Double umbilical cord blood transplantation is increasingly applied in the treatment of adult patients with high-risk hematological malignancies and has been associated with improved engraftment as compared to that provided by single unit cord blood transplantation. The mechanism of improved engraftment is, however, still incompletely understood as only one unit survives. In this multicenter phase II study we evaluated engraftment, early chimerism, recovery of different cell lineages and transplant outcome in 53 patients who underwent double cord blood transplantation preceded by a reduced intensity conditioning regimen. Primary graft failure occurred in one patient. Engraftment was observed in 92% of patients with a median time to neutrophil recovery of 36 days (range, 15-102). Ultimate single donor chimerism was established in 94% of patients. Unit predominance occurred by day 11 after transplantation and early CD4<sup>+</sup> T-cell chimerism predicted for unit survival. Total nucleated cell viability was also associated with unit survival. With a median follow up of 35 months (range, 10-51), the cumulative incidence of relapse and non-relapse mortality rate at 2 years were 39% and 19%, respectively. Progression- free survival and overall survival rates at 2 years were 42% (95% confidence interval, 28-56) and 57% (95% confidence interval, 43-70), respectively. Double umbilical cord blood transplantation preceded by a reduced intensity conditioning regimen using cyclophosphamide/fludarabine/4 Gy total body irradiation results in a high engraftment rate with low non-relapse mortality. Moreover, prediction of unit survival by early CD4<sup>+</sup> lymphocyte chimerism might suggest a role for CD4<sup>+</sup> lymphocyte mediated unit-versus-unit alloreactivity. NTR1573.,
Erasmus MC: University Medical Center Rotterdam

Somers, J. A. E., Braakman, E., van der Holt, B., Petersen, E., Marijt, E., Huisman, C., … Cornelissen, J. (2014). Rapid induction of single donor chimerism after double umbilical cord blood transplantation preceded by reduced intensity conditioning: Results of the HOVON 106 phase II study. Haematologica, 99(11), 1753–1761. doi:10.3324/haematol.2014.106690