tune immunity without the risks of widespread suppression.
Wendell Lim uses synthetic biology to reprogram cellular behaviors and develop therapeutic cells.
Noah Berger, University of California San Francisco
Lim and his team designed bespoke suppressor cells to counter one of the major risks of CAR T cell therapy, which is the potential to attack healthy tissue. The researchers outfitted CD4+ T cells with synthetic Notch (synNotch) receptors—highly programmable chimeric receptors that detect target antigens and when bound, prompt the cell to release tailored molecular payloads.2 By leveraging the precise antigen recognition of the synNotch system, these engineered cells are poised to locally suppress immune responses and prevent collateral damage to non-target cells.
Researchers used synNotch receptors that recognized the model antigen CD19, which is implicated in autoimmune diseases and B cell cancers, then engineered cells to release a variety of different immune modulators when the receptors were activated. Then, they tested how well suppressor cells carrying different payloads could counteract cytotoxic CAR T cells in vitro.
The researchers cultured these different types of suppressor T cells along with leukemia target cells and CAR T cells, then measured the target cell survival. Engineered suppressor cells that produced suppressive cytokines including interleukin-10, programmed death ligand 1, and transforming growth factor–β1 (TGFβ1), along with inflammatory cytokine sinks such as interleukin-2 receptor subunit CD25, most effectively reduced CAR T cell proliferation and target cell death.
“Traditionally, a lot of these synNotch receptors have been applied to get better tumor cell killing or [activate] a stronger response. Here, they took it in a different [direction] to create a suppressor cell that can turn down immune responses,” said Jason Lohmueller, a synthetic biologist and immunologist at the University of Pittsburgh, who was not involved in the study.
To refine their strategy, the researchers compared whether packaging these suppressive factors within a single cell or splitting them between two cells would be more effective. Their results were clear: a single-cell system carrying TGFβ1 and CD25 significantly outperformed the two-cell approach, leading to greater CAR T cell suppression and improved survival of target cells.
How would these synthetic suppressor cells perform in living tissues? The researchers examined CAR T cell-mediated cancer cell killing in a two-tumor mouse model. Their goal was to test whether synthetic cells could protect CD19+ tumors from CAR T cells, while leaving CD19- tumors subject to CAR T cell killing. The team injected mice on both flanks: one flank with a tumor expressing human epidermal growth factor receptor 2 (Her2), which CAR T cells recognize and destroy, and the opposite flank with a tumor presenting both Her2 and CD19, designed to trigger a protective response. The results were promising as a proof-of-concept in shielding specific tissues from harm. Synthetic suppressor T cells successfully curbed CAR T cell proliferation in the dual-antigen tumor, demonstrating their ability to regulate immune responses in vivo. Meanwhile, the CAR T cells continued to attack the CD19- tumors.
The team also tested whether their engineered suppressor T cells could prevent immune rejection in transplantation. Using a pancreatic islet transplantation model, they implanted enhanced beta cells that expressed CD19 into mice. Fourteen days later, they injected either CAR T cells alone or with synthetic suppressor cells and tracked organoid survival via fluorescent imaging. On their own, CAR T cells recognized and rapidly destroyed the transplants, but when the suppressor cells were present, the beta cells were protected. Notably, the suppressor cells surrounded CAR T cells, preventing them from clustering and killing cells. A glucose challenge test confirmed the transplants' function, as mice with suppressor cells showed high C-peptide levels, indicating active insulin-producing beta cells. These results demonstrated the potential for generating synthetic suppressor T cells capable of immune suppression by blocking local cytotoxic T cells.
“I'm excited to see where things go from here. I think it really could be a game-changing type of therapy, where they could use this to treat different autoimmune diseases or help with organ transplantation,” said Lohmueller. He also noted his curiosity about pairing these receptors with suicide switches to control the duration of these suppressor T cells in the body for therapy.
“These immune cells are really a very flexible platform that can be potentially used to do many different things,” said Lim.
Lim and his team plan to further investigate this platform’s potential across different diseases, such as multiple sclerosis, and explore the interactions of these synthetic T cells with other antibodies and B cells. The researchers believe these engineered cells could complement cell therapies by offering protective effects in addition to the actions of killer cells, potentially enhancing treatment efficiency.
- Reddy NR, et al. Engineering synthetic suppressor T cells that execute locally targeted immunoprotective programs. Science. 2024;386(6726):eadl4793.
- Morsut L, et al. Engineering customized cell sensing and response behaviors using synthetic Notch receptors. Cell. 2016;164(4):780-791.
