Mechanisms of Thymic Repair of In Vitro-Induced Precursor T Cells as a Haploidentical Hematopoietic Stem Cell Transplantation Regimen

Abstract

Haploidentical hematopoietic stem cell transplantation (haplo-HSCT) is currently an effective treatment for malignant hematological disease, but the immune deficiency and severe infection triggered by slow immune reconstitution are the main causes of high mortality and transplant failure. One of these outstanding problems is thymus damage, which is associated with graft-versus-host disease (GVHD), and preconditioning including irradiation and chemotherapy. Therefore, rapid repair of damaged thymus and rapid proliferation of thymus-derived donor T cells after transplantation are key to solving the problem. This study is designed to accelerate the recovery of thymus-derived T cells after transplantation. Wild-type mice with normal immunity were used as recipients in a haplo-HSCT mouse model to mimic clinical haplo-HSCT. A modified cell culture system using Notch ligand Delta4 and IL-7 was established that is capable of inducing and amplifying the differentiation and proliferation of hematopoietic stem cells into precursor T (preT) cells in vitro. Haplo-HSCT protocol included the preT and G-CSF mobilized donor splenic mononuclear cells (MNC) co-infusion or MNC alone. Thymic GVHD, thymic repair, and thymus-derived T cell development were compared in two groups by polychromatic immunofluorescence tracking, flow cytometry and detection of T cell receptor Vβ. The thymus homing and T-cell regeneration of allogenic preT cells were observed. The functions of preT cells in accelerating immune reconstitution, restoring thymic architecture, weakening GVH effects, and enhancing immuno-tolerance after transplantation were demonstrated. Further studies revealed that allogeneic preT cells induced by a culture system containing IL7 and Delta4 highly express ccr9 and RANKL. Interestingly, the RANK expression was promoted after preT cells' thymus homing. These results suggested that the RANK/RANKL pathway may play an important role in thymus homing. Our results provide a potential therapeutic option to optimize haplo-HSCT. It further opened up a new field of T cell therapy for artificial induction of allogeneic precursor T cells in vitro to repair the damaged thymus from irradiation and chemotherapy, and to compensate for the recovery of immune function in patients with immune deficiency caused by multiple reasons.