Orthotopic liver allografts in the rat. The influence of strain combination on the fate of the graft

Effects of major histocompatibility complex matching on graft survival in allogeneic rat limb transplantation

Differences in the major histocompatibility complex (MHC) between recipients and donors present a problem because of immunologic responses in graft rejection. The purpose of this study is to clarify the efficacy of MHC matching against acute graft rejection of allogeneic limb transplants in rats. Right hindlimb transplantations were performed using various MHC-mismatched pairs of inbred rats. The rats were classified into 5 groups according to the differences in subregions of the RT1 (rat MHC) between the recipient and the donor: group 1, RT1-A,B,D barrier (the differences of RT1-A,B,D subregions); group 2, RT1-A barrier; group 3, RT1-B,D barrier; group 4, RT1-B barrier; and group 5, RT1-D barrier. The mean survival time significantly decreased in group 1 and increased in group 4. The results suggest that MHC matching clearly improves survival of transplanted limbs. Specifically, both RT1-A and D matching is the most effective compatibility in prolonging survival time of allogeneic limb transplants in rats.

Posttransplant administration of donor leukocytes induces long-term acceptance of kidney or liver transplants by an activation-associated immune mechanism

Donor leukocytes play a dual role in rejection and acceptance of transplanted organs. They provide the major stimulus for rejection, and their removal from the transplanted organ prolongs its survival. Paradoxically, administration of donor leukocytes also prolongs allograft survival provided that they are administered 1 wk or more before transplantation. Here we show that administration of donor leukocytes immediately after transplantation induced long-term acceptance of completely MHC-mismatched rat kidney or liver transplants. The majority of long-term recipients of kidney transplants were tolerant of donor-strain skin grafts. Acceptance was associated with early activation of recipient T cells in the spleen, demonstrated by a rapid increase in IL-2 and IFN-gamma at that site followed by an early diffuse infiltrate of activated T cells and apoptosis within the tolerant grafts. In contrast, IL-2 and IFN-gamma mRNA were not increased in the spleens of rejecting animals, and the diffuse infiltrate of activated T cells appeared later but resulted in rapid graft destruction. These results define a mechanism of allograft acceptance induced by donor leukocytes that is associated with activation-induced cell death of recipient T cells. They demonstrate for the first time that posttransplant administration of donor leukocytes leads to organ allograft tolerance across a complete MHC class I plus class II barrier, a finding with direct clinical application.

Cyclophosphamide-induced tolerance in rat orthotopic liver transplantation

BACKGROUND

We previously established a cyclophosphamide (CP)-induced tolerance system in rodent skin graft models. In this study, we applied this system to rat liver transplantation.

METHODS

Lewis recipients were inoculated on day -2 with spleen and bone marrow cells (SC+BMC) from Dark Agouti (DA) donors, followed by 100 mg/kg CP on day 0. On day 25, DA livers were orthotopically grafted. We assessed the alloresponses to the donors of the long-term surviving recipients, using the second skin grafting and in vitro assay.

RESULTS

The recipients that had been treated with SC+BMC and CP survived for more than 165 days. None of control group that received SC+BMC alone (mean survival times [MST]=13.8 days), CP alone (MST=40.0), SC+BMC from third-party PVG rats and CP (MST=45.0), or no treatment (MST=13.8) survived over 50 days. The donor-specific tolerance was confirmed by second skin grafts onto recipients with permanent DA liver grafts, which accepted DA skins (MST>75) but not PVG (MST=8.3). However, the lymphocytes from the tolerant recipients showed alloresponse to DA in vitro. To investigate whether the T helper type 2 deviation contributed to this "split tolerance," we assessed the production of cytokines in mixed lymphocyte reaction. Interleukin 2 and interferon-gamma were detected but interleukin 4 was not.

CONCLUSIONS

These data showed that this protocol induced split tolerance in rat liver transplantation and, furthermore, the mechanism of split tolerance was not due to T helper 2 deviation.

Effect of liver transplantation on islet allografts: up-regulation of Fas ligand and apoptosis of T lymphocytes are associated with islet graft tolerance

BACKGROUND

Liver allografts in spontaneously tolerant strain combinations can protect other organs of the same donor origin from rejection and reverse ongoing rejection in previously placed grafts. The aims of this study were to examine whether liver allografts have the same protective effect on islet allografts and to investigate the underlying mechanisms.

METHODS

PVG islets were transplanted beneath the kidney capsule of streptozotocin-induced diabetic DA rats with or without liver allografting. The cellular infiltrate, and the extent of apoptosis and of Fas ligand (FasL) expression in the islet grafts were evaluated on days 2, 4, and 7 after transplantation by means of immunostaining and the in situ terminal deoxynucleotide transferase-mediated dUTP nick end labeling assay. Donor and recipient mixed lymphocyte reactions (MLR) were determined at 7 days or 100 days after islet transplantation.

RESULTS

Islet allografts transplanted alone were rapidly rejected within 5-8 days. Rejection was delayed, but not prevented, when islets were transplanted simultaneously with the liver. Liver transplantation 1 month before islet transplantation resulted in long-term survival (>100 days) of islet grafts in three of seven animals, whereas the other four died of liver rejection with functional islet grafts. Liver transplantation on day 4 after islet grafting reversed ongoing islet rejection and led to indefinite islet graft survival in three of seven cases. There was a progressive increase of cellular infiltration in all of the islet allografts, but the intensity of the infiltrate did not correlate with the outcome of the islet allografts. Islet rejection was characterized by an early dominance of monocytes/macrophages and CD25+ T cells in the infiltrates, a high incidence of apoptotic beta cells in grafts, and a sensitized status in the MLR. Tolerance of islet allografts was associated with increased numbers of dendritic cells in the graft infiltrates, up-regulation of FasL, and prominent apoptosis of alloreactive leukocytes in the islet grafts, as well as donor-specific MLR suppression in long-term survivors.

CONCLUSIONS

These results demonstrate that the extent of the protective effect of liver transplantation on islet allografts varies with the time of liver grafting, ranging from delay in islet rejection to complete islet acceptance. Islet graft tolerance induced by liver transplantation is the result of an immune process that involves up-regulation of Fas ligand expression on, and apoptosis of, islet graft infiltrating lymphocytes.

History of clinical transplantation

The emergence of transplantation has seen the development of increasingly potent immunosuppressive agents, progressively better methods of tissue and organ preservation, refinements in histocompatibility matching, and numerous innovations in surgical techniques. Such efforts in combination ultimately made it possible to successfully engraft all of the organs and bone marrow cells in humans. At a more fundamental level, however, the transplantation enterprise hinged on two seminal turning points. The first was the recognition by Billingham, Brent, and Medawar in 1953 that it was possible to induce chimerism-associated neonatal tolerance deliberately. This discovery escalated over the next 15 years to the first successful bone marrow transplantations in humans in 1968. The second turning point was the demonstration during the early 1960s that canine and human organ allografts could self-induce tolerance with the aid of immunosuppression. By the end of 1962, however, it had been incorrectly concluded that turning points one and two involved different immune mechanisms. The error was not corrected until well into the 1990s. In this historical account, the vast literature that sprang up during the intervening 30 years has been summarized. Although admirably documenting empiric progress in clinical transplantation, its failure to explain organ allograft acceptance predestined organ recipients to lifetime immunosuppression and precluded fundamental changes in the treatment policies. After it was discovered in 1992 that long-surviving organ transplant recipients had persistent microchimerism, it was possible to see the mechanistic commonality of organ and bone marrow transplantation. A clarifying central principle of immunology could then be synthesized with which to guide efforts to induce tolerance systematically to human tissues and perhaps ultimately to xenografts.

Immunosuppressive drugs: the first 50 years and a glance forward

During the past 50 years, many immunosuppressive drugs have been described. Often their mechanisms of action were established long after their discovery. Eventually these mechanisms were found to fall into five groups: (i) regulators of gene expression; (ii) alkylating agents; (iii) inhibitors of de novo purine synthesis; (iv) inhibitors of de novo pyrimidine synthesis; and (v) inhibitors of kinases and phosphatases. Glucocorticoids exert immunosuppressive and anti-inflammatory activity mainly by inhibiting the expression of genes for interleukin-2 and other mediators. Cyclophosphamide metabolites alkylate DNA bases and preferentially suppress immune responses mediated by B-lymphocytes. Methotrexate and its polyglutamate derivatives suppress inflammatory responses through release of adenosine; they suppress immune responses by inducing the apoptosis of activated T-lymphocytes and inhibiting the synthesis of both purines and pyrimidines. Azathioprine metabolites inhibit several enzymes of purine synthesis. Mycophenolic acid and mizoribine inhibit inosine monophosphate dehydrogenase, thereby depleting guanosine nucleotides. Mycophenolic acid induces apoptosis of activated T-lymphocytes. A leflunomide metabolite and Brequinar inhibit dihydroorotate dehydrogenase, thereby suppressing pyrimidine nucleotide synthesis. Cyclosporine and FK-506 (Tacrolimus) inhibit the phosphatase activity of calcineurin, thereby suppressing the production of IL-2 and other cytokines. In addition, these compounds have recently been found to block the JNK and p38 signaling pathways triggered by antigen recognition in T-cells. In contrast, rapamycin inhibits kinases required for cell cycling and responses to IL-2. Rapamycin also induces apoptosis of activated T-lymphocytes. Immunosuppressive and anti-inflammatory compounds in development include inhibitors of p38 kinase and of the type IV isoform of cyclic AMP phosphodiesterase which is expressed in lymphocytes and monocytes.A promising future application of immunosuppressive drugs is their use in a regime to induce tolerance to allografts. The role of leukocytes in grafts, and the induction of apoptosis of clones of responding T-lymphocytes, is discussed.

Antiviral and immunomodulatory effects of desferrioxamine in cytomegalovirus-infected rat liver allografts with rejection

BACKGROUND

Cytomegalovirus (CMV) infection is associated with acute and chronic allograft rejection. We have recently shown that rat CMV increases portal inflammation and bile duct destruction in a model of rat liver allograft rejection. Desferrioxamine (DFO), an iron chelator and antioxidant, has recently been demonstrated to have antiviral as well as immunomodulatory effects in vitro. We therefore investigated whether DFO inhibits (a) CMV infection and (b) graft destruction in our rat model.

METHOD

One day after liver transplantation, PVG (RT1c) into BN(RT1n), the rats were infected with rat CMV (RCMV, Maastricht strain; 10(5) plaque-forming units i.p.). The effects of 100 mg/kg body weight and 200 mg/kg body weight DFO were examined.

RESULTS

In the untreated group, the grafts were uniformly RCMV culture-positive. In the group receiving 200 mg/kg DFO, RCMV replication was effectively inhibited. Inflammatory response in the graft, and especially the number of macrophages, was significantly reduced by DFO. Portal inflammation and bile duct destruction were also significantly reduced. In the untreated group, the bile duct epithelial cells were found to be strongly positive for tumor necrosis factor-alpha and this expression was clearly decreased by DFO. In addition, DFO significantly inhibited vascular cell adhesion molecule-1 expression on sinusoidal endothelial cells.

CONCLUSIONS

Our in vivo transplant study strongly supports the inhibitory effects of metal chelators on CMV infection and their possible usefulness in the treatment of CMV-induced pathogenic changes.

Outcome of different models of multiorgan transplantation in rats

In the PVG (RT1(c)) to DA(RT1(a)) rat combination, liver allografts are spontaneously accepted across a complete MHC barrier while cardiac and renal allografts are rejected. We postulated that this spontaneous liver acceptance was associated with the large quantity of antigen in the transplanted liver. In order to test this hypothesis, we have developed three different models of multiorgan transplantation: model I, triple heart transplantation; model II, triple kidney transplantation; and model III, double heart and double kidney transplantation. The results showed that prolongation of heart and kidney allografts was achieved with the increasing number of organs transplanted. The models proved reliable and useful in transplant immunological studies.

Origin, kinetics, and function of chimeric B lymphocytes in liver allografts

BACKGROUND

Despite the well-recognized concordance of chimerism with spontaneous acceptance of rat liver allografts, the active role and the identity of chimeric cells mediating liver allograft tolerance are unknown. Because resting B cells are endowed with a tolerogenic antigen-presenting capacity, we assessed whether donor B cells propagated from the grafted liver may be responsible for liver allograft tolerance.

METHODS

Dark Agouti or Lewis rats were grafted with Lewis or Dark Agouti livers as a tolerogenic or a rejection combination, respectively. We followed the kinetics of donor B cells in recipients by flow cytometry, and we examined the fate of liver allografts depleted of passenger B cells in either B cell-sufficient or -deficient recipients. B-cell depletion was achieved by treatment of animals with polyclonal goat anti-rat IgM antibody from birth.

RESULTS

During the first 3 days after liver allografting, donor B cells rapidly migrated from graft-infiltrating cells and appeared in systemic circulation in both the tolerogenic and rejection combinations. However, systemic chimerism was detectable in the tolerogenic combination by day 14, whereas it was undetectable in the rejection combination by day 7. In graft-infiltrating cells, a significant expansion of chimeric IgM+ (newly formed) B cells was observed on day 5 in the tolerogenic, but not in the rejection, combination. However, depletion of B cells from liver grafts and the absence of antibodies failed to alter the outcome of liver allograft survival in the tolerogenic or immunogenic combination.

CONCLUSION

Although intragraft chimeric B cells proliferated in tolerogenic liver allografts, their clonal expansion does not seem to be essential for the promotion of liver allograft tolerance.

Paradoxical early immune activation during acceptance of liver allografts compared with rejection of skin grafts in a rat model of transplantation

Liver allografts in many animal models are often spontaneously accepted across a complete histocompatibility barrier without requirement for immunosuppression. In contrast, skin allografts are usually rejected, even across minor histocompatibility barriers. To identify the mechanism of liver allograft acceptance we have compared skin rejection with liver acceptance in DA rat strain recipients of PVG donors, a major histocompatibility complex (MHC) class I plus II mismatch. In spite of the established role of draining lymph nodes (LN) in induction of rejection of skin allografts, there was much greater involvement of LN after liver than after skin transplantation. Few donor cells migrated to these organs from transplanted skin but many cells migrated from transplanted liver. There was also a paradoxical increase in interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) mRNA in LN and spleen of liver allograft recipients that greatly exceeded their expression in skin allograft recipients. For example, there were 2. 7+/-1.6x104 molecules of IFN-gamma per 106 molecules of beta-actin mRNA in the LN draining liver allografts 1 day after transplantation compared with 2.0+/-0.3x103 molecules/106 beta-actin in LN draining skin allografts and 8.1+/-1.8x102 molecules/106 beta-actin in LN draining skin isografts. Examination of the graft showed that infiltration and cytokine mRNA up-regulation occurred more slowly in the transplanted skin than in liver but progressed inexorably in skin grafts until rejection. These results show that liver acceptance is associated with a paradoxical marked early activation then subsequent decline of the immune response.

Cytomegalovirus infection is associated with increased inflammation and severe bile duct damage in rat liver allografts

It has been suggested that cytomegalovirus (CMV) infection is involved in allograft rejection. In liver transplantation, it has been suggested that CMV is associated with the development of vanishing bile duct syndrome (VBDS), and persistent CMV has been found in liver grafts that develop chronic rejection. In this experimental study, the effect of rat CMV (RCMV) infection on intragraft changes was investigated in a rat model of acute liver allograft rejection. Liver transplantations were performed in a rat strain combination of PVG (RT1c) --> BN (RT1n). No immunosuppression was given. One group of animals was infected with RCMV Maastricht Strain (10(5) plaque-forming units, intraperitoneally), and another group was left uninfected. The grafts were examined histologically after the rats were killed on postoperative days 7 through 9 at the early phase and days 20 through 30 at the late phase of rejection. Immunohistochemical studies were performed to demonstrate the immunological activation markers major histocompatibility complex class II and interleukin 2 receptors, intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1), and their ligands. RCMV infection was demonstrated from the grafts by culture and direct antigen detection. In liver allografts undergoing acute rejection, CMV significantly increased portal inflammation and caused more severe bile duct damage than in the uninfected grafts. CMV was also linked to the induction of VCAM-1 in the endothelial cells. The ongoing infection was found to vary over time in the different structures of the liver grafts, including the vascular endothelium and bile ducts. Our results support an association between CMV infection and the immunological mechanisms of rejection, as well as the role of CMV in the development of bile duct damage in liver allografts.

Increased immune responses to regenerating partial liver grafts in the rat

The relationship between liver regeneration and the induction of the immune response is uncertain. We hypothesize that the altered environment of the regenerating liver allograft increases the immune response to the allograft. In DA (RT1a) to LEW (RT1I) rats, hepatectomized, small-for-size and whole, normal-for-size liver transplants were performed. Naive and 70% hepatectomized LEW served as controls. Animals were assessed for survival, mass restoration, and host alloresponses. Although 30% partial allografts regenerated well to achieve a volume nearly equal to that of recipient's native liver in 7 days, survival was significantly shorter than that of the recipients of whole grafts (8.8 +/- 0.4 vs 10.3 +/- 1.2 days, n = 6, P < 0.02). When compared on Day 4 after transplantation, histologic examination revealed a more vigorous cellular infiltration in the sinusoidal area in the partial liver transplant group. Phenotypic analysis of thymocytes showed a predominance of more mature phenotypes in the partial group, including more prominent decrease in the frequency of CD4, CD8-double-positive cells and increase in that of alpha beta TCRhigh cells. Proliferative activity of thymocytes in response to Con A was higher in the partial group than in the whole group. MLR of splenocytes against donor-type antigens was higher in the partial group, whereas reactivity against third party was the same as in other groups. These data suggest that host cellular responses to the allograft are enhanced in the regenerating, small-for-size liver graft. These findings have implications in the clinical management of liver recipients with damaged or small for size livers.

Tolerance to rat liver allografts: IV. Acceptance depends on the quantity of donor tissue and on donor leukocytes

Liver allografts in some rat strains are often spontaneously accepted across a complete major histocompatibility barrier without the requirement for immunosuppression while other nonliver allografts are rejected. In previous studies, we have shown that spontaneous acceptance is dependent on liver passenger leukocytes. Depletion of passenger leukocytes by donor irradiation allows rejection, with DA recipients of irradiated PVG livers having a median survival time (MST) of 16 days. Here we show that, in this model, spontaneous acceptance is reconstituted by intravenous injection of donor leukocytes. Intravenous injection of 3-5x10(7) PVG liver leukocytes significantly prolonged DA survival time (MST=96 days, P=0.026), as did 5x10(7) spleen leukocytes (MST>100 days, P=0.002). Deletion of T cells from the reconstituting inoculum reduced survival time (MST=78 days, P=0.039), whereas deletion of B cells or monocytes/macrophages had no effect on survival time. In contrast, PVG hearts are regularly rejected by DA recipients, and PVG liver or spleen leukocytes, even at doses of greater than 3x10(8) cells/recipient, were unable to induce heart acceptance. To investigate the possibility that acceptance of the irradiated liver but not the heart might be due to the large mass of the liver, two kidneys and two hearts of PVG origin were transplanted to each DA recipient together with 1.5x10(8) PVG leukocytes. These organs survived for greater than 200 days, thereby showing that a large mass of donor tissue, in association with donor leukocytes, leads to acceptance of organs that are rejected if transplanted singly. It appears likely that spontaneous liver transplant tolerance is a high-dose or activation-associated immune phenomenon.

Allograft acceptance and rejection, mediated by a liver suppressor factor, LSF-1, purified from serum of liver transplanted rats

In certain rat strain combinations liver allografts are spontaneously accepted without immunosuppression and induce donor-specific tolerance to further skin and heart graft in the recipient. Such an effect is also transferrable using serum from orthotopically liver transplanted rats (OLT serum). In the OLT serum of one such combination, DA (RT1a) donor into PVG (RT1c) recipient, a 40 kDa protein (liver suppressor factor, LSF-1) has been identified and shown to be immunosuppressive in vitro. The aim of the present study is to investigate the immunological effect of LSF-1 and a polyclonal antibody (anti-LSF-1) against this molecule, in a rat heterotopic heart transplant (HHT) model and OLT model, respectively. Intramuscular injection of 300 micrograms of LSF-1, 1 h postoperatively, into a PVG recipient of either a DA or BN (RT1n) cardiac allograft caused significant prolongation of graft survival. Intravenous injection of polyclonal rabbit sera raised against an N-terminal peptide of LSF-1 (anti-LSF-1), within 1 h postoperatively, had variable effects on the survival of DA liver grafts in PVG recipients. In 5/6 cases injection of between 1 and 2 ml of anti-LSF-1 resulted in death of the recipient. Histological examination of the liver showed severe rejection with lymphoid cell infiltration of the portal tract and sinusoids and extensive damage to the parenchyma. All control rats survived for more than 60 days without any signs of rejection. The anti-LSF-1 polyclonal antibody prevented the injection of tolerance in the normally tolerogenic model (DA into PVG). This, together with the in vivo results, suggests a role for LSF-1 in the induction of tolerance.

Liver allograft rejection in rats depleted of CD8+ cells

The mechanism(s) of rejection or tolerance induction is a competitive, complex process that presumably involves interactions between multiple subpopulations of T lymphocytes. We investigated the roles of CD8+ cytolytic and CD4+ helper T cells in rat strains that tolerate liver allografts and that differ at both the major histocompatibility complex (MHC) (RT1) and minor histocompatibility genes. Orthotopic liver transplantation (OLT) with arterial reconstruction was performed with Brown Norway (BN) (RT1n) donors and Lewis (RT1(1)) recipients, some of which were untreated, others treated with anti-CD8 antibody, and still others treated with anti-CD4 antibody. Liver graft rejection was monitored for 28 days on the basis of two criteria: (1) serum levels of AST enzyme at 3-day intervals and (2) liver biopsies at weekly intervals and at the time of sacrifice at the end of the study period. In the untreated control group, an elevation of AST was found to peak at day 6 after grafting, and it remained elevated until day 28 (AST 542 +/- 72 U/l). Histologically, signs of severe rejection were first observed on day 9; these changed to moderate rejection about day 21 and to mild rejection by day 28, when the animals were sacrificed. Recipients pre-treated with anti-CD8 demonstrated a significant elevation of AST within 6 days that, unlike in the control recipients, continued to rise sharply through the observation period (AST 1127 +/- 181 U/l, P = 0.009 vs control group). Liver biopsies showed mild rejection at day 9 and moderate rejection at days 21 through 28. Recipients pretreated with anti-CD4 showed a time course of enzyme elevation and severity of rejection that was not significantly different from that observed in the control group. However, anti-CD4 treatment resulted in only 75% depletion of CD4+ cells in peripheral blood as compared to complete elimination of CD8+ cells following anti-CD8 treatment. Functional studies of spleen and liver-infiltrating lymphocytes obtained after 28 days showed low proliferative response in mixed lymphocyte culture with both BN and PVG stimulator spleen and lymph node cells. These results suggest that in this donor/recipient combination, removal of CD8+ cells increases the severity of rejection as demonstrated by a progressive rise in AST and histology. Moreover, OLT in this combination results in a profound, nonspecific inhibition of proliferative T-cell responses to MHC alloantigens.

Graft-versus-host disease after small bowel transplantation is associated with host colonic injury

The present studies were undertaken to evaluate the histologic effects of graft-versus-host disease on the host colon after small bowel transplantation. Graft-versus-host disease was produced in six Lewis x Brown Norway F1 rats by performing vascularized, out-of-continuity small bowel transplants from parental Lewis donors. Host proximal and distal colon were sampled 14 days after operation when signs of graft-versus-host disease, including weight loss and splenomegaly, were present. Tissue was assessed histologically by blinded observer and compared to eight sham-operated controls. Three histologic features were noted to be statistically increased in diseased animals: (1) mucin loss; (2) crypt abscesses; and (3) large lymphoid aggregates in the mucosa and submucosa. These features were more commonly noted in the distal rather than the proximal colon. Another group of five grafted animals treated with cyclosporine A (10 mg/kg/day intramuscularly) still lost weight but did not display overt signs of graft-versus-host disease and had normal-sized spleens. There was normal mucin content and no evidence of crypt abscesses in these treated animals, although large lymphoid aggregates were present. It is concluded that mucin loss, crypt abscesses, and large lymphoid aggregates are characteristics of graft-versus-host disease-induced colonic injury in this model and that these changes are most evident in the distal colon. Cyclosporine A therapy does not completely reverse the histological changes of colonic graft-versus-host disease. This model may be useful in studying the mechanisms by which immune mediated colitides preferentially affect the distal colon.

[Immunologic tolerance after experimental liver transplantation]

Allografts in the rat liver are rejected less vigorously than other primarily vascularized allografts; they show a better survival rate and induce donor-specific unresponsiveness or tolerance in some donor-recipient combinations. This overview focuses on the immunologic mechanisms of this privileged status of liver allografts. A variety of possible mechanisms, such as generation of suppressor T-cells, humoral factors and microchimerism, has been related to the observed hyporeactivity. A further analysis of these phenomena may enhance the development of clinical organ transplantation protocols that allow for establishment of donor-specific unresponsiveness without the need for life-long immunosuppression.

Variable chimerism, graft-versus-host disease, and tolerance after different kinds of cell and whole organ transplantation from Lewis to brown Norway rats

The bidirectional paradigm of tolerance involving reciprocal host vs. graft and graft vs. host reactions was examined after Lewis (LEW)-->Brown Norway (BN) transplantation of different whole organs (liver, intestine, heart, and kidney) or of 2.5 x 10(8) LEW leukocytes obtained from bone marrow, spleen, lymph nodes, and thymus. The experiments were performed without immunosuppression or under 14 daily doses of postoperative tacrolimus, which were continued in weekly doses to 100 days in a "continuous treatment" subgroup, and to 27 days in a short treatment group. Without immunosuppression, all organs and cell suspensions failed to engraft or were acutely rejected. GVHD (usually fatal) was always caused when either the long or short treatment was used for recipients of intestinal grafts and cell suspensions of spleen and lymph nodes. In contrast, both immunosuppressive protocols allowed engraftment of bone marrow cells, liver, heart, and kidney without clinical GVHD, whereas thymus cell suspensions and small doses of whole blood neither engrafted nor caused GVHD. At 100 days, now drug-free for 73 days, the liver, bone marrow, and heart recipients were tolerant in that they accepted all challenge LEW heart and/or liver grafts for 100 more days despite in vitro evidence of donor-specific reactivity (split tolerance). At 200 days, histopathologic studies of the challenge livers were normal no matter what the priming graft. However, the still-beating challenge hearts had a spectrum from normal to severe chronic rejection that defined the tolerogenicity of the original primary grafts: liver best-->bone marrow next-->heart least. Both the GVHD propensity and tolerogenicity in these experiments were closely associated with recipient tissue chimerism 30 and 100 days after the experiments began. The tissue chimerism was invariably multilineage, but the GVHD outcome was associated with T cell over-representation. These observations provide guidelines that should be considered in devising leukocyte augmentation protocols for human whole organ recipients. The results are discussed in relation to the historical tolerance studies of Billingham, Brent, and Medawar; Good; Monaco; and Calne.

Variable chimerism, graft-versus-host disease, and tolerance after different kinds of cell and whole organ transplantation from Lewis to brown Norway rats

The bidirectional paradigm of tolerance involving reciprocal host vs. graft and graft vs. host reactions was examined after Lewis (LEW)-->Brown Norway (BN) transplantation of different whole organs (liver, intestine, heart, and kidney) or of 2.5 x 10(8) LEW leukocytes obtained from bone marrow, spleen, lymph nodes, and thymus. The experiments were performed without immunosuppression or under 14 daily doses of postoperative tacrolimus, which were continued in weekly doses to 100 days in a "continuous treatment" subgroup, and to 27 days in a short treatment group. Without immunosuppression, all organs and cell suspensions failed to engraft or were acutely rejected. GVHD (usually fatal) was always caused when either the long or short treatment was used for recipients of intestinal grafts and cell suspensions of spleen and lymph nodes. In contrast, both immunosuppressive protocols allowed engraftment of bone marrow cells, liver, heart, and kidney without clinical GVHD, whereas thymus cell suspensions and small doses of whole blood neither engrafted nor caused GVHD. At 100 days, now drug-free for 73 days, the liver, bone marrow, and heart recipients were tolerant in that they accepted all challenge LEW heart and/or liver grafts for 100 more days despite in vitro evidence of donor-specific reactivity (split tolerance). At 200 days, histopathologic studies of the challenge livers were normal no matter what the priming graft. However, the still-beating challenge hearts had a spectrum from normal to severe chronic rejection that defined the tolerogenicity of the original primary grafts: liver best-->bone marrow next-->heart least. Both the GVHD propensity and tolerogenicity in these experiments were closely associated with recipient tissue chimerism 30 and 100 days after the experiments began. The tissue chimerism was invariably multilineage, but the GVHD outcome was associated with T cell over-representation. These observations provide guidelines that should be considered in devising leukocyte augmentation protocols for human whole organ recipients. The results are discussed in relation to the historical tolerance studies of Billingham, Brent, and Medawar; Good; Monaco; and Calne.

The essential roles of parenchymal tissues and passenger leukocytes in the tolerance induced by liver grafting in rats

Liver allografts in pigs and rodents are uniquely capable of inducing tolerance to themselves and to other grafts of donor tissues, instead of succumbing to the acute rejection that follows transplantation of other allogeneic tissues. We demonstrate here, using normal and chimaeric rat liver grafts, that both the allogeneic liver parenchyma and the intrahepatic leukocytes of donor type contribute to the establishment of long-term tolerance, each component being essential and complementary. The essential role of hepatic parenchyma may be related to its continuous release of soluble transplantation antigens that facilitate tolerogenesis. We suggest that clinical attempts at tolerance induction by the infusion of donor bone marrow-derived leukocytes may likewise be facilitated by the coadministration of soluble transplantation antigens of donor type.

31P NMR assessment of orthotopic liver rejection in a rat model

31P NMR spectroscopy was used serially to study rejecting (ACl-->LEW) and nonrejecting (ACl-->ACl) orthotopic liver transplants in rats. Recipients were evaluated on post-transplant days 1, 3, 5, 7, 9, and 11. The relative changes in phosphomonoester (PME), inorganic phosphate (Pi), high-energy phosphates and pH were studied. The earliest significant difference between the rejecting and nonrejecting groups was a decrease in the alpha-NTP peak area on Day 5. This was followed by significant decreases in beta-NTP and pH, and increases in PME and Pi on Day 7. High-resolution 31P NMR spectra of perchloric acid extracts demonstrated the PME increase to be due mainly to elevated phosphoethanolamine. Using the parameter (Pi + PME)/(alpha + beta + gamma-NTP), rejecting livers were distinguished from nonrejecting livers at a moderate stage of rejection.

Host immune suppression after small bowel/liver transplantation in rats

Simultaneous liver grafting in the Lewis (RT1)-to-DA (RT1) rat strain combination protects small intestinal grafts from rejection. The present study examined host immune responses after combined small bowel/liver transplantation (SBL) in this model. Orthotopic liver transplantation and heterotopic small intestinal transplantation were performed simultaneously and compared with isolated small bowel allografts (SBA) and isolated small bowel isografts (SBI). All rats were sacrificed on postoperative day (POD) 7 or 14 for immunological and histological studies. The mean time to rejection of the SBA was 6.6 +/- 0.3 days. In contrast, there was no clinical or histological evidence of intestinal rejection in SBL recipients during the 14 days of follow-up. The SBL recipients showed clinical and histological evidence of graft-versus-host disease (GVHD). Lymphocyte proliferation and IL-2 production in response to donor antigens were suppressed after SBL transplantation compared with the SBA or the SBI controls (P < 0.05). Cell-mediated cytotoxicity and lymphocytotoxic antibody production against donor cells were also significantly inhibited in the SBL recipients compared with the SBA control group (P < 0.05). We conclude that SBL transplantation in the Lewis-to DA rat strain combination: (1) suppresses host alloimmune responses, (2) prevents early intestinal rejection, and (3) favors the development of GVHD.

Host immune suppression after small bowel/liver transplantation in rats

Simultaneous liver grafting in the Lewis (RT1)-to-DA (RT1) rat strain combination protects small intestinal grafts from rejection. The present study examined host immune responses after combined small bowel/liver transplantation (SBL) in this model. Orthotopic liver transplantation and heterotopic small intestinal transplantation were performed simultaneously and compared with isolated small bowel allografts (SBA) and isolated small bowel isografts (SBI). All rats were sacrificed on postoperative day (POD) 7 or 14 for immunological and histological studies. The mean time to rejection of the SBA was 6.6 +/- 0.3 days. In contrast, there was no clinical or histological evidence of intestinal rejection in SBL recipients during the 14 days of follow-up. The SBL recipients showed clinical and histological evidence of graft-versus-host disease (GVHD). Lymphocyte proliferation and IL-2 production in response to donor antigens were suppressed after SBL transplantation compared with the SBA or the SBI controls (P < 0.05). Cell-mediated cytotoxicity and lymphocytotoxic antibody production against donor cells were also significantly inhibited in the SBL recipients compared with the SBA control group (P < 0.05). We conclude that SBL transplantation in the Lewis-to DA rat strain combination: (1) suppresses host alloimmune responses, (2) prevents early intestinal rejection, and (3) favors the development of GVHD.

Nitric oxide production in host-versus-graft and graft-versus-host reactions in the rat

The present study was designed to determine whether .N = O produced in vivo during the rejection of histoincompatible tissues might permit serum NO2-/NO3- levels to serve as markers of a rejection reaction. Rat syngeneic and allogeneic liver, heart, bone marrow/spleen cell, small bowel, skin, and sponge matrix grafts were performed and the stable end-products of .N = O, NO2-/NO3-, were serially assayed in the serum of the grafted animals. A significant rise of serum NO2-/NO3- levels in the allografted animals preceded the onset of clinical signs of rejection or graft-versus-host disease, with the exception of the skin and sponge matrix graft models, where elevated serum NO2-/NO3- levels were never observed. In all transplant models, normal serum NO2-/NO3- levels were observed at all times in animals that received syngeneic grafts. Furthermore, treatment of allograft recipients with the immunosuppressive agents FK 506 or cyclosporine A inhibited .N = O production. Determination of serum creatinine levels demonstrated that the elevated serum NO2-/NO3- levels were not caused by kidney dysfunction. Serum NO2-/NO3- levels might be useful early serum markers of the initiation of a rejection reaction or graft-versus-host disease when functional markers of graft dysfunction are not apparent.

Corneal transplantation--immunologically privileged status

Despite the relatively high success rate and the word 'immunologically privileged site', it has been known that a corneal graft can induce an allograft rejection reaction. This is especially true in the rat where orthotopic penetrating corneal grafts in certain strain combinations are rejected even when transplanted in avascular bed. Reliable microsurgical techniques, together with the availability of inbred or congenic strains and a rapidly developing knowledge of its major histocompatibility complex (MHC) and immune system in general, have made the rat a prime species in which to study the immunological events after corneal grafting. This review describes recent progress in understanding the immunological mechanisms behind corneal graft rejection. The topics discussed include the rat MHC (RT1) antigens and their distribution in the cornea; different responder status in fully allogeneic strain combinations, including the importance of multiple non-MHC antigens; and the role of antigen-presenting cells (APCs) in corneal graft rejection.

The immune response to intracerebral neural grafts

Neural transplantation offers a potential therapeutic approach to a variety of neurological disorders, most notably those of a degenerative nature. However, the degree of immunological privilege (i.e. isolation from an immune response) in the brain, which is not absolute, may be a significant impediment to the survival of histoincompatible grafts. The nature of this privilege, together with the specific immune events leading to neural graft rejection, are discussed. As a consequence of this immune-mediated rejection, immunosuppression in some form might be necessary to guarantee long-term graft survival. Various strategies are being explored to suppress the immune response to neural grafts, not only for future use in clinical therapies, but also to bring intracerebral allo- and xenotransplantation to the attention of the general neurobiologist.

The immunosuppressive effect of hepatocytes in rats--assessment by heart allograft survival and delayed-type-hypersensitivity reactions

The immunosuppressive effect of hepatocytes was examined experimentally by heart allograft and delayed-type-hypersensitivity (DTH) reactions. The hepatocyte inoculation (1 X 10(7) of BDE (of the major histocompatibility complex haplotype RT1u), LEW (RT1l), and DA (RT1a) into the spleens of LEW rats significantly prolonged the survival of BDE heart allografts to 14.3 +/- 2.7 (mean +/- SD), 9.2 +/- 0.8, and 10.8 +/- 2.3 days respectively, compared with 6.7 +/- 0.8 days in controls (p less than 0.01). Moreover, the BDE hepatocytes had a significantly prolonged survival compared to the LEW (p less than 0.01) and DA (0.02 less than p less than 0.05) groups. BDE hepatocyte (donor specific) inoculation 4 and 7 days before priming with the spleen cells reduced DTH responses in the LEW rats to 44.6 +/- 4.8 per cent, and 74.2 +/- 8.0 per cent, respectively. DA hepatocyte inoculation (third party) 4 and 7 days prior to priming reduced DTH responses to 72.5 +/- 11.5 per cent, and 76.5 +/- 11.9 per cent, respectively. All DTH responses were significantly suppressed after hepatocyte inoculation compared to 100 per cent in the controls (p less than 0.01). Moreover, the inoculation of BDE hepatocytes (donor specific) 4 days prior to the priming significantly reduced DTH responses compared to the group primed 7 days before (p less than 0.01). From these results we concluded that hepatocytes produced not only non-specific but also donor specific immunosuppressive effects through T cell immune reaction. Moreover, donor specific immunosuppressive effects were induced at least 4 to 7 days after hepatocyte inoculation.