Kinetic analysis of pulmonary neutrophil retention in vivo using the multiple-indicator-dilution technique

Neutrophil Adaptations upon Recruitment to the Lung: New Concepts and Implications for Homeostasis and Disease

Neutrophils have a prominent role in all human immune responses against any type of pathogen or stimulus. The lungs are a major neutrophil reservoir and neutrophilic inflammation is a primary response to both infectious and non-infectious challenges. While neutrophils are well known for their essential role in clearance of bacteria, they are also equipped with specific mechanisms to counter viruses and fungi. When these defense mechanisms become aberrantly activated in the absence of infection, this commonly results in debilitating chronic lung inflammation. Clearance of bacteria by phagocytosis is the hallmark role of neutrophils and has been studied extensively. New studies on neutrophil biology have revealed that this leukocyte subset is highly adaptable and fulfills diverse roles. Of special interest is how these adaptations can impact the outcome of an immune response in the lungs due to their potent capacity for clearing infection and causing damage to host tissue. The adaptability of neutrophils and their propensity to influence the outcome of immune responses implicates them as a much-needed target of future immunomodulatory therapies. This review highlights the recent advances elucidating the mechanisms of neutrophilic inflammation, with a focus on the lung environment due to the immense and growing public health burden of chronic lung diseases such as cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD), and acute lung inflammatory diseases such as transfusion-related acute lung injury (TRALI).

The counter-intuitive role of the neutrophil in the acute respiratory distress syndrome

INTRODUCTION

Neutrophils are the primary effectors of the innate immune system but are profoundly histotoxic cells. The acute respiratory distress syndrome (ARDS) is considered to be a prime example of neutrophil-mediated tissue injury.

SOURCES OF DATA

The information presented in this review is acquired from the published neutrophil cell biology literature and the longstanding interest of the senior authors in ARDS pathogenesis and clinical management.

AREAS OF AGREEMENT

Investigators in the field would agree that neutrophils accumulate in high abundance in the pulmonary microcirculation, lung interstitium and alveolar airspace of patients with ARDS. ARDS is also associated with systemic neutrophil priming and delayed neutrophil apoptosis and clearance of neutrophils from the lungs. In animal models, reducing circulating neutrophil numbers ameliorates lung injury.

AREAS OF CONTROVERSY

Areas of uncertainty include how neutrophils get stuck in the narrow pulmonary capillary network-whether this reflects changes in the mechanical properties of primed neutrophils alone or additional cell adhesion molecules, the role of neutrophil sub-sets or polarization states including pro-angiogenic and low-density neutrophils, whether neutrophil extracellular trap (NET) formation is beneficial (through bacterial capture) or harmful and the potential for neutrophils to participate in inflammatory resolution. The latter may involve the generation of specialized pro-resolving molecules (SPMs) and MMP-9, which is required for adequate matrix processing.

GROWING POINTS

Different and possibly stable endotypes of ARDS are increasingly being recognized, yet the relative contribution of the neutrophil to these endotypes is uncertain. There is renewed and intense interest in understanding the complex 'new biology' of the neutrophil, specifically whether this cell might be a valid therapeutic target in ARDS and other neutrophil-driven diseases and developing understanding of ways to enhance the beneficial role of the neutrophil in the resolution phase of ARDS.

AREAS TIMELY FOR DEVELOPING RESEARCH

Aside from treatment of the precipitating causes of ARDS, and scrupulous fluid, infection and ventilation management, there are no pharmacological interventions for ARDS; this represents an urgent and unmet need. Therapies aimed at reducing overall neutrophil numbers risk secondary infection; hence better ways are needed to reverse the processes of neutrophil priming activation, hyper-secretion and delayed apoptosis while enhancing the pro-resolution functions of the neutrophil.

Modeling Indicator Dispersion in Extracorporeal Blood Lines

The measurement of indicators such as saline diluted by blood flow provides important information on transport characteristics during extracorporeal blood treatments. When saline is injected and measured using the extracorporeal system, the effects caused by dispersion within the extracorporeal system have to be taken into consideration in order to adequately identify intracorporeal transport characteristics. It was the aim of this study to quantify the extracorporeal contribution and to obtain a transport function for specified sections of the extracorporeal system.

The dispersion of saline following an impulsive input was measured in arterial and venous segments of customary extracorporeal blood lines with different distribution volumes (Vt=23–87 mL) using a range of different blood flows (Qb=200–450 ml/min). The dispersion was analyzed using a modified Gamma distribution function characterized by three parameters n, k, and τ, where n is real, positive, and n 1, where k=Q b/Vt*n2, and where trefers to the indicator appearance time at the sampling site. The parameters n, k, and t were identified by fitting the model function to experimental data.

The value of n was 2.3±0.5 and largely independent of the type of line segment, Qb, or Vt. τ showed a strong dependence on Vt and Qb which was described by τ= Vt/Qb*(n-1)/n.

Thus, with a given n, and when Vt and Qb are known, the transport function for saline in important sections of the extracorporeal circulation can be determined for specific experimental conditions. With this information indicator dilution curves measured in extracorporeal blood lines can be corrected for extracorporeal effects.

Mathematical modeling reveals kinetics of lymphocyte recirculation in the whole organism

The kinetics of recirculation of naive lymphocytes in the body has important implications for the speed at which local infections are detected and controlled by immune responses. With a help of a novel mathematical model, we analyze experimental data on migration of 51Cr-labeled thoracic duct lymphocytes (TDLs) via major lymphoid and nonlymphoid tissues of rats in the absence of systemic antigenic stimulation. We show that at any point of time, 95% of lymphocytes in the blood travel via capillaries in the lung or sinusoids of the liver and only 5% migrate to secondary lymphoid tissues such as lymph nodes, Peyer's patches, or the spleen. Interestingly, our analysis suggests that lymphocytes travel via lung capillaries and liver sinusoids at an extremely rapid rate with the average residence time in these tissues being less than 1 minute. The model also predicts a relatively short average residence time of TDLs in the spleen (2.5 hours) and a longer average residence time of TDLs in major lymph nodes and Peyer's patches (10 hours). Surprisingly, we find that the average residence time of lymphocytes is similar in lymph nodes draining the skin (subcutaneous LNs) or the gut (mesenteric LNs) or in Peyer's patches. Applying our model to an additional dataset on lymphocyte migration via resting and antigen-stimulated lymph nodes we find that enlargement of antigen-stimulated lymph nodes occurs mainly due to increased entrance rate of TDLs into the nodes and not due to decreased exit rate as has been suggested in some studies. Taken together, our analysis for the first time provides a comprehensive, systems view of recirculation kinetics of thoracic duct lymphocytes in the whole organism.

Exercise and leukocyte interchange among central circulation, lung, spleen, and muscle

Circulating leukocytes increase rapidly with exercise then quickly decrease when the exercise ends. We tested whether exercise acutely led to bidirectional interchange of leukocytes between the circulation and the lung, spleen, and active skeletal muscle. To accomplish this it was necessary to label a large number of immune cells (granulocytes, monocytes, and lymphocytes) in a way that resulted in minimal perturbation of cell function. Rats were injected intravenously with a single bolus of carboxyfluorescein diacetate succinamidyl ester (CFSE) dye which is rapidly and irreversibly taken up by circulating cells. The time course of the disappearance of labeled cells and their reappearance in the circulation following exercise was determined via flow cytometry. The majority of circulating leukocytes were labeled at 4h. post-injection and this proportion slowly declined out to 120 h. At both 24 and 120 h, running resulted in an increase in the proportion of labeled leukocytes in the circulation. Analysis of the skeletal muscle, spleen and lung indicated that labeled leukocytes had accumulated in those tissues and were mobilized to the circulation in response to exercise. This indicates that there is an ongoing exchange of leukocytes between the circulation and tissues and that exercise can stimulate their redistribution. Exchange was slower with muscle than with spleen and lung, but in all cases, influenced by exercise. Exercise bouts redistribute leukocytes between the circulation and the lung, spleen and muscle. The modulatory effects of exercise on the immune system may be regulated in part by the systemic redistribution of immune cells.

Endotoxin and polycyclic aromatic hydrocarbons in ambient fine particulate matter from Fresno, California initiate human monocyte inflammatory responses mediated by reactive oxygen species

In urban areas, a correlation between exposure to particulate matter (PM) from air pollution and increased cardiovascular morbidity and mortality has been observed. Components of PM include bacterial contaminants, transition metals, salts, polycyclic aromatic hydrocarbons (PAH), and carbonaceous material, which could interact with various cell types to produce systemic responses when inhaled. We examined the effects of PM collected from Fresno, California on activation of human monocytes and their interaction with vascular endothelium, a key event in atherogenesis. PM exposure increased cytokine expression and secretion from monocytes and enhanced monocyte adhesion to human aortic endothelial cells, both of which were attenuated by neutralizing endotoxin. PM also increased monocyte CYP1a1 expression, and inhibition of the aryl hydrocarbon receptor reduced the CYP1a1 and inflammatory responses. PM-treated monocytes accumulated intracellular reactive oxygen species (ROS), and antioxidants attenuated inflammatory and xenobiotic responses. Finally, supernatants from PM-treated pulmonary microvascular endothelial cells induced monocyte inflammatory responses that were not a consequence of endotoxin transfer. These results suggest that certain components of urban PM, namely endotoxin and PAH, activate circulating monocytes directly or indirectly by first stimulating other cells such as pulmonary endothelial cells, providing several mechanisms by which PM inhalation could induce pulmonary and/or systemic inflammation.

Modeling neutrophil transport in pulmonary capillaries

Neutrophils can be retained in the pulmonary microvasculature due to their low deformability, resulting in having a higher concentration there than in the systemic circulation, even in normal lungs. It is thought that this high concentration of the cells facilitates their effective recruitment to sites of inflammation. Thus, in order to understand their role in the immune system in the lungs, where blood comes in contact with outer air via thin septa of alveoli, it is important to clarify their flow characteristics in the pulmonary capillary bed. However, in contrast to erythrocytes in systemic capillaries, little research has been performed on the flow of neutrophils in pulmonary capillaries. This may be partly because no complete rheological model of the cell has been established yet, and partly because pulmonary capillaries are very short and closely interconnected, forming a complicated three-dimensional network, in addition to difficulty in in vivo experimental observations. Moreover, the neutrophils change their mechanical properties and show active motion in response to some chemoattractants. In this article, various proposed rheological models of the neutrophil, flow models of a cell through a single capillary segment, and alveolar capillary network models are introduced, aiming at the numerical simulation of neutrophil transport in the pulmonary microvasculature.

Effect of sternotomy and extracorporeal circulation on pulmonary neutrophil kinetics in pigs

Pulmonary margination of neutrophils may contribute to lung damage after extracorporeal circulation for cardiac surgery. We evaluated single-pass pulmonary neutrophil kinetics using the multiple indicator-dilution technique in control pigs (n = 10), after sternotomy alone (sterno, n = 10) or after 30 min of observation following a period of 90 min extracorporeal circulation (n = 7). Blood neutrophils increased in the control and sterno groups (p < 0.05) but remained unchanged in the extracorporeal circulation group. The transfer coefficient for neutrophil margination from the circulating to the lung-marginated pool (k(c-m)) and pulmonary neutrophil clearance (Cl(c-m)) were similar between the three groups. There was an inverse correlation between k(c-m) and the degree of lung tissue perfusion evaluated from the tracer-accessible extravascular lung water (r = -0.54, p < 0.01). There was no arterio-venous gradient of neutrophils in any of the groups, suggesting a dynamic equilibrium of the margination/demargination processes. We conclude that extracorporeal circulation does not significantly modify single pass pulmonary neutrophil kinetics 30 min after reperfusion. The rate of neutrophil margination to the tracer-accessible lung tissue suggests that lung tissue de-recruitment is associated with increased neutrophil margination.

Sequential recruitment of neutrophils into lung and bronchoalveolar lavage fluid in LPS-induced acute lung injury

Infiltration of activated neutrophils [polymorphonuclear leukocytes (PMN)] into the lung is an important component of the inflammatory response in acute lung injury. The signals required to direct PMN into the different compartments of the lung have not been fully elucidated. In a murine model of LPS-induced lung injury, we investigated the sequential recruitment of PMN into the pulmonary vasculature, lung interstitium, and alveolar space. Mice were exposed to aerosolized LPS and bronchoalveolar lavage fluid (BAL), and lungs were harvested at different time points. We developed a flow cytometry-based technique to assess in vivo trafficking of PMN in the intravascular and extravascular lung compartments. Aerosolized LPS induced consistent PMN migration into all lung compartments. We found that sequestration in the pulmonary vasculature occurred within the first hour. Transendothelial migration into the interstitial space started 1 h after LPS exposure and increased continuously until a plateau was reached between 12 and 24 h. Transepithelial migration into the alveolar air space was delayed, as the first PMN did not appear until 2 h after LPS, reaching a peak at 24 h. Transendothelial migration and transepithelial migration were inhibited by pertussis toxin, indicating involvement of Galphai-coupled receptors. These findings confirm LPS-induced migration of PMN into the lung. For the first time, distinct transmigration steps into the different lung compartments are characterized in vivo.