CSF1R inhibition promotes neuroinflammation and behavioral deficits during graft-versus-host disease in mice

ABSTRACT

Chronic graft-versus-host disease (cGVHD) remains a significant complication of allogeneic hematopoietic stem cell transplantation. Central nervous system (CNS) involvement is becoming increasingly recognized, in which brain-infiltrating donor major histocompatibility complex (MHC) class II+ bone marrow-derived macrophages (BMDM) drive pathology. BMDM are also mediators of cutaneous and pulmonary cGVHD, and clinical trials assessing the efficacy of antibody blockade of colony-stimulating factor 1 receptor (CSF1R) to deplete macrophages are promising. We hypothesized that CSF1R antibody blockade may also be a useful strategy to prevent/treat CNS cGVHD. Increased blood-brain barrier permeability during acute GVHD (aGVHD) facilitated CNS antibody access and microglia depletion by anti-CSF1R treatment. However, CSF1R blockade early after transplant unexpectedly exacerbated aGVHD neuroinflammation. In established cGVHD, vascular changes and anti-CSF1R efficacy were more limited. Anti-CSF1R-treated mice retained donor BMDM, activated microglia, CD8+ and CD4+ T cells, and local cytokine expression in the brain. These findings were recapitulated in GVHD recipients, in which CSF1R was conditionally depleted in donor CX3CR1+ BMDM. Notably, inhibition of CSF1R signaling after transplant failed to reverse GVHD-induced behavioral changes. Moreover, we observed aberrant behavior in non-GVHD control recipients administered anti-CSF1R blocking antibody and naïve mice lacking CSF1R in CX3CR1+ cells, revealing a novel role for homeostatic microglia and indicating that ongoing clinical trials of CSF1R inhibition should assess neurological adverse events in patients. In contrast, transfer of Ifngr-/- grafts could reduce MHC class II+ BMDM infiltration, resulting in improved neurocognitive function. Our findings highlight unexpected neurological immune toxicity during CSF1R blockade and provide alternative targets for the treatment of cGVHD within the CNS.

A hepatic network of dendritic cells mediates CD4 T cell help outside lymphoid organs

While CD4+ T cells are a prerequisite for CD8+ T cell-mediated protection against intracellular hepatotropic pathogens, the mechanisms facilitating the transfer of CD4-help to intrahepatic CD8+ T cells are unknown. Here, we developed an experimental system to investigate cognate CD4+ and CD8+ T cell responses to a model-antigen expressed de novo in hepatocytes and reveal that after initial priming, effector CD4+ and CD8+ T cells migrate into portal tracts and peri-central vein regions of the liver where they cluster with type-1 conventional dendritic cells. These dendritic cells are locally licensed by CD4+ T cells and expand the number of CD8+ T cells in situ, resulting in larger effector and memory CD8+ T cell pools. These findings reveal that CD4+ T cells promote intrahepatic immunity by amplifying the CD8+ T cell response via peripheral licensing of hepatic type-1 conventional dendritic cells and identify intrahepatic perivascular compartments specialized in facilitating effector T cell-dendritic cell interactions.

mRNA vaccine against malaria tailored for liver-resident memory T cells

Malaria is caused by Plasmodium species transmitted by Anopheles mosquitoes. Following a mosquito bite, Plasmodium sporozoites migrate from skin to liver, where extensive replication occurs, emerging later as merozoites that can infect red blood cells and cause symptoms of disease. As liver tissue-resident memory T cells (Trm cells) have recently been shown to control liver-stage infections, we embarked on a messenger RNA (mRNA)-based vaccine strategy to induce liver Trm cells to prevent malaria. Although a standard mRNA vaccine was unable to generate liver Trm or protect against challenge with Plasmodium berghei sporozoites in mice, addition of an agonist that recruits T cell help from type I natural killer T cells under mRNA-vaccination conditions resulted in significant generation of liver Trm cells and effective protection. Moreover, whereas previous exposure of mice to blood-stage infection impaired traditional vaccines based on attenuated sporozoites, mRNA vaccination was unaffected, underlining the potential for such a rational mRNA-based strategy in malaria-endemic regions.

Apoptotic cell fragments locally activate tingible body macrophages in the germinal center

Germinal centers (GCs) that form within lymphoid follicles during antibody responses are sites of massive cell death. Tingible body macrophages (TBMs) are tasked with apoptotic cell clearance to prevent secondary necrosis and autoimmune activation by intracellular self antigens. We show by multiple redundant and complementary methods that TBMs derive from a lymph node-resident, CD169-lineage, CSF1R-blockade-resistant precursor that is prepositioned in the follicle. Non-migratory TBMs use cytoplasmic processes to chase and capture migrating dead cell fragments using a "lazy" search strategy. Follicular macrophages activated by the presence of nearby apoptotic cells can mature into TBMs in the absence of GCs. Single-cell transcriptomics identified a TBM cell cluster in immunized lymph nodes which upregulated genes involved in apoptotic cell clearance. Thus, apoptotic B cells in early GCs trigger activation and maturation of follicular macrophages into classical TBMs to clear apoptotic debris and prevent antibody-mediated autoimmune diseases.

Liver-specific deletion of miR-181ab1 reduces liver tumour progression via upregulation of CBX7

MiR-181 expression levels increased in hepatocellular carcinoma (HCC) compared to non-cancerous tissues. MiR-181 has been widely reported as a possible driver of tumourigenesis but also acts as a tumour suppressor. In addition, the miR-181 family regulates the development and function of immune and vascular cells, which play vital roles in the progression of tumours. More complicatedly, many genes have been identified as miR-181 targets to mediate the effects of miR-181. However, the role of miR-181 in the development of primary tumours remains largely unexplored. We aimed to examine the function of miR-181 and its vital mediators in the progression of diethylnitrosamine-induced primary liver cancers in mice. The size of liver tumours was significantly reduced by 90% in global (GKO) or liver-specific (LKO) 181ab1 knockout mice but not in hematopoietic and endothelial lineage-specific knockout mice, compared to WT mice. In addition, the number of tumours was significantly reduced by 50% in GKO mice. Whole-genome RNA-seq analysis and immunohistochemistry showed that epithelial-mesenchymal transition was partially reversed in GKO tumours compared to WT tumours. The expression of CBX7, a confirmed miR-181 target, was up-regulated in GKO compared to WT tumours. Stable CBX7 expression was achieved with an AAV/Transposase Hybrid-Vector System and up-regulated CBX7 expression inhibited liver tumour progression in WT mice. Hepatic CBX7 deletion restored the progression of LKO liver tumours. MiR-181a expression was the lowest and CBX7 expression the highest in iClust2 and 3 subclasses of human HCC compared to iClust1. Gene expression profiles of GKO tumours overlapped with low-proliferative peri-portal-type HCCs. Liver-specific loss of miR-181ab1 inhibited primary liver tumour progression via up-regulating CBX7 expression, but tumour induction requires both hepatic and non-hepatic miR-181. Also, miR-181ab1-deficient liver tumours may resemble low-proliferative periportal-type human HCC.

The liver contains distinct interconnected networks of CX3CR1 + macrophages, XCR1 + type 1 and CD301a + type 2 conventional dendritic cells embedded within portal tracts

Portal tracts are key intrahepatic structures where leukocytes accumulate during immune responses. They contain the blood inflow, which includes portal blood from the gut, and lymphatic and biliary outflow of the liver, and as such represent a key interface for potential pathogen entry to the liver. Myeloid cells residing in the interstitium of the portal tract might play an important role in the surveillance or prevention of pathogen dissemination; however, the exact composition and localization of this population has not been explored fully. Our in‐depth characterization of portal tract myeloid cells revealed that in addition to T lymphocytes, portal tracts contain a heterogeneous population of MHCII^high myeloid cells with potential antigen presenting cell (APC) function. These include a previously unreported subset of CSF1R‐dependent CX3CR1⁺ macrophages that phenotypically and morphologically resemble liver capsular macrophages, as well as the two main dendritic cell subsets (cDC1 and cDC2). These cells are not randomly distributed, but each subset forms interconnected networks intertwined with specific components of the portal tract. The CX3CR1⁺ cells were preferentially detected along the outer border of the portal tracts, and also in the portal interstitium adjacent to the portal vein, bile duct, lymphatic vessels and hepatic artery. cDC1s abounded along the lymphatic vessels, while cDC2s mostly surrounded the biliary tree. The specific distributions of these discrete subsets predict that they may serve distinct functions in this compartment. Overall, our findings suggest that portal tracts and their embedded cellular networks of myeloid cells form a distinctive lymphoid compartment in the liver that has the potential to orchestrate immune responses in this organ.

ROCK2 inhibition attenuates profibrogenic immune cell function to reverse thioacetamide-induced liver fibrosis

Background & Aims

Fibrosis, the primary cause of morbidity in chronic liver disease, is induced by pro-inflammatory cytokines, immune cell infiltrates, and tissue resident cells that drive excessive myofibroblast activation, collagen production, and tissue scarring. Rho-associated kinase 2 (ROCK2) regulates key pro-fibrotic pathways involved in both inflammatory reactions and altered extracellular matrix remodelling, implicating this pathway as a potential therapeutic target.

Methods

We used the thioacetamide-induced liver fibrosis model to examine the efficacy of administration of the selective ROCK2 inhibitor KD025 to prevent or treat liver fibrosis and its impact on immune composition and function.

Results

Prophylactic and therapeutic administration of KD025 effectively attenuated thioacetamide-induced liver fibrosis and promoted fibrotic regression. KD025 treatment inhibited liver macrophage tumour necrosis factor production and disrupted the macrophage niche within fibrotic septae. ROCK2 targeting in vitro directly regulated macrophage function through disruption of signal transducer and activator of transcription 3 (STAT3)/cofilin signalling pathways leading to the inhibition of pro-inflammatory cytokine production and macrophage migration. In vivo, KDO25 administration significantly reduced STAT3 phosphorylation and cofilin levels in the liver. Additionally, livers exhibited robust downregulation of immune cell infiltrates and diminished levels of retinoic acid receptor-related orphan receptor gamma (RORγt) and B-cell lymphoma 6 (Bcl6) transcription factors that correlated with a significant reduction in liver IL-17, splenic germinal centre numbers and serum IgG.

Conclusions

As IL-17 and IgG–Fc binding promote pathogenic macrophage differentiation, together our data demonstrate that ROCK2 inhibition prevents and reverses liver fibrosis through direct and indirect effects on macrophage function and highlight the therapeutic potential of ROCK2 inhibition in liver fibrosis.

Lay summary

By using a clinic-ready small-molecule inhibitor, we demonstrate that selective ROCK2 inhibition prevents and reverses hepatic fibrosis through its pleiotropic effects on pro-inflammatory immune cell function. We show that ROCK2 mediates increased IL-17 production, antibody production, and macrophage dysregulation, which together drive fibrogenesis in a model of chemical-induced liver fibrosis. Therefore, in this study, we not only highlight the therapeutic potential of ROCK2 targeting in chronic liver disease but also provide previously undocumented insights into our understanding of cellular and molecular pathways driving the liver fibrosis pathology.

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ROCK2 inhibition with the small-molecule inhibitor KD025 prevents and reverses hepatoxin-induced liver fibrosis.

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ROCK2 inhibition attenuates profibrogenic immune function.

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KD025 exerts direct effects on liver macrophages resulting in decreased TNF secretion and impeded migration.

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KD025 administration attenuates T cell IL-17 production and B-cell IgG production, which indirectly contributes to downregulation of profibrogenic macrophage function.

The self-peptide repertoire plays a critical role in transplant tolerance induction

While direct allorecognition underpins both solid organ allograft rejection and tolerance induction, the specific molecular targets of most directly alloreactive CD8+ T cells have not been defined. In this study, we used a combination of genetically engineered major histocompatibility complex class I (MHC I) constructs, mice with a hepatocyte-specific mutation in the class I antigen-presentation pathway, and immunopeptidomic analysis to provide definitive evidence for the contribution of the peptide cargo of allogeneic MHC I molecules to transplant tolerance induction. We established a systematic approach for the discovery of directly recognized pMHC epitopes and identified 17 strongly immunogenic H-2Kb-associated peptides recognized by CD8+ T cells from B10.BR (H-2k) mice, 13 of which were also recognized by BALB/c (H-2d) mice. As few as 5 different tetramers used together were able to identify a high proportion of alloreactive T cells within a polyclonal population, suggesting that there are immunodominant allogeneic MHC-peptide complexes that can account for a large component of the alloresponse.

TCR Affinity Controls the Dynamics but Not the Functional Specification of the Antimycobacterial CD4+ T Cell Response

The quality of T cell responses depends on the lymphocytes' ability to undergo clonal expansion, acquire effector functions, and traffic to the site of infection. Although TCR signal strength is thought to dominantly shape the T cell response, by using TCR transgenic CD4⁺ T cells with different peptide:MHC binding affinity, we reveal that TCR affinity does not control Th1 effector function acquisition or the functional output of individual effectors following mycobacterial infection in mice. Rather, TCR affinity calibrates the rate of cell division to synchronize the distinct processes of T cell proliferation, differentiation, and trafficking. By timing cell division-dependent IL-12R expression, TCR affinity controls when T cells become receptive to Th1-imprinting IL-12 signals, determining the emergence and magnitude of the Th1 effector pool. These findings reveal a distinct yet cooperative role for IL-12 and TCR binding affinity in Th1 differentiation and suggest that the temporal activation of clones with different TCR affinity is a major strategy to coordinate immune surveillance against persistent pathogens.

Glycolipid-peptide vaccination induces liver-resident memory CD8+ T cells that protect against rodent malaria

Liver resident-memory CD8⁺ T cells (T_RM cells) can kill liver-stage Plasmodium-infected cells and prevent malaria, but simple vaccines for generating this important immune population are lacking. Here, we report the development of a fully synthetic self-adjuvanting glycolipid-peptide conjugate vaccine designed to efficiently induce liver T_RM cells. Upon cleavage in vivo, the glycolipid-peptide conjugate vaccine releases an MHC I-restricted peptide epitope (to stimulate Plasmodium-specific CD8⁺ T cells) and an adjuvant component, the NKT cell agonist α-galactosylceramide (α-GalCer). A single dose of this vaccine in mice induced substantial numbers of intrahepatic malaria-specific CD8⁺ T cells expressing canonical markers of liver T_RM cells (CD69, CXCR6, and CD101), and these cells could be further increased in number upon vaccine boosting. We show that modifications to the peptide, such as addition of proteasomal-cleavage sequences or epitope-flanking sequences, or the use of alternative conjugation methods to link the peptide to the glycolipid improved liver T_RM cell generation and led to the development of a vaccine able to induce sterile protection in C57BL/6 mice against Plasmodium berghei sporozoite challenge after a single dose. Furthermore, this vaccine induced endogenous liver T_RM cells that were long-lived (half-life of ~425 days) and were able to maintain >90% sterile protection to day 200. Our findings describe an ideal synthetic vaccine platform for generating large numbers of liver T_RM cells for effective control of liver-stage malaria and, potentially, a variety of other hepatotropic infections.

Multiple receptors converge on H2-Q10 to regulate NK and γδT-cell development

Class Ib major histocompatibility complex (MHC) is an extended family of molecules, which demonstrate tissue-specific expression and presentation of monomorphic antigens. These characteristics tend to imbue class Ib MHC with unique functions. H2-Q10 is potentially one such molecule that is overexpressed in the liver but its immunological function is not known. We have previously shown that H2-Q10 is a ligand for the natural killer cell receptor Ly49C and now, using H2-Q10-deficient mice, we demonstrate that H2-Q10 can also stabilize the expression of Qa-1b. In the absence of H2-Q10, the development and maturation of conventional hepatic natural killer cells is disrupted. We also provide evidence that H2-Q10 is a new high affinity ligand for CD8αα and controls the development of liver-resident CD8αα γδT cells. These data demonstrate that H2-Q10 has multiple roles in the development of immune subsets and identify an overlap of recognition within the class Ib MHC that is likely to be relevant to the regulation of immunity.

Pulmonary immunization with a recombinant influenza A virus vaccine induces lung-resident CD4+ memory T cells that are associated with protection against tuberculosis

The lung is the primary site of infection with the major human pathogen, Mycobacterium tuberculosis. Effective vaccines against M. tuberculosis must stimulate memory T cells to provide early protection in the lung. Recently, tissue-resident memory T cells (T_RM) were found to be phenotypically and transcriptional distinct from circulating memory T cells. Here, we identified M. tuberculosis-specific CD4⁺ T cells induced by recombinant influenza A viruses (rIAV) vaccines expressing M. tuberculosis peptides that persisted in the lung parenchyma with the phenotypic and transcriptional characteristics of T_RMs. To determine if these rIAV-induced CD4⁺ T_RM were protective independent of circulating memory T cells, mice previously immunized with the rIAV vaccine were treated with the sphingosine-1-phosphate receptor modulator, FTY720, prior to and during the first 17 days of M. tuberculosis challenge. This markedly reduced circulating T cells, but had no effect on the frequency of M. tuberculosis-specific CD4⁺ T_RMs in the lung parenchyma or their cytokine response to infection. Importantly, mice immunized with the rIAV vaccine were protected against M. tuberculosis infection even when circulating T cells were profoundly depleted by the treatment. Therefore, pulmonary immunization with the rIAV vaccine stimulates lung-resident CD4⁺ memory T cells that are associated with early protection against tuberculosis infection.

An Atypical Parvovirus Drives Chronic Tubulointerstitial Nephropathy and Kidney Fibrosis

The occurrence of a spontaneous nephropathy with intranuclear inclusions in laboratory mice has puzzled pathologists for over 4 decades, because its etiology remains elusive. The condition is more severe in immunodeficient animals, suggesting an infectious cause. Using metagenomics, we identify the causative agent as an atypical virus, termed "mouse kidney parvovirus" (MKPV), belonging to a divergent genus of Parvoviridae. MKPV was identified in animal facilities in Australia and North America, is transmitted via a fecal-oral or urinary-oral route, and is controlled by the adaptive immune system. Detailed analysis of the clinical course and histopathological features demonstrated a stepwise progression of pathology ranging from sporadic tubular inclusions to tubular degeneration and interstitial fibrosis and culminating in renal failure. In summary, we identify a widely distributed pathogen in laboratory mice and establish MKPV-induced nephropathy as a new tool for elucidating mechanisms of tubulointerstitial fibrosis that shares molecular features with chronic kidney disease in humans.

ALK7 expression in prolactinoma is associated with reduced prolactin and increased proliferation

Prolactinoma represents the most frequent hormone-secreting pituitary tumours. These tumours appear in a benign form, but some of them can reach an invasive and aggressive stage through an unknown mechanism. Discovering markers to identify prolactinoma proliferative and invading character is therefore crucial to develop new diagnostic/prognostic strategies. Interestingly, members of the TGFβ-Activin/BMP signalling pathways have emerged as important actors of pituitary development and adult function, but their role in prolactinomas remains to be precisely determined. Here, using a heterotopic allograft model derived from a rat prolactinoma, we report that the Activins orphan type I receptor ALK7 is ectopically expressed in prolactinomas-cells. Through immunohistological approaches, we further confirm that normal prolactin-producing cells lack ALK7-expression. Using a series of human tumour samples, we show that ALK7 expression in prolactinomas cells is evolutionary conserved between rat and human. More interestingly, our results highlight that tumours showing a robust expression of ALK7 present an increased proliferation as address by Ki67 expression and retrospective analysis of clinical data from 38 patients, presenting ALK7 as an appealing marker of prolactinoma aggressiveness. Beside this observation, our work pinpoints that the expression of prolactin is highly heterogeneous in prolactinoma cells. We further confirm the contribution of ALK7 in these observations and the existence of highly immunoreactive prolactin cells lacking ALK7 expression. Taken together, our observations suggest that Activin signalling mediated through ALK7 could therefore contribute to the hormonal heterogeneity and increased proliferation of prolactinomas.

Direct recognition of hepatocyte-expressed MHC class I alloantigens is required for tolerance induction

Adeno-associated viral vector-mediated (AAV-mediated) expression of allogeneic major histocompatibility complex class I (MHC class I) in recipient liver induces donor-specific tolerance in mouse skin transplant models in which a class I allele (H-2Kb or H-2Kd) is mismatched between donor and recipient. Tolerance can be induced in mice primed by prior rejection of a donor-strain skin graft, as well as in naive recipients. Allogeneic MHC class I may be recognized by recipient T cells as an intact molecule (direct recognition) or may be processed and presented as an allogeneic peptide in the context of self-MHC (indirect recognition). The relative contributions of direct and indirect allorecognition to tolerance induction in this setting are unknown. Using hepatocyte-specific AAV vectors encoding WT allogeneic MHC class I molecules, or class I molecules containing a point mutation (D227K) that impedes direct recognition of intact allogeneic MHC class I by CD8+ T cells without hampering the presentation of processed peptides derived from allogeneic MHC class I, we show here that tolerance induction depends upon recognition of intact MHC class I. Indirect recognition alone yielded a modest prolongation of subsequent skin graft survival, attributable to the generation of CD4+ Tregs, but it was not sufficient to induce tolerance.

Deletion of sphingosine kinase 1 inhibits liver tumorigenesis in diethylnitrosamine-treated mice

Primary liver cancer is the 3rd leading cause of cancer deaths worldwide with very few effective treatments. Sphingosine kinase 1 (SphK1), a key regulator of sphingolipid metabolites, is over-expressed in human hepatocellular carcinoma (HCC) and our previous studies have shown that SphK1 is important in liver injury. We aimed to explore the role of SphK1 specifically in liver tumorigenesis using the SphK1 knockout (SphK1^-/-) mouse. SphK1 deletion significantly reduced the number and the size of DEN-induced liver cancers in mice. Mechanistically, fewer proliferating but more apoptotic and senescent cells were detected in SphK1 deficient tumors compared to WT tumors. There was an increase in sphingosine rather than a decrease in sphingosine 1-phosphate (S1P) in SphK1 deficient tumors. Furthermore, the STAT3-S1PR pathway that has been reported previously to mediate the effect of SphK1 on colorectal cancers was not altered by SphK1 deletion in liver cancer. Instead, c-Myc protein expression was down-regulated by SphK1 deletion. In conclusion, this is the first in vivo evidence that SphK1 contributes to hepatocarcinogenesis. However, the downstream signaling pathways impacting on the development of HCC via SphK1 are organ specific providing further evidence that simply transferring known oncogenic molecular pathway targeting into HCC is not always valid.

A Liver Capsular Network of Monocyte-Derived Macrophages Restricts Hepatic Dissemination of Intraperitoneal Bacteria by Neutrophil Recruitment

The liver is positioned at the interface between two routes traversed by pathogens in disseminating infection. Whereas blood-borne pathogens are efficiently cleared in hepatic sinusoids by Kupffer cells (KCs), it is unknown how the liver prevents dissemination of peritoneal pathogens accessing its outer membrane. We report here that the hepatic capsule harbors a contiguous cellular network of liver-resident macrophages phenotypically distinct from KCs. These liver capsular macrophages (LCMs) were replenished in the steady state from blood monocytes, unlike KCs that are embryonically derived and self-renewing. LCM numbers increased after weaning in a microbiota-dependent process. LCMs sensed peritoneal bacteria and promoted neutrophil recruitment to the capsule, and their specific ablation resulted in decreased neutrophil recruitment and increased intrahepatic bacterial burden. Thus, the liver contains two separate and non-overlapping niches occupied by distinct resident macrophage populations mediating immunosurveillance at these two pathogen entry points to the liver.

Up-regulation of LFA-1 allows liver-resident memory T cells to patrol and remain in the hepatic sinusoids

Liver-resident CD8+ T cells are highly motile cells that patrol the vasculature and provide protection against liver pathogens. A key question is: how can these liver CD8+ T cells be simultaneously present in the circulation and tissue-resident? Because liver-resident T cells do not express CD103 - a key integrin for T cell residence in epithelial tissues - we investigated other candidate adhesion molecules. Using intra-vital imaging we found that CD8+ T cell patrolling in the hepatic sinusoids is dependent upon LFA-1-ICAM-1 interactions. Interestingly, liver-resident CD8+ T cells up-regulate LFA-1 compared to effector-memory cells, presumably to facilitate this behavior. Finally, we found that LFA-1 deficient CD8+ T cells failed to form substantial liver-resident memory populations following Plasmodium or LCMV immunization. Collectively, our results demonstrate that it is adhesion through LFA-1 that allows liver-resident memory CD8+ T cells to patrol and remain in the hepatic sinusoids.

The RhoGAP protein ARHGAP18/SENEX localizes to microtubules and regulates their stability in endothelial cells

Localization of a regulator of RhoGTPases (ARHGAP18) is important for microtubule stability and endothelial cell function. The localization is demonstrated by advanced imaging and biochemical techniques.

RhoGTPases are important regulators of the cell cytoskeleton, controlling cell shape, migration and proliferation. Previously we showed that ARHGAP18 in endothelial cells is important in cell junctions. Here we show, using structured illumination microscopy (SIM), ground-state depletion (GSD), and total internal reflection fluorescence microscopy (TIRF) that a proportion of ARHGAP18 localizes to microtubules in endothelial cells, as well as in nonendothelial cells, an association confirmed biochemically. In endothelial cells, some ARHGAP18 puncta also colocalized to Weibel–Palade bodies on the microtubules. Depletion of ARHGAP18 by small interfering RNA or analysis of endothelial cells isolated from ARHGAP18-knockout mice showed microtubule destabilization, as evidenced by altered morphology and decreased acetylated α-tubulin and glu-tubulin. The destabilization was rescued by inhibition of ROCK and histone deacetylase 6 but not by a GAP-mutant form of ARHGAP18. Depletion of ARHGAP18 resulted in a failure to secrete endothelin-1 and a reduction in neutrophil transmigration, both known to be microtubule dependent. Thrombin, a critical regulator of the Rho-mediated barrier function of endothelial cells through microtubule destabilization, enhanced the plasma membrane–bound fraction of ARHGAP18. Thus, in endothelial cells, ARHGAP18 may act as a significant regulator of vascular homeostasis.

The pro-fibrotic role of dipeptidyl peptidase 4 in carbon tetrachloride-induced experimental liver injury

Liver fibrosis is a progressive pathological process involving inflammation and extracellular matrix deposition. Dipeptidyl peptidase 4 (DPP4), also known as CD26, is a cell surface glycoprotein and serine protease. DPP4 binds to fibronectin, can inactivate specific chemokines, incretin hormone and neuropeptides, and influences cell adhesion and migration. Such properties suggest a pro-fibrotic role for this peptidase but this hypothesis needs in vivo examination. Experimental liver injury was induced with carbon tetrachloride (CCl₄) in DPP4 gene knockout (gko) mice. DPP4 gko had less liver fibrosis and inflammation and fewer B cell clusters than wild type mice in the fibrosis model. DPP4 inhibitor-treated mice also developed less liver fibrosis. DNA microarray and PCR showed that many immunoglobulin (Ig) genes and some metabolism-associated transcripts were differentially expressed in the gko strain compared with wild type. CCl₄-treated DPP4 gko livers had more IgM⁺ and IgG⁺ intrahepatic lymphocytes, and fewer CD4⁺, IgD⁺ and CD21⁺ intrahepatic lymphocytes. These data suggest that DPP4 is pro-fibrotic in CCl₄-induced liver fibrosis and that the mechanisms of DPP4 pro-fibrotic action include energy metabolism, B cells, NK cells and CD4⁺ cells.

Suicidal emperipolesis: a process leading to cell-in-cell structures, T cell clearance and immune homeostasis

"Suicidal emperipolesis" is one of the most recently reported processes leading to cell-in-cell structures that promote cell death. This process was discovered in studies investigating the fate of autoreactive CD8 T cells activated within the liver. Recently, we reported that activated T cells invaded hepatocytes, formed transient cell-in-cell structures, and were rapidly degraded within endosomal/lysosomal compartments by a non-apoptotic pathway. Importantly, pharmacological inhibition of this process caused intrahepatic accumulation of tissue-reactive T cells and breach of immune tolerance. The characterization of the molecular mechanisms of suicidal emperipolesis is still in its infancy, but initial studies suggest this phenomenon is distinct from other reported cell-in-cell structures. As opposed to the formation of other cell-in-cell structures, suicidal emperipolesis takes place in a non-malignant environment, and without obvious pathology. It is therefore the first cell-in-cell structure described to have a role in maintaining homeostasis in normal physiology in higher organisms. T cell emperipolesis within hepatocytes has also been observed by pathologists in a range of chronic human liver pathologies. As T cell-in-hepatocyte structures resulting from suicidal emperipolesis are very transiently observed in normal physiology, their accumulation during liver disease would suggest that severe tissue injury is promoted by, or associated with, defective T cell clearance. In this review, we compare "suicidal emperipolesis" to other processes leading to cell-in-cell structures, and consider its potential biological roles in maintaining immune homeostasis and tolerance in the context of the hepatic environment.

Effector T cell function rather than survival determines extent and duration of hepatitis in mice

BACKGROUND & AIMS

Acute hepatitis is often mediated by cytotoxic T lymphocytes (CTLs); however, the intrinsic parameters that limit CTL-mediated liver injury are not well understood.

METHODS

To investigate whether acute liver damage is limited by molecules that decrease the lifespan or effector function of CTLs, we used a well-characterized transgenic (Tg) mouse model in which acute liver damage develops upon transfer of T cell receptor (TCR) Tg CD8 T cells. Recipient Tg mice received donor TCR Tg T cells deficient for either the pro-apoptotic molecule Bim, which regulates CTL survival, or suppressor of cytokine signaling-1 (SOCS-1), which controls expression of common gamma chain cytokines; the effects of anti-PD-L1 neutralizing antibodies were also assessed.

RESULTS

Use of Bim-deficient donor T cells and/or PD-L1 blockade increased the number of intrahepatic T cells without affecting the degree and kinetic of acute hepatitis. In contrast, SOCS-1-deficient T cells induced a heightened, prolonged acute hepatitis caused by their enhanced cytotoxic function and increased expansion. Although they inflicted more severe acute liver damage, SOCS-1-deficient T cells never precipitated chronic hepatitis and became exhausted.

CONCLUSIONS

The degree of acute hepatitis is regulated by the function of CD8 T cells, but is not affected by changes in CTL lifespan. Although manipulation of the examined parameters affected acute hepatitis, persistent hepatitis did not ensue, indicating that, in the presence of high intrahepatic antigen load, changes in these factors in isolation were not sufficient to prevent T cell exhaustion and mediate progression to chronic hepatitis.

Immune outcomes in the liver: Is CD8 T cell fate determined by the environment?

The liver is known for its tolerogenic properties. This unique characteristic is associated with persistent infection of the liver by the hepatitis B and C viruses. Improper activation of cellular adaptive immune responses within the liver and immune exhaustion over time both contribute to ineffective cytotoxic T cell responses to liver-expressed antigens in animal models, and likely play a role in incomplete clearance of chronic hepatitis virus infections in humans. However, under some conditions, functional immune responses can be elicited against hepatic antigens, resulting in control of hepatotropic infections. In order to develop improved therapeutics in immune-mediated chronic liver diseases, including viral hepatitis, it is essential to understand how intrahepatic immunity is regulated. This review focuses on CD8 T cell immunity directed towards foreign antigens expressed in the liver, and explores how the liver environment dictates the outcome of intrahepatic CD8 T cell responses. Potential strategies to rescue unresponsive CD8 T cells in the liver are also discussed.

Selective Treg reconstitution during lymphopenia normalizes DC costimulation and prevents graft-versus-host disease

Regulatory T cells (Tregs) have been shown to enhance immune reconstitution and prevent graft-versus-host disease (GVHD) after hematopoietic stem cell transplantation; however, it is unclear how Tregs mediate these effects. Here, we developed a model to examine the mechanism of Treg-dependent regulation of immune reconstitution. Lymphopenic mice were selectively reconstituted with Tregs prior to transfer of conventional CD4+ T cells. Full Treg reconstitution prevented the rapid oligoclonal proliferation that gives rise to pathogenic CD4 effector T cells, while preserving the slow homeostatic form of lymphopenia-induced peripheral expansion that repopulates a diverse peripheral T cell pool. Treg-mediated CTLA-4-dependent downregulation of CD80/CD86 on DCs was critical for inhibition of rapid proliferation and was a function of the Treg/DC ratio achieved by reconstitution. In an allogeneic BM transplant model, selective Treg reconstitution before T cell transfer also normalized DC costimulation and provided complete protection against GVHD. In contrast, cotransfer of Tregs was not protective. Our results indicate that achieving optimal recovery from lymphopenia should aim to improve early Treg reconstitution in order to increase the relative number of Tregs to DCs and thereby inhibit spontaneous oligoclonal T cell proliferation.

The level of antigen expression in the liver predicts the fate and functional outcome of antigen-specific CD8 T cells (LYM5P.706)

A wide spectrum of CD8 T cell responses can be elicited against antigens (Ag) expressed in the liver, ranging from tolerance to full effector function. Identifying the factors involved would allow us to predict the outcome of liver transplantation and viral hepatitis. To investigate how CD8 T cells respond to liver Ags, we have developed recombinant adeno-associated viral vectors that, when injected into mice, allow de novo Ag expression in a variable number of hepatocytes. Within the first day of Ag expression by at least 70% of hepatocytes, most naïve Ag-specific CD8 T cells were activated in the liver via direct presentation by hepatocytes. CD8 T cell activation in lymph nodes and spleen via cross presentation was detected a day later, suggesting that primary activation of CD8 T cells against hepatocyte-expressed Ag is compartmentalised. These T cells proliferated, developed into CTLs in the first week but were unable to kill all Ag-expressing hepatocytes. Most effector T cells were rapidly deleted while those surviving became exhausted and unresponsive over time. In contrast, when less than 10% of hepatocytes expressed the targeted Ag, CD8 T cell were gradually activated in liver and lymphoid tissues but were not deleted. Instead, they eliminated all Ag-expressing hepatocytes and maintained their effector function overtime. In summary, this study shows that intrahepatic Ag load dictates both the fate and function of CD8 T cells recognising hepatocyte-expressed Ags.

Malaria and the liver: immunological hide-and-seek or subversion of immunity from within?

During the pre-erythrocytic asymptomatic phase of malarial infection, sporozoites develop transiently inside less than 100 hepatocytes that subsequently release thousands of merozoites. Killing of these hepatocytes by cytotoxic T cells (CTLs) confers protection to subsequent malarial infection, suggesting that this bottleneck phase in the parasite life cycle can be targeted by vaccination. During natural transmission, although some CTLs are generated in the skin draining lymph nodes, they are unable to eliminate the parasite, suggesting that the liver is important for the sporozoite to escape immune surveillance. The contribution of the organ to this process is unclear. Based on the known ability of several hepatic antigen-presenting cells (APCs) to induce primary activation of CD8 T cells and tolerance, malarial antigens presented by both infected hepatocytes and/or hepatic cross-presenting APCs should result in tolerance. However, our latest model predicts that due to the low frequency of infected hepatocytes, some T cells recognizing sporozoite epitopes with high affinity should differentiate into CTLs. In this review, we discuss two possible models to explain why CTLs generated in the liver and skin draining lymph nodes are unable to eliminate the parasite: (1) sporozoites harness the tolerogenic property of the liver; (2) CTLs are not tolerized but fail to detect infected cells due to sparse infection of hepatocytes and the very short liver stage. We propose that while malaria sporozoites might use the ability of the liver to tolerize both naive and effector cells, they have also developed strategies to decrease the probability of encounter between CTLs and infected liver cells. Thus, we predict that to achieve protection, vaccination strategies should aim to boost intrahepatic activation and/or increase the chance of encounter between sporozoite-specific CTLs and infected hepatocytes.

Caveolae control the anti-inflammatory phenotype of senescent endothelial cells

Senescent endothelial cells (EC) have been identified in cardiovascular disease, in angiogenic tumour associated vessels and in aged individuals. We have previously identified a novel anti-inflammatory senescent phenotype of EC. We show here that caveolae are critical in the induction of this anti-inflammatory senescent state. Senescent EC induced by either the overexpression of ARHGAP18/SENEX or by H₂O₂ showed significantly increased numbers of caveolae and associated proteins Caveolin-1, cavin-1 and cavin-2. Depletion of these proteins by RNA interference decreased senescence induced by ARHGAP18 and by H₂O₂. ARHGAP18 overexpression induced a predominantly anti-inflammatory senescent population and depletion of the caveolae-associated proteins resulted in the preferential reduction in this senescent population as measured by neutrophil adhesion and adhesion protein expression after TNFα treatment. In confirmation, EC isolated from the aortas of CAV-1(-/-) mice failed to induce this anti-inflammatory senescent cell population upon expression of ARHGAP18, whereas EC from wild-type mice showed a significant increase. NF-κB is one of the major transcription factors mediating the induction of E-selectin and VCAM-1 expression, adhesion molecules responsible for leucocyte attachment to EC. TNFα-induced activation of NF-κB was suppressed in ARHGAP18-induced senescent EC, and this inhibition was reversed by Caveolin-1 knock-down. Thus, out results demonstrate that an increase in caveolae and its component proteins in senescent ECs is associated with inhibition of the NF-kB signalling pathway and promotion of the anti-inflammatory senescent pathway.

miR-181a mediates TGF-β-induced hepatocyte EMT and is dysregulated in cirrhosis and hepatocellular cancer

BACKGROUND & AIMS

Epithelial-mesenchymal transition (EMT) has been implicated in the processes of embryogenesis, tissue fibrosis and carcinogenesis. Transforming growth factor-β (TGF-β) has been identified as a key driver of EMT and plays a key role in the pathogenesis of cirrhosis and hepatocellular carcinoma (HCC). The aim was to identify microRNA (miR) expression in TGF-β-induced hepatocyte EMT.

METHODS

We treated a human hepatocyte cell line PH5CH8 with TGF-β to induce an EMT-like change in phenotype and then identified dysregulated miRs using TaqMan Low Density Arrays. MiR expression was altered using miR-181a mimic and inhibitor in the same system and gene changes were identified using TaqMan gene arrays. MiR-181a gene expression was measured in human and mouse cirrhotic or HCC liver tissue samples. Gene changes were identified in rAAV-miR-181a-expressing mouse livers using TaqMan gene arrays.

RESULTS

We identified miR-181a as a miR that was significantly up-regulated in response to TGF-β treatment. Over-expression of a miR-181a mimic induced an in vitro EMT-like change with a phenotype similar to that seen with TGF-β treatment alone and was reversed using a miR-181a inhibitor. MiR-181a was shown to be up-regulated in experimental and human cirrhotic and HCC tissue. Mouse livers expressing rAAV-miR-181a showed genetic changes associated with TGF-β signalling and EMT.

CONCLUSIONS

MiR-181a had a direct effect in inducing hepatocyte EMT and was able to replace TGF-β-induced effects in vitro. MiR-181a was over-expressed in cirrhosis and HCC and is likely to play a role in disease pathogenesis.

Altered MENIN expression disrupts the MAFA differentiation pathway in insulinoma

The protein MENIN is the product of the multiple endocrine neoplasia type I (MEN1) gene. Altered MENIN expression is one of the few events that are clearly associated with foregut neuroendocrine tumours (NETs), classical oncogenes or tumour suppressors being not involved. One of the current challenges is to understand how alteration of MENIN expression contributes to the development of these tumours. We hypothesised that MENIN might regulate factors maintaining endocrine-differentiated functions. We chose the insulinoma model, a paradigmatic example of well-differentiated pancreatic NETs, to study whether MENIN interferes with the expression of v-MAF musculoaponeurotic fibrosarcoma oncogene homologue A (MAFA), a master glucose-dependent transcription factor in differentiated β-cells. Immunohistochemical analysis of a series of human insulinomas revealed a correlated decrease in both MENIN and MAFA. Decreased MAFA expression resulting from targeted Men1 ablation was also consistently observed in mouse insulinomas. In vitro analyses using insulinoma cell lines showed that MENIN regulated MAFA protein and mRNA levels, and bound to Mafa promoter sequences. MENIN knockdown concomitantly decreased mRNA expression of both Mafa and β-cell differentiation markers (Ins1/2, Gck, Slc2a2 and Pdx1) and, in parallel, increased the proliferation rate of tumours as measured by bromodeoxyuridine incorporation. Interestingly, MAFA knockdown alone also increased proliferation rate but did not affect the expression of candidate proliferation genes regulated by MENIN. Finally, MENIN variants with missense mutations detected in patients with MEN1 lost the WT MENIN properties to regulate MAFA. Together, our findings unveil a previously unsuspected MENIN/MAFA connection regarding control of the β-cell differentiation/proliferation balance, which could contribute to tumorigenesis.

Differential migration of passenger leukocytes and rapid deletion of naive alloreactive CD8 T cells after mouse liver transplantation

Donor passenger leukocytes (PLs) from transplanted livers migrate to recipient lymphoid tissues, where they are thought to induce the deletion of donor-specific T cells and tolerance. Difficulties in tracking alloreactive T cells and PLs in rats and in performing this complex surgery in mice have limited progress in identifying the contribution of PL subsets and sites and the kinetics of T cell deletion. Here we developed a mouse liver transplant model in which PLs, recipient cells, and a reporter population of transgenic CD8 T cells specific for the graft could be easily distinguished and quantified in allografts and recipient organs by flow cytometry. All PL subsets circulated rapidly via the blood as soon as 1.5 hours after transplantation. By 24 hours, PLs were distributed differently in the lymph nodes and spleen, whereas donor natural killer and natural killer T cells remained in the liver and blood. Reporter T cells were activated in both liver and lymphoid tissues, but their numbers dramatically decreased within the first 48 hours. These results provide the first unequivocal demonstration of the differential recirculation of liver PL subsets after transplantation, and show that alloreactive CD8 T cells are deleted more rapidly than initially reported. This model will be useful for dissecting early events leading to the spontaneous acceptance of liver transplants.

Regulation of dipeptidyl peptidase 8 and 9 expression in activated lymphocytes and injured liver

AIM

To investigate the expression of dipeptidyl peptidase (DPP) 8 and DPP9 in lymphocytes and various models of liver fibrosis.

METHODS

DPP8 and DPP9 expression were measured in mouse splenic CD4⁺ T-cells, CD8⁺ T-cells and B-cells (B220⁺), human lymphoma cell lines and mouse splenocytes stimulated with pokeweed mitogen (PWM) or lipopolysaccharide (LPS), and in dithiothreitol (DTT) and mitomycin-C treated Raji cells. DPP8 and DPP9 expression were measured in epidermal growth factor (EGF) treated Huh7 hepatoma cells, in fibrotic liver samples from mice treated with carbon tetrachloride (CCl₄) and from multidrug resistance gene 2 (Mdr2/Abcb4) gene knockout (gko) mice with biliary fibrosis, and in human end stage primary biliary cirrhosis (PBC).

RESULTS

All three lymphocyte subsets expressed DPP8 and DPP9 mRNA. DPP8 and DPP9 expression were upregulated in both PWM and LPS stimulated mouse splenocytes and in both Jurkat T- and Raji B-cell lines. DPP8 and DPP9 were downregulated in DTT treated and upregulated in mitomycin-C treated Raji cells. DPP9-transfected Raji cells exhibited more annexin V⁺ cells and associated apoptosis. DPP8 and DPP9 mRNA were upregulated in CCl₄ induced fibrotic livers but not in the lymphocytes isolated from such livers, while DPP9 was upregulated in EGF stimulated Huh7 cells. In contrast, intrahepatic DPP8 and DPP9 mRNA expression levels were low in the Mdr2 gko mouse and in human PBC compared to non-diseased livers.

CONCLUSION

These expression patterns point to biological roles for DPP8 and DPP9 in lymphocyte activation and apoptosis and in hepatocytes during liver disease pathogenesis.

Both passenger leucocytes and hepatic parenchyma contribute to activation and deletion of graft-reactive CD8 T cells in liver transplantation (P2130)

Unlike most solid organs, liver transplants are spontaneously accepted across MHC mismatch and induce donor-specific tolerance. The mechanisms and site of tolerance induction remain unclear. Both recipient lymphoid tissues (RLT) where donor passenger leukocytes (PL) migrate, and the liver parenchyma itself are thought to contribute to this process by inducing abortive activation of alloreactive T cells. To determine the relative contribution of these compartments to alloreactive T cell fate, we developed a murine liver transplant model in which we traced the fate of PL and a naïve alloreactive CD8 T cell reporter population specific for donor MHC. Ly5.2+C57BL/6 livers were transplanted into allogeneic Ly5.1+B10.BR recipients. Directly allograft reactive Des-TCR transgenic T cells specific for donor H-2Kb were adoptively transferred as a reporter population. Donor and recipient leukocyte location and fate were traced by flow cytometry and radiolabelling. All Des T cells were rapidly activated in RLT and liver but numbers dropped dramatically within 48h, with cells resident in RLT predominantly dying in situ, while most circulating Des T cells were deleted in the liver. Intrahepatic clearance was associated with degradation of Des T cells in hepatocyte lysosomal compartments. In conclusion, these results show that both PL and the hepatic parenchyma contribute to deletion of graft reactive T cells, and reveal a novel mechanism of tolerance induction within the hepatic allograft.

The magnitude of acute hepatitis is more limited by regulation of cytokine signalling than apoptosis of liver-infiltrating effector CD8 T cells (P4174)

Acute hepatitis is often mediated by CD8 T cells that kill target cells or secrete hepatotoxic cytokines. However, the parameters regulating CD8 T cell-mediated acute hepatitis are not well understood. We used the well-characterized Met-Kb Tg mouse model of acute hepatitis, in which hepatitis is mediated by lymph node activated TCR Tg CD8 recognizing their cognate antigen in the liver. To investigate whether cytokine regulation or apoptosis of CD8 T cells were critical in limiting acute hepatocellular injury, adoptive transfer experiments were performed using TCR Tg T cells deficient for either suppressor of cytokine signaling (SOCS-1) or the pro-apoptotic molecule Bim. Although there was a substantial accumulation of Bim-/- Tg cells, the outcome of hepatitis remained unchanged, suggesting that effector T cell survival was not a critical parameter in limiting liver damage. In contrast, SOCS-1-/- Tg T cells induced a heightened severity of hepatitis than their wt counterparts. SOCS-1-/- Tg cells isolated from the liver displayed upregulated IL-2Rα, required for the high affinity IL-2 receptor, and was associated with higher levels of IFN-γ expression, CTL activity, and proliferation rates, consistent with enhanced effector function. These data support a critical role for cytokines in CTL function, and demonstrate that the propensity of CD8 T cells to mediate acute hepatitis is determined by the quality rather than the number of CTLs infiltrating the liver.

Two lymph nodes draining the mouse liver are the preferential site of DC migration and T cell activation

BACKGROUND & AIMS

Lymph nodes (LNs) play a critical role in host defence against pathogens. In rodents, lymphatic anatomy and drainage have been characterized for many different organs. Surprisingly, the LNs draining the mouse liver have not been clearly identified. This knowledge is of central importance to allow accurate characterization of immune responses to pathogens infecting the liver. It is also important for exploring immune responses in hepatic tumour models, and mechanisms underlying the relative tolerogenic properties of the liver. In this study, we used both anatomical and immunological approaches to identify the LN(s) draining the mouse liver.

METHODS

Evans Blue and purified dendritic cells were directly injected into the hepatic parenchyma.

RESULTS

Using Evans Blue, we identified three LNs adjacent to the liver that stained with the dye within the first 5 min, which we termed portal, coeliac, and first mesenteric LNs. We also provide evidence that dendritic cells (DCs) injected under the liver capsule preferentially migrate to the coeliac and portal nodes, leading to local activation of antigen-specific naïve CD8 and CD4 T cells, suggesting this is a route of lymphatic drainage from the liver. Consistent with this result, cell-associated antigen injected under the liver capsule was also cross-presented to CD8 T cells in these nodes.

CONCLUSIONS

These results suggest for the first time that the coeliac and portal nodes are the main LNs draining the liver, and that DCs exiting the liver can elicit primary T cell activation within these lymph nodes; first mesenteric nodes play a secondary role. We propose this nomenclature to be used as common designations for the observed structures.

Role of the hepatic parenchyma in liver transplant tolerance: a paradigm revisited

Unlike other solid organs, liver transplants are spontaneously accepted in a wide range of animal models. In the clinic, transplanted livers also display privileged immunological properties allowing weaning of immunosuppression therapy in up to 20% of selected patients. To explain this phenomenon, many studies have focused on the role of donor-derived 'passenger' leukocytes that are thought to induce antigen-specific tolerance by migrating from the graft into recipient secondary lymphoid tissues. Although convincing evidence exists that these cells are able to elicit antiallograft T cell hyporesponsiveness, several studies argue against an exclusive role for this cell population and even question whether it is critical in conferring donor MHC-specific tolerance. Instead, these studies suggest that the hepatic parenchyma plays a more critical role in this phenomenon. In this review we will reinterpret the results of old and more recent literature in light of recent advances in the field of liver immunology to explain the contribution of both passenger leukocytes and liver tissue in the liver tolerance effect.

Permanent genetic resources added to Molecular Ecology Resources Database 1 August 2010-30 September 2010

This article documents the addition of 229 microsatellite marker loci to the Molecular Ecology Resources Database. Loci were developed for the following species: Acacia auriculiformis × Acacia mangium hybrid, Alabama argillacea, Anoplopoma fimbria, Aplochiton zebra, Brevicoryne brassicae, Bruguiera gymnorhiza, Bucorvus leadbeateri, Delphacodes detecta, Tumidagena minuta, Dictyostelium giganteum, Echinogammarus berilloni, Epimedium sagittatum, Fraxinus excelsior, Labeo chrysophekadion, Oncorhynchus clarki lewisi, Paratrechina longicornis, Phaeocystis antarctica, Pinus roxburghii and Potamilus capax. These loci were cross-tested on the following species: Acacia peregrinalis, Acacia crassicarpa, Bruguiera cylindrica, Delphacodes detecta, Tumidagena minuta, Dictyostelium macrocephalum, Dictyostelium discoideum, Dictyostelium purpureum, Dictyostelium mucoroides, Dictyostelium rosarium, Polysphondylium pallidum, Epimedium brevicornum, Epimedium koreanum, Epimedium pubescens, Epimedium wushanese and Fraxinus angustifolia.

The impact of poloxamer 407 on the ultrastructure of the liver and evidence for clearance by extensive endothelial and kupffer cell endocytosis

Poloxamer 407 (P407) is a non-ionic detergent that is used widely in pharmaceutical formulations and personal care products. In animals, P407 causes hyperlipidaemia. P407 is taken up by the liver and causes loss of fenestrations in liver sinusoidal endothelial cells (LSEC), which contributes to the pathogenesis of hyperlipidaemia. Here the short-term (1-15 days) effects of P407 on all liver cells were investigated in mice using electron and light microscopy. As expected, P407 was associated with hyperlipidaemia. Kupffer cells became massively engorged with vacuoles and took on a marked honeycomb morphology. LSECs also became engorged with vacuoles and endocytosis was activated. The diameter of lipoproteins in the space of Disse was less than those in the lumen, consistent with a filtering effect of fenestrations. Defenestration of the LSEC was noted. Hepatocyte endocytosis of lipoproteins and P407 particles was also noted; however, hepatocyte steatosis was not evident. Hepatic stellate cells did not appear to be abnormal. In conclusion, P407 is taken up by the liver mostly through endocytosis by LSECs and Kupffer cells.

Spontaneous acceptance of mouse kidney allografts is associated with increased Foxp3 expression and differences in the B and T cell compartments

Spontaneous acceptance of organ allografts can identify novel mechanisms of drug-free transplantation tolerance. Spontaneous acceptance occurs in both mouse kidney transplants and rat liver transplants however the early immune processes of mouse kidney acceptance have not been studied. Acceptance of C57BL/6 strain kidney allografts in fully MHC-incompatible B10.BR recipients was compared with rejection (REJ) of heart allografts in the same strain combination. Graft infiltrate and antibody deposition were examined by immunohistochemical staining. Expression of mRNA was measured by quantitative real-time PCR. Apoptosis was examined by TUNEL staining. The majority of kidney allografts were accepted long-term and induced tolerance (TOL) of donor-strain skin grafts, showing that acceptance was not due to immune ignorance. There was an extensive infiltrate of T cells in the TOL kidney that exceeded the level in REJ hearts but subsequently declined. The main differences were deposition of IgG2a antibody in REJ that was absent in TOL, more B cells infiltrating TOL kidneys and a progressive increase in the ratio of CD8:CD4 cells during rejection. There was also significantly greater Foxp3 mRNA expression in TOL. Kidneys from RAG-/- donors were accepted, showing that donor lymphocytes were not necessary for acceptance. Neutralising antibodies to TGF-β administered from day 0 to day 6 did not prevent TOL. On the basis of cytokine expression and apoptosis there was no evidence for immune deviation or deletion as mechanisms of acceptance. In accord with the findings of spontaneous acceptance of liver allografts in rats, the main difference between mouse kidney TOL and heart REJ was in the B cell compartment. The major difference to rat liver allograft acceptance was that apoptosis of infiltrate did not appear to play a role. Instead, increased Foxp3 expression in TOL kidneys implies that regulatory T cells might be important.

Invading macrophages play a major role in the liver progenitor cell response to chronic liver injury

BACKGROUND & AIMS

Although a strong association between liver progenitor cells (LPCs) and inflammation exists in many chronic liver diseases, the exact role of the immune system in LPC-mediated hepatic regeneration remains unclear. A number of pro-inflammatory factors were identified in cytokine knockout mice in which the LPC response was attenuated but neither the mechanism nor the producing cells are known.

METHODS

To identify the critical immune cells and cytokines required in the LPC response, we compared two diet-induced models of liver injury with two recently established transgenic models of immune-mediated hepatitis.

RESULTS

Despite severe inflammation being observed in all models, the generation of LPCs was highly dependent on the cause and kinetics of liver damage. The LPC response was associated with an increase of macrophages and CD8(+) T cells but not natural killer cells. T cell-deficient mice were able to mount a LPC response, albeit delayed, suggesting that T cells are not essential. Mice mounting an LPC response showed elevated numbers of Kupffer cells and invading CX(3)CR1(high)CCR2(high) macrophages secreting persistent high levels of tumour necrosis factor alpha (TNFalpha), a major cytokine involved in the LPC response.

CONCLUSIONS

Liver macrophages are an important determinant of LPC expansion during liver regeneration in models of diet- and immune-mediated liver injury. Invading macrophages in particular provide pro-mitogenic cytokines such as TNFalpha that underpin the process. LPC themselves are a source of chemokines (CCL2, CX(3)CL1) that attract infiltrating macrophages.

Mechanisms of T cell death in the liver: to Bim or not to Bim?

Despite being a non-lymphoid organ, the liver displays immunological properties distinct from other solid organs and is associated with the induction of T cell tolerance. This property has been demonstrated in several clinical settings including transplantation and hepatotropic viral infections, such as those induced by hepatitis B and C viruses. Many models have been proposed to explain the 'liver tolerance effect', but the molecular and cellular mechanism(s) mediating this phenomenon remain unknown. Using transgenic mouse models, we have previously shown that the liver is the only non-lymphoid organ able to retain and activate naïve CD8+ T cells independently of lymphoid tissues in an antigen-specific manner. These findings, confirmed by other groups, have opened new possibilities to explain the remarkable capacity of the liver to induce antigen-specific tolerance in transplantation and following infection by hepatotropic viruses, such as the hepatitis C and B viruses. In our models, T cells activated by hepatocytes that proliferate die by neglect in a Bim-dependent manner. This paper will thus review the evidence showing Bim playing a critical role following intrahepatic primary T cell activation.

Liver tolerance and the manipulation of immune outcomes

In recent years it has become apparent that the liver holds a distinct immunological position. Previously described as a "graveyard" for T cells activated in the periphery, emerging evidence indicates that this organ may have a more active role in mediating tolerance. Attenuated immune responses in the liver can be beneficial in the transplantation setting, as liver transplants are more readily accepted than other organ allografts even in the absence of immunosuppressive drugs. However, the ability of the liver to induce immunological unresponsiveness could be exploited by some pathogens, such as the hepatitis C virus (HCV), to establish chronic infections with potentially fatal outcomes. Understanding the mechanisms controlling the balance between intrahepatic tolerance and immunity is critical in order to design new strategies to enhance acceptance of solid organ allografts and to promote efficient immune responses against HCV. In this article, we will review current knowledge of the mechanisms regulating intrahepatic immunity and discuss how these mechanisms might potentially be targeted to achieve advantageous clinical outcomes in transplantation and persistent hepatotropic infections.

Intrahepatic murine CD8 T-cell activation associates with a distinct phenotype leading to Bim-dependent death

BACKGROUND & AIMS

Chronic infections by hepatotropic viruses such as hepatitis B and C are generally associated with an impaired CD8 T-cell immune response that is unable to clear the virus. The liver is increasingly recognized as an alternative site in which primary activation of CD8 T cells takes place, a property that might explain its role in inducing tolerance. However, the molecular mechanism by which intrahepatically activated T cells become tolerant is unknown. Here, we investigated the phenotype and fate of naïve CD8 T cells activated by hepatocytes in vivo.

METHODS

Transgenic mouse models in which the antigen is expressed in lymph nodes and/or in the liver were adoptively transferred with naïve CD8 T cells specific for the hepatic antigen.

RESULTS

Liver-activated CD8 T cells displayed poor effector functions and a unique CD25(low) CD54(low) phenotype. This phenotype was associated with increased expression of the proapoptotic protein Bim and caspase-3, demonstrating that these cells are programmed to die following intrahepatic activation. Importantly, we show that T cells deficient for Bim survived following intrahepatic activation.

CONCLUSIONS

This study identifies Bim for the first time as a critical initiator of T-cell death in the liver. Thus, strategies inhibiting the up-regulation of this molecule could potentially be used to rescue CD8 T cells, clear the virus, and reverse the outcome of viral chronic infections affecting the liver.