Yersinia pestis is the causative agent of plague in humans. We have discovered a chromosomally located bicistronic operon that encodes a novel pair of interacting surface proteins called YadB and YadC (collectively referred to as YadB-C) that belong to the trimeric autotransporter family. YadB-C promotes invasion of epithelioid cells and J774A.1 macrophage-like cells;but unlike most trimeric autotransporters appears not to promote adherence. Interestingly, invasive activity requires the presence of the surface aspartyl protease Pla, which also is an adhesin/invasin, independent of its protease activity. In addition, Pla selectively degrades YadB, causing YadB-C complexes to disappear, and its presence is required for release of a fragment of YadC into the culture medium. Like Pla, YadB-C is crucial for virulence in bubonic plague;however, it is not essential for virulence in pneumonic plague. This raises the possibility that the dominant virulence function of these proteins is directed against acute inflammation, which occurs rapidly in bubonic plague but only late in pneumonic plague. The small plasmid that encodes Pla is unique to Y. pestis, and its acquisition is believed to have been a key step in the evolution of Y. pestis from Y. pseudotuberculosis. Thus yadBC may be another link to the high virulence of Y. pestis. We hypothesize that YadB, YadC, and Pla constitute a novel ternary virulence mechanism to counteract inflammatory defenses that are mobilized early in bubonic plague. The goal of this proposed R21 project is to define the essential roles of the 3 components of this system. In Aim 1 we will exploit a set of Y. pestis strains carrying variants of these components to determine the roles of Pla, YadC, and YadB in YadB-C function in vitro. In Aim 2, we will characterize infections of normal mice and mice lacking key inflammatory cells to develop a hypothesis for the innate defense target of YadB-C. PUBLIC HEALTH RELEVANCE: These studies will contribute to our broad understanding of pathogenesis in the face of an acute inflammatory response. Accordingly, the study of YadB-C will inform studies in many other bacterial systems in addition to improving our understanding of the pathogenesis of plague.