Background and objectives: Alveolar macrophages (AMs) are targets for respiratory syncytial virus (RSV) infection in vivo and in vitro. However, only a minority of AMs are permissive to acute RSV infection in vitro, and it is unknown whether this permissiveness may be related to the degree of cellular maturation that is achieved in vivo.
Methods: By using density gradient centrifugation, in which the degree of AM maturation is inversely related to buoyant density, we prepared three subpopulations of guinea pig AMs (designated as hypodense, intermediate-density, 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 sonication and quantified by plaque assay, and intracellular localization of RSV proteins was evaluated by immunogold electron microscopy.
Results: High-density AMs had a significantly higher proportion of RSV-positive cells than hypodense AMs (p < 0.001), with intermediate-density AMs having intermediate values. The amounts of intracellular virus significantly increased from hypodense to intermediate density to high-density AMs (p < 0.001). Hypodense cells showed immunogold labeling principally within phagolysosomes, whereas intermediate-density and high-density cells showed immunolabeling 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 replication.