Abstract
BACKGROUND: Liver allografts are accepted across major histocompatibility complex (MHC) barriers in mice and induce donor-specific tolerance without requirement for immunosuppressive therapy. There is evidence that passenger leukocytes may play a key role in tolerance induction. Flt-3 ligand (FL) is a recently cloned hematopoietic cytokine that strikingly augments functional dendritic cells (DCs) within lymphoid and nonlymphoid tissue. METHODS: The expression of costimulatory molecules and MHC class II antigen on DCs isolated from livers of FL-treated B10 (H2b) mice (10 microg/day; 10 days) was examined by flow cytometric analysis, and their allostimulatory activity assessed in primary mixed leukocyte cultures. B10 livers from FL-treated donors were transplanted orthotopically into naive C3H (H2k) recipients. Donor cells (MHC class II+) in recipient spleens were identified by immunohistochemistry. Antidonor cytotoxic T lymphocyte activity, and both natural killer and lymphokine-activated killer cell activities of graft nonparenchymal cells and host splenocytes were determined using isotope release assays. Apoptotic activity within liver grafts was determined by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling. RESULTS: DCs isolated from livers of FL-treated donor mice exhibited increased cell surface expression of CD40, CD80, CD86, and IAb, and augmented T cell allostimulatory activity compared with controls. Within 24 hr of organ transplantation, the numbers of donor IAb+ cells within recipient spleens was augmented substantially compared with normal liver recipients. Livers from FL-treated donors were rejected acutely (median survival time, 5 days), whereas control B10 liver allografts survived >100 days. Nonparenchymal cells from rejecting grafts 4 days after transplantation exhibited increased antidonor cytotoxic T lymphocyte, natural killer, and lymphokine-activated killer cell activities compared with cells from spontaneously accepted grafts. This augmented cytotoxic reactivity was associated with histologic evidence of injury to bile duct epithelium and vascular endothelium that was not readily evident in controls. CONCLUSION: Thus, although normal livers provide allostimulatory signals sufficient to elicit an antidonor immune response, regulatory mechanisms that may include apoptosis of graft-infiltrating T cells, and that are overcome by augmenting the number of functional donor DCs, may account for inherent liver tolerogenicity.