By stimulating cancer cells to produce and release an enzyme that activates the cGAS-STING signaling pathway in nearby immune cells, MIT researchers found a way to force tumors to trigger their own destruction.
The researchers used lipid nanoparticles (LNPs) to deliver cyclic GMP-AMP (cGAMP) synthase (cGAS) mRNA to cancer cells. This results in production of the endogenous stimulator of interferon genes (STING) agonist cGAMP, which is then released and transferred to neighboring immune cells. STING is a protein that helps to trigger immune responses.
Studies in mouse cancer models found that activating the cGAS-STING signaling pathway in this way worked even better when combined with existing immune checkpoint blockade immunotherapy, promoting antitumor immunity and controlling tumor growth.
The team suggests their approach may avoid the side effects of delivering large doses of a STING activator and could more easily be applied to develop a treatment for use in patients. “Our approach harnesses the tumor’s own machinery to produce immune-stimulating molecules, creating a powerful antitumor response,” said Natalie Artzi, PhD, a principal research scientist at MIT’s Institute for Medical Engineering and Science, an associate professor of medicine at Harvard Medical School, a core faculty member at the Wyss Institute for Biologically Inspired Engineering at Harvard.
“By increasing cGAS levels inside cancer cells, we can enhance delivery efficiency—compared to targeting the more scarce immune cells in the tumor microenvironment—and stimulate the natural production of cGAMP, which then activates immune cells locally,” Artzi noted. “This strategy not only strengthens antitumor immunity but also reduces the toxicity associated with direct STING agonist delivery, bringing us closer to safer and more effective cancer immunotherapies.”
Artzi is senior author of the team’s published paper in Proceedings of the National Academy of Sciences (PNAS), titled “Restoration of cGAS in cancer cells promotes antitumor immunity via transfer of cancer cell–generated cGAMP,” in which they concluded “These findings highlight how cancer cells can be used to actively contribute to their own elimination and may be a broadly applicable strategy for delivery of other reprogramming molecules to cancer cells and wider therapeutic combinations.” Alexander Cryer, PhD, a visiting scholar at IMES, is the study’s first author.
When STING is activated, it turns on a pathway that initiates production of type one interferons, which are cytokines that stimulate immune cells. Many research groups, including Artzi’s, have explored the possibility of artificially stimulating the pathway using STING agonists, which could help immune cells to recognize and attack tumor cells. This approach has worked well in animal models, but it has had limited success in clinical trials, in part because the required doses can cause harmful side effects. “Stimulator of interferon genes (STING) agonists are a promising class of innate immune agonists and can promote antitumor immunity,” the authors wrote. “However, direct administration can cause deleterious side effects.”
While working on a project exploring new ways to deliver STING agonists, Cryer became intrigued when learning from previous work that cancer cells can produce a STING activator known as cGAMP. The cells then secrete cGAMP, which can activate nearby immune cells.
“Part of my philosophy of science is that I really enjoy using endogenous processes that the body already has, and trying to utilize them in a slightly different context,” Cryer noted. “Evolution has done all the hard work. We just need to figure out how push it in a different direction. Once I saw that cancer cells produce this molecule, I thought: maybe there’s a way to take this process and supercharge it.”
Within cells, the production of cGAMP is catalyzed by an enzyme called cGAS. To get tumor cells to activate STING in immune cells, the researchers developed lipid nanoparticles to deliver the messenger RNA that encodes cGAS. “We used lipid nanoparticles (LNPs) to deliver mRNA coding for cGAS which catalyzes the production of cGAMP,” they wrote. When cGAS detects double-stranded DNA in the cell body, which can be a sign of either infection or cancer-induced damage, it begins producing cGAMP.
“It just so happens that cancer cells, because they’re dividing so fast and not particularly accurately, tend to have more double-stranded DNA fragments than healthy cells,” Cryer added. The tumor cells then release cGAMP into tumor microenvironment (TME) where it can be taken up by neighboring immune cells and activate their STING pathway.
Using a mouse model of melanoma, the researchers evaluated their new strategy’s potential to kill cancer cells. They injected mRNA encoding cGAS, encapsulated in lipid nanoparticles, into tumors. One group of mice received this treatment alone, while another received a checkpoint blockade inhibitor, and a third received both treatments.
Given on their own, cGAS and the checkpoint inhibitor each significantly slowed tumor growth. However, the best results were seen in the mice that received both treatments.
“Treatment of syngeneic murine melanoma with cGAS LNPs reduced tumor growth significantly and further benefit was observed upon combination with immune checkpoint blockade (anti-PD-1),” the investigators stated. In that combination treatment group, tumors were completely eradicated in 30% of the mice, while none of the tumors were fully eliminated in the groups that received just one treatment.
An analysis of the immune response showed that the mRNA treatment stimulated production of interferon as well as many other immune signaling molecules. By taking advantage of endogenous cell–cell communication processes, the most numerous cell type in the TME, the cancer cell, became a source of potent immunomodulatory molecules, which were able to activate neighboring cells, including immune cells,” the team stated. A variety of immune cells, including macrophages and dendritic cells, were activated. These cells help to stimulate T cells, which can then destroy cancer cells.
The researchers were able to elicit these responses with just a small dose of cancer cell-produced cGAMP, which could help to overcome one of the potential obstacles to using cGAMP on its own as therapy, which is that large doses are required to stimulate an immune response, but these doses can lead to widespread inflammation, tissue damage, and autoimmune reactions. When injected on its own, cGAMP tends to spread through the body and is rapidly cleared from the tumor, while in this study, the mRNA nanoparticles and cGAMP remained at the tumor site.
“The side effects of this class of molecule can be pretty severe, and one of the potential advantages of our approach is that you’re able to potentially subvert some toxicity that you might see if you’re giving the free molecules,” Cryer said. “By mimicking endogenous processes, our approach may subvert toxicity that has been observed by dosing cGAMP, synthetic STING agonists, or deleterious immune effects such as T-cell apoptosis,” the authors stated.
The researchers now hope to work on adapting the delivery system so that it could be given as a systemic injection, rather than injecting it into the tumor. They also plan to test the mRNA therapy in combination with chemotherapy drugs or radiotherapy that damage DNA, which could make the therapy even more effective because there could be even more double-stranded DNA available to help activate the synthesis of cGAMP.
“Our approach stands to combine adroitly with clinically approved therapies such as ICB to maintain effective T cell function, as well double-as radiotherapy and chemotherapy that induce strand breaks or replication errors in DNA, leading to more cytosolic dsDNA,” they noted.
