Molecular Obstacles to Clinical Translation of iPSCs Natalia Tapia, Hans R. Schöler  Cell Stem Cell  Volume 19, Issue 3, Pages 298-309 (September 2016) DOI: 10.1016/j.stem.2016.06.017 Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 1 Immune Response to Autologous iPSCs and Their Progeny (A) The subcutaneous injection of iPSCs into the hind leg of genetically matched mice was described to induce an immune response, which initially suggested that isogenic iPSCs might not be tolerated by the immune system (Zhao et al., 2011). (B) Transplantation of terminally differentiated tissues isolated from syngeneic iPSC-derived chimeric mice into the skin and bone marrow of syngeneic mice was shown to elicit limited immunogenicity, demonstrating that iPSC-derived tissues are not immunogenic (Araki et al., 2013). (C) Transplantation of in vitro iPSC derivatives into the subcapsular renal space of syngeneic mice was tolerated by the immune system, leading to the conclusion that in vitro iPSC-differentiated cells are not immunogenic in autologous recipients (Guha et al., 2013). (D) Humanized mice displayed different immune responses to autologous iPSC progeny obtained through in vitro differentiation and transplanted into clinically relevant sites. These findings suggest that the expression of immunogenic antigens in in vitro iPSC derivatives depends on the maturity level of the cell type obtained with a specific differentiation protocol (Zhao et al., 2015). Cell Stem Cell 2016 19, 298-309DOI: (10.1016/j.stem.2016.06.017) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 2 Genomic and Epigenetic Stability of hiPSCs Somatic mosaicism is amplified during clonal generation and expansion of iPSCs. This initial genetic variability comprises harmless as well as oncogenic modifications. In addition, de novo genetic mutations or epigenetic abnormalities might also be introduced during the reprogramming process. Thus, rigorous selection screening on established iPSC clones must be performed to identify and discard lines that contain genomic insults that might compromise subsequent molecular and clinical studies. Several reports have described genomic anomalies after differentiation, thus requiring a final screening step to be included to confirm the genomic integrity of iPSC derivatives before their use in therapeutic settings. Further studies need to distinguish genomic variations without clinical relevance from those with tumorigenic potential. Cell Stem Cell 2016 19, 298-309DOI: (10.1016/j.stem.2016.06.017) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 3 Reprogramming Fidelity in iPSC Technology versus Other Reprogramming Methods (A) An enucleated oocyte can reprogram a somatic cell nucleus into a blastocyst-stage embryo from which SCNT ESCs can be derived. The reprogramming process progresses through active and passive demethylation and under physiological levels of oocyte transcription factors (TFs). Of note, the oocyte reprograms the somatic cell nucleus to a totipotent state from which pluripotency is subsequently established. SCNT ESCs display epigenetic memory, reprogramming errors, imprinting aberrations, CNVs, and SNPs. (B) The forced expression of four TFs is sufficient to reprogram differentiated cells into iPSCs. The reprogramming mechanism requires passive demethylation and nonphysiological levels of TFs. In contrast to SCNT, the iPSC technology reprograms somatic cells directly to a pluripotency state. Like SCNT ESCs, iPSCs exhibit epigenetic memory, reprogramming errors, imprinting aberrations, CNVs, and SNPs. (C) Specific culture conditions can reprogram germline stem cells into gPSCs through passive demethylation and physiological levels of TFs. Like SCNT ESCs and iPSCs, gPSCs exhibit epigenetic memory and reprogramming errors. N.D., not determined. Cell Stem Cell 2016 19, 298-309DOI: (10.1016/j.stem.2016.06.017) Copyright © 2016 Elsevier Inc. Terms and Conditions