DONOR PRETREATMENT WITH FLT-3 LIGAND AUGMENTS ANTIDONOR CYTOTOXIC T-LYMPHOCYTE, NATURAL-KILLER, AND LYMPHOKINE-ACTIVATED KILLER-CELL ACTIVITIES WITHIN LIVER ALLOGRAFTS AND ALTERS THE PATTERN OF INTRAGRAFT APOPTOTIC ACTIVITY

Background, Liver allografts are accepted across major histocompatibil ity 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 str ikingly augments functional dendritic cells (DCs) within lymphoid and nonlymphoid tissue. Methods. The expression of costimulatory molecules and MIIC class II antigen on DCs isolated di sm livers of FH-treated B10 (H2(b)) mice (10 mu g/day; 10 days) was examined by flow cytometri c analysis, and their allo-stimulatory activity assessed in primacy mi xed leukocyte cultures. B10 livers from FL-treated donors were transpl anted orthotopically into naive C3H (H2(k)) recipients. Donor cells (M HC class II+) in recipient spleens were identified by immunwohistochem istry. Antidonor cytotoxic T lymphocyte activity, and both natural kil ler and lymphokine-activated killer cell activities of graft nonparenc hymal cells and host splenocytes were determined using isotope release assays. Apoptotic activity within liver grafts was determined by term inal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labelin g. Results. DCs isolated from livers of FL-treated donor mice exhibite d increased cell surface expression of CD40, CD80, CD86, and IA(b), an d augmented T cell allostimulatory activity compared with controls. Wi thin 24 hr of organ transplantation, the numbers of donor IA(b+) cells within recipient spleens was augmented substantially compared with no rmal liver recipients. Livers from FL-treated donors were rejected acu tely (median survival time, 5 days), whereas control B10 liver allogra fts survived >100 days. Nonparenchymal cells from rejecting grafts 4 d ays after transplantation exhibited increased antidonor cytotoxic T ly mphocyte, natural killer, and lymphokine-activated killer cell activit ies compared with cells from spontaneously accepted grafts. This augme nted cytotoxic reactivity was associated with histologic evidence of i njury to bile duct epithelium and vascular endothelium that was not re adily evident in controls. Conclusion. Thus, although normal livers pr ovide allostimulatory signals sufficient to elicit an antidonor immune response, regulatory mechanisms that may include apoptosis of graft-i nfiltrating T cells, and that are overcome by augmenting the number of functional donor DCs, may account for inherent liver tolerogenicity.