The molecular mechanism of apoptosis induced by xenogeneic cytotoxicity

In order to clarify the role of natural killer (NK) cells in delayed xenograft rejection (DXR) of discordant xenotransplantation, we used in vitro xenogeneic combination of human NK cells and pig kidney target cells (PK15), and investigated the mechanism of xenogeneic cytotoxicity caused by human NK cells. In the presence of decomplemented human serum or human IgG, freshly isolated human peripheral blood lymphocytes (PBLs) caused both membrane (Cr-51 release) and DNA (H-3 release) damage on PK15. In contrast, only membrane damage was detected in the presence of normal human serum. To clarify the participation of perforin/granzymes-cell mediated cytotoxicity (P/G-CMC), when EGTA or concanamycin B (CMB) was added to the cytotoxicity assays, both cytotoxicities were completely inhibited by these drugs in a dose-dependent manner. In terms of the involvement of Fas/FasL-based cytotoxicity (F-CMC), while the cytotoxicity assays were performed in the presence of antagonistic antihuman FasL mAb, this antibody was not able to block the cytotoxicity. From these results, it is concluded that xenogeneic cytotoxicity is due to NK cell dependent ADCC (antibody-dependent cell-mediated cytotoxicity), and their effector mechanism can cause apoptosis on target cells via P/G-CMC.