Objective This study aimed at evaluating whether alloantigens in the form of donor cell chimerism or engrafted skin were required for the maintenance of a tolerance state after in utero marrow transplantation.
Methods Low-level chimeras (peripheral chimerism of <10%) were generated by in utero marrow transplantation in a C57BL/6 (H-2Kb) into FVB/N (H-2Kq) strain combination and examined for their tolerant state by skin transplantation. We collected chimeras with skin tolerance of >120 days, followed up their peripheral chimerism, and reappraised their tolerance state by secondary skin transplantation one month after the removal of engrafted primary donor skin. At killing of tolerant mice, chimerism in the marrow, spleen, thymus, lymph node and peritoneum was also examined.
Results Among 38 tolerant mice, 26 spontaneously lost peripheral chimerism by the age of 18 months. Eight with the loss of peripheral chimerism were subjected to secondary skin transplantation one month after the removal of engrafted syngeneic and donor-specific skins. Two died within 7 days after skin re-transplantation and the others kept acceptance of second-set syngeneic and donor-specific skins for >120 days. Twelve skin-tolerant mice that lost peripheral chimerism were killed to examine tissue chimerism. Three of them had no detectable donor cells in any tissue examined.
Conclusions Skin tolerance could last in the absence of donor cell chimerism and engrafted donor skin. Thus, there was immunological memory of donor-specific tolerance, independent of alloantigens from chimeric cells or engrafted donor skin.