FIG. 3.

Identification of VLP surface loops important for neutralizing activity in human sera by titration of VLPs. Wild-type and hybrid VLPs were titrated and incubated with one of four human sera overnight. These samples were then tested for residual activity in a pseudovirus neutralization assay. Lower optical density readings indicated that neutralization activity was not adsorbed by pretreatment with VLPs. Higher optical density readings indicated that neutralization activity was adsorbed or that there was no neutralizing activity in the serum. Serum A was known to be nonreactive with HPV16 VLPs (not shown). A control for each experiment was serum that was not pretreated with VLPs and that was used to neutralize pseudovirus infection (on the right side of each graph). The symbols represent the same VLP preparations as in Fig. 2.