Can fetal and newborn allografts survive in an immunocompetent host?

This study explores methods of prolonging allograftsurvival by varying the ontogeny of the donor tissue (fetal, newborn, and adult), and the recipient (newborn and adult) in a series of outbred Sprague-Dawley rats. Allografts of renal or adrenal tissue (1 mm²) were implanted under the renal capsule of the recipient animal. Six experimental groups were constructed with the adult as the recipient in the first three, and four- to six-day-old newborn rat pups in the last three groups. A total of 212 animals were grafted and the animals were killed between 7 and 83 days later, and we carried out morphometric and histologic analyses of all grafts. In Group I (adult donor → adult host), all 17 grafts implanted for ten days or longer were completely rejected. In Group II, newborn tissue was implanted into 23 adults. By nine days after implantation, 17 grafts were fully rejected and the average graft had decreased in size by 68%±78.7% (P<.05 compared with their initial size). In contrast, when fetal renal or adrenal grafts were implanted into 93 adults (Group III) we saw a 17.6±9.7 fold increase in graft size when recipients were killed at least 7 days after implantation (P<.05 compared with their initial size). When we used the newborn as a recipient, we found that all 20 adult grafts (Group IV) were rejected within 10 days. When newborn tissue was implanted into 15 newborns (Group V) all 15 animals rejected their grafts within ten days. When 44 fetal grafts were implanted into newborn recipients (Group VI) there was a modest increase in size of 7.1±7.5 fold when the grafts were assessed between 8 to 10 days after implantation (P<.05 compared with their initial size). However, 29.6% of these grafts were fully rejected. We conclude that selected fetal tissue can grow and mature in the adult recipient without immunosuppression. Adult or newborn tissue, however, grafted into similar adults were promptly rejected. Further, we found that the newborn is not a privileged host for either adult or newborn grafts. Although the newborn does allow for the growth of selected fetal tissue, it is not more immunotolerant than the adult recipient.