Chimerism kinetics in pediatric hematopoietic stem cell transplantation: from biological basis to advanced monitoring technologies

Monitoring donor-recipient chimerism following hematopoietic stem cell transplantation (HSCT) is essential for assessing engraftment success, early identification of graft failure and relapse, and guiding immunomodulating interventions. Recent advancements in molecular technologies, particularly digital droplet PCR (ddPCR) and next-generation sequencing (NGS), have markedly improved the sensitivity and precision of chimerism detection, allowing the identification of recipient cells at levels below 0.1%. These advancements enable more accurate and dynamic monitoring compared to traditional methods, which are limited in terms of sensitivity and specificity. In pediatric patients, the interpretation of chimerism results is complicated by unique factors, including thymic recovery, exposure to serotherapy agents such as ATG or alemtuzumab, and constraints related to small blood volume. These factors affect the kinetics of donor cell engraftment and the dynamics of mixed chimerism in different hematopoietic lineages. This review presents the current understanding of the biological basis of chimerism, compares traditional and advanced detection methodologies, and details lineage-specific chimerism kinetics in children. It also provides practical guidance for interpreting serial chimerism data to inform timely and preemptive clinical interventions. Owing to the limited pediatric-specific data, adult findings were integrated where appropriate, underscoring the urgent need for pediatric validation, assay harmonization, and standardized implementation protocols to optimize transplant outcomes in children.