Inter-alpha-inhibitor as marker for neutrophil proteinase activity: an in vitro investigation

Human neutrophil proteinases have been implicated in the pathogenesis of a wide variety of inflammatory diseases. The degradation of plasma proteins such as coagulation and fibrinolysis factors has been attributed to the excessive release of elastase in septicemia and in other conditions in which heightened proteolysis occurs. Inter-alpha-inhibitor (IalphaI) is particularly sensitive to cleavage by leukocyte proteinases. For this reason, the determination of IalphaI has been proposed as a method for evaluating plasma protein proteolysis by neutrophil enzymes. In this article we provide evidence that intact residual IalphaI can be accurately quantified by enzyme-linked immunosorbent assay (ELISA) determination without interference from fragments released from IalphaI by incubation with triggered neutrophils. We demonstrate that under these conditions IalphaI was quickly and steadily proteolyzed in a cell dose-dependent manner. Alpha-1 proteinase inhibitor (alpha1PI) partially protected IalphaI; however, the proteolysis persisted when IalphaI was incubated with stimulated neutrophils in the presence of a large relative excess of alpha1PI over the amount of elastase theoretically present in cells. For the same amount of alpha1PI, serum provided a better protection than alpha1PI alone but did not completely inhibit the IalphaI degradation. Therefore, ELISA determination of IalphaI might be useful for monitoring the in vivo activity of neutrophil proteinases in systemic proteolytic states.