More than 22 million individuals in the United States have asthma(42
). Of these, approximately 5% to 10% have severe disease that is difficult to control despite treatment with high doses of inhaled corticosteroids plus long-acting β2-agonists or oral corticosteroids(43
). Limited alternative treatment options exist for these individuals who are refractory to standard therapies. Additional controller medications that can be utilized as add-on therapy are limited to anti-IgE monoclonal antibodies and leukotriene modifiers, such as leukotriene receptor antagonists and 5-lipoxygenase inhibitors(42
). Therefore, new treatment options are needed for asthmatics, especially for those with severe disease who experience significant morbidity and have high health care-related costs.
Apolipoprotein A–I, a major constituent of high density lipoproteins, can prevent and reverse atherosclerosis by mediating cholesterol efflux from lipid-laden macrophages(1
). ApoA-I also attenuates inflammation in atherosclerosis by removing pro-inflammatory oxidized phospholipids from low density lipoproteins and arterial cell walls(1
). ApoA-I has also been shown to have anti-inflammatory effects on a variety of cell types that play an important role in the pathogenesis of asthma, such as dendritic cells, T cells, neutrophils and macrophages. For example, apoA-I prevents dendritic cell maturation, reduces T lymphyocyte and neutrophil activation, suppresses macrophage cytokine production, and blocks T cell-monocyte interactions(46
). Taken together, these findings suggest that the anti-inflammatory effects of apolipoprotein A–I might be utilized in a therapeutic fashion to attenuate airway inflammation in asthma.
The expense and difficulty in preparing sufficient quantities of pure, pharmaceutical quality apoA-I protein have limited the development of apoA-I as a therapeutic agent(3
). To address this problem, several apoA-I mimetic peptides that retain the beneficial effects of apoA-I and HDL on cholesterol efflux and atherosclerosis have been developed(1
). Consistent with this, administration of apoA-I mimetic peptides have been shown to have anti-inflammatory effects in models of atherosclerosis and cardiac ischemia-reperfusion injury, as well as to attenuate endothelial dysfunction(52
). Furthermore, apoA-I mimetic peptides have demonstrated anti-inflammatory properties in murine models of viral infection and collagen-induced arthritis(57
Since airway inflammation plays a major role in the pathogenesis of asthma, we assessed whether administration of an apoA-I mimetic peptide could suppress inflammatory and immune responses in a HDM-challenge model of asthma. We utilized the 5A apoA-I mimetic peptide, which is a bihelical amphipathic peptide that mediates cholesterol efflux and reduces atherosclerosis via the ABCA1 transporter(3
). Each helix is comprised of 18 amino acids linked by a proline(5
). In contrast to other apoA-I mimetic peptides that are cytotoxic based upon their ability to insert into cell membranes and disrupt the lipid bilayer, the 5A peptide does not induce hemolysis of red blood cells(5
). Here, we demonstrate that the 5A apoA-I mimetic peptide dramatically inhibits the induction of many of the key pathologic features of house dust mite-induced asthma, including airway inflammation and airway hyperreactivity. The 5A apoA-I mimetic peptide also reduced the severity of several key manifestations of airway remodeling, such as goblet cell hyperplasia and expression of the MUC5AC mucin gene expression and genes encoding type I and type III collagens. The ability of the 5A apoA-I mimetic peptide to inhibit airway inflammation was mediated by multiple mechanisms that included both the attenuated expression of Th2- and Th17-type cytokines, as well as the reduced expression of chemokines that promote the chemotaxis of T cells, dendritic cells, and eosinophils. Furthermore, the 5A apoA-I mimetic peptide inhibited the recruitment of alternatively activated macrophages to the lungs of HDM-challenged mice. In contrast, the 5A apoA-I mimetic peptide did not inhibit HDM-induced increases in serum IgE levels, which is consistent with the conclusion that the mechanism by which 5A attenuates asthma is not a consequence of impaired allergic sensitization.
The mechanism by which the 5A peptide mediates its inhibitory effects on the induction of asthma may be mediated by its interaction with the ATP-binding cassette (ABC) transporter A1 (ABCA1). Consistent with this, the 5A peptide has been shown to mediate enhanced lipid efflux from HeLa cells, as well as inhibit TNF-mediated NF-κB activation in vascular endothelial cells, in an ABCA1-dependent fashion(5
). ABCA1 is expressed by several cell types in the lung, including airway smooth muscle cells, type I and type II pneumocytes, and pulmonary macrophages(63
). ABCA1 plays an important role in the maintenance of normal lung lipid composition, structure and function, as evidenced by a phenotype of cholesterol accumulation and alveolar proteinosis in ABCA1 knockout mice(64
). An alternative mechanism by which the 5A apoA-I mimetic peptide may mediate its effects is via binding to pro-inflammatory proteins and lipids. For example, apoA-I can associate with lipopolysaccharide binding protein (LBP) and thereby allow HDL to neutralize bacterial lipopolysaccharides(69
). An additional possibility is that the 5A peptide may interact with other apolipoprotein receptors that recognize apolipoprotein ligands containing amphipathic helical structures.
It is important to address several points regarding our study. First, the effects of the 5A peptide were accomplished at a dose of 1 mg/kg/day, which is significantly lower than the 30 mg/kg dose that has been utilized to promote reverse cholesterol transport in a murine model of atherosclerosis(59
). Second, our model utilized a 4 week period of exposure to house dust mite to assess the effect of the 5A peptide on several key manifestations of airway remodeling. Additional experiments using models with longer periods of exposure to house dust mite could also be utilized to characterize the effects of the 5A peptide on additional manifestations of airway remodeling, such as angiogenesis(60
). Third, we utilized an invasive measurement of airway resistance to determine the effects of the 5A peptide on airway hyperreactivity, rather than a non-invasive method, such as unrestrained plethysmography, which may not directly correlate with changes in airway resistance(61
). Lastly, although our study was not designed to assess toxicity related to administration of the 5A peptide, no untoward effects were noted.
In summary, we have shown that administration of a 5A apoA-I mimetic peptide attenuates the induction of many of the key pathogenic features of house dust mite-induced asthma, including airway inflammation and airway hyperreactivity. These results identify apoA-I mimetic peptides, such as 5A, as a novel therapeutic strategy that could be developed to treat asthmatic patients who do not respond to standard therapies, such as those with severe asthma.