A synthetic segment of surfactant protein A: structure, in vitro surface activity, and in vivo efficacy

Mechanism of Anti-Inflammatory Activity of TLR4-Interacting SPA4 Peptide

The TLR4-interacting SPA4 peptide suppresses inflammation. We assessed the structural and physicochemical properties and binding of SPA4 peptide to TLR4-MD2. We also studied the changes at the whole transcriptome level, cell morphology, viability, secreted cytokines and chemokines, and cell influx in cell systems and mouse models challenged with LPS and treated with SPA4 peptide. Our results demonstrated that the SPA4 peptide did not alter the cell viability and size and only moderately affected the transcriptome of the cells. Computational docking and rendering suggested that the SPA4 peptide intercalates with LPS-induced TLR4-MD2 complex. Results with alanine mutations of D-2 amino acid and NYTXXXRG-12-19 motif of SPA4 peptide suggested their role in binding to TLR4 and in reducing the cytokine response against LPS stimulus. Furthermore, therapeutically administered SPA4 peptide significantly suppressed the secreted levels of cytokines and chemokines in cells and bronchoalveolar lavage fluids of LPS-challenged mice. The results suggest that the SPA4 peptide intercalates with LPS-induced TLR4 complex and signaling for the suppression of inflammation.

Delivery and performance of surfactant replacement therapies to treat pulmonary disorders

Lung surfactant is crucial for optimal pulmonary function throughout life. An absence or deficiency of surfactant can affect the surfactant pool leading to respiratory distress. Even if the coupling between surfactant dysfunction and the underlying disease is not always well understood, using exogenous surfactants as replacement is usually a standard therapeutic option in respiratory distress. Exogenous surfactants have been extensively studied in animal models and clinical trials. The present article provides an update on the evolution of surfactant therapy, types of surfactant treatment, and development of newer-generation surfactants. The differences in the performance between various surfactants are highlighted and advanced research that has been conducted so far in developing the optimal delivery of surfactant is discussed.

New Synthetic Surfactants: The Next Generation?

Surfactant preparations have been proven to improve clinical outcome of infants at risk for or having respiratory distress syndrome (RDS). In clinical trials, ani mal-derived surfactant preparations reduce the risk of pneumothorax and mortality when compared to non-protein-containing synthetic surfactant preparations. In part, this is thought to be due to the presence of surfactant proteins in animal-derived surfactant preparations. Four native surfactant proteins have been identified. The hydrophobic surfactant proteins B (SP-B) and C (SP-C) are tightly bound to phospholipids. These proteins have important roles in maintaining the surface tension-lowering properties of pulmonary surfactant. Surfactant protein A (SP-A) and D (SP-D) are extremely hydrophilic and are not retained in the preparation of any commercial animal-derived surfactant products. These proteins are thought to have a role in recycling surfactant and improving host defense. There is concern that animal-derived products may have some batch-to-batch variation regarding the levels of native pulmonary surfactant proteins. In addition, there is concern regarding the hypothetical risk of transmission of viral or unconventional infectious agents from an animal source. New surfactant preparations, composed of synthetic phospholipids and essential hydrophobic surfactant protein analogs, have been developed. These surfactant protein analogs have been produced by peptide synthesis and recombinant technology to provide a new class of synthetic surfactants that may be a suitable alternative to animal-derived surfactants. Preliminary clinical studies have shown that treatment with these novel surfactant preparations can ameliorate RDS and improve clinical outcome. Clinicians will need to further understand any differences in clinical effects between available products.

Surfactant therapy for respiratory distress syndrome in premature neonates: a comparative review

Exogenous surfactant therapy has been part of the routine care of preterm neonates with respiratory distress syndrome (RDS) since the beginning of the 1990s. Discoveries that led to its development as a therapeutic agent span the whole of the 20th century but it was not until 1980 that the first successful use of exogenous surfactant therapy in a human population was reported. Since then, randomized controlled studies demonstrated that surfactant therapy was not only well tolerated but that it significantly reduced both neonatal mortality and pulmonary air leaks; importantly, those surviving neonates were not at greater risk of subsequent neurological impairment. Surfactants may be of animal or synthetic origin. Both types of surfactants have been extensively studied in animal models and in clinical trials to determine the optimum timing, dose size and frequency, route and method of administration. The advantages of one type of surfactant over another are discussed in relation to biophysical properties, animal studies and results of randomized trials in neonatal populations. Animal-derived exogenous surfactants are the treatment of choice at the present time with relatively few adverse effects related largely to changes in oxygenation and heart rate during surfactant administration. The optimum dose of surfactant is usually 100 mg/kg. The use of surfactant with high frequency oscillation and continuous positive pressure modes of respiratory support presents different problems compared with its use with conventional ventilation. The different components of surfactant have important functions that influence its effectiveness both in the primary function of the reduction of surface tension and also in secondary, but nonetheless just as important, role of lung defense. With greater understanding of the individual surfactant components, particularly the surfactant-associated proteins, development of newer synthetic surfactants has been made possible. Despite being an effective therapy for RDS, surfactant has failed to have a significant impact on the incidence of chronic lung disease in survivors. Paradoxically the cost of care has increased as surviving neonates are more immature and consume a greater proportion of neonatal intensive care resources. Despite this, surfactant is considered a cost-effective therapy for RDS compared with other therapeutic interventions in premature infants.

Surface activity of a synthetic lung surfactant containing a phospholipase-resistant phosphonolipid analog of dipalmitoyl phosphatidylcholine

Surface activity and sensitivity to inhibition from phospholipase A2 (PLA2), lysophosphatidylcholine (LPC), and serum albumin were studied for a synthetic C16:0 diether phosphonolipid (DEPN-8) combined with 1.5% by weight of mixed hydrophobic surfactant proteins (SP)-B/C purified from calf lung surfactant extract (CLSE). Pure DEPN-8 had better adsorption and film respreading than the major lung surfactant phospholipid dipalmitoyl phosphatidylcholine and reached minimum surface tensions <1 mN/m under dynamic compression on the Wilhelmy balance and on a pulsating bubble surfactometer (37 degrees C, 20 cycles/min, 50% area compression). DEPN-8 + 1.5% SP-B/C exhibited even greater adsorption and had overall dynamic surface tension lowering equal to CLSE on the bubble. In addition, films of DEPN-8 + 1.5% SP-B/C on the Wilhelmy balance had better respreading than CLSE after seven (but not two) cycles of compression-expansion at 23 degrees C. DEPN-8 is structurally resistant to degradation by PLA2, and DEPN-8 + 1.5% SP-B/C maintained high adsorption and dynamic surface activity in the presence of this enzyme. Incubation of CLSE with PLA2 led to chemical degradation, generation of LPC, and reduced surface activity. DEPN-8 + 1.5% SP-B/C was also more resistant than CLSE to direct biophysical inhibition by LPC, and the two were similar in their sensitivity to biophysical inhibition by serum albumin. These findings indicate that synthetic surfactants containing DEPN-8 combined with surfactant proteins or related synthetic peptides have potential utility for treating surfactant dysfunction in inflammatory lung injury.

Component-specific surface and physiological activity in bovine-derived lung surfactants

Composition, surface activity and effects on pressure-volume (P-V) mechanics are examined for lavaged calf lung surfactant (LS) and the clinical exogenous surfactants Infasurf and Survanta. Lavaged LS and Infasurf had closely-matching compositions of phospholipids and neutral lipids. Survanta had higher levels of free fatty acids and triglycerides consistent with its content of added synthetic palmitic acid and tripalmitin. Infasurf and Survanta both contained less total protein than LS because of extraction with hydrophobic solvents, but the total protein content relative to phospholipid in Survanta was about 45% lower than in Infasurf. This difference was primarily due to surfactant protein (SP)-B, which was present by ELISA at a mean weight percent relative to phospholipid of 1.04% in LS, 0.90% in Infasurf, and 0.044% in Survanta. Studies on component fractions separated by gel permeation chromatography showed that SP-B was a major contributor to the adsorption, dynamic surface activity, and P-V mechanical effects of Infasurf, which approached whole LS in magnitude. Survanta had lower adsorption, higher minimum surface tension, and a smaller effect on surfactant-deficient P-V mechanics consistent with minimal contributions from SP-B. Addition of 0.05% by weight of purified bovine SP-B to Survanta did not improve surface or physiological activity, but added 0.7% SP-B improved adsorption, dynamic surface tension lowering, and P-V activity to levels similar to Infasurf. The SP-B content of lung surfactants appears to be a crucial factor in their surface activity and efficacy in improving surfactant-deficient pulmonary P-V mechanics.

Function and inhibition sensitivity of the N-terminal segment of surfactant protein B (SP-B1-25) in preterm rabbits

BACKGROUND

Surfactant protein B (SP-B) is an essential component of pulmonary surfactant, but shorter SP-B sequences may exert equivalent surface activity.

METHODS

We synthesised a peptide based on the amino-terminal domain of SP-B (SP-B1-25), a full length SP-B1-78, and a full length palmitoylated SP-C peptide (SP-C1-35) and compared the in vivo function and sensitivity to plasma inhibition of preparations consisting of mixtures of phospholipids with SP-B1-25 or SP-B1-78 and/or SP-C1-35 to Survanta. Preterm rabbits born at 27 days of gestation were treated at birth with surfactant and ventilated for 60 minutes. At 15 minutes half of them received plasma intratracheally. Dynamic compliance was monitored every 15 minutes and postmortem pressure-volume curves were measured to define lung mechanics.

RESULTS

Dynamic compliance and postmortem lung volumes were highest after treatment with a surfactant consisting of an SP-B peptide and SP-C1-35 or Survanta. Plasma instillation decreased dynamic compliance and lung volumes sharply, but the most effective activity was by prior instillation of surfactants containing SP-B1-25.

CONCLUSION

These experiments suggest that the N-terminal domain of SP-B (SP-B1-25) exhibits in vitro and in vivo surface activity and is relatively insensitive to plasma inhibition.

The present status of exogenous surfactant for the newborn

This year is the 20th anniversary of the first successful trial of exogenous surfactant for respiratory distress syndrome in the newborn and it is perhaps a good time to review recent advances in basic science and clinical practice as they relate to surfactant therapy.

Commercial versus native surfactants. Surface activity, molecular components, and the effect of calcium

Despite their broad clinical use, there is no standardized comparative study on the functional, biochemical, and morphologic differences of the various commercial surfactants in relation to native surfactant. We investigated these parameters in Alveofact, Curosurf, Exosurf, and Survanta, and compared them with native bovine (NBS) and porcine (NPS) surfactant. For Curosurf and Alveofact the concentrations necessary for minimal surface tensions < 5 mN/m were six to 12 times higher (1.5 and 3 mg/ml, respectively) than with NPS and NBS. Exosurf and Survanta only reached 22 and 8 mN/m, respectively. Increasing calcium to nonphysiologic concentrations artificially improved the function of Alveofact and Curosurf, but it had little effect on Exosurf and Survanta. Impaired surface activity of commercial versus native surfactants corresponded with their lack in surfactant protein SP-A and decreased SP-B/C. The higher surface activity of Curosurf compared with Alveofact corresponded with its higher concentration of dipalmitoylphosphatidylcholine (DPPC). Despite their enrichment in DPPC Survanta and Exosurf exhibited poor surface activity because of low or absent SP-B/C. Ultrastructurally, Curosurf and Alveofact consisted mainly of lamellar and vesicular structures, which were also present in NPS and NBS. Exosurf contained crystalline structures only, whereas the DPPC-enriched Survanta contained separate lamellar/vesicular and crystalline structures. We conclude that in vitro surface activity of commercial surfactants is impaired compared with native surfactants at physiologic calcium concentrations. In the presence of SP-B/C, surface activity corresponds to the concentration of DPPC. Our data underscore the importance of a standardized protocol at physiologic calcium concentrations for the in vitro assessment of commercial surfactants.

Addition of alpha1-antitrypsin to surfactant improves oxygenation in surfactant-deficient rats

During its life cycle, surfactant converts from highly surface active, large aggregates to less surface active, smaller aggregates. This process is probably regulated by a serine protease. We tested whether adding alpha1-antitrypsin (alpha1-AT), an antiprotease, to surfactant improves its in vivo function. alpha1-AT was added to Survanta, to a standard phospholipid (PL) mixture, and to a synthetic surfactant (BC mixture = PL mixture + synthetic surfactant proteins B and C) at a dose of 100 mg alpha1-AT per 75 mg PL. Adding alpha1-AT did not affect in vitro surface activity, except for that of the PL mixture. Adult rats were ventilated with 100% O2, at a tidal volume of 7.5 ml/kg and a ventilatory rate of 60 breaths/ min. The rats' lungs were lavaged with saline until the PaO2 dropped below 100 mm Hg, at which time 100 mg/kg of surfactant with or without alpha1-AT or alpha1-AT alone was instilled. After 1 h of ventilation the rats were killed, pressure-volume curves were generated, and the rats' lungs were relavaged. Surfactant treatment improved oxygenation in the order: BC mixture > Survanta > PL mixture. Addition of alpha1-AT equalized oxygenation in all three alpha1-AT groups, but decreased respiratory system compliance in the groups given Survanta and PL mixture. Particle sizing of the final lung lavages showed preservation of large surfactant aggregates after treatment with alpha1-AT. These data suggest that the addition of alpha1-AT to surfactant can exert a positive effect on oxygenation and surfactant metabolism in surfactant-deficient rats.

Sensitivity of synthetic surfactants to albumin inhibition in preterm rabbits

Surfactant can be inhibited in vivo by plasma proteins invading the alveolar space during acute lung injury. The resistance to protein inhibition of surfactant preparations with various synthetic surfactant proteins B and C (B and C) was tested in preterm rabbits. Surfactants consisted of a palmitic acid containing phospholipid mixture (PL) with full-length SP-B peptide (B1-78), one of two SP-B mutants (Bserine and BR236C), the synthetic SP-B mimic KL4 (UCLA-KL4), a natural SP-B (Bbovine), synthetic palmitoylated SP-C peptide (C1-35), a combination of B1-78 + C1-35, a combination of BR236C + C1-35, and the clinical surfactant Survanta. Preterm rabbits born at 28 days of gestation were ventilated and received 100 mg/kg of albumin intratracheally at 30 min and 100 mg/kg of surfactant at 45 min after birth. Dynamic lung compliance (tidal volume/mean airway pressure) decreased from 0.82 to 0.57 mL/kg/cm H2O after albumin instillation and to 0.43 mL/kg/cm H2O over a 60-min period after saline placebo. Treatment with B1-78 + C1-35 and BR236C + C1-35 surfactant and Survanta returned dynamic compliance to prealbumin values, B1-78, BR236C, Bbovine, and C1-35 surfactant stabilized dynamic compliance, but PL, Bserine, and UCLA-KL4 surfactant were unable to prevent a further deterioration in dynamic compliance. These data suggest that a combination of synthetic surfactant peptides B1-78 and C1-35 and the clinical surfactant Survanta confer a high degree of resistance to surfactant inhibition by human albumin in ventilated preterm rabbits.

Structural changes of surfactant protein A induced by cations reorient the protein on lipid bilayers

Surfactant protein A (SP-A) is an octadecameric hydrophilic glycoprotein and is the major protein component of pulmonary surfactant. This protein complex plays several roles in the body, such as regulation of surfactant secretion, recycling and adsorption of surfactant lipids, and non-serum-induced immune response. Many of SP-A's activities are dependent upon the presence of cations, especially calcium. Here, we have studied in vitro the effect of cations on the interaction of purified bovine SP-A with phospholipid vesicles made of dipalmitoylphosphatidylcholine and unsaturated phosphatidylcholine. We have found that SP-A octadecamers exist in an "opened-bouquet" conformation in the absence of cations and interact with lipid membranes via one or two globular headgroups. Calcium-induced structural changes in SP-A lead to the formation of a clearly identifiable stem in a "closed-bouquet" conformation. This change, in turn, seemingly results in all of SP-A's globular headgroups interacting with the lipid membrane surface and with the stem pointing away from the membrane surface. These results represent direct evidence that the headgroups of SP-A (comprising carbohydrate recognition domains), and not the stem (comprising the amino-terminus and collagen-like region), interact with lipid bilayers. Our data support models of tubular myelin in which the headgroups, not the tails, interact with the lipid walls of the lattice.

Collectins and pulmonary host defense

The surfactant-associated proteins SP-A and SP-D are members of a family of collagenous host defense lectins, designated collectins. There is increasing evidence that these pulmonary epithelial-derived proteins are important components of the innate immune response to microbial challenge, and that they participate in other aspects of immune and inflammatory regulation within the lung. The collectins bind to glycoconjugates and/or lipid moieties expressed by a wide variety of microorganisms and certain other organic particles in vitro. Although binding may facilitate microbial clearance through aggregation or other direct effects on the organism, SP-A and SP-D also have the capacity to modulate leukocyte function and, in some circumstances, to enhance their killing of microorganisms. The biologic activity of cell wall components, such as gram-negative bacterial polysaccharides, may be altered by interactions with collectins. Complementary or cooperative interactions between SP-A and SP-D could contribute to the efficiency of this defense system. Collectins may play particularly important roles in settings of inadequate or impaired specific immunity. Acquired or genetic alterations in the levels of active proteins within the airspaces and distal airways may increase susceptibility to infection.

Protein composition of synthetic surfactant affects gas exchange in surfactant-deficient rats

Synthetic surfactant peptides offer an opportunity to standardize the protein composition of surfactant. We tested the effect of phospholipids (PL) with synthetic full-length SP-B1-78 (B), mutant B (Bser), KL4 peptide (UCLA-KL4), and palmitoylated SP-C1-35 (C) on oxygenation and lung function in a surfactant-deficient rat model. Sixty-four adult rats were ventilated with 100% oxygen, a tidal volume of 7.5 mL/kg, and a rate of 60/min. Their lungs were lavaged with saline until the arterial PO2 dropped below 80 torr, when 100 mg/kg surfactant was instilled. Surfactant preparations included: PL (PL surfactant), PL + 3% B (B surfactant), PL + 3% B and 1% C (BC surfactant), PL + 3% UCLA-KL4 (KL4 surfactant), PL + 3% Bser (Bser surfactant), and PL + 3% B and 1% UCLA-KL4 (BKL4 surfactant). Sixty minutes after surfactant instillation, positive end-expiratory pressure was applied for 5 min, and pressure-volume curves were determined in situ. The six surfactant preparations had a minimum surface tensions <10 mN/m on a Langmuir/Wilhelmy balance. Instillation of PL, Bser, and BKL4 surfactant increased mean arterial/alveolar PO2 (aADO2) ratios by 50-100% over postlavage values, whereas KL4 surfactant increased aADO2 ratios by 118%, B surfactant by 191%, and BC surfactant by 225%. Lung volumes at 30 cm H2O pressure were highest after treatment with BC surfactant, intermediate after B and KL4 surfactants, and lowest after BKL4, Bser, and PL surfactants. These data suggest that a surfactant preparation with a combination of synthetic B and C peptides surpasses synthetic B and KL4 surfactants in improving oxygenation and lung compliance in surfactant-deficient rats.

Synthetic mimics of surfactant proteins B and C: in vitro surface activity and effects on lung compliance in two animal models of surfactant deficiency

Synthetic surfactant peptides SP-B1-78 and SP-C1-31 in a standard phospholipid mixture have been employed to examine the correlation between in vitro surface activity and in vivo function of synthetic surfactant preparations in the isolated rat lung and premature rabbit models of respiratory distress syndrome. Monolayer techniques showed that SP-B peptides have a high propensity for association with a phospholipid structure. By dynamic respreading, synthetic SP-B and SP-C showed rapid spreading and attained low surface tensions. Used as replacement surfactants in two animal models, these synthetic surfactant preparations partially restored lung compliance in lavaged rats and premature rabbits better than a pure phospholipid preparation and to a degree comparable to clinical surfactant, measured by pressure/volume curves. Our data confirm that in vitro functional determinations of synthetic surfactant peptides are instrumental in the preparation of replacement surfactants, and that dispersions thus selected represent viable therapeutic alternatives to current treatments for respiratory distress syndrome.

Inhibition of bacterial growth by synthetic SP-B1-78 peptides

Residues 12-34 of mature human pulmonary surfactant protein B (SP-B1-78) are 68% homologous to residues 48-72 of the frog peptide antibiotic dermaseptin b I. We examined the effects of SP-B1-78 on the growth of Escherichia coli in order to find whether full length SP-B1-78 might act as a peptide antibiotic. We found that SP-B1-78 peptide inhibited growth of E. coli (MIC = 210 micrograms.ml-1), but that the SP-B variant [R/K-->S]SP-B1-78 was less potent (MIC = 500 micrograms.ml-1).