Switching on an immune pathway in cancer cells with a new mRNA therapy reprograms the immune system in complex tumor environments to launch a broader attack
By Benjamin Boettner
(BOSTON) — Cancer cells develop various strategies to paralyze immune cells to evade their attack in the complex tumor microenvironment (TME). Using one such strategy, they cripple their own production of a small signaling molecule known as cGAMP, which, if released into the TME, can be taken up by immune cells that then build up a first line of defense against cancer cells, commonly referred to as the “innate immune response.”
To accomplish this, cancer cells reduce or shut down the expression of the so-called cGAS enzyme that usually synthesizes cGAMP when it encounters double-stranded DNA (dsDNA) that is not supposed to be there, such as DNA from invading pathogens or cellular DNA that has been damaged. “It just so happens that cancer cells, because they are dividing so fast and not particularly accurately, tend to have more double-stranded DNA fragments than healthy cells and thus have a strong need to reduce cGAS activity,” explained Alexander Cryer, Ph.D., the first author of a new study published in PNAS, and a research fellow at the Wyss Institute and Instructor in Medicine at Brigham and Women’s Hospital working with Wyss Associate Institute Director Natalie Artzi, Ph.D. Normally, although not in TMEs with strongly reduced cGAS expression, the released cGAMP enters innate immune cells in the TME, and binds and activates the protein STING, which in turn triggers the production of interferon and other immune proteins that coordinately build up the innate immune response – STING accordingly stands for “stimulator of interferon genes.
It has therefore been an attractive goal of cancer immunologists to find an immunotherapeutic approach that can activate the “cGAS-STING pathway” in TME-resident immune cells. However, the most obvious way, namely, activating STING directly, using small molecule agonist drugs, has proven challenging because these compounds are poorly taken up by immune cells, rapidly cleared from the TME, and can cause systemic inflammation. Moreover, many tumor cells exhibit impaired cGAS–STING signaling, which limits their responsiveness to such agonists and reduces their overall therapeutic efficacy.
Hitting the gas in cancer cells
Now, Artzi’s team has brought the problem back to cancer cells. “Cancer cells comprise a significant portion of the TME but are often under-utilized for immunotherapy,” said Artzi, who in addition to being a Core Faculty member at the Wyss Institute, is also a Principal Research Scientist at MIT, and the Hansjörg Wyss Professor of Biologically Inspired Engineering at Harvard Medical School and Brigham and Women’s Hospital in Boston, where most of this work was done. Previously, her team also devised a nanoparticle-based method to create a bioactive depot of STING agonists in tumor cells that enables the sustained activation of nearby immune cells. In their new study, the researchers, however, took a different approach.
Cancer cells comprise a significant portion of the TME but are often under-utilized for immunotherapy…Our findings with cGAS LNPs highlight how cancer cells can be used to contribute to their own elimination.
By targeting mouse melanoma cells with lipid nanoparticles (LNPs) that they loaded with mRNA encoding cGAS as well as a piece of dsDNA to activate the synthesized cGAS enzyme (cGAS LNPs), Artzi, Cryer, and the team were able to coerce cancer cells into producing significant amounts of cGAMP, which essentially overrides their immune-evading capability. After adding the supernatant that covered the cultured targeted melanoma cells, which contains newly synthesized cGAS and all other secreted proteins, to immune cells, they observed a potent activation of a STING response, compared to control LNPs lacking the cGAS mRNA. In addition, cGAS also directly traveled via physical cell-cell contact sites from targeted melanoma cells to immune cells.
Next, to investigate whether their cGAS restoration in cancer cells would also occur in vivo and enable tumor control, the researchers treated mice carrying melanoma tumors by injecting cGAS LNPs directly into tumor masses. The mRNA therapy reduced tumor growth in the animals and resulted in the activation of a broader range of immune cells, including cytotoxic CD8+ T cells, natural killer cells, macrophages, and dendritic cells. “The spectrum of activated immune cells in the TME and nearby lymph nodes showed us that our approach can also engage the adaptive immune system, which is a promising sign of antitumor immunity,” said Cryer, who spearheaded the project. “This happens with just a small dose of cancer cell-produced cGAMP, which may help circumvent the side effects of the required high doses of STING agonists that can cause unwanted inflammation, tissue damage, and autoimmune reactions.”
Combination of benefits
T cells are often silenced in the TME due to a so-called checkpoint blockade. Cancer cells produce so-called checkpoint proteins that bind to specific receptor molecules on the surface of T cells, thereby rendering them ineffective. Checkpoint inhibitors – a hugely successful form of clinically approved immunotherapy, including for the treatment of melanoma – block checkpoint proteins or their receptors to release the brakes on T cells again.
Artzi’s team therefore treated the mouse melanoma model with a combined therapy of cGAS LNPs and the checkpoint inhibitor drug anti-PD-1, which further improved the therapeutic outcomes. In 30% of the mice, melanoma tumors were completely eradicated, while tumor growth was significantly slowed or halted, but tumors were never completely eliminated in mice that received just one of the treatments.
“Our findings with cGAS LNPs highlight how cancer cells can be used to contribute to their own elimination. A future cGAS LNP therapy could help jolt the innate immune system in the TME into action and improve the benefits of checkpoint inhibitor therapy in patients,” said Artzi. Her team is planning to adapt the delivery system for cGAS mRNA to allow the therapy to be administered as a systemic injection. In addition, the researchers will test whether cGAS mRNA therapy can synergize with chemotherapy drugs or radiation drugs that damage DNA in cancer cells to increase cGAS activation.
“This study nicely complements our work on disease-agnostic Duplex RNA technology supported by ARPA-H in which we leverage our drug delivery platform to target cells with a newly identified small dsRNA that activates interferon signaling via a different route directly in target cells,” said Artzi. “There are several ways to bridle a horse, i.e., engage the innate immune system – which one is the right one will depend on the disease and situation.”
Artzi is also Head of Structural Nanomedicine at Mass General Brigham and co-founded the Targeted Nucleic Acid Delivery Working Group at the Wyss Institute that serves as a forum to facilitate internal and industry collaborations on promising drug payload-delivery method matches for specific disease applications. “This special PNAS issue on Structural Nanomedicine featuring our work highlights how this burgeoning field is gaining momentum in a growing number of disease areas – an exciting development that we are committed to being a vital part of at the Wyss Institute and wider Harvard ecosystem,” said Artzi. She noted that “very interestingly, it’s also becoming obvious that mRNA treatments, similar to the one we developed in this study, can exhibit domino effects in disease areas they weren’t initially designed for. This insight first surfaced in recent studies on how COVID-19 mRNA vaccines surprisingly benefit cancer patients who are treated with cancer immunotherapies.”
Other authors on the study were Pere Dosta, Michelle Dion, Leonardo Soto, Eliz Amar-Lewis, Gabriela Carmona, Alejandro Pérez, Diego Aguilar, Triana Huerta, Beatriz Ruiz, Nathalie Hernandez, and Yael Soria.