Donor γδ T Lymphocytes Promote Allogeneic Engraftment Across the Major Histocompatibility Barrier in Mice

T cells that express the αβ T-cell receptor are thought to be the T-cell population primarily responsible for facilitating alloengraftment. The role of γδ+ T cells that comprise only a minority of mature T cells in promoting allogeneic engraftment, however, has not been extensively studied. The purpose of this study was to determine whether γδ T cells were capable of facilitating alloengraftment in murine recipients of major histocompatibility complex-mismatched marrow grafts. We developed a model where engraftment of C57BL/6 × 129/F2 (H-2b) marrow in sublethally irradiated (800 cGy) recipients (AKR/J, H-2k) is dependent on the presence of mature donor T cells in the marrow graft. In this model, donor T-cell engraftment was significantly augmented by as few as 1 × 105 αβ T cells. The role of γδ T cells was then investigated using transgenic donors (C57BL/6 × 129 background) in which a portion of the T-cell receptor–β chain gene was deleted by gene targeting so that these mice lack αβ T cells. Addition of 10 × 106 naive γδ T cells to T-cell depleted marrow grafts was required to significantly increase alloengraftment, although donor T cells averaged <50% of total splenic T cells. To determine whether higher doses of γδ T cells would improve donor engraftment and eradicate residual host T cells, γδ T cells were ex vivo expanded with a γδ T-cell–specific monoclonal antibody and interleukin-2 and then transplanted into irradiated recipients. Transplantation of ≥ 160 × 106 activated γδ T cells was necessary to consistently and significantly augment donor cell chimerism and enhance hematopoietic reconstitution when compared to control mice, but host T cells persisted in these chimeras. Addition of 2.5 × 104 mature αβ T cells, which alone were incapable of facilitating engraftment, to T-cell depleted marrow grafts containing 160 × 106 activated γδ T cells resulted in long-term (<100 day) complete donor engraftment, indicating that limiting numbers of αβ T cells were required in the marrow graft for the eradication of residual host T cells. Using serial weight curves and B-cell reconstitution as end points, clinically significant graft-versus-host disease was not observed in these chimeras under these experimental conditions. These data show that, whereas less potent than αβ T cells, γδ T cells are able to promote engraftment and enhance hematopoietic reconstitution in allogeneic marrow transplant recipients.