Vitamin A derivative suppresses immune response and cancer vaccine efficacy

Scientists at the Princeton University Branch of the Ludwig Institute for Cancer Research have identified novel mechanisms by which a metabolic derivative of vitamin A-all-trans retinoic acid-compromises both the body’s normal anti-cancer immune response and, in a different context, the efficacy of a promising type of cancer vaccine. The role of Vitamin A metabolites, otherwise known as retinoids, has long been controversial in both health and disease. Described in two publications, the findings help resolve this controversy and advance the first candidate drugs to switch off the biochemical signaling pathway they engage within cells.

One study, published in the current issue of Nature Immunology and led by Ludwig Princeton’s Yibin Kang and graduate student Cao Fang, describes how retinoic acid produced by the immune system’s dendritic cells (DCs) alters them to induce a dangerous tolerance of tumors. This tolerance, the researchers show, diminishes the efficacy of otherwise promising immunotherapies known as dendritic cell vaccines. They also report the design and preclinical assessment of a candidate drug that inhibits retinoic acid production by both cancer cells and DCs. The compound, KyA33, not only boosts the efficacy of DC vaccines in preclinical studies but also holds promise as an independent cancer immunotherapy.

The second study, led by a former graduate student in Kang’s lab, Mark Esposito, and recently reported in the journal iScience, describes the rational design and preclinical development of drugs that inhibit retinoic acid production and so turn off retinoid signaling. While retinoids have been known to scientists for more than a century, efforts to develop viable drugs to block their signaling have so far met with failure. The process of drug-discovery developed in this study provided the blueprint for the design of KyA33.

“Taken together, our findings reveal the broad influence retinoic acid has in attenuating vitally important immune responses to cancer,” said Kang. “In exploring this phenomenon, we also solved a longstanding challenge in pharmacology by developing safe and selective inhibitors of retinoic acid signaling and established preclinical proof of concept for their use in cancer immunotherapy.”

A deadly tolerance

Retinoic acid is produced by an enzyme known as ALDH1a3, which is often overexpressed in human cancer cells, or by its sibling ALDH1a2 in certain subtypes of DCs.

The molecule activates a receptor in the cell’s nucleus to initiate a molecular signaling cascade that alters gene expression. Its production by DCs is known to induce the generation of immune cells known as regulatory T cells (Tregs) in the gut that restrain potentially dangerous autoimmune reactions. However, its effect on DCs themselves was not known.

DCs are perhaps best known for their critical role in orchestrating protective immune responses. DCs patrol the body looking for signs of infection and cancer. Upon detecting such threats, these itinerant cells process and present fragments of disease-associated proteins-or antigens-to activate T cells, which then target sick and cancerous cells.

DC cancer vaccines are typically produced by generating DCs from their precursors in the blood. These immature blood cells are first isolated from a patient and then grown in lab cultures in the presence of cancer antigens from that patient’s tumor. The expectation is that such “primed” dendritic cells should elicit potent anti-tumor responses when transferred back into the patient.

Trouble is, they often do not, even though researchers have made significant headway in identifying the right cancer antigens to use for such purposes. Fang, Kang and colleagues, including Esposito and Princeton Branch Director Joshua Rabinowitz, figured out why this is the case.

“We discovered that under conditions commonly employed to produce DC vaccines, differentiating dendritic cells begin expressing ALDH1a2, producing high levels of retinoic acid,” said Fang. “The nuclear signaling pathway it activates then suppresses DC maturation, diminishing 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 seen in clinical trials.”

To make matters worse, the retinoic acid secreted by DCs also favors the development of macrophages that are less efficient than DCs in combating cancer cells. The accumulation of these cells instead of DCs further undermines the efficacy of DC vaccines.

The researchers show that the genetic disruption of ALDH1a2 expression or its pharmacological inhibition with KyA33 restores the maturation and anti-tumor function of DCs. DC vaccines formulated in the presence of KyA33 elicit strong antigen-specific immune responses, delaying the onset of tumors in mouse models of melanoma and slowing their progression. When given directly to mice, the inhibitor also works independently as an immunotherapy, suppressing tumor growth.

Resolving an old paradox

The development of these ALDH1a2/3 inhibitors itself is a notable accomplishment. Of the dozen classic nuclear receptor signaling pathways, the one activated by retinoic acid was the first such pathway discovered but remains the only one that has not yet been successfully targeted by a drug.

The iScience paper describes a hybrid computational and large-scale drug screening approach Esposito, Kang and colleagues took to develop their inhibitors. With the unique tool offered by these novel compounds, the researchers were able to solve the apparent paradox of retinoid nuclear signaling in cancer.

Retinoic acid has been shown to induce the growth arrest and death of cancer cells in laboratory cell cultures, a finding that has imbued vitamin A with anti-cancer agency in the popular imagination. On the other hand, multiple lines of evidence, including the findings of major clinical trials, indicate that high intake of vitamin A actually increases the incidence of cancer (and cardiovascular disease) and related mortality. Moreover, elevated expression of ALDH1A enzymes in tumors is associated with poor survival across multiple types of cancer. To resolve this paradox, much research has attempted, with little success, to dissociate the role of ALDH1A enzymes in cells from retinoic acid production.

“Our study reveals the mechanistic basis for this paradox,” said Esposito. “We’ve shown that ALDH1a3 is overexpressed in diverse cancers to generate retinoic acid, but that cancer cells lose their responsiveness to retinoid receptor signaling, avoiding its potential anti-proliferative or differentiating effects. This explains, in part, the paradox of vitamin A’s effects on cancer growth.”

The other part, Esposito, Kang and colleagues found, is that retinoic acid does not influence the cancer cells themselves but is rather secreted into the tumor microenvironment to suppress the anti-cancer immune response. One way it does so is by disrupting T cell responses to cancer.

To demonstrate this, the researchers showed that these novel ALDH1a3 inhibitors serve as a potent immunotherapy in mouse models of cancer by stimulating the immune system to attack tumors.

“By developing candidate drugs that safely and specifically inhibit nuclear signaling through the retinoic acid pathway, we are paving the way for a novel therapeutic approach to cancer,” said Kang.

Esposito and Kang have launched a biotechnology company, Kayothera, to bring their novel ALDH1A inhibitors to the clinic for multiple diseases known to be driven by retinoic acid, including cancer, diabetes and cardiovascular disease.