Introduction: Semi-allogeneic bone marrow and spleen cells (BMC/SC) induce transplant tolerance in neonatal mice whereas allogeneic BMC/SC cause lethal graft-versus-host disease (GVHD). To prevent aGVHD and induce transplant tolerance with allo-SC/BMC we reprogrammed donor/host CD4 T cells by co-stimulation blockade to preserve regulatory CD4 T cells while recapitulating depleted donor/host CD8 T cells.
Methods: C3H (H-2k) neonatal mice were injected iv with total or depleted B6 (H-2b) GFP+ SC/BMC. CD8 T and CD49b NK cells were depleted from donor inocula using StemCell Technologies kits. In vivo neonatal host CD8 T cells were depleted (Mab 53–6.7) and donor/host CD4 T cells reprogrammed (CD154 Mab MR1). Trafficking and interactions of injected cells were monitored by microscopy. Tolerance induction was assessed by transplanting treated mice as adults with donor-type hearts.
Results: Neonatal mice injected with GFP+ allo-SC/BMC developed aGVHD, with diarrhea, reduced growth and early death. GFP+ cells proliferated and spread throughout injected mice indicating systemic inflammation. Depletion of donor/host CD8 T cells together with co-stimulation blockade of donor/host CD4 T cells (CD154) resulted in GFP signal being reduced and restricted to lymphoid organs (day 6). High resolution microscopy showed donor T and B cells positioned respectively in PALS and follicular regions of host spleen; lack of Ki67 immunostaining indicated many donor cells were not proliferating. Donor DC were detected in host thymus (day 6) suggesting a possible role in central tolerance. Donor-type B6 hearts transplanted into neonatally-treated adult C3H mice (n=5) (CD8 depleted/CD4 co-stimulation blockade) all continued to beat at 100 days post-transplant, 3 grafts with maximal strength and 2 grafts with diminished strength. In untreated control mice (n=3) donor-type hearts stopped beating by day 10. H&E staining of a strongly beating B6 heart graft at 100 days showed undamaged cardiomyocytes with little if any cellular infiltrate and absence of macrophages and T cells by immunostaining.
Conclusion: Reprograming CD4 T cells while depleting/recapitulating CD8 T cells in allo-SC/BMC treated neonates leads to 100% cardiac allograft survival at 100days; low beat scores in 2/5 grafts suggests further optimization is required. These findings provide insight into robust tolerance induction in neonates.
This project was supported with funding from the Canadian Institutes for Health Research and the Gordon English family.