IN VIVO HEMATOPOIETIC CELL ENGRAFTMENT IS MODULATED BY DPPIV/CD26 INHIBITION AND RHEB2 OVEREXPRESSION

Abstract

Hematopoietic cell transplantation (HCT) is an important modality used to treat patients with hematologic diseases and malignancies. A better understanding of the biological processes controlling hematopoietic cell functions such as migration/homing, proliferation and self-renewal is required for improving HCT therapies. This study focused on the role of two biologically relevant proteins, dipeptidylpeptidase IV (DPPIV/CD26) and Ras homologue enriched in brain 2 (Rheb2), in modulating hematopoietic cell engraftment. The first goal of this study was to determine the role of the protein DPPIV/CD26 in modulating the engraftment of human umbilical cord blood (hUCB) CD34+ stem/progenitor cells using a NOD/SCID mouse xenograft model, and based upon previous work demonstrating a role for this enzyme in Stromal-Derived Factor-1/CXCL12 mediated migration and homing. Related to this first goal, pretreatment with an inhibitor of DPPIV/CD26 peptidase activity increased engraftment of hUCB CD34+ cells in vivo in recipient Non Obese Diabetic/Severe Combined Immunodeficiency (NOD/SCID) mice while not disturbing their differentiation potential following transplantation. These results support using DPPIV/CD26 inhibition as a strategy for enhancing the efficacy of cord blood transplantation. The second goal was to determine, by overexpression, the role of the Rheb2 in affecting the balance between proliferation and in vivo repopulating activity of mouse hematopoietic cells. Rheb2 is known to activate the mammalian target of rapamycin (mTOR) pathway, a pathway important in hematopoiesis. Rheb2 overexpression increased the proliferation and mTOR signaling of two hematopoietic cell lines, 32D and BaF3, in response to delayed IL-3 addition. In primary mouse hematopoietic cells, Rheb2 overexpression enhanced the proliferation and expansion of hematopoietic progenitor cells (HPCs) and phenotypic hematopoietic stem cells (HSCs) in vitro. In addition, HPC survival was enhanced by Rheb2 overexpression. Using in vivo competitive repopulation assays, Rheb2 overexpression transiently expanded immature HPC/HSC populations shortly after transplantation, but reduced the engraftment of total transduced cells. These findings support previous work showing that signaling proteins able to enhance the proliferative status of hematopoietic stem cells often cause exhaustion of self-renewal and repopulating ability. These studies of hematopoietic engraftment modulated by both of these molecules provide information which may be important to future work on HCT.