Gut lymph hypothesis of early shock and trauma-induced multiple organ dysfunction syndrome: A new look at gut origin sepsis

Crosstalk between gut microbiota and sepsis

Sepsis is an overwhelming inflammatory response to microbial infection. Sepsis management remains a clinical challenge. The role of the gut microbiome in sepsis has gained some attention. Recent evidence has demonstrated that gut microbiota regulate host physiological homeostasis mediators, including the immune system, gut barrier function and disease susceptibility pathways. Therefore, maintenance or restoration of microbiota and metabolite composition might be a therapeutic or prophylactic target against critical illness. Fecal microbiota transplantation and supplementation of probiotics are microbiota-based treatment methods that are somewhat limited in terms of evidence-based efficacy. This review focuses on the importance of the crosstalk between the gastrointestinal ecosystem and sepsis to highlight novel microbiota-targeted therapies to improve the outcomes of sepsis treatment.

Nutrition and the gut microbiome during critical illness: A new insight of nutritional therapy

Changes in the microbiome in response to environmental influences can affect the overall health. Critical illness is considered one of the major environmental factors that can potentially influence the normal gut homeostasis. It is associated with pathophysiological effects causing damage to the intestinal microbiome. Alteration of intestinal microbial composition during critical illness may subsequently compromise the integrity of the intestinal epithelial barrier and intestinal mucosa absorptive function. Many factors can impact the microbiome of critically ill patients including ischemia, hypoxia and hypotension along with the iatrogenic effects of therapeutic agents and the lack of enteral feeds. Factors related to disease state and medication are inevitable and they are part of the intensive care unit (ICU) exposure. However, a nutritional intervention targeting gut microbiota might have the potential to improve clinical outcomes in the critically ill population given the extensive vascular and lymphatic links between the intestines and other organs. Although nutrition is considered an integral part of the treatment plan of critically ill patients, still the role of nutritional intervention is restricted to improve nitrogen balance. What is dismissed is whether the nutrients we provide are adequate and how they are processed and utilised by the host and the microbiota. Therefore, the goal of nutrition therapy during critical illness should be extended to provide good quality feeds with balanced macronutrient content to feed up the entire body including the microbiota and host cells. The main aim of this review is to examine the current literature on the effect of critical illness on the gut microbiome and to highlight the role of nutrition as a factor affecting the intestinal microbiome-host relationship during critical illness.

Pathophysiology and protective approaches of gut injury in critical illness

The gut is a complex organ that has played an important role in digestion, absorption, endocrine functions, and immunity. The gut mucosal barriers consist of the immunologic barrier and nonimmunologic barrier. During critical illnesses, the gut is susceptible to injury due to the induction of intestinal hyperpermeability. Gut hyperpermeability and barrier dysfunction may lead to systemic inflammatory response syndrome. Additionally, gut microbiota are altered during critical illnesses. The etiology of such microbiome alterations in critical illnesses is multifactorial. The interaction or systemic host defense modulation between distant organs and the gut microbiome is increasingly studied in disease research. No treatment modality exists to significantly enhance the gut epithelial integrity, permeability, or mucus layer in critically ill patients. However, multiple helpful approaches including clinical and preclinical strategies exist. Enteral nutrition is associated with an increased mucosal barrier in animal and human studies. The trophic effects of enteral nutrition might help to maintain the intestinal physiology, prevent atrophy of gut villi, reduce intestinal permeability, and protect against ischemia-reperfusion injury. The microbiome approach such as the use of probiotics, fecal microbial transplantation, and selective decontamination of the digestive tract has been suggested. However, its evidence does not have a high quality. To promote rapid hypertrophy of the small bowel, various factors have been reported, including the epidermal growth factor, membrane permeant inhibitor of myosin light chain kinase, mucus surrogate, pharmacologic vagus nerve agonist, immune-enhancing diet, and glucagon-like peptide-2 as preclinical strategies. However, the evidence remains unclear.

Oncostatin M Plays a Critical Role in Survival after Acute Intestinal Ischemia: Reperfusion Injury

Background: Acute intestinal ischemia-reperfusion injury (AIIRI) is a devastating clinical condition relevant to multiple diseases processes, including sepsis, trauma, transplantation, and burns. An AIIRI is a contributor to the development of multiple organ dysfunction syndrome (MODS). Oncostatin M (OSM)/oncostatin M receptor (OSMR) signaling is an unrecognized and novel candidate pathway for the mediation of MODS. In this study, we hypothesized that OSM mediates the injury mechanism of AIIRI leading to MODS. Methods: Wild-type (WT) and OSMR-knockout (OSMR^-/-) C57BL/6 mice underwent AIIRI using a well-established model of selective occlusion of the superior mesenteric artery (SMA). Serum cytokine concentrations were measured using a multiplex detection system. Further tissue analysis was conducted with polymerase chain reaction, enzyme-linked immunosorbent assay, Western blots, and histologic review. Results: Survival was significantly higher in WT than in OSMR^-/- groups at 30 minutes of ischemia with 2 hours of reperfusion (100% versus 42.9%; P = 0.015). No significant differences in the degree of local intestinal injury was seen in the two groups. In contrast, the degree of lung injury, as evidenced by myeloperixodase activity, was lower in OSMR^-/- animals in the early AIIRI groups. There was a greater degree of renal dysfunction in OSMR^-/- mice. Oncostatin M mediated interleukin (IL)-10 upregulation, with WT animals having significantly lower IL-10 concentrations (52.04 ± 23.06 pg/mL versus 324.37 ± 140.35 pg/mL; P = 0.046). Conclusion: Oncostatin M signalling is essential during acute intestinal ischemia-reperfusion injury. An OSMR deficiency results in decreased early lung injury but increased renal dysfunction. There was a significantly increased mortality rate after AIIRI in mice with OSMR deficiency. Augmentation of OSM may be a novel immunomodulatory strategy for AIIRI.

Gut Microbiome in Critical Illness (Review)

Radical changes in the composition, diversity and metabolic activity of gut microbiome in critically ill patients most probably affect adversely the outcome of treatment. Microbiota dysfunction may be a predictor and presumably the main cause of infectious complications and sepsis. Clinicists use objective scales for evaluation of patient condition severity including specific parameters of disorders of organs and systems; however, microbiota function is not considered specific and, hence, not evaluated. Technical capabilities of the recent decade have allowed characterizing the intestinal microbiota and that helped understanding the ongoing processes. The authors have analyzed data about the role of intestinal microbiota as a metabolic 'reactor' during critical states, possible complications related to misbalance of 'harmful' and 'beneficial' bacteria, and examined potential of a targeted therapy aimed directly at correction of intestinal microbiota. Search for papers was carried out using Scopus and Web of Science databases 2001 to 2018 years: (Gut Microbiota) AND (Critically ill OR Intensive care unit), key words taken for the search were: intestinal microbiota, metabolism, sepsis, antibiotics, critically ill patients, multiple organ failure. A number of questions in understanding of the interaction between gut microbiome and host remain open. It is necessary to take into account interference of microbial metabolism while assessing metabolome of patients with sepsis. Among low-molecular compounds found in blood of sepsis patients, special attention should be paid to molecules that can be classified as ‘common metabolites’ of humans and bacteria, for example, degradation products of aromatic compounds, which many-fold rise in blood of septic patients. It is necessary to take into consideration and experimentally model changes in the human internal environment, which occur during radical transformation of microbiome in critically ill patients. Such approach brings in new prospects for objective monitoring of diseases by evaluating metabolic profile at a particular moment of time based on integral indices reflecting the status of microbiome/metabolome system, which will supply new targets for therapeutic intervention in future.

Gut microbiota and host defense in critical illness

PURPOSE OF REVIEW

The review aims to discuss emerging evidence in the field of microbiome-dependent roles in host defense during critical illness with a focus on lung, kidney, and brain inflammation.

RECENT FINDINGS

The gut microbiota of critical ill patients is characterized by lower diversity, lower abundances of key commensal genera, and in some cases overgrowth by one bacterial genera, a state otherwise known as dysbiosis. Increasing evidence suggests that microbiota-derived components can reach the circulatory system from the gut and modulate immune homeostasis. Dysbiosis might have greater consequences for the critically ill than previously imagined and could contribute to poor outcome. Preclinical studies suggest that impaired communication across the gut - organ axes is associated with brain, lung - and kidney failure.

SUMMARY

In health, a diverse microbiome might enhance host defense, while during critical illness, the dysbiotic microbiome might contribute to comorbidity and organ dysfunction. Future research should be aimed at further establishing the causes and consequences of dysbiosis seen in the critically ill, which will provide perspective for developing new strategies of intervention.

The anatomy and physiology of the terminal thoracic duct and ostial valve in health and disease: potential implications for intervention

The thoracic duct (TD) transports lymph drained from the body to the venous system in the neck via the lymphovenous junction. There has been increased interest in the TD lymph (including gut lymph) because of its putative role in the promotion of systemic inflammation and organ dysfunction during acute and critical illness. Minimally invasive TD cannulation has recently been described as a potential method to access TD lymph for investigation. However, marked anatomical variability exists in the terminal segment and the physiology regarding the ostial valve and terminal TD is poorly understood. A systematic review was conducted using three databases from 1909 until May 2017. Human and animal studies were included and data from surgical, radiological and cadaveric studies were retrieved. Sixty-three articles from the last 108 years were included in the analysis. The terminal TD exists as a single duct in its terminal course in 72% of cases and 13% have multiple terminations: double (8.5%), triple (1.8%) and quadruple (2.2%). The ostial valve functions to regulate flow in relation to the respiratory cycle. The patency of this valve found at the lymphovenous junction opening, is determined by venous wall tension. During inspiration, central venous pressure (CVP) falls and the valve cusps collapse to allow antegrade flow of lymph into the vein. During early expiration when CVP and venous wall tension rises, the ostial valve leaflets cover the opening of the lymphovenous junction preventing antegrade lymph flow. During chronic disease states associated with an elevated mean CVP (e.g. in heart failure or cirrhosis), there is a limitation of flow across the lymphovenous junction. Although lymph production is increased in both heart failure and cirrhosis, TD lymph outflow across the lymphovenous junction is unable to compensate for this increase. In conclusion the terminal TD shows marked anatomical variability and TD lymph flow is controlled at the ostial valve, which responds to changes in CVP. This information is relevant to techniques for cannulating the TD, with the aid of minimally invasive methods and high resolution ultrasonography, to enable longitudinal physiology and lymph composition studies in awake patients with both acute and chronic disease.

The microbiome and critical illness

The central role of the microbiome in critical illness is supported by a half century of experimental and clinical study. The physiological effects of critical illness and the clinical interventions of intensive care substantially alter the microbiome. In turn, the microbiome predicts patients' susceptibility to disease, and manipulation of the microbiome has prevented or modulated critical illness in animal models and clinical trials. This Review surveys the microbial ecology of critically ill patients, presents the facts and unanswered questions surrounding gut-derived sepsis, and explores the radically altered ecosystem of the injured alveolus. The revolution in culture-independent microbiology has provided the tools needed to target the microbiome rationally for the prevention and treatment of critical illness, holding great promise to improve the acute and chronic outcomes of the critically ill.

A sphingosine-1 phosphate agonist (FTY720) limits trauma/hemorrhagic shock-induced multiple organ dysfunction syndrome

BACKGROUND

Trauma/hemorrhagic shock (T/HS) is one of the major consequences of battlefield injury as well as civilian trauma. FTY720 (sphingosine-1-phosphate agonist) has the capability to decrease the activity of the innate and adaptive immune systems and, at the same time, maintain endothelial cell barrier function and vascular homeostasis during stress. For this reason, we hypothesize that FTY720, as part of resuscitation therapy, would limit T/HS-induced multiple organ dysfunction syndrome in a rodent T/HS model.

METHODS

Rats subjected to trauma/sham shock (T/SS) or T/HS (30 mm Hg × 90 min) were administered FTY720 (1 mg/kg) post-T/HS during volume resuscitation. Lung injury (permeability to Evans blue dye), polymorphonuclear leukocyte (PMN) priming (respiratory burst activity), and red blood cell (RBC) rigidity were measured. In addition, lymph duct-cannulated rats were used to quantify the effect of FTY720 on gut injury (permeability and morphology) and the biologic activity of T/HS versus T/SS lymph on PMN-RBC and RBC deformability.

RESULTS

Trauma/hemorrhagic shock-induced increased lung permeability, PMN priming, and RBC rigidity were all abrogated by FTY720. The systemic protective effect of FTY720 was only partially at the gut level, because FTY720 did not prevent T/HS-induced gut injury (morphology or permeability); however, it did abrogate T/HS lymph-induced increased respiratory burst and RBC rigidity.

CONCLUSIONS

FTY720 limited T/HS-induced multiple organ dysfunction syndrome (lung injury, red cell injury, and neutrophil priming) as well as T/HS lymph bioactivity, although it did not limit gut injury.

Mechanisms of Acute Alcohol Intoxication-Induced Modulation of Cyclic Mobilization of [Ca²⁺] in Rat Mesenteric Lymphatic Vessels

BACKGROUND

We have demonstrated that acute alcohol intoxication (AAI) increases the magnitude of Ca(2+) transients in pumping lymphatic vessels. We tested the contribution of extracellular Ca(2+) via L-type Ca(2+) channels and intracellular Ca(2+) release from the sarcoplasmic reticulum (SR) to the AAI-induced increase in Ca(2+) transients.

METHODS AND RESULTS

AAI was produced by intragastric administration of 30% alcohol to conscious, unrestrained rats; isovolumic administration of water served as the control. Mesenteric lymphatic vessels were isolated, cannulated, and loaded with Fura-2 AM to measure changes in intracellular Ca(2+). Measurements were made at intraluminal pressures of 2, 6, and 10 cm H2O. L-type Ca(2+) channels were blocked with nifedipine; IP-3 receptors were inhibited with xestospongin C; and SR Ca(2+) release and Ca(2+) pool (Ca(2+) free APSS) were achieved using caffeine. Nifedipine reduced lymphatic Ca(2+) transient magnitude in both AAI and control groups at all pressures tested, but reduced lymphatic contraction frequency only in the control group. Xestospongin C did not significantly change any of the Ca(2+) parameters in either group; however, fractional shortening increased in the controls at low transmural pressure. RyR (ryanodine receptor) activation with caffeine resulted in a single contraction with a greater Ca(2+) transient in lymphatics from AAI than those from controls. SR Ca(2+) pool was also greater in lymphatics isolated from AAI- than from control animals.

CONCLUSIONS

These data suggest that 1) L-type Ca(2+) channels contribute to the AAI-induced increase in lymphatic Ca(2+) transient, 2) blockage of IP-3 receptors could increase calcium sensitivity, and 3) AAI increases Ca(2+) storage in the SR in lymphatic vessels.

Does recombinant factor XIII eliminate early manifestations of multiple-organ injury after experimental burn similarly to gut ischemia-reperfusion injury or trauma-hemorrhagic shock?

The authors have previously shown that recombinant factor XIII (rFXIII) eliminates early manifestations of multiple-organ injury caused by experimental superior mesenteric artery occlusion or trauma-hemorrhagic shock. The aim of the present study was to test the hypothesis that rFXIII provides similar protective effect in experimental burn injury. Rats were randomly divided into five groups (eight animals per group): group 1: burn + placebo treatment; group 2: burn + rFXIII pretreatment; group 3: burn + rFXIII treatment; group 4: sham burn + placebo treatment, and group 5: sham burn + rFXIII treatment. Burn (40% of TBSA) was achieved by immersing the back and abdomen of a rat into 97°C water for 10 and 5 seconds, respectively. Infusion of rFXIII (1 mg/kg) or placebo was performed immediately after burn/sham burn in treatment groups or 24 hours before burn and repeated immediately after it in pretreatment group. Endpoint parameters measured 3 hours after burn/sham burn included muscle blood flow and PO2, lung permeability, gut histology, lung and gut myeloperoxidase activity, neutrophil respiratory burst, and FXIII activity. Both treatment and pretreatment with rFXIII partially preserved microvascular blood flow in the muscle. Muscle PO2 in pretreated rats did not differ from that in shams. Pretreatment but not treatment with rFXIII preserved lung permeability. rFXIII did not have any protective effect on other endpoint parameters. In contrast to superior mesenteric artery occlusion and trauma-hemorrhagic shock experimental models, rFXIII at the doses tested has a limited effect on preventing early manifestations of multiple-organ injury after experimental burn.

The canonical Notch signaling was involved in the regulation of intestinal epithelial cells apoptosis after intestinal ischemia/reperfusion injury

Notch signaling plays a critical role in the maintenance of intestinal homeostasis. The aim of the present study was to investigate the role of Notch signaling in the apoptosis of intestinal epithelial cells after intestinal ischemia reperfusion (I/R) injury. Male C57BL/6 mice were subjected to sham operation or I/R injury. Intestinal tissue samples were collected at 12 h after reperfusion. TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling) staining showed that intestinal I/R injury induced significantly increased apoptosis of intestinal epithelial cells. Meanwhile, the mRNA expression of Jagged1, DLL1, Notch2, and Hes5, and protein expression of NICD2 and Hes5 were increased significantly after I/R injury in intestinal epithelial cells. In an in vitro IEC-6 culture model, flow cytometry analyses showed that inhibition of Notch signaling by γ-secretase inhibitor DAPT and the suppression of Hes5 expression using siRNA both significantly increased the apoptosis of IEC-6 cells under the condition of hypoxia/reoxygenation (H/R). In conclusion, the Notch2/Hes5 signaling pathway was activated and involved in the regulation of intestinal epithelial cells apoptosis in intestinal I/R injury.

Trauma-hemorrhagic shock induces a CD36-dependent RBC endothelial-adhesive phenotype

OBJECTIVE

Microvascular dysfunction is a key element in the development of the multiple organ dysfunction syndrome. Although the mechanisms for this response are unclear, RBC adhesion to endothelium may initiate intravascular occlusion leading to ischemic tissue injury. Thus, we tested the hypothesis that trauma-hemorrhage induces RBC-endothelial cell adhesion.

DESIGN

Prospective in vivo and in vitro animal study and analysis of patient blood samples.

SETTING

University research laboratory and hospital emergency and trauma units.

INTERVENTION

We initially assayed RBC adhesion to endothelial cells in vitro using RBCs obtained from rats subjected to trauma-hemorrhagic shock or sham shock as well as from severely injured trauma patients. Subsequently, we measured the role of putative RBCs and endothelial cell receptors in the increased RBC-endothelial cell adhesive response.

MAIN RESULTS

In both rats and humans, trauma-hemorrhagic shock increased RBC adhesion to endothelium as well as increasing several putative RBC surface adhesion molecules including CD36. The critical factor leading to RBC-endothelial cell adhesion was increased surface RBC CD36 expression. Adhesion of trauma-hemorrhagic shock RBCs was mediated, at least in part, by the binding of RBC CD36 to its cognate endothelial receptors (αVβ3 and VCAM-1). Gut-derived factors carried in the intestinal lymphatics triggered these trauma-hemorrhagic shock-induced RBC changes because 1) preventing trauma-hemorrhagic shock intestinal lymph from reaching the systemic circulation abrogated the RBC effects, 2) in vitro incubation of naïve whole blood with trauma-hemorrhagic shock lymph replicated the in vivo trauma-hemorrhagic shock-induced RBC changes while 3) injection of trauma-hemorrhagic shock lymph into naïve animals recreated the RBC changes observed after actual trauma-hemorrhagic shock.

CONCLUSIONS

1) Trauma-hemorrhagic shock induces rapid RBC adhesion to endothelial cells in patients and animals. 2) Increased RBC CD36 expression characterizes the RBC-adhesive phenotype. 3) The RBC phenotypic and functional changes were induced by gut-derived humoral factors. These novel findings may explain the microvascular dysfunction occurring after trauma-hemorrhagic shock, sepsis, and other stress states.

Early trauma-hemorrhage-induced splenic and thymic apoptosis is gut-mediated and toll-like receptor 4-dependent

Immune depression after trauma-hemorrhage has been implicated as an important factor in the pathogenesis of sepsis and septic-organ failure. Although recent studies have implicated immune-cell apoptosis as an important factor in the evolution of this posttrauma immune-suppressed state, neither the initial triggers that induce this response nor the cellular pathways through which these triggering pathways act have been fully defined. Thus, the current study tests the hypothesis that acute splenic and thymic immune-cell apoptosis developing after trauma-hemorrhagic shock (T/HS) is due to gut-derived factors carried in intestinal lymph and that this T/HS lymph-induced immune depressed state is mediated through Toll-like receptor 4 (TLR4). The first set of experiments documented that T/HS caused both thymic and splenic immune-cell apoptosis as measured by TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) and caspase-3 immunohistochemistry and that this increase in apoptosis was totally abrogated by mesenteric lymph duct ligation. In subsequent experiments, mesenteric lymph collected from animals subjected to T/HS or trauma-sham shock were injected into TLR4-deficient (TLR4mut) mice or their wild-type (WT) littermates. Trauma-hemorrhagic shock, but not trauma-sham shock, lymph caused splenic apoptosis in the WT mice. However, the TLR4mut mice were resistant to T/HS lymph-induced splenic apoptosis. Furthermore, the WT, but not the TLR4mut mice developed splenic apoptosis after actual T/HS. In conclusion, gut-derived factors appear to initiate a sequence of events that leads to an acute increase in splenic and thymic immune-cell apoptosis, and this process is TLR4-dependent.

Activation of toll-like receptor 4 is necessary for trauma hemorrhagic shock-induced gut injury and polymorphonuclear neutrophil priming

Interactions of toll-like receptors (TLRs) with nonmicrobial factors play a major role in the pathogenesis of early trauma-hemorrhagic shock (T/HS)-induced organ injury and inflammation. Thus, we tested the hypothesis that TLR4 mutant (TLR4 mut) mice would be more resistant to T/HS-induced gut injury and polymorphonuclear neutrophil (PMN) priming than their wild-type littermates and found that both were significantly reduced in the TLR4 mut mice. In addition, the in vivo and ex vivo PMN priming effect of T/HS intestinal lymph observed in the wild-type mice was abrogated in TLR4 mut mice as well the TRIF mut-deficient mice and partially attenuated in Myd88 mice, suggesting that TRIF activation played a more predominant role than MyD88 in T/HS lymph-induced PMN priming. Polymorphonuclear neutrophil depletion studies showed that T/HS lymph-induced acute lung injury was PMN dependent, because lung injury was totally abrogated in PMN-depleted animals. Because the lymph samples were sterile and devoid of endotoxin or bacterial DNA, we investigated whether the effects of T/HS lymph was related to endogenous nonmicrobial TLR4 ligands. High-mobility group box 1 protein 1, heat shock protein 70, heat shock protein 27, and hyaluronic acid all have been implicated in ischemia-reperfusion-induced tissue injury. None of these "danger" proteins appeared to be involved, because their levels were similar between the sham and shock lymph samples. In conclusion, TLR4 activation is important in T/HS-induced gut injury and in T/HS lymph-induced PMN priming and lung injury. However, the T/HS-associated effects of TLR4 on gut barrier dysfunction can be uncoupled from the T/HS lymph-associated effects of TLR4 on PMN priming.

Gut-origin sepsis: evolution of a concept

The concept of bacterial translocation and gut-origin sepsis as a cause of systemic infectious complications and the multiple organ dysfunction syndrome (MODS) in surgical and ICU patients has emerged over the last several decades, although the exact clinical relevance of these phenomena continues to be debated. Thus, the goal of this review is to trace the evolution of gut-origin sepsis and gut-induced MODS and put these disorders and observations into clinical perspective. Additionally, the mechanisms leading to gut-derived complications are explored as well as therapeutic options to limit or prevent these complications. From this work, several major conclusions emerge. First, that bacterial translocation occurs clinically and is responsible for increased infectious complications in patients undergoing major abdominal surgery. However, the phenomenon of bacterial translocation is not sufficient to explain the development of MODS in ICU patients. Instead, the development of MODS in these high-risk patients is likely due to gut injury and the systemic spread of non-microbial, tissue-injurious factors that reach the systemic circulation via the intestinal lymphatics. These observations have resulted in the gut-lymph hypothesis of MODS.

The anatomic sites of disruption of the mucus layer directly correlate with areas of trauma/hemorrhagic shock-induced gut injury

BACKGROUND

The intestinal mucus layer is an important but understudied component of the intestinal barrier. Consequently, we tested the hypothesis that the anatomic sites of loss of the mucus layer would directly correlate with sites of intestinal villous injury after trauma-hemorrhagic shock (T/HS) and may, therefore, serve as loci of gut barrier failure. Consequently, to investigate this hypothesis, we used Carnoy's fixative solution to prepare fixed tissue blocks where both the gut morphology and the mucus layer could be assessed on the same tissues slides.

METHODS

Male Sprague-Dawley rats were subjected to a laparotomy (trauma) and 90 minutes of sham shock (T/SS) or 35 mm Hg × 90 minutes of actual shock (T/HS). Three hours after resuscitation, the rats were killed, and samples of the terminal ileum were processed by fixation in Carnoy's solution. Gut injury was evaluated by determining the percentage of villi injured. The status of the intestinal mucus layer was quantified by determining the percentage of the villi covered by the mucus and the mucus thickness.

RESULTS

Histologic analysis of gut injury showed that the incidence of gut injury was ∼10-fold higher in the T/HS than the T/SS rats (T/SS=2.5% ± 0.5% vs. T/HS=22.4% ± 0.5% of injured villi; p<0.01). The T/SS rats had 98% of their ileal mucosa covered with a mucus layer, and this was decreased after T/HS to 63% ± 3% (T/HS vs. T/SS; p<0.001). Furthermore, loss of the mucus layer was found to directly correlate with villous injury with a regression coefficient of r=0.94 (p<0.001).

CONCLUSION

This study shows that T/HS significantly reduces the intestinal mucus layer and causes villous injury and that a correlation exists between specific anatomic sites of T/HS-induced loss of the mucus layer and gut injury.

Trauma hemorrhagic shock-induced lung injury involves a gut-lymph-induced TLR4 pathway in mice

Background

Injurious non-microbial factors released from the stressed gut during shocked states contribute to the development of acute lung injury (ALI) and multiple organ dysfunction syndrome (MODS). Since Toll-like receptors (TLR) act as sensors of tissue injury as well as microbial invasion and TLR4 signaling occurs in both sepsis and noninfectious models of ischemia/reperfusion (I/R) injury, we hypothesized that factors in the intestinal mesenteric lymph after trauma hemorrhagic shock (T/HS) mediate gut-induced lung injury via TLR4 activation.

Methods/Principal Findings

The concept that factors in T/HS lymph exiting the gut recreates ALI is evidenced by our findings that the infusion of porcine lymph, collected from animals subjected to global T/HS injury, into naïve wildtype (WT) mice induced lung injury. Using C3H/HeJ mice that harbor a TLR4 mutation, we found that TLR4 activation was necessary for the development of T/HS porcine lymph-induced lung injury as determined by Evan's blue dye (EBD) lung permeability and myeloperoxidase (MPO) levels as well as the induction of the injurious pulmonary iNOS response. TRIF and Myd88 deficiency fully and partially attenuated T/HS lymph-induced increases in lung permeability respectively. Additional studies in TLR2 deficient mice showed that TLR2 activation was not involved in the pathology of T/HS lymph-induced lung injury. Lastly, the lymph samples were devoid of bacteria, endotoxin and bacterial DNA and passage of lymph through an endotoxin removal column did not abrogate the ability of T/HS lymph to cause lung injury in naïve mice.

Conclusions/Significance

Our findings suggest that non-microbial factors in the intestinal mesenteric lymph after T/HS are capable of recreating T/HS-induced lung injury via TLR4 activation.

Anticoagulants influence the in vitro activity and composition of shock lymph but not its in vivo activity

Many models of trauma-hemorrhagic shock (T/HS) involve the reinfusion of anticoagulated shed blood. Our recent observation that the anticoagulant heparin induces increased mesenteric lymph lipase activity and consequent in vitro endothelial cell cytotoxicity prompted us to investigate the effect of heparin-induced lipase activity on organ injury in vivo as well as the effects of other anticoagulants on mesenteric lymph bioactivity in vitro and in vivo. To investigate this issue, rats subjected to trauma-hemorrhage had their shed blood anticoagulated with heparin, the synthetic anticoagulant arixtra (fondaparinux sodium), or citrate. Arixtra, in contrast to heparin, did not increase lymph lipase activity or result in high levels of endothelial cytotoxicity. Yet, the arixtra-treated rats subjected to T/HS still manifested lung injury, neutrophil priming, and red blood cell dysfunction, which was totally abrogated by lymph duct ligation. Furthermore, the injection of T/HS mesenteric lymph, but not sham-shock lymph, collected from the arixtra rats into control mice recreated the pattern of lung injury, polymorphonucleocyte (PMN) priming, and red blood cell dysfunction observed after actual shock. Consistent with these observations, citrate-anticoagulated rats subjected to T/HS developed lung injury, and the injection of mesenteric lymph from the citrate-anticoagulated T/HS rats into control mice also resulted in lung injury. Based on these results, several conclusions can be drawn. First, heparin-induced increased mesenteric lymph lipase activity is not responsible for the in vivo effects of T/HS mesenteric lymph. Second, heparin should be avoided as an anticoagulant when studying the biology or composition of mesenteric lymph because of its ability to cause increases in lymph lipase activity that increase the in vitro cytotoxicity of these lymph samples.

Hypoxia-inducible factor plays a gut-injurious role in intestinal ischemia reperfusion injury

Gut injury and loss of normal intestinal barrier function are key elements in the paradigm of gut-origin systemic inflammatory response syndrome, acute lung injury, and multiple organ dysfunction syndrome (MODS). As hypoxia-inducible factor (HIF-1) is a critical determinant of the physiological and pathophysiological response to hypoxia and ischemia, we asked whether HIF-1 plays a proximal role in the induction of gut injury and subsequent lung injury. Using partially HIF-1α-deficient mice in an isolated superior mesenteric artery occlusion (SMAO) intestinal ischemia reperfusion (I/R) injury model (45 min SMAO followed by 3 h of reperfusion), we showed a direct relationship between HIF-1 activation and intestinal I/R injury. Specifically, partial HIF-1α deficiency attenuated SMAO-induced increases in intestinal permeability, lipid peroxidation, mucosal caspase-3 activity, and IL-1β mRNA levels. Furthermore, partial HIF-1α deficiency prevented the induction of ileal mucosal inducible nitric oxide synthase (iNOS) protein levels after SMAO and iNOS deficiency ameliorated SMAO-induced villus injury. Resistance to SMAO-induced gut injury was also associated with resistance to lung injury, as reflected by decreased levels of myeloperoxidase, IL-6 and IL-10 in the lungs of HIF-1α(+/-) mice. In contrast, a short duration of SMAO (15 min) followed by 3 h of reperfusion neither induced mucosal HIF-1α protein levels nor caused significant gut and lung injury in wild-type or HIF-1α(+/-) mice. This study indicates that intestinal HIF-1 activation is a proximal regulator of I/R-induced gut mucosal injury and gut-induced lung injury. However, the duration and severity of the gut I/R insult dictate whether HIF-1 plays a gut-protective or deleterious role.

Intravenous injection of mesenteric lymph produced during hemorrhagic shock decreases RBC deformability in the rat

OBJECTIVE

To test the hypothesis that gut-derived factors carried in trauma-hemorrhagic shock (T/HS) lymph are sufficient to induce red blood cells (RBC) injury, to investigate their potential mechanisms of action, and to define the time post-T/HS that these factors appear in the lymph.

METHODS

Mesenteric lymph collected from T/HS or trauma-sham shock (T/SS) rats over different time periods was injected intravenously into male rats at a rate of 1 mL/h for 3 hours. RBC deformability was measured using laser-assisted ektacytometer to calculate the elongation index. From the shear-stress elongation curve, the stress required for the erythrocytes to reach 50% of their maximal elongation was also determined. RBC deformability was measured before lymph infusion and at 1 hour and 3 hours after the initiation of lymph infusion. The effect of the lymph samples (5% v/v) was also determined in vitro by incubating naïve whole blood with the lymph samples. The potential role of T/HS lymph-induced RBC oxidant injury mediated by inducible nitric oxide synthase (iNOS)-generated oxidants and/or white blood cells (WBC) was investigated using iNOS inhibitors and WBC depletion, respectively. In all the in vivo studies, five to seven rats were studied per group.

RESULTS

The intravenous injection of T/HS lymph but not T/SS lymph caused in vivo RBC injury. The biological activity of T/HS lymph varied over time with the RBC-injurious factors being produced only during the first 3 hours postshock. The in vivo inhibition of iNOS did not prevent lymph-induced RBC injury. T/HS lymph incubated in vitro with naïve whole blood resulted in RBC injury, but this injury was not observed in blood depleted of WBC.

CONCLUSIONS

These results indicate that T/HS lymph produced during the initial 3-hour postshock period is sufficient to induce RBC injury in otherwise normal rats and that the lymph-induced RBC injury is not dependent on activation of the iNOS pathway but seems to require WBC.

Gut lymph and lymphatics: a source of factors leading to organ injury and dysfunction

Major trauma, shock, sepsis, and other conditions can lead to the acute respiratory distress syndrome (ARDS), which may progress to the highly lethal multiple organ dysfunction syndrome (MODS). Although a number of therapeutic strategies have been initiated, their success has been limited largely due to an incomplete understanding of the biology of MODS. However, recent studies indicate that the intestinal lymphatics serve as the primary route for nonbacterial, tissue injurious gut-derived factors, which can induce acute ARDS and MODS. The gut lymph hypothesis of ARDS and MODS thus helps clarify several important issues. First, because the lung is the first organ exposed to mesenteric lymph and not the liver (i.e., mesenteric lymph enters the subclavian vein via the thoracic duct, which, in turn, empties directly into the heart and lungs), it would explain the clinical observation that the lung is generally the first organ to fail. Second, this hypothesis provides new pathophysiologic information, thereby providing a basis for novel therapies. Finally, by studying the composition of lymph, MODS-inducing factors can be isolated and identified.

Loss of the intestinal mucus layer in the normal rat causes gut injury but not toxic mesenteric lymph nor lung injury

There is substantial evidence that gut barrier failure is associated with distant organ injury and systemic inflammation. After major trauma or stress, the factors and mechanisms involved in gut injury are unknown. Our primary hypothesis is that loss of the intestinal mucus layer will result in injury of the normal gut that is exacerbated by the presence of luminal pancreatic proteases. Our secondary hypothesis is that the injury produced in the gut will result in the production of biologically active mesenteric lymph and consequently distant organ (i.e., lung) injury. To test this hypothesis, five groups of rats were studied: 1) uninstrumented naive rats; 2) control rats in which a ligated segment of distal ileum was filled with saline; 3) rats with pancreatic proteases placed in their distal ileal segments; 4) rats with the mucolytic N-acetylcysteine (NAC) placed in their distal ileal segments; and 5) rats exposed to NAC and pancreatic proteases in their ileal segments. The potential systemic consequences of gut injury induced by NAC and proteases were assessed by measuring the biological activity of mesenteric lymph as well as gut-induced lung injury. Exposure of the normal intestine to NAC, but not saline or proteases, led to increased gut permeability, loss of mucus hydrophobicity, a decrease in the mucus layer, as well as morphological evidence of villous injury. Although proteases themselves did not cause gut injury, the combination of pancreatic proteases with NAC caused more severe injury than NAC alone, suggesting that once the mucus barrier is impaired, luminal proteases can injure the now vulnerable gut. Because comparable levels of gut injury caused by systemic insults are associated with gut-induced lung injury, which is mediated by biologically active factors in mesenteric lymph, we next tested whether this local model of gut injury would produce active mesenteric lymph or lead to lung injury. It did not, suggesting that gut injury by itself may not be sufficient to induce distant organ dysfunction. Therefore, loss of the intestinal mucus layer, especially in the presence of intraluminal pancreatic proteases, is sufficient to lead to injury and barrier dysfunction of the otherwise normal intestine but not to produce gut-induced distant organ dysfunction.

HIF-1 mediates pathogenic inflammatory responses to intestinal ischemia-reperfusion injury

Acute lung injury (ALI) and the development of the multiple organ dysfunction syndrome (MODS) are major causes of death in trauma patients. Gut inflammation and loss of gut barrier function as a consequence of splanchnic ischemia-reperfusion (I/R) have been implicated as the initial triggering events that contribute to the development of the systemic inflammatory response, ALI, and MODS. Since hypoxia-inducible factor (HIF-1) is a key regulator of the physiological and pathophysiological response to hypoxia, we asked whether HIF-1 plays a proximal role in the induction of gut injury and subsequent lung injury. Utilizing partially HIF-1α-deficient mice in a global trauma hemorrhagic shock (T/HS) model, we found that HIF-1 activation was necessary for the development of gut injury and that the prevention of gut injury was associated with an abrogation of lung injury. Specifically, in vivo studies demonstrated that partial HIF-1α deficiency ameliorated T/HS-induced increases in intestinal permeability, bacterial translocation, and caspase-3 activation. Lastly, partial HIF-1α deficiency reduced TNF-α, IL-1β, cyclooxygenase-2, and inducible nitric oxide synthase levels in the ileal mucosa after T/HS whereas IL-1β mRNA levels were reduced in the lung after T/HS. This study indicates that prolonged intestinal HIF-1 activation is a proximal regulator of I/R-induced gut mucosal injury and gut-induced lung injury. Consequently, these results provide unique information on the initiating events in trauma-hemorrhagic shock-induced ALI and MODS as well as potential therapeutic insights.

Intestinal mucus layer preservation in female rats attenuates gut injury after trauma-hemorrhagic shock

BACKGROUND

We tested the hypothesis that females are more resistant to trauma-hemorrhagic shock (T/HS)-induced gut injury than males, and this is related to better preservation of their intestinal mucus layer, which is influenced in turn by the estrus cycle stage at the time of injury.

METHODS

Male, proestrus and diestrus female rats underwent a laparotomy (trauma) and 90 minutes of shock ( approximately 35 mm Hg). At 3 hours after reperfusion, terminal ileum was harvested and stained with Carnoy's Alcian Blue for mucus assessment, hematoxylin and eosin, and periodic acid schiff for villous and goblet cell morphology and injury. Ileal permeability was measured in separate intestinal segments using the ex vivo everted gut sac technique.

RESULTS

When compared with males, proestrus female rats were significantly more resistant to T/HS-induced morphologic gut injury, as reflected in both a lower incidence of villous injury (14% vs. 22%; p < 0.05) and a lesser grade of injury (1.0 vs. 2.8; p < 0.05) as well as preservation of gut barrier function (17.9 vs. 32.2; p < 0.05). This resistance to gut injury was associated with significant preservation of the mucus layer (87% vs. 62%; p < 0.05) and was influenced by the estrus cycle stage of the female rats. There was a significant inverse correlation between mucus layer coverage and the incidence (r = 0.9; p < 0.0001) and magnitude (r = 0.89; p < 0.0001) of villous injury and gut permeability (r = 0.74; p < 0.001).

CONCLUSIONS

The resistance of female rats to T/HS-induced intestinal injury and dysfunction was associated with better preservation of the intestinal mucus barrier and was to some extent estrus cycle-dependent. Preservation of the mucus barrier may protect against shock-induced gut injury and subsequent distant organ injury by limiting the ability of luminal contents such as bacteria and digestive enzymes from coming into direct contact with the epithelium.

Mesenteric lymph duct ligation improves survival in a lethal shock model

The goal of this study was to test the hypothesis that factors released from the gut and carried in the mesenteric lymph contribute to mortality in a lethal gut I/R model. To test this hypothesis, a lethal splanchnic artery occlusion (SAO) shock model was used in male Sprague-Dawley rats. In the first set of experiments, ligation of the mesenteric lymph duct (LDL), which prevents gut-derived factors carried in the intestinal lymphatics from reaching the systemic circulation, significantly improved 24-h survival after a 20-min SAO insult (0% vs. 60% survival; P < 0.05). This increase in survival in the LDL-treated rats was associated with a blunted hypotensive response. Because increased iNOS-induced NO levels have been implicated in SAO-induced shock, we measured plasma nitrite/nitrate levels and liver iNOS protein levels in a second group of animals. Ligation of the mesenteric lymph duct significantly abrogated the SAO-induced increase in plasma nitrite/nitrate levels and the induction of hepatic iNOS (P < 0.05). In an additional series of studies, we documented that LDL increased not only 24-h but also long-term 7-day survival. During the course of these studies, we made the unexpected finding that Sprague-Dawley rats from different animal vendors had differential resistance to SAO, and that the time of the year that the experiments were carried out also influenced the results. Nonetheless, in conclusion, these studies support the hypothesis that factors carried in the mesenteric lymph significantly contribute to the development of irreversible shock after SAO.