Induction of transplantation tolerance in haploidenical transplantation under reduced intensity conditioning: the role of ex-vivo generated donor CD8+ T cells with central memory phenotype.

Haploidentical hematopoietic stem cell transplantation (HSCT) offers the advantage of readily available family member donors for nearly all patients. A 'megadose' of purified CD34+ hematopoietic stem cells is used to overcome the host's residual immunity surviving the myeloablative conditioning, while avoiding severe GVHD. However, the number of CD34+ cells that can be harvested is insufficient for overcoming the large numbers of host T cells remaining after reduced intensity conditioning (RIC). Therefore, combining a 'megadose' of CD34+ HSCT with other tolerizing cells could potentially support and promote successful engraftment of haploidentical purified stem cell transplantation under a safer RIC. One approach to address this challenge could be afforded by using Donor CD8 T cells directed against 3rd-party stimulators, bearing an ex-vivo induced central memory phenotype (Tcm). These Tcm cells,depleted of GVH reactivity, were shown to be highly efficient in overcoming host T cells mediated rejection and in promoting fully mismatched bone-marrow (BM) engraftment, in HSCT murine models. This is likely due to the marked lymph node homing of the Tcm, their strong proliferative capacity and prolonged persistence in BM transplant recipients. Thus, combining anti 3rd-party Tcm cell therapy with a 'megadose' of purified CD34+ stem cells, could offer a safer RIC protocol for attaining hematopoietic chimerism in patients with hematological diseases and as a platform for organ transplantation or cell therapy in cancer patients.