Fig 3.

Neutralization of NDV by complement proceeds by C3 deposition. (A) Purified NDV/egg was incubated with NHS and overlaid on a 10 to 26% Opti-Prep gradient, and fractions were collected from the bottom following density gradient centrifugation. Virus-only and virus-NHS (Virus+NHS) fractions were immunoblotted with NDV polyclonal sera: bands specific for the NDV fusion (F) and matrix (M) protein were detected. Virus-NHS fractions were also immunoblotted for the C3 protein. NHS-only fractions were also probed for C3. The positions of C3bα′ and C3bβ are indicated. (B) Purified NDV/egg particles were treated with 1:25-diluted C8-depleted serum. C3 deposition was detected with a primary C3 antibody and secondary 12-nm colloidal gold-conjugated species-specific antibody. Transmission electron microscopic analysis show colloidal gold binding to NHS-treated particles NHS-2°(), whereas no binding is visible on the control treated with only secondary (2° only) antibody. Scale bars, 0.1 μm.