Regulatory T Cells and Hematopoiesis in Bone Marrow Transplantation

CD4+CD25+FoxP3+ regulatory T cells (Treg) possess the capacity to modulate both adaptive and innate immunity. Due to their suppressive nature, Treg cells have been studied and tested in a variety of scenarios in an attempt to ameliorate undesired immune responses. While graft versus host disease (GVHD) has in fact emerged as the first clinical application for human Treg cells (Riley et al. 2009), equally important are issues concerning hematopoietic engraftment and immune reconstitution. Currently, little is known about the effect(s) that regulatory T cells may exert outside the immune system in this context. Based on cytokine effector molecules they can produce we hypothesized that Treg cells could regulate hematopoietic phenomena. The studies portrayed in this dissertation demonstrate that Treg cells can differentially affect the colony forming activity of myeloid and erythroid progenitor cells. In-vitro as well as in-vivo findings demonstrate the ability of Tregs to inhibit and augment the differentiation of primitive and intermediate myeloid (interleukin (IL)-3 driven) and late erythroid (erythropoietin driven) hematopoietic progenitor cells, respectively. The inhibitory and enhancing affects appeared to be mediated by independent pathways, the former requiring cell-cell contact, major histocompatibility complex (MHC) class II expression on marrow cells and involving transforming growth factor beta (TGF-beta), whereas the latter required interleukin (IL)-9 and was not contact dependent. Strikingly, we observed that in addition to regulating hematopoietic activity in normal primary BM cells, Tregs were also capable of suppressing colony forming activity by the myelogenous leukemia cell line NFS-60. Furthermore, studies involving endogenous Treg manipulations in-situ (i.e. depletion of these cells) resulted in elevated overall myeloid colony activity (CFU-IL3) and diminished colony numbers of erythroid precursors (CFU-E) in recipients following BMT. Consistent with these results, it was found that upon co-transplant with limiting numbers of bone marrow cells, exogenously added Treg cells exert in-vivo regulation of myeloid and erythroid CFU activity during the initial weeks post-transplantation. This regulation of hematopoietic activity by freshly generated Tregs upon transplantation led to the elaboration of a second hypothesis; following lethal total body irradiation (TBI) the host microenvironment facilitates regulatory T cell activation/effector function. Substantial evidence has accumulated in support of this hypothesis, for example we demonstrate up-regulation of surface molecules such as GARP and CD150/SLAM, which have been previously reported as indicators of Treg activation following TCR signaling and co-stimulation, occurs in donor (reporter) Treg populations. Acquisition of an activated phenotype and hence of effector/modulatory function is consistent with the previous in-vivo observations, indicating that both recipient and donor Treg cells can influence hematopoietic progenitor cell activity post-transplant. Finally, the present studies may be of great relevance in the emerging field of Treg cell based immunotherapy for prevention and/or treatment of HSCT complications