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1-Antitrypsin Polymerizes in the Lung and Acts as a Neutrophil Chemoattractant*
* From the Department of Medicine (Drs. Mulgrew, Taggart, Greene, ONeill, and McElvaney, and Mr. Lawless), Respiratory Research Division, Royal College of Surgeons in Ireland, Beaumont Hospital, Dublin, Ireland; and the Pulmonary and Critical Care Medicine Division (Dr. Brantly), University of Florida School of Medicine, Gainesville, FL.
Correspondence to: Noel G. McElvaney, MD, Department of Medicine, Royal College of Surgeons in Ireland, Beaumont Hospital, Dublin 9, Ireland; e-mail: gmcelvaney{at}rcsi.ie
| Abstract |
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1-antitrypsin (A1AT) is an abundant protein that is synthesized in the liver and is secreted into the plasma. From the plasma, A1AT diffuses into various body compartments, including the lung where it provides much of the antiprotease protection. The current understanding of the pathogenesis of emphysema in A1AT-deficient individuals focuses on the polymerization of mutant protein within the liver, which results in a deficiency of circulating A1AT and a protease-antiprotease imbalance in the lungs. Methods and results: In this study, we evaluated BAL fluid samples from five healthy volunteers, five individuals with ZA1AT deficiency, and an individual with the PiZZ phenotype who had received a liver transplant. We show that the lung itself is a source of A1AT. In addition, the Z protein formed in the lung polymerizes, and these polymers are detectable in lung epithelial lining fluid by enzyme-linked immunosorbent assay and Western blot analysis. Finally, we show that polymeric ZA1AT is a potent neutrophil chemoattractant that is similar to polymerized MA1AT.
Conclusions: Our findings suggest that the polymerization of locally produced ZA1AT is a contributory factor to the lung inflammation experienced by those with A1AT deficiency and that standard antiprotease therapies may not address this problem.
Key Words: chemoattractant polymerization Z
1-antitrypsin
| Introduction |
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1-antitrypsin (A1AT) deficiency is a condition that is characterized by low serum levels of A1AT12 and a substantially increased risk for the development of emphysema by the fourth or fifth decade.34 There are also risks for the development of hepatic disease56 and cutaneous panniculitis.7 The A1AT gene is expressed in cells of several lineages, with the highest expression existing in hepatocytes. Serum A1AT is almost totally derived from hepatic production, but A1AT is also actively transcribed and secreted by other cells, including neutrophils, blood monocytes, macrophages, pulmonary alveolar cells, and intestinal epithelial cells.8910
It is thought that the hepatic damage in A1AT deficiency results mainly from the retention and abnormal processing of polymerized ZA1AT in the liver.111213 In this context, efforts to increase secretion may have a therapeutic effect.14 On the other hand, the lung disease associated with A1AT deficiency is thought to result from the markedly decreased antiprotease protection in the lung due to the inadequate secretion of polymerized ZA1AT from the liver with subsequent diminution of plasma and lung A1AT levels. Little is known about the local production and secretion of A1AT in the lung. This may be important because, although the secreted protein may be expected to have some antiprotease activity, albeit reduced, there is also evidence that A1AT, if polymerized, may be proinflammatory.15 This fact has important consequences for therapy in patients with the condition. Presently, the lung disease associated with A1AT deficiency is treated with the standard therapies for COPD in addition to A1AT augmentation therapy. Augmentation therapy, by increasing the serum levels of A1AT, will in turn increase the A1AT levels on the epithelial and interstitial surfaces of the lung, thereby potentially preventing or hindering the progression of emphysema.161718 IV augmentation therapy with plasma-purified A1AT has been approved by the US Food and Drug Administration for the treatment of lung disease associated with A1AT deficiency since 1987.
In this study, we evaluate whether the local secretion of A1AT onto the surface of the lung is significant, whether Z protein in the lungs of individuals with A1AT deficiency is polymerized, and finally whether the secreted protein has a proinflammatory effect that may undermine the antiprotease effect of exogenously administered A1AT. To do this, we evaluated individuals with severe A1AT deficiency (Z homozygotes), nondeficient individuals (M homozygotes), and one individual with the Z deficiency phenotype who underwent liver transplantation. In this individual, the phenotype of the A1AT in his serum should reflect that of his transplanted liver, while any ZA1AT protein detected in his lungs would reflect local production and secretion.
| Materials and Methods |
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Determination of A1AT Levels
A1AT levels in serum and BAL fluid were determined using a double-sandwich enzyme-linked immunosorbent assay (ELISA) technique, as previously described.19 This was performed using a polyclonal antibody that was capable of detecting all forms of A1AT (1:1000 dilution of serum) as well as with a monoclonal antibody (1:100 dilution of serum) developed by Wallmark et al.20 This antibody was raised against PiZZ hepatocytic A1AT and was initially thought to be specific to the Z protein. However, some studies21 have shown that this antibody detects polymerized A1AT as well as A1AT complexed with neutrophil elastase (NE) with no apparent affinity for native, latent, or cleaved forms of A1AT. In addition, this antibody has been used routinely to identify ZA1AT-deficient serum samples, and false-positive results have never been detected.21 Fluid was derived from BAL performed according to standardized guidelines set out by Klech and Pohl.22 In brief, during fiberoptic bronchoscopy aliquots of 3 x 60 mL were injected into a division of the right middle lobe of the lung and reaspirated immediately. BAL fluid was filtered through sterile gauze and centrifuged at 1,100 revolutions per minute for 10 min. The supernatant was divided into aliquots and stored at 80°C.
Western Blot Analysis of A1AT Protein
BAL specimens were separated by 8% nondenaturing polyacrylamide gel electrophoresis (PAGE) and were transferred onto a nitrocellulose membrane. After probing with monoclonal antibody to ZA1AT diluted 1:500, antimouse IgG-alkaline phosphatase conjugate (I-Block; Promega; Madison, WI) was added (1:7500), and the blot developed with a chemiluminescent reagent (CDP-Star; Applied Biosystems; Foster City, CA).
Generation of Polymerized ZA1AT
The purification of ZA1AT was accomplished using a modification of the method described by Sugiura et al.23 Briefly, 950 mL plasma obtained from a single PIZZ individual, and verified by isoelectric focusing,24 was fractionated using ammonium sulfate. The fraction, which was obtained with an ammonium sulfate saturation of between 50% and 80%, was passed through a cibacron blue column (Sepharose F3GA-CL-4B column; Amersham Biosciences; Little Chalfont, UK). The fractions containing A1AT were then further purified by chromatography on a diethylaminoethanol column (Sephacel; Amersham Biosciences) by eluting from this column using a linear gradient from 0 to 250 mmol/L NaCl. The final purification steps involved chromatography on a 50 x 100 column (Sephacryl S 200 HP XK; Amersham Biosciences), and passage through antihuman albumin and antihuman
1-acid glycoprotein columns. The identity and purity of the ZA1AT protein was confirmed by sodium dodecyl sulfate gel electrophoresis, isoelectric focusing at pH 4 to 5, nephelometry and amino acid analysis, and protein sequencing (Harvard University Microchemistry Facility; Cambridge, MA). The Z protein was polymerized by incubation at a final concentration of 0.1 to 0.5 mg/mL in 50 mmol/L Tris (pH 7.4) and 50 mmol/L KCl at 37°C over a 10-day time-course. Polymerization was confirmed using 8% nondenaturing PAGE. Polymerized Z protein was separated into aliquots and stored at 20°C prior to its use in further experiments.
Neutrophil Isolation
Neutrophils were isolated from heparinized (10 U/mL; Sarstedt; Wexford, Ireland) venous blood, as previously described.25
Chemotaxis Assays
Neutrophil migration in response to polymerized A1AT was evaluated using wells (Costar Transwells; Corning; Rochester, NY) with polycarbonate filters and a 3-µm pore size. Briefly, stimuli were placed into the lower chamber of the well, and neutrophils were placed in the top chamber. Following chemotaxis, the wells were removed and washed in phosphate-buffered saline solution, and the nonadherent neutrophils were removed with a sterile swab. The wells then were stained (Diff-Quik; Clin-Tech; Essex, UK) and rinsed in sterile water, and the cells were counted by microscopy. The data were presented as the mean ± SE, and t tests were performed using specific software (Prism 3.0; GraphPad Software; San Diego, CA).
| Results |
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Five PiMM individuals were also studied. All were healthy nonsmokers (ie, FEV1, 100.5 ± 3% predicted; FEV1/FVC ratio, 81 ± 15% predicted; DLCO, 77 ± 2% predicted). All patients were men, with an age range of 30 to 60 years. BAL fluid analysis revealed total cell counts of 4 x 106 to 2 x 107 cells, with 0 to 2% neutrophils present.
One male patient was PiZZ (age, 64 years) and had received a liver transplant 9 years previously. He had no history of smoking and no history of significant lung disease. Spirometry revealed the following: FEV1, 83% predicted; FEV1/FVC ratio, 73% predicted; DLCO, 67% predicted. BAL fluid analysis revealed 3 x 107 cells, with 21% neutrophils present.
A1AT Levels and Western Blot Analysis of A1AT Protein
Serum levels of A1AT were obtained using a polyclonal antibody against A1AT (Fig 1)
and a monoclonal antibody, which is specific for polymerized/elastase-complexed A1AT (Fig 2
, top, A, and middle, B). As MA1AT protein does not polymerize at body temperature, it can be inferred that the A1AT detected by this antibody is either polymerized Z protein or ZA1AT or MA1AT complexed with NE. The serum levels of A1AT (Fig 1, left, A: PiZZ patients, 9.98 ± 1.72 µmol/L; PiMM patients, 66.08 ± 13.86 µmol/L; transplant patient, 54 µmol/L), which were measured using the polyclonal antibody, were as expected.23 Using the polyclonal antibody, BAL levels of A1AT were as follows: PiZZ patients, 1.412 ± 0.05 µmol/L/epithelial lining fluid (ELF); PiMM patients, 3.712 ± 0.45 µmol/L/ELF; transplant patient, 10.9 µmol/L/ELF (Fig 1, right, B). The monoclonal antibody did not detect A1AT protein in the serum of nondeficient individuals or in the transplanted individual, but did detect A1AT protein in the serum of Z individuals, suggesting that the ZA1AT protein in these individuals was either polymerized or complexed with NE (Fig 2, top, A; optical density [OD], 0.496 ± 0.007). ZA1AT was also detectable in the BAL fluid from Z individuals (OD, 0.4424 ± 0.0123) and the transplant patient (OD, 0.4672 ± 0.0137) using the monoclonal antibody (Fig 2, middle, B). However, it was not clear whether the ZA1AT protein detected by ELISA using the monoclonal antibody was polymerized or complexed with NE. Therefore, the BAL samples were separated by PAGE, and Western blot analysis of these BAL fluids was performed using the monoclonal antibody. The results show that the majority of the A1AT protein in the Z-deficient individuals (Fig 2, bottom, C, lane 2) was polymerized, not complexed to NE, due to the slow mobility of the ZA1AT on the gel (A1AT-NE complexes migrate with a similar mobility to that of monomeric A1AT).21 Likewise, there were also large amounts of polymerized A1AT in the transplant patient BAL fluid (Fig 2, bottom, C, lane 3) but not in the BAL fluid from the PiMM patient (Fig 2, bottom, C, lane 4). The presence of ZA1AT polymers in the BAL fluid from A1AT-deficient individuals has been demonstrated previously by transverse urea gradient gel methodology, and we have now confirmed this finding by ELISA and Western blot analysis.26
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| Discussion |
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How important is the presence of airway polymerized Z protein chemoattraction in vivo? Some of the Z protein in serum is polymerized. This possibly represents an opposing chemoattraction to that of the polymerized ZA1AT protein on the lung epithelial surface.21 However, although the net effect of these opposing forces is difficult to quantify, it should be noted that even PiZZ patients with near-normal lung function have high neutrophil concentrations on their respiratory epithelial surfaces.27 Previously, this increased neutrophil burden was attributed to leukotriene B4 or IL-8 release from neutrophils or epithelial cells.2829 Our findings emphasize that neutrophil accumulation in the lungs of A1AT-deficient individuals is multifactorial and that chemoattraction due to polymerized Z protein is another potential cause of this neutrophil-dominated inflammation.
The findings in this study have implications for the therapy of this disorder. In the past, it was assumed that all of the A1AT present on the epithelial or interstitial surfaces of the lung was derived from serum. It was therefore rationalized that in order to treat A1AT deficiency in the lung one had to increase serum levels of A1AT above a putative therapeutic threshold, and that this A1AT then would diffuse into the various lung compartments, protecting them against NE-mediated damage. This study shows that A1AT is also produced locally in the lung and that this production is quite significant. The Z protein, although less competent than the M protein, is an antiprotease in its native state,30 raising the possibility that this local production could be augmented to therapeutic effect. This has been attempted in the past, although with the intent of increasing the liver secretion of A1AT using typhoid vaccine, estrogen-progesterone combinations, tamoxifen, or danazol with varying degrees of success.313233 However, we have shown in this study that the ZA1AT present in the BAL fluid of the transplant patient and in that of deficient patients is not only polymerized, and thus inactive, but also could act as a potent neutrophil chemoattractant.
A more rational approach to therapy would be to attempt to inhibit the polymerization of the Z protein both intracellularly and extracellularly. This would increase the secretion of active nonpolymerized Z protein from the liver, potentially ameliorating the liver disease and, from the respiratory epithelial surface, providing anti-NE protection and avoiding the proinflammatory effects of polymerized ZA1AT. This approach is feasible as has been shown by a 2002 study,34 where a 6-mer peptide was selectively annealed to ZA1AT, preventing its polymerization without interfering with the function of other similar proteins. However, ZA1AT, even in its unpolymerized form, exhibits diminished anti-NE capacity, and such a therapeutic approach may have to be accompanied by standard augmentation therapy.
| Footnotes |
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Abbreviations: A1AT =
1-antitrypsin; DLCO = diffusing capacity of the lung for carbon monoxide; ELF = epithelial lining fluid; ELISA =enzyme-linked immunosorbent assay; IL = interleukin; NE =neutrophil elastase; OD = optical density; PAGE = poly-acrylamide gel electrophoresis
Received for publication September 3, 2003. Accepted for publication January 21, 2004.
| References |
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1-globulin pattern of serum in
1-antitrypsin deficiency. Scand J Clin Lab Invest 15,132-140[CrossRef][ISI]
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