Co­transplantation of tonsil­derived mesenchymal stromal cells in bone marrow transplantation promotes thymus regeneration and T cell diversity following cytotoxic conditioning.

ABSTRACT

Bone marrow (BM) transplantation (BMT) represents a curative treatment for various hematological disorders. Prior to BMT, a large amount of the relevant anticancer drug needed to be administered to eliminate cancer cells. However, during this pre­BMT cytotoxic conditioning regimen, hematopoietic stem cells in the BM and thymic epithelial cells were also destroyed. The T cell receptor (TCR) recognizes diverse pathogen, tumor and environmental antigens, and confers immunological memory and self­tolerance. Delayed thymus reconstitution following pre­BMT cytotoxic conditioning impedes de novo thymopoiesis and limits T cell­mediated immunity. Several cytokines, such as RANK ligand, interleukin (IL)­7, IL­22 and stem cell factor, were recently reported to improve thymopoiesis and immune function following BMT. In the present study, it was found that the co­transplantation of tonsil­derived mesenchymal stromal cells (T­MSCs) with BM­derived cells (BMCs) accelerated the recovery of involuted thymuses in mice following partial pre­BMT conditioning with busulfan­cyclophosphamide treatment, possibly by inducing FMS­like tyrosine kinase 3 ligand (FLT3L) and fibroblast growth factor 7 (FGF7) production in T­MSCs. The co­transplantation of T­MSCs with BMCs also replenished the CD3+ cell population by inhibiting thymocyte apoptosis following pre­BMT cytotoxic conditioning. Furthermore, T­MSC co­transplantation improved the recovery of the TCR repertoire and led to increased thymus­generated T cell diversity.