Microchimerism, donor dendritic cells, and alloimmune reactivity in recipients of Flt3 ligand-mobilized hemopoietic cells: Modulation by tacrolimus

Flt3 ligand (FL) is a potent hemopoietic growth factor that strikingly enha nces stem tells and dendritic cells (DC) in vivo. We examined the impact of infusing FL-mobilized bone marrow (BM) cells on microchimerism and anti-do nor reactivity in normal and tacrolimus-immunosuppressed, noncytoablated al logeneic recipients, BM from B10 (H2(b)) mice given FL (10 mu g/day; days 0 -8; FL-BM) contained a 7-fold higher incidence of potentially tolerogenic i mmature CD11c(+) DC (CD40(low), CD80(low), CD86(low), MHC IIlow) that induc ed alloantigen-specific T cell hyporesponsiveness in vitro. C3H (H2(k)) mic e received 50 x 10(6) normal or FL-BM cells (day 0) and tacrolimus (2 mg/kg /day; days 0-12), On day 15, enhanced numbers of donor (IA(b+)) cells were detected in the thymi and spleens of FL-BM recipients. Tacrolimus markedly enhanced microchimerism, which declined as a function of time. Ex vivo sple nocyte proliferative and CTL responses and Th1 cytokine (IFN-gamma) product ion in response to donor alloantigens were augmented by FL-BM infusion, but reduced by tacrolimus, Systemic infusion of purified FL-BM immature DC, eq uivalent in number to that in corresponding whole BM, confirmed their capac ity to sensitize, rather than tolerize, recipient T cells in vivo. In vitro , tacrolimus suppressed GM-CSF-stimulated growth of myeloid DC from normal BM much more effectively than from FL-BM without affecting MHC class D or c ostimulatory molecule expression. infusion of normal B10 BM cells at the ti me of transplant prolonged C3H heart allograft survival, whereas FL-BM cell s did not. A therapeutic effect of tacrolimus on graft survival was observe d in combination with normal, but not FL-BM cells, These Endings suggest th e need for alternative immunosuppressive strategies to calcineurin inhibiti on to enable the engraftment, survival, and immunomodulatory function of FL -enhanced, immature donor DC.