FIG. 1.

Expression time course of WNV prME protein in a stably transfected Drosophila S2 cell line. Cells were transfected with the insertless parent vector pMT/BiP/V5HisA (filled squares), a vector expressing green fluorescent protein (open diamonds), pMT/Bip/WNVpreME/V5His (filled circles), or pMT/Bip/WNVpreME (filled triangles) along with the selection vector pCoHYGRO and placed under selection for 19 days. One-sixth of the cells were transferred to a 25-cm² flask and grown in selective medium at 28°C containing an inducer (333 μM CuSO₄). Cell culture medium (0.5 ml) was harvested at various times after the addition of the inducer, and the presence of WNV antigen was detected by an antigen capture EIA following a 1:4 dilution into sample buffer. The total cell density was measured at 1, 2, 6, and 20 days postinduction (open squares). Only those cells transfected with WNV expression constructs produced detectable antigen in the culture medium.