Why do protease inhibitors enhance leucocyte migration inhibition to the antigen PPD?

Epsilon amino caproic acid (EACA) and Aprotinin significantly increase the inhibition to PPD in the leucocyte migration test (Burden, Stacey, Wood & Bell, 1978). This is not due to increased leucocyte inhibition factor (LIF) generation. The proteolytic inhibitors prevent the loss of activity of LIF due to prolonged incubation in the presence of serum. Both have a significant effect on the target cells (Polymorphonuclear leucocytes) used in the assay to exaggerate the migration inhibition to LIF. These actions can be explained by decreased proteolytic breakdown of LIF.