Gal-positive PERV is sensitive to inactivation by purified NAb (A) and human serum (B). The sensitivities of Gal-positive (filled symbols) and Gal-null (open symbols) PERV were assessed using cell-free LacZ pseudotype infection of Gal-null ST-IOWA−/− cells. (C) The infectivity of Gal-positive PERV-A is reduced, but not eliminated, following exposure to NAb. Replication-competent PERV-A was produced in either wild-type ST-IOWA+/+ cells (○, •), Gal-null ST-IOWA−/− cells (▪, □), or 293 cells (▴, ▵) and treated with either NAb at 50 μg/ml (open symbols) or fetal bovine serum (filled symbols). The treated supernatant was used to infect 293 cells, and infection was monitored by RT assay.