The role of gamma delta T cells in haematopoietic stem cell transplantation.

Although haematopoietic stem cell transplantation (HSCT) is a potential curative treatment for haematological malignancies, it is still a procedure associated with substantial morbidity and mortality due to toxicity, graft-versus-host disease (GVHD) and relapse. Recent attempts of developing safer transplantation modalities increasingly focuses on selective cell depletion and graft engineering with the aim of retaining beneficial immune donor cells for the graft-versus-leukaemia (GVL) effect. In this context, the adoptive and especially innate effector functions of γδ T cells together with clinical studies investigating the effect of γδ T cells in relation to HSCT are reviewed. In addition to phospho-antigen recognition by the γδ T cell receptor (TCR), γδ T cells express receptors of the natural killer (NK) and natural cytotoxicity (NCR) families enabling them to recognize and kill leukaemia cells. Antigen recognition independent from the major histocompatibility complex (MHC) allows for the theoretical possibility of mediating GVL without an allogeneic response in terms of GVHD. Early studies on the impact of γδ T cells in HSCT have reported conflicting results. Recent studies, however, do suggest an overall favourable effect of high γδ T cell immune reconstitution after HSCT; patients with elevated numbers of γδ T cells had a significantly higher overall survival rate and a decreased rate of acute GVHD compared to patients with low or normal γδ T cell counts. Further research in terms of effector mechanisms, subtypes and tissue distribution during the course of HSCT is needed to assess the potentially beneficial effects of γδ T cells in this setting.