Inhibition of factor XIIIa-mediated incorporation of
fibronectin into fibrin by pulmonary surfactant
Andreas
Elssner,
Gertraud
Mazur, and
Claus
Vogelmeier
Division for Pulmonary Diseases, Department of Internal Medicine I,
Klinikum Grosshadern, Ludwig-Maximilians-University of Munich, 81377 Munich, Germany
 |
ABSTRACT |
Intra-alveolar
deposition of exudated plasma proteins is a hallmark of acute and
chronic inflammatory lung diseases. In particular, fibrin and
fibronectin may provide a primary matrix for fibrotic lung remodeling
in the alveolar compartment. The present study was undertaken to
explore the effect of two surfactant preparations on the incorporation
of fibronectin into fibrin. We observed that surfactant phospholipids
are associated with insoluble fibrin, factor XIIIa-cross-linked fibrin,
and cross-linked fibrin with incorporated fibronectin. Factor
XIIIa-mediated binding of fibronectin to fibrin was noticeably altered
in the presence of surfactant. Coincubation with two different
commercially available surfactants but not with
dipalmitoylphosphatidylcholine alone resulted in a reduction of
fibronectin incorporation into fibrin clots by approximately one-third.
This effect was not dependent on the calcium concentration. We conclude
that 1) factor XIIIa-cross-linked fibrin-fibronectin is able to incorporate surfactant phospholipids in
amounts comparable to fibrin clots without fibronectin and 2) the binding of fibronectin to
fibrin is partially inhibited in the presence of pulmonary surfactant.
plasma proteins; surfactant phospholipids; adult respiratory
distress syndrome; fibrosis
 |
INTRODUCTION |
LEAKAGE OF PLASMA PROTEINS into alveoli and formation
of hyaline membranes are commonly found in adult respiratory distress syndrome (ARDS) and chronic inflammatory lung diseases
such as idiopathic pulmonary fibrosis (3, 7, 20). In particular, fibrin
and fibronectin may provide a primary matrix for fibrotic lung
remodeling (1, 2, 6, 8, 23, 41). The intra-alveolar exudate leads to a
dysfunction of the surfactant system. It is therefore assumed that
under such circumstances regulation of alveolar surface tension is
disturbed (for a review see Ref. 35). Among the proteins capable of
inhibiting surface tension-lowering properties of surfactant,
fibrinogen and fibrin monomer surpass the potency of, e.g., albumin and
hemoglobin (36). The predominant inhibitory mechanism may be the
incorporation of lipophilic surfactant components into growing
fibrin clots (34). The likelihood of intra-alveolar clot formation in
ARDS is enhanced by a shift in the hemostatic balance toward a
predominance of coagulation (10, 19, 29). An increased expression of
procoagulative factors such as tissue factor and factor VII may be
responsible for the extrinsic activation pathway of coagulation within
the alveolus. Tissue factor and factor VII may be produced by alveolar
epithelial cells (14) as well as by activated macrophages (10, 29). In
addition, reduced fibrinolytic activity via an urokinase-type activator
and elevated levels of plasminogen activator inhibitor-1 and
2-antiplasmin were found in
inflammatory lung diseases (5, 9, 19). Whereas leakage from capillaries
appears to be the predominant source of intra-alveolar plasma proteins,
fibronectin may also be locally secreted by alveolar macrophages (32,
33). Usually, covalent cross-linking of fibrin and the incorporation of
0.7% (percentage of clot mass)
2-antiplasmin and 4.4%
fibronectin is mediated by factor XIIIa (11, 27). The fibronectin
binding site of fibrin mediated by factor XIIIa is located in the
carboxy-terminal region of the
-chain (26). The corresponding
binding region on fibronectin is located in the amino terminus of the
molecule (18). Because the fibrin split product
D-dimer has been found in
bronchoalveolar lavage fluid obtained from humans with acute lung
injury (12), covalent cross-linking of fibrin may also take place in
the alveolus. This hypothesis is supported by the occurrence of
cross-linked fibrin in the terminal airways of rats with
bleomycin-induced lung injury (31). With these findings as a
background, the present study was conducted to evaluate the interaction
of fibrin-fibronectin with pulmonary surfactant. We studied the
influence of two commercially available surfactants, a bovine lung
extract (Alveofact) and a porcine lung extract (Curosurf), on the
incorporation of fibronectin into fibrin.
 |
METHODS |
Materials. Alveofact was kindly
provided by Thomae (Biberach, Germany). Curosurf was a gift from Serono
Pharma (Unterschleissheim, Germany). Purified bovine fibrinogen
(plasminogen and factor XIII free, >95% clotting
ability) was generously donated by Dr. H. Keuper
(Behringwerke, Marburg, Germany). Human thrombin (specific activity 120 U/mg) and human fibronectin (pure; Boehringer Mannheim, Mannheim,
Germany), human factor XIII (Fibrogammin, Centeon Pharma, Marburg,
Germany), dipalmitoylphosphatidylcholine (DPPC; Sigma, Munich,
Germany), 125I-labeled fibrinogen
(200 µCi/mg),
[14C]DPPC (154 µCi/mg; both from Amersham Buchler, Braunschweig, Germany),
125I-fibronectin (220 µCi/mg),
and Coomassie blue colloidal stain (both from ICN Biomedicals,
Eschwege, Germany) were commercially available. Microcentrifuge tubes
with 10-µm polypropylene mesh filters were obtained from Whatman
(Maidstone, UK).
Preparation of reaction mixtures. For
examination of phospholipid incorporation into fibrin, surfactant and
plasma proteins (fibrinogen and fibronectin) were mixed and diluted
with buffer (20 mM Tris, 0.9% NaCl, and 3 mM
CaCl2) to a final sample volume of 0.5 ml. All samples contained Alveofact at a constant concentration of 2 mg/ml of phospholipids. Fibrinogen was added at different concentrations ranging from 0.1 to 2 mg/ml in either the presence or
absence of 1 U/ml of factor XIII. Alternatively, in addition to factor
XIII, 0.2 mg/ml of fibronectin was admixed. In addition, a
fibrinogen-free preparation was made consisting of 2 mg/ml of surfactant, fibronectin in varying concentrations from 0.05 to 1 mg/ml,
and 1 U/ml of factor XIII. In all samples except controls, coagulation
was started by the addition of 0.5 U/ml of thrombin. The reaction
temperature was 37°C, and the incubation time was 3 h. Before
initiation of coagulation, the mixtures were vortexed for 1 min
followed by a 10-min period of gentle shaking at 37°C.
To investigate the influence of surfactant on the incorporation of
fibronectin into fibrin, 2 mg/ml of fibrinogen and 0.2 mg/ml of
fibronectin (diluted with buffer as described) were coagulated by
adding 1 U/ml of factor XIII and 0.5 U/ml of thrombin in the presence
and absence of surfactant. Before admixture, the thrombin samples were
vortexed and preincubated as described above. Surfactant stock
solutions (Alveofact and Curosurf) were admixed to a final concentration of 5 mg/ml of phospholipids. The reaction time ranged from 5 to 60 min. In addition, dose-effect characteristics were studied
by varying the surfactant concentrations from 0.1 to 5 mg/ml at an
incubation time of 1 h. In this experiment, the effect of the two
surfactants was compared with that of DPPC (0.1-5 mg/ml). In
another experiment designed to study the effects of varying calcium
concentrations, 2 mg/ml of fibrinogen, 0.2 mg/ml of fibronectin, and 5 mg/ml of Curosurf were incubated with factor XIII and thrombin for 3 h
as described while CaCl2 was added
at different concentrations ranging from 0 to 10 mM.
Determination of phospholipid, fibrinogen, and
fibronectin content in the clotted material. Either
[14C]DPPC (10 nCi/mg
surfactant phospholipids),
125I-fibrinogen, or
125I-fibronectin (30 nCi/ml each)
was added to the reaction mixtures. After the incubation period, the
mixtures were spun at 300 g through a
10-µm polypropylene mesh filter to separate the clot from the clot
liquor. 14C and
125I counts were identified in the
filtrate and related to the radioactivity assessed before the start of
coagulation in each experiment. The cross-linking reaction was not
affected by the radioactivity because pilot studies all showed similar
results with different amounts of labeled and unlabeled proteins as
well as of phospholipids. In other control experiments, similar results
were observed by calculating the content of phospholipids and protein
based on the measurement of organic phosphorus and photometric
determination of absorbance.
SDS-PAGE. For gel electrophoresis,
coagulation was stopped by adding an equal volume of SDS-PAGE sample
buffer containing 4% SDS, 20 mM EDTA, and 4% 2-mercaptoethanol in 8 M
urea and 0.02% bromphenol blue. After complete dissolution of the
clot, 10 µl of the mixture were subjected to 7.5% polyacrylamide
separation gels. The running buffer was 0.025 M Tris and 0.192 M
glycine, pH 8.3, with 0.1% SDS. The gels were fixed with 40%
methanol, 10% acetic acid, and 50% deionized water. Staining was
performed with Coomassie blue colloidal stain and destaining with 10%
acetic acid and 90% deionized water.
Control of enzymatic reactions.
Thrombin was dissolved to a 10 U/ml stock solution and divided into
aliquots. Thrombin activity was monitored by the use of a cromogenic
substrate-based commercial assay. For all other proteins (fibrinogen,
fibronectin, and factor XIII), solutions of the lyophilized materials
were prepared from the same batches on the day of the experiment.
 |
RESULTS |
Incorporation of phospholipids into polymerizing
fibrin. In these experiments, >95% of the fibrinogen
had to be clotted to insoluble fibrin after the incubation time of 3 h.
Fibrin was therefore retained on the filter, and a quasi
fibrinogen-free clot liquor was released into the filtrate. As
anticipated, when 2 mg/ml of
[14C]DPPC-enriched
surfactant (Alveofact) were added, the Alveofact was incorporated into
the growing fibrin clot in a dose-dependent manner. Fibrin at a dose of
0.5 mg/ml provoked an
50% loss in [14C]DPPC from the
soluble phase, whereas the polymerization of 2 mg/ml of fibrin resulted
in >90% depletion of DPPC from the clot liquor (Fig.
1A).
Adding factor XIII to produce covalently cross-linked fibrin caused a
dose-dependent effect that paralleled that of noncovalently
cross-linked fibrin (Fig. 1B). No
significant change was observed when fibronectin was admixed (Fig.
1C). In contrast, no retention of
DPPC on the filter and full recovery in the soluble filtrate were
achieved by incubating 2 mg/ml of Alveofact, fibronectin in varying
concentrations from 0.05 to 1 mg/ml, and 1 U/ml of factor XIII in the
absence of fibrin(ogen) (Fig. 1D).

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Fig. 1.
Dose-dependency of dipalmitoylphosphatidylcholine (DPPC;
14C labeled) association with
growing fibrin clots. Alveofact (2 mg/ml) was mixed with
[14C]DPPC and
increasing concentrations of fibrinogen. Complete conversion of
fibrinogen to fibrin was achieved after incubation of mixture with 0.5 U/ml of thrombin for 3 h at 37°C. Clotting to fibrin was performed
in absence of factor (F) XIIIa (A),
in presence of 1 U/ml of F XIIIa to get covalently cross-linked fibrin
(B), and in presence of F XIIIa and
0.2 mg/ml of fibronectin (C). As a
control, fibronectin in concentrations ranging from 0.05 to 1 mg/ml was
incubated with 0.5 U/ml of thrombin and 1 U/ml of F XIIIa in absence of
fibrinogen (D). To separate clotted
material from clot liquor, samples were filtered. Radioactivity was
assessed in filtrate, and counts are given as a percentage of
radioactivity initially present in incubation medium. Each point is
mean ± SE of at least 4 independent experiments; error bars not
shown are within symbol.
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Inhibition of incorporation of fibronectin into fibrin
by surfactant. An incubation time (5-60
min)-dependent rise in the amount of fibrin-fibronectin hybrids could
be evidenced by SDS-PAGE (Fig. 2). In the
clot filtration experiments, incubation of 2 mg/ml of fibrinogen with
0.2 mg/ml of fibronectin resulted in the incorporation of
35% of
the fibronectin after 10 min and
45% after 60 min of incubation
through the action of activated factor XIII (Fig.
3). The amount of incorporated fibronectin
could not be enhanced by increasing the incubation time to 24 h. With the addition of 5 mg/ml of Curosurf, the amount of fibronectin incorporated into fibrin was markedly reduced. After an initial incorporation of
30% in the first 10 min, no significant increase could be achieved by increasing the incubation time (Fig. 3). Similar
results were observed with 5 mg/ml of Alveofact, although there was a
slightly lower incorporation of fibronectin compared with that with
Curosurf (Fig. 3). No substantial loss of fibronectin in the filtrate
and, therefore, no incorporation into fibrin were noticed in the
absence of factor XIII in the incubation mixture (Fig. 3). The same was
true in the absence of calcium and in the absence of fibrinogen (data
not shown in detail). Dose-effect characteristics of Curosurf and
Alveofact were studied with concentrations ranging from 0.1 to 5 mg/ml
of phospholipids during a 60-min incubation period. For Curosurf, a
significant increase in fibronectin in the soluble phase (indicating
nonincorporation into fibrin) was observed between 0.5 and 1 mg/ml of
phospholipids, and the maximum amount of fibronectin in the filtrate
occurred at 5 mg/ml of phospholipids. No significant inhibitory effect
could be demonstrated at concentrations < 1 mg/ml. With Alveofact,
similar results were obtained (Fig. 4). In
contrast, with DPPC alone, no inhibitory effect could be found (Fig.
4). The inhibition of factor XIIIa-mediated binding of fibronectin to
fibrin was also evidenced by SDS-PAGE. Coincubation with 5 mg of
Alveofact during coagulation resulted in a markedly reduced band for
the fibronectin
-complex (Fig. 5). There
was optimal fibronectin incorporation into fibrin in the presence and
absence of 5 mg/ml of Curosurf at a 3-5 mmol/l calcium
concentration (Fig. 6). Adding more calcium
to the incubation mixture was not accompanied by an increase in
fibronectin incorporation into fibrin whether or not the surfactant was
present.

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Fig. 2.
F XIIIa-mediated incorporation of fibronectin into fibrin as documented
by SDS-PAGE under reducing conditions. Fibrinogen (2 mg/ml) was
incubated with 0.2 mg/ml of fibronectin, 1 U/ml of F XIIIa, and 0.5 U/ml of thrombin. Coagulation was stopped by adding sample buffer (see
METHODS) after incubation periods of
5 (lane 1), 10 (lane 2), 30 (lane
3), and 60 (lane
4) min.
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Fig. 3.
Incorporation of fibronectin into fibrin is dependent on incubation
time in absence of F XIII (A), in
presence (+) of Alveofact (B), in
presence of Curosurf (C), and in
absence of surfactant (D). Except
for control where F XIII was left out, 2 mg/ml of fibrinogen were mixed
with fibronectin (0.2 mg/ml, 125I
labeled) and coagulation occurred over different time intervals after
0.5 U/ml of thrombin and 1 U/ml of F XIII were added. Alternatively,
either Alveofact or Curosurf (5 mg of phospholipids each) was added to
initial solution. Each point is mean ± SE of at least 4 independent
experiments.
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Fig. 4.
Influence of Curosurf, Alveofact, and DPPC alone on fibronectin (0.2 mg/ml) incorporation into fibrin. Complete conversion of fibrinogen (2 mg/ml) into insoluble fibrin was achieved by incubation with 1 U/ml of
F XIII and 0.5 U/ml of thrombin for 1 h at 37°C. Surfactants and
DPPC were added in different doses ranging from 0 to 5 mg/ml. Samples
were filtered, and radioactivity was assessed in filtrate. Each point
is mean ± SE of at least 4 independent experiments; error bars not
shown are within symbol.
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Fig. 5.
SDS-PAGE (reducing conditions) of fibronectin incorporation into fibrin
by F XIIIa in presence and absence of pulmonary surfactant.
Lane 1, fibronectin and fibrinogen
( -, -, and -chains) before start of coagulation (2 mg/ml
fibrinogen and 0.2 mg/ml fibronectin); lane
2, coagulation of fibrin with incorporated fibronectin
in presence of - -cross-links between -chains of fibrin and
development of complexes between fibronectin and -chains of fibrin;
lane 3, same protocol except for
addition of 5 mg/ml of Curosurf before start of coagulation by F
XIIIa.
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Fig. 6.
Dependence of fibronectin incorporation into fibrin on calcium
concentration in presence and absence of 5 mg/ml of surfactant
(Curosurf). Fibrinogen (2 mg/ml) and fibronectin (0.2 mg/ml) were
clotted by 0.5 U/ml of thrombin and 1 U/ml of F XIII at different
calcium concentrations from 0 to 10 mmol/l. Clotted material was
separated from clot liquor by filtering samples. Radioactivity was
assessed in filtrate. Each point is mean ± SE of at least 4 independent experiments.
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 |
DISCUSSION |
In the present work, we observed incorporation of surfactant
phospholipids into a growing fibrin network. This was demonstrable in
factor XIII-free fibrinogen coagulated by thrombin and factor XIIIa-cross-linked fibrin as well as in cross-linked fibrin with incorporated fibronectin. In all dose-effect experiments, ~50% of
the DPPC present at a surfactant concentration of 2 mg/ml of phospholipids was retained in the clotted material at a fibrin concentration of 0.5 mg/ml, whereas >90% of the DPPC was retained in
the clot when 2 mg/ml of fibrin were used. Interestingly, the presence
of two different commercially available surfactants (Alveofact and
Curosurf) during the process of coagulation led to an inhibition of
fibronectin incorporation into fibrin.
With the development of hyaline membranes with an intra-alveolar
accumulation of plasma proteins, a well-known morphological feature of
inflammatory lung diseases (3, 7), Balis et al. (4) first reported a
coagulative type of surfactant depletion. It has been shown that this
depletion is a result of an association of surfactant phospholipids
with polymerizing fibrin (34). Plasma proteins and, in particular,
fibrinogen and fibrin monomer cause a deterioration of the biophysical
properties of pulmonary surfactant (35, 36). The "trapping" of
surfactant phospholipids, mainly DPPC but also surfactant apoprotein
(SP) B (A. Elssner and W. Seeger, unpublished data),
results in a loss of surface activity that surpasses the effect of
fibrinogen and fibrin monomer by more than two orders of magnitude
(34). In plasma coagulation under physiological conditions,
~4-5% fibronectin is incorporated into the clot due to the
action of activated factor XIII (27). In agreement with these findings,
the calculated fibronectin content within our fibrin clots was
4-5%. In addition, we chose a ratio of fibronectin to fibrin
(0.2:2 mg/ml) that is found physiologically in human plasma (26, 27).
Admixture of either Alveofact or Curosurf reduced the incorporation
rate of fibronectin to approximately one-third compared with
coagulation in the absence of surfactant. Whereas the time-dependent fibronectin incorporation into fibrin was slightly reduced with Alveofact, a significant difference between Alveofact (a calf lung
extract) and Curosurf (a pig lung extract) could not be demonstrated. This finding is not surprising considering the similarities in the
chemical composition of the two surfactants, which both predominantly contain polar lipids (mainly DPPC) and
1% hydrophobic SP-B and SP-C
(information provided by the manufacturers). In summary, when surfactant was admixed, a distinct reduction in fibronectin incorporation into fibrin was reproducible, whereas a difference between the two surfactant extracts used could not be found.
Interestingly, the addition of DPPC alone showed no significant effect
on the binding of fibronectin to fibrin. Thus SP-B and SP-C, which are present in minor amounts in both surfactants used, are probably involved in the observed inhibitory effect. The interaction of surfactant lipids and SPs may lead to the change in the composition of
the clot.
In agreement with earlier observations, we found no influence of
surfactant on the generation of fibrin by thrombin- or factor XIII-induced cross-linking of fibrin. This implies that the enzymatic activity of thrombin and factor XIII themselves are not compromised by
surfactant. The activation of factor XIII and, therefore, covalent incorporation of fibronectin into fibrin are strongly calcium dependent
(25). Under the conditions we used, the optimum calcium concentration
for fibronectin incorporation was 5 mM, which was independent from the
presence or absence of surfactant. The inhibition of fibronectin
incorporation into fibrin due to the admixed surfactant could not be
overcome by enhancing the calcium concentration. Hence it is unlikely
that the inhibitory effect of surfactant was due to the binding of
calcium ions to negatively charged surfactant components. Thus other
possible mechanisms need to be discussed. 1) The alterations in the mechanical
properties of fibrin caused by embedded surfactant may lead to changes
in the three-dimensional arrangement of the fibrin network, with
concomitant changes in the binding sites for fibronectin.
2) Surfactant components may directly interfere with the binding sites of fibronectin. This could be
due to electrostatic or hydrophobic interactions in these parts of the
macromolecular fibrin network. 3)
Surfactant components may inhibit the activity of factor XIIIa at the
site of the fibronectin molecule itself. Surfactant also interferes
with enzymatic reactions important for fibrinolysis: synthetic
surfactant inhibits cleavage of fibrinogen by plasmin (15) and natural
surfactant can block plasmin-induced fibrin lysis (37). Another in
vitro study (16) showed that incorporation of a bovine surfactant
extract into fibrin provokes retardation of clot lysis through the
inhibition of plasmin, trypsin, or elastase.
The question is whether the demonstrated inhibitory effect of
surfactant on fibronectin incorporation has biological relevance. Based
on data obtained from studies with bronchoalveolar lavage fluid, the
surfactant concentrations we used are within or even below the
estimated concentration in the alveolar lining layer (35, 38, 40).
Therefore, we and other investigators (17) believe that intra-alveolar
coagulation takes place in an highly surfactant-enriched milieu.
However, some skepticism is warranted. The lowest ratio of surfactant
phospholipids to fibrinogen (mg/mg) at which we saw an effect was 0.5. Based on data from rat lungs (42), the concentration of surfactant
phospholipids in a lung that is flooded to functional residual capacity
(FRC) with fluid would be 0.33 mg/ml (estimated FRC of ~3 ml). In
cases where the locally produced amount of fibronectin can be neglected
in acute lung injury, the intra-alveolar fibrinogen and fibronectin
must be of plasma origin and the maximum concentrations of the proteins in the alveolar lining layer should be in the range of the
physiological concentrations in plasma. Thus the maximum concentration
of fibrinogen would be around 3 mg/ml. Hence alveoli that are flooded
to their FRC with plasma would contain material with a surfactant
phospholipid-to-fibrinogen ratio of 0.1 mg/mg. This is
five times less than the lowest ratio we showed to have an effect.
However, this calculation does not take into account that diffuse
microatelectasis due to collapsed alveoli is a typical feature of the
acute phase of ARDS. Thus the leakage of smaller amounts (<3 mg/ml)
of fibrinogen may be sufficient to result in deterioration of the
alveolar surfactant function and consequently contribute to the
development of ARDS. In summary, our data may have relevance in
diseases with an intra-alveolar accumulation of fibrin such as ARDS. On
the other hand, under the condition of plasma-flooded alveoli, the
surfactant phospholipid-to-fibrinogen ratio may be lower than the ratio
at which we saw an effect on the incorporation of fibronectin into fibrin.
What can be expected when incorporation of fibronectin into fibrin is
inhibited by surfactant in vivo within the alveolus, e.g., in ARDS? It
is known that the addition of fibronectin to fibrin in physiological
amounts increases fibrin fiber thickness and network permeability (28,
30). In addition, altered mechanical properties of fibrin clots were
recently described when coagulation takes place in a
surfactant-enriched milieu. The fibrin network with incorporated
surfactant displayed a decrease in the elastic modulus of arising
fibrin polymers and an increased hydraulic conductivity in conjunction
with an increased pore size, suggesting an altered architecture (17).
Thus it could be speculated that these mechanical alterations of fibrin
clots in the presence of surfactant are partially attributable to
reduced fibronectin incorporation into fibrin, considering the minor
amounts of fibronectin and factor XIII in the fibrinogen preparation
used in that study. Fibrin and fibronectin may provide a provisional
matrix for ongoing repair in lung injury. Type II cells are believed to
reepepithelialize injured alveoli through integrin-mediated adherence
to fibrin-fibronectin matrices or basement membranes (22, 39). On the
other hand, a fibrin-fibronectin matrix may initiate fibrotic
remodeling via an invasion of fibroblasts (1, 2, 6, 8, 23, 41), leading
to a so-called collapse induration (7). Because fibroblasts bind more
tightly to fibronectin when it is cross-linked to fibrin (13),
surfactant may have an antifibroproliferative effect. Another aspect is
that the addition of fibronectin to fibrin was shown to enhance
macrophage binding in vitro (21). Therefore, the presence of surfactant
within fibrin-fibronectin matrices could reduce the clearance of clots
by alveolar macrophages within alveoli. This may prevent further plasma
leakage, bleeding, and more lung damage due to persistent hyaline
membranes. On the other hand, the reduced clearance of clots by
surfactant may inhibit rapid restoration of gas exchange. Fibrin
matrices that had been depleted of factor XIII and/or fibronectin were
shown to be a superior matrix for macrophage migration over fibrin
matrices with incorporated fibronectin (24). Consequently, surfactant may facilitate the movement of macrophages within damaged alveoli. In
summary, our findings support the hypothesis that coagulation within
the surfactant-containing alveolar milieu is different from clotting of
plasma proteins in other compartments.
 |
FOOTNOTES |
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: A. Elssner,
Dept. of Internal Medicine I, Div. for Pulmonary Diseases, Klinikum
Grosshadern, Ludwig-Maximilians-Univ. of Munich, Marchioninistrasse 15, 81377 Munich, Germany (E-mail:
Andreas.Ellsner{at}med1.med.uni-muenchen.de).
Received 11 February 1998; accepted in final form 29 December
1998.
 |
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