Substrate hydrolysis by immune complex-activated neutrophils: effect of physical presentation of complexes and...

The ability of rat leukocytes to hydrolyze a radiolabeled, surface-bound protein substrate in a solid phase assay was determined, and various factors that influence the process were measured. Unstimulated leukocytes hydrolyzed very little substrate. When the cell suspension was mixed with zymosan particles or incubated with preformed immune complexes, the amount of substrate hydrolysis increased dramatically. Not surprisingly, immune complexes at equivalence proved to be the most effective in eliciting the response. Immune complexes attached to the surface along with the protein substrate were able to effectively induce hydrolysis, though they were not as effective as immune complexes in suspension. Three protease inhibitors, alpha 1-antitrypsin, alpha 2-macroglobulin, and soybean trypsin inhibitor, which were able to neutralize nearly all of the protease activity in rat neutrophil lysates, were tested for their ability to inhibit immune complex-induced protein hydrolysis. It was found that when the inhibitors were surface bound along with the substrate protein, they were effective in preventing the neutrophils from hydrolyzing the protein. However, when the same inhibitors were present in the fluid phase, they were much less effective. The relative ineffectiveness of fluid phase protease inhibitors to block the protease activity of contact-activated leukocytes may explain how immune complex injury can take place in the presence of high concentrations of serum inhibitors.