Impaired mesenteric leukocyte recruitment in experimental portal hypertension in the rat

Cardiovascular dysfunction in sepsis at the dawn of emerging mediators

Subcellular dysfunction and impaired metabolism derived from the complex interaction of cytokines and mediators with cellular involvement are on the basis of the cardiovascular response to sepsis. The lethal consequences of an infection are intimately related to its ability to spread to other organ sites and the immune system of the host. About one century ago, William Osler (1849-1919), a Canadian physician, remarkably defined the sequelae of the host response in sepsis: "except on few occasions, the patient appears to die from the body's response to infection rather than from it." Cardiac dysfunction has received considerable attention to explain the heart failure in patients progressing from infection to sepsis, but our understanding of the processes remains limited. In fact, most concepts are linked to a mechanical concept of the sarcomeric structure, and physiological data seems to be often disconnected. Cytokines, prostanoids, and nitric oxide release are high direct impact factors, but coronary circulation and cardiomyocyte physiology also play a prominent role in modulating the effects of monocyte adhesion and infiltration. Damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs) are involved in the host response. The identification of microRNAs, as well as the cyclic activation of the inflammatory cascade, has further added complexity to the scene. In this review, we delineate the current concepts of cellular dysfunction of the cardiomyocyte in the setting of sepsis and consider potential therapeutic strategies.

Novel role of group VIB Ca2+-independent phospholipase A2γ in leukocyte-endothelial cell interactions: An intravital microscopic study in rat mesentery

BACKGROUND

Phospholipase A2 (PLA2) is associated with a variety of inflammatory processes related to polymorphonuclear neutrophil (PMN)-endothelial cell interactions. However, the cellular and molecular mechanisms underlying the interactions and the causative isoform(s) of PLA2 remain elusive. In addition, we recently showed that calcium-independent PLA2γ (iPLA2γ), but not cytosolic PLA2 (cPLA2), is responsible for the cytotoxic functions of human PMN including respiratory bursts, degranulation, and chemotaxis. We therefore hypothesized that iPLA2γ is a prerequisite for the PMN recruitment cascade into the site of inflammation. The aim of this study was to elucidate the roles of the three major phospholipases A2, iPLA2, cPLA2 and secretory PLA2, in leukocyte rolling and adherence and in the surface expression of β2-integrins in vivo and in vitro in response to well-defined stimuli.

METHODS

Male Wistar rats were pretreated with PLA2 inhibitors selective for iPLA2β, iPLA2γ, cPLA2, or secretory PLA2. Leukocyte rolling/adherence in the mesenteric venules superfused with platelet-activating factor (PAF) were quantified by intravital microscopy. Furthermore, isolated human PMNs or whole blood were incubated with each PLA2 inhibitor and then activated with formyl-methionyl-leucyl-phenylalanine (fMLP) or PAF. PMN adherence was assessed by counting cells bound to purified fibrinogen, and the surface expression of lymphocyte function-associated antigen 1 and macrophage antigen 1 (Mac-1) was measured by flow cytometry.

RESULTS

The iPLA2γ-specific inhibitor almost completely inhibited the fMLP/PAF-induced leukocyte adherence in vivo and in vitro and also decreased the fMLP/PAF-stimulated surface expression of Mac-1 by 60% and 95%, respectively. In contrast, the other inhibitors did not affect these cellular functions.

CONCLUSION

iPLA2γ seems to be involved in leukocyte/PMN adherence in vivo and in vitro as well as in the up-regulation of Mac-1 in vitro in response to PAF/fMLP. This enzyme is therefore likely to be a major regulator in the PMN recruitment cascade.

Markers of bacterial translocation in end-stage liver disease

Bacterial translocation (BT) refers to the passage of viable bacteria or bacterial products from the intestinal lumen, through the intestinal epithelium, into the systemic circulation and extraintestinal locations. The three principal mechanisms that are thought to be involved in BT include bacterial overgrowth, disruption of the gut mucosal barrier and an impaired host defence. BT is commonly observed in liver cirrhosis and has been shown to play an important role in the pathogenesis of the complications of end stage liver disease, including infections as well as hepatic encephalopathy and hepatorenal syndrome. Due to the importance of BT in the natural history of cirrhosis, there is intense interest for the discovery of biomarkers of BT. To date, several such candidates have been proposed, which include bacterial DNA, soluble CD14, lipopolysaccharides endotoxin, lipopolysaccharide-binding protein, calprotectin and procalcitonin. Studies on the association of these markers with BT have demonstrated not only promising data but, oftentimes, contradictory results. As a consequence, currently, there is no optimal marker that may be used in clinical practice as a surrogate for the presence of BT.

Pathological bacterial translocation in cirrhosis: pathophysiology, diagnosis and clinical implications

Bacterial translocation (BT) is defined by the passage of viable indigenous bacteria from the intestinal lumen to mesenteric lymph nodes (MLNs) and other territories, and its diagnostic criteria rely on the isolation of viable bacteria in MLNs. Small intestinal overgrowth, increased intestinal permeability and immunological alterations are the main factors involved in its pathogenesis. BT is obviously difficult to identify in patients with cirrhosis, and alternative methods have been proposed instead. Bacterial DNA detection and species identification in serum or ascitic fluid has been proposed as a reliable marker of BT. Bacterial products, such as endotoxin, or bacterial DNA can translocate to extra-intestinal sites and promote an immunological response similar to that produced by viable bacteria. Therefore, pathological BT plays an important role in the pathogenesis of the complications of cirrhosis, not only in infections, but by exerting a profound inflammatory state and exacerbating the haemodynamic derangement. This may promote in turn the development of hepatorenal syndrome, hepatic encephalopathy and other portal hypertension-related complications. Therapeutic approaches for the prevention of BT in experimental and human cirrhosis are summarized. Finally, new investigations are needed to better understand the pathogenesis and consequences of translocation by viable bacteria (able to grow in culture), or non-viable BT (detection of bacterial fragments with negative culture) and open new therapeutic avenues in patients with cirrhosis.

The tripeptide feG inhibits leukocyte adhesion

Background

The tripeptide feG (D-Phe-D-Glu-Gly) is a potent anti-inflammatory peptide that reduces the severity of type I immediate hypersensitivity reactions, and inhibits neutrophil chemotaxis and adhesion to tissues. feG also reduces the expression of β1-integrin on circulating neutrophils, but the counter ligands involved in the anti-adhesive actions of the peptide are not known. In this study the effects of feG on the adhesion of rat peritoneal leukocytes and extravasated neutrophils to several different integrin selective substrates were evaluated.

Results

The adhesion of peritoneal leukocytes and extravasated neutrophils from rats to adhesive proteins coated to 96-well plates was dependent upon magnesium (Mg²⁺) ion, suggestive of integrin-mediated adhesion. feG inhibited leukocyte adhesion, but only if the cells were stimulated with PAF (10^-9M), indicating that feG's actions in vitro require cell activation. In the dose range of 10^-10M to 10^-12M feG inhibited the adhesion of peritoneal leukocytes to fibrinogen and fibronectin, but not IgG, vitronectin or ICAM-1. feG inhibited the binding of extravasated neutrophils to heparin, IgG, fibronectin and CD16 antibody. Antigen-challenge of sensitized rats reduced the adhesion of peritoneal leukocytes to most substrates and abolished the inhibitory effects of feG. However, pretreating the animals with intraperitoneal feG (100 μg/kg) 18 h before collecting the cells from the antigen-challenged animal restored the inhibition of adhesion by in vitro feG of peritoneal leukocytes and extravasated neutrophils to fibronectin.

Conclusion

The modulation of leukocyte adhesion by feG appears to involve actions on αMβ2 integrin, with a possible interaction with the low affinity FcγRIII receptor (CD16). The modulation of cell adhesion by feG is dual in nature. When administered in vivo, feG prevents inflammation-induced reductions in cell adhesion, as well as restoring its inhibitory effect in vitro. The mechanism by which in vivo treatment with feG exerts these effects remains to be elucidated.

Decreased leukocyte recruitment in the mesenteric microcirculation of rats with cirrhosis is partially restored by treatment with peginterferon: an in vivo study

BACKGROUND/AIMS

Patients with liver cirrhosis are predisposed to develop bacterial infections. An essential process in inflammatory responses is the recruitment of circulating leukocytes through the activation of adhesion molecules. Interferon-alpha2a is a cytokine reported to influence the expression of adhesion molecules. We investigated the effect of peginterferon-alpha2a (PegIFN-alpha(2a)) in vivo on the leukocyte recruitment in the mesenteric microcirculation of cirrhotic rats after lipopolysaccharide exposure.

METHODS

Leukocyte rolling, adhesion and extravasation were visualized by intravital microscopy in sham-operated and common bile duct ligated (CBDL) rats. PegIFN-alpha(2a) was administered to influence leukocyte recruitment. Endothelial P-selectin, E-selectin and ICAM-1 expression were studied by immunohistochemistry.

RESULTS

CBDL placebo rats showed significantly impaired rolling, adhesion and extravasation of leukocytes compared to Sham-operated placebo rats. Endothelial P-selectin, E-selectin and ICAM-1 expressions in CBDL placebo rats were significantly reduced compared to Sham-operated placebo rats. PegIFN-alpha(2a) 18 microg normalized number of rolling leukocytes in CBDL rats, without influencing on adhering and extravasated leukocytes. PegIFN-alpha(2a) upregulates the expression of P-selectin and E-selectin in CBDL rats, but ICAM-1 expression remained significantly lower than in Sham rats.

CONCLUSIONS

Leukocyte recruitment is significantly impaired in the mesenteric microcirculation of cirrhotic rats. This deficiency appears to result from a reduced endothelial P-selectin, E-selectin and ICAM-1 expression. Peginterferon-alpha(2a) treatment normalizes rolling of leukocytes in cirrhotic rats by upregulation of P-selectin and E-selectin expressions, but has no influence on adhesion and extravasation possibly due to the absence of effect on ICAM-1 expression.

[Physiopathology of bacterial translocation and spontaneous bacterial peritonitis in cirrhosis]

The key pathogenic mechanism initiating spontaneous bacterial peritonitis (SBP) is bacterial translocation (BT), a process through which enteric bacteria cross the intestinal barrier and infect the mesenteric lymph nodes, thus entering the blood circulation and ascitic fluid. The high rate of bacterial translocation in cirrhosis is due to injury to the three pilars composing the intestinal mucosal barrier (the balance of intraluminal bacterial flora, the integrity of the intestinal epithelial barrier, and the local immune system). Blood dissemination and microbial growth in ascitic fluid resulting from SBP are a consequence of damage to the immune system in cirrhosis. Hyperproduction of proinflammatory cytokines and other vasoactive substances contributes to the arterial vasodilation and renal failure that frequently complicate the course of SBP. Even in the absence of SBP, translocation of bacteria and bacterial products from the intestinal lumen contribute to systemic inactivation of immune cells in cirrhosis.

Intestinal barrier dysfunction in developing liver cirrhosis: An in vivo analysis of bacterial translocation

AIM

Transmucosal passage of bacteria across the intestine, the essential step for bacterial translocation, has been identified as a key event in the pathogenesis of life-threatening infections in cirrhosis. Existing animal models of liver cirrhosis only provide indirect information about the pathogenesis of such infections. The aim of this study has been to assess transmucosal passage and bacterial translocation directly in vivo using a rat model of developing liver cirrhosis.

METHODS

Male Sprague Dawley rats were randomly assigned to two groups: controls and animals with developing liver cirrhosis induced by s.c. injection of carbon tetrachloride. In anesthetized animals a suspension of green fluorescent protein (GFP)-tagged E. coli was administered into the terminal ileum. Time intervals necessary for translocation of E. coli into the mucosa and muscularis were assessed by intravital microscopy and translocation of E. coli in mesentery, liver and spleen was determined microbiologically.

RESULTS

Bacterial kinetics at the level of the mucosa and muscularis showed significant enhancement in cirrhotic rats compared to the controls (P < 0.001). GFP-expressing E. coli were detected in the mesentery, liver and spleen of animals with cirrhosis taken one hour after E. coli administration. However, cultures of control animals remained sterile.

CONCLUSION

Intravital microscopy of fluorescent bacteria represents a novel approach to studying bacterial translocation in vivo. Here we report that this technique can be used to visualize bacterial transit in in vivo and gives further support to the transmucosal passage of bacteria across the intestine correlating with bacterial translocation in CCl(4)-induced liver cirrhosis.

Gut microflora in the pathogenesis of the complications of cirrhosis

The gut flora plays an important role in the pathogenesis of the complications of cirrhosis. Cirrhotic patients are prone to develop bacterial infections, mainly the 'spontaneous' infection of ascites or spontaneous bacterial peritonitis. Other complications of cirrhosis, such as variceal haemorrhage and ascites, occur mostly or solely as a consequence of portal hypertension. Portal pressure increases initially as a consequence of an increased intrahepatic resistance but, once collaterals have formed, high portal pressure is maintained by an increased splanchnic blood inflow secondary to vasodilatation. Splanchnic vasodilatation is the initiating event in the hyperdynamic circulatory state that aggravates the complications of cirrhosis. The gut flora plays a role in both the development of infections and in the hyperdynamic circulatory state of cirrhosis and, although less prominently, it also plays a role in the pathogenesis of hepatic encephalopathy. This chapter presents evidence regarding gut flora and its modification in the pathogenesis and management of these complications of cirrhosis.

Platelet-activating factor promotes mucosal apoptosis via FasL-mediating caspase-9 active pathway in rat small intestine after ischemia-reperfusion

Platelet activating factor (PAF) is a proinflammatory lipid mediator for inflammatory response. It is unclear whether PAF is involved in the very complex process of ischemia-reperfusion (I/R) induced mucosal apoptosis in small intestine. Intestinal I/R was induced in rats intestine by 60 min occlusion of the superior mesenteric artery, followed by a 60 min reperfusion. I/R induced mucosal apoptosis and PAF activity but inhibited PAF-acetylhydrolase activity. Increases in interleukin-6 (IL-6) and decreases in IL-10 were observed. Western blot analysis showed that I/R induced expressions of platelet endothelial cell adhesion molecule-1 (PECAM-1) and Fas and Fas ligand (FasL) proteins, cleaved Bid, and enhanced the release of cytochrome c from mitochondria to activate caspase-9. Pretreatment of PAF antagonist BN-52021 attenuated these changes, except the increase in Fas. The results showed that I/R-inhibited mucosal PAF-acetylhydrolase activity resulted in an increase of activated PAF. The activated PAF increased the mucosal IL-6 and PECAM-1, enhanced the expression of FasL but not Fas, and led to the cleavage of Bid and the release of cytochrome c from mitochondria to activate caspase-9. This finding suggests that PAF promotes mucosal apoptosis after I/R in the rat small intestine partly through FasL mediating caspase-9 active pathway.

Platelets modulate ischemia/reperfusion-induced leukocyte recruitment in the mesenteric circulation

P-selectin-dependent leukocyte-endothelial cell adhesion has been implicated in the pathogenesis of ischemia/reperfusion (I/R) injury in several vascular beds, including the gut. Because platelet-endothelial (P/E) cell adhesion also occurs in postischemic venules, the possibility exists that the expression of P-selectin on the surface of platelets that are adherent to venular endothelial cells may mediate the leukocyte recruitment elicited by I/R. P-selectin expression [dual radiolabeled monoclonal antibody (MAb) technique] and neutrophil accumulation [myeloperoxidase (MPO) activity] were measured in the postischemic small intestine of untreated rats and rats treated with either antiplatelet serum (APS) or MAbs directed against either P-selectin, GPIIb/IIIa, or fibrinogen. The increases in P-selectin expression and tissue MPO normally elicited by I/R were significantly attenuated in the different treatment groups, suggesting that I/R-induced neutrophil recruitment is a platelet-dependent, P-selectin-mediated process. Intravital microscopy was then employed to examine this process relative to leukocyte-endothelial cell adhesion in postischemic rat mesenteric venules. The recruitment of adherent and emigrated leukocytes after I/R was attenuated by pretreatment with a MAb against, either P-selectin, GPIIb/IIIa, or fibrinogen, as well as an Arg-Gly-Asp peptide. Whereas thrombocytopenia greatly blunted leukocyte emigration, it did not alter the leukocyte adherence response to I/R. These findings suggest that platelet-associated P-selectin contributes to the accumulation of leukocytes in postischemic tissue via a mechanism that alters transendothelial leukocyte migration.

Neutrophil adhesion is impaired in the mesentery but not in the liver sinusoids of portal hypertensive rats

Altered leukocyte/cytokine response to inflammation has been observed in human and experimental portal hypertension. The aim of this study was to characterize leukocyte adhesion in portal hypertensive (PPVL) rats stimulated with endotoxin. Leukocyte rolling, adhesion, and migration assessed by intravital microscopy were impaired in mesenteric venules after lipopolysaccharide administration (150 microg/kg) in PPVL vs. sham-operated rats. Analysis of leukocyte L-selectin expression and soluble L-selectin showed that this defective adhesion was related to increased L-selectin shedding. In vitro experiments using isolated leukocytes treated with phorbol 12-myristate 13-acetate showed that monocytes and neutrophils but not lymphocytes were hyperreactive to cell activation, as measured by CD11b overexpression and increased L-selectin shedding in PPVL rats. However, neutrophil emigration in liver sinusoids and in the lung 3 h after endotoxin injection were similar in both groups of animals. Thus the alterations in leukocyte activation and adhesion molecule expression observed in this study may contribute to a better understanding of the higher susceptibility and severity of bacterial infections in cirrhotic patients with portal hypertension.

Differential role of selectins in experimental colitis

BACKGROUND AND AIMS

The role of selectins in experimental colitis remains unknown. The aims of this study were to characterize the time-course expression of selectins in trinitrobenzene sulfonic acid (TNBS)-induced colitis, the functional role of selectins in colonic leukocyte-endothelial cell interactions, and the therapeutic usefulness of selectin blockade in this model.

METHODS

Control and TNBS-induced colitic rats were studied. Expression of P- and E-selectin was assessed by the radiolabeled antibody technique, and L-selectin by flow cytometry. Leukocyte-endothelial cell interactions were studied in colonic venules by using intravital microscopy under basal conditions and after P-, E-, or L-selectin immunoblockade. Additional groups of animals were treated with anti-P-selectin antibody, a nonbinding antibody, or dexamethasone, for 7 days.

RESULTS

P-selectin and E-selectin expression were markedly up-regulated in colitic rats. Increased leukocyte rolling was abrogated by anti-P-selectin, but only attenuated by anti-E- or anti-L-selectin antibodies. Only pretreatment with anti-P- selectin decreased leukocyte adhesion. Animals chronically treated with dexamethasone, but not with anti- P-selectin, had significantly lower macroscopic and histologic damage scores, colon weight, and myeloperoxidase (MPO) activity than those treated with nonbinding antibody.

CONCLUSIONS

P-selectin plays a key role on leukocyte rolling and its blockade attenuates leukocyte adhesion in TNBS-induced colitis. However, treatment with an anti-P-selectin antibody does not significantly improve colitis.