Effects of Psa and F1 on the Adhesive and Invasive Interactions of Yersinia pestis with Human Respiratory Tract Epithelial Cells

Since the last plague pandemic at the end of the 19th century, its bacterial agent, Yersinia pestis, has been maintained in rodents in several Asian, African, and American countries, including the United States (17, 40). Bubonic plague results from the transmission of Y. pestis by flea bites. In contrast, primary pneumonic plague is acquired when a mammalian host inhales particles or aerosols carrying Y. pestis. Although plague is currently not a major public health problem in developed countries and has been suggested to be less contagious than commonly believed (32), the spread of Y. pestis by aerosols could cause a cluster of human cases of primary pneumonic plague with potential amplification of the outbreak (27). The major adhesins and invasins of enteropathogenic Yersinia pseudotuberculosis and Yersinia enterocolitica (YadA, Ail, and Inv) are not expressed by Y. pestis strains (15, 45, 51). Thus, how Y. pestis attaches to and translocates through the epithelial layer of the respiratory tract to reach deeper tissues and the bloodstream following airborne transmission remains unknown. Interestingly, Y. pestis exhibits an extensive extracellular lifestyle due to the intracellular delivery of several antiphagocytic effector proteins by its type III secretion system (T3SS-1 or Yops regulon) (5-7, 14, 54, 56). Moreover, two antigenic surface structures exported by usher-chaperone proteins characteristic of fimbrial biogenesis systems have been shown to inhibit Y. pestis uptake by macrophages (19, 26). Each of these structures, designated capsular antigen fraction 1 (F1) and pH 6 antigen (Psa), consists of the homopolymeric association of a single-subunit protein (Caf1 and PsaA, respectively) on the bacterial surface (36, 59). PsaA can assemble into distinct thin individual fimbrial strands, bundles of fimbriae associated along their lengths, or structureless aggregates, as shown by electron microscopy (12, 36). The F1 capsular antigen is made of linear fibers of Caf1 subunits that cannot be detected as fimbrial organelles (59), not unlike the Dr adhesins that were designated afimbrial adhesins (2). Although the expression of redundant antiphagocytic virulence factors suggests that Y. pestis remains and replicates mainly extracellularly, Y. pestis has been reported to invade human epithelial HeLa cells (16) and to be phagocytosed by macrophages, where it can replicate (9, 11, 31, 36, 37, 41, 52). It has been suggested that intracellular survival and replication occur mainly during the early stages of host colonization and invasion (42). Phagocytosed Y. pestis isolates have also been found to have cytotoxic and apoptotic effects on macrophages (22, 60). Cultured epithelial cell lines have been used previously to investigate the epithelioadhesive property of Y. pestis and the involvement of Psa (16, 58). Here, we determined whether Y. pestis adheres to human respiratory tract epithelial cells and possibly penetrates these cells. Whether and how the F1 and Psa surface structures modulate such interactions were the focus of this study.