Role of LPS in the hepatic microvascular dysfunction elicited by cecal ligation and puncture in mice

Advances in Rodent Experimental Models of Sepsis

In the development of therapeutic strategies for human diseases, preclinical experimental models have a key role. However, the preclinical immunomodulatory therapies developed using rodent sepsis were not successful in human clinical trials. Sepsis is characterized by a dysregulated inflammation and redox imbalance triggered by infection. Human sepsis is simulated in experimental models using methods that trigger inflammation or infection in the host animals, most often mice or rats. It remains unknown whether the characteristics of the host species, the methods used to induce sepsis, or the molecular processes focused upon need to be revisited in the development of treatment methods that will succeed in human clinical trials. Our goal in this review is to provide a survey of existing experimental models of sepsis, including the use of humanized mice and dirty mice, and to show how these models reflect the clinical course of sepsis. We will discuss the strengths and limitations of these models and present recent advances in this subject area. We maintain that rodent models continue to have an irreplaceable role in studies toward discovering treatment methods for human sepsis.

Vascular Endothelial Cells and Innate Immunity

In addition to the roles of endothelial cells (ECs) in physiological processes, ECs actively participate in both innate and adaptive immune responses. We previously reported that, in comparison to macrophages, a prototypic innate immune cell type, ECs have many innate immune functions that macrophages carry out, including cytokine secretion, phagocytic function, antigen presentation, pathogen-associated molecular patterns-, and danger-associated molecular patterns-sensing, proinflammatory, immune-enhancing, anti-inflammatory, immunosuppression, migration, heterogeneity, and plasticity. In this highlight, we introduce recent advances published in both ATVB and many other journals: (1) several significant characters classify ECs as novel immune cells not only in infections and allograft transplantation but also in metabolic diseases; (2) several new receptor systems including conditional danger-associated molecular pattern receptors, nonpattern receptors, and homeostasis associated molecular patterns receptors contribute to innate immune functions of ECs; (3) immunometabolism and innate immune memory determine the innate immune functions of ECs; (4) a great induction of the immune checkpoint receptors in ECs during inflammations suggests the immune tolerogenic functions of ECs; and (5) association of immune checkpoint inhibitors with cardiovascular adverse events and cardio-oncology indicates the potential contributions of ECs as innate immune cells.

Platelets in Sepsis: An Update on Experimental Models and Clinical Data

Beyond their important role in hemostasis, platelets play a crucial role in inflammatory diseases. This becomes apparent during sepsis, where platelet count and activation correlate with disease outcome and survival. Sepsis is caused by a dysregulated host response to infection, leading to organ dysfunction, permanent disabilities, or death. During sepsis, tissue injury results from the concomitant uncontrolled activation of the complement, coagulation, and inflammatory systems as well as platelet dysfunction. The balance between the systemic inflammatory response syndrome (SIRS) and the compensatory anti-inflammatory response (CARS) regulates sepsis outcome. Persistent thrombocytopenia is considered as an independent risk factor of mortality in sepsis, although it is still unclear whether the drop in platelet count is the cause or the consequence of sepsis severity. The role of platelets in sepsis development and progression was addressed in different experimental in vivo models, particularly in mice, that represent various aspects of human sepsis. The immunomodulatory function of platelets depends on the experimental model, time, and type of infection. Understanding the molecular mechanism of platelet regulation in inflammation could bring us one step closer to understand this important aspect of primary hemostasis which drives thrombotic as well as bleeding complications in patients with sterile and infectious inflammation. In this review, we summarize the current understanding of the contribution of platelets to sepsis severity and outcome. We highlight the differences between platelet receptors in mice and humans and discuss the potential and limitations of animal models to study platelet-related functions in sepsis.

Bone marrow-derived mesenchymal stem cells ameliorate liver injury in a rat model of sepsis by activating Nrf2 signaling

Sepsis is a fatal condition that leads to serious systemic inflammation and multiple organ dysfunction syndromes. This study was designed to investigate the possible therapeutic effect of bone marrow-derived mesenchymal stem cells (BMSCs) on sepsis-induced liver injury. We also aimed to examine the role of Nrf2 activation in modulating the response to sepsis following BMSCs treatment. Twenty-four adult male albino rats were assigned to: control, lipopolysaccharide (LPS) and LPS-stem cell groups. Liver samples were processed for light and electron microscope examinations. Immunohistochemical localization of BAX, proliferating cell nuclear antigen and nuclear factor-erythroid 2-related factor 2 (Nrf2) was carried out. Liver homogenates were prepared for assessment of reduced glutathione, glutathione peroxidase, tumor necrosis factor-alpha and interleukin-6 and also real-time PCR analysis of Nrf2 expression. BMSCs treatment improved the histopathological changes of the liver, enhanced tissue regeneration and decreased apoptosis following sepsis. We reported highly significant enhancement in Nrf2 expressions at mRNA and protein levels in the LPS-stem cell group compared with the LPS group. The up regulation of Nrf2 was probably implicated in decreasing inflammatory cytokine levels and counteracting oxidative stress induced by sepsis. Thus, BMSCs therapies could be a viable approach to treat sepsis-induced liver damage by activating Nrf2 signaling.

Targeting the annexin 1-formyl peptide receptor 2/ALX pathway affords protection against bacterial LPS-induced pathologic changes in the murine adrenal cortex

Hypothalamo-pituitary-adrenocortical dysfunction contributes to morbidity and mortality in a high proportion of patients with sepsis. Here, we provide new insights into the underlying adrenal pathology. Using a murine model of endotoxemia (LPS injection), we demonstrate that adrenal insufficiency is triggered early in the disease. LPS induced a local inflammatory response in the adrenal gland within 4 hours of administration, coupled with increased expression of mRNAs for annexin A1 (AnxA1) and the formyl peptide receptors [(Fprs) 1, 2, and 3], a loss of lipid droplets in cortical cells (index of availability of cholesterol, the substrate for steroidogenesis), and a failure to mount a steroidogenic response to ACTH. Deletion of AnxA1 or Fpr2/3 in mice prevented lipid droplet loss, but not leukocyte infiltration. LPS increased adrenal myeloid differentiation primary response gene 88 and TLR2 mRNA expression, but not lymphocyte antigen 96 or TLR4. By contrast, neutrophil depletion prevented leukocyte infiltration and increased AnxA1, Fpr1, and Fpr3 mRNAs but had no impact on lipid droplet loss. Our novel data demonstrate that AnxA1 and Fpr2 have a critical role in the manifestation of adrenal insufficiency in this model, through regulation of cholesterol ester storage, suggesting that pharmacologic interventions targeting the AnxA1/FPR/ALX pathway may provide a new approach for the maintenance of adrenal steroidogenesis in sepsis.

Laser speckle contrast imaging for assessment of abdominal visceral microcirculation in acute peritonitis: does sequential impairments exist?

OBJECTIVE

It is believed that the microcirculation of multiple organs is impaired during acute peritonitis, however whether distinct susceptibilities of visceral microvasculature exist is still unknown. The present study aims to verify whether the microcirculatory alterations occur sequentially among multiple abdominal viscera during acute peritonitis.

MATERIALS AND METHODS

Acute peritonitis was achieved on 29 Sprague-Dawley rats through colon ascendens stent peritonitis (CASP) model. With laser speckle contrast imaging (LSCI), the microcirculation of the liver, ileum and renal cortex was monitored in each rat at baseline before CASP sepsis and continued monitoring at 4h, 8h, or 12h after the surgery. Another 9 rats served for sham operation. One-way analysis of variance with a post hoc Dunnett's test was used for analysis.

RESULTS

The ileum microcirculation was impaired earliest from 342.1±61.0 laser speckle perfusion unit (LSPU) at baseline to 271.7±74.0 LSPU at 4h (P<0.05), while the decline of renal microcirculation was not obvious until 8h after peritonitis (289.1±111.2 vs 376.2±53.4, P<0.05). However hepatic microcirculation was not significantly changed during 12h of observation period.

CONCLUSION

The microcirculation of various viscera has shown distinct susceptibilities to acute peritonitis: the ileum is more susceptible than the kidney, while the hepatic microcirculation seems to be the most resistant to peritonitis.

An evolving new paradigm: endothelial cells--conditional innate immune cells

Endothelial cells (ECs) are a heterogeneous population that fulfills many physiological processes. ECs also actively participate in both innate and adaptive immune responses. ECs are one of the first cell types to detect foreign pathogens and endogenous metabolite-related danger signals in the bloodstream, in which ECs function as danger signal sensors. Treatment with lipopolysaccharide activates ECs, causing the production of pro-inflammatory cytokines and chemokines, which amplify the immune response by recruiting immune cells. Thus, ECs function as immune/inflammation effectors and immune cell mobilizers. ECs also induce cytokine production by immune cells, in which ECs function as immune regulators either by activating or suppressing immune cell function. In addition, under certain conditions, ECs can serve as antigen presenting cells (antigen presenters) by expressing both MHC I and II molecules and presenting endothelial antigens to T cells. These facts along with the new concept of endothelial plasticity suggest that ECs are dynamic cells that respond to extracellular environmental changes and play a meaningful role in immune system function. Based on these novel EC functions, we propose a new paradigm that ECs are conditional innate immune cells. This paradigm provides a novel insight into the functions of ECs in inflammatory/immune pathologies.

Mechanisms and functional consequences of liver failure substantially differ between endotoxaemia and faecal peritonitis in rats

BACKGROUND

Many of the concepts describing molecular mechanisms of sepsis-induced liver failure are derived from endotoxin models. However, the biological significance of such models is questionable as the complexity of clinical sepsis and associated organ failure is only partially replicated.

AIMS

Comparison of cytokine response, leucocyte recruitment, oxidative stress and markers of hepatic organ dysfunction in rat models of endotoxaemia or peritoneal contamination and infection (PCI).

METHODS

Endotoxemia and polymicrobial sepsis were induced in rats by intraperitoneal injection of lipopolysaccharide (LPS) or stool suspension, respectively.

RESULTS

Both insults produced clinical and laboratory signs of multiple organ dysfunction, including hepatic excretory dysfunction. However, TNF alpha, oxidative stress responses and the degree of cell death were significantly higher in endotoxaemia compared to PCI (e.g. serum TNF levels (pg/ml) at 1.5 h post-insult: sham 5 ± 1.4, LPS 1 mg/kg bw 2176.92 ± 373.78, sepsis below detection limit; P P < 0.05). Cholestasis was significantly more pronounced in polymicrobial sepsis whereas serum bilirubin in endotoxaemic animals did not differ from sham-operated controls (plasma levels of bilirubin (μmol/L) at 15 h after the insult: sham 7.1 ± 0.6, LPS 30 mg/kg 9.1 ± 0.6, sepsis 15.2 ± 1.3).

CONCLUSIONS

Polymicrobial sepsis produces profound hepatocellular dysfunction in the absence of traditional cytokine-mediated mechanisms of cellular injury. This questions the central role of cytokines and the ensuing oxidative stress as key molecular events in mediating liver dysfunction.

IRF3 contributes to sepsis pathogenesis in the mouse cecal ligation and puncture model

Much remains to be learned regarding which components of the innate immune response are protective versus detrimental during sepsis. Prior reports demonstrated that TLR9 and MyD88 play key roles in the CLP mouse model of sepsis; however, the role of additional PRRs and their signaling intermediates remains to be explored. In a prior report, we demonstrated that the signal adaptor IRF3 contributes to the systemic inflammatory response to liposome:DNA. We hypothesized that IRF3 might likewise promote sepsis in the CLP model. Here, we present results demonstrating that IRF3-KO mice have reduced disease score, mortality, hypothermia, and bacterial load following CLP versus WT counterparts. This is paired with reduced levels of systemic inflammatory mediators in IRF3-KO mice that undergo CLP. We demonstrate that peritoneal cells from WT CLP mice produce more cytokines than IRF3-KO counterparts on a per-cell basis; however, there are more cells in the peritoneum of IRF3-KO CLP mice. Finally, we show that IRF3 is activated in macrophages cultured with live or sonicated commensal bacteria. These results demonstrate that IRF3 plays a detrimental role in this mouse model of sepsis.

Effects of hepatocyte CD14 upregulation during cholestasis on endotoxin sensitivity

Cholestasis is frequently related to endotoxemia and inflammatory response. Our previous investigation revealed a significant increase in plasma endotoxin and CD14 levels during biliary atresia. We therefore propose that lipopolysacharides (LPS) may stimulate CD14 production in liver cells and promote the removal of endotoxins. The aims of this study are to test the hypothesis that CD14 is upregulated by LPS and investigate the pathophysiological role of CD14 production during cholestasis. Using Western blotting, qRT-PCR, and promoter activity assay, we demonstrated that LPS was associated with a significant increase in CD14 and MD2 protein and mRNA expression and CD14 promoter activity in C9 rat hepatocytes but not in the HSC-T6 hepatic stellate cell line in vitro. To correlate CD14 expression and endotoxin sensitivity, in vivo biliary LPS administration was performed on rats two weeks after they were subjected to bile duct ligation (BDL) or a sham operation. CD14 expression and endotoxin levels were found to significantly increase after LPS administration in BDL rats. These returned to basal levels after 24 h. In contrast, although endotoxin levels were increased in sham-operated rats given LPS, no increase in CD14 expression was observed. However, mortality within 24 h was more frequent in the BDL animals than in the sham-operated group. In conclusion, cholestasis and LPS stimulation were here found to upregulate hepatic CD14 expression, which may have led to increased endotoxin sensitivity and host proinflammatory reactions, causing organ failure and death in BDL rats.

Platelet-derived adenosine 5'-triphosphate suppresses activation of human hepatic stellate cell: In vitro study

AIM

  Activated hepatic stellate cells (HSC) play a critical role in liver fibrosis. Suppressing abnormal function of HSC or reversion from activated to quiescent form is a hopeful treatment for liver cirrhosis. The interaction between platelets and HSC remains unknown although platelets go through hepatic sinusoids surrounded by HSC. This study aimed at clarifying the hypothesis that platelets control activation of HSC.

METHODS

  We used human platelets, platelet extracts, and primary or immortalized human HSC. We examined the effect of platelets on the activation, DNA synthesis, type I collagen production, and fibrosis-relating gene expressions of HSC. We investigated what suppressed activation of HSC within platelets and examined the mechanism of controlling activation in vitro.

RESULTS

  Platelets and platelet extracts suppressed activation of HSC. Platelets decreased type I collagen production without affecting DNA synthesis. Platelets increased the expression of matrix metallopeptidase 1. As platelet extracts co-cultured with an enzyme of degrading adenosine 5'-triphosphate (ATP) suppressed activation, we detected adenine nucleotides within platelets or on their surfaces and confirmed the degradation of adenine nucleotides by HSC and the production of adenosine. Adenosine and platelets increased the intracellular cyclic adenosine 5'-monophosphate (cAMP), which is important in quiescent HSC. A great amount of adenosine and ATP also suppressed activation of HSC.

CONCLUSION

  Activation of human HSC is suppressed by human platelets or platelet-derived ATP via adenosine-cAMP signaling pathway in vitro. Therefore, platelets have the possibility to be used in the treatment of liver cirrhosis.

The impact of resuscitated fecal peritonitis on the expression of the hepatic bile salt transporters in a porcine model

Sepsis is often associated with cholestatic liver dysfunction caused by changes in the expression profile of hepatic bile salt transporters. However, in rodent endotoxin models, the role of ischemic hepatitis caused by liver hypoperfusion cannot be delineated. We hypothesized that hepatocytes change their expression pattern of bile salt transporters during early severe sepsis despite adequate resuscitation. Fifteen anesthetized and instrumented pigs were randomized to either fecal peritonitis (n = 8) or control (n = 7). Resuscitation was performed by hydroxyethyl starch and norepinephrine infusion. Hemodynamic parameters and markers of cholestatic and ischemic hepatic dysfunction were recorded. At baseline and after 21 h, messenger RNA (mRNA) and protein expression of bile salt transporters was determined by quantitative real-time polymerase chain reaction and immunohistochemistry, respectively, on in vivo liver biopsies. All resuscitated septic pigs developed a normotensive hyperdynamic circulation with increased portal flow. After 21 h of peritonitis, no signs of biochemical or histological cholestasis were present. Na-taurocholate cotransporting polypeptide and bile salt export pump mRNA were downregulated by 83% (P = 0.001) and 67% (P = 0.001), respectively, in comparison with controls, whereas multidrug resistance-associated protein 4 (MRP4) mRNA was upregulated by 85% (P = 0.02). Bile salt export pump and MRP2 staining were downregulated in septic pigs. During early porcine fluid-resuscitated sepsis, hepatic basolateral influx (Na-taurocholate cotransporting polypeptide) and canalicular efflux (bile salt export pump) of bile salts were downregulated without hemodynamic signs of hepatic hypoperfusion or biochemical signs of cholestasis. In parallel, the basolateral escape transport (MRP4) was markedly upregulated, possibly as an early adaptive response to counteract hepatocellular accumulation of toxic bile acids.

Western diet enhances hepatic inflammation in mice exposed to cecal ligation and puncture

Background

Obese patients display an exaggerated morbidity during sepsis. Since consumption of a western-style diet (WD) is a major factor for obesity in the United States, the purpose of the present study was to examine the influence of chronic WD consumption on hepatic inflammation in mice made septic via cecal ligation and puncture (CLP). Feeding mice diets high in fat has been shown to enhance evidence of TLR signaling and this pathway also mediates the hepatic response to invading bacteria. Therefore, we hypothesized that the combined effects of sepsis and feeding WD on TRL-4 signaling would exacerbate hepatic inflammation. Male C57BL/6 mice were fed purified control diet (CD) or WD that was enriched in butter fat (34.4% of calories) for 3 weeks prior to CLP. Intravital microscopy was used to evaluate leukocyte adhesion in the hepatic microcirculation. To demonstrate the direct effect of saturated fatty acid on hepatocytes, C3A human hepatocytes were cultured in medium containing 100 μM palmitic acid (PA). Quantitative real-time PCR was used to assess mRNA expression of tumor necrosis factor-alpha (TNF-α, monocyte chemotactic protein-1 (MCP-1), intercellular adhesion molecule-1 (ICAM-1), toll-like receptor-4 (TLR-4) and interleukin-8 (IL-8).

Results

Feeding WD increased firm adhesion of leukocytes in the sinusoids and terminal hepatic venules by 8-fold six hours after CLP; the increase in platelet adhesion was similar to the response observed with leukocytes. Adhesion was accompanied by enhanced expression of TNF-α, MCP-1 and ICAM-1. Messenger RNA expression of TLR-4 was also exacerbated in the WD+CLP group. Exposure of C3A cells to PA up-regulated IL-8 and TLR-4 expression. In addition, PA stimulated the static adhesion of U937 monocytes to C3A cells, a phenomenon blocked by inclusion of an anti-TLR-4/MD2 antibody in the culture medium.

Conclusions

These findings indicate a link between obesity-enhanced susceptibility to sepsis and consumption of a western-style diet.

Polymicrobial sepsis and endotoxemia promote microvascular thrombosis via distinct mechanisms

BACKGROUND

We reported recently that endotoxemia promotes microvascular thrombosis in cremaster venules of wild-type mice, but not in mice deficient in toll-like receptor 4 (TLR4) or von Willebrand factor (VWF).

OBJECTIVE

To determine whether the clinically relevant model of polymicrobial sepsis induced by cecal ligation/perforation (CLP) induces similar responses via the same mechanisms as endotoxemia.

METHODS

We used a light/dye-injury model of thrombosis in the cremaster microcirculation of wild-type mice and mice deficient in toll-like receptor-4 (C57BL/10ScNJ), toll-like receptor 2 (TLR2), or VWF. Mice underwent CLP or sham surgery, or an intraperitoneal injection of endotoxin (LPS) or saline. In the CLP model, we assessed the influence of fluid replacement on thrombotic responses.

RESULTS

Both CLP and LPS enhanced thrombotic occlusion in wild-type mice. In contrast to LPS, CLP enhanced thrombosis in TLR4- and VWF-deficient strains. While TLR2-deficient mice did not demonstrate enhanced thrombosis following CLP, LPS enhanced thrombosis in these mice. LPS, but not CLP, increased plasma VWF antigen relative to controls. Septic mice, particularly those undergoing CLP, developed significant hemoconcentration. Intravenous fluid replacement with isotonic saline prevented the hemoconcentration and prothrombotic responses to CLP, though fluids did not prevent the prothrombotic response to LPS.

CONCLUSIONS

Polymicrobial sepsis induced by CLP and endotoxemia promote microvascular thrombosis via distinct mechanisms; enhanced thrombosis induced by CLP requires TLR2 but not TLR4 or VWF. The salutary effects of intravenous fluid replacement on microvascular thrombosis in polymicrobial sepsis remain to be characterized.

Leptin induces an inflammatory phenotype in lean Wistar rats

The present study addressed the hypothesis that leptin promotes leukocyte trafficking into adipose tissue. Accordingly, male Wistar rats were treated with saline or recombinant rat leptin (1 mg/kg) via the tail vein. Leukocyte trafficking in mesenteric venules was quantified by intravital microscopy. Treatment with leptin resulted in a 3- and 5-fold increases in rolling and firm adhesion, respectively. Compared to vehicle controls, leptin enhanced mRNA levels of IL-6 (8-fold) and MCP-1 (5-fold) in mesenteric adipose tissue (MAT). Similar increases in these markers were observed in mesenteric venules and in liver. Finally, the direct effect of leptin was assessed in C3A hepatocytes treated with leptin for 24 hours (7.8 ng/mL–125 ng/mL). Consistent with observations in vivo, production of ICAM-1, MCP-1, and IL-6 by hepatocytes was increased significantly. These findings support the hypothesis that leptin directly initiates inflammation in the local environment of mesenteric adipose tissue as well as systemically.

Sepsis-induced intestinal microvascular and inflammatory responses in obese mice

Although clinical obesity is associated with increases in the morbidity and mortality of sepsis, little is known about the mechanisms that underlie the influence of obesity on sepsis. The objective of this study was to determine (a) whether obesity is associated with exaggerated inflammatory and thrombogenic responses in the intestinal microvasculature of septic mice and (b) whether these microvascular alterations are related to changes in the serum levels of cytokines that are produced by adipose tissue. Intravital microscopy was used to quantify leukocyte and platelet adhesion in intestinal postcapillary venules of lean wild-type (WT) mice, and two murine models of obesity, that is, ob/ob and db/db mice. Sepsis was induced by cecal ligation and perforation (CLP). Serum cytokine levels were measured using a cytometric bead assay, whereas adipokines were quantified using enzyme-linked immunosorbent assay. Cecal ligation and perforation elicited significant increases in the adhesion of leukocytes and platelets in venules of lean WT mice. These CLP-induced adhesive interactions were much more pronounced in the microvasculature of both ob/ob and db/db mice. Cecal ligation and perforation was associated with significant increases in serum cytokines in both WT and ob/ob mice, but such changes were not detected in db/db mice. However, db/db (but not WT or ob/ob) mice did exhibit significant increases in serum leptin and adiponectin levels after CLP. Sepsis promotes more intense inflammatory and thrombogenic responses in the gut microcirculation of obese mice than in their lean counterparts. The obesity-enhanced microvascular dysfunction in septic mice shows no consistent correlation with serum cytokines or adipokines.

The importance of immune dysfunction in determining outcome in acute liver failure

Acute liver failure (ALF) shares striking similarities with septic shock with regard to the features of systemic inflammation, progression to multiple organ dysfunction and functional immunoparesis. While the existence of opposing systemic pro- and anti-inflammatory profiles resulting in organ failure and immune dysfunction are well recognised in septic shock, characterization of these processes in ALF has only recently been described. This review explores the evolution of the systemic inflammation in acute liver failure, its relation to disease progression, exacerbation of liver injury and development of innate immune dysfunction and extra-hepatic organ failure as sequelae. Defects in innate immunity are described in hepatic and extra-hepatic compartments. Clinical studies measuring levels of pro- and anti-inflammatory cytokines and expression of the antigen presentation molecule HLA-DR on monocytes, in combination with ex-vivo experiments, demonstrate that the persistence of a compensatory anti-inflammatory response syndrome, leading to functional monocyte deactivation, is a central event in the evolution of systemic immune dysfunction. Accurate immune profiling in ALF may permit the development of immunomodulatory strategies in order to improve outcome in this condition.