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| - Stable mixed donor/host chimerism has been reliably established in dogs given a sublethal dose of 2 Gy total body irradiation (TBI) before and immunosuppression with mycophenolate mofetil (MMF) or rapamycin combined with cyclosporine (CSP) after marrow transplantation from DLA-identical littermates (HCT). When TBI was reduced to 1 Gy, only transient engraftment was observed. Here we asked whether stable engraftment after 1 Gy TBI could be accomplished by reducing host vs. donor immune responsiveness through preceding CD154 blockade and infusion of donor peripheral blood mononuclear cells (PBMC). The anti-human CD154 antibody, 5c8, cross-reacted with canine lymphocytes and blocked allo-immune responses in vitro. Based on pharmacokinetic studies, six dogs received a single i.v. injection of 5 mg/kg anti-CD154 antibody (day -5) followed one day later by donor PBMC. On day 0, dogs were given 1 Gy TBI and DLA-identical marrow grafts. Postgrafting immunosuppression consisted of MMF and CSP. All six dogs showed initial engraftment, which was sustained in three of the six for >26 weeks, while three dogs rejected their grafts after 9, 22, and 24 weeks, respectively, and survived with autologous recovery. Graft survival was significantly improved over that among 11 historical controls conditioned with 1 Gy TBI and given either MMF or rapamycin with CSP after HCT, all of which rejected their grafts between 3 and 12 weeks (P = 0.03). Preceding donor PBMC infusion and CD154 blockade improved survival of DLA-identical marrow grafts after 1 Gy TBI.
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