Endotoxin-induced suppression of pulmonary antibacterial defenses against Staphylococcus aureus

Acyloxyacyl hydrolase promotes pulmonary defense by preventing alveolar macrophage tolerance

Although alveolar macrophages (AMs) play important roles in preventing and eliminating pulmonary infections, little is known about their regulation in healthy animals. Since exposure to LPS often renders cells hyporesponsive to subsequent LPS exposures ("tolerant"), we tested the hypothesis that LPS produced in the intestine reaches the lungs and stimulates AMs, rendering them tolerant. We found that resting AMs were more likely to be tolerant in mice lacking acyloxyacyl hydrolase (AOAH), the host lipase that degrades and inactivates LPS; isolated Aoah-/- AMs were less responsive to LPS stimulation and less phagocytic than were Aoah+/+ AMs. Upon innate stimulation in the airways, Aoah-/- mice had reduced epithelium- and macrophage-derived chemokine/cytokine production. Aoah-/- mice also developed greater and more prolonged loss of body weight and higher bacterial burdens after pulmonary challenge with Pseudomonas aeruginosa than did wildtype mice. We also found that bloodborne or intrarectally-administered LPS desensitized ("tolerized") AMs while antimicrobial drug treatment that reduced intestinal commensal Gram-negative bacterial abundance largely restored the innate responsiveness of Aoah-/- AMs. Confirming the role of LPS stimulation, the absence of TLR4 prevented Aoah-/- AM tolerance. We conclude that commensal LPSs may stimulate and desensitize (tolerize) alveolar macrophages in a TLR4-dependent manner and compromise pulmonary immunity. By inactivating LPS in the intestine, AOAH promotes antibacterial host defenses in the lung.

Decreased Risk of Ventilator-Associated Pneumonia in Sepsis Due to Intra-Abdominal Infection

RATIONALE

Experimental studies suggest that intra-abdominal infection (IAI) induces biological alterations that may affect the risk of lung infection.

OBJECTIVES

To investigate the potential effect of IAI at ICU admission on the subsequent occurrence of ventilator-associated pneumonia (VAP).

METHODS

We used data entered into the French prospective multicenter Outcomerea database in 1997-2011. Consecutive patients who had severe sepsis and/or septic shock at ICU admission and required mechanical ventilation for more than 3 days were included. Patients with acute pancreatitis were not included.

MEASUREMENTS AND MAIN RESULTS

Of 2623 database patients meeting the inclusion criteria, 290 (11.1%) had IAI and 2333 (88.9%) had other infections. The IAI group had fewer patients with VAP (56 [19.3%] vs. 806 [34.5%], P<0.01) and longer time to VAP (5.0 vs.10.5 days; P<0.01). After adjustment on independent risk factors for VAP and previous antimicrobial use, IAI was associated with a decreased risk of VAP (hazard ratio, 0.62; 95% confidence interval, 0.46-0.83; P<0.0017). The pathogens responsible for VAP were not different between the groups with and without IAI (Pseudomonas aeruginosa, 345 [42.8%] and 24 [42.8%]; Enterobacteriaceae, 264 [32.8%] and 19 [34.0%]; and Staphylococcus aureus, 215 [26.7%] and 17 [30.4%], respectively). Crude ICU mortality was not different between the groups with and without IAI (81 [27.9%] and 747 [32.0%], P = 0.16).

CONCLUSIONS

In our observational study of mechanically ventilated ICU patients with severe sepsis and/or septic shock, VAP occurred less often and later in the group with IAIs compared to the group with infections at other sites.

Acute lung inflammation in Klebsiella pneumoniae B5055-induced pneumonia and sepsis in BALB/c mice: a comparative study

Lungs play an important role in the body's defense against a variety of pathogens, but this network of immune system-mediated defense can be deregulated during acute pulmonary infections. The present study compares acute lung inflammation occurring during Klebsiella pneumoniae B5055-induced pneumonia and sepsis in BALB/c mice. Pneumonia was induced by intranasal instillation of bacteria (10(4) cfu), while sepsis was developed by placing the fibrin-thrombin clot containing known amount of bacteria (10(2) cfu) into the peritoneal cavity of animals. Mice with sepsis showed 100% mortality within five post-infection days, whereas all the animals with pneumonia survived. In animals suffering from K. pneumoniae B5055-induced pneumonia, all the inflammatory parameters (TNF-α, IL-1α, MPO, MDA, and NO) were found to be maximum till third post-infection day, after that, a decline was observed, whereas in septic animals, all the above-mentioned markers of inflammation kept on increasing. Histopathological study showed presence of alternatively activated alveolar macrophages (or foam cells) in lungs of mice with pneumonia after third post-infection day, which might have contributed to the induction of resolution of inflammation, but no such observation was made in lungs of septic mice. Hence, during pneumonia, controlled activation of macrophages may lead to resolution of inflammation.

Soluble L-selectin at levels present in septic patients diminishes leukocyte-endothelial cell interactions in mice in vivo: a mechanism for decreased leukocyte delivery to remote sites in sepsis

OBJECTIVE

Recent in vivo studies of both septic humans and animals demonstrate that leukocyte delivery is attenuated to sites remote from the primary infection. The mechanisms for this are not entirely clear. L-selectin is integral to rolling, the first step in leukocyte recruitment to an inflammatory site. L-selectin is shed from leukocytes in sepsis, resulting in increased levels of soluble L-selectin in plasma (2.33 microg/mL). This study investigates the effects of soluble L-selectin at levels found in sepsis on leukocyte trafficking in vivo.

DESIGN

Prospective, controlled trial.

SETTING

Surgical research laboratory in a university hospital.

SUBJECTS

Swiss white male mice of 25-35 g.

INTERVENTIONS

Mice were randomized to one of three study groups: soluble L-selectin 2.33, soluble L-selectin 8.0, or albumin. Intravital microscopy was performed on postcapillary venules of 20-40 microm in diameter in the cremaster muscle of mice. Leukocyte-endothelial cell interactions (rolling, adherence, and rolling velocity) were measured pre- and post- (1, 15, 30, and 45 mins) intravenous infusion of human recombinant soluble L-selectin (2.33 and 8.0 microg/mL) or human albumin (8.0 microg/mL).

MEASUREMENTS AND MAIN RESULTS

The intravenous administration of soluble L-selectin to a systemic concentration of 2.33 microg/mL diminished rolling significantly. Soluble L-selectin at 8.0 microg/mL decreased rolling and increased rolling velocity to a greater degree. Injection of albumin did not alter leukocyte-endothelial cell interactions at any time point. No difference between groups in blood pressure, shear rate, or leukocyte counts was detected.

CONCLUSIONS

Soluble L-selectin diminishes leukocyte rolling at levels present in sepsis (2.33 microg/mL). This effect is dose dependent, and could not be explained by differences in blood pressure, shear rate, or leukocyte counts. These findings identify increased soluble L-selectin levels as one of the mechanisms for decreased leukocyte delivery and exudation to remote sites in septic patients.

Gut ischemia/reperfusion activates lung macrophages for tumor necrosis factor and hydrogen peroxide production

BACKGROUND

Gut ischemia followed by reperfusion (I/R) is implicated as a prime initiating event in the mechanism of multiple organ failure after trauma and hemorrhagic shock. Several lines of evidence indicate that macrophages are involved in this prime event. Our purpose was to evaluate hydrogen peroxide (H2O2) and tumor necrosis factor (TNF) production and phagocytosis by lung macrophages in a gut I/R model of multiple organ failure in rats.

METHODS

In the experimental group (I/R), Wistar rats (n = 35) were anesthetized and subjected to a median laparotomy, and the superior mesenteric artery was clamped for 45 minutes followed by 60 minutes of reperfusion. In the control group (LAP) (n = 37), animals underwent sham laparotomy. After the period of reperfusion, bronchoalveolar lavage (BAL) was performed and the resulting BAL cells were assayed for H2O2 production using the horseradish peroxidase-mediated red phenol oxidation method. TNF release was determined using the L929 cells bioassay. Zymosan phagocytosis by BAL macrophages was quantitated using phase microscopy.

RESULTS

H2O2 release in BAL cells of I/R rats (19.90 +/- 7.98 nmol/L/2 x 10(5) cells) is statistically higher than in the LAP group (10.92 +/- 5.01 nmol/L per 2 x 10(5) cells) (p = 0.0155), and the TNF production by BAL cells of the I/R group (38.09 +/- 20.79 units per 10(6) cells) was significantly higher than that of LAP rats (17.16 +/- 13.35 units per 10(6) cells) (p = 0.0281). Phagocytic activity of BAL mac. Macrophages of I/R rats was not statistically different from LAP animals.

CONCLUSION

These results suggest that BAL macrophage play a role in the mechanism of acute lung injury after trauma and hemorrhagic shock.

Effects of cytokines and endotoxin on the intracellular growth of bacteria

Patients with unresolving acute respiratory distress syndrome (ARDS) have persistently elevated levels of proinflammatory cytokines in the lungs and circulation and increased rates of bacterial infections. Phagocytic cells hyperactivated with lipopolysaccharide (LPS), which induces high levels of proinflammatory cytokines in monocytic cells, are inefficient in killing ingested bacteria despite having intact phagocytic activity. On the other hand, phagocytic cells that are activated with an analogue of LPS that does not induce the expression of proinflammatory cytokines effectively ingest and kill bacteria. We hypothesized that in the presence of high concentrations of proinflammatory cytokines, bacteria may adapt and utilize cytokines to their growth advantage. To test our hypothesis, we primed a human monocytic cell line (U937) with escalating concentrations of the proinflammatory cytokines tumor necrosis factor alpha, interleukin-1beta (IL-1beta), and IL-6 and with LPS. These cells were then exposed to fresh isolates of three common nosocomial pathogens: Staphylococcus aureus, Pseudomonas aeruginosa, and an Acinetobacter sp. In human monocytes primed with lower concentrations of proinflammatory cytokines (10 to 250 pg) or LPS (1 and 10 ng), intracellular bacterial growth decreased. However, when human monocytes were primed with higher concentrations of proinflammatory cytokines (1 to 10 ng) or LPS (1 to 10 micrograms), intracellular growth of the tested bacteria increased significantly (P <0.0001). These results were reproduced with peripheral blood monocytes obtained from normal healthy volunteers. The specificity of the cytokine activity was demonstrated by neutralizing the cytokines with specific antibodies. Our findings provide a possible mechanism to explain the frequent development of bacterial infections in patients with an intense and protracted inflammatory response.

Effect of alcohol on bacterial translocation in rats

Bacterial translocation has been proposed to be important in the pathophysiology of sepsis, as well as to be a consequence of sepsis. To study the effect of alcohol on bacterial translocation from the gut, normal Sprague-Dawley rats were administered alcohol by gavage by two regimens: Acute (3.7 g/kg, one dose) or Subacute (1 of 2 doses, 2.4 or 3.7 g/kg/day once daily for 14 days). Mesenteric lymph node cultures were performed, and portal venous blood was assayed for endotoxin. Ileal and cecal permeability studies were performed in the Acute and Subacute groups using fluorescein isothiocyanate-labeled dextrans of either 4,000 or 70,000 kDa size. As an index of the effect of systemic endotoxin, tissues from mesenteric lymph nodes, liver, and intestinal Peyer's patches were assayed for the presence of mRNA for tumor necrosis factor. Additionally, because extrapulmonary sepsis has been shown to suppress pulmonary antibacterial defenses, animals in the Subacute group were challenged by aerosol inoculation with Pseudomonas aeruginosa to determine bacterial clearance and alveolar cellular responses. The results show that neither of the alcohol regimens resulted in bacterial growth from mesenteric lymph nodes or portal blood. Animals in the Subacute group had more endotoxin present in portal blood than did the Control group (92.9 pg/ml vs. 40.2 pg/ml; p < 0.02). None of the animals had demonstrable mRNA for tumor necrosis factor in any of the tissues assayed. There were no demonstrable increases in ileal or cecal permeability for either the small or large molecular weight dextran in either alcohol group. Furthermore, there was no delay in the clearance of P. aeruginosa from the lung in the Subacute group, but these animals recruited fewer neutrophils into the airspaces in response to this challenge than did the Control animals. Thus, alcohol intoxication does not result in bacterial translocation from the gut in this model. Despite higher levels of portal venous endotoxin in the animals in the Subacute alcohol group, no adverse systemic consequences of this phenomenon could be demonstrated.

Ventilator-associated pneumonia due to Pseudomonas aeruginosa

OBJECTIVE

Ventilator-associated pneumonia (VAP) caused by Pseudomonas aeruginosa has been associated with higher case fatality rates than VAP caused by other bacterial etiologies. The causes of this excess mortality are unclear.

DESIGN

Retrospective review of 38 consecutive ventilated patients with Pseudomonas pneumonia, documented by highly reliable methods. Charts of five additional patients were unavailable for review.

SETTING

Medical ICUs of a university-affiliated Veterans Affairs Medical Center and a university-affiliated municipal hospital.

MEASUREMENTS

Prospectively collected hospital admission acute physiologic and chronic health examination (APACHE) II scores and cause of ICU admission. Retrospectively calculated organ failure and APACHE scores, VAP score. Clinical and microbiologic variables. Antibiotic treatment and outcome. Direct cause of death by standard definitions.

RESULTS

Overall mortality was 69% (26/38), significantly higher than the APACHE II predicted mortality of 42.6% (p=0.037). At least 38% (10/26) of deaths were directly attributable to Pseudomonas VAP. Multivariate analysis of factors associated with death found infectious cause for ICU admission (odds ratio [OR]=8.67; 95% confidence interval [CI], 0.86 to 85.94) and number of organ dysfunctions on the day of diagnosis (OR=1.73, 95% CI, 1.02 to 2.92) were significant. Septic shock from Pseudomonas VAP, septic shock from subsequent infection, and multiple organ dysfunction syndrome were the most common immediate causes of death. Mortality increased linearly with increasing APACHE III score on the day of diagnosis. Of initial antibiotic regimens, 67% (26/36) were considered failures. Persistent pneumonia occurred in 35% of patients while recurrent pneumonia was unusual (1/38).

CONCLUSIONS

Development of Pseudomonas pneumonia results in a mortality rate in excess of that due to the presenting illness. The attributable mortality determined by several means appears to approach 40%. The excess mortality appears to be related to the host defense response to the pneumonia rather than any characteristic of the pneumonia. Even standard antibiotic regimens fail frequently and do not prevent the excess mortality. Since at least 38% of deaths can be directly attributable to the Pseudomonas pneumonia, improvement in therapy is needed.

Influence of cytostatic agents on the pulmonary defence of mice infected with Klebsiella pneumoniae and on the efficacy of treatment with ceftriaxone

The effect of cytostatic treatment on the cellular defence and the efficacy of treatment with ceftriaxone in Klebsiella pneumoniae pneumonia was studied. Mice, made monocytopenic and granulocytopenic by cyclophosphamide or monocytopenic by etoposide, were infected intratracheally with K. pneumoniae (approximately 10(4) CFU) and then treated with ceftriaxone. At various intervals, the numbers of bacteria in the broncho-alveolar lavage (BAL) fluid and in lungs homogenised after lavage were determined. Cyclophosphamide reduced the numbers of granulocytes in the BAL fluid significantly but reduced only slightly the number of alveolar macrophages at the time of inoculation, 12 and 15 h later. The number of CFU in cyclophosphamide-treated mice was higher than that in controls, being significant in the homogenised lungs at 15 h after infection. In etoposide-treated mice, the numbers of alveolar phagocytes in BAL did not differ from those in control mice, whereas the number of bacteria was lower (only significantly in BAL fluid at 15 h after infection) than that in the controls. In this short experimental infection cytostatic treatment did not affect the outgrowth of Klebsiella pneumoniae substantially or the efficacy of treatment with ceftriaxone.

Endotoxin primes perfused rabbit lungs for enhanced vasoconstrictor response to staphylococcal alpha-toxin

The major pore-forming exotoxin of Staphylococcus aureus, staphylococcal alpha-toxin, causes thromboxane-mediated pulmonary hypertension and prostanoid-independent protracted vascular leakage in perfused rabbit lungs. We asked whether lung responsiveness to the staphylococcal agent would be altered by a preceding period of endotoxin priming. Isolated rabbit lungs were perfused with Krebs-Henseleit buffer in the presence or absence of 100 ng/ml Salmonella abortus equii endotoxin for up to 5 h. The lipopolysaccharide exposure evoked the release of large quantities of tumor necrosis factor into the vascular and alveolar spaces but did not significantly alter pulmonary artery pressure, organ weight, or the repeatedly assessed capillary filtration coefficient (Kfc). Two and 4 h after endotoxin administration, alpha-toxin (10 to 30 ng/ml) was bolus-injected into the pulmonary artery. Toxin-evoked prostanoid generation (TxB2, 6-keto-PGF1 alpha) and pressor responses were markedly accelerated and enhanced in endotoxin-primed lungs, both for the 2 h and the 4 h priming period. No significant influence of endotoxin was noted when applied simultaneously with alpha-toxin. Cyclooxygenase inhibition suppressed the alpha-toxin-evoked pressure rise in both endotoxin-primed and nonprimed lungs. Endotoxin priming did not influence the alpha-toxin-induced protracted increase in Kfc values, assessed in the presence of cyclooxygenase inhibition. We conclude that endotoxin primes rabbit lungs for enhanced prostanoid generation and pulmonary hypertension in response to S. aureus alpha-toxin. Such cooperativity of endotoxin priming and exotoxin triggering may be relevant in critically ill patients suffering from both endotoxemia and gram-positive sepsis.

Lipopolysaccharide-induced stimulation of alveolar macrophage opsonin-independent phagocytosis

Alveolar macrophage (AM) opsonin-independent phagocytosis may be an important process by which bacteria are cleared from the airspaces. Although it has been shown that clearance of bacteria from the lung is altered in association with endotoxemia, the effect of endotoxemia on opsonin-independent phagocytosis is unknown. Because alterations in AM opsonin-independent phagocytosis could potentially influence bacterial clearance in the lung, we studied the effects of the intravenous injection of Escherichia coli lipopolysaccharide (LPS) into rats on AM opsonin-independent phagocytosis of latex particles. AM obtained by bronchoalveolar lavage (BAL) 2 or 4 hr after injection of LPS demonstrated phagocytosis comparable to AM from control animals. In contrast, AM obtained 12 hr after injection of LPS demonstrated a nearly threefold increase in phagocytosis. Depletion of serum complement with Naja naja cobra venom factor (CVF) did not alter LPS-induced augmentation of phagocytosis. Furthermore, phagocytosis of AM obtained by BAL 2 or 12 hr after activation of complement by injection of CVF was not significantly different from that of AM from control animals, indicating that complement activation alone was not sufficient to stimulate opsonin-independent phagocytosis. These observations suggest that opsonin-independent phagocytosis may represent an important route of bacterial uptake by AM following endotoxemia, and that LPS-induced stimulation of AM opsonin-independent phagocytosis may occur independently of activation of complement by LPS.