(A). Serial dilutions of E16 (mouse IgG2b) were mixed with PBS, 5% fresh or heat-inactivated mouse serum, incubated with WNV RVP, and added to FcγRIIa+ K562 cells. Forty-eight hours later, cells were analyzed by flow cytometry for GFP expression. The data are expressed as the fold enhancement of infection compared to no antibody. (B). Experiments were performed as in panel (A) except that fresh C1q−/− or C3−/− serum or purified C1q was mixed with the mouse E16 mAb. (C, D). Experiments were performed as in panel (B) except the epitope matched DIII-specific (C) E24 (mouse IgG2a) or (D) E34 (mouse IgG1) mAbs were used. Adapted from (126).