Alpha 1-Antitrypsin Does Not Inhibit Human Monocyte Caspase-1

BACKGROUND: Alpha 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. METHODS: Purified 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. RESULTS: THP-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. CONCLUSIONS: A1AT does not inhibit human monocyte caspase-1.