Human herpes virus 6 infection is a hallmark of cord blood transplant in adults and may participate to delayed engraftment: a comparison with matched unrelated donors as stem cell source.

Occurrence of CMV, EBV and human herpes virus 6 (HHV6) infections and immune reconstitution were compared in 15 adult patients receiving a cord blood transplantation (CBT) and 40 patients who received an allogeneic transplantation from a matched unrelated donor (MUD). Herpes virus DNA quantifications in the blood (459 samples) were performed before and then monthly up to 9 months after transplant and the main lymphocytes populations were counted at 3, 6 and 9 months. Incidence of HHV6 infection was significantly higher in the CBT group (80 vs 42.5%; P<0.0001), with higher viral load (P<0.0001). In multivariate analysis, the use of a CBT and a myeloablative conditioning regimen were found to increase the risk of HHV6 infection (odds ratio (OR)=5.4, P=0.02 and OR=3.5, P=0.04, respectively). Incidences of CMV were similar between the two groups whereas MUD increased the risk of EBV infection, in univariate analysis only. HHV6 reactivation translated toward delayed neutrophils and plts engraftment in the two groups. MUD and CBT do not share the same immune reconstitution patterns, notably for B and CD8 lymphocytes and NK cells. There is a strong and specific relationship between HHV6 infection and the use of cord blood cells.