Neonatal inoculation of mice with semi-allogeneic lymphohematopoietic cells produces a state of highly specific allograft tolerance. Phenotypically, by both in vivo and in vitro criteria, antigen-reactive cells specific for the tolerated antigens appear to be clonally deleted from intact, tolerant mice. However, a series of adoptive transfer experiments using mice rendered tolerant of various H-2 alloantigens revealed that tolerance of Ia (class II) antigens is maintained by a different mechanism than tolerance of K/D (class I) antigens. Long-term acceptance of Ia-disparate grafts by recipients of Ia-tolerant lymphoid cells suggested that an active process (rather than passive clonal deletion) mediates and maintains this type of tolerance. No comparable success was achieved when tolerance of isolated class I or entire H-2 haplotype disparity was examined, suggesting that clonal deletion might be operative in these combinations. Modest prolongation of skin-graft survival was observed in adoptive transfer recipients of lymphoid cells from donors tolerant of I-JECSD disparity. These data are compatible with the hypothesis that the central I region (JE) promotes tolerance induction to associated strong IA- and D-region alloantigens by activating a suppression mechanism.
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