Permissiveness of guinea pig alveolar macrophage subpopulations to acute respiratory syncytial virus infection in vitro

Background and objectives: Alveolar macrophages (AMs) are targets for respi ratory synctial virus (RSV) infection ii vice and Ira vitro. However, only a minority of AMs are permissive to acute RSV infection ii vitro, and it is unknown whether this permissiveness may be related to the degree of cellul ar maturation that is achieved in vivo, Methods: By using density gradient centrifugation, in which the degree of A M maturation is inversely related to buoyant density, we prepared three sub populations of guinea pig AMs (designated as hypodense, intermediate-densit y, and high-density AMs), Twenty-four hours after exposure to RSV in vitro, the percentage of RSV-positive cells in each subpopulation was determined by immunocytochemistry intracellular,virus was released from cells by sonic ation and quantified by plaque assay, and intracellular localization of RSV proteins was evaluated by immunogold electron microscopy, Results: High-density AM Is had a significantly higher proportion of RSV-po sitive cells than hypodense AMs (p < 0.001), with intermediate-density AMs having intermediate values. The amounts of intracellular virus significantl y increased from hypodense to intermediate density to high-density AMs (p < 0.001), Hypodense cells showed immunogold labeling principally within phag olysosomes, whereas intermediate-density and high-density cells showed immu nolabeling of free cytoplasmic viral proteins and nucleocapsids, Conclusions: The permissiveness of guinea pig AMs to acute RSV infection in vitro is inversely related to their degree of maturation achieved in vivo, In addition, these results suggest that immature, high-density AMs support RSV replication whereas more mature, hypodense AMs may restrict viral repl ication.