Introduction: Induction of HLA (MHC)-mismatched mixed chimerism (MC) is a promising approach for organ transplantation immune tolerance, but MC can develop split tolerance in which recipients accept donor hematopoietic cells but reject donor-type solid organ transplant. The mechanisms remain unclear and requires further study.
Methods: Induction of MHC-mismatched mixed chimerism (MC) in murine models was achieved using a published regimen consisting of conditioning with cyclophosphamide, pentostatin, and rabbit anti-murine thymocyte globulin and infusion of BM and CD4+ T-depleted spleen cells as well as transplanted with heart and/or skin graft from WT or PD-L1-/- BALB/c donors. The recipients were monitored for MC status as well as heart and skin graft survival, and further analyzed for mechanisms of tolerance.
Results: With clinically applicable murine models, we show that while MC with wild-type donor hematopoietic cells (WT MC) provides tolerance to donor-type heart and skin grafts, MC with PD-L1-/- hematopoietic cells (PD-L1-/- MC) showed split tolerance by rejecting the WT donor-type organ grafts while maintaining mixed chimerism. A split tolerance was also observed in WT MC transplanted with PD-L1-/- grafts. The PD-L1 deficiency in donor APCs were associated with expansion of host-type Tcon cells and reduction of host-type Helios-Nrp1+ pTreg cells. On the other hand, depletion of host-type Treg cells results in reduction of donor-type CD8+ DCs and down-regulating PD-L1 expression by the residual CD8+ DCs.
Conclusions: These results indicate that, in MHC-mismatched MC, PD-L1 expressed by donor-type tolerogenic DCs augments pTreg differentiation and expansion, thereby preventing split tolerance.