Another important finding is that, in spite of the immune responses mounted by the host brain, a substantial number of TH+ cells survived in the allografts. This is consistent with previous clinical reports of human fetal cell transplantation. Postmortem analyses of the patients revealed robust survival of DA neurons in spite of the fact that numerous immune cells were present around the graft (Kordower et al., 1997). In two double-blind clinical trials, immunosuppressive drugs were never used (Freed et al., 2001) or were withdrawn after 6 months (Olanow et al., 2003). In these cases, the cells from multiple fetuses were used without HLA matching, but more than 50,000 TH+ cells had survived after several years. Our quantitative PCR (qPCR) study in vitro showed that the expression of MHC-I increased in response to IFN-γ, but the expression level was still 1/10 that of untreated monkey peripheral blood cells (Figure 2B). The in vivo studies revealed that the serum level of IFN-γ increased at 2 months, and CD45+ cells (including CD8+ cells) accumulated in the allografts 3.5–4 months after the transplant. On the other hand, the levels of INF-γ in the cerebrospinal fluid (CSF) and the levels of tumor necrosis factor α (TNF-α) in both the serum and CSF were below the limit of detection by ELISA (data not shown). An immunofluorescence study did not reveal any apparent expression of MHC-I by the grafted cells (Figure S4A). Therefore, it is possible that the immune response in the primate brain was not strong enough to reject all of the donor cells. These findings closely correlate with the results of previous murine experiments (Hudson et al., 1994; Shinoda et al., 1995). To apply our findings to a more clinically relevant setting, we investigated the expression of HLA-I during neural differentiation of human ESCs (hESCs) and iPSCs by qPCR (Figure S4D). The expression level was 1/100 compared with that of human peripheral blood cells in both hESCs and iPSCs, and it was similarly elevated in response to IFN-γ. It is difficult to precisely compare immunogenicity in monkeys with that in humans, but the low expression level of MHC-I by the donor cells may account for the mild rejection in both monkey and human neural transplantation.
double-blind clinical trials
two
withdrawn
6 months
untreated monkey peripheral blood cells
expression of MHC-I
1/10
serum level of IFN-γ increased
2 months
months
CD45+ cells
accumulated in the allografts
3.5–4 months
months
human peripheral blood cells in both hESCs and iPSCs
expression of HLA-I during neural differentiation of human ESCs (hESCs) and iPSCs
1/100