Galectin-9 promotes immune persistence of tyrosine kinase inhibitor-tolerant cells in oncogene-driven non-small cell lung cancer

Lung cancer remains the leading cause of cancer-related death worldwide, often because the disease is found too late for a cure. For patients whose tumors are driven by specific genetic errors, such as mutations in the EGFR gene, doctors have powerful tools called tyrosine kinase inhibitors. These are small-molecule drugs that act like a precise key, turning off the broken switch that tells the cancer cells to grow. In many cases, these drugs work dramatically, shrinking tumors and giving patients years of life. Yet, a frustrating pattern emerges: the cancer almost always returns. Even when a drug seems to wipe out the visible tumor, a tiny, hidden group of cells survives. These survivors, known as drug-tolerant persisters, lie dormant or grow slowly, eventually reawakening to cause a relapse that is harder to treat than the original disease. The central challenge for modern medicine is not just killing the bulk of the cancer, but finding a way to eliminate these invisible survivors before they can strike again.

A team of researchers at Hyogo Medical University and collaborating institutions has uncovered a specific mechanism these survivors use to hide from the body's natural defenses, and they have identified a way to strip away that protection. In their study, they focused on the innate immune system, the body's first line of defense that includes natural killer cells. Unlike the adaptive immune system, which learns to recognize specific cancer markers over time, natural killer cells can attack abnormal cells immediately without needing to be taught. The researchers discovered that when lung cancer cells are exposed to tyrosine kinase inhibitors, they do something unexpected: they produce a massive amount of a protein called galectin-9. This protein acts as a shield. When it appears on the surface of the cancer cells, it binds to receptors on the natural killer cells and effectively tells them to stand down, preventing them from killing the cancer. By blocking this signal, the researchers found they could wake up the natural killer cells and allow them to hunt down and destroy the drug-tolerant survivors.

The investigation began by observing what happens inside lung cancer cells when they are treated with drugs like osimertinib or alectinib. The researchers found that these drugs, while stopping the cells from dividing, actually trigger a genetic switch inside the nucleus of the surviving cells. This switch, controlled by a regulator called IRF-1, turns on the gene that produces galectin-9. In the lab, when the researchers exposed cancer cells to these drugs, the levels of galectin-9 rose sharply. They then tested whether this protein was the reason the cells could survive the immune system's attack. When they added natural killer cells to a dish of cancer cells that had been treated with the drug, the immune cells struggled to kill the cancer. However, when the researchers used a genetic tool to remove the ability of the cancer cells to make galectin-9, the natural killer cells became highly effective, wiping out the survivors completely. This suggested that the drug itself was inadvertently helping the cancer hide by boosting the production of this protective protein.

To see if this dynamic held true in a living organism, the team moved to experiments using mice that lacked a specific type of immune cell but still possessed natural killer cells. They injected these mice with lung cancer cells that either could or could not produce galectin-9. When the mice were treated with the cancer drug, the tumors shrank in both groups. But as soon as the treatment stopped, the tumors in the mice with the normal, galectin-producing cancer cells began to grow back quickly. In contrast, the mice with the galectin-deficient cancer cells showed a much slower return of the disease. The researchers then took this a step further by testing a therapeutic antibody designed to block galectin-9. When they combined this antibody with the standard cancer drug, the tumors were suppressed for a significantly longer time compared to the drug alone. This confirmed that the antibody could neutralize the cancer's shield, allowing the body's own natural killer cells to finish the job.

The researchers also looked at human patients to see if this pattern matched reality. They examined tissue samples from 57 patients with a specific type of lung cancer who had been treated with osimertinib. They found that patients whose tumors had high levels of galectin-9 did not respond differently at first; the drug worked just as well for them as for those with low levels. However, the long-term outcome was very different. Patients with high levels of galectin-9 saw their cancer return much sooner, with a median progression-free survival of 18 months, compared to 45 months for those with low levels. This clinical data aligned perfectly with the lab findings: the protein did not stop the drug from working initially, but it allowed the remaining cancer cells to survive the immune system's cleanup efforts and regrow later.

The study suggests that the path to a lasting cure for this type of lung cancer may lie in combining existing drugs with a new strategy to disable this specific shield. The researchers demonstrated that the drug-tolerant cells rely on galectin-9 to evade the innate immune system, and that removing this protection allows natural killer cells to eliminate the survivors. While the study was conducted in laboratory settings and animal models, and the human data was retrospective, the consistency across these different methods points to a clear biological truth. The findings indicate that targeting galectin-9 could transform the treatment of oncogene-driven lung cancer, turning a temporary remission into a durable one by ensuring that the few cells that survive the initial drug attack are not left to hide and multiply. This approach offers a potential way to overcome the inevitable resistance that currently limits the success of even the most advanced targeted therapies.