Detection of lysozyme-like enzymatic activity secreted by an immune-responsive mosquito cell line

Using molecular approaches, we have recently shown that the C7-10 mosquito cell line from Aedes albopictus, and the Aag-2 line from Aedes aegypti, secrete a variety of immune peptides into the culture medium, including cecropins, defensins, transferrin, and lysozyme. The diversity of these peptides makes it difficult to quantify the relative activities of each molecule, because possible synergistic interactions may occur. Using a microtiter plate assay with live bacteria, we now show that C7-10 cells secrete an activity that is more potent against the Gram-positive bacterium, Micrococcus luteus, than against Gram-negative Escherichia coli. This lysozyme-like activity is accompanied by production of a lyric zone in an agarose plate assay containing commercially available, lyophilized M. luteus. Properties of the lysozyme-like activity from C7-10 cells included a broad pH optimum from 5.5 to 6.5, and heat-sensitivity above 42°C. Amounts of secreted activity increased during the initial 24 h of incubation with heat-killed bacteria. During this induction, lysozyme-like activity was found primarily in the cell culture supernatant.