New nanoparticle therapy overcomes T cell exhaustion in solid tumors

Engineers at the University of Pennsylvania have developed a new type of lipid nanoparticle (LNP) that could one day serve as a universal immunotherapy for cancers that form solid tumors, including common variants such as cancers of the breast, liver and colon.

One of the greatest challenges in immunotherapy is the exhaustion of T cells, the white blood cells responsible for detecting and destroying cancer cells. Many tumors produce an enzyme called IDO that dampens immune activity. Over time, exposure to the harsh environment inside tumors further weakens T cells.

The new particles counter both effects at once. By delivering a drug that blocks IDO together with mRNA that instructs cells to produce an immune-activating protein, the engineered nanoparticles reinvigorate exhausted T cells, enabling them to attack tumors without the need for costly and time-consuming, patient-specific adjustments. 

In animal models of colon cancer, the approach eliminated established tumors and protected against recurrence, suggesting that the immune system developed a lasting memory of the cancer cells even though the therapy did not directly target tumor-specific markers.

"Traditionally, immunotherapies have been highly specific," says Michael J. Mitchell, Associate Professor in Bioengineering (BE) and senior author of a study in Nature Nanotechnology describing the particles. "This more general approach works by simply re-energizing T cells, whose exhaustion has been a bottleneck for developing solid-tumor immunotherapies." 

Overcoming T cell exhaustion

T cells are central players in the body's defense against cancer. When functioning properly, they patrol tissues, identify abnormal cells and destroy them. But inside solid tumors, that system begins to break down.

Tumors create a hostile environment that deprives immune cells of nutrients and floods them with suppressive signals. Over time, T cells exposed to cancer lose their ability to proliferate, produce key signaling molecules and kill malignant cells, a state known as T-cell exhaustion.

While treatments such as CAR-T therapy have shown remarkable success against certain blood cancers, they have proven far less effective against tumors that grow within organs. Even when T cells recognize cancer, they often lack the metabolic energy and molecular support needed to sustain an effective attack.

Inside a solid tumor, T cells are like cars trying to drive with one foot on the brake and almost no fuel in the tank. These particles release the brake and refuel the T cells at the same time." 

Qiangqiang Shi, postdoctoral fellow in BE and study's co-first author

Engineering a dual-function nanoparticle

Traditionally, lipid nanoparticles have served as delivery vehicles, transporting genetic cargo into cells, where those instructions are translated into proteins that help fight disease.

The Penn team took a different approach. Rather than simply packaging two separate components together, they chemically linked a drug that inhibits the immune suppressant IDO to a key LNP component: the ionizable lipid, which helps the particle enter cells and release its cargo. 

While other researchers have attached similar drugs to LNP components such as cholesterol, this is the first report of one being conjugated to the ionizable lipid itself. "By building the drug directly into the lipid, we created a single, unified therapeutic system," says Jinjin Wang, a postdoctoral fellow in BE and co-author of the study. "The lipid doesn't just help deliver a therapy, it becomes part of the therapy, too." 

The result is a "prodrug" lipid nanoparticle, or pLNP, that releases an IDO-blocking drug within the tumor while also instructing the tumor's own cells to produce interleukin-12 (IL-12), a powerful immune-stimulating protein.

Extensive testing confirmed that simply mixing the two therapies was not enough. "We tested seven different control groups," adds Hannah Geisler, a doctoral student in BE and co-author of the study. "Putting both components into one particle produced a much stronger immune response than delivering them separately."

Promising pre-clinical results

While pLNPs have yet to be tested in humans, the researchers demonstrated promising results in the lab. In cancer cells, pLNPs triggered far higher production of IL-12, the immune-stimulating protein, than conventional lipid nanoparticles.

In mice, the new particle not only arrested the growth of colon tumors, but nearly eliminated them within 30 days. Mice that received only one of the key components - the IDO inhibitor or the IL-12 mRNA - showed only partial tumor control, underscoring the importance of delivering both therapies in a single particle.

Treated tumors contained higher numbers of CD8⁺ "killer" T cells, fewer immune-suppressive regulatory T cells and lower levels of PD-1, a marker of T-cell exhaustion - all signs of a reinvigorated immune response. Previously "cold" tumors, which typically evade immune detection, were transformed into "hot," inflamed tumors rich in immune activity.

What's more, injecting pLNPs directly into tumors resulted in minimal toxicity. In contrast, intravenous delivery produced moderate tumor suppression, but also elevated circulating inflammatory cytokines and liver stress markers, side effects historically associated with IL-12 therapy.

Perhaps most strikingly, the immune response extended beyond the treated tumor. In mice bearing tumors on both sides of the body, injecting particles into one tumor caused the other to regress. Mice that had cleared their tumors also resisted tumor regrowth.

"We were targeting one tumor, but we saw immune activity throughout the body," says Shi. "That told us the treatment was not just acting locally, it was retraining the immune system."

Next steps

Although the findings are encouraging, the therapy remains in the preclinical stage. The researchers are now exploring ways to expand the platform's versatility and improve its translational potential.

One avenue involves testing additional immune-stimulating mRNAs beyond IL-12, broadening the range of immune signals the particle can deliver. The team is also investigating new chemical linkers that respond to different features of the tumor microenvironment, such as acidity, enzymes or oxidative stress, allowing drug release to be tuned even more precisely.

Another key goal is improving systemic delivery. While intratumoral injection proved highly effective with minimal toxicity, intravenous administration remains the most common clinical route. The researchers are exploring ways to enhance tumor targeting after intravenous injection, potentially by adding tumor-specific antibodies to reduce liver accumulation and increase delivery to tumors.

"Our platform is designed to be adaptable," says Mitchell. "We've shown it can restore immune function inside solid tumors. The next step is to refine and expand it so that it can be safely and effectively translated to the clinic."