Adult stem cells--perspectives in treatment of metabolic diseases

There is currently great excitement and expectation concerning the differential potential of adult stem cells or adult cells with capacity of differentiation. As the body of work concerning transdifferentiation of somatic stem cells and bone marrow derived stem cells grows, the number of critics increases steadily questioning the reliability of reported findings. So scientists are now challenged more and more to prove that resulting differentiated somatic cells originated from somatic adult stem cell through a transdifferentiation process. Phenomenons such as fusion of cells have to be ruled out and the origin of the differentiated cell has to be determined by specific techniques i.e. in situ hybridisation. Cellular mimicry through uptake of specific factors out of the medium is questioned to be the reason for cells staining positive for Insulin. Some multipotent adult stem cells can cross lineage boundaries and differentiate into somatic cells of other lineages after being relocated. Bone marrow cells have been described to have the greatest plasticity among adult stem cells regenerating damaged liver or myocardium. It has been proposed that the differentiation of bone marrow derived adult stem cells occurs naturally even in healthy organs as a physiologic process of tissue-regeneration. Others believe that organ damage is essential to induce transdifferentiation by release of organ specific microenvironmental factors. We here try to constitute necessary data which should be demonstrated to give substantial evidence for transdifferentiation of newly characterized cells including exclusion of fusion, phagocytosis or DNA uptake, description of the outset cell, differentiation into all three germ layers and functional parameters.

Adhesion and Rac1-dependent Regulation of Biglycan Gene Expression by Transforming Growth Factor-β

Both transforming growth factor-beta (TGF-beta)-induced expression of biglycan (BGN) and activation of p38 MAPK have been implicated in cellular adhesion and migration. Here, we analyzed the role of adhesive events and the small GTPase Rac1 in TGF-beta regulation of BGN. TGF-beta1 induction of BGN expression and activation of p38 was abolished or strongly reduced when cells were kept in suspension or exposed to either the actin cytoskeleton-disrupting agent cytochalasin D or a specific chemical Rac1 inhibitor. Ectopic expression of a dominant negative mutant (T17N) of Rac1 abrogated both TGF-beta-induced p38 MAPK activation and BGN up-regulation but did not affect TGF-beta-induced phosphorylation of Smad3 or transcriptional induction of Growth Arrest DNA Damage 45beta, previously shown to be crucial for TGF-beta regulation of BGN. Overexpression of wild type Rac1 greatly enhanced the TGF-beta effect on BGN in adherent cells, whereas ectopic expression of constitutively active Rac1 (Q61L) activated p38 and in the presence of exogenous TGF-beta was able to rescue BGN expression in nonadherent cells. Endogenous Rac1 was activated by TGF-beta treatment in PANC-1 cells in an adhesion-dependent fashion. Like Rac1-T17N, the NADPH oxidase inhibitor diphenylene iodonium and the tyrosine kinase inhibitor herbimycin A blocked TGF-beta-induced p38 activation and BGN expression, suggesting that Rac1 exerts its effect on BGN and p38 through increasing NADPH oxidase activity and subsequent production of reactive oxygen species. These results show that the TGF-beta effect on BGN is dependent on cell adhesion and that activated Rac1, presumably acting through NADPH oxidase(s), is necessary but not sufficient for TGF-beta-induced BGN expression.

Detection and clinical implications of minimal residual disease in gastro-intestinal cancer

INTRODUCTION

Metastatic dissemination is an important factor for the prognosis of patients with gastro-intestinal cancer. Exact staging is crucial to determine appropriate multimodal therapeutic strategies. At present, the sensitivity of routinely performed diagnostic techniques is suboptimal for the detection of minimal residual disease (MRD) and occult metastases since the number of disseminated tumour cells (DTCs) is mostly marginal. To amend the verification of DTCs, immunohistochemical and molecular methods were applied to retrieve epithelial cell-specific proteins in non-epithelial tissue of different body compartments or fluids. Many groups have eagerly focussed on the identification of new markers and novel tests, yet specificity and sensitivity of these methods as well as robustness in the clinical setting are frequently missing.

MATERIALS AND METHODS

This review critically evaluates the prognostic impact of MRD in patients with pancreatic, colorectal and gastric cancer by outlining those studies showing diagnostic results of DTC detection in lymph nodes, bone marrow, venous blood and peritoneal lavage, some of which present novel strategies.

CONCLUSION

The analysed data concerning MRD in gastro-intestinal cancers reveal that results are undesirably heterogeneous. From a critical point of view, many clinical studies missed their chance because of small cohort size; moreover, methodological standardisation is generally lacking. On the other hand, the very encouraging results achieved so far, together with the comprehensive analyses of a few research groups, foster the prediction that DTC/MRD issues will soon expand the standard TNM classification.

2005 Dr. Gary J. Becker Young Investigator Award: Periprocedural Oral Administration of the Leflunomide Analogue FK778 Inhibits Neointima Formation in a Double-injury Rat Model of Restenosis

PURPOSE

To test the efficacy of limited oral administration of the new leflunomide analogue FK778 for suppression of neointima proliferation in a double-injury restenosis model in the rat.

MATERIALS AND METHODS

For induction of aortic lesions, silicon cuffs were placed operatively around the infrarenal aortas of Lewis rats. After 21 days, the aortic cuffs were removed and the lesions were dilated with 2-F Fogarty catheters inserted via the left common carotid artery. The novel immunosuppressant FK778 was administered at a dose of 5 mg/kg body weight (group 1) or 15 mg/kg body weight (group 2) in a total of 38 animals. For both doses, three different periinterventional time periods, each with a 5-day course of oral FK778, were defined as follows: (i) days -2 to 2, (ii) days 1-5, and (iii) days 7-11, with six or seven rats in each group. After 3 weeks, intima/media ratios were assessed morphometrically and immunohistochemistry for quantification of intimal alpha-actin expression was performed.

RESULTS

In both dose groups, there was a trend toward inhibition of neointima formation when the 5-day course of FK778 was started before or 1 day after the intervention. However, in the lower-dose group, inhibition of neointima was not statistically significant regardless of the time frame of treatment (groups 1a-c). With the higher dose, suppression of intimal hyperplasia was significant when FK778 was administered between days 1 and 5 after angioplasty (group 2b; P<.01). Expression of alpha-actin in the intima of FK778-treated rats was significantly reduced when the drug was started 2 days before angioplasty in group 1a (P<.05) or 1 day after angioplasty in both dosage groups (group 1b, P<.01; group 2b, P<.05).

CONCLUSION

In the double-injury rat model presented, balloon-mediated proliferation of smooth muscle cells in the intima with consecutive intimal thickening was influenced by FK778 in a dose-dependent manner. However, long-term studies are needed to exclude a delay of vascular healing in this particular model.

Differentiation of in vitro-modified human peripheral blood monocytes into hepatocyte-like and pancreatic islet-like cells

BACKGROUND & AIMS

Adult stem cells provide a promising alternative for the treatment of diabetes mellitus and end-stage liver diseases. We evaluated the differentiation potential of human peripheral blood monocytes into hepatocyte-like and pancreatic islet-like cells.

METHODS

Monocytes were treated with macrophage colony-stimulating factor and interleukin 3 for 6 days, followed by incubation with hepatocyte and pancreatic islet-specific differentiation media. Cells were characterized by flow cytometry, gene-expression analysis, metabolic assays, and transplantation for their state of differentiation and tissue-specific functions.

RESULTS

In response to macrophage colony-stimulating factor and interleukin 3, monocytes resumed cell division in a CD115-dependent fashion, which was associated with a down-regulation of the PRDM1 and ICSBP genes. These programmable cells of monocytic origin were capable of differentiating into neohepatocytes, which closely resemble primary human hepatocytes with respect to morphology, expression of hepatocyte markers, and specific metabolic functions. After transplantation into the liver of severe combined immunodeficiency disease/nonobese diabetic mice, neohepatocytes integrated well into the liver tissue and showed a morphology and albumin expression similar to that of primary human hepatocytes transplanted under identical conditions. Programmable cells of monocytic origin-derived pancreatic neoislets expressed beta cell-specific transcription factors, secreted insulin and C peptide in a glucose-dependent manner, and normalized blood glucose levels when xenotransplanted into immunocompetent, streptozotocin-treated diabetic mice. Programmable cells of monocytic origin retained monocytic characteristics, notably CD14 expression, a monocyte-specific methylation pattern of the CD115 gene, and expression of the transcription factor PU.1.

CONCLUSIONS

The ability to reprogram, expand, and differentiate peripheral blood monocytes in large quantities opens the real possibility of the clinical application of programmable cells of monocytic origin in tissue repair and organ regeneration.

IL-2 Antagonists: The European Perspective

Acute rejection remains the main risk factor following intestinal transplantation. New immunosuppressive agents have substantially reduced the incidence of severe acute rejection. The question arises, which is the most powerful immunosuppressive combination with the lowest incidence of side effects? According to International Intestinal Transplant Registry data, anti-IL-2 antibodies are slightly advantageous compared with antilymphocyte preparations with respect to long-term patient survival. However, different antilymphocyte preparations are used in different doses and at different time points. The anti-IL-2 antibodies daclizumab and basiliximab were also used in different protocols. Therefore, final results on efficacy are awaited. The most important difference between IL-2 antibodies and antilymphocyte preparations is the suppression of CD4+ CD25+ T lymphocytes by anti-IL-2 antibodies. Antilymphocyte preparations do not affect CD4+ CD25+ T cells. Because regulatory CD4+ CD25+ T cells are essential for tolerance induction, protocols attempting tolerance may omit anti-IL-2 antibodies in the future.

Expression of E-selectin and its transcripts during intestinal ischemia-reperfusion injury in pigs

BACKGROUND

Ischemia-reperfusion injury (IRI) can result in severe organ dys- or nonfunction. Interaction of leukocytes and endothelial cells mediated by E-selectin appears to be a key step for disturbed microcirculation. Therefore we studied gene and protein expression as well as localization of E-selectin during intestinal IRI.

METHODS

Intestinal tissue samples were obtained from extracorporeal perfused intestines (cold ischemia time [CIT] 2 or 20 hours, each n = 5) and additionally in intestinal transplanted pigs (CIT 2 or 20 hours, each n = 1). Mucosal damage was graded according to the Chiu classification. E-selectin mRNA was determined by PCR and quantitative RT-PCR. Localization of E-selectin mRNA was performed by in situ hybridization and of the protein by immunohistochemistry.

RESULTS

Histologically, mucosal damage occurred during reperfusion and was earlier and more severe after 20 hours of CIT. E-selectin mRNA expression was detected by PCR already after laparotomy and was elevated after reperfusion. Interestingly, mRNA expression was already increased after 20 hours of CIT. E-selectin mRNA was localized to the luminal surface of muscular, submucosal, and mucosal endothelial cells and the protein was detected on submucosal arterial endothelium as early as 2 hours after reperfusion.

CONCLUSION

Prolongation of CIT results in more severe mucosal damage during reperfusion, which is associated with protein expression of E-selection that might be used as a marker for activated endothelial cells. Increased E-selectin mRNA at end of 20 hours of CIT might indicate a preactivated state of endothelial cells potentially triggered by bacterial translocation or products.

Kinetics and localization of interleukin-2, interleukin-6, heat shock protein 70, and interferon gamma during intestinal-rerfusion injury

BACKGROUND

Intestinal ischemia-reperfusion injury (IRI) represents an exaggerated inflammatory cascade with a complex pathophysiology. IL-2, IL-6, HSP70, and INF-gamma are mediators of the inflammatory process. Therefore, we investigated their kinetics and localization during intestinal IRI.

METHODS

Pig intestinal specimens were obtained during cold preservation (cold ischemia time 2 hours) and extracorporeal perfusion. Mucosal damage was graded according to the Chiu classification. MRNA expression was determined by Northern blot (IL-2, IL-6, IFN-gamma) or by quantitative RT-PCR (IL-6, HSP70) and localized by in situ hybridization.

RESULTS

Histologically, mucosal damage occurred during reperfusion. Expression of IL-2 mRNA was up-regulated after HTK perfusion and was highest at the start and 7 hours after reperfusion. Expression of IL-6 mRNA increased at 2 hours after reperfusion and HSP70 at 3 hours after reperfusion. IFN-gamma mRNA was expressed after HTK perfusion, with expression of this cytokine increasing to 1 hour after the start of reperfusion, and decreasing thereafter. IL-2 mRNA was localized to endothelial cells (EC) and leukocytes and in close relation to ganglion cells (GC): IL-6 mRNA in EC, smooth muscle cells (SMC), and GC: HSP70 mRNA in EC and SMC; and IFN-gamma mRNA in leukocytes.

CONCLUSION

IL-2, IL-6, HSP70, and INF-gamma are parameters of early mRNA expression during intestinal IRI. EC, SMC, leukocytes, and GC have been identified as sources of transcripts that might afford potential targets for intervention strategies to attenuate IRI.

Transforming growth factor-beta (TGF-beta) type I receptor/ALK5-dependent activation of the GADD45beta gene mediates the induction of biglycan expression by TGF-beta

We have recently shown that induction of biglycan (BGN) expression by transforming growth factor-beta1 (TGF-beta1) required sequential activation of both Smad and p38 mitogen-activated protein kinase signaling (Ungefroren, H., Lenschow, W., Chen, W.-B., and Kalthoff, H. (2003) J. Biol. Chem. 278, 11041-11049). Here, we have analyzed the receptors through which TGF-beta1 controls expression of BGN and GADD45beta, the latter of which is postulated to link early Smad signaling to delayed activation of p38. Ectopic expression of a dominant-negative mutant of the TGF-beta type II receptor in PANC-1 cells abrogated TGF-beta-induced BGN up-regulation. Similarly, inhibition of the TGF-beta type I receptor/ALK5 with either SB431542 or by enforced stable expression of a kinase-dead mutant greatly attenuated the TGF-beta effect on both BGN and GADD45beta expression in PANC-1 and MG-63 cells. The enhancing effect of ALK5 on TGF-beta-mediated GADD45beta and BGN expression and on GADD45beta promoter activity was also dependent on its ability to activate Smad signaling, because an ALK5 mutant defective in Smad activation (TbetaRImL45) but with an otherwise functional kinase domain failed to mediate these responses. The TGF-beta/ALK5 effect on p38 activation and BGN expression was mimicked by overexpression of GADD45beta alone (in the absence of TGF-beta stimulation) and suppressed upon antisense inhibition of GADD45beta expression. These results show that TGF-beta induces BGN expression through (the Smad-activating function of) ALK5 and GADD45beta and suggest that the sensitivity of MyD118 to activation by TGF-beta, which varies between tissues, ultimately determines the strength of the TGF-beta effect on BGN.

Stem cell–mediated tolerance inducing strategies in organ transplantation

The scope of possible tools to modulate the recipients immune response towards tolerance induction basically includes deletional and non deletional mechanisms, which are currently targeted by various strategies including monoclonal antibodies, cytokine deviation, chimerism induction and the support of regulating T-cells. Here we summarize the main findings in the field derived from experimental animal studies and currently performed clinical studies. This review focuses to give a clinically relevant overview over relevant tolerance inducing concepts, taking into consideration risk profiles and clinical efficacy associated with specific immunosuppressive regiments currently applied in the clinical setting of transplantation.

[Modification of the liver resection technique with radiofrequency coagulation]

INTRODUCTION

Since the introduction of liver resection, intraoperative blood loss is considered to be a major risk factor for perioperative morbidity and mortality. Radiofrequency energy-an established technique for in situ ablation-was used to facilitate liver resection by creating a nearly bloodless tissue plane.

METHODS

After thorough manual and sonographic exploration of the liver, the planned and marked resection plane was coagulated by radiofrequency and then transected. Seven patients with liver metastases of colorectal carcinoma were operated on employing this technique.

RESULTS

A nearly bloodless transection of the parenchyma could be achieved in all cases. Only two patients received blood transfusions: one due to a low preoperative hemoglobin and one due to extended additional abdominal surgery. Procedure-related complications included one case of a second-degree burn to the thigh and one case of postoperative bile leakage requiring hemihepatectomy.

CONCLUSIONS

Radiofrequency-assisted resection offers a valuable additional option in liver surgery, especially in patients with an increased risk of intraoperative bleeding (cirrhosis, coagulopathy).

Immunosuppression following intestinal transplantation

Acute rejection is still the main risk factor following intestinal transplantation. Potent immunosuppression decreases rejection frequency, but may increase immunosuppression-related complications. Isolated small intestinal transplantation was performed in 14 adult patients with short bowel syndromes. Immunosuppression included tacrolimus and rapamycin in combination with steroids for 6 months after ATG or daclizumab induction therapy. In addition to protocol biopsies, cellular immune status and soluble immune parameters were used to guide immunosuppression. CMV and EBV markers were determined on a routine basis. Ten of 14 patients (71%) survived for 1 to 38 months (median 26 months). Eight patients are at home, in good physical condition, completely on enteral nutrition. Among the 5 patients (36%) who developed acute rejection, 2 patients with early postoperative events underwent graft removal and 1 patient died due to multiple organ failure. Two patients developed severe acute rejection episodes at 10 and 24 months following transplantation. Both patients recovered following OKT3 rescue therapy and increased baseline immunosuppression with repeated methylprednisolone and infliximab treatment. Infections included peritonitis (n = 3), pneumonia (n = 3), central line infection (n = 5), urinary tract (n = 2), CMV (n = 2), and EBV (n = 4). Two patients developed anastomotic leaks at the esophageal and coloanal anastomosis. In conclusion, acute rejection episodes can be controlled by potent immunosuppression using tacrolimus in combination with rapamycin. Immunosuppression-associated complications, including infections, were in an acceptable range. However, even late after transplantation, reduction in immunosuppression may lead to severe rejection without major clinical symptoms.

Isolated adult intestinal transplantation using portal venous drainage

There is some evidence that portal venous drainage may offer immunologic and metabolic advantages in small bowel transplantation. Isolated small bowel transplantation was performed in 14 adult patients. In all cases, the donor pancreas was transplanted into another patient. During the donor procedure, the superior mesenteric artery and vein were separated below the division of the inferior pancreaticoduodenal artery and below the veins of the pancreatic head. An arterial interposition graft was used in all cases. One donor mesenteric artery was reconstructed in 6 patients; two arteries in 5 patients; and three arteries in 3 patients. Proximal arteries of the graft were ligated and the upper part of the jejunum resected. In 10 patients, a direct anastomosis was performed in an end-to side fashion between donor superior mesenteric vein (SMV) and recipient inferior mesenteric vein (IMV). In 2 patients, a branch of the superior mesenteric vein was used and 2 patients required a venous interposition graft to confluence using the donor iliac vein. Patency of the venous anastomosis was documented by magnetic resonance imaging (MRI) angiography after 6 months. No vascular complications have been observed to date. Portal venous drainage is technically feasible in most cases. An anastomosis to the recipient IMV offers the advantage of being direct despite the short donor vein segment. Furthermore, donor and recipient vessels are well matched for size. Using microsurgical techniques, vascular complications may be avoided.

Long-term culture and differentiation of rat embryonic stem cell-like cells into neuronal, glial, endothelial, and hepatic lineages

The in vitro differentiation of mouse embryonic stem cells into different somatic cell types such as neurons, endothelial cells, or myocytes is a well-established procedure. Long-term culture of rat embryonic stem cells is known to be hazardous, and attempts to differentiate these cells in vitro so far have been unsuccessful. We herein describe stable long-term culture of an alkaline phosphatase-positive rat embryonic stem cell-like cell line (RESC) and its differentiation into neuronal, endothelial, and hepatic lineages. RESCs were characterized by typical growth in single cells as well as in embryoid bodies when cultured in the presence of leukemia inhibitory factor. RESC expressed stage-specific-embryonic antigen-1 and the major histocompatibility complex class I molecule. For neuronal differentiation, cells were incubated with medium containing 10(-6) M retinoic acid for 14 days. For endothelial differentiation, RESCs were grown on Matrigel for 14 days, and for induction of hepatocyte-specific antigen expression, RESCs were grown in medium supplemented with fibroblast growth factor-4. Differentiated cells exhibited typical morphological changes and expressed neuronal (nestin, mitogen-activated protein-2, synaptophysin), glial (S100, glial fibrillary acid protein), endothelial (panendothelial antibody, CD31) and hepatocyte-specific (alpha-fetoprotein [alphaFP], albumin, alpha-1-antitrypsin, CK18) antigens. In addition, expression of hepatocyte-specific genes (alphaFP, transthyretin, carbamoyl-phosphate synthetase, and coagulation factor-2) was detected by reverse transcription polymerase chain reaction. We were able to culture RESCs under stable, long-term conditions and to initiate programmed differentiation of RESCs to endothelial, neuronal, glial, and hepatic lineages in the rat species.

Tolerance-inducing strategies in transplantation surgery?current status and perspectives

BACKGROUND

Life-long immunosuppressive medication has to be administered to the majority of solid-organ recipients after transplantation of genetically mismatched organs in order to circumvent acute graft loss due to alloreactive rejection responses triggered by the host's immune system. However, life-long suppression of the immune system implicitly limits the host's ability to respond appropriately to infectious, fungal and carcinogenic threats. Simultaneously non-targeted inhibition of immunological defense mechanisms coincides with substantial morbidity and mortality for the host. Thus, for the past five decades research in the field of transplantation medicine has focused on innovative strategies to induce graft tolerance to donor alloantigens, a state in which the recipient's lymphocytes have learned to accept the foreign organ or tissue as "self" or "non-dangerous" without the need of chronic immunosuppression. Achieving that specific goal of donor-specific tolerance would not only minimize the risk of the recipient to suffer from serious side effects resulting from continuous immunosuppressive therapy, but would also prevent loss of long-term graft function caused by chronic rejection processes. Recently, numerous insights into the dynamic interrelationships of host immune responses elicited by donor antigen-presentation, either on the graft itself or on specialized antigen-presenting cells, have substantially broadened our understanding of the cascade of events that result in the acquisition of tolerance.

METHOD

We highlight areas of research that are currently particularly helpful not only to set up new strategies to induce donor-specific tolerance or long-term graft acceptance, but also to identify and describe parameters which serve to characterize those patients who have acquired a state of tolerance and are safe to be weaned off from their immunosuppressive regimen.

[Stem cells and liver replacement]

Reproducible stem cell programmability, either embryonic or adult in nature, will open a variety of clinical application strategies for the treatment of tissue injuries, degenerated organs and body components, respectively, in the future. In addition, stem cells will be used to complement metabolically or genetically caused diseases. Pluripotent plasticity of stem cells describes their ability to be programmed into target cells encountered in all three germ layers during embryologic development. Here, we review future application modalities for the clinical use of cellular-derived products and therapeutics for patients presenting with liver diseases. This overview is designed as a state-of-the-art survey taking into account current knowledge and published work. It includes a diversified description of biological advantages and disadvantages of different types of stem cells, considers related ethical aspects and outlines clinically required parameters for the use of stem cells in humans. Successful application of stem cell-associated therapeutics in man will be closely related with ensuing patient risk profiles, stringent indications, long-term patient outcomes and will be determined by cost-benefit efficacy.

Future strategies for tolerance induction: a comparative study between hematopoietic stem cells and macrophages

So far, clinical benefit of hematopoietic stem cell induced donor-specific tolerance across major histocompatibility complex (MHC) barriers was hampered by either graft rejection or graft-versus-host disease. An alternative approach focuses on the use of donor-derived cells that bear an inherent mechanism to circumvent allospecific rejection upon injection into non-immunosuppressed hosts. Using a myeloablative conditioning model in the rat, full donor chimeric recipients were generated and their potential to induce long-term cardiac allograft survival was compared with the fate of grafts transferred to non-immunosuppressed host rats pretreated with donor-macrophages derived from the peritoneal cavity in the LEW to DA inbred strain combination. The induction of full multilineage long-term donor-chimerism (> 150 days) after initial host conditioning using two doses of cyclophosphamide and one shot of busulphan prevented acute graft rejection, whereas non-chimeric animals experienced acute and complete rejection. Conversely, vigorous T-cell depletion is required to protect conditioned animals from lethal graft-versus-host disease. Instead, the use of donor intraperitoneal macrophages achieved a state of transient chimerism and subsequent long-term graft survival in fully immunocompetent rats without the need of lethal conditioning regimens. In conclusion, the complex immunologic interactions, observed after allogeneic bone marrow transplantation as a means to induce donor chimerism, and subsequent long-term graft acceptance can be avoided if appropriate cell populations can be identified that, by themselves, induce a transient state of donor chimerism prevailing long enough to deviate allospecific immune responses, as outlined in this study.

Spontaneous tolerance: experience with the rat liver transplant model

Soluble major histocompatibility complex (MHC) class I antigens released from hepatocytes and the passenger leukocyte population of the liver allograft have both been considered as important contributors for spontaneous liver tolerance upon allogeneic transplantation into fully MHC-mismatched hosts. This study was conducted to delineate the role of "passenger leukocytes" (PL) as well as local intra-graft defence mechanisms of long-term accepted liver allografts in more detail. Orthotopic liver transplantation was performed in male inbred rats as follows (n = 4-6): (i) Lewis (LEW; RT1.(l)) --> LEW; (ii) DA (RT1.(av1)) --> DA; (iii) DA --> LEW; (iv) LEW --> DA; (v) LEW (10-Gy whole body irradiation [WBI], d-7) --> DA; (vi) LEW (10-Gy WBI, d-7) --> LEW; (vii) LEW (10-Gy WBI, d-7) --> LEW (parked for 36 hours) --> DA; and (viii) LEW (10-Gy WBI, d-7) --> DA (parked for 36 hours) --> DA. The model specifically investigated the role of PLs as potential contributors to liver tolerance as WBI destroys this nonresident liver population. Characterization of Fas/FasL expression and the frequency of apoptotic cell death was performed by immunohistochemistry and TUNEL staining. Reverse transcriptase-polymerase chain reaction, by the use of Fas and FasL-specific cDNA probes, was performed on isolated hepatocytes of tolerized and rejected livers at various time points after transplantation. Animal survival in the various experimental groups was calculated using Kaplan-Meier survival statistics and related log-rank statistics as follows: (i) < 100 days; (ii) > or = 100 days; (iii) > 100 days; (iv) 11.3 +/- 1.0 day; (v) 9.0 +/- 0.5 days; (vi) > 100 days; (vii) 5/6 > 100 days; and (viii) 8.0 +/- 1.5 days. Immunohistochemistry revealed high numbers of proliferating cells in tolerized liver allografts. Apoptotic cell death of hepatocytes could be detected in both rejecting and to a lower extent in tolerized animals. Conversely, only tolerized but not rejected liver allografts revealed upregulation of FasL-expression on hepatic parenchymal cells from day 3 onwards. Irradiated LEW livers, in turn, lose their ability to survive in allogeneic DA hosts (group v) whereas they survive in syngeneic hosts (group vi), indicating that irradiation itself does not destroy the liver parenchyma. Reconstitution of irradiated LEW livers with syngeneic (group vii) but not with allogeneic (group viii) PLs restored tolerance induction. The underlying mechanisms of immune-privilege observed with liver allografts appear to share characteristics of clonal exhaustion suggesting that alloreactive lymphocytes are depleted by AICD via the FasL/Fas signal transduction pathway. The high frequency of apoptotic lymphocytes found in the portal tract of tolerized (but not rejected) LEW grafts supposes that functional FasL expression on graft hepatocytes mediates specific elimination of graft-directed effector lymphocytes. This mechanism constitutes peripheral deletion as one of the possible tolerogenic mechanisms involved. Chimerical liver grafts consisting of donor (LEW) parenchyma and host (DA) passenger leukocytes lose their tolerogenic capacity. In contrast, syngeneic reconstitution with LEW-PLs, restores liver graft acceptance upon transplantation into allogeneic DA hosts. This phenomenon is not relying on the induction of micro- or macrochimeric hosts, as no LEW PLs were found in spleen, thymus or the blood compartment from long-term surviving DA rats. Thus, non-resident liver cells contribute significantly to liver graft acceptance. Subsequently, liver tolerance appears to be mainly induced in the graft itself.