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Over the past ten years, T cell immunotherapy has emerged as one of the most promising developments in cancer treatment. These therapies work by training a patient’s own immune system to detect and destroy dangerous cells. Despite their success, scientists have struggled to fully explain how these treatments function at a molecular level. This lack of understanding has slowed progress, especially since T cell therapies work well for only a small number of cancer types and fail in most others, for reasons that have remained unclear. Gaining insight into their modus operandi could help make these therapies effective for far more patients.
Scientists at The Rockefeller University have now uncovered crucial details about the T cell receptor (TCR), a protein complex embedded in the cell membrane that plays a central role in T cell therapies. Using cryo-EM, researchers from the Laboratory of Molecular Electron Microscopy studied the receptor in a biochemical setting designed to closely resemble its native milieu. They discovered that the TCR behaves like a jack-in-the-box, staying compact until it encounters an antigen or another suspicious particle, at which point it rapidly opens. This behavior contradicts what earlier cryo-EM studies of the receptor had shown.
The findings, published in Nature Communications, could help researchers improve and expand the use of T cell immunotherapies.
“This new fundamental understanding of how the signaling system works may help re-engineer that next generation of treatments,” says first author Ryan Notti, an instructor in clinical investigation in Walz’s lab and a special fellow in the Department of Medicine at Memorial Sloan Kettering Cancer Center, where he treats patients with sarcomas, or cancers that arise in soft tissue or bone.
“The T cell receptor is really the basis of virtually all oncological immunotherapies, so it’s remarkable that we use the system but really have had no idea how it actually works — and that’s where basic science steps in,” says Walz, a world expert in cryo-EM imaging. “This is some of the most important work to ever come out of my lab.”
How T Cells Detect Threats
Walz’s lab focuses on producing detailed images of macromolecular complexes, especially proteins found in cell membranes that help cells communicate with their surroundings. The TCR is one such complex. Made up of multiple proteins, it enables T cells to recognize antigens displayed by human leukocyte antigen (HLA) complexes on other cells. This recognition process is what T cell therapies rely on to mobilize the immune system against cancer.
Although scientists have known the individual parts of the TCR for many years, the earliest steps that trigger its activation have remained elusive. Notti, who works as both a physician and a researcher, found this gap especially troubling because many of his sarcoma patients were not benefiting from T cell immunotherapies.
“Determining that would help us understand how the information gets from outside the cell, where those antigens are being presented by HLAs, to the inside of the cell, where signaling turns on the T cell,” he says.
Notti earned his Ph.D. in structural microbiology at Rockefeller before moving into oncology, and he suggested to Walz that they investigate this unanswered question together.
Rebuilding the TCR’s Natural Environment
Walz’s team is known for creating custom membrane environments that closely mimic the natural surroundings of membrane proteins. “We can change the biochemical composition, the thickness of the membrane, the tension and curvature, the size — all kinds of parameters that we know have an influence on the embedded protein,” Walz says.
For this study, the researchers set out to observe the TCR in conditions that closely resemble those inside a living cell. They placed the receptor into a nanodisc, a tiny disc-shaped section of membrane held in solution by a scaffold protein wrapped around its edge. Assembling the full receptor was difficult, and “getting all eight of these proteins properly assembled into the nanodisc was challenging,” Notti says.
Previous structural studies of the TCR had relied on detergent, which often strips away the surrounding membrane. Walz notes that this was the first time the receptor complex had been restored to a membrane environment for detailed imaging.
Seeing the Receptor Switch On
Once the TCR was embedded in the nanodisc, the researchers used cryo-EM to visualize it. The images showed that the receptor remains closed and compact when inactive. When it encounters an antigen-presenting molecule, however, the structure opens and extends outward, resembling a wide-reaching motion.
The result surprised the team. “The data that were available when we began this research depicted this complex as being open and extended in its dormant state,” Notti explains. “As far as anyone knew, the T cell receptor didn’t undergo any conformational changes when binding to these antigens. But we found that it does, springing open like a sort of jack-in-the-box.”
The researchers believe two factors made this discovery possible. First, they carefully recreated the TCR’s in vivo membrane environment using the right lipid mixture. Second, they reinserted the receptor into a membrane using nanodiscs before conducting cryo-EM imaging. They found that an intact membrane keeps the receptor in a closed position until activation occurs. In earlier studies, detergent may have removed this restraint, allowing the receptor to open prematurely.
“It was important that we used a lipid mixture that resembled that of the native T cell membrane,” says Walz. “If we had just used a model lipid, we wouldn’t have seen this closed dormant state either.”
Implications for Cancer Therapies and Vaccines
The team believes their findings could help improve treatments that rely on T cell receptors. “Re-engineering the next generation of immunotherapies tops the charts in terms of unmet clinical needs,” Notti says. “For example, adoptive T cell therapies are being used successfully to treat certain very rare sarcomas, so one could imagine using our insights to re-engineer the sensitivity of those receptors by tuning their activation threshold.”
Walz also sees potential applications beyond cancer therapy. “This information may be used for vaccine design as well,” he adds. “People in the field can now use our structures to see refined details about the interactions between different antigens presented by HLA and T cell receptors. Those different modes of interaction might have some implication for how the receptor functions — and ways to optimize it.”
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