Transplantation of RPE cells may be a treatment

Transplantation of RPE cells may be a treatment for retinal diseases, such as age-related macular degeneration (AMD). Many experimental clinical applications of allogeneic RPE cells for the treatment of AMD have been attempted (Algvere, 1997; Algvere et al., 1999; Kaplan et al., 1999; Peyman et al., 1991). The clinical application of iPSC-derived RPE (iPS-RPE) cells for AMD treatment was started in our associated hospital in 2014. Before transplantation studies of iPSCs are undertaken, questions concerning the survival of RPE cells in situ and the presence of immune attacks after retinal surgery must be addressed. It is assumed that MHC molecules on RPE cells, including cells derived from iPSCs, might be the main tyrosol in allogeneic inflammatory reactions. In previous reports (Mochizuki et al., 2013; Sugita, 2009; Sugita and Streilein, 2003; Sun et al., 2003), immune cells such as T cells were stimulated or inhibited by exposure to RPE cells. The dual effects of RPE cells are regulated by MHC and co-stimulatory molecules on RPE cells. Retinal antigen-specific T cells are stimulated by exposure to RPE cells that express MHC class II (MHC-II) on their surface (Sun et al., 2003). RPE cells maintain immune privilege in the eye (Mochizuki et al., 2013; Sugita, 2009), but allogeneic RPE grafts are immunogenic after ocular transplantation.
Results
Discussion In the present study, we established RPE cells from iPSCs in MHC homozygote animals. We found no immune attacks in iPSC-derived RPE allografts of MHC-matched animal models without immunosuppression. By contrast, there were inflammatory immune rejections around the graft and retinal tissue damage together with inflammatory cell invasion in MHC-mismatched monkeys. In an immunohistochemical examination, the transplanted iPS-RPE cells of MHC-mismatched allografts did not survive in the subretinal space, and the retinal tissues exhibited hypertrophic changes, e.g., inflammatory nodules with many inflammatory cells, such as Iba1+ microglia/macrophages, inflammatory MHC-II+ APCs, and CD3+ T cells that invaded the region around the RPE grafts. On the other hand, Iba1+ cells, MHC-II+ cells, and CD3+ cells poorly invaded around the transplanted retina, and the allografts survived if we used MHC-matched allografts. In addition, T cells directly recognized allogeneic iPS-RPE cells in vitro as well as B cells (positive controls), whereas the T cells failed to recognize RPE cells from MHC homozygous donors when there was an MHC-matched combination between T cells and iPS-RPE cells. The process of T cell activation after transplantation could be both direct and indirect. MHC-II-expressing APCs directly activate alloreactive T cells after transplantation. Alloreactive T cells of the direct type have T cell receptors (TCRs) that can directly recognize allogeneic MHC-I (mainly CD8) or MHC-II (mainly CD4), and these T cells are important mediators of allograft immune rejection. Most solid-tissue grafts contain MHC-II+ APCs, i.e., dendritic cells and macrophages/microglia, which promptly migrate from the graft once it is in place. RPE cells do not contain CD11b+, CD11c+, CD14+, or CD45+ cells (Sugita and Streilein, 2003). In the case of solid-tissue grafts with MHC-II+ APCs, by trafficking to draining lymph nodes these donor-derived cells activate alloreactive T cells with TCRs that directly bind donor MHC antigens. In the indirect pathway, T cells recognize graft MHC-II allogeneic antigens that have been processed and presented by host APCs. Ocular resident dendritic cells, macrophages, retinal microglia, and also RPE cells have the capacity to present retinal antigens to naive and sensitized T cells such as professional APCs (Butler and McMenamin, 1996; Percopo et al., 1990). Our results showed that the rejection process after allogeneic RPE transplantation in the subretinal space could be achieved by the direct and indirect activation of T cells. As shown in this study, our iPS-RPE cells expressed MHC-I and MHC-II molecules (first signal). In addition, the cells expressed CD276 (B7-H3) molecules (second signal). Although there was no expression of CD40, CD80, CD86, or B7-H2 co-stimulatory molecules on iPS-RPE cells, the cells constitutively expressed MHC and co-stimulatory molecules B7-H3, and can activate T cells through the first and second signals. Although the function of B7-H3 molecules is still controversial, B7-H3 molecules that express APC can promote T cell-mediated immune responses and the development of acute and chronic immune rejection in allografts after transplantation (Chapoval et al., 2001). Our results indicated that the co-stimulatory signals from B7-H3 molecules on iPS-RPE cells could mediate allogeneic T cell activation. As the next step, we must consider other antigens (e.g., minor antigens and iPSC-specific antigens) and other immune cells (e.g., CD8+ CTL, NK cells, B cells, and complement factors) to develop successful transplantation techniques for retinal diseases. The presentation of aberrant peptides (minor antigens) by MHC molecules could lead to significant T cell stimulation. There is substantial literature on major and minor antigen mismatch in bone marrow transplants (Dickinson et al., 2002; Goulmy et al., 1996; Tseng et al., 1999; Voogt et al., 1988). There are no previous reports of the correlation between minor histocompatibility antigens and T lymphocytes after retinal transplantation. The minor histocompatibility antigens, e.g., HA-1, are immunogenic allogeneic antigens responsible for graft-versus-host disease (GVHD) in HLA-matched bone marrow transplantation (Tseng et al., 1999; Voogt et al., 1988). Laurin et al. (2010) previously showed that minor histocompatibility antigen-associated alloreactivity plays a critical role in the development of GVHD and graft-versus-leukemia (GVL) after HLA-identical hematopoietic stem cell transplantation. They showed by IFN-γ assay that the alloreactivity from donor versus recipient or donor versus mismatched minor antigens was associated with acute GVHD and GVL. We currently have no evidence for the correlation between minor histocompatibility antigen and T cells after RPE transplantation, but we might be able to monitor the minor histocompatibility antigen-related immune rejection if we use similar methods using peripheral T cells in transplanted patients.