Pulmonary surfactant

Pulmonary Surfactant: A Mighty Thin Film

Pulmonary surfactant is a critical component of lung function in healthy individuals. It functions in part by lowering surface tension in the alveoli, thereby allowing for breathing with minimal effort. The prevailing thinking is that low surface tension is attained by a compression-driven squeeze-out of unsaturated phospholipids during exhalation, forming a film enriched in saturated phospholipids that achieves surface tensions close to zero. A thorough review of past and recent literature suggests that the compression-driven squeeze-out mechanism may be erroneous. Here, we posit that a surfactant film enriched in saturated lipids is formed shortly after birth by an adsorption-driven sorting process and that its composition does not change during normal breathing. We provide biophysical evidence for the rapid formation of an enriched film at high surfactant concentrations, facilitated by adsorption structures containing hydrophobic surfactant proteins. We examine biophysical evidence for and against the compression-driven squeeze-out mechanism and propose a new model for surfactant function. The proposed model is tested against existing physiological and pathophysiological evidence in neonatal and adult lungs, leading to ideas for biophysical research, that should be addressed to establish the physiological relevance of this new perspective on the function of the mighty thin film that surfactant provides.

Correlation between serum surfactant protein-D level with respiratory compliance and acute respiratory distress syndrome in critically ill COVID-19 Patients: A retrospective observational study

Background

Acute respiratory distress syndrome (ARDS) is one of the manifestations of severe coronavirus disease 2019 (COVID-19) with low respiratory compliance and poor oxygenation as main characteristics and mortality rate of 50%-94%. Surfactants, including surfactant protein D (SP-D), have a role in maintaining respiratory compliance. This study aimed to analyze the relationship between serum SP-D levels with respiratory compliance and ARDS in patients with critically ill COVID-19 pneumonia.

Methods

This study was a cross-sectional study. Subjects were adult reverse transcription-polymerase chain reaction-confirmed COVID-19 patients who had ARDS treated with invasive mechanical ventilation. All data were obtained from medical records. Statistical analysis was done using Spearman test, Mann-Whitney test, and receiver operating characteristic curve.

Results

Serum level of SP-D was significantly correlated with static respiratory compliance (P = 0.009; correlation coefficient [rs] = 0.467). Serum SP-D levels correlated with ARDS severity (P < 0.001). SP-D levels had a very strong diagnostic value for ARDS severity, with an optimal cutoff value of 44.24 ng/mL (sensitivity 92.3%; specificity 94.1%). ARDS severity also correlated significantly with respiratory compliance (P = 0.005; correlation coefficient 0.496).

Conclusion

Higher serum SP-D levels were associated with lower respiratory compliance, ARDS severity, and may be utilized diagnostically to identify patients with severe ARDS.

Examining lung mechanical strains as influenced by breathing volumes and rates using experimental digital image correlation

BACKGROUND

Mechanical ventilation is often employed to facilitate breathing in patients suffering from respiratory illnesses and disabilities. Despite the benefits, there are risks associated with ventilator-induced lung injuries and death, driving investigations for alternative ventilation techniques to improve mechanical ventilation, such as multi-oscillatory and high-frequency ventilation; however, few studies have evaluated fundamental lung mechanical local deformations under variable loading.

METHODS

Porcine whole lung samples were analyzed using a novel application of digital image correlation interfaced with an electromechanical ventilation system to associate the local behavior to the global volume and pressure loading in response to various inflation volumes and breathing rates. Strains, anisotropy, tissue compliance, and the evolutionary response of the inflating lung were analyzed.

RESULTS

Experiments demonstrated a direct and near one-to-one linear relationship between applied lung volumes and resulting local mean strain, and a nonlinear relationship between lung pressures and strains. As the applied air delivery volume was doubled, the tissue surface mean strains approximately increased from 20 to 40%, and average maximum strains measured 70-110%. The tissue strain anisotropic ratio ranged from 0.81 to 0.86 and decreased with greater inflation volumes. Local tissue compliance during the inflation cycle, associating evolutionary strains in response to inflation pressures, was also quantified.

CONCLUSION

Ventilation frequencies were not found to influence the local stretch response. Strain measures significantly increased and the anisotropic ratio decreased between the smallest and greatest tidal volumes. Tissue compliance did not exhibit a unifying trend. The insights provided by the real-time continuous measures, and the kinetics to kinematics pulmonary linkage established by this study offers valuable characterizations for computational models and establishes a framework for future studies to compare healthy and diseased lung mechanics to further consider alternatives for effective ventilation strategies.

Reduced Prenatal Pulmonary Lymphatic Function Is Observed in Clp1 K/K Embryos With Impaired Motor Functions Including Fetal Breathing Movements in Preparation of the Developing Lung for Inflation at Birth

Embryonic lungs must be inflated immediately after birth to establish respiration. In addition to pulmonary surfactant, recently, we have revealed lymphatic function as a previously unknown regulator of prenatal lung compliance that prepares the embryonic lung for inflation at birth. It is well-documented that the late gestation embryo performs episodic breathing-like movements called as fetal breathing movements (FBMs), but the physiological importance of these events is not clear. Here we aimed to study the physiological role of FBMs in preparation for air inflation at birth. Clp1^K/K late gestation embryos develop a progressive loss of spinal motor neurons associated with axonal degeneration and denervation of neuromuscular junctions serving as an ideal genetic model to test the possible role of FBMs. We demonstrated that Clp1^K/K newborns show impaired motor function resulting in fatal respiratory failure after birth. Next, we showed that the alveolar septa are thicker, and the alveolar area is reduced in Clp1^K/K late gestation embryos, while the expression of molecular markers of lung development are not affected. Importantly, pulmonary lymphatic vessels are dilated and the prenatal pulmonary lymphatic function is reduced in Clp1^K/K late gestation embryos. Our results have revealed that Clp1^K/K mice show impaired motor functions including FBMs, and late gestation Clp1^K/K embryos display reduced prenatal lymphatic function and impaired lung expansion represented as thickened alveolar septa and reduced alveolar area in preparation of the developing lung for inflation at birth. These findings suggest a possible mechanism that FBMs, similarly to breathing movements after birth, stimulate prenatal lymphatic function in pulmonary collecting lymphatics lacking smooth muscle coverage to prepare the developing lung for inflation and gas exchange at birth. Moreover, these results raise the possibility that stimulating FBMs during late gestation might be an effective way to reduce the risk of the development of neonatal respiratory failure.

Transcriptional landscape of pulmonary lymphatic endothelial cells during fetal gestation

The genetic programs responsible for pulmonary lymphatic maturation prior to birth are not known. To address this gap in knowledge, we developed a novel cell sorting strategy to collect fetal pulmonary lymphatic endothelial cells (PLECs) for global transcriptional profiling. We identified PLECs based on their unique cell surface immunophenotype (CD31+/Vegfr3+/Lyve1+/Pdpn+) and isolated them from murine lungs during late gestation (E16.5, E17.5, E18.5). Gene expression profiling was performed using whole-genome microarrays, and 1,281 genes were significantly differentially expressed with respect to time (FDR q < 0.05) and grouped into six clusters. Two clusters containing a total of 493 genes strongly upregulated at E18.5 were significantly enriched in genes with functional annotations corresponding to innate immune response, positive regulation of angiogenesis, complement & coagulation cascade, ECM/cell-adhesion, and lipid metabolism. Gene Set Enrichment Analysis identified several pathways coordinately upregulated during late gestation, the strongest of which was the type-I IFN-α/β signaling pathway. Upregulation of canonical interferon target genes was confirmed by qRT-PCR and in situ hybridization in E18.5 PLECs. We also identified transcriptional events consistent with a prenatal PLEC maturation program. This PLEC-specific program included individual genes (Ch25h, Itpkc, Pcdhac2 and S1pr3) as well as a set of chemokines and genes containing an NF-κB binding site in their promoter. Overall, this work reveals transcriptional insights into the genes, signaling pathways and biological processes associated with pulmonary lymphatic maturation in the fetal lung.

Lymphatic function is required prenatally for lung inflation at birth

Neonatal mice lacking lymphatic vessels due to loss of lymphangiogenic factor CCBE1 or VEGFR3 function fail to inflate their lungs, suggestive of respiratory failure in infants with congenital pulmonary lymphangiectasia.

Mammals must inflate their lungs and breathe within minutes of birth to survive. A key regulator of neonatal lung inflation is pulmonary surfactant, a lipoprotein complex which increases lung compliance by reducing alveolar surface tension (Morgan, 1971). Whether other developmental processes also alter lung mechanics in preparation for birth is unknown. We identify prenatal lymphatic function as an unexpected requirement for neonatal lung inflation and respiration. Mice lacking lymphatic vessels, due either to loss of the lymphangiogenic factor CCBE1 or VEGFR3 function, appear cyanotic and die shortly after birth due to failure of lung inflation. Failure of lung inflation is not due to reduced surfactant levels or altered development of the lung but is associated with an elevated wet/dry ratio consistent with edema. Embryonic studies reveal active lymphatic function in the late gestation lung, and significantly reduced total lung compliance in late gestation embryos that lack lymphatics. These findings reveal that lymphatic vascular function plays a previously unrecognized mechanical role in the developing lung that prepares it for inflation at birth. They explain respiratory failure in infants with congenital pulmonary lymphangiectasia, and suggest that inadequate late gestation lymphatic function may also contribute to respiratory failure in premature infants.

Pressure-volume curves and compliance in acute lung injury: evidence of recruitment above the lower inflection point

Measuring elastic pressure-volume (Pel-V) curves of the respiratory system and the volume recruited by a positive end-expiratory pressure (PEEP) allows one to study the pressure range over which recruitment occurs in acute lung injury (ALI), and to explain how recruitment affects the compliance. Pel-V curves were measured with the low flow inflation technique in 11 patients mechanically ventilated for ALI. Curve I was recorded during inflation from the volume attained after a prolonged expiration (6 s) at PEEP (9.0 +/- 2.2 cm H2O), and Curve II after expiration to the elastic equilibrium volume at zero end-expiratory pressure (ZEEP). By using the end-expiratory volume of the breaths, the curves were aligned on a common volume axis to determine the effect of a single complete expiration. In each patient, Curve II (from ZEEP) was shifted toward lower volumes than Curve I. The volume shift, probably due to derecruitment, was 205 +/- 100 ml at 15 cm H2O (p < 0.01) and 78 +/- 93 ml at 30 cm H2O (p < 0.01); thus, during inflation from ZEEP, the volume deficit was successively regained over a pressure range up to at least 30 cm H2O. At any pressure, compliance was higher on the curve from ZEEP than from PEEP, by 10.0 +/- 8.7 ml/cm H2O at 15 cm H2O (p < 0.01), and by 5.4 +/- 5.5 at 30 cm H2O (p < 0.01). It is concluded that in ALI, a single expiration to ZEEP leads to lung collapse. High compliance during insufflation from ZEEP indicates that lung recruitment happens far above the lower inflection point of the Pel-V curve.

Studies on asphyxia: on the changes of the alveolar walls of rats in the hypoxic state. II. The hypoxic state produced by carbon dioxide and methane gases

Experimental studies were presented here concerning death by asphyxia due to the inhalation of carbon dioxide and methane gases. The morphological changes were almost the same as those demonstrated in our previous report. The authors concluded that the morphological changes in the lung tissue were not attributable to the chemical specificity of gases used in the experiment but to the decrease of oxygen.

Lung injury in acute pancreatitis: primary inhibition of pulmonary phospholipid synthesis

Alterations in the pulmonary surfactant system are partly responsible for the respiratory insufficiency seen with acute pancreatitis. In this model of cerulein-induced pancreatitis in rats, we utilized a new stable isotope metabolic tracer technique to examine one aspect of the pulmonary surfactant system and its relationship to associated lung injury. We have demonstrated primary, early depression of lung phospholipid synthesis reflected in both lung tissue and alveolar washings. We suggest that this quantitative change in pulmonary surfactant synthetic rate may partly explain the occurrence of respiratory failure with acute pancreatitis.

Use of the drop volume of amniotic fluid in estimating the risk for respiratory distress syndrome in the newborn infant

The present study describes the testing and function of the drop-volume method in the analysis of fetal lung maturity with use of amniotic fluid. Elevated surface tension resulting from a lack of surface-active phospholipids (surfactant) is the primary etiologic defect in the development of respiratory distress syndrome. The drop-volume method quantifies the surface tension of amniotic fluid with use of the fact that the volume of a falling drop of liquid is proportional to the quantity of surfactant in the solution. The drop-volume method requires only 2 minutes and 2 ml of amniotic fluid and predicts fetal lung maturity with an accuracy equal to or greater than that of other tests currently in use.

Membrane regulation of liver and lung microsomes under low oxygen tension

A highly monitorized animal model has been developed for the study of the influence of low oxygen tension on lipid composition, microviscosity and regulation of enzyme activities involved in the phospholipid synthesis of hepatic and pulmonary microsomes. Microviscosity decreased in liver microsomes whereas no difference was shown in that of microsomal membrane core of hypoxemic lung. Nevertheless, phospholipid and cholesterol content of both liver and lung membranes changed significantly. Microsomal membranes of hypoxemic liver increased the unsaturation degree of fatty acids, whereas hypoxemic lung membranes become more saturated, mainly due to the increase of palmitic acid. The adaptive response of lung was confirmed by the high increase of the deacylation-reacylation mechanism.

Rapid isolation of type II pneumocytes with magnetic removal of macrophages

A method is described for the rapid preparation of lung cell fractions enriched in type II alveolar pneumocytes. Isolated perfused rabbit lungs are exposed to Fe3O4 by tracheal lavage, which permits pulmonary alveolar macrophages to phagocytize the particles. Alveolar epithelial cells are then selectively freed from the basement membrane matrix by critical placement of collagenase and elastase. Detached cells are harvested either by repeated tracheal lavage or by mincing the lobes and filtering freed cells through a series of nylon mesh sieves. Iron oxide-containing macrophages are then removed from the harvested cells by a strong magnetic field. A final sizing of the macrophage-depleted suspension yields a preparation enriched in alveolar type II cells. Eight million viable cells (95% type II) were obtained per rabbit lung when harvested by lavage, while 32 x 10(6) (88% type II) cells were obtained from minced lungs. These values for cell yield and relative purity are comparable to previously described separation methods that depend upon differences in cell density or size. A major advantage of the magnetic separation procedure is the substantially shortened preparation time, typically 2 hr instead of 4. The viability (90-95%), oxygen consumption (88 nmol/10(6) cells/hr), and incorporation of [14C]acetate and [14C]choline (0.44 and 0.115 nmol/10(6) cells/hr, respectively) indicate that these cells will be suitable for pharmacologic and toxicologic investigation.

A new method of evaluating fetal lung maturity: the enzymatic lecithin determination in amniotic fluid

The concentration of phospholipids is well suited as indicator for the prognosis of a possibly postnatal respiratory distress syndrome. The method used most frequently up to now has been the determination of the lecithin/sphingomyelin ratio (L/S ratio) by thin layer chromatography. We have developed a specific assay for the quantitative determination of lecithin in amniotic fluid, which yields absolute concentration values and does not require the determination of a concentration ratio. Lecithin is hydrolized by phospholipase C and alkaline phosphatase. Choline is determined afterwards by a highly specific choline kinase from yeast. The total time required is less than 2 h. The usual lecithin concentration present in the 35th to 38th wk of gestation can be determined with a coefficient of variation of 2-3% (n=30). Fetal lung maturity can be expected at a lecithin concentration above 4.7-5.1 mg/100 ml. The method compares well with the L/S ratio. Detailed data about clinical significance will be presented. Good precision accuracy and simple handling make enzymatic lecithin determinations suitable for routine use.

The gram-negative bacillary pneumonias

With increasing age, chronic underlying disease, and debility, the oropharyngeal flora are colonized with aerobic gram-negative bacilli. In this debilitated population, gram-negative bacillary pneumonias (GNBP) are increasingly common. GNBP account for two of every three pneumonia deaths today. As a group, the mortality of GNBP is about 50%. Although the original epidemiologic surveys were done 15 years ago, there is little evidence for an improving case fatality rate despite the appearance of aminoglycoside antibiotics, carbenicillin, and cephalosporins. In susceptible patients, GNBP pneumonias occur both in the community and as nosocomial infections. Recognition of the dangers of contaminated reservoir nebulizers or other similar devices used in inhalation therapy has led to epidemiologic measures within hospitals that have markedly decreased the incidence of this nosocomial GNBP. The role of Gram stain and culture of expectorated sputum and similar examinations of specimens obtained by transtracheal aspiration, fiberoptic bronchoscopy, and lung biopsy in the diagnosis of GNBP are discussed in this review (see Criteria for Diagnosis). In the presence of pulmonary emphysema, congestive heart failure, mixed gram-negative bacillary infections, or the use of immunosuppressive drugs, typical characteristics of individual GNBP may not be apparent. Typical features of Pseudomonas aeruginosa, Escherichia coli, Enterobacter, Proteus, Hemophilus, and anaerobic pulmonary infections are described. Early recognition and institution of appropriate antibacterial agents are emphasized, particularly in GNBP caused by Pseudomonas aeruginosa, Escherichia coli, or Friedländer's bacillus, where the mortality approaches 70%. The mortality of GNBP, including Enterobacter, Proteus, Hemophilus, and anaerobic GNBP, is about 20%. The latter figure is the same as the mortality of pneumococcal pneumonia in similar patients.

The effect of smoke inhalation on pulmonary surfactant

This paper details efforts to define the primary pathophysiology of acute smoke inhalation without the variables of infection, burns, or fluid resuscitation. A standard dose of smoke (wood and kerosene) was delivered at 37 C to mongrel dogs. The parameters studied included blood gases, carboxyhemoglobin, pulmonary and systemic hemodynamics, respiratory mechanics, surface tension area curves as an indication of surfactant activity, and in vivo photomicroscopy. The FiO2 of the smoke was 17 volumes per cent; the carbon monoxide 17,000 ppm. Immediately following smoke exposure, dense, nonsegmental atelectasis developed. Hemodynamic changes were insignificant, but the PaO2 fell to 49 mmHg; the right to left shunt rose from 5 to 41%. Surfactant reduction was significant: enough to cause an increase in the minimum surface tension from 7 to 22 dynes/cm. This surfactant loss may explain the atelectasis seen and the marked instability of subpleural alveolar walls. The data collected are consistent and support the acute inactivation of surfactant as one of the primary pathophysiologic events in smoke inhalation. The clinical correlation is good; surfactant loss may explain why victims of smoke inhalation are so vulnerable to fluid administration if they have thermal burns as well effectiveness of medical devices.

Pulmonary responses to atmospheric pollutants. II. Effect of petrol vapour inhalation on secretion of pulmonary surfactant

Inhalation of air contaminated with petrol vapour has been shown to produce reduced surfactant levels in the lungs of rats. Pulmonary surfactant was obtained by endobronchial lavage followed by salt extraction and freeze drying to obtain the dry, hydrophobic product. During 45 days of continuous exposure, the lowest yield of surfactant was obtained after 15 days of treatment. During the following 30 days of treatment, the surfactant yield reached a relatively constant level, approximately half the mean value for control animals. Chromatographic analysis indicated no qualitative alteration in the phospholipid components of surfactant with increasing times of exposure to the irritant. It has been possible to correlate biochemical evidence of toxic lung injury with signs of cellular damage obtained from ultrastructural studies.

Pulmonary infection of mice with Staphylococcus aureus

The survival of Staphylococcus aureus in the lungs of mice was studied under various conditions. Doses of 10(7) to 10(9) washed staphylococci were quantitatively introduced into the lungs after intratracheal inoculation in mice under either ether or sodium pentobarbital anesthesia. Mice were sacrificed at intervals, the lungs were excised and homogenized, and the cocci were enumerated by plate count. The 50% lethal dose was 6 x 10(8) cocci per mouse, and mice died within 24 h but without proliferation of the inoculum. Mice given 10(8) cocci intratracheally under pentobarbital anesthesia regularly survived and eliminated the organisms over a 48-h period. The use of ether anesthesia resulted in persistence of the inoculum for up to 48 h, but the organisms were then eliminated. Inability to proliferate did not appear to result from a lack of iron because pretreatment of the mice with ferric ammonium citrate or Imferon did not alter inoculum survival. Staphylococci inoculated intratracheally in mice infected with influenza virus 3 to 21 days previously showed no enhanced persistence or multiplication. Cocci preclumped with fibrinogen, inocula mixed with 10 times the number of Formalin-killed staphylococci, or inocula of the encapsulated Smith strain did not survive any better than conventional inocula, suggesting that phagocytosis might not be the sole mechanism for elimination. However, a sedimentable fraction from normal or infected lung homogenates proved either inhibitory or cidal for staphylococci in vitro.

Effect of tidal volume and positive end-expiratory pressure on compliance during mechanical ventilation

In 12 patients requiring therapy with mechanical ventilation for acute respiratory failure, total static compliance (Cst) increased from 29 +/- 4 ml/cm H2O at a tidal volume (TV) of 5 ml/kg to 42 +/- 7 ml/cm H2O at a TV of 15 ml/kg. Similarly, Cst increased from 42 +/- 7 ml/cm H2O to 52 +/- 8 ml/cm H2O between 0 and 6 cm H2O of positive end-expiratory pressure (PEEP). At high levels of pulmonary inflation (ie, high PEEP and large TV) compliance decreased. The changes of total respiratory compliance with TV were mainly due to changes in pulmonary compliance. With PEEP, the functional residual capacity increased, and specific compliance did not change. Two mechanisms may be responsible for the changes in compliance. First, varying TV or PEEP will alter the position of tidal ventilation on the pressure-volume curve, resulting in an increase in compliance with increasing TV and PEEP up to a point, where overdistention occurs and compliance decreases. Secondly, the function of the surface-lowering substance may be altered in acute pulmonary parenchymal disease, thus disturbing the regulation of surface tension over the range of pulmonary inflation studied.

Conversion of lamellar body membranes into tubular myelin in alveoli of fetal rat lungs

Fluid-filled lumina of fetal rat lungs contain lamellar bodies (LBs) as well as tubular myelin (TM), both of which are thought to be stores of phospholipid-rich pulmonary surfactant. The alveolar epithelium is believed to secrete LBs, but neither the origin nor the mechanism of TM formation is entirely certain. The main objective of this study was to determine the relationship between secreted LBs and TM and to define membrane phenomena which occur during TM formation. I examined lung tissues of 20-21 day-old fetuses (day 22 = term) using transmission and high voltage transmission electron microscopy and cytochemistry. My findings indicate that secreted LBs, identified by the presence of an acid-phosphatase reactive core, are the precursor of TM. Secreted LBs are highly organized structures which contain structurally specialized areas, one of which is a "mini-lattice" structure similar to TM. During TM formation, fuzzes or 8.0-nm diameter particles appear on transition membranes, although LB membranes appear to lack both structures. Similar particles are present on TM membranes and are generally associated with membrane intersections. My results provide evidence that TM is formed from LBs within the alveolar lumen by mechanisms which may be complex.

A continuous tumor-cell line from a human lung carcinoma with properties of type II alveolar epithelial cells

The A549 tumor-cell line, initiated from a human alveolar cell carcinoma, has been continuously propagated in vitro for more than 3 years (more than 1,000 cell generations). These cells have a human karyotype and appear to have been derived from a single parent cell. All A549 cells examined by electron microscopy at both early and late passage levels contain multilamellar cytoplasmic inclusion bodies typical of those found in type II alveolar epithelial cells of the lung. At early and late passage levels, the cells synthesize lecithin with a high percentage of disaturated fatty acids utilizing the cytidine diphosphocholine pathway; such a pattern of phospholipid synthesis is expected for cells believed to be responsible for pulmonary surfactant synthesis. The A549 cell line should permit in vitro analysis of human surfactant synthesis and secretion and possibly provide a source of human surfactant for therapeutic intervention in pulmonary disease states characterized by surfactant deficiency.

Stimualtion of glycerolphosphate phosphatidyltransferase activity in fetal rabbit lung by cortisol administration

Phosphatidylglycerol is an important component of pulmonary surfactant. Previous studies have shown that direct administration of corticosteroids of thyroxine to the fetus during the latter part of gestation results in accelerated lung maturation with increased surfactant production. We have shown that administration of cortisol to fetal rabbits at 24 days' gestation results 3 days later in a significant increase in the activity of pulmonary glycerolphosphate phosphatidyltransferase, an enzyme involved in the synthesis of phosphatidylglycerol. The activity of the liver enzyme was not affected. Choline phosphotransferase, CDPdiglyceride-inositol phosphatidyltransferase, lysophosphatidic acid acyltransferase and lysolecithin acyltransferase activities were not altered significantly by cortisol treatment. Thyroxine treatment had no effect on any of the enzymes of phospholipid or fatty acid biosynthesis studied.

The destruction of type 2 pneumocytes by airborne influenza PR8-A virus; its effect on surfactant and lecithin content of the pneumonic lesions of mice

Influenzal pneumonia has been studied in mice subjected to sublethal doses of airborne PR8-A influenza virus. Electron microscopy revealed that the virus propagated in and at the same time destroyed the ciliated and nonciliated bronchial cells and the types 1 and 2 alveolar pneumocytes. The regenerating bronchial membranes were metaplastic and grew peripherally into the surrounding alveolar ducts and alveoli to form epithelial nodules which caused obstruction and collapse of the involved lobes. The development of the lung lesions was correlated with phospholipid (lecithin) levels in consolidated and unconsolidated infected and normal lungs. As the lungs became more and more consolidated, there was a corresponding and significant decrease in the amount of phospholipid (dipalmitoyl lecithin) compared to the amount of normal or unconsolidated infected tissue. The destruction of the type 2 pneumocytes by the influenza virus and their failure to regenerate is considered to be the reason for the low phospholipid levels in the involved lobes, and thus an important cause of post-influenzal collapse in mice. The above adds additional evidence to the view that the type 2 pneumocytes are a major source of surfactant in mammalian lungs.