Francisella tularensis, the causative agent of tularemia, is a highly virulent organism, especially when exposure occurs by inhalation. Recent data suggest that Francisella interacts directly with alveolar epithelial cells. Although F. tularensis causes septicemia and can live extracellularly in a murine infection model, there is little information about the role of the vascular endothelium in the host response. We hypothesized that F. tularensis would interact with pulmonary endothelial cells as a prerequisite to the clinically observed recruitment of neutrophils to the lung. Using an in vitro Transwell model system, we studied interactions between live F. tularensis LVS (Ft LVS) and a pulmonary microvascular endothelial cell (PMVEC) monolayer. Organisms invaded the endothelium and were visualized within individual endothelial cells using confocal microscopy. Although these bacterial-endothelial cell interactions did not elicit production of the proinflammatory chemokines, polymorphonuclear leukocytes (PMN) were stimulated to transmigrate across the endothelium in response to Ft LVS. Moreover, transendothelial migration altered the phenotype of recruited PMN with reduced capacity to activate the NADPH oxidase and to release elastase in response to subsequent stimulation, as compared to PMN that traversed PMVEC in response to Streptococcus pneumoniae. The blunting of PMN responsiveness required PMN transendothelial migration, but did not require PMN uptake of Ft LVS, was not dependent on the presence of serum-derived factors, and was not reproduced by Ft LVS-conditioned medium. We speculate that the capacity of Ft LVS-stimulated PMVEC to support transendothelial migration of PMN without triggering release of IL-8 and MCP-1 and to suppress the responsiveness of transmigrated PMN to subsequent stimulation could contribute to the dramatic virulence seen during inhalational challenge with Francisella.