Bone marrow-derived host cells in murine cardiac allografts

Despite the success of current immunosuppressive agents in controlling acute rejection in the field of transplantation, chronic rejection continues to limit survival and quality of life in heart transplant recipients. Transplant vascular disease (TVD) is a rapidly progressive form of atherosclerosis that occurs in the vessels of solid organ transplants. The pathogenesis of TVD has a multifactorial basis arising from a complicated interplay between immunological and non-immunological factors. Together, these factors result in endothelial cell (EC) damage and accumulation of modified smooth muscle cells (SMC) in the arterial walls of the transplanted heart. Recent reports have shown discordant data on the ability of host stem cells to migrate to sites of damage following cardiac transplantation and repopulate cells of the vessel wall. Many questions remain unanswered, such as, the relative contributions of different sources of host progenitor cells to vascular regeneration and whether these new cells subsequently alter vascular function and ameliorate the pathogenesis of TVD. As well, the effect of immunosuppressive therapy commonly used following cardiac transplantation and the effect of cytokine treatment with granulocyte-colony stimulating factor (G-CSF) on the host cell response to the allograft remains to be determined. Therefore, the major focus of this dissertation is to investigate the contribution of host BM-derived cells to the replacement of cells in the blood vessels of cardiac allografts. A murine heterotopic heart transplant model of TVD will be used to study the role of host BM-derived cells in the vasculature of cardiac allografts. The effect of immunosuppressive treatment with tacrolimus on the host BM-derived cell response to the allograft and vascular chimerism will be examined. As well, G-CSF-induced mobilization of host BM-derived cells will be used to determine whether increased circulating levels of host progenitor cells leads to altered rates of vascular chimerism in the vessels of the allografts following transplantation. Finally, the therapeutic potential of using autologous progenitor cells, injected following cardiac transplantation, will be examined to determine whether these injected cells are able to seed to damaged areas within the vasculature, contribute to cell chimerism and subsequently improve graft outcome. Using BM-GFP transgenic mice as recipients of heterotopic heart transplants, we found that host BM-derived progenitor cells contributed to both EC and SMC replacement in transplanted hearts. The rate of re-endothelialization was found to diminish significantly from 11.8% ± 2.5% at 14 days post-transplant to 4.0% ± 1.2% (p< or =0.05) 30 days post-transplant. Also, immunosuppressive treatment of cardiac allografts with tacrolimus was not found to affect the frequency of re-endothelialization or SMC replacement by host BM-derived cells. G-CSF treatment of BM-derived progenitor cells in vitro was found to improve survival, proliferation and angiogenesis of the infused cells despite treatment with immunosuppressive agents. G-CSF pretreatment of BM-GFP transgenic recipient mice prior to heterotopic heart transplantation resulted in the same rate of re-endothelialization at 14 days post-transplant as non-pretreated allografts. However, at the 30 day post-transplant timepoint, there was a higher rate of re-endothelialization in G-CSF pretreated allografts (9.3% ± 2.2%) relative to non-pretreated allografts (3.4% ± 1.6%). In addition, G-CSF pretreated allografts demonstrated less intimal narrowing in vessels of the transplanted heart relative to vessels in non-pretreated control allografts. In addition, there was no evidence of vascular chimerism by the lineage negative cells injected into hearts at both 14 and 30 days post-transplantation, with no evidence of a beneficial effect from the injection of these BM-derived progenitor cells on TVD progression. At both timepoints of 14 and 30 days post-transplantation, there was no observed improvement in either the percent luminal narrowing or the intima to media ratio in the transplanted hearts of allografts between the control injected versus the lin- cell injected groups. As well, repeated injections of the lineage negative cells following transplantation did not result in any change in graft outcome. In summary, the results obtained in this work provide valuable insights into the contribution of host BM-derived progenitor cells to the replacement of cells within the damaged vasculature following cardiac transplantation. In particular, cytokine-induced mobilization of circulating host BM-derived progenitor cells with G-CSF was found to augment EC repopulation and ameliorate the development of allograft lesions. In addition, injected lineage negative, BM-derived progenitor cells were not found to seed to sites of damage in a manner similar to endogenous BM-derived progenitor cells and were not found to contribute to vascular cell replacement in transplanted hearts.Medicine, Faculty ofPathology and Laboratory Medicine, Department ofGraduat