Researchers at the Princeton University Branch of the Ludwig Institute for Cancer Research have discovered new ways in which a vitamin A-derived molecule can impair the immune system’s ability to fight cancer. The molecule, known as all-trans-retinoic acid, was found to weaken natural anti-cancer immune responses and, under certain conditions, reduce the effectiveness of a promising cancer vaccine. Vitamin A metabolites, also known as retinoids, have long been a source of debate due to their mixed effects on health and disease. The new findings described in two scientific articles help to clarify this long-standing controversy. They also led to the development of the first experimental drugs designed to interrupt the cellular signaling pathway triggered by retinoic acid.
How Retinoic Acid Undermines Cancer Vaccines
One of the studies, published in Nature Immunology, was led by Ludwig Princeton researcher Yibin Kang and PhD student Cao Fang. The team found that retinoic acid produced by dendritic cells (DCs), important immune cells responsible for activating immune defenses, can reprogram these cells to promote tolerance to tumors. This tolerance significantly reduces the effectiveness of dendritic cell vaccines, a type of immunotherapy that trains the immune system to recognize and attack cancer. The researchers also described the development and preclinical testing of a drug that blocks retinoic acid production in both cancer cells and DCs. The compound KyA33 improved the efficacy of DC vaccines in animal studies and also showed potential as a stand-alone cancer immunotherapy.
A second study, led by former Kang lab PhD student Mark Esposito and published in the journal iScience, focused on developing drugs that inhibit retinoic acid production and completely stop retinoid signaling. Although scientists have been studying retinoids for more than a century, attempts to develop drugs that safely block their signaling have repeatedly failed. The approach described in this study combined computational modeling with large-scale drug screening. This strategy provided the framework for the development of KyA33 and represented a significant advance in the study of a signaling pathway that had resisted drug development for decades.
“Overall, our results show that retinoic acid has a far-reaching impact on attenuating vital immune responses to cancer,” said Kang. “By exploring this phenomenon, we have also solved a long-standing challenge in pharmacology by developing safe and selective inhibitors of retinoic acid signaling and establishing a preclinical proof of concept for their use in cancer immunotherapy.”
Why Dendritic Cells are Important for Cancer Defense
Retinoic acid is produced by an enzyme called ALDH1a3, which is often found in high concentrations in human cancer cells. A related enzyme, ALDH1a2, produces retinoic acid in certain subsets of DCs. Once retinoic acid is produced, it activates a receptor in the cell nucleus and triggers a signaling cascade that alters gene activity. In the gut, this process is known to promote the formation of regulatory T cells (Tregs), which help to prevent harmful autoimmune reactions. Until now, however, scientists were unaware of how retinoic acid affects the dendritic cells themselves. Dendritic cells play a central role in coordinating immune responses. They continuously monitor the body for signs of infection or cancer. When they detect a threat, they process fragments of abnormal proteins and present them as antigens to T cells, which then seek out and destroy diseased or cancerous cells.
Dendritic cell vaccines are produced by taking immature immune cells from a patient’s blood and growing them in the laboratory together with antigens from that patient’s tumor. These prepared cells are then returned to the patient with the aim of eliciting a strong anti-tumor immune response. Despite improvements in the identification of suitable cancer antigens, these vaccines often do not have the desired effect. Fang, Kang and their colleagues, including Esposito and Princeton Branch Director Joshua Rabinowitz, set out to explore the reasons for this.
How Vaccine Production Triggers Immunosuppression
“We discovered that under the conditions commonly used to produce DC vaccines, differentiating dendritic cells begin to express ALDH1a2 and produce high levels of retinoic acid,” Fang said. “The resulting activated nuclear signaling pathway then suppresses DC maturation and reduces the ability of these cells to trigger anti-tumor immunity. This previously unknown mechanism likely contributes to the largely suboptimal performance of DC and other cancer vaccines that has been repeatedly observed in clinical trials.” However, the problem does not end there. The retinoic acid released by dendritic cells also promotes the formation of macrophages, which are less effective against cancer. As these macrophages accumulate in place of functional dendritic cells, the overall efficacy of dendritic cell vaccines is further reduced.
Restoring the Immune Defense With a New Drug
The researchers showed that blocking ALDH1a2, either by genetic techniques or with KyA33, restores the maturation of dendritic cells and their ability to activate the immune defense. DC vaccines produced in the presence of KyA33 elicited strong, targeted immune responses in mouse models of melanoma. These responses delayed tumor development and slowed cancer progression. When administered directly to mice, KyA33 also acted as an independent immunotherapy and reduced tumor growth by stimulating the immune system.
The development of inhibitors targeting ALDH1a2 and ALDH1a3 represents a significant scientific achievement. Of the twelve classic nuclear receptor signaling pathways, the retinoic acid pathway was the first to be discovered and the only one that has not yet been successfully treated with drugs. The study in iScience describes in detail the computational and experimental approach used to overcome this challenge. With these new compounds, the researchers were finally able to explain a long-standing paradox associated with vitamin A and cancer.
In laboratory experiments, retinoic acid can cause cancer cells to stop growing or die, contributing to the assumption that vitamin A has anti-cancer properties. However, large clinical studies and other evidence show that high vitamin A intake increases the risk of cancer (and cardiovascular disease) and increases mortality. High levels of ALDH1A enzymes in tumors are also associated with poorer survival in many cancers. Previous attempts to separate the functions of ALDH1A enzymes from retinoic acid production have largely failed.
Towards New Treatments for Cancer and Beyond
“Our study reveals the mechanistic basis for this paradox,” Esposito said. “We have shown that ALDH1a3 is overexpressed in various cancers to generate retinoic acid, but that cancer cells lose their responsiveness to retinoid receptor signaling, bypassing its potential antiproliferative or differentiating effects. This partly explains the paradox of vitamin A’s effects on cancer growth.” The researchers also found that retinoic acid primarily affects the immune environment around tumors rather than the cancer cells themselves. By penetrating the tumor microenvironment, retinoic acid suppresses immune responses, including the activity of T cells that normally fight cancer cells.
To confirm this, the team showed that ALDH1a3 inhibitors stimulated potent immune attacks against tumors in mouse models, demonstrating their potential as effective immunotherapies. “By developing drug candidates that safely and specifically inhibit nuclear signaling through the retinoic acid pathway, we are paving the way for a novel therapeutic approach to cancer treatment,” said Kang. Esposito and Kang have since founded the biotechnology company Kayothera to bring these ALDH1A inhibitors to the clinical testing phase. The company aims to develop treatments for various diseases that are affected by retinoic acid, including cancer, diabetes and cardiovascular disease.