Determinants of lung bacterial clearance in mice after acute hypoxia

Mechanisms controlling bacterial infection in myeloid cells under hypoxic conditions

Various factors of the tissue microenvironment such as the oxygen concentration influence the host-pathogen interaction. During the past decade, hypoxia-driven signaling via hypoxia-inducible factors (HIF) has emerged as an important factor that affects both the pathogen and the host. In this chapter, we will review the current knowledge of this complex interplay, with a particular emphasis given to the impact of hypoxia and HIF on the inflammatory and antimicrobial activity of myeloid cells, the bacterial responses to hypoxia and the containment of bacterial infections under oxygen-limited conditions. We will also summarize how low oxygen concentrations influence the metabolism of neutrophils, macrophages and dendritic cells. Finally, we will discuss the consequences of hypoxia and HIFα activation for the invading pathogen, with a focus on Pseudomonas aeruginosa, Mycobacterium tuberculosis, Coxiella burnetii, Salmonella enterica and Staphylococcus aureus. This includes a description of the mechanisms and microbial factors, which the pathogens use to sense and react to hypoxic conditions.

Hypoxia-mediated impairment of the mitochondrial respiratory chain inhibits the bactericidal activity of macrophages

In infected tissues oxygen tensions are low. As innate immune cells have to operate under these conditions, we analyzed the ability of macrophages (Mφ) to kill Escherichia coli or Staphylococcus aureus in a hypoxic microenvironment. Oxygen restriction did not promote intracellular bacterial growth but did impair the bactericidal activity of the host cells against both pathogens. This correlated with a decreased production of reactive oxygen intermediates (ROI) and reactive nitrogen intermediates. Experiments with phagocyte NADPH oxidase (PHOX) and inducible NO synthase (NOS2) double-deficient Mφ revealed that in E. coli- or S. aureus-infected cells the reduced antibacterial activity during hypoxia was either entirely or partially independent of the diminished PHOX and NOS2 activity. Hypoxia impaired the mitochondrial activity of infected Mφ. Inhibition of the mitochondrial respiratory chain activity during normoxia (using rotenone or antimycin A) completely or partially mimicked the defective antibacterial activity observed in hypoxic E. coli- or S. aureus-infected wild-type Mφ, respectively. Accordingly, inhibition of the respiratory chain of S. aureus-infected, normoxic PHOX(-/-) NOS2(-/-) Mφ further raised the bacterial burden of the cells, which reached the level measured in hypoxic PHOX(-/-) NOS2(-/-) Mφ cultures. Our data demonstrate that the reduced killing of S. aureus or E. coli during hypoxia is not simply due to a lack of PHOX and NOS2 activity but partially or completely results from an impaired mitochondrial antibacterial effector function. Since pharmacological inhibition of the respiratory chain raised the generation of ROI but nevertheless phenocopied the effect of hypoxia, ROI can be excluded as the mechanism underlying the antimicrobial activity of mitochondria.

Changing Pattern of Ventilator Settings in Patients Without Acute Lung Injury

STUDY OBJECTIVES

To determine whether the widely accepted concept of using lower tidal volume (Vt) values in patients with ARDS or obstructive lung disease has affected the pattern of ventilator settings in mechanically ventilated patients who do not have one of these conditions.

DESIGN AND PATIENTS

We performed a retrospective chart review of all patients who had experienced out-of-hospital cardiac arrest and had received ventilatory support for > or = 1 day at a university-affiliated county hospital during the years 1990, 1991, 1992, 1995, 1998, 1999, and 2000.

RESULTS

In 139 such patients, the mean final Vt values used on the first day of mechanical ventilation were 11.7, 12.4, 11.3, 9.6, 9.7, 9.2, and 9.8 mL/kg in those years, respectively. Multivariate analysis revealed that increasing year (beta-coefficient = -0.24; p = 0.001) and the presence of pulmonary edema (beta-coefficient = -1.2; p = 0.001) were independent predictors of the use of lower Vt values. Patients managed with a low Vt (ie, < 10 mL/kg; mean [+/- SD] Vt, 8.4 +/- 1.3 mL/kg) had a significantly higher incidence of atelectasis than the patients who were managed with traditional, larger Vt values (ie, > or = 10 mL/kg; mean Vt, 11.8 +/- 1.5 mL/kg) [61.1% vs 36.7%, respectively; p = 0.02]. Multivariate analysis revealed that the mean Vt used on days 1, 2, and 3 (<10 mL/kg or > or = 10 mL/kg) was the only predictor of the development of atelectasis during the first 3 days of mechanical ventilation (odds ratio, 0.33; p = 0.015). There was no difference in the incidence of pneumonia, the number of days spent receiving mechanical ventilation, Pao(2)/fraction of inspired oxygen ratio, or respiratory system compliance between the low Vt group and the traditional Vt group.

CONCLUSION

Currently, physicians at our hospital use lower Vt values than they have in the past. This is associated with the increase in the incidence of atelectasis in the patients who received ventilation using low Vt values.

Phagocytosis and ATP levels in alveolar macrophages during acute hypoxia

Pulmonary alveolar macrophages (PAM) function as phagocytes of inhaled particulate matter and microorganisms at the air-tissue interface of lung alveoli. Changes in cellular ATP concentrations ([ATP]) and phagocytic function during acute hypoxia may be important in conditions associated with low alveolar O2. We proposed that acute hypoxia would decrease phagocytosis and reduce [ATP] in freshly isolated PAM. Phagocytic function (fluorescent microscopic technique determining percent phagocytosis in live cells) was monitored by recording uptake and retention of glutaraldehyde-fixed red blood cells (GRBC) in isolated rabbit PAM during acute incubations in air (20% O2) or hypoxia (1.7% O2). Macrophage [ATP] were determined spectrophotometrically. Acute hypoxia for 30 to 150 min decreased phagocytic function 30 to 56% in PAM without significantly affecting cell adherence and viability. Pre-exposure of PAM to hypoxia before addition of GRBC resulted in an even greater reduction in phagocytosis (97% decrease by 30 min), and recovery of phagocytic function occurred 60 to 90 min after returning PAM to air. The cellular retention of phagocytosed GRBC (percentage of PAM with GRBC and number of GRBC/PAM) was reduced 30% by 1 h of hypoxia. Compared with [ATP] of PAM in air, [ATP] of PAM exposed to hypoxia were reduced 55 and 35% at 30 and 60 min, respectively. Compared with [ATP] of cells with GRBC in air at 0 and 30 min, PAM with GRBC in hypoxia for 30 min had, respectively, 61 and 40% lower [ATP]. By 60 min with GRBC, PAM [ATP] in air and hypoxia were similar but were 50% lower than [ATP] at time 0.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrapulmonary growth of Staphylococcus aureus in rats during induced atelectasis

Intrinsic pulmonary antibacterial defenses are mediated by alveolar macrophages and by noncellular factors. Mechanical ventilation in the resting tidal volume range leads to alterations in the physical characteristics of alveolar surfactant, alveolar instability, regional hypoxia, and systemic hypoxemia. While a number of experimental manipulations diminish the activity of the intrinsic antibacterial defense system, the effects of mechanical ventilation per se have not been systematically evaluated previously. We found that normal rats ventilated without sighing (periodic large breaths) manifested severe defects in pulmonary clearance of Staphylococcus aureus during 6-h experiments, such that growth of the inoculum occurred. Addition of a timer-controlled mechanism to cause the animals to sigh every 2 min, without other modifications in the experimental conditions, caused significant improvement in clearance. Analysis of cellular response, compartmentalization of viable bacteria, surfactant quantities and sedimentation characteristics, and protein influx indicated that the defect in clearance paralleled alterations in the physical state of surfactant and alveolar stability but was not strongly correlated with alterations in the other parameters we measured. The data show that defective pulmonary bacterial clearance is rapidly induced by measures which alter alveolar stability and suggest that intrinsic pulmonary defenses require maintenance of normal air-liquid interfaces for optimal function.

Atelectasis in childhood

The clinician who identifies atelectasis in a child must determine the mechanism by which atelectasis developed, the functional significance of the collapsed lung, and the etiology responsible for its presence. This article provides a framework to help physicians address these issues and thereby elect the most rational diagnostic and therapeutic interventions at the most appropriate time.

Nosocomial infections

Recent developments in the understanding of nosocomial infection in general and nosocomial respiratory infections in particular are elucidated. Although the discussion focuses on aerobic bacteria, data are presented to indicate the growing realization that unusual and newly discovered microorganisms play a significant role in hospital-acquired infections. Strategies for the control or prevention of nosocomial infections are highlighted.

Adult respiratory distress syndrome--a review

Adult respiratory distress syndrome (ARDS) represents a common pathway of damage to the lungs by a wide variety of different agents. The important aetiological factors and mechanisms of lung injury are considered. Electron microscopic features as well as light microscopy are described. Factors that may modify the pathological picture are discussed. Probably the most important of these is oxygen. This gas is said to produce interstitial pulmonary fibrosis but this concept is once again questioned.

Early clearance of pneumococci from the lungs of decomplemented rats

Pneumococcal types which exhibited varying degrees of interaction with the complement system in vitro wee aerosolized into normal and decomplemented rats, and the rate of killing of pneumococci was monitored by serial cultures of whole lung homogenates. The clearance of pneumococci from the alveoli did not correlate with the ability of the pneumococci to be opsonized by complement in vitro. Similarly, rats depleted of complement in the serum and the bronchoalveolar lavage fluid maintained their ability to rapidly inactivate aerosolized pneumococci. These results indicate that the early phase of pneumococcal killing in the alveoli does not require complement and suggest that in the nonimmune host, phagocytosis by alveolar macrophages can be achieved without complement-dependent opsonization or, alternatively, that extracellular factors rapidly inactivate inhaled pneumococci.

Effect of hydrogen sulfide on bacterial inactivation in the rat lung

This study evaluated the effect of a low concentration of hydrogen sulfide on the rat antibacterial defense system. Rats exposed to 45 ppm hydrogen sulfide (threshold limit value for hydrogen sulfide = 10 ppm) for 4 or 6 hr exhibited a significant (P less .01) reduction in the inactivation of viable staphylococci deposited in the lungs during a bacterial aerosol challenge. Pre-exposure of rats to 46 ppm hydrogen sulfide for 2 hr, however, did not alter intrapulmonary staphylococcal inactivation. We hypothesize that impairment of the alveolar macrophage is the basis of these findings.

Early bacterial clearance from murine lungs. Species-dependent phagocyte response

Two sets of phagocytic cells are available to defend the lung against inhaled bacteria. Both resident alveolar macrophages and granulocytes from the circulation have been observed in pulmonary air spaces after the deposition of bacteria; their functional roles, however, have been defined. We rendered mice selectively granulocytopenic with heterologous antiserum in order to ascertain the relative contributions of these two groups of cells in intrapulmonary bacterial killing. The clearance of Staphylococcus aureus was unimpaired in granulocytopenic animals, confirming the primary role of the alveolar macrophages in the killing of these organisms. In contrast, granulocytopenic animals cleared only 10.0+/-7.0% of an inoculum of Klebsiella pneumoniae compared with 33.0+/-4.0% clearance in normal animals (P < 0.02), and Pseudomonas aeruginosa proliferated to 513% of baseline levels in granulocytopenic animals, whereas normal mice cleared 26.8+/-10.6% of the inoculum. These findings indicate that circulating granulocytes play a major role in the clearance of the latter two organisms. This variation in cellular response to different bacterial species suggests that the defense of the lung against pathogenic bacteria is more complex than has been previously assumed.