Haploidentical stem cell transplantation: the always present but overlooked donor

Haploidentical stem cell transplantation is a treatment option for the approximately 70% of patients who do not have an HLA-identical sibling donor. The availability of a haploidentical donor in most families is a potential advantage, both for avoiding the need to find an alternative unrelated donor, and for the potentially more potent graft-versus-tumor effect that can be induced. The early complications of severe graft-versus-host disease (GVHD) following T-cell replete stem cell transplantation (SCT), and graft failure and recurrent malignancy (after T-cell depleted SCT) have limited the applications of this approach. Newer strategies employing T-cell depletion of the graft, using either very high-dose peripheral blood stem cells and/or more intensive conditioning therapy have overcome some of the problems of conventional transplantation. Nonmyeloablative SCT approaches have overcome some of the morbidity and mortality associated with the early complications of SCT and have been associated with favorable engraftment and GVHD profiles. Induction of mixed lymphohematopoietic chimerism as a platform for adoptive cellular immunotherapy (via delayed donor lymphocyte infusions) may have important application in avoiding early GVHD, while ultimately capturing a very potent graft-versus-tumor effect. Current strategies are focusing on improvement of immune reconstitution and prevention of recurrence of the underlying malignancy.