Due to the poor capacity for articular cartilage to regenerate, its damage tends to result in progressively degenerating conditions such as osteoarthritis. To repair the damage, the transplantation of allogeneic human induced pluripotent stem cell (iPSC)-derived cartilage is being considered. However, although allogeneic cartilage transplantation is effective, immunological reactions can occur. One hypothetical solution is to delete the expression of major histocompatibility complex (MHC) class I molecules to reduce the immunological reactions. For this purpose, we deleted the β2 microglobulin (B2M) gene in a cynomolgus monkey (crab-eating monkey [Macaca fascicularis]) iPS cells (cyiPSCs) to obtain B2M-/- cyiPSCs using the CRISPR/Cas9 system. Western blot analysis confirmed B2M-/- cyiPSCs lacked B2M protein, which is necessary for MHC class I molecules to be transported to and expressed on the cell surface by forming multimers with B2M. Flow cytometry analysis revealed no B2M-/- cyiPSCs expressed MHC class I molecules on their surface. The transplantation of B2M-/- cyiPSCs in immunodeficient mice resulted in teratoma that contained cartilage, indicating that the lack of MHC class I molecules on the cell surface affects neither the pluripotency nor the chondrogenic differentiation capacity of cyiPSCs. By modifying the chondrogenic differentiation protocol for human iPSCs, we succeeded at differentiating B2M+/+ and B2M-/- cyiPSCs toward chondrocytes followed by cartilage formation in vitro, as indicated by histological analysis showing that B2M+/+ and B2M-/- cyiPSC-derived cartilage were positively stained with safranin O and expressed type II collagen. Flow cytometry analysis confirmed that MHC class I molecules were not expressed on the cell surface of B2M-/- chondrocytes isolated from B2M-/- cyiPSC-derived cartilage. An in vitro mixed lymphocyte reaction assay showed that neither B2M+/+ nor B2M-/- cyiPSC-derived cartilage cells stimulated the proliferation of allogeneic peripheral blood mononuclear cells. On the contrary, osteochondral defects in monkey knee joints that received allogeneic transplantations of cyiPSC-derived cartilage showed an accumulation of leukocytes with more natural killer cells around B2M-/- cyiPSC-derived cartilage than B2M+/+ cartilage, suggesting complex mechanisms in the immune reaction of allogeneic cartilage transplanted in osteochondral defects in vivo. Impact statement The transplantation of allogeneic induced pluripotent stem cell (iPSC)-derived cartilage is expected to treat articular cartilage damage, although the effects of major histocompatibility complex (MHC) in immunological reactions have not been well studied. We succeeded at creating B2M-/- cynomolgus monkey (cy)iPSCs and cyiPSC-derived cartilage that lack MHC class I molecules on the cell surface. B2M-/- cyiPSC-derived cartilage cells did not stimulate the proliferation of allogeneic peripheral blood mononuclear cells in vitro. On the contrary, the transplantation of B2M-/- cyiPSC-derived cartilage into osteochondral defects in monkey knee joints resulted in survival of transplants and accumulation of leukocytes, including natural killer cells, suggesting complex mechanisms for the immune reaction.
Keywords: MHC; allogeneic transplantation; cartilage; chondrocytes; iPS cells; monkey.