334 FACTORS AFFECTING HEMATOPOIETIC ENGRAFTMENT OF MONKEY EMBRYONIC STEM CELLS IN SHEEP FETUSES

Abstract

Previously, we generated monkey/sheep haematopoietic chimeras by in utero transplantation (IUT) of monkey embryonic stem (ES); however, the factors that control how the ES cells successfully engraft and differentiate into haematopoietic tissue in sheep fetuses remain uncertain. Here, we examined factors that might influence donor cells and recipient sheep and affect successful ES cell engraftment. We transplanted either undifferentiated monkey ES cells or ES-derived cells at an early haematopoietic differentiation stage into sheep fetuses. The latter cells were allowed to differentiate by culturing on OP9 cell layers for 6 days. Cells were transplanted into the liver or subcutaneous tissue of recipient sheep fetuses at 43 to 50 or 51 to 67 days of gestation (full term = 147 days) using ultrasound to identify the site for transplantation. After birth, monkey haematopoietic engraftment in the bone marrow was analysed in 40 lambs using colony-PCR with cells grown in methylcellulose in the presence of defined cytokines; teratoma formation was analysed by biopsy and immunohistochemistry. We found that haematopoietic engraftment was only observed when ES-derived cells at the early differentiation stage were transplanted into fetal livers at 51 to 67 days of gestation (6/9). However, teratoma formation with mature monkey tissue structures was only observed following transplantation of undifferentiated ES cells into fetal subcutaneous tissues at 43 to 50 days of gestation (4/6), but that was not observed when both types of cells were transplanted into the liver (0/18) or at 51 to 67 days of gestation (0/24). These results demonstrate that the differentiation status of the donor cells, the transplantation site, and the age of the fetus at transplantation are important factors in engraftment and differentiation into haematopoietic tissue or teratoma formation in sheep fetuses.