Effects of cryopreservation on tracheal allograft antigenicity in dogs

Prolonged cryopreservation (~8-10 months) has been shown to reduce the antigenicity of tracheal allografts due to subsequent denuding of epithelium. In the present study, tracheal epithelium was assessed after variable periods of cryopreservation. Immunosuppressant-free allotransplantation was then undertaken to evaluate the impact of cryopreservation on tracheal antigenicity in dogs.

Tracheal rings [7-8] were removed from mongrel adult dogs for cryopreservation (1-10 months, -85 °C) and grafting. Before transplantation, one ring was sectioned from each end for histologic examination. The residual five-ring segments of mediastinal trachea were then transplanted into recipient dogs after 1-7 months (group 1, n=9) or 8-10 months (group 2, n=6) of cryopreservation. Anastomotic sites and allografts were covered by omental pedicles. No immunosuppressants whatsoever were administered.

In microscopic views, the ciliated tracheal epithelium of most grafts showed variable loss but was generally intact after cryopreservation, still demonstrating major histocompatibility complex (MHC)-II positivity. By postoperative bronchoscopy, allografts in both groups had largely developed lethal strictures. In group 1, eight dogs were sacrificed or died within 50 days post-transplantation, whereas survival times in group 2 were somewhat longer, with three dogs surviving for >60 days. Upon sacrifice, histologic preparations of grafted tissue in both groups were typically denuded of epithelium, with marked lymphocytic/monocytic submucosal infiltrates. Tracheal cartilage had been absorbed or destroyed.

After cryopreservation, some degree of tracheal epithelium loss maybe expected, but complete denudation is not obligatory. Retained epithelial antigenicity is thus capable of triggering rejection, resulting in transplant failures. Although prolonging transplant survival to an extent, fatal rejection of tracheal allografts was not preventable by prior cryopreservation.