Current status of haploidentical stem cell transplantation.

Haploidentical stem cell transplantation is an alternative transplant strategy for patients without an human leucocyte antigen-matched donor. The historical experience with haploidentical stem cell transplantation has been characterised by the immunological consequences of crossing a major histocompatibility barrier, namely graft-versus-host disease (GVHD), graft rejection and impaired immune reconstitution. Ex vivo T-cell depletion of the graft may reduce the risk of GVHD, but at the expense of a higher risk of engraftment failure and relapse of the underlying malignancy. Myeloablative transplant strategies using vigorously T-cell-depleted 'megadose' stem cells appear to have improved the outcomes of selected patients with acute leukaemia. Non-myeloablative stem cell transplantation strategies, in which mixed chimaerism as a platform for adoptive cellular immunotherapy is intentionally induced, show promise for limiting GVHD and lessening transplant-related mortality. Future approaches, based on promising preclinical and early clinical observations, may include selective allodepletion of the graft, and manipulation of the cellular environment post-transplant using selected cellular populations or immunomodulatory soluble factors.