Although we utilized two hiPSC

Although we utilized two hiPSC lines derived from two different types of somatic cells, specifically skin fibroblasts (eKA3) and peripheral blood mononuclear naloxone hydrochloride (Ff-I01), obtained from independent donors, the immune responses mediated by hiPSC-NS/PCs were similar between the two cell types. This result is in contrast to the results of a previous report study showing that the immunogenicity of iPSC-derived derivatives was dependent on the parental somatic cells used to generate the iPSCs (Liu et al., 2013b). In that study, Liu et al. compared the immune reactions of umbilical cord mesenchymal cell (UMC)-derived iPSC-NS/PCs (UMC-NS/PCs) with those of skin fibroblast derivatives and showed that UMC-NS/PCs were indeed less immunogenic than skin fibroblast-derived iPSC-NS/PCs in stimulating the proliferation of PBMCs (Liu et al., 2013b). This discrepancy may arise because UMCs are immune tolerant (De Schauwer et al., 2014; Qiu et al., 2014). Thus, further research is required to compare the immunogenic properties of hiPSC-NS/PCs derived from other somatic tissues. In recent years, several groups have carried out immunological studies involving the transplantation of iPSC-derived cells in non-human primate models (Morizane et al., 2013; Kamao et al., 2014; Kawamura et al., 2016). One model is the cynomolgus macaque, which is similar to humans in terms of gene structure and major histocompatibility complex (MHC) polymorphisms (Kono et al., 2014; Shiina et al., 2015). According to Kawamura et al., cardiomyocytes derived from MHC-homozygous iPS cells do not suppress the allogeneic immune reaction due to the lack of MHC matching, despite the application of multiple immunosuppressants. In contrast, our group, using common marmosets, demonstrated the survival of transplanted allogeneic ESC-NS/PCs in the injured spinal cord with only a low dose of a single immunosuppressant; furthermore, transplantation promoted functional recovery (Iwai et al., 2015). In a human clinical trial, transplanted allogeneic human fetal NS/PCs promoted functional motor recovery following SCI at one year after injection with only a single immunosuppressant administered for 9weeks (Shin et al., 2015). As hiPSC-NS/PCs possess similar characteristics to fetal and hESC-NS/PCs in terms of antigen-presenting and immunomodulatory functions, hiPSC-NS/PC immunogenicity is potentially lower than that of iPSC-derived cardiomyocytes. Taken together, the degree of the immune response and the effects of HLA-matching on the immune response differ among transplanted cell types and transplantation sites. In terms of future therapeutic prospects, auto-transplantation therapy employing hiPSCs may be difficult due to the cost and labor required, as previously described (Zimmermann et al., 2012; Aoi and Stacey, 2015). Consequently, a clinical trial is currently planned to test allogeneic HLA-matched transplantation with banked HLA homozygous hiPSCs in Japan (Okita et al., 2011; Taylor et al., 2012). Matching for HLA-A, -B and -DRB1 attenuates immune rejection and reduces the use of immunosuppressant drugs in organ and umbilical cord blood transplantation (Kurtzberg et al., 2008; Opelz and Döhler, 2010). Therefore, hiPSCs will be established from HLA-A, -B and -DRB1 homozygous donors. As people of Japanese descent have relatively homogeneous genetic backgrounds, the establishment of an HLA-homozygous cell bank may prove advantageous. There are several limitations of the current study. Our data were obtained through in vitro analyses. Therefore, it remains unclear how allogeneic hiPSC-NS/PCs will affect the host immunoreaction when they are transplanted into an injured human spinal cord. It is crucial to further verify the applicability of these in vitro results in a clinical setting. In the first-in-human trial of hiPSC-NS/PC transplantation for SCI patients, we plan to perform NS/PC-PBMC MLR prior to transplantation. We also plan to conduct positron emission tomography (PET) analyses with [11C] PK11195, which binds to activated microglia (Vowinckel et al., 1997) and reflects inflammation and the immune reaction (Morizane et al., 2013) after transplantation. Based on the evaluation of the correlation between the in vitro analysis prior to transplantation and the in vivo analysis after transplantation, it could be clarified whether the immune rejection can be predicted with NS/PC-PBMC MLR in advance of transplantation. In the present study, although we only treated NS/PCs with TNFα and/or IFNγ, a number of studies have demonstrated that various biochemical processes involving numerous mediators cause inflammation in the acute injured spinal cord (Nakamura et al., 2003; Yang et al., 2005; Nishimura et al., 2013). Other undefined inflammatory mediators may also be involved in the regulation of surface antigens related to NS/PC immunogenicity in vivo. Moreover, HLA antigens other than HLA-A, -B, and -DRB1 and minor histocompatibility antigens (Spierings, 2014; Kwun et al., 2015) were not evaluated in the present study. Further investigations will enable us to elucidate the differences between autologous and allogeneic transplantation or HLA-matched and -mismatched transplantation in the future.