Trafficking of APC from liver allografts of Flt3L-treated donors: augmentation of potent allostimulatory cells in recipient lymphoid tissue is associated with a switch from tolerance to rejection.

Livers transplanted across major histocompatibility complex (MHC) barriers in mice are normally accepted without recipient immune suppression, and induce a state of functional tolerance. However, markedly increasing functional dendritic cells (DC) in the 'passenger leucocyte' population by donor pretreatment with the hematopoietic growth factor Flt3-ligand (Flt3L; 10 microg/day for 10 days) results in acute allograft rejection. In this study, molecular, immunohistochemical and flow cytometric analysis of donor cell traffick into recipient lymphoid tissue 24 h after liver transplantation (C57BL/10 [H2b]-->C3H [H2k]) was performed. In addition, the capacity of donor-derived cells in these tissues to stimulate host T cell proliferation was examined. Reverse transcriptase polymerase chain reaction analysis revealed increases in donor genomic DNA in both thymi and spleens of mice given livers from Flt3L-treated donors compared to controls. Donor MHC class II+ (IAb+) cells in spleens were strikingly elevated (10-fold) in the former group. Two-colour flow cytometry revealed a similar increase in donor-derived H-2Kb+/I-Ab+ cells, and in the incidence of donor leucocytes expressing CD40, CD80, and CD86. CD11c+ DC comprised approximately 40% of the I-Ab+ cells in spleens of mice given livers from Flt3L-treated donors. These changes were associated with the presence, in spleens, of potent allostimulatory activity for naive recipient strain T cells, that was not observed in normal liver recipients. Elicitation of allograft rejection, associated with enhanced trafficking of stimulatory donor antigen-presenting cells (APC), in particular DC, suggests that normal liver graft survival and tolerance induction may be linked to failure/counter-regulation of APC-driven stimulation of effective anti-donor T cell responses.