Presumptive BSE cases with an aberrant prion protein phenotype in Switzerland, 2011: Lack of prion disease in experimentally inoculated cattle and bovine prion protein transgenic mice.

Bovine spongiform encephalopathy (BSE) is caused by different prion strains that are discriminated by the molecular characteristics of the pathological prion protein. In 2011, Switzerland reported two presumptive cases of BSE in cattle with a prion protein phenotype different from previously described strains, and it was unclear whether these findings were related to a transmissible disease and have implications on animal and public health. In this study, brain tissues of these cases were inoculated into transgenic mice expressing the bovine prion protein (BoPrP-Tg110) and into cattle. Clinical and pathological investigations as well as molecular testing did not provide evidence for the presence of BSE in the Swiss cases after two passages in BoPrP-Tg110 mice and a challenge period of 3.5 years in cattle. This lack of disease transmission suggests that the Swiss 2011 cases were not affected by a prion disease and were unrelated to the feed-born BSE epidemic.

Experimental transmission of h-type bovine spongiform encephalopathy to bovinized transgenic mice.

To characterize the biological and biochemical properties of H-type bovine spongiform encephalopathy (BSE), a transmission study with a Canadian H-type isolate was performed with bovinized transgenic mice (TgBoPrP), which were inoculated intracerebrally with brain homogenate from cattle with H-type BSE. All mice exhibited characteristic neurologic signs, and the subsequent passage showed a shortened incubation period. The distribution of disease-associated prion protein (PrP(Sc)) was determined by immunohistochemistry, Western blot, and paraffin-embedded tissue (PET) blot. Biochemical properties and higher molecular weight of the glycoform pattern were well conserved within mice. Immunolabeled granular PrP(Sc), aggregates, and/or plaque-like deposits were mainly detected in the following brain locations: septal nuclei, subcallosal regions, hypothalamus, paraventricular nucleus of the thalamus, interstitial nucleus of the stria terminalis, and the reticular formation of the midbrain. Weak reactivity was detected by immunohistochemistry and PET blot in the cerebral cortex, most thalamic nuclei, the hippocampus, medulla oblongata, and cerebellum. These findings indicate that the H-type BSE prion has biological and biochemical properties distinct from those of C-type and L-type BSE in TgBoPrP mice, which suggests that TgBoPrP mice constitute a useful animal model to distinguish isolates from BSE-infected cattle.

Experimental classical bovine spongiform encephalopathy: definition and progression of neural PrP immunolabeling in relation to diagnosis and disease controls.

Tissues from sequential-kill time course studies of bovine spongiform encephalopathy (BSE) were examined to define PrP immunohistochemical labeling forms and map disease-specific labeling over the disease course after oral exposure to the BSE agent at two dose levels. Study was confined to brainstem, spinal cord, and certain peripheral nervous system ganglia-tissues implicated in pathogenesis and diagnosis or disease control strategies. Disease-specific labeling in the brainstem in 39 of 220 test animals showed the forms and patterns observed in natural disease and invariably preceded spongiform changes. A precise temporal pattern of increase in labeling was not apparent, but labeling was generally most widespread in clinical cases, and it always involved neuroanatomic locations in the medulla oblongata. In two cases, sparse labeling was confined to one or more neuroanatomic nuclei of the medulla oblongata. When involved, the spinal cord was affected at all levels, providing no indication of temporal spread within the cord axis or relative to the brainstem. Where minimal PrP labeling occurred in the thoracic spinal cord, it was consistent with initial involvement of general visceral efferent neurons. Labeling of ganglia involved only sensory ganglia and only when PrP was present in the brainstem and spinal cord. These experimental transmissions mimicked the neuropathologic findings in BSE-C field cases, independent of dose of agent or stage of disease. The model supports current diagnostic sampling approaches and control measures for the removal and destruction of nervous system tissues in slaughtered cattle.

Bacterial infections in pigs experimentally infected with Nipah virus.

Nipah virus (NiV; Paramyxoviridae) caused fatal encephalitis in humans during an outbreak in Malaysia in 1998/1999 after transmission from infected pigs. Our previous study demonstrated that the respiratory, lymphatic and central nervous systems are targets for virus replication in experimentally infected pigs. To continue the studies on pathogenesis of NiV in swine, six piglets were inoculated oronasally with 2.5 x 10(5) PFU per animal. Four pigs developed mild clinical signs, one exudative epidermitis, and one neurologic signs due to suppurative meningoencephalitis, and was euthanized at 11 days post-inoculation (dpi). Neutralizing antibodies reached in surviving animals titers around 1280 at 16 dpi. Nasal and oro-pharyngeal shedding of the NiV was detected between 2 and 17 dpi. Virus appeared to be cleared from the tissues of the infected animals by 23 dpi, with low amount of RNA detected in submandibular and bronchial lymph nodes of three pigs, and olfactory bulb of one animal. Despite the presence of neutralizing antibodies, virus was isolated from serum at 24 dpi, and the viral RNA was still detected in serum at 29 dpi. Our results indicate slower clearance of NiV from some of the infected pigs. Bacteria were detected in the cerebrospinal fluid of five NiV inoculated animals, with isolation of Streptococcus suis and Enterococcus faecalis. Staphylococcus hyicus was isolated from the skin lesions of the animal with exudative epidermitis. Along with the observed lymphoid depletion in the lymph nodes of all NiV-infected animals, and the demonstrated ability of NiV to infect porcine peripheral blood mononuclear cells in vitro, this finding warrants further investigation into a possible NiV-induced immunosuppression of the swine host.

Estimating the temporal relationship between PrPSc detection and incubation period in experimental bovine spongiform encephalopathy of cattle.

This study examines tissues from sequential-kill, time-course pathogenesis studies to refine estimates of the age at which disease-specific PrP (PrP(Sc)) can first be detected in the central nervous system (CNS) and related peripheral nervous system ganglia of cattle incubating bovine spongiform encephalopathy (BSE). Such estimates are important for risk assessments of the age at which these tissues should be removed from cattle at slaughter to prevent human and animal exposure to BSE infection. Tissues were examined from cattle dosed orally with 100 or 1 g BSE-infected brain. Incubation period data for the doses were obtained from attack rate and the sequential-kill studies. A statistical model, fitted by maximum likelihood, accounted for the differences in the lognormal incubation period and the logistic probability of infection between different dose groups. Initial detection of PrP(Sc) during incubation was invariably in the brainstem and the earliest was at 30 and 44 months post-exposure for the 100 g- and 1 g-dosed sequential-kill study groups, respectively. The point at which PrP(Sc) in 50 % of the animals would be detected by immunohistochemistry applied to medulla-obex was estimated at 9.6 and 1.7 months before clinical onset for the 100 g- and 1 g-dosed cattle, respectively, with a low probability of detection in any of the tissues examined at more than 12 months before clinical onset. PrP(Sc) was detected inconsistently in dorsal root ganglia, concurrent with or after detection in CNS, and not at all in certain sympathetic nervous system ganglia.

Modulation of simian immunodeficiency virus neuropathology by dopaminergic drugs.

Drug abuse and human immunodeficiency virus (HIV) infection seem to cause cumulative damage in the central nervous system (CNS). Elevated extracellular dopamine is thought to be a prime mediator of the reinforcing effects of addictive substances. To investigate the possible role of increased dopamine availability in the pathogenesis of HIV dementia, simian immunodeficiency virus (SIV)-infected monkeys were treated with dopaminergic drugs (selegiline or L-DOPA). Both substances increased intracerebral SIV expression, combined with aggravation of infection-related neuropathology and ultrastructural alterations of dendrites in dopaminergic areas (spongiform polioencephalopathy) in asymptomatic animals. Moreover, this treatment resulted in enhanced TNF-alpha expression in the brains of SIV-infected animals. These findings indicate a synergistic interaction between dopamine and SIV infection on microglia activation, leading to increased viral replication and production of neurotoxic substances. Our results suggest that increased dopamine availability through dopaminergic medication or addictive substances may potentiate HIV dementia.

Macaque animal model for HIV-induced neurological disease.

The pathogenesis of HIV-induced neurological disorders is still incompletely understood. Since many aspects of this disease are difficult to explore in humans, animal models are necessary to fill the gaps in our knowledge. Based on the high concordance with the human system, the SIV-infection of macaques currently provides the best animal model to study pathogenesis, therapy and prevention of HIV-infection. In this review, important features of the CNS-infection in this model are outlined. Recent virological, immunological, neurophysiological and neurochemical findings obtained with this animal model are presented and key factors in the development of neurological disease are identified.

Activation of microglia cells is dispensable for the induction of rat retroviral spongiform encephalopathy.

In the course of retroviral CNS infections, microglia activation has been observed frequently, and it has been hypothesized that activated microglia produce and secrete neurotoxic products like proinflammatory cytokines, by this promoting brain damage. We challenged this hypothesis in a rat model for neurodegeneration. In a kinetic study, we found that microglia cells of rats neonatally inoculated with neurovirulent murine leukemia virus (MuLV) NT40 became infected in vivo to maximal levels within 9-13 days postinoculation (d.p.i.). Beginning from 13 d.p.i., degenerative alterations, i.e., vacuolization of neurons and neuropil were found in cerebellar and other brain-stem nuclei. Elevated numbers of activated microglia cells--as revealed by immunohistochemical staining with monoclonal antibody ED1--were first detected at 19 d.p.i. and were always locally associated with degenerated areas but not with nonaltered, yet infected, brain regions. Both neuropathological changes and activated microglia cells increased in intensity and numbers, respectively, with ongoing infection but did not spread to other than initially affected brain regions. By ribonuclease protection assays, we were unable to detect differences in the expression levels of tumor-necrosis-factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), and interleukin-6 (IL-6) in microglia cells nor in total brains from infected versus uninfected rats. Our results suggest that the activation of microglia in the course of MuLV neurodegeneration is rather a reaction to, and not the cause of, neuronal damage. Furthermore, overt expression of the proinflammatory cytokines TNF-alpha, IL-1beta, and IL-6 within the CNS is not required for the induction of retroviral associated neurodegeneration in rats.

Enhancement of central nervous system pathology in early simian immunodeficiency virus infection by dopaminergic drugs.

Human immunodeficiency virus infection (HIV) at late stages of the disease is accompanied by neurological complications, including motor, behavioral and cognitive impairment. Using simian immunodeficiency virus (SIV)-infected rhesus monkeys, an animal model of HIV infection, we found that during the asymptomatic SIV infection dopamine (DA) deficits are early components of central nervous system (CNS) dysfunction. To investigate the role of the DA system in SIV infection and to restore the DA deficiency, we administered selegiline, an agent with DAergic and neuroprotective properties, to SIV-infected monkeys. Selegiline increased DA availability but induced CNS vacuolization, SIV encephalitic lesions, and enhanced CNS viral replication during early SIV infection. The pathological changes seem to be mediated by DA, as treatment with L-DOPA, the precursor of DA, had similar effects. We propose that any natural or induced DAergic dysregulation which results in increased DA availability may potentiate HIV-associated neurological disease (ND). Our findings raise new questions regarding the pathogenesis of HIV-ND and generate concerns about the safety of dopaminergic drugs in the clinical management of HIV-infected patients.

Total numbers of lymphocyte subsets in different lymph node regions of uninfected and SIV-infected macaques.

Human immunodeficiency virus (HIV) infection leads to a decline of CD4+ T-cells in blood. Because blood represents only a small proportion of the total lymphocyte pool, it is important to investigate other lymphoid organs. So far, only relative proportions of lymphocyte subsets in single peripheral lymph node (LN) regions of HIV-infected patients and simian immunodeficiency virus (SIV)-infected macaques have been documented. We have therefore quantified the absolute numbers of lymphocyte subsets in blood and six different LN regions of 10 uninfected and 26 SIV-infected macaques. In addition, we have determined the expression of markers of activation and differentiation. Already, in uninfected monkeys, there were significant differences in the cellular composition of different LN regions. Infection with SIV resulted in drastic changes in the proportion as well as absolute numbers of different lymphocyte subsets. Moreover, the relative contribution of the single LN regions to the total lymphocyte pool was also altered.

Inhibition of Ras farnesylation by lovastatin leads to downregulation of proliferation and migration in primary cultured human glioblastoma cells.

BACKGROUND: Malignant astrocytomas are the most common primary intracranial human tumors. All therapeutic approaches are limited due to their high proliferative capacity and their ability to diffusely invade the brain. Amplification of tyrosine kinase receptors and their signaling pathways have been implicated as contributing to the molecular pathogenesis of astrocytomas, providing possible new targets for therapeutic intervention. In particular, astrocytomas, although lacking oncogenic Ras mutations, have elevated levels of activated Ras. Lovastatin, an inhibitor of the beta-hydroxy-beta-methylglutary CoA reductase (HMG-CoA-reductase), is currently used to treat patients with hypercholesterolemia. In addition, it inhibits isoprenylation of several members of the Ras superfamily of proteins and therefore has multiple cellular effects including the reduction of proliferation. MATERIALS AND METHODS: In this study, we investigated the impact of lovastatin on two human glioma cell lines and on 15 primary cell cultures established from biopsies of patients with glioblastoma multiforme (GBM,) Proliferation of glioma cell lines and primary tumor cells was determined by cell counting and by using the MTT assay. The cell morphology was analyzed by staining of actin filaments with phalloidin. Apoptosis was measured using the TUNEL assay. To investigate the influence of this drug on glioma cell motility, tumor cell migration was investigated using three dimensional spheroid disintegration assays. In addition, tumor cell invasion was analyzed with a confrontational assay between tumor spheroids and rat brain aggregates. RESULTS: Inhibition of farnesyl biosynthesis using lovastatin led to a block in Ras mediated signaling, indicated by lower MAPK activity. Consequently, tumor cell proliferation was reduced up to 80%. Lovastatin appeared to decrease glioma viability by inducing apoptosis, as indicated by morphological changes and increase of TUNEL positive cells. Lovastatin acts through isoprenoid depletion, because supplementation of the media with 50-100 microM mevalonate restored all tau eta epsilon effects. Invasion of tumor cells into brain tissue was not effected while migration was reduced beta upsilon about 30-40% in cells treated with high concentrations (> or = 100 microM) of lovastatin. This was surprising because drug treatment at lower concentrations led to a disruption of the actin cytoskeleton, as indicated by Phalloidin staining. CONCLUSION: Our data strongly suggest that inhibition of elevated Ras activity by lovastatin effectively targets the MAPK and probably other signaling pathways thus offering a pharmacological based approach for a potential treatment of human astrocytomas.

Brain choline acetyltransferase reduction in SIV infection. An index of early dementia?

HIV infection at late stages is associated with neurological complications including impaired motor and cognitive functions. We used simian immunodeficiency (SIV)-infected rhesus monkeys, an animal model of HIV infection, to investigate changes in choline acetyltransferase (ChAT) activity, a biochemical marker of cognitive function, in post-mortem brains during early, asymptomatic SIV infection and AIDS. ChAT activity was dramatically reduced in putamen and hippocampus already during asymptomatic infection. In animals with AIDS, ChAT activity was further decreased. The reduction of ChAT was not related to brain viral load or CNS pathological lesions. Our results demonstrate deficits in ChAT activity already during the first months of SIV infection and imply that cognitive dysfunction may occur early in immunodeficiency viral infections.

Relationship between viral load in blood, cerebrospinal fluid, brain tissue and isolated microglia with neurological disease in macaques infected with different strains of SIV.

The role of the viral burden in the brain for the pathogenesis of human immunodeficiency virus-associated neurological disorders is still unclear. To address this issue, we have quantified the viral load in plasma, cerebrospinal fluid (CSF) and brain tissue of macaques infected with simian immunodeficiency virus (SIV). We discovered that the viral strain used for infection determines the replicative capacity in microglial cells as well as the extent of neuropathological lesions and the occurrence of neurological symptoms. Moreover, the viral load in the brain parenchyma correlated with the development of overt neurological disease whereas the one in plasma did not. By comparing the viral load in three different compartments, we demonstrated that the viral burden in the CSF is influenced both by the viral replication in the periphery as well as in the brain parenchyma. According to these results, it is not the absolute amount of viral load in the CSF but rather the viral antigen contributed by the viral production within the brain which correlates with the development of neurological disease. In longitudinal studies, we observed that this autochthonous virus production, as evidenced by a ratio of the viral load in CSF to the one in plasma, takes place for a prolonged period of time before overt neurological signs are manifested. This finding suggests that this ratio could be used as a prognostic marker for immunodeficiency virus-induced neurological disease.

Donor-derived alloantigen-presenting cells persist in the liver allograft during tolerance induction.

The predictive value of chimerism was evaluated in three different transplantation models in the rat without immunosuppression: small bowel- (SBTx), liver- (LTx), and liver/small bowel transplantation (LSBTx) were performed in the Brown Norway (BN)-to-Lewis-(LEW) strain combination. Immunohistochemistry and flow cytometry were used to identify donor cells in the recipient's spleen. Their number did not change significantly during transient rejection or tolerance after LTx and LSBTx. However, the amount of donor-derived nonparenchymal cells within the liver allograft including antigen-presenting cells (APCs), such as dendritic and Kupffer cells, clearly mirrored the recipient's immune status: as expected, their number decreased during rejection, but recovered considerably during and after tolerance induction. We conclude that donor cells in the periphery of the recipient correlate with the presence of the allograft, but do not seem to influence graft acceptance actively. However, the kinetics of the detected donor APC population in the liver suggests their important role in modifying the recipient's immune response towards tolerance.

Abundant defective viral particles budding from microglia in the course of retroviral spongiform encephalopathy.

A pathogenetic hallmark of retroviral neurodegeneration is the affinity of neurovirulent retroviruses for microglia cells, while degenerating neurons are excluded from retroviral infections. Microglia isolated ex vivo from rats peripherally infected with a neurovirulent retrovirus released abundant mature type C virions; however, infectivity associated with microglia was very low. In microglia, viral transcription was unaffected but envelope proteins were insufficiently cleaved into mature viral proteins and were not detected on the microglia cell surface. These microglia-specific defects in envelope protein translocation and processing not only may have prevented formation of infectious virus particles but also may have caused further cellular defects in microglia with the consequence of indirect neuronal damage. It is conceivable that similar events play a role in neuro-AIDS.

Alterations in neurotrophin and neurotrophin receptor gene expression patterns in the rat central nervous system following perinatal Borna disease virus infection.

Infection of newborn rats with Borna disease virus (BDV) leads to persistence in the absence of overt signs of inflammation. BDV persistence, however, causes cerebellar hypoplasia and hippocampal dentate gyrus neuronal cell loss, which are accompanied by diverse neurobehavioral abnormalities. Neurotrophins and their receptors play important roles in the differentiation and survival of hippocampal and cerebellar neurons. We have examined whether BDV can cause alterations in the neurotrophin network, thus promoting neuronal damage. We have used RNase protection assay to measure mRNA levels of the neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3), and their trkC and trkB receptors, as well as the growth factors insulin-like growth factor I (IGF-1) and basic fibroblast growth factor (bFGF), in the cerebellum and hippocampus of BDV-infected and control rats at different time points p.i. Reduced mRNA expression levels of NT-3, BDNF and NGF were found after day 14 p.i. in the hippocampus, but not in the cerebellum, of newborn infected rats. Three weeks after infection, trkC mRNA expression levels were reduced in both hippocampus and cerebellum of infected rats, whereas decreased trkB mRNA levels were only observed in the cerebellum. Reduced trkC mRNA expression was confined to the dentate gyrus of the hippocampus, as assessed by in situ hybridization. TUNEL assay revealed massive apoptotic cell death in the dentate gyrus of infected rats at days 27 and 33 p.i. Increased numbers of apoptotic cells were also detected in the cerebellar granular layer of infected rats after 8 days p.i. Moreover, a dramatic loss of cerebellar Purkinje cells was seen after day 27 p.i. Our results support the hypothesis, that BDV-induced alterations in neurotrophin systems might contribute to selective neuronal cell death.

Effects of Selegiline in a retroviral rat model for neurodegenerative disease.

Upon inoculation into neonatal rats, murine leukemia virus (MuLV) NT40 causes a non-inflammatory degeneration of the central nervous system. While microglia cells appear to be the major target cells within the brain parenchyma for neurovirulent MuLV, degenerating neurons do not express retroviral gene products. In order to protect rats from neuronal damage we treated retrovirally infected rats once with monoamine oxidase (MAO) B inhibitor Selegiline which--under different conditions--exerts neuroprotective effects. Unexpectedly, when administered at 17 days post-infection (d.p.i.) a single intraperitoneal dose of Selegilin (1 mg/kg bodyweight) significantly shortened the incubation period for neurological disease. In contrast, Selegiline given in a lower dosage (0.05 mg/kg bodyweight) and/or at a different time point (13 d.p.i.) at the low (0.05 mg/kg bodyweight) and the high dose (1.0 mg/kg bodyweight) had no effect on the outcome of neurological disease. Animals treated with Selegiline (1.0 mg/kg bodyweight at 17 d.p.i.) contained higher amounts of viral loads in the CNS, higher numbers of brain cells expressing major histocompatibility complex class II molecules, and exhibited inhibition of MAO-B in comparison to untreated yet infected (control) animals. Supposedly, Selegiline activated the major target cell population of the CNS for MuLV-NT40, microglia, with the consequence of enhanced susceptibility for retroviral infection and triggered endogenous mechanism(s) involved in the pathogenesis of retroviral neurodegeneration.

Modulation of acute coronavirus-induced encephalomyelitis in gamma-irradiated rats by transfer of naive lymphocyte subsets before infection.

Clinical course, recovery of infectious virus from brain tissue and histopathology of the central nervous system were examined in gamma-irradiated Lewis rats reconstituted by naive lymphocytes before infection with coronavirus MHV-4 (strain JHM). Up to 9 days past infection, no differences were seen between immunologically competent and immuno-deficient animals in terms of onset and progression of neurological disease. However, in the latter animals neurological symptoms were dominated by signs of encephalitis instead of paralytic disease as usually seen in immunocompetent animals. Nevertheless, despite high titers of infectious virus in the CNS of immunodeficient animals only mild histopathological changes were noticeable. In contrast, infectious virus in the CNS of immunologically competent animals was below the detection limit of the assay. Paralytic disease and tissue destruction were T lymphocyte mediated because gamma-irradiated rats that were reconstituted by CD4+ or CD8+ T lymphocyte enriched cells in the absence of B lymphocytes revealed an earlier onset of clinical symptoms and a more rapid deterioration of their clinical state compared to fully competent animals. Whereas in CD4+ T cell reconstituted animals infectious virus was moderately reduced and tissue destruction as well as inflammatory changes in the CNS were focal, in CD8+ T cell reconstituted animals vacuolizing white matter inflammation was diffuse without reduction of infectious virus in brain tissue. From the presented data we conclude that in the acute stage of JHMV-induced encephalomyelitis of Lewis rats: (i) tissue destruction and paralytic clinical symptomatology are mainly T cell-mediated; (ii) CD4+ T lymphocytes can directly contribute to reduction of viral load in the brain and (iii) only coordinated action of both, the T and the B cell compartment enables animals to survive the infection and recover from disease.

Microglial/macrophage expression of interleukin 10 in human glioblastomas.

Interleukin 10 (IL-10) expression has been found to be correlated with the extent of malignancy in gliomas. In vitro, IL-10 increases proliferation and migratory capacity in human glioma cell lines. In this study, we localized the site of IL-10 synthesis in gliomas to cells of microglial origin. Biopsy specimens from 11 patients with malignant glioma were processed on native tissues and at early cell culture passages (0-4). IL-10 mRNA was analyzed by RT-PCR and in situ hybridization. Protein was quantitatively assessed by ELISA in cell culture supernatants, and cells expressing IL-10 were determined by a combination of immunohistochemistry for CD68 (specific for microglia/macrophage lineage) and IL-10 in situ hybridization. IL-10 mRNA decreased from passage 0 to 4 in all samples and was undetectable beyond passage 5. Such downregulation of mRNA leads to a steep decrease of IL-10 protein in culture supernatants (below detection level, 0.05 ng/ml, beyond passage 1). The combination of in situ hybridization for IL-10 and CD68 immunostaining revealed that only cells of the microglia/macrophage lineage produced IL-10 mRNA. Our results identify microglia/macrophage cells as the major source of IL-10 expression in gliomas which decreases markedly during early passages of primary cultures of human gliomas due to a progressive reduction of microglia/macrophages present.

Apoptosis of T lymphocytes in liver and/or small bowel allografts during tolerance induction.

BACKGROUND: Apoptosis of parenchymal cells has been described during allograft rejection. Immunologically privileged tissue in the mouse has been found to prevent rejection by initiating apoptosis of infiltrating lymphocytes. The aim of this study was to investigate whether apoptosis may play a role in T-cell regulation during rejection and subsequent tolerance induction after liver transplantation (LTx) and combined liver/small bowel transplantation (LSBTx). METHODS: LTx and LSBTx (Brown Norway-->Lewis) were performed without immunosuppression. Cell migration, activation, and apoptosis were investigated by means of sequential histology, immunohistochemistry, and the terminal deoxynucleotidyl transferase-mediated dUTP-digoxigenin nick end labeling assay. Donor (Brown Norway) and third-party (Dark Agouti) cardiac allografts were transplanted into LSBTx recipients to determine specific tolerance. RESULTS: Transient acute cellular rejection occurred after LTx and LSBTx and was followed by specific tolerance. The kinetics of apoptosis were similar in liver allografts after LTx and LSBTx, but differed from the processes in small bowel allografts after LSBTx. Apoptosis of parenchymal cells in the grafted livers correlated directly with interleukin-2 receptor expression of the infiltrating T cells. During the late phase of rejection, a peak of apoptosis in the lymphocyte infiltrate was demonstrated, characterized as predominantly apoptotic CD8+ T lymphocytes. CONCLUSIONS: These results demonstrate that specific tolerance is achieved in both LTx and LSBTx after a transient rejection crisis. Apoptosis is involved in graft rejection and tolerance induction. Activation of T lymphocytes correlates with parenchymal cell apoptosis in the allograft. T-cell inactivation seems to result in apoptosis of cytotoxic T cells and tolerance, which appears to be unique to the liver allograft.