Objective: Anti-CD154 (MR1) monoclonal antibody (mAb) and rapamycin
(RAPA) treatment were both shown to improve survival of concordant ratto-
mouse islet xenografts. The present study investigated rat-to-mouse islet
xenograft survival when MR1 and RAPA treatment were combined and
analyzed the role of CD4+CD25+Foxp3+ T regulatory cells (Treg) in the
induction and maintenance of the ensuing tolerance.
Research Design and Methods: RAPA MR1 treated mice received additional
IL-2, anti-IL2 mAb (n=14) or anti CD25 mAb either early (0-28 day) or
late (100-128 day) post-transplantation. Treg were characterised in the
blood, spleen, draining lymph nodes and within the graft of tolerant mice
and rejecting mice.
Results: Fourteen days of combination therapy with MR1 and RAPA
allowed indefinite islet graft survival. Additional administration of
IL-2, anti-IL-2 mAb or depleting anti-CD25 mAb at the time of
transplantation resulted in rejection, 100%, 100% and 89% respectively,
whereas administration at 100 ays post transplantation lead to lower
rejection rates (10%, 25% and 40% respectively). Tolerant mice showed
an increase of Treg within the graft and in draining lymph nodes early
post transplantation, whereas 100 days post transplantation no significant
increase of Treg was observed. Rejecting mice showed a transient
increase of Treg in the xenograft and secondary lymphoid organs
which disappeared within 7 days after rejection. Treg isolated from both
tolerant and rejecting recipients showed pronounced suppressive function
in vitro.
Conclusion: Treg might play a critical role in the induction of tolerance
early, but not in the maintenance of tolerance late after concordant islet
xenotransplantation.