James F. Lewis

Surfactant protein A inhibits T cell proliferation via its collagen-like tail and a 210-kDa receptor

Investigation of possible mechanisms to describe the hyporesponsiveness of pulmonary leukocytes has led to the study of pulmonary surfactant and its constituents as immune suppressive agents. Pulmonary surfactant is a phospholipid-protein mixture that reduces surface tension in the lung and prevents collapse of the alveoli. The most abundant protein in this mixture is a hydrophilic molecule termed surfactant-associated protein A (SP-A). Previously, we showed that bovine (b) SP-A can inhibit human T lymphocyte proliferation and interleukin-2 production in vitro. Results presented in this investigation showed that different sources of human SP-A and bSP-A as well as recombinant rat SP-A inhibited human T lymphocyte proliferation in a dose-dependent manner. A structurally similar collagenous protein, C1q, did not block the in vitro inhibitory action of SP-A. The addition of large concentrations of mannan to SP-A-treated cultures also did not disrupt inhibition, suggesting that the effect is not mediated by the carbohydrate recognition domain of SP-A. Use of recombinant mutant SP-As revealed that a 36-amino acid Arg-Gly-Asp (RGD) motif-containing span of the collagen-like domain was responsible for the inhibition of T cell proliferation. A polyclonal antiserum directed against an SP-A receptor (SP-R210) completely blocked the inhibition of T cell proliferation by SP-A. These results emphasize a potential role for SP-A in dampening lymphocyte responses to exogenous stimuli. The data also provide further support for the concept that SP-A maintains a balance between the clearance of inhaled pathogens and protection against collateral immune-mediated damage.

A Search for Subgroups of Patients With ARDS Who May Benefit From Surfactant Replacement Therapy: A Pooled Analysis of Five Studies With Recombinant Surfactant Protein-C Surfactant (Venticute)

BACKGROUND Studies to date have shown no survival benefit for the use of exogenous surfactant to treat patients with the ARDS. To identify specific patient subgroups for future study, we performed an exploratory post hoc analysis of clinical trials of recombinant surfactant protein-C (rSP-C) surfactant (Venticute; Nycomed GmbH; Konstanz, Germany). METHODS We performed a pooled analysis of all five multicenter studies in which patients with ARDS due to various predisposing events were treated with rSP-C surfactant. Patients received either usual care (n = 266) or usual care plus up to four intratracheal doses (50 mg/kg) of rSP-C surfactant (n = 266). Factors influencing the study end points were analyzed using descriptive statistics, analysis of covariance, and logistic regression models. RESULTS ARDS was most often associated with pneumonia or aspiration, sepsis, and trauma or surgery. For the overall patient population, treatment with rSP-C surfactant significantly improved oxygenation (p = 0.002) but had no effect on mortality (32.6%). Multivariate analysis showed age and acute physiology and chronic health evaluation (APACHE) II score to be the strongest predictors of mortality. In the subgroup of patients with severe ARDS due to pneumonia or aspiration, surfactant treatment was associated with markedly improved oxygenation (p = 0.0008) and improved survival (p = 0.018). CONCLUSIONS rSP-C surfactant improved oxygenation in patients with ARDS irrespective of the predisposition. Post hoc evidence of reduced mortality associated with surfactant treatment was obtained in patients with severe respiratory insufficiency due to pneumonia or aspiration. Those patients are the focus of a current randomized, blinded, clinical trial with rSP-C surfactant.

Recombinant surfactant protein C-based surfactant for patients with severe direct lung injury.

RATIONALE Patients with acute lung injury have impaired function of the lung surfactant system. Prior clinical trials have shown that treatment with exogenous recombinant surfactant protein C (rSP-C)-based surfactant results in improvement in blood oxygenation and have suggested that treatment of patients with severe direct lung injury may decrease mortality. OBJECTIVES Determine the clinical benefit of administering an rSP-C-based synthetic surfactant to patients with severe direct lung injury due to pneumonia or aspiration. METHODS A prospective randomized blinded study was performed at 161 centers in 22 countries. Patients were randomly allocated to receive usual care plus up to eight doses of rSP-C surfactant administered over 96 hours (n = 419) or only usual care (n = 424). MEASUREMENTS AND MAIN RESULTS Mortality to 28 days after treatment, the requirement for mechanical ventilation, and the number of nonpulmonary organ failure-free days were not different between study groups. In contrast to prior studies, there was no improvement in oxygenation in patients receiving surfactant compared with the usual care group. Investigation of the possible reasons underlying the lack of efficacy suggested a partial inactivation of rSP-C surfactant caused by a step of the resuspension process that was introduced with this study. CONCLUSIONS In this study, rSP-C-based surfactant was of no clinical benefit to patients with severe direct lung injury. The unexpected lack of improvement in oxygenation, coupled with the results of in vitro tests, suggest that the administered suspension may have had insufficient surface activity to achieve clinical benefit.

Role of exogenous surfactant in acute lung injury

ObjectivesTo outline both the preclinical and clinical data demonstrating surfactant alterations in acute lung injury, which provide the rationale for testing exogenous surfactant administration in this setting. We also review the results of the randomized, controlled clinical trials conducted to date that have evaluated this therapy in patients with acute respiratory distress syndrome, and we review the various factors that may have affected the outcomes of these trials. Future areas for surfactant research will also be addressed. Data Synthesis and ExtractionA review of the literature utilizing a MEDLINE search was performed using the key words: surfactant, surfactant administration, acute respiratory distress syndrome, and lung injury. Personal views are presented and references to unpublished clinical data are made based on the authors’ access to this data. ConclusionsExogenous surfactant administration has proven inconsistent as a therapeutic modality for patients with acute respiratory distress syndrome. This is because of the severity of the injury at the time of treatment and because of the variable surfactant preparations, dosing regimes, and delivery methods used in the different trials. Future research efforts will focus on determining the optimal timing of surfactant administration in patients at risk of developing acute respiratory distress syndrome with the aim of preventing progressive lung dysfunction and determining whether surfactant treatments need to be tailored to the specific patient in question. Moreover, with the recognition that surfactant also plays an important role in host defense, the future for surfactant therapy is exciting.

Carbon dioxide attenuates pulmonary impairment resulting from hyperventilation.

ObjectiveDeliberate elevation of Paco2 (therapeutic hypercapnia) protects against lung injury induced by lung reperfusion and severe lung stretch. Conversely, hypocapnic alkalosis causes lung injury and worsens lung reperfusion injury. Alterations in lung surfactant may contribute to ventilator-associated lung injury. The potential for CO2 to contribute to the pathogenesis of ventilator-associated lung injury at clinically relevant tidal volumes is unknown. We hypothesized that: 1) hypocapnia would worsen ventilator-associated lung injury, 2) therapeutic hypercapnia would attenuate ventilator-associated lung injury; and 3) the mechanisms of impaired compliance would be via alteration of surfactant biochemistry. DesignRandomized, prospective animal study. SettingResearch laboratory of university-affiliated hospital. SubjectsAnesthetized, male New Zealand Rabbits. InterventionsAll animals received the same ventilation strategy (tidal volume, 12 mL/kg; positive end-expiratory pressure, 0 cm H2O; rate, 42 breaths/min) and were randomized to receive Fico2 of 0.00, 0.05, or 0.12 to produce hypocapnia, normocapnia, and hypercapnia, respectively. Measurements and Main ResultsAlveolar-arterial oxygen gradient was significantly lower with therapeutic hypercapnia, and peak airway pressure was significantly higher with hypocapnic alkalosis. However, neither static lung compliance nor surfactant chemistry (total surfactant, aggregates, or composition) differed among the groups. ConclusionsAt clinically relevant tidal volume, CO2 modulates key physiologic indices of lung injury, including alveolar-arterial oxygen gradient and airway pressure, indicating a potential role in the pathogenesis of ventilator-associated lung injury. These effects are surfactant independent.

Pharmacotherapy for Prevention and Treatment of Acute Respiratory Distress Syndrome: Current and Experimental Approaches

The acute respiratory distress syndrome (ARDS) arises from direct and indirect injury to the lungs and results in a life-threatening form of respiratory failure in a heterogeneous, critically ill patient population. Critical care technologies used to support patients with ARDS, including strategies for mechanical ventilation, have resulted in improved outcomes in the last decade. However, there is still a need for effective pharmacotherapies to treat ARDS, as mortality rates remain high. To date, no single pharmacotherapy has proven effective in decreasing mortality in adult patients with ARDS, although exogenous surfactant replacement has been shown to reduce mortality in the paediatric population with ARDS from direct causes. Several promising therapies are currently being investigated in preclinical and clinical trials for treatment of ARDS in its acute and subacute, exudative phases. These include exogenous surfactant therapy, β2-adrenergic receptor agonists, antioxidants, immunomodulating agents and HMG-CoA reductase inhibitors (statins). Recent research has also focused on prevention of acute lung injury and acute respiratory distress in patients at risk. Drugs such as captopril, rosiglitazone and incyclinide (COL-3), a tetracycline derivative, have shown promising results in animal models, but have not yet been tested clinically. Further research is needed to discover therapies to treat ARDS in its late, fibroproliferative phase. Given the vast number of negative clinical trials to date, it is unlikely that a single pharmacotherapy will effectively treat all patients with ARDS from differing causes. Future randomized controlled trials should target specific, more homogeneous subgroups of patients for single or combination therapy.

Effects of inhaled nitric oxide in a rat model of Pseudomonas aeruginosa pneumonia.

Objective Antimicrobial effects of nitric oxide (NO) have been demonstrated in vitro against a variety of infectious pathogens, yet in vivo evidence of a potential therapeutic role for exogenous NO as an antimicrobial agent is limited. Thus, we assessed the effects of inhaled NO on pulmonary infection, leukocyte infiltration, and NO synthase (NOS) activity in a rat model of Pseudomonas aeruginosa pneumonia. Design Controlled animal study. Setting Research laboratory of an academic institution. Subjects Male Sprague-Dawley rats. Interventions After intratracheal instillation of either P. aeruginosa or saline (sham), rats were randomly exposed to either 40 ppm of inhaled NO or room air (RA) for 24 hrs before they were killed. Measurements and Main Results Inhaled NO in pneumonia rats markedly reduced pulmonary bacterial load (0.02 ± 0.01% vs. 0.99 ± 0.59% of bacterial input in pneumonia with room air, p < .05) and pulmonary myeloperoxidase activity, a marker of leukocyte infiltration (21.7 ± 3.8 vs. 55.0 ± 8.1 units in pneumonia with room air, p < .05), but had no effect on systemic hemodynamics or gas exchange. Pneumonia was associated with enhanced pulmonary NOS activity (8.8 ± 2.4 vs. 0.2 ± 0.1 pmol citrulline/min/mg protein in sham, p < .01) and increased plasma levels of nitrites/nitrates (NOx−; 45 ± 7 vs. 16 ± 3 &mgr;mol/L in sham, p < .01). Inhaled NO therapy attenuated the pneumonia-induced increase in pulmonary calcium-independent NOS activity (p < .05) and markedly increased plasma NOx− levels. Exposure of P. aeruginosa in culture to 40 ppm of ambient NO confirmed a delayed antibacterial effect of NO in vitro. Conclusions Inhaled NO has an important antibacterial effect both in vitro and in vivo against P. aeruginosa and is associated with reduced pulmonary leukocyte infiltration in vivo. These results in a rat model of P. aeruginosa pneumonia suggest that future studies should address the possible clinical effects of inhaled NO therapy in pneumonia.

Role of cholesterol in the biophysical dysfunction of surfactant in ventilator-induced lung injury

Mechanical ventilation may lead to an impairment of the endogenous surfactant system, which is one of the mechanisms by which this intervention contributes to the progression of acute lung injury. The most extensively studied mechanism of surfactant dysfunction is serum protein inhibition. However, recent studies indicate that hydrophobic components of surfactant may also contribute. It was hypothesized that elevated levels of cholesterol significantly contribute to surfactant dysfunction in ventilation-induced lung injury. Sprague-Dawley rats (n = 30) were randomized to either high-tidal volume or low-tidal volume ventilation and monitored for 2 h. Subsequently, the lungs were lavaged, surfactant was isolated, and the biophysical properties of this isolated surfactant were analyzed on a captive bubble surfactometer with and without the removal of cholesterol using methyl-beta-cyclodextrin. The results showed lower oxygenation values in the high-tidal volume group during the last 30 min of ventilation compared with the low-tidal volume group. Surfactant obtained from the high-tidal volume animals had a significant impairment in function compared with material from the low-tidal volume group. Removal of cholesterol from the high-tidal volume group improved the ability of the surfactant to reduce the surface tension to low values. Subsequent reconstitution of high-cholesterol values led to an impairment in surface activity. It is concluded that increased levels of cholesterol associated with endogenous surfactant represent a major contributor to the inhibition of surfactant function in ventilation-induced lung injury.

Emerging therapies for treatment of acute lung injury and acute respiratory distress syndrome

In 2007, Bosma et. al provided a comprehensive review of emerging therapies for the acute respiratory distress syndrome (ARDS), a condition which continues to carry a mortality rate of greater than 30%. Over the past several years, the development of novel and effective therapeutic agents for ARDS remains disappointing, and unfortunately, no recent therapeutic interventions have demonstrated a clear benefit. Herein, the results of several of these early and late phase clinical trials are reviewed, the majority of which address known maladaptive processes that have been deemed critical in ARDS pathophysiology. Based on the ongoing futility of current therapeutic models to yield effective therapies, it is speculated whether or not novel treatment paradigms, which address distinctly different aspects of this disease paradigm, may be warranted.

Mitigation of injury in canine lung grafts by exogenous surfactant therapy.

BACKGROUND Exogenous surfactant therapy of lung donors improves the preservation of normal canine grafts. The current study was designed to determine whether exogenous surfactant can mitigate the damage in lung grafts induced by mechanical ventilation before procurement. METHODS AND RESULTS Five donor dogs were subjected to 8 hours of mechanical ventilation (tidal volume 45 ml/kg). This produced a significant decrease in oxygen tension (p = 0.007) and significant increases in bronchoscopic lavage fluid neutrophil count (p = 0.05), protein concentration (p = 0.002), and the ratio of poorly functioning small surfactant aggregates to superiorly functioning large aggregates (p = 0.02). Five other animals given instilled bovine lipid extract surfactant and undergoing mechanical ventilation in the same manner demonstrated no significant change in oxygen tension values, lavage fluid protein concentration, or the ratio of small to large aggregates. All 10 lung grafts were then stored for 17 hours at 4 degrees C. Left lungs were transplanted and reperfused for 6 hours. After 6 hours of reperfusion the ratio of oxygen tension to inspired oxygen fraction was 307 +/- 63 mm Hg in lung grafts administered surfactant versus 73 +/- 14 mm Hg in untreated grafts (p = 0.007). Furthermore, peak inspired pressure was significantly (p < 0.05) lower in treated animals from 90 to 360 minutes of reperfusion. Analysis of lavage fluid of transplanted grafts after reperfusion revealed small to large aggregate ratios of 0.17 +/- 0.04 and 0.77 +/- 0.17 in treated versus untreated grafts, respectively (p = 0.009). CONCLUSIONS Instillation of surfactant before mechanical ventilation reduced protein leak, maintained a low surfactant small to large aggregate ratio, and prevented a decrease of oxygen tension in donor animals. After transplantation, surfactant-treated grafts had superior oxygen tension values and a higher proportion of superiorly functioning surfactant aggregate forms in the air space than untreated grafts. Exogenous surfactant therapy can protect lung grafts from ventilation-induced injury and may offer a promising means to expand the donor pool.