SURFACTANT AND ITS ROLE IN PULMONARY DISEASE
Professor Ron Slocombe, School of Veterinary Science.
University of Melbourne, Victoria, Australia.
Abstract: Over 40 years ago it was recognized that the qualities of lung epithelial
lining fluid in reducing surface tension were essential for life. A recent word
search using “PubMed” revealed over 8 thousand citations for pulmonary
surfactant. Early research was directed toward the phospholipid components of
surfactant and its effects on surface tension. In the last decade, an increasingly
diverse number of important biologic processes involving surfactant components
have been identified. Surfactant lipids and proteins are now known to have roles
in modulating fluid flux across air-tissue interfaces, in the recognition of
pathogens and in either inducing or suppressing inflammation. An extensive
series of reviews on pulmonary surfactant was published in November, 1998 and
much of the emerging information about the surfactant proteins is published
here. (see Biochimica et Biophysica Acta (1998), 1408:1-289.) In addition, the
isolation and elucidation of roles for surfactant proteins in health and disease
offers new hope for serologic testing to diagnose interstitial lung disease, as the
appearance of these proteins in increasing amounts in the blood correlates with
interstitial lung injury. Currently the application of these methods to veterinary
medicine is limited by the lack of suitable reagents, and most current data on
animals is restricted to experimental laboratory animals, reptiles and horses.
The measurement and use of surfactant components in the diagnosis and
therapy of lung disease presents exciting new possibilities within the next
decade.
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The nature and function of surfactant lipids.
Pulmonary epithelial lining fluid (ELF), surfactant, in mammals has a relatively
constant composition of about 90% lipid and 10% protein. Biophysical properties
relating to surface tension reduction reside principally with the phospholipids in
surfactant, and of these dipalmitoylphosphatidylcholine (DPPC) is the most
important, constituting about 50% of the total phospholipid and responsible for
most of the surface-tension reducing capacity. Other phospholipids are present
in smaller amounts and in some species there are significant alterations in minor
phospholipids with maturation from the fetal state to the adult.
Neutral lipids, including cholesterol and esterified cholesterol, and fatty acids
are present in surfactant in low amounts, but have no known function. It has
been postulated that these and the minor phospholipids might enhance the
reconstitution of DPPC if displaced from surface films. The study of surfactant
lipids has been hampered by difficulties in accurately and easily determining
individual lipid components. In addition, for an adequate understanding of the
role of surfactant lipids in vivo, integrated studies that analyze biochemical
composition and relate these to biophysical properties are needed. To date, this
has not been done in animals.
Production, use and degredation of surfactant lipids.
Surfactant lipids are produced by alveolar type II pneumocytes and to a lesser
extent by bronchiolar non-ciliated epithelial cells. These lipids are synthesized
into preformed intracytoplasmic lamellar bodies, secreted into the aqueous
subphase of ELF, where they unravel to form tubular myelin figures, and from
tubular myelin are exchanged with surface film phospholipids. The movement of
DPPC from lamellar bodies to surface active film is too rapid for simple diffusion.
Neutral lipids and hydrophobic surfactant proteins are though to facilitate this
process. This is essential because cyclic stretching and compression of ELF
during rhythmic breathing causes displacement of surface lipids back into the
subphase, particularly during lung collapse, and without rapid restitution of the
surface-active molecules, successive cycles of lung deflation would inexorably
lead to further lung collapse. The surfactant lipids, dispersed in the subphase,
may also be degraded by both alveolar macrophages and pneumocytes, due to
the action of the serine protease, surfactant convertase. In lavage fluids, dense
aggregates of surfactant are thought to represent tubular myelin and light
unilamellar vesicles degraded, inactive surfactant. Surfactant may be lost by
airway removal, further degraded or resynthesised into lamellar bodies along
with surfactant proteins. This intimate relationship between recycled
components of ELF, macrophages and epithelial cells provides pathways in
addition to those involved in surface tension for antigen processing and
recognition.
Manipulation of surfactant lipids.
Immature fetal lungs and those severely damaged by acute interstitial lung
disease may develop oedema and collapse because of surfactant dysfunction.
The influence of corticosteroids in promoting maturation of type II pneumocytes
in preterm lungs is well known. However, exogenous administration of surfactant
lipids in cases of lung disease is not widely practiced in veterinary medicine.
Purified DPPC has marginal effectiveness as a surface tension reductant, and
although significant improvements in activity are produced by the addition of
hydrophobic surfactant proteins, these proteins differ sufficiently between
species and therefore exogenous complex surfactants designed for human use
may not be as effective in animals. Several studies have investigated whether
dietary manipulations can alter surfactant lipid composition. Manipulation of
dietary lipids or vitamin A in general has minimal effect on surfactant
composition, and presumably has no functional effect on biophysical properties.
Alteration of the lipid composition of surfactant with changing temperature
occurs in reptiles but the function of such changes remains unclear.
Surfactant lipids, inflammation and infectious agents.
Inactive or inadequate surfactant lipids lead to lung collapse and predisposes to
pulmonary edema. Surfactant is not only subject to degradation by surfactant
convertase, but oxygen radicals, phospholipases, leukocyte elastases and other
proteolysins generated during inflammation can reduce its activity. Inhibition of
proteolysis by addition of 1 antitrypsin prolongs surfactant function and
improves oxygenation in surfactant deficient rats. Inflammation associated with
allergic lung disease has also been shown to increase ELF protein and inversely
correlate with surfactant activity, leading to increased airway obstruction.
Surfactant dysfunction often occurs following lung transplantation, ARDS and in
ischemia-reperfusion injury (IRI). Previously, it has been assumed that
surfactant was inactivated by proteolysins in edema fluid that developed as a
consequence of inflammation, but recently experimental IRI studies found that
surfactant dysfunction preceded protein leak and edema formation. Surfactant
dysfunction occurs with septic pneumonia, and replacement improves lung
function and speeds clearance of organisms. Some of these effects may be
indirect, since better reduction in surface tension in pneumonic lung lessens
edema, improves oxygenation, and facilitates mucociliary clearance. Surfactant
has additional beneficial effects, suppressing the growth of bacteria directly,
serving as an opsonin for phagocytosis and affecting the generation of nuclear
factor B, critical in the regulation of many cytokines. Some of these functions
may be more closely related to actions of surfactant proteins rather than lipid
components. Surfactant dysfunction also occurs in horses that develop EIPH,
and in horses following transport stress, when it is associated with sepsis.
The hydrophilic Surfactant Proteins.
There are 2 proteins in this group, SP-A and SP-D. Both belong to the collectin
family of proteins, responsible in part for innate immunity and mucosal defences.
These proteins may be especially important in preventing infections in neonates
prior to the development of aquired immunity. SP-A is secreted independently of
lamellar bodies, is also secreted by Clara cells and to a lesser extent by
conducting airway epithelium. It appears to be under feedback control and
influences the amount of total surfactant present. SP-A is the most abundant
surfactant protein and is required for surfactant to effectively lower surface
tension. Excess SP-A in the subphase hastens lipid degradation and recycling
and suppresses further lipid secretion by Type II cells. Receptors for SP-A and
SP-D are present on both type II cells and alveolar macrophages and SP-A
enhances the chemotaxis, phagocytosis and killing of micro-organisms in the
lungs. There are conflicting reports of the influence of SP-A, -D on cytokine
production, the range of inflammatory cells that carry receptors for it and whether
inflammatory cells exposed to SP-A, -D release oxygen radicals and enzymes.
SP-A may also assist in protecting surfactant from inactivation by plasma
proteins during inflammation. SP-A and SP-D bind carbohydrates and lipids and
this binding has been associated with antimicrobial functions, including a range
of viruses, bacteria, endotoxin, fungi and Pneumocystis. SP-A deficient animals
are susceptible to lung infections but do not develop respiratory failure through
surface tension abnormalities. Both SP-A and SP-D bind pollen and mite
antigens, influence lymphocyte function and therefore may modulate allergic
reactions in the lungs. Serum levels of these proteins rise with alveolar injury,
infection and SP-A also in association with lung carcinoma. Equine SP-D has
recently been isolated.
The hydrophobic Surfactant Proteins.
There are 2 proteins in this group, SP-B belonging to the saposin family of
proteins, and SP-C. These proteins are thought to have similar functions and
are critical for the effective secretion, maintenance of tubular myelin and surface
films, and recycling of the surface-active lipid components of surfactant.
Inherited deficiencies in SP-B are associated with fatal neonatal respiratory
failure. Incorporation of synthetic SP-B into artificial surfactant preparations
dramatically improves efficacy.
Reviews
1. Pulmonary surfactant. (1998) Biochimica et Biophysica Acta 1408:1-289.
2. Doyle, IR., Nicholas, TE., and Bersten, AD. (1999) Partitioning lung and plasma
proteins: Circulating surfactant proteins as biomarkers of alveolocapillary
permeability. Clin Exp Pharmacol Physiol. 26: 185-197.
3. Van Iwaarden, JF. (1993) Surfactant and the pulmonary defense system. In B
Robertson, LMG Golde and JJ Batenburg, editors. Pulmonary surfactant: from
molecular biology to clinical practice. Elsevier, Amsterdam, 215-225.
Recent articles on surfactant in animals
1. Belai, Y. et al (1999) Addition of 1-antitrypsin to surfacatant improves oxygenation
in surfactant deficient rats. Am J Crit Care Med 159: 917-923.
2. Gunther, A et al (1999) Surfactant subtype conversion is related to loss of
surfacatant Apoprotein B and surface activity in large surfacatant aggregates:
experimental and clinical studies. Am J Crit Care Med 159: 244-251.
3. Harrod, KS. et al (1999) SP-A enhances viral clearance and inhibits inflammation
after pulmonary adenoviral infection. AJP 277: L580-L588.
4. Herting, E. et al (1999) Combined treatment with surfactant and specific
immunoglobulin reduces bacterial proliferation in experimental neonatal group B
streptococcal pneumonia. Am J Crit Care Med 159: 1862-1867.
5. Hickman-Davis, J. et al (1999) Surfactant protein A mediated mycolplasmacidal
activity of alveolar macrophages by peroxynitrite. Proc Nat Acad Sci 96: 4953-4958.
6. Hobo S. et al (1997) Effect of transportation on the composition of broncho-alveolar
lavage fluid obtained from horses. Am J Vet Res 58: 531-534.
7. Hobo S. et al (1999) Purification and biochemical characterization of equine
pulmonary surfactant protein D. Am J Vet Res 60: 368-372.
8. Morrison KE. et al (1999) Functional and compositional changes in pulmonary
surfactant in response to exercise. Equine Vet J Suppl 30: 62-66.
9. Ochs M. et al (1999) Ultrastructural alterations in intra-alveolar surfacatant subtypes
after experimental ischemia and reperfusion. Am J Crit Care Med 160: 718-724.
10. Orgeig S. et al (1997) Surfactant regulates pulmonary fluid balance in reptiles. Am
J Physiol 273/42: R2013-R2021.
11. Savov, J. et al (1999) Mechanical ventilation of rat lung: effect of surfactant forms.
AJP 277: L320-L326.