FIG. 5.
Protective responses after VLP immunization. (A) Neutralizing antibody titers in sera preboost and postboost from mice immunized with VLP-H/G+F/F (the initial immunization contained 6.4 μg F protein, the boost contained 2 μg F protein) or infectious RSV (6 × 105 PFU for both initial and boost immunizations). Aliquots of sera from mice at day 35 were pooled for preboost titers. Mice were boosted at day 43, and aliquots of sera harvested at day 56 were pooled for postboost titers. Neutralizing antibody titers, log2 of the dilution resulting in 60% reduction in virus titer, were determined as described in Materials and Methods. Results shown are the average of duplicate plaque assays and are representative of results of two separate mouse immunization experiments. (B) Virus titers in lungs of mice at 4 days postchallenge with infectious RSV. Mice were immunized with VLP-H/G+F/F (6 μg F protein/mouse) i.m. or infectious RSV (6 × 105 PFU/mouse) i.n. Two other sets of mice were not immunized. At day 52, mice were challenged with RSV (1.5 × 106 PFU/mouse). Four days postchallenge, lungs were harvested and virus titers were determined by plaque assay. One set of mice was not immunized or challenged and represents the negative control.