The Role of Glycosylated Alpha-1 Antitrypsin in Resolution of Inflammation
Alpha-1 antitrypsin (AAT) is a 52 kDa, glycoprotein, whose main function is to inhibit neutrophil elastase (NE), maintaining the protease/anti-protease balance in the lung. However, this acute phase protein has been shown to possess immuno-regulatory properties, independent of this anti-protease activity. The glycans of AAT alter during community acquired pneumonia, due to increased levels of sialic acid (rpAAT). The aim of this study was to determine the cause and consequence of rpAAT production.
The cause of rpAAT production was investigated in hepatocytes, and it was observed that interleukin (IL-) 6 induced increased protein expression of beta-galactoside alpha-2,6-sialyltransferase 1 (ST6GAL1), an enzyme responsible for the addition of sialic acid. Additionally, it was discovered that IL-10 causes a decrease in the protein expression of ST6GAL1. As rpAAT is produced during resolution of inflammation, the consequence of rpAAT production was investigated. It was uncovered that rpAAT binds a higher level of chemokines than the less sialylated AAT, as IL-8 and neutrophil activating peptide 2 (NAP-2), were bound more strongly to rpAAT. These are drivers of neutrophil activation and migration. A key finding of this study was that by binding these chemokines, rpAAT decreases the ability of IL-8 and NAP-2 to engage with the CXCR receptors on the neutrophil membrane. Furthermore, rpAAT reduced both neutrophil migration and degranulation in response to IL-8 and NAP-2 to a greater extent than less sialylated AAT. However, rpAAT can be overwhelmed in excessive levels of inflammation.
rpAAT is produced during resolution of acute inflammation indicating that rpAAT is produced to aid resolution. Our findings add to our understanding of AAT, revealing a natural way by which the body acts to control excessive inflammation. This knowledge has implications for the use of AAT as a therapy to treat conditions dominated by neutrophil driven inflammation.