The Tumorigenicity of Multipotent Adult Germline Stem Cells Transplanted into the Heart Is Affected by Natural Killer Cells and by Cyclosporine A Independent of Its Immunosuppressive Effects

Abstract

Transplantation of stem cells represents an upcoming therapy for many degenerative diseases. For clinical use, transplantation of pluripotent stem cell-derived cells should lead to integration of functional grafts without immune rejection or teratoma formation. Our previous studies showed that the risk of teratoma formation is highly influenced by the immune system of the recipients. In this study, we have observed a higher teratoma formation rate when undifferentiated so-called multipotent adult germline stem cells (maGSCs) were transplanted into the heart of T, B, and natural killer (NK) cell-deficient RAG2−/−γc−/− mice than in RAG2−/− mice, which still have NK cells. Notably, in both strains, the teratoma formation rate was significantly reduced by the immunosuppressive drug cyclosporine A (CsA). Thus, CsA had a profound effect on teratoma formation independent of its immunosuppressive effects. The transplantation into RAG2−/− mice led to an activation of NK cells, which reached the maximum 14 days after transplantation and was not affected by CsA. The in vivo-activated NK cells efficiently killed YAC-1 and also maGSC target cells. This NK cell activation was confirmed in C57BL/6 wild-type mice whether treated with CsA or not. Sham operations in wild-type mice indicated that the inflammatory response to open heart surgery rather than the transplantation of maGSCs activated the NK cell system. An activation of NK cells during the transplantation of stem cell-derived in vitro differentiated grafts might be clinically beneficial by reducing the risk of teratoma formation by residual pluripotent cells.

Immunological Properties of Murine Parthenogenetic Stem Cells and Their Differentiation Products

“…However, the patterns of more or less lysis by LAK or NK cells from the different donor strains was not explainable by a stronger or weaker engagement of inhibitory NK receptors by self-MHC molecules on PSCs. This result is in agreement with our finding that the PSC lines, similarly to murine ESCs, maGSCs, and iPSCs ( 32 , 36 , 38 , 39 ), largely lack MHC class I molecules at the plasma membrane as determined by flow cytometry. Instead, the PSC lines expressed ligands of activating NK receptors also similarly as the other pluripotent stem cell types ( 32 , 36 , 38 ).…”

“…This result is in agreement with our finding that the PSC lines, similarly to murine ESCs, maGSCs, and iPSCs ( 32 , 36 , 38 , 39 ), largely lack MHC class I molecules at the plasma membrane as determined by flow cytometry. Instead, the PSC lines expressed ligands of activating NK receptors also similarly as the other pluripotent stem cell types ( 32 , 36 , 38 ). The cytotoxic activity of NK cells toward target cells is controlled by the balance of signals from inhibitory and activating NK receptors ( 46 ).…”

“…This phenotype is similar to other pluripotent stem cell types, including ESCs, iPSCs, and maGSCs ( 37 ). Moreover, we found previously that the transplantation of maGSCs into the myocardium of mice by open heart surgery activated NK cells and significantly increased their ability to kill maGSCs in direct ex vivo assays ( 38 ). It has also been reported that human cardiac-derived stem/progenitor cells were resistant to resting allogeneic NK cells but moderately triggered killing by cytokine-activated NK cells ( 51 ).…”

“…In view of these phenotypes, NK cells could be an interesting player after transplantation of stem cell-derived grafts because they can specifically target undifferentiated cells, which are at risk to elicit tumors in the recipients ( 18 ). NK cells that can kill undifferentiated cells, which might contaminate grafts in trace amounts, could increase the safety of transplantations ( 38 , 58 ) largely independent of the pluripotent stem cell type that is used for the generation of grafts. However, individual differences between stem cell lines and variations in the efficacy of the NK cells as well as immunosuppressive treatments applied after transplantation might modulate the threshold of undifferentiated cells that could be tolerated in a graft ( 52 ).…”

“…So far, the analysis of the immunological properties of PSCs is lacking behind the other pluripotent stem cell types. We have shown previously that mouse ESCs, iPSCs, and maGSCs are targets for IL-2-activated allogeneic as well as syngeneic NK cells ( 32 , 36 , 37 ) and that NK cells can impair the growth of teratomas upon transplantation of the stem cells ( 20 , 36 , 38 ). Since in vitro differentiation increased the resistance of these pluripotent stem cell types ( 36 ), NK cells might increase the safety of transplantations because they can target undifferentiated cells while ignoring differentiated cells.…”