Activated platelets inhibit neutrophil elastase mainly through SLPI. (A) Thrombin-stimulated platelets were incubated with a synthetic elastase substrate and purified human neutrophil elastase. Enzyme activity, measured as the linear rate of increase in A₄₁₀, was measured spectrophotometrically after inclusion of serial 2-fold dilutions of suspensions of activated wild-type platelets. Elastase activity was inhibited in proportion to the platelet concentration, whereas resting platelets lacked this activity (data not shown). (B) Platelets from SLPI^-/- or β1 tubulin^-/- mice consistently showed reduced inhibition of in vitro elastase activity. The data shown are representative of 5 independent experiments with platelets pooled from variable numbers of mice to ensure equal input numbers.