Seventeen children with acute leukemia that had already resisted treatment got a peptide vaccine after a stem-cell transplant, and most of them were alive three years later. The vaccine trained their immune systems to hunt a protein that leukemia cells make in excess, and the children whose immune systems answered the call did far better than the ones whose did not.
The result comes from a phase II study published in Blood Advances ↗, the American Society of Hematology journal, posted online June 16. The trial was registered as UMIN000005319. It is small and single-arm, which limits how hard the numbers can be pushed. The pattern inside it is what carries the signal.
The target and the idea
The protein is WT1, short for Wilms' tumor 1. It is overexpressed in most acute leukemias and sits at low levels in healthy tissue. That contrast makes it a long-standing target for cancer vaccines. Show the immune system a fragment of WT1 and it learns to kill cells that display it. A peptide vaccine does exactly that. It presents short synthetic stretches of the protein, and the body raises cytotoxic T cells, the immune cells that destroy cancerous and infected cells, against anything carrying that marker.
The setting matters as much as the target. These were children with refractory acute leukemia, disease that standard therapy had failed to control. The vaccine was given as maintenance after an allogeneic hematopoietic stem-cell transplant. Maintenance means the treatment is meant to prevent relapse after the transplant, not to knock down active disease. The median age was 8.3 years, with a range from 2 to 18. Each child received one of two WT1 peptide vaccines by injection into the skin.
What happened
The primary endpoint was three-year overall survival, and it came in at 70.6 percent (95 percent confidence interval 43.1 to 86.6). The authors compared that against a historical benchmark of 30 percent for refractory pediatric leukemia after transplant. About 71 percent alive at three years against an expected 30 percent is the headline. For children whose disease had already failed earlier treatment, that is a wide margin.
The more telling number is inside the cohort. Twelve children were clinical responders, meaning they stayed in complete remission a year after the vaccinations started, and their three-year survival was 91.7 percent. When the authors sorted patients by whether their immune systems actually reacted to the vaccine, the split sharpened. The eleven immune responders, defined as those whose WT1-specific cytotoxic T cells rose at least 1.455-fold from baseline, had 91 percent three-year survival. The five immune non-responders had 40 percent (P = 0.027).
The T-cell measurements track the same story. In responders, the frequency of WT1-specific killer T cells in the blood climbed from 0.25 percent before vaccination to 1.07 percent after, peaking around week 12 (P less than 0.001). In non-responders it barely moved, from 0.12 to 0.20 percent (P = 0.42). The vaccine's benefit did not come simply from receiving it. It came from whether the immune system learned the lesson.
The catch, and the part worth keeping
A single-arm study of 17 patients judged against a historical control is hypothesis-generating, not definitive. Children who mount a strong immune response may be healthier in ways that also predict survival. The responder gap is a clue about mechanism, not proof that the vaccine caused it. The authors note that elevated WT1-specific T cells before vaccination already predicted a favorable outcome. That cuts both ways. It suggests the blood test finds children primed to do well regardless of the vaccine.
What survives those caveats is a usable signal. The immune readout was not a side measurement. It sorted survival cleanly, on a peptide platform tested for two decades without a clear pediatric maintenance win. No child stopped treatment because of the vaccine, and the side effects, including graft-versus-host disease, were manageable. For a refractory pediatric leukemia population with few options after transplant, a maintenance vaccine whose effect can be read in a blood test is worth a controlled trial. This study is the argument for one.