The possible mechanism of reduced antigenicity of the tracheal bioartificial organ treated by cryopreservation

Purpose of study: It is reported that the trachea bioartificial organ which treated by cryopreservation can be used for tracheal replacement without immunosuppressants. However, the mechanism of reduced antigenicity is still unclear. We investigated this issue in rat tracheal grafts after cryopreservation employing light microscopic observation, scanning electron microscopic and transmission electron microscopic examination. Methods: Rat tracheal grafts were preserved in a cryopreservative solution at -80°C. After 1 week, 1 month, 3 months and 6 months (n=6 in each group), the epithelium of the grafts were examined by light microscopic and scanning electron microscopic examination. The expression of major histocompatibility complex-II (MHC-II) was determined using frozen sections. And the ultrastructure of dendritic cells (DCs) was observed by transmission electron microscopy. Fresh tracheal grafts (n=6) were evaluated in the same way as control. Results: The epithelium of the tracheal grafts was still remained even 6 months after cryopreservation, but the cells was reduced gradually. MHC-II antigen could be determined even at 6 months, but the expression was lower than that on the fresh grafts. The DCs in the grafts displayed vacuolation, degeneration of lysosomes and decreased enzyme activity. Conclusions: The main antigenic structure of the tracheal grafts– the epithelium and DCs was damaged during cryopreservation, thus explaining the reduction of antigenicity.