Beneath the sword of Damocles: regenerative medicine and the shadow of immunogenicity

Accept or Reject: The Role of Immune Tolerance in the Development of Stem Cell Therapies and Possible Future Approaches

In 2011, Goldring and colleagues published a review article describing the potential safety issues of novel stem cell-derived treatments. Immunogenicity and immunotoxicity of the administered cell product were considered risks in the light of clinical experience of transplantation. The relative immunogenicity of mesenchymal stem cells, embryonic stem cells (ESCs), and induced pluripotent stem cells (iPSCs) was being addressed through in vitro and in vivo models. But the question arose as to whether the implanted cells needed to be identical to the recipient in every respect, including epigenetically, to evade immune recognition? If so, this set a high bar which may preclude use of many cells derived from iPSCs which have vestiges of a fetal phenotype and epigenetic memory of their cell of origin. However, for autologous iPSCs, the immunogenicity reduces once the surface antigen expression profile becomes close to that of the parent somatic cells. Therefore, a cell product containing incompletely differentiated cells could be more immunogenic. The properties of the administered cells, the immune privilege of the administration site, and the host immune status influence graft success or failure. In addition, the various approaches available to characterize potential immunogenicity of a cell therapy will be discussed.

Human-Induced Pluripotent Stem Cell Technology and Cardiomyocyte Generation: Progress and Clinical Applications

Human-induced pluripotent stem cells (hiPSCs) are reprogrammed cells that have hallmarks similar to embryonic stem cells including the capacity of self-renewal and differentiation into cardiac myocytes. The improvements in reprogramming and differentiating methods achieved in the past 10 years widened the use of hiPSCs, especially in cardiac research. hiPSC-derived cardiac myocytes (CMs) recapitulate phenotypic differences caused by genetic variations, making them attractive human disease models and useful tools for drug discovery and toxicology testing. In addition, hiPSCs can be used as sources of cells for cardiac regeneration in animal models. Here, we review the advances in the genetic and epigenetic control of cardiomyogenesis that underlies the significant improvement of the induced reprogramming of somatic cells to CMs; the methods used to improve scalability of throughput assays for functional screening and drug testing in vitro; the phenotypic characteristics of hiPSCs-derived CMs and their ability to rescue injured CMs through paracrine effects; we also cover the novel approaches in tissue engineering for hiPSC-derived cardiac tissue generation, and finally, their immunological features and the potential use in biomedical applications.

Immunological Properties of Murine Parthenogenetic Stem Cell-Derived Cardiomyocytes and Engineered Heart Muscle

Pluripotent parthenogenetic stem cells (pSCs) can be derived by pharmacological activation of unfertilized oocytes. Homozygosity of the major histocompatibility complex (MHC) in pSCs makes them an attractive cell source for applications in allogeneic tissue repair. This was recently demonstrated for pSC-based tissue-engineered heart repair. A detailed analysis of immunological properties of pSC-derived cardiomyocytes and engineered heart muscle (EHM) thereof is, however, lacking. The aim of this study was to determine baseline and cytokine-inducible MHC class I and MHC class II as well as programmed death ligand-1 (PDL-1) and co-stimulatory protein (CD40, CD80, CD86) expression in pSC-derived cardiomyocytes and pSC-EHM in vitro and in vivo. Cardiomyocytes from an MHC-homologous (H2^d/d) pSC-line were enriched to ~90% by making use of a recently developed cardiomyocyte-specific genetic selection protocol. MHC class I and MHC class II expression in cardiomyocytes could only be observed after stimulation with interferon gamma (IFN-γ). PDL-1 was markedly upregulated under IFN-γ. CD40, CD80, and CD86 were expressed at low levels and not upregulated by IFN-γ. EHM constructed from H2^d/d cardiomyocytes expressed similarly low levels of MHC class I, MHC class II, and costimulatory molecules under basal conditions. However, in EHM only MHC class I, but not MHC class II, molecules were upregulated after IFN-γ-stimulation. We next employed a cocultivation system with MHC-matched and MHC-mismatched splenocytes and T-cells to analyze the immune stimulatory properties of EHMs. Despite MHC-mismatched conditions, EHM did not induce splenocyte or T-cell proliferation in vitro. To evaluate the immunogenicity of pSC-derived cardiomyocytes in vivo, we implanted pSC-derived embryoid bodies after elimination of non-cardiomyocytes (cardiac bodies) under the kidney capsules of MHC-matched and -mismatched mice. Spontaneous beating of cardiac bodies could be observed for 28 days in the matched and for 7 days in the mismatched conditions. Teratomas formed after 28 days only in the MHC-matched conditions. Immunohistochemistry revealed single clusters of CD3-positive cells in the border zone of the implant in the mismatched conditions with few CD3-positive cells infiltrating the implant. Taken together, MHC-matched pSC-cardiomyocyte allografts show little immune cell activation, offering an explanation for the observed long-term retention of pSC-EHM allografts in the absence of immunosuppression.