Cord blood transplants are known to tolerate genetic mismatches better than other donor sources. But one specific HLA pairing may sharply raise the risk of severe graft-versus-host disease.
Umbilical cord blood transplantation has expanded treatment options for patients with blood cancers and other life-threatening hematological disorders, especially when a fully matched donor cannot be found (1✔ ✔Trusted Source
High-Risk Human Leukocyte Antigen Mismatch Combinations Responsible for Severe Acute Graft-Versus-Host Disease in Cord Blood Transplantation
Go to source
).
Cord blood is often considered more forgiving of genetic mismatches than other stem cell sources. Even so, serious immune complications remain a major concern.
Now, researchers at Fujita Health University report that one specific donor-recipient genetic mismatch may sharply increase the risk of the most severe form of acute graft-versus-host disease (aGVHD). In this condition, immune cells from the donor begin attacking the patient’s own tissues.
A research team led by Associate Professor Takakazu Kawase from the Department of Immune Regenerative Medicine at the International Center for Cell and Gene Therapy, Fujita Health University, Japan, conducted one of the largest studies of its kind. The work was carried out on behalf of the Japanese Society for Transplantation and Cellular Therapy (JSTCT) HLA Working Group.
The researchers examined how individual human leukocyte antigen (HLA) mismatch combinations affect outcomes after unrelated cord blood transplantation (UCBT). They analyzed nationwide registry data from 7,462 Japanese patients aged 16 years and older who underwent their first UCBT.
The study was first made available online on October 3, 2025. It was later published in Transplantation and Cellular Therapy (Volume 32, Issue 1) on January 1, 2026.
When One Genetic Pairing Raises the Risk of Severe Immune Complications
In stem cell transplantation, HLA molecules play a central role in immune recognition. They help the immune system distinguish between what belongs in the body and what does not. For years, donor selection has focused mainly on reducing the total number of HLA mismatches. Fewer mismatches were generally assumed to mean lower risk.
But the number alone does not tell the full story. Not all mismatches provoke the same immune response. Certain combinations appear to trigger much stronger immune activation, and in some cases this can escalate into severe acute graft-versus-host disease (aGVHD).
To examine this more carefully, the researchers applied advanced statistical models. These were adjusted for patient age, disease status, conditioning regimen, graft cell dose, and the overall number of mismatches, helping ensure that routine clinical differences did not distort the findings.
What they uncovered was highly specific. A previously unrecognized mismatch — HLA-C03:04 in the donor paired with HLA-C14:02 in the recipient — was associated with a threefold increase in the risk of severe (grade III–IV) aGVHD. The association remained statistically significant even after strict correction for multiple testing. The hazard ratio was 3.09.
Importantly, high-risk mismatch combinations previously reported in unrelated bone marrow transplantation did not show the same effect in cord blood transplantation, highlighting a unique risk profile for UCBT.
“This study shows that even in cord blood transplantation, where HLA mismatches are generally better tolerated, specific HLA combinations can provoke very strong immune reactions,” said Dr. Kawase. “Identifying these high-risk mismatches gives us an opportunity to improve donor selection and reduce life-threatening complications.”
How One Donor–Recipient Mismatch Can Trigger Dangerous Immune Reactions
The researchers also examined the broader clinical impact of aGVHD. They used time-dependent analysis to examine how different grades of aGVHD influenced survival. The results were nuanced.
Grade II–IV aGVHD was, in some cases, associated with improved survival. This may reflect beneficial immune effects against underlying disease. However, the pattern shifted when the condition became more severe.
Once grade III–IV aGVHD developed, overall survival declined significantly. The risk of death increased by about 80%, with a hazard ratio of 1.82.
These findings highlight an important distinction. Preventing severe GVHD may be far more critical than simply treating it after it begins.
The real-world implications are considerable. If this newly identified high-risk mismatch is factored into cord blood unit selection algorithms, clinicians may be able to avoid donor–recipient combinations that carry a heightened risk of severe GVHD. This could be particularly important when alternative units are available. Over time, this approach is expected to improve transplant safety and outcomes.
Reflecting on the motivation behind the study, Dr. Kawase explained, “We have been engaged in long-term analysis of transplant registry data with the goal of building solid evidence that improves patient outcomes. In earlier work, we were the first to identify high-risk HLA mismatch combinations in unrelated bone marrow transplantation, and this study is a continuation of that effort in cord blood transplantation.”
Looking ahead, the team believes that continued research into genetic and immunological factors driving severe GVHD will help refine transplant strategies further. Such advances could significantly improve survival and quality of life for patients undergoing stem cell transplantation in the next 5 to 10 years.
Reference:
- High-Risk Human Leukocyte Antigen Mismatch Combinations Responsible for Severe Acute Graft-Versus-Host Disease in Cord Blood Transplantation – (https://www.sciencedirect.com/science/article/pii/S2666636725014812?via%3Dihub)
Source-Transplantation and Cellular Therapy