Figure 1.

Kinetics of immune reconstitution and transplant-related complications in children following UCB transplantation. The post-transplantation period can be divided in three phases. The pre-engraftment period (Phase I, days 0–30) is characterized by general immunosuppression (i.e., depletion of lymphocytes in the recipient by ATG), neutropenia, low platelet counts, and high susceptibility to fungal infections. Host residual APCs progressively disappear during Phase I. Massive proliferation leads NK cells to reach normal levels within 1 month post-transplantation. They may represent 80% of the recipient’s PBLs during the post-engraftment period (Phase II, days 31–100). During Phase II, patients experience high susceptibility to bacterial infections, viral infections (i.e., CMV), and acute GvHD. Some UCB T cells are believed to persist in the recipient and proliferate through HE. This period culminates with the de novo generation of T cells through thymopoiesis and the end of the 100-day high-risk window. The late phase (Phase III) is characterized by a higher incidence of VZV infection/reactivation and a progressive reconstitution of B cell and T-cell subsets, which can reach normal levels at 6–9 months post-transplant [figure and legend originally published by Merindol and colleagues (226); used with the permission of H. Soudeyns and the Journal of Leukocyte Biology (Copyright FASEB Office of Publications, Bethesda, MD, USA)].