Several human leukocyte subsets including natural killer (NK) cells, cytotoxic T lymphocytes (CTL), and polymorphonuclear neutrophils (PMN) participate in cellular immune responses directed against vascularized pig-to-human xenografts. As these leukocytes express the death receptor Fas either constitutively (PMN) or upon activation (NK, CTL), we explored in vitro whether the transgenic expression of membrane-bound human Fas ligand (mFasL) on porcine fetal fibroblasts is a valuable strategy to protect porcine xenografts. cDNA of mFasL carrying the deletion at the cleavage site with metalloproteinase and lacking the death domain in its cytoplasmic tail was subcloned into pCAGGS expression vector driven by the chicken β-actin promoter containing blastidin- resistance cassette. The mFasL expression vector was transfected into mini-pig fetal fibroblasts by lipofection method. Blastidin-resistant cells were screened by PCR and FISH. The expression of mFasL was confirmed by Western blot and FACS with the mouse anti-human FasL antibody. Interaction of two transgenic clonal cell lines with human leukocytes was analyzed using functional assay for cytotoxicity. mFasL expressed on porcine fetal fibroblasts protected porcine fetal fibroblasts against killing mediated by human NK cells. The rate of NK cell mediated cytotoxicity was significantly reduced in transgenic clonal cells (54±10.80%) compared to normal minipig fetal fibroblasts. This result indicated that grafts of transgenic pigs expressing mFasL could control the cellular immune response to xenografts, and create a window of opportunity to facilitate xenograft survival.

Pigs may be considered as a suitable organ source for its characteristics in xenotransplantation if significant immunological barriers can be overcome. However, xenograft could be rejected by T cells, especially CD8+ cytotoxic T lymphocytes (CTL)-mediated response, because these elements show great cytotoxicity against xenograft by recognizing Swine Leukocyte Antigen (SLA)-I. Human cytomegalovirus (HCMV) encodes unique short (US) 11 gene, which interferes with cellular immune responses by inducing rapid degradation of newly synthesized heavy chains (HC) of MHC class I from endoplasmic reticulum (ER) to the cytosol. In this study we established two US11 clonal cell lines by transfection into minipig fetal fibroblasts and confirmed the integration of US11 gene by PCR and FISH. The reduction of Swine Leukocyte Antigen (SLA)-I which was expressed on the cell surface by US11 was also detected by flow cytometry assay. The level (14.6 % to 21.2%) of SLA-I expression in US11 clonal cell lines was decreased relative to the control. The reconstructed embryos were produced with these clonal cells and transferred to nine surrogate gilts. Ultrasound examination of recipient surrogates on days 35 after embryo transfer confirmed established pregnancies in two recipients. One recipient delivered one piglet with normal birth weight. PCR analysis revealed that transgene vector was integrated in the offspring genome. Transgene-expression analysis and CTL assay are currently underway. The present results show that transgenic pig was produced with US11 cDNA for controlling cell-mediated rejection. This result indicated that grafts of transgenic pigs expressing human cytomegalovirus protein US11 could control the cellular immune response to xenografts, and create a window of opportunity to facilitate xenograft survival. This research was supported by the BioGreen 21 Program (#20110301-061-541- 001-05-00), Rural Development Administration, Republic of Korea.