In Vitro Intracellular Trafficking of Virulence Antigen during Infection by Yersinia pestis

Yersinia pestis, the causative agent of plague, encodes several essential virulence factors on a 70 kb plasmid, including the Yersinia outer proteins (Yops) and a multifunctional virulence antigen (V). V is uniquely able to inhibit the host immune response; aid in the expression, secretion, and injection of the cytotoxic Yops via a type III secretion system (T3SS)-dependent mechanism; be secreted extracellularly; and enter the host cell by a T3SS-independent mechanism, where its activity is unknown. To elucidate the intracellular trafficking and target(s) of V, time-course experiments were performed with macrophages (MΦs) infected with Y. pestis or Y. pseudotuberculosis at intervals from 5 min to 6 h. The trafficking pattern was discerned from results of parallel microscopy, immunoblotting, and flow cytometry experiments. The MΦs were incubated with fluorescent or gold conjugated primary or secondary anti-V (antibodies [Abs]) in conjunction with organelle-associated Abs or dyes. The samples were observed for co-localization by immuno-fluorescence and electron microscopy. For fractionation studies, uninfected and infected MΦs were lysed and subjected to density gradient centrifugation coupled with immunoblotting with Abs to V or to organelles. Samples were also analyzed by flow cytometry after lysis and dual-staining with anti-V and anti-organelle Abs. Our findings indicate a co-localization of V with (1) endosomal proteins between 10–45 min of infection, (2) lysosomal protein(s) between 1–2 h of infection, (3) mitochondrial proteins between 2.5–3 h infection, and (4) Golgi protein(s) between 4–6 h of infection. Further studies are being performed to determine the specific intracellular interactions and role in pathogenesis of intracellularly localized V.