Insect cell stimulation by LPS requires the activity of cell-released proteases

The hemocyte line BTI-EA-1174-A from the lepidopteran insect Estigmene acraea responds to bacterial lipopolysaccharide (LPS) by an enhanced phagocytic reaction and a dose-dependent increase of lysozyme release [Wittwer et al., Dev Comp Immunol 21:323 (1997)]. This paper provides evidence for a strong proteolytic activity in cell culture supernatants occurring after addition of LPS (1 mg/ml). The proteolysis is caused by cell-released proteases and seems to be necessary for cell activation. Its inhibition by alpha 2-macroglobulin results in a dose-dependent reduction in cellular response strength. Phagocytic reactions, as well as lysozyme release, are lowered to about half in the presence of 0.0001 mg/ml alpha 2-macroglobulin. A nearly complete abolishment of activation was achieved with final concentrations of 1.0 mg/ml alpha 2-macroglobulin. The data presented allow us to conclude that the LPS-triggered proteolytic activity is an important part of the activation process; it occurs outside of the cells and delivers immune response activating factors.