Complete Metabolic Response to Combined Immune Checkpoint Inhibition after Progression of Metastatic Colorectal Cancer on Pembrolizumab: A Case Report.

DNA mismatch repair deficient (dMMR) and microsatellite instable (MSI) metastatic colorectal cancer (mCRC) can be successfully treated with FDA- and EMA-approved immune checkpoint inhibitors (ICI) pembrolizumab and nivolumab (as single agents targeting the anti-programmed cell death protein-1 (PD-1)) or combinations of a PD-1 inhibitor with ipilimumab, a cytotoxic T-lymphocyte-associated protein 4 (CTLA-4)-targeting antibody. The best treatment strategy beyond progression on single-agent ICI therapy remains unclear. Here, we present the case of a 63-year-old male with Lynch-syndrome-associated, microsatellite instability-high (MSI-H) mCRC who achieved a rapid normalization of his tumor markers and a complete metabolic remission (CMR), currently lasting for ten months, on sequential ICI treatment with the combination of nivolumab and ipilimumab followed by nivolumab maintenance therapy after progression on single-agent anti-PD-1 ICI therapy. The therapy was well-tolerated, and no immune-related adverse events occurred. To the best of our knowledge, this is the first case of a sustained metabolic complete remission in an MSI-H mCRC patient initially progressing on single-agent anti-PD-1 therapy. Thus, dMMR mCRC patients might benefit from sequential immune checkpoint regimens even with long-term responses. However, further sophistication of clinical algorithms for treatment beyond progression on single-agent ICI therapy in MSI-mCRC is urgently needed.

[Risk Factors for Permanent Postoperative Hypoparathyreoidism]. / Risikofaktoren für den permanenten postoperativen Hypoparathyreoidismus.

BACKGROUND: Permanent postoperative hypoparathyreoidism remains the most frequent complication after total thyreoidectomy with severe long-term morbidity, impairment of quality of life and economic implications. OBJECTIVE: Identification of risk factors for permanent postoperative hypoparathyreoidism. MATERIAL AND METHODS: 420 patients received total thyreoidectomy in our endocrine centre between 08/2012 und 08/2014, of whom 382 were included in the study. 117 patients underwent a follow-up investigation between 8 and 32 months postoperatively. RESULTS: We determined a low parathyroid hormone level on postoperative day 1, non-application of a drain and a prolonged postoperative hospital stay as being associated with permanent postoperative hypoparathyreoidism. No association was found between postoperative hypoparathyroidism and autotransplantation of a parathyroid gland. Associations were strong but not significant. DISCUSSION: We identified associated factors for permanent postoperative hypoparathyreoidism. Larger multicentre studies should be performed for validation as possible relevant risk factors. Knowledge of risk factors might help to avoid this complication and to manage close follow-up and therapy in patients affected.

Oral tolerance originates in the intestinal immune system and relies on antigen carriage by dendritic cells.

Oral tolerance induction is a key feature of intestinal immunity, generating systemic nonresponsiveness to ingested antigens. In this study, we report that orally applied soluble antigens are exclusively recognized in the intestinal immune system, particularly in the mesenteric lymph nodes. Consequently, the initiation of oral tolerance is impeded by mesenteric lymphadenectomy. Small bowel transplantation reveals that mesenteric lymph nodes require afferent lymph to accomplish the recognition of orally applied antigens. Finally, oral tolerance cannot be induced in CCR7-deficient mice that display impaired migration of dendritic cells from the intestine to the mesenteric lymph nodes, suggesting that immunologically relevant antigen is transported in a cell-bound fashion. These results demonstrate that antigen transport via afferent lymphatics into the draining mesenteric lymph nodes is obligatory for oral tolerance induction, inspiring new therapeutic strategies to exploit oral tolerance induction for the prevention and treatment of autoimmune diseases.

The circadian rhythm of glucocorticoids is regulated by a gating mechanism residing in the adrenal cortical clock.

In mammals, the master clock of the suprachiasmatic nuclei (SCN) and subordinate clocks found throughout the body coordinate circadian rhythms of behavior and physiology. We characterize the clock of the adrenal, an important endocrine gland that synchronizes physiological and metabolic rhythms. Clock gene expression was detected in the outer adrenal cortex prefiguring a role of the clock in regulating gluco- and mineral corticoid biogenesis. In Per2/Cry1 double mutant mice, which lack a circadian clock, hypothalamus/pituitary/adrenal axis regulation was defective. Organ culture and tissue transplantation suggest that the adrenal pacemaker gates glucocorticoid production in response to adrenocorticotropin (ACTH). In vivo the adrenal circadian clock can be entrained by light. Transcriptome profiling identified rhythmically expressed genes located at diverse nodes of steroid biogenesis that may mediate gating of the ACTH response by the adrenal clock.

Chemokine receptor CXCR5 supports solitary intestinal lymphoid tissue formation, B cell homing, and induction of intestinal IgA responses.

Solitary intestinal lymphoid tissue (SILT) comprises a spectrum of phenotypically diverse lymphoid aggregates interspersed throughout the small intestinal mucosa. Manifestations of SILT range from tiny lymphoid aggregates almost void of mature lymphocytes to large structures dominated by B cells. Large SILT phenotypically resemble a single Peyer's patch follicle, suggesting that SILT might contribute to intestinal humoral immune responses. In this study, we track the fate of individual SILT in vivo over time and analyze SILT formation and function in chemokine receptor CXCR5-deficient mice. We show that, in analogy to Peyer's patches, formation of SILT is invariantly determined during ontogeny and depends on CXCR5. Young CXCR5-deficient mice completely lack SILT, suggesting that CXCR5 is essential for SILT formation during regular postnatal development. However, microbiota and other external stimuli can induce the formation of aberrant SILT distinguished by impaired development of B cell follicles in CXCR5-deficient mice. Small intestinal transplantation and bone marrow transplantation reveal that defect follicle formation is due to impaired B cell homing. Moreover, oral immunization with cholera toxin or infection with noninvasive Salmonella fail to induce efficient humoral immune responses in CXCR5-deficient mice. Bone marrow transplantation of CXCR5-deficient recipients with wild-type bone marrow rescued B cell follicle formation in SILT but failed to restore full humoral immune responses. These results reveal an essential role of CXCR5 in Peyer's patch and SILT development and function and indicate that SILT do not fully compensate for the lack of Peyer's patches in T cell-dependent humoral immune responses.

Littoral cell angioma of the spleen in a patient with previous pulmonary sarcoidosis: a TNF-α related pathogenesis?

BACKGROUND: Littoral cell angioma (LCA) is a rare vascular tumor of the spleen. Generally thought to be benign, additional cases of LCA with malignant features have been described. Thus, its malignant potential seems to vary and must be considered uncertain. The etiology remains unclear, but an immune dysregulation for the apparent association with malignancies of visceral organs or immune-mediated diseases has been proposed. CASE PRESENTATION: We report a case of LCA in a 43-year old male patient who presented with a loss of appetite and intermittent upper abdominal pain. Computed tomography showed multiple hypoattenuating splenic lesions which were hyperechogenic on abdominal ultrasound. Lymphoma was presumed and splenectomy was performed. Pathological evaluation revealed LCA. CONCLUSIONS: LCA is a rare, primary vascular neoplasm of the spleen that might etiologically be associated with immune dysregulation. In addition, it shows a striking association with synchronous or prior malignancies. With about one-third of the reported cases to date being co-existent with malignancies of visceral organs or immune-mediated diseases, this advocates for close follow-ups in all patients diagnosed with LCA. To our knowledge, this report is the first one of LCA associated with previous pulmonary sarcoidosis and hypothesizes a TNF-α related pathogenesis of this splenic tumor.

Clinical Impact of Stump Closure Reinforced With Hemopatch on the Prevention of Clinically Relevant Pancreatic Fistula After Distal Pancreatectomy: A Multicenter Randomized Trial.

Introduction: Postoperative pancreatic fistula (POPF) is the most dreaded complication after distal pancreatectomy (DP). This multicenter randomized trial evaluated the efficacy, safety, and tolerance of Hemopatch in preventing clinically relevant (grades B/C according to the ISGPS classification) POPF after DP. Material and methods: After stump closure, patients were randomized to affix Hemopatch to the stump or not. Statistical significance was set at 0.025. Clinical significance was defined as the number of patients needed to treat (NNT) to avoid 1 B/C POPF. Results: Of 631 eligible patients, 360 were randomized and 315 analyzed per protocol (155 in the standard closure group; 160 in the Hemopatch group). The rates of B/C POPF (the primary endpoint) were 23.2% and 16.3% (P = 0.120), while the number of patients with 1 or more complications (including patients with B/C POPF) was 34.8% and 24.4% (P = 0.049) in the standard and Hemopatch groups, respectively. In patients with hand-sewn stump and main duct closure, the rates were 26.2% versus 10.0% (P = 0.014) and 23.3% versus 7.7% (P = 0.015) in the standard and Hemopatch groups, respectively. The NNT in these 2 subgroups was 6 and 6.4, respectively. Conclusion: The results of the first randomized trial evaluating Hemopatch-reinforced pancreatic stump after DP to prevent type B/C POPF do not allow us to conclude that the risk of B/C POPF was lower. Based on the NNT, however, routine use of Hemopatch after DP may result in fewer complications (including POPF) overall, especially in cases with hand-sewn closure of the pancreatic stump or main pancreatic duct.

ADAP deficiency combined with costimulation blockade synergistically protects intestinal allografts.

Adhesion and degranulation promoting adapter protein (ADAP) plays an important role in T cell activation. ADAP deficiency was recently found to prolong heart graft survival in mice. We investigated the role of ADAP in intestinal transplantation and the synergistic effect of ADAP deficiency and Costimulation blockade (CB). T cell proliferation and cytotoxic T lymphocyte (CTL) activity were determined. MHC mismatched intestinal allografts was transplanted heterotopically. Anti-CD40L antibody was applied to the recipient. Upon stimulation with allogenic dendritic cells (DC), ADAP-deficient (ADAP-/-) T cells displayed impaired proliferative responses compared with that of wild-type (WT) T cells. In contrast, the CTL activity in ADAP-/- mice was comparable with that of WT mice. Rejection of intestinal allografts was ameliorated, but not prevented in ADAP-/- mice. Although CB alone was not sufficient to mitigate the rejection, the combination of CB and ADAP deficiency profoundly inhibited rejection. This was accompanied by less infiltration and activation of host lymphocytes in the gut-associated lymphoid tissue of intestinal allografts. ADAP deficiency combined with CB protected the intestinal allografts synergistically. ADAP could be a novel target in the induction phase of the immune responses in organ transplantation.

Inactivation of T-cell receptor-mediated integrin activation prolongs allograft survival in ADAP-deficient mice.

BACKGROUND: Signaling through the T cell receptor (TCR) leads to profound changes in the function and properties of T cells, including integrin activation. Adhesion and degranulation promoting adapter protein (ADAP) is an adapter protein linking T cell receptor stimulation to integrin activation. We aim to clarify how disruption of TCR-mediated integrin activation affects alloreactive immune responses. METHODS: In vitro T cell proliferation and the cytokine production was determined. In vivo cytotoxic T lymphocyte (CTL) activity was measured as well. Allogenic skin and heart transplantation was used to test the in vivo role of ADAP in alloimmune responses. Histology and flow cytometry was applied to analyze the graft infiltrating lymphocytes. RESULTS: Upon stimulation with allogenic dendritic cells ADAP-deficient T cells displayed impaired proliferative responses compared to wild type (WT) T cells. This was accompanied by significantly decreased production of the cytokine interleukin-2. In contrast, the in vivo CTL activity in ADAP-deficient mice was comparable to that of WT mice. Consistently, we observed a prolongation of fully major histocompatibility complex (MHC)-mismatched heart transplants in ADAP deficient mice. Protection of allogenic heart grafts in ADAP-deficient mice was accompanied by a decrease in the infiltration, proliferation and activation of T cells in the allograft. However, no effect was observed after fully MHC-mismatched skin transplantation. CONCLUSIONS: We have shown that although ADAP is dispensable for the rejection of allografts, ADAP function plays an important role for the efficacy of graft rejection. ADAP's main function appears to affect the induction phase of the immune response.

Radiation protocols determine acute graft-versus-host disease incidence after allogeneic bone marrow transplantation in murine models.

PURPOSE: Effects of radiation sources used for total body irradiation (TBI) on Graft-versus-Host Disease (GvHD) induction were examined. MATERIALS AND METHODS: In a T cell receptor (TCR) transgenic mouse model, single fraction TBI was performed with different radiation devices ((60)Cobalt; (137)Cesium; 6 MV linear accelerator), dose rates (0.85; 1.5; 2.9; 5 Gy/min) and total doses before allogeneic bone marrow transplantation (BMT). Recipients were observed for 120 days. Different tissues were examined histologically. RESULTS: Acute GvHD was induced by a dose rate of 0.85 Gy/min ((60)Cobalt) and a total dose of 9 Gy and injection of 5 x 10(5) lymph node cells plus 5 x 10(6) bone marrow cells. Similar results were obtained using 6 MV linear accelerator- (linac-) photons with a dose rate of 1.5 Gy/min and 0.85 Gy/min, a total dose of 9.5 Gy and injection of same cell numbers. TBI with (137)Cesium (dose rate: 2.5 Gy/min) did not lead reproducibly to lethal acute GvHD. CONCLUSIONS: Experimental TBI in murine models may induce different immunological responses, depending on total energy, total single dose and dose rate. GvHD might also be induced by TBI with low dose rates.

Protection of mouse small bowel allografts by FTY720 and costimulation blockade.

BACKGROUND: The clinical application of small bowel transplantation (SBTx) is hampered by its pronounced immunogenicity. We aimed to test the hypothesis that prolonged sequestration of lymphocytes in secondary lymphoid organs may enhance the alloprotective effect of costimulation blockade. METHODS: For this purpose, recipients of intestinal allografts were treated with MR1, FTY720, combined FTY720 plus MR1, or were left untreated. Grafts were examined 6 and 14 days after transplantation by applying a histologic rejection score, multiparameter-immunofluorescent staining, and flow cytometry. RESULTS: FTY720 or MR1 monotherapy did not prevent the rejection of mouse intestinal allografts, whereas combined therapy with FTY720 plus MR1 profoundly inhibited rejection at day 6 and day 14 after transplantation. In FTY720-treated mice infiltration of host lymphocytes in graft mesenteric lymph nodes, Peyer's patches, intraepithelial lymphocytes, and lamina propria lymphocytes (LPLs) was reduced on day 6. Anti-CD40L antibody improved the rejection score at day 14 but had no effect at day 6. Importantly, host CD8 T-cell infiltration in graft LPLs was significantly reduced compared with all other groups. CONCLUSION: FTY720 plus MR1 effectively inhibited intestinal allograft rejection in mice, possibly by enhancing the alloprotective effects of costimulation blockade by prolonged sequestration of lymphocytes in secondary lymphoid organs.

Influence of radiation protocols on graft-vs-host disease incidence after bone-marrow transplantation in experimental models.

Bone-marrow transplantation is an approved curative treatment for many hemato- and oncologic diseases. Nevertheless, the severe acute clinical course of graft-vs-host disease (GVHD) after allogeneic bone-marrow transplantation is frequently fatal, and is to date not curable. Acute GVHD must, therefore, be prevented from the start of the bone-marrow transplantation by immunosuppressive medication, causing sometimes serious side effects. Therefore, new preventive strategies are tested, starting with animal experiments. Often mice are chosen for this kind of trial, and the clinical protocol of bone-marrow transplantation is transferred into the experimental settings. The first step to induce an acute GVHD is whole-body irradiation of the recipients. Several methods are available for this purpose: the most common is a 60cobalt source (gamma-irradiation); less common are a 137cesium source (gamma-irradiation) and a linear (particle) accelerator (photons). Differences between these radiation techniques can occur and can unexpectedly interfere with the results of the experiments. In this chapter, the materials and methods for bone-marrow transplantation in mice, with particular emphasis on the different radiation techniques, are explained; furthermore, the advantages and disadvantages in regard to the underlying physical principles will be discussed.

Detection of H-2 alleles by PCR-RFLP-an alternative to flow cytometry in the screening of transgenic mice.

An increasing number of experimental models is based on well-defined transgenic mice in medical and biological research. Particularly in settings in which transgenic recombinants are used, a fast and reliable method is needed to screen for a defined H-2 background. For this purpose, flow cytometry with specific monoclonal antibodies is the standard procedure. However, epitopes of closely related rodent strains show only minor variations affecting the production of specific discriminating antibodies. Therefore, cross-reactivity of antibodies against specific major histocompatibility complex (MHC) leads to unreliable results in settings with closely related strains. In need of a method with high reliability, we have designed a screening assay based on polymerase chain reaction (PCR) followed by restriction fragment length polymorphisms (RFLP) to discriminate the MHC class I antigens H-2K(d), -K(b), -K(k), which are sequence variants of the H-2K gene. A part of the mus musculus MHC gene coding for H-2K-covering exons 4 and 5 with MHC-differentiating restriction sites-was amplified. Subsequent restriction digest of the PCR products allows to discriminate the three aforementioned alleles and to identify homozygous as well as heterozygous haplotypes. To distinguish transgenic mice defined by certain MHC backgrounds, the PCR-RFLP method is simple, cost-effective, specific, and reliable and can be used independently or in addition to other methods in any laboratory.

Prolongation of allograft survival in ccr7-deficient mice.

BACKGROUND: Lymphocyte homing to secondary lymphoid organs is thought to be required for initiation of the alloreactive immune response. Because CCR7 is the essential chemokine receptor responsible for lymphocyte and dendritic cell homing to secondary lymphoid organs, allograft survival was analyzed in CCR7-deficient (CCR7) mice. METHODS: Heterotopic heart and skin allotransplantation was performed in CCR7 and wild-type (WT) recipients. Graft survival was monitored daily. Grafts and draining lymph nodes were analyzed by immunohistology and flow cytometry at different time points. Groups of mice were splenectomized at the day of allotransplantation. RESULTS: A significant though modest prolongation of allograft survival in CCR7 recipients was observed for heart grafts (WT, 7.3 +/- 0.5 days; CCR7, 10.7 +/- 2.8 days) and skin grafts (WT, 8.9 +/- 0.9 days; CCR7, 12.3 +/- 0.9 days). This was accompanied by a delay in the cellular infiltration of allografts. T-cell accumulation and expansion in the draining lymph nodes in CCR7 recipients was severely impaired. Splenectomy had only a moderate prolongation effect on allograft survival in CCR7 mice. CONCLUSIONS: These results suggest that CCR7-dependent processes support allograft rejection yet are dispensable for the rejection response.

Allogeneic adrenocortical transplantation: glucocorticosteroid-independent immunomodulatory properties of adrenal cortex cells.

BACKGROUND: Hormone substitution for the treatment of adrenocortical insufficiency does not adequately substitute the physiologic circadian secretion of corticosteroids and leads to long-term sequelae and reduced quality of life. The lack of adaptation to physical and psychologic stress situations may lead to life-threatening Addison's crises. Allogeneic transplantation of adrenal cortex could offer an intriguing alternative. Adrenocortical grafts were demonstrated to proliferate and produce corticosteroids in physiologic concentrations after transplantation. METHODS: K -transgenic murine lymphocytes and allogeneic adrenal cortex cells were cocultured in mixed lymphocyte reactions to examine the alloimmune response; lymphocytes from T-cell receptor transgenic mice and normal mice, respectively, served as responder cells. The effects of corticosteroids secreted by adrenocortical cells were antagonized by the steroid receptor antagonist mifepristone, whereas the impact of cell-cell interactions was differentiated with transwell culture systems. RESULTS: Coculture of adrenal cortex cells in mixed lymphocyte reactions markedly suppressed lymphocyte proliferation. Transwell cultures demonstrated that adrenocortical cells exerted their effects by a soluble factor that was only partially antagonized by mifepristone. CONCLUSION: In vitro, the presence of adrenocortical cells potently suppressed allogeneic immune responses. This effect was not exclusively the result of the secretion of corticosteroids, indicating an additional immunomodulatory property of adrenocortical cells.

beta2-Adrenoceptor-mediated suppression of human intestinal mast cell functions is caused by disruption of filamentous actin dynamics.

Previous studies indicated potent inhibitory effects of beta2-adrenoceptor (beta2AR) activation on the immunological mediator release of mast cells (MC). Here, we studied effects of beta2AR agonists on human MC mediator release, and in particular on MC proliferation, adhesion, and migration. MC were isolated from human intestinal mucosa, purified, and cultured in the presence of stem cell factor (SCF). beta2AR activation by epinephrine, norepinephrine, and salbutamol suppressed the IgE receptor-dependent release of histamine, lipid mediators, and TNF-alpha, and inhibited SCF-dependent MC proliferation and migration. Moreover, beta2-adrenergic stimulation interfered with MC adhesion to fibronectin and human endothelial cells. Using fluorescent phallacidin, we found that beta2AR activation reduced the amount of filamentous actin (F-actin) within minutes, whereas MC stimulation by either IgE receptor cross-linking or SCF caused F-actin accumulation. Interestingly, this activation-induced F-actin increase was abolished by previous beta2-adrenergic stimulation. Finally, we demonstrated that disruption of the F-actin cytoskeleton by latrunculin B mimicked the effects of beta2AR agonists on MC adhesion and migration. Our results argue for an important role of F-actin interference in beta2AR-mediated MC inhibition. Furthermore, the data support the concept of neuroimmune interactions regulating intestinal MC distribution, density, and functionality in vivo.

Cryptopatches and isolated lymphoid follicles: dynamic lymphoid tissues dispensable for the generation of intraepithelial lymphocytes.

In comparison to secondary lymphoid organs, gut-associated lymphoid tissues such as isolated lymphoid follicles (ILF) and cryptopatches (CP) have been less intensively studied. To gain a better insight into processes regulating organization and function of these structures, which are believed to participate in immune responses and extrathymic T cell development, we characterized the lymphoid structures of the murine small intestine in more detail. The size and cellular composition of small intestinal lymphoid aggregations were analyzed in C57BL/6 and BALB/c wild-type and lymphotoxin (LT)-deficient mice, by flow cytometry, histology and automated multi-color immunofluorescence microscopy evaluating large coherent areas of the intestine. These evaluations demonstrate that aggregated lymphoid structures in the small intestine vary in size and cellular composition, with a majority of structures not matching the current definitions of CP or ILF. Accordingly, significant variations depending on species, age and mouse strain were observed. Furthermore, small bowel transplantation revealed a rapid exchange of B but not T cells between host and grafted tissue. Moreover, LT-deficient animals lack any intestinal lymphoid aggregations yet possess the complete panel of intraepithelial lymphocytes (IEL). In summary, our observations disclose intestinal lymphoid aggregations as dynamic structures with a great deal of inborn plasticity and demonstrate their dispensability for the generation of IEL.

Transplantation of adrenal tissue fragments in a murine model: functional capacities of syngeneic and allogeneic grafts.

Oral hormone substitution for the treatment of Addison's disease inadequately replaces the physiologic circadian secretion of corticosteroids. Alternative therapeutic approaches are reimplantation of healthy autologous adrenal tissue and allogeneic transplantation (Tx), respectively. The aim of our study was to evaluate the functional capacity of adrenal grafts and the influence of intercellular adhesion molecule-1 (ICAM-1) on graft survival. Fragmented adrenal glands of wild-type B10.BR (H-2k) and wild-type or ICAM-1-deficient BALB/c (H-2d) mice were transplanted underneath the kidney capsule of adrenalectomized B10.BR mice [complete major histocompatibility complex (MHC) haplotype disparity in the latter]. Postoperatively, the endocrine function of the adrenal grafts was evaluated by the following parameters: (1) survival analysis of the recipients (termination at day 70 after Tx); (2) reverse transcription-polymerase chain reaction expression analysis of aldosterone synthase (zona glomerulosa-specific) and of 11b-hydroxylase (zona fasciculata-specific); and (3) measurement of basal adrenocorticotropic hormone (ACTH) stimulated serum corticosterone levels. Expression of both enzyme-specific mRNAs was detected in the grafts at any time during the post-Tx period. The adrenal grafts of syngeneic and surviving MHC-disparate mice displayed a similar basal hormone secretion, which was about 60% lower than that in sham-operated animals. In the transplanted mice, ACTH-stimulated corticosterone measurement revealed a 5- to 10-fold decreased functional reserve capacity. ICAM-1 deficiency significantly prolonged the survival of adrenal grafts. Fragmented adrenal grafts are able to maintain physiologic basal corticosterone levels but had markedly reduced reserve capacity. Nevertheless, the results give rise to hopes that autologous or MHC-compatible allogeneic transplantation of adrenal grafts may replace oral hormone substitution in humans.

Cytokine-dependent bystander hepatitis due to intrahepatic murine CD8 T-cell activation by bone marrow-derived cells.

BACKGROUND & AIMS: Intrahepatic accumulation of CD8+ T cells following antigen-specific activation has been demonstrated in a number of transgenic models and also in extrahepatic viral infections. In some transgenic models, intrahepatic accumulation of cytotoxic T lymphocytes is associated with hepatitis. This observation suggests that hepatocellular damage may occur in some forms of immune-mediated hepatitis on the basis of a "bystander injury," whereby cytotoxic T lymphocytes accumulating in the liver mediate injury to hepatocytes in a nonspecific manner. Mouse transgenic models were therefore developed to investigate whether bystander damage to non-antigen-bearing hepatocytes occurs in vivo. METHODS: T cell receptor transgenic T cells were adoptively transferred into transgenic mice ubiquitously expressing the specific antigen, or into bone marrow radiation chimeras in which hepatocytes did not express the antigen. RESULTS: Selective accumulation of transgenic CD8+ T cells in the liver of intact recipients could be detected within 2 hours of transfer, despite ubiquitous antigenic expression. T cells retained in the liver were activated and induced hepatitis. Similar results were obtained using bone marrow chimeras, suggesting that antigen expression by hepatocytes was not required either for intrahepatic accumulation or for subsequent hepatitis. This "bystander hepatitis" was dependent on tumor necrosis factor alpha and interferon gamma. CONCLUSIONS: Intrahepatic accumulation of activated CD8+ T cells and subsequent hepatitis can result from primary activation of CD8+ T cells by liver resident bone marrow-derived cells, inducing bystander damage to non-antigen-bearing hepatocytes. This mechanism may play a role in some forms of biologically significant hepatitis, including autoimmune hepatitis and hepatitis associated with extrahepatic diseases.