The field of regenerative medicine is quickly advancing. Therapeutic applications of hESC-derived oligodendrocyte progenitor cells (www.geron.com
) and hESC-derived retinal pigment epithelial cells (www.advancedcell.com
) have recently been initaited in patients with acute spinal cord injury and Stargardt’s macular dystrophy, respectively. More Phase I clinical trials are expected within the next 5–10 years (Lomax et al., 2007
). One issue critical to the realization of such goals is the elimination of the immunologic barrier that presently precludes the successful application of cell based regenerative therapy (Carpenter et al., 2009
; Chidgey et al., 2008
). The focus of this study was to characterize the immunogenic properties of ESCs, iPSCs, and their differentiated derivatives, and to evaluate the efficacy of blockade of leukocyte costimulatory molecules as a way to induce transplanted cell engraftment and survival.
Future clinical applications of pluripotent cells for regenerative therapy will likely involve allogeneic transplantation setting. However, at the present time a comprehensive study of hESC immunogenicity in humans is not yet feasible due to ethical reasons and safety constrains. As a next best option, we initially focused on the allogeneic transplantation scenario. We demonstrated that costimulatory blockade is an effective approach to induce engraftment of mESCs in a murine host. However, conclusions drawn from mESCs possibly may not reliably be extrapolated to hESCs. One major difference between the two cell population is that in the undifferentiated state mESCs express undetectably low levels of MHC-I (H2-Kb
) (Abdullah et al., 2007
; Bonde and Zavazava, 2006
), whereas hESCs demonstrate low but detectable levels of MHC-I expression. Similarly, differentiation of hESCs induces increased MHC-I expression. For these reasons, it was important to also demonstrate the immunosuppressive efficacy of costimulatory blockade to prevent the rejection of undifferentiated hESCs as well as spontaneously differentiated hESCs and in vitro differentiated hESC-ECs and miPSC-NSCs.
Both undifferentiated and spontaneously differentiated hESCs were rejected in the absence of immunosuppression and demonstrated stable engraftment at all time points assayed in the presence of costimulatory blockade treatment. In the absence of immunosuppression, hESC-ECs were rejected by day 7, whereas treatment with costimulatory blockade permitted hESC-EC survival similar to NOD/SCID mice. Overall, costimulatory blockade is more advantageous than more common forms of immunosuppression (e.g., tacrolimus, sirolimus) because it involves only a brief period of administration, produces minimal systemic toxicity, and induces superior long-term engraftment of murine and human pluripotent cells.
As an alternative approach to circumvent cellular rejection following transplantation, the use of hiPSCs has been suggested because they can be derived from the recipient and thus may not provoke an immune response (Byrne, 2008
). However, it may not be economically feasible to offer this type of treatment to the population at large, nor logistically feasible to safely develop autologous hiPSCs for transplantation in patients with acute injury such as spinal cord trauma, stroke, or myocardial infarction. In the future, it is possible that allogeneic hiPSC transplantation would be necessary in certain scenarios, which therefore would necessitate the development of immunotolerance strategies. At present, the immunogenic properties of hiPSCs remain largely unknown, as no data exist regarding the immune response towards hiPSCs. The only prior study to investigate the immune properties of iPSCs focused on miPSCs and their susceptibility to NK-cell mediated immune rejection (Dressel et al., 2010
). To our knowledge, this is the first
study investigating the immunogenic properties of hiPSCs. We demonstrate that xenogeneic hiPSCs are rejected under similar kinetics as hESCs and that immunosuppression with costimulatory blockade successfully mitigates this immune rejection. Similarly, allogeneic transplantation of undifferentiated miPSCs or differentiated miPSC-NSCs results in immune rejection by 21 days post transplantation, whereas engraftment in animals treated with costimulatory blockade was similar to NOD/SCID mice. This is important because if future clinical applications of iPSC-based therapies involve an allogeneic transplantation setting, costimulatory blockade may be a viable immunosuppressive approach to mitigate the allogeneic immune response.
In summary, this study demonstrates that a short course of costimulatory blockade treatment is sufficient to induce engraftment of allogeneic mESCs and miPSCs as well as xenogeneic hESCs, hiPSCs, and their differentiated derivates. Our data suggest that costimulatory blockade permits transplanted cell engraftment by decreasing the expression of pro-inflammatory cytokines (e.g., IL-2, Tnfrsf9), decreasing the polarization of naive T cells towards a type I phenotype, increasing the establishment of a pro-apoptotic phenotype, and inducing clonal anergy. Further demonstrations of successful management of transplant rejection as shown here will help realize the full potential of stem cell-based regenerative therapies in the future.