Immune responses to osteochondral allografts. Current knowledge and future directions.

Immune responses directed against osteochondral allograft-associated immunogens have been demonstrated in a variety of animals models and, to a more limited degree, in human recipients. The most extensively studied, and presumably the most potent source of sensitization, are cell-surface transplantations found on the heterogeneous cell types ubiquitous to skeletal tissues. By virtually all criteria used for evaluation, the immunogenicity resulting from histocompatibility antigens is reduced by deep-freezing the allograft prior to implantation, and responses are diminished even further when tissues are freeze-dried. There is preliminary evidence to suggest that matrix components, particularly proteoglycan subunits, are also capable of sensitizing osteochondral graft recipients. The magnitude and significance of these immunogens are matters of speculation. Collagen may also act as an osteochondral allograft immunogen. The significance of osteochondral allograft immunogenicity, as compared with biomechanical factors in the incorporation of a bone graft, has not yet been elucidated. The effect of immune responses on clinical allograft success, if it is as important as it appears, should profoundly influence future osteochondral allograft research in reconstructive joint surgery.