FLT-3 Ligand Increases Microchimerism but Can Prevent the Therapeutic Effect of Donor Bone Marrow in Transiently Immunosuppressed Cardiac Allograft Recipients

132 Purpose: The aim of this study was to examine the influence of the recently cloned hematopoietic growth factor flt3-ligand (FL) on microchimerism and organ allograft survival in transiently immunosuppressed recipients given donor bone marrow (BM) cells at the time of transplant. FL administration has been shown to mobilise stem and progenitor cells and to strikingly increase numbers of functional dendritic cells in vivo. Methods: C3H (H2k) mice received 50 × 106 B10 (H2b) BM cells either alone or with FL (10µg/day), tacrolimus (2mg/kg/day) or both agents for 7 days. Donor MHC class II+ (IAb+) cells were quantitated in spleens by immunohistochemical analysis, and donor class II DNA detected in recipient BM by PCR. Heterotopic heart transplants to the ear pinnae of C3H recipients were performed using neonatal B10 donors. Graft survival was monitored by daily examination for contractile activity. Anti-donor reactivity was quantitated by 3-day mixed leukocyte reactions (MLR) and by cytotoxic T lymphocyte (CTL) assay using 51Cr- labelled target cells.Results: Donor cells were rare in the BM alone and BM + FL groups, whereas there was a substantial increase in chimerism in the BM + tacrolimus group. Addition of FL to BM + tacrolimus led to a further 8-fold increase in donor cells and enhanced donor DNA compared to the BM + tacrolimus group. This increase in donor cells was almost 500-fold compared to BM alone. C3H recipients of B10 heart allografts given perioperative B10 BM and tacrolimus(days 0-13) exhibited a markedly extended median graft survival time(MST; 42 days) compared to those given tacrolimus alone (MST: 22 days). Addition of FL (10 µg/day; 7 days) to BM + tacrolimus prevented the beneficial effect of donor BM (MST: 18 days). BM alone or BM + FL resulted in uniform early heart graft failure (MST < 8 days). Functional studies revealed maximal anti-donor MLR and CTL activities in the BM and BM + FL treated groups, with minimal activity in the tacrolimus-treated groups.Conclusion: Thus, dramatic growth factor-induced increases in chimerism achieved under cover of short-term immunosuppression may result in augmented anti-donor reactivity and reduced graft survival after immunosuppressive drug withdrawal. With FL, this may reflect striking augmentation of immunostimulatory dendritic cells.