A three-dimensional (3-D) lung aggregate model was developed from A549 human lung epithelial cells by using a rotating-wall vessel bioreactor to study the interactions between Pseudomonas aeruginosa and lung epithelial cells. The suitability of the 3-D aggregates as an infection model was examined by immunohistochemistry, adherence and invasion assays, scanning electron microscopy, and cytokine and mucoglycoprotein production. Immunohistochemical characterization of the 3-D A549 aggregates showed increased expression of epithelial cell-specific markers and decreased expression of cancer-specific markers compared to their monolayer counterparts. Immunohistochemistry of junctional markers on A549 3-D cells revealed that these cells formed tight junctions and polarity, in contrast to the cells grown as monolayers. Additionally, the 3-D aggregates stained positively for the production of mucoglycoprotein while the monolayers showed no indication of staining. Moreover, mucin-specific antibodies to MUC1 and MUC5A bound with greater affinity to 3-D aggregates than to the monolayers. P. aeruginosa attached to and penetrated A549 monolayers significantly more than the same cells grown as 3-D aggregates. Scanning electron microscopy of A549 cells grown as monolayers and 3-D aggregates infected with P. aeruginosa showed that monolayers detached from the surface of the culture plate postinfection, in contrast to the 3-D aggregates, which remained attached to the microcarrier beads. In response to infection, proinflammatory cytokine levels were elevated for the 3-D A549 aggregates compared to monolayer controls. These findings suggest that A549 lung cells grown as 3-D aggregates may represent a more physiologically relevant model to examine the interactions between P. aeruginosa and the lung epithelium during infection.Cell culture models have been widely used to study the infectious process of Pseudomonas aeruginosa. The most frequently used in vitro model of lung epithelia is the monolayer, where cells are grown on flat plastic surfaces. While this system has provided important insight into the fundamentals of hostpathogen interactions, it is not without limitations. Studies show that when cells are removed from their host tissue and grown as monolayers on impermeable surfaces, they undergo dedifferentiation and lose specialized functions (13). This is thought to be, in part, the result of the disassociation of cells from their native three-dimensional (3-D) tissue structure in vivo to their two-dimensional propagation as monolayers on plastic surfaces (13,43). Given the inherent limitations of conventional monolayers, the availability of models preserving properties of in vivo tissues that are easily manipulated would benefit the exploration of host-pathogen interactions.The rotating-wall vessel (RWV) bioreactor (Fig. 1) is an optimized suspension cell culture technology designed for growing 3-D cells under conditions that promote many of the specialized features of in vivo tissues (16,38,43). The principal design feature...