Incomplete tolerance induced in Xenopus by larval tissue allografting: evidence from immunohistology and mixed leucocyte culture.

Application of adult skin allografts to Xenopus larvae has been a favoured protocol for probing the development of self-tolerance. A more physiologic approach is presented here that examines the immunologic outcome of grafting semi- or fully allogeneic larval skin or spleen to age-matched, larval Xenopus (X. laevis/X. gilli clonal hybrids). Following such grafting at 2 or 4 weeks-of-age, young froglets (4-5-months-old) are generally unable to reject second-set skin transplants, but destroy third-party skin vigorously, the MHC class II-rich spleen proving especially effective at inducing this tolerance. In contrast, following larval grafting of semiallogeneic tissues, mixed leucocyte culture performed at the end of metamorphosis (6 weeks) and again at 6 months reveals splenocyte reactivity toward donor-strain stimulators. Immunohistological findings extend this observation of anti-donor reactivity (suggesting incomplete tolerance) to the graft site. Thus despite excellent health when viewed externally, apparently tolerated second-set skin transplants display localised infiltration (especially into the epidermis) by CD8+ T cells and increased numbers of MHC class I and II-expressing cells by 3 weeks post-grafting. The immunologic implications of these findings are discussed.