DIFFERENT HEALING RATES OF BONE AUTOGRAFTS, SYNGENEIC GRAFTS, AND ALLOGRAFTS IN AN EXPERIMENTAL RAT MODEL

Matching of donors and recipients for tissue antigens is vitally impor tant for successful transplantation of essentially all organs and tiss ues, the major exception being bone. The importance of tissue-typing f or the healing of bone allografts remains, however, a controversial is sue as development of both humoral and cell-mediated immunity against the grafted bone has been observed in some experimental systems. In th e present study, we compared the healing patterns of frozen antigen-mi smatched allografts, frozen antigen-matched allografts (syngeneic graf ts), and fresh cortical bone autografts in an experimental rat model. Histomorphometry of the graft-host interface revealed that new bone fo rmation started significantly earlier in autografts than in allografts or syngeneic grafts. By 2 weeks, the level of new bone formation in t he syngeneic grafts had reached that in autografts. Antigen-mismatched allografts, however, continued to exhibit a retarded formation of new bone throughout the union process. These histomorphometric observatio ns were confirmed by molecular biologic analyses for the mRNA levels o f type I collagen, which increased earlier and reached a higher level in autografts than in allografts. Use of syngeneic grafts resulted in a longer persistence of type I collagen mRNA expression in the healing tissue than in antigen-mismatched allografts. No apparent differences were seen between allografts and autografts in the expression of type III collagen. No cartilage-specific type II collagen mRNA was observe d, indicating that antigen-mismatching or preservation by freezing did not alter the basic mechanism of the interface healing process, altho ugh it did slow down the beginning of the process. The experiments sug gest that a major antigen mismatch between donor and recipient affects the temporal gene expression of extracellular bone matrix and delays new bone formation at the graft-host interface of cortical bone allogr afts.