Abnormal polyamine metabolism is unique to the neuropathic forms of MPS: potential for biomarker development and insight into pathogenesis.

The mucopolysaccharidoses (MPS) are rare genetic disorders marked by severe somatic and neurological symptoms. Development of treatments for the neurological manifestations of MPS has been hindered by the lack of objective measures of central nervous system disease burden. Identification of biomarkers for central nervous system disease in MPS patients would facilitate the evaluation of new agents in clinical trials. High throughput metabolite screening of cerebrospinal fluid (CSF) samples from a canine model of MPS I revealed a marked elevation of the polyamine, spermine, in affected animals, and gene therapy studies demonstrated that reduction of CSF spermine reflects correction of brain lesions in these animals. In humans, CSF spermine was elevated in neuropathic subtypes of MPS (MPS I, II, IIIA, IIIB), but not in subtypes in which cognitive function is preserved (MPS IVA, VI). In MPS I patients, elevated CSF spermine was restricted to patients with genotypes associated with CNS disease and was reduced following hematopoietic stem cell transplantation, which is the only therapy currently capable of improving cognitive outcomes. Additional studies in cultured neurons from MPS I mice showed that elevated spermine was essential for the abnormal neurite overgrowth exhibited by MPS neurons. These findings offer new insights into the pathogenesis of CNS disease in MPS patients, and support the use of spermine as a new biomarker to facilitate the development of next generation therapeutics for MPS.

Neuroinflammatory targets and treatments for epilepsy validated in experimental models.

A large body of evidence that has accumulated over the past decade strongly supports the role of inflammation in the pathophysiology of human epilepsy. Specific inflammatory molecules and pathways have been identified that influence various pathologic outcomes in different experimental models of epilepsy. Most importantly, the same inflammatory pathways have also been found in surgically resected brain tissue from patients with treatment-resistant epilepsy. New antiseizure therapies may be derived from these novel potential targets. An essential and crucial question is whether targeting these molecules and pathways may result in anti-ictogenesis, antiepileptogenesis, and/or disease-modification effects. Therefore, preclinical testing in models mimicking relevant aspects of epileptogenesis is needed to guide integrated experimental and clinical trial designs. We discuss the most recent preclinical proof-of-concept studies validating a number of therapeutic approaches against inflammatory mechanisms in animal models that could represent novel avenues for drug development in epilepsy. Finally, we suggest future directions to accelerate preclinical to clinical translation of these recent discoveries.

Liver-directed gene therapy corrects cardiovascular lesions in feline mucopolysaccharidosis type I.

Patients with mucopolysaccharidosis type I (MPS I), a genetic deficiency of the lysosomal enzyme α-l-iduronidase (IDUA), exhibit accumulation of glycosaminoglycans in tissues, with resulting diverse clinical manifestations including neurological, ocular, skeletal, and cardiac disease. MPS I is currently treated with hematopoietic stem cell transplantation or weekly enzyme infusions, but these therapies have significant drawbacks for patient safety and quality of life and do not effectively address some of the most critical clinical sequelae, such as life-threatening cardiac valve involvement. Using the naturally occurring feline model of MPS I, we tested liver-directed gene therapy as a means of achieving long-term systemic IDUA reconstitution. We treated four MPS I cats at 3-5 mo of age with an adeno-associated virus serotype 8 vector expressing feline IDUA from a liver-specific promoter. We observed sustained serum enzyme activity for 6 mo at ∼ 30% of normal levels in one animal, and in excess of normal levels in three animals. Remarkably, treated animals not only demonstrated reductions in glycosaminoglycan storage in most tissues, but most also exhibited complete resolution of aortic valve lesions, an effect that has not been previously observed in this animal model or in MPS I patients treated with current therapies. These data point to clinically meaningful benefits of the robust enzyme expression achieved with hepatic gene transfer that extend beyond the economic and quality of life advantages over lifelong enzyme infusions.

Neonatal tolerance induction enables accurate evaluation of gene therapy for MPS I in a canine model.

High fidelity animal models of human disease are essential for preclinical evaluation of novel gene and protein therapeutics. However, these studies can be complicated by exaggerated immune responses against the human transgene. Here we demonstrate that dogs with a genetic deficiency of the enzyme α-l-iduronidase (IDUA), a model of the lysosomal storage disease mucopolysaccharidosis type I (MPS I), can be rendered immunologically tolerant to human IDUA through neonatal exposure to the enzyme. Using MPS I dogs tolerized to human IDUA as neonates, we evaluated intrathecal delivery of an adeno-associated virus serotype 9 vector expressing human IDUA as a therapy for the central nervous system manifestations of MPS I. These studies established the efficacy of the human vector in the canine model, and allowed for estimation of the minimum effective dose, providing key information for the design of first-in-human trials. This approach can facilitate evaluation of human therapeutics in relevant animal models, and may also have clinical applications for the prevention of immune responses to gene and protein replacement therapies.

Neonatal Systemic AAV Induces Tolerance to CNS Gene Therapy in MPS I Dogs and Nonhuman Primates.

The potential host immune response to a nonself protein poses a fundamental challenge for gene therapies targeting recessive diseases. We demonstrate in both dogs and nonhuman primates that liver-directed gene transfer using an adeno-associated virus (AAV) vector in neonates induces a persistent state of immunological tolerance to the transgene product, substantially improving the efficacy of subsequent vector administration targeting the central nervous system (CNS). We applied this approach to a canine model of mucopolysaccharidosis type I (MPS I), a progressive neuropathic lysosomal storage disease caused by deficient activity of the enzyme α-l-iduronidase (IDUA). MPS I dogs treated systemically in the first week of life with a vector expressing canine IDUA did not develop antibodies against the enzyme and exhibited robust expression in the CNS upon intrathecal AAV delivery at 1 month of age, resulting in complete correction of brain storage lesions. Newborn rhesus monkeys treated systemically with AAV vector expressing human IDUA developed tolerance to the transgene, resulting in high cerebrospinal fluid (CSF) IDUA expression and no antibody induction after subsequent CNS gene therapy. These findings suggest that inducing tolerance to the transgene product during a critical period in immunological development can improve the efficacy and safety of gene therapy.

Intrathecal gene therapy corrects CNS pathology in a feline model of mucopolysaccharidosis I.

Enzyme replacement therapy has revolutionized the treatment of the somatic manifestations of lysosomal storage diseases (LSD), although it has been ineffective in treating central nervous system (CNS) manifestations of these disorders. The development of neurotrophic vectors based on novel serotypes of adeno-associated viruses (AAV) such as AAV9 provides a potential platform for stable and efficient delivery of enzymes to the CNS. We evaluated the safety and efficacy of intrathecal delivery of AAV9 expressing α-l-iduronidase (IDUA) in a previously described feline model of mucopolysaccharidosis I (MPS I). A neurological phenotype has not been defined in these animals, so our analysis focused on the biochemical and histological CNS abnormalities characteristic of MPS I. Five MPS I cats were dosed with AAV9 vector at 4-7 months of age and followed for 6 months. Treated animals demonstrated virtually complete correction of biochemical and histological manifestations of the disease throughout the CNS. There was a range of antibody responses against IDUA in this cohort which reduced detectable enzyme without substantially reducing efficacy; there was no evidence of toxicity. This first demonstration of the efficacy of intrathecal gene therapy in a large animal model of a LSD should pave the way for translation into the clinic.

Liver surface depressions in the presence of diaphragmatic muscular bands on trans-illumination.

Traditional descriptions of liver anatomy refer to a smooth, convex surface contacting the diaphragm. Surface depressions are recognized anatomic variants. There are many theories to explain the cause of the depressions. We discuss the theory that these are caused by hypertrophic muscular bands in the diaphragm.

Efficient CNS gene delivery by intravenous injection.

We administered recombinant SV40-derived viral vectors (rSV40s) intravenously to mice with or without prior intraperitoneal injection of mannitol to deliver transgenes to the central nervous system (CNS). We detected transgene-expressing cells (mainly neurons) most prominently in the cortex and spinal cord; prior intraperitoneal mannitol injection increased CNS gene delivery tenfold. Intravenous injection of rSV40s, particularly with mannitol pretreatment, resulted in extensive expression of multiple transgenes throughout the CNS.

HIV-1 Tat neurotoxicity: a model of acute and chronic exposure, and neuroprotection by gene delivery of antioxidant enzymes.

HIV-associated neurocognitive disorder (HAND) is an increasingly common, progressive disease characterized by neuronal loss and progressively deteriorating CNS function. HIV-1 gene products, particularly gp120 and Tat elicit reactive oxygen species (ROS) that lead to oxidant injury and cause neuron apoptosis. Understanding of, and developing therapies for, HAND requires accessible models of the disease. We have devised experimental approaches to studying the acute and chronic effects of Tat on the CNS. We studied acute exposure by injecting recombinant Tat protein into the caudate-putamen (CP). Ongoing Tat expression, which more closely mimics HIV-1 infection of the brain, was studied by delivering Tat-expression over time using an SV40-derived gene delivery vector, SV(Tat). Both acute and chronic Tat exposure induced lipid peroxidation and neuronal apoptosis. Finally, prior administration of recombinant SV40 vectors carrying antioxidant enzymes, copper/zinc superoxide dismutase (SOD1) or glutathione peroxidase (GPx1), protected from Tat-induced apoptosis and oxidative injury. Thus, injection of recombinant HIV-1 Tat and the expression vector, SV(Tat), into the rat CP cause respectively acute or ongoing apoptosis and oxidative stress in neurons and may represent useful animal models for studying the pathogenesis and, potentially, treatment of HIV-1 Tat-related damage.

Role of CCR5 and its ligands in the control of vascular inflammation and leukocyte recruitment required for acute excitotoxic seizure induction and neural damage.

Chemokines may play a role in leukocyte migration across the blood-brain barrier (BBB) during neuroinflammation and other neuropathological processes, such as epilepsy. We investigated the role of the chemokine receptor CCR5 in seizures. We used a rat model based on intraperitoneal kainic acid (KA) administration. Four months before KA injection, adult rats were given femoral intramarrow inoculations of SV (RNAiR5-RevM10.AU1), which carries an interfering RNA (RNAi) against CCR5, plus a marker epitope (AU1), or its monofunctional RNAi-carrying homologue, SV(RNAiR5). This treatment lowered expression of CCR5 in circulating cells. In control rats, seizures induced elevated expression of CCR5 ligands MIP-1α and RANTES in the microvasculature, increased BBB leakage and CCR5(+) cells, as well as neuronal loss, inflammation, and gliosis in the hippocampi. Animals given either the bifunctional or the monofunctional vector were largely protected from KA-induced seizures, neuroinflammation, BBB damage, and neuron loss. Brain CCR5 mRNA was reduced. Rats receiving RNAiR5-bearing vectors showed far greater repair responses: increased neuronal proliferation, and decreased production of MIP-1α and RANTES. Controls received unrelated SV(BUGT) vectors. Decrease in CCR5 in circulating cells strongly protected from excitotoxin-induced seizures, BBB leakage, CNS injury, and inflammation, and facilitated neurogenic repair.

Blood-brain barrier abnormalities caused by HIV-1 gp120: mechanistic and therapeutic implications.

The blood-brain barrier (BBB) is compromised in many systemic and CNS diseases, including HIV-1 infection of the brain. We studied BBB disruption caused by HIV-1 envelope glycoprotein 120 (gp120) as a model. Exposure to gp120, whether acute [by direct intra-caudate-putamen (CP) injection] or chronic [using SV(gp120), an experimental model of ongoing production of gp120] disrupted the BBB, and led to leakage of vascular contents. Gp120 was directly toxic to brain endothelial cells. Abnormalities of the BBB reflect the activity of matrix metalloproteinases (MMPs). These target laminin and attack the tight junctions between endothelial cells and BBB basal laminae. MMP-2 and MMP-9 were upregulated following gp120-injection. Gp120 reduced laminin and tight junction proteins. Reactive oxygen species (ROS) activate MMPs. Injecting gp120 induced lipid peroxidation. Gene transfer of antioxidant enzymes protected against gp120-induced BBB abnormalities. NMDA upregulates the proform of MMP-9. Using the NMDA receptor (NMDAR-1) inhibitor, memantine, we observed partial protection from gp120-induced BBB injury. Thus, (1) HIV-envelope gp120 disrupts the BBB; (2) this occurs via lesions in brain microvessels, MMP activation and degradation of vascular basement membrane and vascular tight junctions; (3) NMDAR-1 activation plays a role in this BBB injury; and (4) antioxidant gene delivery as well as NMDAR-1 antagonists may protect the BBB.

HIV-1 gp120 upregulates matrix metalloproteinases and their inhibitors in a rat model of HIV encephalopathy.

Matrix metalloproteinases (MMPs) are implicated in diverse processes, such as neuroinflammation, leakiness of the blood-brain barrier (BBB) and direct cellular damage in neurodegenerative and other CNS diseases. Tissue destruction by MMPs is regulated by their endogenous tissue inhibitors (TIMPs). TIMPs prevent excessive MMP-related degradation of extracellular matrix components. In a rat model of human immunodeficiency virus (HIV)-related encephalopathy, we described MMP-2 and MMP-9 upregulation by HIV-1 envelope gp120, probably via gp120-induced reactive oxygen species. Antioxidant gene delivery blunted gp120-induced MMP production. We also studied the effect of gp120 on TIMP-1 and TIMP-2 production. TIMP-1 and TIMP-2 levels increased 6 h after gp120 injection into rat caudate-putamen (CP). TIMP-1 and TIMP-2 colocalized mainly with neurons (92 and 95%, respectively). By 24 h, expression of these protease inhibitors diverged, as TIMP-1 levels remained high but TIMP-2 subsided. Gene delivery of the antioxidant enzymes Cu/Zn superoxide dismutase or glutathione peroxidase into the CP before injecting gp120 there reduced levels of gp120-induced TIMP-1 and TIMP-2, recapitulating the effect of antioxidant enzymes on gp120-induced MMP-2 and MMP-9. A significant correlation was observed between MMP/TIMP upregulation and BBB leakiness. Thus, HIV-1 gp120 upregulated TIMP-1 and TIMP-2 in the CP. Prior antioxidant enzyme treatment mitigated production of these TIMPs, probably by reducing MMP expression.

Blood-brain barrier abnormalities caused by exposure to HIV-1 gp120--protection by gene delivery of antioxidant enzymes.

HIV-1 effects on the blood-brain barrier (BBB) structure and function are still poorly understood in animal models based on direct administration of recombinant HIV proteins. We therefore injected HIV-1 envelope glycoprotein, gp120, into rat caudate-putamens (CPs) and examined vascular integrity and function. Gp120 coimmunostained with endothelial cell marker, CD31. It induced apoptosis of endothelial cells in vitro and in vivo. BBB function was assessed by administering Evans Blue (EB) intravenously before injecting gp120. EB leaked near the site of gp120 administration. Within 1h after intra-CP gp120 injection, structures positive for endothelial markers ICAM-1 and RECA-1 were greatly decreased. Vascular density assessed by laminin immunostaining remained decreased 1 month after gp120 injection. RECA-1-positive cells expressed hydroxynonenal, a marker of lipid peroxidation and rSV40-mediated gene delivery of antioxidant enzymes protected the BBB from gp120-related injury. Extravasated IgG accumulated following intra-CP SV(gp120) injection, an experimental model of continuing gp120 exposure. Thus: acute and chronic exposure to gp120 disrupts the BBB; gp120-mediated BBB abnormalities are related to lesions of brain microvessels; and gp120 is directly toxic to brain endothelial cells.

Gene transfer to the cerebellum.

There are several diseases for which gene transfer therapy to the cerebellum might be practicable. In these studies, we used recombinant Tag-deleted SV40-derived vectors (rSV40s) to study gene delivery targeting the cerebellum. These vectors transduce neurons and microglia very effectively in vitro and in vivo, and so we tested them to evaluate gene transfer to the cerebellum in vivo. Using a rSV40 vector carrying human immunodeficiency virus (HIV)-Nef with a C-terminal FLAG epitope, we characterized the distribution, duration, and cell types transduced. Rats received test and control vectors by stereotaxic injection into the cerebellum. Transgene expression was assessed 1, 2, and 4 weeks later by immunostaining of serial brain sections. FLAG epitope-expressing cells were seen, at all times after vector administration, principally detected in the Purkinje cells of the cerebellum, identified as immunopositive for calbindin. Occasional microglial cells were tranduced; transgene expression was not detected in astrocytes or oligodendrocytes. No inflammatory or other reaction was detected at any time. Thus, SV40-derived vectors can deliver effective, safe, and durable transgene expression to the cerebellum.

A rat model of human immunodeficiency virus 1 encephalopathy using envelope glycoprotein gp120 expression delivered by SV40 vectors.

Human immunodeficiency virus 1 (HIV-1) encephalopathy is thought to result in part from the toxicity of HIV-1 envelope glycoprotein gp120 for neurons. Experimental systems for studying the effects of gp120 and other HIV proteins on the brain have been limited to the acute effects of recombinant proteins in vitro or in vivo in simian immunodeficiency virus-infected monkeys. We describe an experimental rodent model of ongoing gp120-induced neurotoxicity in which HIV-1 envelope is expressed in the brain using an SV40-derived gene delivery vector, SV(gp120). When it is inoculated stereotaxically into the rat caudate putamen, SV(gp120) caused a partly hemorrhagic lesion in which neuron and other cell apoptosis continues for at least 12 weeks. Human immunodeficiency virus gp120 is expressed throughout this time, and some apoptotic cells are gp120 positive. Malondialdehyde and 4-hydroxynonenal assays indicated that there was lipid peroxidation in these lesions. Prior administration of recombinant SV40 vectors carrying antioxidant enzymes, copper/ zinc superoxide dismutase or glutathione peroxidase, was protective against SV(gp120)-induced oxidative injury and apoptosis. Thus, in vivo inoculation of SV(gp120) into the rat caudate putamen causes ongoing oxidative stress and apoptosis in neurons and may therefore represent a useful animal model for studying the pathogenesis and treatment of HIV-1 envelope-related brain damage.

HIV-1 gp120 neurotoxicity proximally and at a distance from the point of exposure: protection by rSV40 delivery of antioxidant enzymes.

Toxicity of HIV-1 envelope glycoprotein (gp120) for substantia nigra (SN) neurons may contribute to the Parkinsonian manifestations often seen in HIV-1-associated dementia (HAD). We studied the neurotoxicity of gp120 for dopaminergic neurons and potential neuroprotection by antioxidant gene delivery. Rats were injected stereotaxically into their caudate-putamen (CP); CP and (substantia nigra) SN neuron loss was quantified. The area of neuron loss extended several millimeters from the injection site, approximately 35% of the CP area. SN neurons, outside of this area of direct neurotoxicity, were also severely affected. Dopaminergic SN neurons (expressing tyrosine hydroxylase, TH, in the SN and dopamine transporter, DAT, in the CP) were mostly affected: intra-CP gp120 caused approximately 50% DAT+ SN neuron loss. Prior intra-CP gene delivery of Cu/Zn superoxide dismutase (SOD1) or glutathione peroxidase (GPx1) protected SN neurons from intra-CP gp120. Thus, SN dopaminergic neurons are highly sensitive to HIV-1 gp120-induced neurotoxicity, and antioxidant gene delivery, even at a distance, is protective.

Safe and Sustained Expression of Human Iduronidase After Intrathecal Administration of Adeno-Associated Virus Serotype 9 in Infant Rhesus Monkeys.

Many neuropathic diseases cause early, irreversible neurologic deterioration, which warrants therapeutic intervention during the first months of life. In the case of mucopolysaccharidosis type I, a recessive lysosomal storage disorder that results from a deficiency of the lysosomal enzyme α-l-iduronidase (IDUA), one of the most promising treatment approaches is to restore enzyme expression through gene therapy. Specifically, administering pantropic adeno-associated virus (AAV) encoding IDUA into the cerebrospinal fluid (CSF) via suboccipital administration has demonstrated remarkable efficacy in large animals. Preclinical safety studies conducted in adult nonhuman primates supported a positive risk-benefit profile of the procedure while highlighting potential subclinical toxicity to primary sensory neurons located in the dorsal root ganglia (DRG). This study investigated the long-term performance of intrathecal cervical AAV serotype 9 gene transfer of human IDUA administered to 1-month-old rhesus monkeys (N = 4) with half of the animals tolerized to the human transgene at birth via systemic administration of an AAV serotype 8 vector expressing human IDUA from the liver. Sustained expression of the transgene for almost 4 years is reported in all animals. Transduced cells were primarily pyramidal neurons in the cortex and hippocampus, Purkinje cells in the cerebellum, lower motor neurons, and DRG neurons. Both tolerized and non-tolerized animals were robust and maintained transgene expression as measured by immunohistochemical analysis of brain tissue. However, the presence of antibodies in the non-tolerized animals led to a loss of measurable levels of secreted enzyme in the CSF. These results support the safety and efficiency of treating neonatal rhesus monkeys with AAV serotype 9 gene therapy delivered into the CSF.

Evaluation of Intrathecal Routes of Administration for Adeno-Associated Viral Vectors in Large Animals.

Delivery of adeno-associated viral (AAV) vectors into the cerebrospinal fluid (CSF) can achieve gene transfer to cells throughout the brain and spinal cord, potentially making many neurological diseases tractable gene therapy targets. Identifying the optimal route of CSF access for intrathecal AAV delivery will be a critical step in translating this approach to clinical practice. We previously demonstrated that vector injection into the cisterna magna is a safe and effective method for intrathecal AAV delivery in nonhuman primates; however, this procedure is not commonly used in clinical practice. More routine methods of administration into the CSF are now being explored, including intracerebroventricular (ICV) injection and injection through a lumbar puncture. In this study, we compared ICV and intracisternal (IC) AAV administration in dogs. We also evaluated vector administration via lumbar puncture in nonhuman primates, with some animals placed in the Trendelenburg position after injection, a maneuver that has been suggested to improve cranial distribution of vector. In the dog study, ICV and IC vector administration resulted in similarly efficient transduction throughout the brain and spinal cord. However, animals in the ICV cohort developed encephalitis associated with a T-cell response to the transgene product, a phenomenon that was not observed in the IC cohort. In the nonhuman primate study, transduction efficiency was not improved by placing animals in the Trendelenburg position after injection. These findings illustrate important limitations of commonly used methods for CSF access in the context of AAV delivery, and will be important for informing the selection of a route of administration for first-in-human studies.

The p38 mitogen-activated protein kinase signaling pathway is coupled to Toll-like receptor 5 to mediate gene regulation in response to Pseudomonas aeruginosa infection in human airway epithelial cells.

In this study, we show that stimulation of human airway epithelial cells (HAECs) by Pseudomonas aeruginosa strain PAO1 induces time- and dose-dependent activation of p38 mitogen-activated protein kinase (MAPK). Activated p38 MAPK stayed in the cytoplasm instead of translocating to the nucleus, as shown by cellular fractionation. p38 MAPK was activated when HAECs were incubated with P. aeruginosa strain PAK and Burkholderia cepacia, while little activation was observed with the isogenic flagellin-free strains PAK/fliC and B. cepacia BC/fliC. The presence of Toll-like receptor 5 (TLR5) in 293 cells mediated PAO1-dependent activation of p38 MAPK, and in HAECs p38 MAPK activation was blocked by the overexpression of a dominant negative TLR5. Two inhibitors of p38 MAPK, SB202190 and SB203580, significantly attenuated PAO1-dependent expression of an NF-kappaB-dependent luciferase reporter gene, suggesting that p38 MAPK activation is required for full activation of NF-kappaB-dependent signaling. Microarray analysis of NF-kappaB target genes revealed up-regulation of multiple genes by PAO1 in HAECs. Reverse transcription-PCR and protein expression analysis were used to show that up-regulation of NF-kappaB-dependent genes induced by PAO1, such as the genes encoding Cox-2 and interleukin-8, was attenuated by SB203580. These results demonstrate a role for p38 MAPK signaling in gene regulation in response to P. aeruginosa via TLR5.

Preterminal host dendritic cells in irradiated mice prime CD8+ T cell-mediated acute graft-versus-host disease.

To understand the relationship between host antigen-presenting cells (APCs) and donor T cells in initiating graft-versus-host disease (GVHD), we followed the fate of host dendritic cells (DCs) in irradiated C57BL/6 (B6) recipient mice and the interaction of these cells with minor histocompatibility antigen- (miHA-) mismatched CD8+ T cells from C3H.SW donors. Host CD11c+ DCs were rapidly activated and aggregated in the T cell areas of the spleen within 6 hours of lethal irradiation. By 5 days after irradiation, <1% of host DCs were detectable, but the activated donor CD8+ T cells had already undergone as many as seven divisions. Thus, proliferation of donor CD8+ T cells preceded the disappearance of host DCs. When C3H.SW donor CD8+ T cells were primed in vivo in irradiated B6 mice or ex vivo by host CD11c+ DCs for 24-36 hours, they were able to proliferate and differentiate into IFN-gamma-producing cells in beta(2)-microglobulin-deficient (beta(2)m(-/-)) B6 recipients and to mediate acute GVHD in beta(2)m(-/-) --> B6 chimeric mice. These results indicate that, although host DCs disappear rapidly after allogeneic bone marrow transplantation, they prime donor T cells before their disappearance and play a critical role in triggering donor CD8+ T cell-mediated GVHD.