Therefore, MHC matching might donate to long-term graft survival in iPSC-based transplantation. and typical immunosuppressants concentrating on T cell activation acquired limited results on managing rejection. Nevertheless, anti-donor antibodies weren’t detected just in the minimal antigen-mismatched transplantation. This murine transplantation model may be used to additional analyze immunological topics for MHC homo-to-hetero iPSC-based transplantation. Subject terms:Allotransplantation, T cells, Induced pluripotent stem cells, MHC == Introduction == In transplantation medicine, including organ or tissue transplantation, promoting graft acceptance and prolonging graft survival are important issues. Rejection is the major cause of graft loss, and many studies focusing on preventing rejection have been performed. Although the major cause of graft loss is human leukocyte antigen (HLA) (major histocompatibility complex, MHC) mismatches, in Melanotan II addition to this, other antigens, such as minor antigens, can also cause rejection1,2. Therefore, allogeneic recipients are usually administered immunosuppressants throughout their lives to control rejection. Pluripotent stem cells (PSCs) used for cell or tissue transplantation are a subject suffered from rejection when transplanted in allogeneic recipients3,4. Using autologous iPSCs is an ideal approach for avoiding rejection3. However, preparation of these cells Melanotan II is time-consuming and costly. In contrast, stocking of off-the-shelf Melanotan II allogeneic iPSCs has been proposed3,4. To reduce the risk of rejection in off-the-shelf iPS-derived cell or tissue transplantation, MHC (HLA)-homozygous iPSCs stock project has been suggested3,4. In preclinical studies, monkey dopaminergic neurons or cardiomyocytes derived from MHC homozygous iPSCs were transplanted into MHC heterozygous recipients5,6. In this type of MHC homo-to-hetero transplantation, although the immune response was decreased, administration of immunosuppressants was still required to prevent rejection. The immune responses in this case are considered to be induced by mismatches of minor antigens. However, the immunological features of the MHC-matched but minor antigen-mismatched transplantation are not well-understood. In Melanotan II fact, MHC-matching is advantageous for reducing the risk of rejection, including chronic antibody mediated rejection (CABMR), HDAC5 in clinical kidney transplantation7. However, there are few cases in which the types of MHCs between donors and recipients are completely matched7. This is because MHC molecules are highly polymorphic, and it is very difficult to find a completely suitable donor for each recipient. For these reasons, the immunological aspects of MHC-matched transplantation have not been fully elucidated. In this study, we firstly established an experimental mouse model in which MHC-matched but minor antigen-mismatched tissue (skin) was transplanted. The immunological characteristics of the transplantation have been examined in detail. The information from this study would be beneficial for developing iPSC-based transplantation. == Results == == The survival of skin grafts depends on the combination of donors and recipients in MHC-matched but minor antigen-mismatched transplantation == We first established a murine experimental model as follows: C3H/He C57BL/10 (B10) F1 (C3B10F1, MHC haplotype: k/b), C57BL/6J (B6) DBA/2 (B6D2F1: b/d), and C3H/He 129X1/Sv F1 (C3129F1: k/b) mice as recipients and B6 (b/b), CBA/N (k/k), 129X1/Sv (b/b), and BALB/c (d/d) mice as donors (Fig.1). It is known that the strength of rejection is different among the type of grafts. Usually, tissue or cellular grafts are suffered from severer rejection than organ grafts in allogeneic transplantation8. Therefore, we chose skin (tissue) transplantation as a model because it is thought that iPSC-derived grafts are composed mainly of cells or tissues rather than of organs9. In C3B10F1 recipients, fully Melanotan II allogeneic BALB/c skin grafts were rejected as early as day 16, but B6 and auto (C3B10F1) skin grafts, were accepted for over 100 days (Fig.1a). This is presumably because the B6 and B10 mouse strains were both derived from C57BL mice (this will be discussed later). In B6D2F1 or C3129F1 recipients, the survival time of minor antigen-mismatched 129X1/Sv (Fig.1b), BALB/c (Fig.1b), or CBA/N grafts (Fig.1c) were slightly prolonged but grafts were rejected by day 27 (Fig.1b,c). However, unexpectedly, only minor antigen-mismatched B6 grafts transplanted into C3129F1 recipients were rejected as early as fully allogeneic BALB/c grafts (Fig.1c). These results indicate that even in the MHC-matched setting, minor antigen mismatch between the donor and recipient is critical for determining the extent of immune rejection. B6 skin grafts on C3129F1 recipients were rejected earliest among minor antigen-mismatched grafts in this study. Therefore, we chose this pair of donors and recipients as the experimental model. == Figure 1. == Combination between donor and recipient is critical for determining the extent of immune rejection in MHC-matched but minor antigen-mismatched transplantation. Auto or allogeneic skin grafts (BALB/c (MHC haplotype: d/d), C57BL/6 (b/b), CBA/N (k/k), or 129X1/Sv (b/b)) were transplanted into MHC-heterozygous recipients. The recipients were (a) C3B10F1 (k/b), (b) B6D2F1 (b/d), and (c) C3129F1 (k/b). **p< 0.01, log-rank test. == Immune cells infiltrate into skin grafts in MHC-matched but minor antigen-mismatched transplantation == Next we examined.