SPI-6 Expression Protects Embryonic Stem Cells from Lysis by Antigen-Specific CD8⁺ Cytotoxic T Lymphocytes

Abstract

Embryonic stem (ES) cells are defined by their capacity of self renewal and the potential to give rise to any differentiated cell type of the entire organism. These properties have made ES cells promising tools for future applications of cell replacement therapy. The possible use of ES cells in replacement therapy clearly demands a comprehensive understanding of the immunological properties of ES cells and their differentiated derivatives the embryoid body cells (EB cells). To date, the immune response to ES cells is still poorly understood. This study addresses the adaptive cellular immune response to undifferentiated ES and EB cells infected with the Lymphocytic Choriomeningitis (LCM) virus, a vertically transmitted pathogen in mice and humans. In contrast to the prevailing view, this study shows that murine ES and EB cells express MHC class I molecules, although at low levels. Interestingly, both ES and EB cells infected with the LCM virus are able to present viral antigens at the cell surface and are effectively recognized by LCM virus-specific cytotoxic T lymphocytes (CTL) in an antigen-specific and MHC class I-restricted manner despite the low level-expression of MHC class I molecules. However, unlike control fibroblasts, LCM virus-infected ES or EB cells are not effectively killed by the highly cytotoxic LCM virus-specific CD8⁺ CTL. <br /> Quantitative real time PCR and Western blot analysis show that both ES and EB cells express high levels of Cathepsin B as well as the serine protease inhibitor (SPI)-6, inhibitors of the CTL-derived cytotoxic effector molecules perforin and granzyme B, respectively. Whereas pharmacological inhibition of Cathepsin B did not increase the susceptibility of ES or EB cells to lysis by CTL, down-regulation of SPI-6 by RNA interference rendered ES cells highly sensitive for CTL-induced cell death. <br /> The results of this study show that LCM virus-infected ES and EB cells present viral antigens and are of sufficient immunogenicity to be effectively recognized by antigen-specific CD8⁺ CTL in an antigen-specific and MHC class I-restricted manner. However, ES and EB cells resist CTL-mediated lysis due to high-level expression of cytoprotective molecules. The immunoprivileged properties of ES cells and their derivatives demands further detailed analysis because they might pose the risk of persistence infection within ES cell-derived transplants or of the non-immune-surveilled growth of malignancies.