SP-A enhances viral clearance and inhibits inflammation after pulmonary adenoviral infection

Kevin S. Harrod, Bruce C. Trapnell, Kazuhisa Otake, Thomas R. Korfhagen, and Jeffrey A. Whitsett

Children's Hospital Medical Center, Division of Neonatology and Pulmonary Biology, Cincinnati, Ohio 45229


    ABSTRACT
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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Surfactant protein A (SP-A) is a member of the collectin family of host defense molecules expressed primarily in the epithelial cells of the lung. To determine the role of SP-A in pulmonary adenoviral infection, SP-A-deficient (SP-A -/-) mice were intratracheally infected with a replication-deficient recombinant adenovirus, Av1Luc1. Lung inflammation was markedly increased in SP-A -/- compared with SP-A +/+ mice and was associated with increased hemorrhage and epithelial cell injury. Polymorphonuclear cells in bronchoalveolar lavage fluid (BALF) were increased in SP-A -/- mice after administration of adenovirus. Coadministration of adenovirus and purified human SP-A ameliorated adenoviral-induced lung inflammation in SP-A -/- mice. Concentrations of tumor necrosis factor-alpha (TNF-alpha ), interleukin (IL)-6, and IL-1beta were increased in BALF of SP-A -/- mice. Likewise, TNF-alpha , IL-6, macrophage inflammatory protein (MIP)-1alpha , monocyte chemotactic protein-1, and MIP-2 mRNAs were increased in lung homogenates from SP-A -/- mice 6 and 24 h after viral administration. Clearance of adenoviral DNA from the lung and uptake of fluorescent-labeled adenovirus by alveolar macrophages were decreased in SP-A -/- mice. SP-A enhances viral clearance and inhibits lung inflammation during pulmonary adenoviral infection, providing support for the importance of SP-A in antiviral host defense.

surfactant protein A; lung epithelium; collectins; alveolar macrophages


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

SURFACTANT PROTEIN A (SP-A) is an abundant 28- to 36-kDa protein expressed primarily by epithelial cells of the lung. SP-A is a member of the C-type lectin subgroup of proteins termed collectins (23). Collectins are mammalian lectins that bind complex carbohydrates, thereby acting as opsonins for various microorganisms. SP-A binds bacteria, including Staphylococcus aureus, Haemophilus influenza type A, Pseudomonas aeruginosa, and group B streptococcus (17, 19). SP-A also binds phospholipids and is critical for the formation of tubular myelin (10). In vitro studies support a role for SP-A in enhancing surfactant activity in vivo, protecting surfactant from inactivation by serum proteins. SP-A-deficient (SP-A -/-) mice were produced by targeted inactivation of the SP-A gene (11). Importantly, homozygous SP-A null mice survive normally under standard laboratory conditions. SP-A -/- mice lack tubular myelin; however, lung function and surface tension of surfactant from SP-A -/- mice are normal under physiological conditions (10). Previous work from this laboratory demonstrated that SP-A -/- mice are more susceptible to pulmonary infection by group B streptococcus and P. aeruginosa (14, 15). In vitro, SP-A neutralizes influenza A virus infection and enhances uptake of influenza A virus by macrophages (3, 4). However, little is understood about the role of SP-A in viral host defense of the lung in vivo.

Adenoviruses are ubiquitous viral pathogens that cause respiratory, gastrointestinal, and genitourinary infections. In the lung, adenoviruses usually cause acute respiratory pathology; however, the virus can also persist as an asymptomatic infection of the respiratory tract. Acutely, adenovirus infection causes infiltration of macrophages and neutrophils into alveolar air spaces. Concentrations of cytokines tumor necrosis factor-alpha (TNF-alpha ), interleukin (IL)-6, and IL-1beta are increased in pulmonary tissues of mice after adenoviral infection, coinciding with the infiltration of macrophages into the lung parenchyma. Because of their tropism for airway epithelial cells, adenoviruses have also been utilized as vectors for gene transfer to the lung. The efficiency and duration of gene expression with recombinant adenoviruses, however, are limited by host inflammation and immune responses. Nevertheless, infection by adenovirus and adenoviral vectors has provided a useful model for studying viral-induced inflammation in the lung.

To assess the role of SP-A in lung inflammation after viral infection, SP-A -/- mice were administered a replication-deficient recombinant adenoviral vector, Av1Luc1, by intratracheal instillation. Clearance of the lung and uptake of the virus by alveolar macrophages were decreased in SP-A -/- mice. Lung inflammation was increased in SP-A -/- mice early during the course of adenoviral infection. Inflammatory responses to the virus were abrogated by intratracheal administration of exogenous human SP-A (hSP-A).


    MATERIALS AND METHODS
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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Mice. SP-A -/- (129J/129J hybrid; see Ref. 11) and wild-type 129J (Jackson Laboratory) mice were housed in the Children's Hospital Research Foundation vivarium under pathogen-free conditions as required by American Association for Accreditation of Laboratory Animal Care guidelines. Animal protocols were reviewed and approved by the Institutional Animal Care and Use Committee of Children's Hospital Medical Center.

Intratracheal administration of adenovirus. Eight- to twelve-week old SP-A -/- and 129J wild-type control mice (n = 6-12 mice/group) were used. The procedure for intratracheal administration of adenovirus vectors was previously described by Zsengeller et al. (33). Briefly, mice were anesthetized with methoxyflurane vapor, and a ventral midline incision was made to expose the trachea. Intratracheal inoculation of 1 × 109 plaque-forming units (pfu) of Av1Luc1, an E1-E3 deleted adenoviral vector (serotype 5) expressing firefly luciferase from the Rous sarcoma virus promoter, in 100 µl of delivery vehicle (10 mM Tris, 1 mM MgCl2, and 10% glycerol, pH 7.4) was performed using a bent, 27-gauge tuberculin syringe (Monoject, St. Louis, MO). The incision was closed with one drop of Nexaband liquid (VPL, Phoenix, AZ), and the mice were allowed to recover. Mice recovered rapidly and remained active after the procedure. At a predetermined time, mice were killed by lethal injection of pentobarbital sodium. A midline incision was made in the abdomen. Exsanguination was accomplished by transection of the inferior vena cava to reduce hemorrhage in the lung. For histological studies and RNA and protein analysis, right upper, right middle, and right lower lobes of the lung were clamped with a hemostat and removed for RNA and protein measurements. The left lung lobe was inflated with 4% paraformaldehyde (Electron Microscopy Sciences, Ft. Washington, PA) and fixed overnight for histological examination.

Inflammatory cells in bronchoalveolar lavage fluid. Inflammatory cell numbers and percentages were evaluated at 6 and 24 h after adenovirus vector administration. Bronchoalveolar lavage fluid (BALF; n = 6-8/group) was obtained after intratracheal instillation of 1 ml of PBS while the lung was in the thoracic cavity. Lungs were reinfused two times with fresh PBS before final collection and pooled. BALF cells were isolated by centrifugation at 500 g and resuspended in 500 µl, and 100 µl of cell suspension were mixed in 100 µl of 0.4% trypan blue (GIBCO BRL, Grand Island, NY) and counted with a hemacytometer. To determine inflammatory cell types in BALF, 5 × 104 cells were mounted on slides by cytospin centrifugation in 100 µl of PBS at 600 rpm for 3 min. Cell types were identified and counted by differential staining microscopy using Diff-Quik (Baxter Healthcare, Miami, FL). Inflammatory cell populations were determined by counting 100 cells, and a percentage was calculated based on five sample sets from three animals per group.

Purification of hSP-A. hSP-A was purified as described previously by Haagsman et al. (8).

Cytokine analysis. Cytokine mRNA abundance was determined by RT-PCR analysis of whole lung total RNA. Briefly, whole lung total RNA was isolated by phenol-chloroform extraction and precipitation by isopropanol with the Phase-Lock protocol (5 Prime-3 Prime, Boulder, CO). Total RNA quantitation was confirmed by gel electrophoresis. Total RNA was converted to cDNA by the reverse transcriptase reaction (GIBCO BRL, Gaithersburg, MD). PCR for cytokine cDNA was performed as described previously (1, 9), using the following primer tandems: beta -actin primer 1, 5'-GTGGGCCG- CTCTAGGCACCAA-3'; beta -actin primer 2, 5'-CTCTTTGATGTCACGCACGATTTC-3'; IL-6 primer 1, 5'-TTGCCTTCTTGGGACTGATGCT-3'; IL-6 primer 2, 5'-GTATCTCTCTGAAGGACTCTGG-3'; TNF-alpha primer 1, 5'-CCAGACCCTCACACTCAGAT-3'; TNF-alpha primer 2, 5'-AACACCCATTCCCTTCACAG-3'; macrophage inflammatory protein (MIP)-1alpha primer 1, 5'-ACTGCCCTTGCTGTTCTT-CTCT-3'; MIP-1alpha primer 2, 5'-AGGCATTCAGTTCCAGGTCAGT-3'; MIP-2 primer 1, 5'-ATGGCCCCTCCCACCTGC-3'; MIP-2 primer 2, 5'-TCAGTTAGCCTTGCCTTTGTT-3'; monocyte chemotactic protein (MCP)-1 primer 1, 5'-ATGCAGGTCCCTGTCATGCTT-3'; MCP-1 primer 2, 5'-CTAGTTCACTGTCACACTGGT-3'. PCR using the OptiPrime reagents (Stratagene, La Jolla, CA) was performed for 25 cycles on a Perkin-Elmer 2400 Gene Amp System thermal cycler with the following parameters: initiation at 94°C for 30 s, annealing temperature at 59°C for 30 s, and elongation temperature of 72°C for 30 s. DNA fragments were separated by agarose gel (2%) electrophoresis and were stained with ethidium bromide to visualize the PCR products.

TNF-alpha , IL-6, and IL-1beta concentrations were assessed in lung homogenates by ELISA according to the manufacturer's recommendations (Endogen, Woburn, MA).

Pulmonary histopathology. Histopathological changes were evaluated 6 and 24 h after intratracheal administration of Av1Luc1. Lungs were inflation fixed at 20 cmH2O pressure. Inflation-fixed lungs were washed three times, embedded in paraffin, sectioned (5 µm), and stained with hematoxylin and eosin for analysis by light microscopy.

PCR analysis of adenoviral DNA. Adenoviral DNA was evaluated in lung homogenates after Av1Luc1 administration. Total lung DNA was isolated from lung homogenates by phenol-chloroform extraction and ethanol precipitation. Total DNA was quantitated, and 1, 0.1, and 0.01 µg were evaluated by PCR amplification of the adenoviral E2a gene. PCR primers were as follows: E2a primer 1, 5'-CGG AAT TCC AAC AGA GGA TAA AAA GCA AGA CC-3'; E2a primer 2, 5'-CGG AAT TCA AGG CCA GCT GCT TGT CCG CTC GG-3'. PCR using the OptiPrime reagents (Stratagene) was performed for 25 cycles on a Perkin-Elmer 2400 Gene Amp System thermal cycler with the following parameters: initiation at 94°C for 30 s, annealing temperature at 55°C for 30 s, and elongation temperature of 72°C for 30 s. DNA fragments were separated by agarose gel (2%) electrophoresis and stained with ethidium bromide to visualize the PCR products. PCR amplification of beta -actin was performed as an internal control using primers from GIBCO BRL according to the manufacturer's recommendations.

Fluorescent labeling of adenovirus. Fluorescent adenovirus was prepared as described previously (13). Briefly, 1 × 1013 particles/ml of adenovirus were incubated for 30 min at room temperature with lyophilized Cy3 dye (Amersham, Arlington Heights, IL) according to the manufacturer's instructions. Cy3-labeled adenovirus was dialyzed two times against 10 mM Tris and 10% glycerol for 24 h and was stored at -80°C.

Fluorescent microscopy and fluorescence-activated cell sorter analysis. Bronchoalveolar lavage (BAL) was evaluated by fluorescence-activated cell sorter (FACS) analysis to determine the uptake of fluorescent adenovirus by macrophages. Cells were isolated from lavage fluid by standard centrifugation and were resuspended in PBS. For fluorescence microscopy, 5-10 × 104 cells were mounted on glass slides by cytospin centrifugation and air-dried under dark conditions. Fluorescence microscopy was performed using a Nikon Microphot-FXA microscope and Nikon Mercury lamp light source. Fluorescence was determined qualitatively by viewing 10 microscopic fields from each sample.

FACS analysis was used for quantitative assessment of fluorescent cells in lung lavage. Briefly, cellular pellets were resuspended in 500 µl of PBS and analyzed using a Becton Dickinson FACScan analyzer and FACScan software. Cells from the lavage of uninfected mice were used to determine the macrophage population as observed by flow cytometry and to assess the background fluorescence of macrophages. For each sample, 1 × 104 events were acquired, and analysis of fluorescence was limited to the gated region corresponding to the macrophages.

Statistical analysis. Statistical analysis for multiple groups was determined by ANOVA using Microsoft Excel computer software. All data are presented as means ± SE. Differences were considered significant at P <=  0.05.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Inflammatory cells were increased in BAL of SP-A -/- mice after adenoviral infection. To assess the role of SP-A in lung inflammation after adenoviral infection, SP-A -/- and wild-type SP-A +/+ mice (pathogen free, 8-13 wk of age) were administered 109 pfu of Av1Luc1 intratracheally. Av1Luc1 is a replication-deficient E1- to E3-deleted recombinant adenovirus expressing firefly luciferase. Both SP-A +/+ and SP-A -/- mice tolerated adenoviral infection and were active within 1 h after the surgical procedure, and no mortality was observed. Inflammatory cell numbers in BALF were assessed at 6 and 24 h after viral infection in SP-A -/- and SP-A +/+ control mice. Six and twenty-four hours after adenoviral infection, cell counts in BALF from SP-A -/- mice were increased compared with cell counts from SP-A +/+ mice (Fig. 1). To determine whether exogenous administration of SP-A influenced cellular infiltrates in SP-A -/- mice, 100 µg of purified hSP-A were coadministered intratracheally with adenovirus. Administration of exogenous SP-A had a more pronounced effect on the decrease in cell counts in BALF from adenoviral-infected SP-A -/- mice compared with the decrease in cell counts in BALF from SP-A +/+ mice 6 and 24 h after infection. In the absence of virus, exogenous administration of hSP-A did not alter cell counts in BALF of SP-A +/+ or SP-A -/- mice at 6 and 24 h.


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Fig. 1.   Increased inflammatory cells in bronchoalveolar lavage fluid (BALF) of surfactant protein A (SP-A)-deficient (-/-) mice after adenoviral infection. Cell counts were determined in BALF from SP-A +/+ (wild-type) and SP-A -/- mice. Responses after intratracheal delivery of 109 plaque-forming units of Av1Luc1 or coadministration of virus and purified human SP-A (hSP-A; 100 µg) 6 (A) and 24 (B) h after infection are shown. Cell counts were significantly increased in BALF from SP-A -/- compared with SP-A +/+ mice 6 and 24 h after infection. Coadministration of hSP-A significantly decreased cell counts in SP-A -/- mice after adenoviral infection. In the absence of virus, exogenous administration of hSP-A did not alter cell counts in BALF of SP-A +/+ or SP-A -/- mice at 6 and 24 h. Cell counts from uninfected SP-A +/+ are shown and are similar to cell counts from BALF of SP-A -/- mice (data not shown). * Values are significantly different from uninfected controls (P <=  0.05); ** significant difference after coadministration of hSP-A (P <=  0.05).

To determine whether SP-A altered individual cell populations in BALF, inflammatory cells from SP-A -/- and SP-A +/+ mice were assessed by differential staining 6 and 24 h after infection. Inflammatory cells in BALF from uninfected SP-A -/- and SP-A +/+ mice consisted predominantly of macrophages (Table 1 and Fig. 2A). Six and twenty-four hours after infection, BALF from SP-A +/+ mice contained predominantly macrophages, with few neutrophils, whereas BALF from SP-A -/- mice contained increased numbers of neutrophils compared with that from SP-A +/+ mice (Table 1 and Fig. 2, B and E). Coadministration of purifed hSP-A to SP-A -/- mice, but not to SP-A +/+ mice, significantly decreased the numbers of neutrophils in BALF at 24 h after infection (Table 1 and Fig. 2, D and G). Administration of exogenous SP-A to uninfected SP-A +/+ mice did not alter inflammatory cell populations.

                              
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Table 1.   Differential cell counts in BALF from SP-A +/+ and SP-A -/- mice after adenoviral infection alone or with coadministration of purified hSP-A



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Fig. 2.   Cytology of BALF after Av1Luc1 infection. Inflammatory cells in BALF from uninfected mice (A), SP-A +/+ mice (B and E), and SP-A -/- mice (C, D, F, and G) were assessed 6 h (B-D) and 24 h (E-G) after infection. Increased neutrophils (arrowheads) are apparent in SP-A -/- mice 6 and 24 h after Av1Luc1 infection. Coadministration of hSP-A and Av1Luc1 to SP-A -/- mice resulted in predominantly macrophage infiltrate (arrows) 6 h (D) and 24 h (G) after infection. Insets at top right: higher magnification of cell types in BALF demonstrating cell morphology.

Cytokine expression was increased in SP-A -/- mice after adenoviral infection. To assess the role of SP-A in the regulation of cytokine and chemokine responses to the virus, total RNA was isolated from lung homogenates of SP-A +/+ and SP-A -/- mice 6 and 24 h after Av1Luc1. TNF-alpha , IL-6, MCP-1, MIP-1alpha , and MIP-2 mRNAs were determined by RT-PCR analysis. Cytokine (TNF-alpha and IL-6) and chemokine (MCP-1, MIP-1alpha , and MIP-2) mRNAs were not detected in lung homogenates of uninfected SP-A -/- mice (Fig. 3). Six hours after infection, TNF-alpha , IL-6, MIP-1alpha , MCP-1, and MIP-2 mRNAs were increased in SP-A -/- compared with SP-A +/+ mice. Twenty-four hours after infection, TNF-alpha , IL-6, MCP-1, and MIP-2 mRNAs from SP-A -/- mice were increased compared with SP-A +/+ mice, whereas MIP-1alpha mRNA was not detected.


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Fig. 3.   Cytokine and chemokine mRNA expression after adenoviral infection in SP-A -/- mice. Cytokine [tumor necrosis factor-alpha (TNF-alpha ) and interleukin (IL)-6] and chemokine [monocyte chemotactic protein (MCP)-1, macrophage inflammatory protein (MIP)-1alpha , and MIP-2] mRNA expression was assessed by RT-PCR in lung homogenates from SP-A -/- and SP-A +/+ mice 6 and 24 h after Av1Luc1 administration. Agarose gel (2%) electrophoresis and ethidium bromide staining were used to visualize PCR products. TNF-alpha , IL-6, MCP-1, MIP-1alpha , and MIP-2 mRNA expression was increased in the lungs of SP-A -/- compared with SP-A +/+ mice 6 and 24 h after infection. Samples shown are representative of n = 12-15 samples/group. Uninf, uninfected.

TNF-alpha , IL-6, and IL-1beta were determined by ELISA in BALF from SP-A -/- and SP-A +/+ mice 6 and 24 h after adenoviral administration (Fig. 4). Six hours after infection, IL-6 was increased in the BALF from SP-A +/+ mice compared with uninfected mice. TNF-alpha , IL-1beta , and IL-6 concentrations were increased in SP-A -/- compared with SP-A +/+ mice 6 h after adenoviral infection. Twenty-four hours after infection, TNF-alpha , IL-6, and IL-1beta concentrations were increased in BALF from SP-A -/- compared with SP-A +/+ mice. Concentrations of TNF-alpha , IL-6, and IL-1beta were not different in uninfected SP-A +/+ or SP-A -/- mice.


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Fig. 4.   Increased cytokine concentrations in BALF of SP-A -/- mice after adenoviral infection. Concentrations of TNF-alpha , IL-1beta , and IL-6 were determined in BALF by ELISA 6 and 24 h after infection with Av1Luc1. TNF-alpha , IL-1beta , and IL-6 were increased in BALF from SP-A -/- compared with SP-A +/+ mice 6 and 24 h after infection. * Significant difference (P <=  0.05) of SP-A -/- from time-matched SP-A +/+ mice.

Increased lung inflammation in SP-A -/- mice after viral infection. Lung histology was evaluated in SP-A +/+ and SP-A -/- mice after viral infection. Administration of Av1Luc1 to SP-A +/+ mice caused a mild monocytic infiltration 6 h after infection (Fig. 5). In contrast, alveolar epithelial cell sloughing and thickening of alveolar septa were observed 6 h after adenoviral infection of SP-A -/- mice, coinciding with increased numbers of monocytes and neutrophils in alveolar air spaces and in the conducting airways. Twenty-four hours after infection, little or no lung inflammation was observed in SP-A +/+ mice. In contrast, inflammation persisted 24 h after infection in the lungs of SP-A -/- mice, consisting of increased numbers of monocytes and neutrophils in the alveolar air spaces and surrounding peripheral bronchioles. Alveolar cell injury and cellular sloughing were also apparent. Red blood cells were observed in the alveolar air spaces, consistent with the disruption of the alveolar-capillary barrier in SP-A -/- mice.


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Fig. 5.   Lung histology after adenoviral infection. Lung histology was assessed in SP-A +/+ (B and D), SP-A -/- (C and E), and uninfected (A) mice after exposure to adenovirus for 6 (B and C) and 24 (D and E) h. Lung inflammation and neutrophil infiltrates (arrows) were increased in the lungs of SP-A -/- mice after viral infection. Data shown represent n = 12 animals/group. Magnification is ×256.

Decreased clearance of adenovirus in SP-A -/- mice. To determine the role of SP-A in clearance of adenovirus from the lung, DNA was isolated from lung homogenates from both SP-A -/- and SP-A +/+ mice 6 and 24 h after adenoviral administration. Semiquantitative PCR was used to estimate adenovirus-specific DNA. Adenovirus-specific DNA was not detected in lung homogenates from uninfected mice (Fig. 6). Six hours after infection, adenoviral DNA levels were readily detectable and similar in the lungs of SP-A +/+ and SP-A -/- mice. However, 24 h after Av1Luc1 administration, adenoviral DNA was increased in lung homogenates of SP-A -/- mice compared with SP-A +/+ mice.


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Fig. 6.   Decreased clearance of adenoviral DNA in SP-A -/- mice. Adenoviral DNA was assessed by PCR in the lungs of uninfected SP-A +/+ mice (minus adenovirus), SP-A +/+ mice (wild type), and SP-A -/- mice after Av1Luc1 administration. Adenovirus-specific PCR products were separated by 2% agarose gel electrophoresis and stained for ethidium bromide (top). PCR for beta -actin DNA is shown as a control for loading (bottom). Adenovirus-specific DNA was increased in the lungs of SP-A -/- mice 24 h after infection. Ad, adenovirus; M, molecularmass marker.

Decreased uptake of adenovirus by alveolar macrophages in SP-A -/- mice. Uptake of fluorescently labeled adenoviral particles was determined by fluorescence microscopy and FACS analysis of BALF cells. Cells were isolated from the lungs of SP-A -/- and SP-A +/+ mice 24 h after administration of Cy3-labeled adenovirus. Cells from BALF of SP-A +/+ mice were highly fluorescent 24 h after intratracheal administration of adenovirus (Fig. 7). Fluorescence was almost exclusively confined to alveolar macrophages. The intensity of fluorescence and number of fluorescent macrophages were decreased in BALF from SP-A -/- mice. Staining of neutrophils in BALF from SP-A -/- mice was not observed.


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Fig. 7.   Decreased uptake of adenovirus by alveolar macrophages in SP-A -/- mice. Cy3-labeled adenoviral particles were intratracheally administered to SP-A +/+ (B) and SP-A -/- (C) mice, and fluorescent-labeled cells were assessed in BALF by fluorescence-activated cell sorter (FACS, A-C, left and middle) and fluorescent microscopy (A-C, right). A: FACS analysis and fluorescent microscopy of BALF from uninfected SP-A +/+ mice. BALF from SP-A +/+ mice infected with a Cy3-labeled adenovirus exhibited an altered profile as assessed by flow cytometry (B, left) compared with SP-A -/- mice (C, left). Macrophages in BALF from SP-A +/+ mice infected with Cy3-labeled adenovirus had increased fluorescence as assessed by FACS (B, middle) compared with SP-A -/- mice (C, middle). Fluorescent-labeled macrophages were increased in BALF from SP-A +/+ (B, right) compared with SP-A -/- (C, right) mice as assessed fluorescent microscopy.

Uptake of Cy3-labeled adenovirus was also determined in cells from BALF of SP-A -/- and SP-A +/+ mice by FACS analysis. Cells from BALF of uninfected mice were used to determine background fluorescence of alveolar macrophages, as well as to determine the separation of the macrophage subpopulations by flow cytometry. Macrophages from SP-A +/+ mice stained intensely 24 h after administration of Cy3-labeled adenovirus (Fig. 7). In contrast, staining of macrophages from SP-A -/- mice 24 h after administration of Cy3-labeled adenovirus was markedly decreased. Thus fluorescence microscopy and FACS analysis demonstrated decreased uptake of the virus by alveolar macrophages from SP-A -/- compared with SP-A +/+ mice.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Increased pulmonary inflammation and cytokine production were observed after intratracheal infection of SP-A -/- mice with Av1Luc1, a replication-deficient adenovirus. Increased neutrophils were observed in the lungs of SP-A -/- mice after infection. Cytokine and chemokine mRNA expressions (TNF-alpha , IL-6, MCP-1, MIP-1alpha , and MIP-2) were increased in the lungs of infected SP-A -/- mice, consistent with the increase in inflammation observed after exposure to the virus. Likewise, concentrations of proinflammatory cytokines TNF-alpha , IL-6, and IL-1beta were increased in BALF of SP-A -/- mice. Coadministration of purified hSP-A with virus inhibited pulmonary inflammation in SP-A -/- mice. Uptake and clearance of adenovirus were impaired in SP-A -/- mice. These data indicate that SP-A enhances clearance and decreases pulmonary inflammation after adenoviral infection in vivo.

The present study demonstrates that SP-A enhances the uptake of adenovirus by alveolar macrophages. In vitro, SP-A binds to a number of gram-positive and gram-negative bacteria, facilitating the uptake of bacteria by phagocytes (17, 24), functioning as an opsonin, and activating phagocytes involved in bacterial clearance (30). SP-A also binds viruses (4, 26); however, the potential role of SP-A in host responses to virus is not well understood. In vitro studies demonstrated that SP-A binds and inhibits infection of herpes simplex I (26) and influenza A virus (4). SP-A enhances phagocytosis of herpes simplex I virus by alveolar macrophages through a mechanism other than the C1q receptor (25), suggesting that SP-A may have an important role in viral clearance after infection in the lung. Worgall et al. (28) demonstrated that macrophages rapidly eliminate adenoviral vectors from the lung, suggesting that alveolar macrophages may constitute an important early defense against viral infection in the lung. The present findings provide further support for the role of SP-A-mediated viral uptake and clearance by alveolar macrophages.

The role of SP-A in modulating inflammation after viral infection has not been previously studied in vivo. In the present work, SP-A reduced inflammation, decreased neutrophilic infiltration, and decreased cytokine production in the lung after adenoviral infection. These findings are consistent with the recent observations that SP-A decreased inflammation after pulmonary infection by bacteria. Increased pulmonary inflammation was observed after intratracheal infection with group B streptococcus or P. aeruginosa in SP-A -/- mice (14, 15). After pulmonary P. aeruginosa infection in SP-A -/- mice, neutrophilic infiltrates were associated with increased cytokine production (15). In the present study, it is unclear whether increased cytokine or chemokine responses are caused by decreased viral clearance or by primary effects of SP-A on host cell responses. SP-A modulates various functions of inflammatory cells in vitro, including chemotaxis (30), cytokine production (18), and proliferation (5). Thus the present findings provide further support for the role of SP-A in pulmonary viral infection but do not clarify the precise mechanisms by which SP-A suppresses inflammation.

In the present study, decreased clearance of virus from the lungs of SP-A -/- mice was associated with increased pulmonary inflammation and increased cytokine production. The mechanisms underlying the association between increased inflammation and decreased viral clearance are unclear at present. Decreased clearance of virus in SP-A -/- mice may result in the increased response of target cells to the virus. Increased adenoviral DNA in the lungs of SP-A -/- mice is consistent with delayed clearance that may result in exposure of target cells to increased numbers of viral particles. It is also possible that the pulmonary inflammation observed in SP-A -/- mice may be related to altered activation of alveolar macrophages after viral infection. For example, McIntosh et al. (18) demonstrated that SP-A inhibited TNF-alpha release from alveolar macrophages after lipopolysaccharide exposure. Likewise, SP-A inhibits IL-2 production by human lymphocytes (5). Thus SP-A may modulate macrophage function and/or activation in the presence of infectious stimuli. SP-A is known to bind to alveolar macrophages and alveolar type II cells (22, 29). Because alveolar type II cells produce various cytokines, chemokines, eicosanoids, and adhesion molecules after injury and infection, it is possible that SP-A modifies various aspects of the inflammatory response by the respiratory epithelium.

Because of its tropism for the respiratory tract, recombinant adenoviruses have been utilized for gene delivery to the lung. However, host immune responses and inflammation to adenoviral vectors severely limit both the efficiency and duration of gene expression in vivo (6, 31-33). In the current study, a replication-deficient adenovirus was used, and thus the role of viral replication in the host response was not considered. The present work demonstrates that SP-A plays an important role in clearance of the adenoviral vector from the lung and provides support for the role of the alveolar macrophage in that process. Otake et al. (20) have recently demonstrated that early, nonspecific inflammatory responses limit adenoviral gene expression independently of specific or acquired immune responses, suggesting that the alveolar macrophage may have an important role in those responses. Likewise, macrophage depletion prolongs adenoviral-mediated gene delivery in vivo (12, 27, 28). Taken together, SP-A appears to play an important role in adenoviral clearance by alveolar macrophages.

Concentrations of SP-A in BALF are decreased during bacterial pneumonia (2, 16) and after respiratory syncytial virus infection in children (16). SP-A levels are also reduced in preterm infants and bronchopulmonary dysplasia and in various interstitial lung diseases (7, 21). The observation that viral clearance is decreased in SP-A -/- mice suggests that decreased SP-A may increase susceptibility to viral infection and subsequent inflammation. Furthermore, whether the administration of SP-A protects against pulmonary infection or inflammation in vivo remains to be determined.

In summary, the presence of SP-A in the lungs of mice decreased inflammation and increased clearance of adenovirus early during the course of pulmonary infection. These studies provide important insights into the role of SP-A in host defense against viral infection in vivo and suggest that SP-A modulates uptake by alveolar macrophages and inflammation after viral infection in the lung.


    ACKNOWLEDGEMENTS

We thank Gary F. Ross for preparation of purified human surfactant protein A and Anne Marie Levine for thoughtful discussion.


    FOOTNOTES

This study was supported by grants from the Parker B. Francis Foundation (K. S. Harrod), the National Heart, Lung, and Blood Institute (NHLBI) Program of Excellence (K. S. Harrod), and NHLBI Grants HL-28623 (J. A. Whitsett), HL-41496 (J. A. Whitsett), and HL-58795 (T. R. Korfhagen).

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: J. A. Whitsett, Children's Hospital Medical Center, Div. of Neonatology and Pulmonary Biology, 3333 Burnet Ave., Cincinnati, OH 45229-3039 (E-mail: whitj0{at}chmcc.org).

Received 23 December 1998; accepted in final form 5 May 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

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Am J Physiol Lung Cell Mol Physiol 277(3):L580-L588
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