Immunogenicity of pluripotent stem cells and their differentiation products

Embryonic stem cells (ESCs) hold great promises for regenerative medicine since they are able to differentiate into any cell type of an adult body. However, transplantation of these cells is associated with at least two severe risks: the risk of tumor formation in the recipient and the risk of an immune rejection. Since ESCs have to be transplanted in an allogeneic setting, major histocompatibility complex (MHC) mismatch could lead to rapid immune rejection of these cells. Recently, the usage of autologous pluripotent stem cells for transplantation became conceivable by description of induced pluripotent stem cells (iPSCs) and multipotent adult germline stem cells (maGSCs). But even autologous transplantations could lead to immune rejection, caused by the expression of differentiation antigens that act as mHC antigens, leading to immunogenicity of these cells. This PhD thesis aims to further characterize the immunological properties of pluripotent stem cells (PSCs) and differentiated cells derived from them. In addition, the different types of PSCs were systematically compared. In particular, the impact of mHC antigens on the susceptibility of PSCs to cytotoxic T cells (CTLs) was analyzed. Moreover, the immunogenicity of different PSCs types was studied, involving the assessment of their ability to activate the immune system as well as their potential to suppress immune functions. In previous studies our group has shown that PSCs can be killed by CTLs, although they were negative for MHC class I expression in flow cytometry. In these experiments, PSCs were pulsed with an antigenic peptide before they were exposed to activated, peptide-specific CTLs. In this thesis endogenously expressed Ovalbumin (OVA) was used as model antigen and CTLs, transgenic for a T cell receptor (TCR) that is specific for an OVA-derived peptide, were used to assess the capability of PSCs to process and present antigens. The analysis of two different, OVA-expressing ESC lines and an iPSC line revealed that they were not able to present OVA-derived antigens, whereas an analyzed maGSC line exhibited a weak antigen presentation capability. Gene expression studies of several proteins that are part of the peptide loading complex revealed that the transporter associated with antigen processing (TAP) genes were only low expressed in pluripotent stem cells. Since peptide loading is crucial for MHC class I stability and also for the transport to cell membrane, this could explain the failure of most stem cells to present antigens. Furthermore, co-culture of OVA-expressing PSCs with naive OVA-specific CD8+ and CD4+ T cells revealed that PSC were not only unable to stimulate those cells but actively inhibited their proliferation. Transplantation studies with OVA-expressing iPSCs in syngeneic hosts revealed that the expression of OVA as a model of a mHC antigen render these cells immunogenic. The transplanted iPSC OVA cells were rejected or tumor growth was significantly impeded in immunocompetent but not in immunodeficient hosts. CTLs could have contributed to this result since OVA-specific CTLs, which were able to kill OVA-expressing RMA cells, were found in the majority of the hosts, especially in those which successfully rejected the OVA-expressing iPSCs.