Stable Mixed Hematopoietic Chimerism in Dogs Conditioned with Donor Antigen, Anti-CD154 Antibody and 100cGy TBI.

Stable mixed chimerism can be established in dogs given a sublethal dose of 200cGy total body irradiation (TBI) before and immunosupression with mycophenolate mofetil (MMF) and Cyclosporine (CSP). When the TBI dose is reduced to 100 cGy, only transient engraftment is observed. Here we asked whether stable engraftment after 100 cGy TBI could be accomplished by first reducing the intensity of host immune responsiveness with the help of an anti-CD154 antibody which blocks the CD40-CD154 pathway.

Accordingly, recipients were given a single i.v. injection of 5 mg/kg anti-CD154 antibody (day -5) followed one day later by combined i.v./sc. injections of 10⁷ peripheral white blood cells/kg from their intended DLA-identical littermate marrow donors. TBI, 100cGy (delivered at 7 cGy/min), was given on day 0 followed by infusion of a median of 4.22x10⁸ marrow cells/kg. Postgrafting immunosuppression consisted of MMF (10mg/kg, BID, days 0 to 28) and CSP (15 mg/kg BID, days -1 to 35). Six dogs have been transplanted. Two dogs are too early after transplant for evaluation, while four dogs have been followed for >9, 12, >18 and >26 weeks. All four dogs had initial engraftment. Three of four dogs have continued to show stable mixed donor/host hematopoietic chimerism among lymphocytes and granulocytes from the peripheral blood. One dog rejected the graft 12 weeks after transplantation. All six transplanted dogs are alive and well. Besides mild allergic reactions to the antibody in five of the six dogs (redness of the eyes, skin rashes and dry mucous membranes), no additional side effects attributed to the pretransplant treatment were observed.

Data were consistent with previous findings in dogs, in which the costimulatory signal between B7 and CD28 was blocked, and support the hypothesis, that stable marrow allografts would be established by combining non-myeloablative pretransplant host immunosuppression and posttransplant immunosupression of both host and donor cells using MMF and CSP.