Rational use of postgrafting immunossuppression for the induction of mixed hematopoietic chimerism (MC) and preservation of a graft-versus-leukemia (GVL) effect

Patients undergoing non-myeloablative allogeneic stem cell transplantation (NST) often convert quickly to full donor hematopoietic chimerism, develop GVHD and are unable to receive donor lymphocyte infusions (DLI). We hypothesized that MC could be achieved and GVL effect of DLI could be preserved through the rational integration of post-transplantation immunosuppression based on their biochemical and molecular mechanisms of action. We were especially interested in the effects of Cyclosporine A (CSA) and Rapamycin (RAPA), which either prevent or promote tolerance induction, respectively, as well as post-transplantation cyclophosphamide (Cy), which selectively depletes lymphocytes proliferating in response to recent antigen stimulation. To test our hypothesis we developed a stringent model of NST in which BALB/c mice (H-2d) received 500 cGy total body irradiation, followed by transplantation of bone marrow cells and 10 million splenocytes from C57BL/6 (H-2b) donors resulting in potent lymphohematopoietic-graft-versus-host reaction (LH-GVHR) and fast conversion to complete donor chimerism. Groups of five mice each were treated with RAPA (1.5 mg/kg), CSA (15 mg/kg), or vehicle, daily for 14 days, and then three times weekly for an additional two weeks. Administration of RAPA or CSA from the day of NST was unable to diminish a potent LH-GVHR, resulting in conversion to full donor chimerism at the time of completion of drug treatment. The administration of a single dose of Cy on day 3 alone or addition of CSA from day 4 also resulted in complete donor chimerism, however, the combination of Cy on day 3 and RAPA from day 4, 10, or 20 post-NST resulted in the preservation of MC (61.7± ; 4.14, 53.83± ; 9.05 and 54.62± ; 10), respectively. Subsequent experiments were designed to measure the outcome of CSA and RAPA administration on LH-GVHR and GVL effect of DLI administered to mixed allogeneic chimeras constructed using the previously described NST conditioning regimen. C57BL-10 mice (H-2b) were transplanted with marrow from MHC-incompatible B10.BR (H-2k) donors and, three weeks later, received 2 x 107 B10.BR splenocytes followed by vehicle, CSA or RAPA in the same schedule as above. The conversion to full donor chimerism in the vehicle treated group and groups receiving CSA from day 0 or 20 after DLI was indicative of a potent LH-GVHR. In contrast, early administration of RAPA from day 0 or day 10 after DLI prevented LH-GVHR and preserved MC (40.9± ; 4.2 and 60.9± ; 9.1) while drug administration from day 20 after DLI administration resulted in conversion to full donor chimerism, respectively. In a similar experimental design, groups of animals treated with RAPA or CSA from day 0 or 20 after DLI, received lethal challenge of 1 x 105 C1498 leukemia cells IV on day 10. RAPA administration from day 0 interfered with the GVL effect in comparison to vehicle treated control group (0/10 vs 7/10 ; p