Indirectly Activated Treg Allow Dominant Tolerance to Murine Skin-grafts Across an MHC Class I Mismatch After a Single Donor Specific Transfusion.

BACKGROUND: Tolerance induced in stringent animal transplant models using donor specific transfusions (DST) has previously required additional immunological manipulation. Here we demonstrate a dominant skin-allograft tolerance model induced by a single DST across an MHC Class I mismatch in an unmanipulated B6 host. METHODS: C57BL/6(H-2) (B6) mice were injected intravenously with splenocytes from B6.C.H-2 (H-2k) (bm1) or F1 (B6xbm1) mice prior to skin transplantation. Mice were transplanted 7 days postinjection with donor (bm1 or F1) and third party B10.BR (H-2) skin-grafts. RESULTS: B6 hosts acutely rejected skin-grafts from B6.C.H-2 (bm1) and F1 (B6xbm1) mice. A single transfusion of F1 splenocytes into B6 mice without any additional immune modulation led to permanent acceptance of F1 skin-grafts. This graft acceptance was associated with persistence of donor cells long-term in vivo. The more rapid removal of DST bm1 cells than F1 cells was reduced by NK cell depletion. Tolerant grafts survived an in vivo challenge with naive splenocytes. Both CD4CD25 and CD4CD25 T cells from F1 DST treated B6 mice suppressed allo-proliferation in vitro. Tolerance was associated with expansion of peripheral Foxp3CD4CD25 Treg and increased Foxp3 expression in tolerant grafts. In tolerant mice Foxp3 Treg arise from the proliferation of indirectly activated natural Foxp3 Treg (nTreg) and depletion of Foxp3 Treg abrogates skin-graft tolerance. CONCLUSIONS: This study demonstrate that the persistence of transfused semiallogeneic donor cells mismatched at MHC Class I can enhance tolerance to subsequent skin allografts through indirectly expanded nTreg leading to dominant tolerance without additional immunological manipulation.