Four or five days after RSV challenge, subsets of mice were euthanized and the lungs were removed into Eagle's modified essential medium. The tissues were then processed through a steel screen to obtain a single-cell suspension, and particulate matter was removed by passage through a 70-μm cell strainer (BD Labware, Franklin Lakes, NJ). The supernatants were collected, and RSV titers in the supernatants were measured by standard plaque assay on subconfluent HEp-2 monolayers. The data are expressed as the PFU per gram of lung tissue.