How immune cell therapy surprisingly cured cancer in a teenager

The tumor was removed by surgery, followed by chemotherapy. Two years later the first metastases were discovered. From then on, the cancer continued to spread, eventually ending up with tumors on the liver and spleen, on the lungs and in the brain.

“I asked myself whether it was ethical to still give hope here,” remembers Christian Seitz. He is senior physician and medical director of cell and gene therapy at the Hopp Children’s Tumor Center Heidelberg (KiTZ). Mailo’s mother had contacted them and asked: “Isn’t there anything else that can be done?”

Seitz’s first reflex was: There’s nothing left to do. But then doctors and scientists at KiTZ Mailos took a closer look at tumor cells and discovered that all cancer cells produce a specific protein molecule, called PRAME. Good news.

Arm immune systems against tumor cells

This opened up the possibility of immune cell therapy, a weapon that has so far hardly been used in the fight against cancer in children. “The idea is to activate the body’s own immune system against the tumor cells,” explains Seitz. There are different methods to encourage the immune system to fight cancer.

In Mailo’s case, the immune system’s defense cells, the T cells, should be genetically modified so that they recognize and specifically attack tumor cells. Genetic modification means: Using a virus, a genetic blueprint is introduced into the cells that are previously obtained from the patient’s blood.

According to this blueprint, a receptor is created on the T cells that is programmed for the tumor’s specific protein molecules – here the PRAME protein. This enables the T cells to recognize and kill the tumor cells.

In June 2025, Mailo will travel from Thuringia, where he, his parents and two siblings live, to Heidelberg, to the children’s clinic at the university hospital. The doctors won’t give the boy three months or more. Immunotherapy remains the only, last chance for the passionate downhill biker who has just completed secondary school and is doing relatively well despite the poor findings.

Therapy for children is “absolutely new territory”

Before the doctors at KiTZ can even treat Mailo, they must obtain approval from the responsible authorities to attempt individual treatment. Never before had a child or adolescent with a solid tumor been treated with cellular immunotherapy. For the KiTZ, the healing attempt meant completely new territory.

There are several immunotherapies that have already been approved, but only for adults. The few immunotherapies available for children can only be used for certain types of cancer. The form of immunotherapy that Mailo was supposed to receive, cellular immunotherapy, only has one approved therapy to date, and that is for the treatment of leukemia, blood cancer.

Christian Seitz remembers that Mailo said he really wanted to reach 2026. “I had to swallow.” On July 2, 2025, the modified immune cells will be administered to Mailo via an infusion. He is in the intensive care unit of the children’s hospital, they want to be prepared for anything. There is a great risk that the immune system will get out of hand when the T cells start their attack.

Three days later, Mailo developed a severe fever. “This is the point in time when the programmed T cells multiply explosively,” explains Seitz. Just twelve days after the cells were administered, the first tumor foci could no longer be seen on the MRI. According to Seitz, 120 days later there is no longer any living tumor tissue to be seen; There were also no tumor cells detectable in the blood. “Looking at it soberly, that wasn’t to be expected,” says the doctor.

Career wish after therapy: doctor

In one photo, Mailo can be seen in the hospital; he appears very thin. He’s wearing a T-shirt with the words “It’s a hill. Get over it. And back down” written on the back. It’s a mountain. Get over him. And then it goes down. “My goal was always clear: I wanted to get back on the bike,” says Mailo.

He has already taken part in a number of downhill biking competitions, which he says gave him strength and a zest for life. Shortly after the immunotherapy, he trained again on the Königstuhl. Now he wants to continue cycling. And he wants to get his high school diploma because what he wants to become is clear: “I want to study medicine in Heidelberg and then become a doctor.”

Preparations are currently underway at KiTZ for a clinical study that will prove that Mailo’s case was not an isolated case, but that this type of immunotherapy also works on other children and adolescents. 15 to 18 young cancer patients will be included there. Seitz urges us to hurry because the treatment options, especially for children with cancer after a relapse, are limited. “These children cannot wait; they need new, effective therapies.”