Abstract
BackgroundAlpha 1-antitrypsin (A1AT) is a 52 kDa serine protease inhibitor produced largely by hepatocytes but also by mononuclear phagocytes. A1AT chiefly inhibits neutrophil elastase and proteinase-3 but has also been reported to have immune modulatory functions including the ability to inhibit caspases. Its clinical availability for infusion suggests that A1AT therapy might modulate caspase related inflammation. Here we tested the ability of A1AT to modulate caspase-1 function in human mononuclear phagocytes.MethodsPurified plasma derived A1AT was added to active caspase-1 in a cell-free system (THP-1 lysates) as well as added exogenously to cell-culture models and human whole blood models of caspase-1 activation. Functional caspase-1 activity was quantified by the cleavage of the caspase-1 specific fluorogenic tetrapeptide substrate (WEHD-afc) and the release of processed IL-18 and IL-1β.ResultsTHP-1 cell lysates generated spontaneous activation of caspase-1 both by WEHD-afc cleavage and the generation of p20 caspase-1. A1AT added to this cell free system was unable to inhibit caspase-1 activity. Release of processed IL-18 by THP-1 cells was also unaffected by the addition of exogenous A1AT prior to stimulation with LPS/ATP, a standard caspase-1 activating signal. Importantly, the A1AT exhibited potent neutrophil elastase inhibitory capacity. Furthermore, A1AT complexed to NE (and hence conformationally modified) also did not affect THP-1 cell caspase-1 activation. Finally, exogenous A1AT did not inhibit the ability of human whole blood samples to process and release IL-1β.ConclusionsA1AT does not inhibit human monocyte caspase-1.
Highlights
Alpha 1-antitrypsin (A1AT) is an acute phase glycoprotein primarily synthesized and secreted by hepatocytes [1]
Release of processed IL-18 by THP-1 cells was unaffected by the addition of exogenous A1AT prior to stimulation with LPS/ATP, a standard caspase-1 activating signal
The A1AT exhibited potent neutrophil elastase inhibitory capacity
Summary
Alpha 1-antitrypsin (A1AT) is an acute phase glycoprotein primarily synthesized and secreted by hepatocytes [1]. A1AT is actively transcribed and secreted in relatively smaller amounts by cells including neutrophils, mononuclear phagocytes, enterocytes [4,5], and human respiratory epithelial cells [6]. It has been reported that A1AT can rescue serum withdrawal-induced apoptosis [7] and inhibition of structural alveolar cell apoptosis in emphysema [8]. Recent studies have suggested that A1AT directly inhibits caspase-1 activation [9,10]. Alpha 1-antitrypsin (A1AT) is a 52 kDa serine protease inhibitor produced largely by hepatocytes and by mononuclear phagocytes. A1AT inhibits neutrophil elastase and proteinase-3 but has been reported to have immune modulatory functions including the ability to inhibit caspases. We tested the ability of A1AT to modulate caspase-1 function in human mononuclear phagocytes
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