The challenge of co-existence : from graft-versus-host disease to stable mixed chimerism after allogeneic hematopoietic stem cell transplantation

Abstract: The only curative treatment strategy for many hematologic and inborn immunodeficiency disorders is an allogeneic hematopoietic stem cell transplantation (HSCT). The treatment involves replacing the entire hematopoietic system of the recipient. If successful, the underlying condition of the patient is resolved, the donor hematopoietic system engrafts and a tolerance between donor- and patient-derived cells is developed. Though the procedure of HSCT has been refined for decades, there are still several severe complications associated to it. Graft-versus-host disease (GVHD) is one of the most common and most feared complications post-HSCT, and is a result of donor graft-derived cells attacking recipient tissue. Despite improved GVHD treatment strategies, severe grade GVHD is still associated with high morbidity and mortality rates. A condition known as mixed chimerism (MC), where recipient hematopoietic cells co-exist with donor hematopoietic cells, may also be considered an adverse event early post-HSCT. This is certainly the case for patients with malignancies as it indicates a potential relapse. However, in rare cases where HSCT is performed due to non-malignant disorders, long-term stable MC may develop without any apparent signs of unfavourable symptoms. The papers in this thesis aim to provide a better understanding of the co-existence of graftand host-derived cells from an immunological perspective. I will focus on GVHD and longterm stable MC post-HSCT particularly. In Paper I, I aimed to identify predictive markers for acute GVHD development. Acute GVHD occurs relatively shortly post-HSCT with potential devastating effects. In this paper, I observed a reduced frequency in mucosal-associated-invariant T (MAIT) cells in donor grafts, given to patients who later developed acute GVHD. Moreover, I could identify a predictive role for high PD-1 and low CD127-expressing T cell frequencies in the donor grafts. Together with increased levels of TNFa in both the donor graft and in patient plasma prior to HSCT, these findings may putatively be used to predict acute GVHD development in patients at time of transplantation. In Paper II, I focused on chronic GVHD, a complication that usually develops later postHSCT presenting with symptoms from several organs. Patients may suffer from chronic GVHD for years, resulting in a diminished quality of life. In this paper, I was able to identify novel cellular subsets via mass cytometry that could be linked to the severity of chronic GVHD. These subsets could also be identified by conventional flow cytometry panels more suitable for routine laboratories. Additionally, similar to the study on acute GVHD, patients with more pronounced chronic GVHD were found to have fewer MAIT cells in their blood. Thus, Paper I and II indicate a potential role for MAIT cells in both acute and chronic GVHD. In Paper III and IV, the focus was long-term stable MC, which is defined as the presence of 5-95% recipient-derived cells, after ≥5 years post-HSCT in this study. Interestingly, patients with long-term stable MC had a similar quality of life, infectious disease burden and response to vaccines compared to patients with full donor chimerism (DC). Fluctuations in recipient chimerism tended to decrease and reach stable levels after around two to five years post-HSCT. Moreover, patients with MC appear to retain functional recipient-derived cells in multiple cellular subsets. Patients with MC also displayed increased levels of IgG3 and reduced lymphocyte expression of ZAP-70, though they were found to be similar to patients with DC in overall immune phenotype.