Antibody Opsonization Enhances Early Interactions between Yersinia pestis and Neutrophils in the Skin and Draining Lymph Node in a Mouse Model of Bubonic Plague

Characterization and CRISPR/Cas9-mediated genetic manipulation of neutrophils derived from Hoxb8-ER-immortalized myeloid progenitors

Neutrophils represent a first line of defense against a wide variety of microbial pathogens. Transduction with an estrogen receptor-Hoxb8 transcription factor fusion construct conditionally immortalizes myeloid progenitor cells (NeutPro) capable of differentiation into neutrophils. This system has been very useful for generating large numbers of murine neutrophils for in vitro and in vivo studies. However, some questions remain as to how closely neutrophils derived from these immortalized progenitors reflect primary neutrophils. Here we describe our experience with NeutPro-derived neutrophils as it relates to our studies of Yersinia pestis pathogenesis. NeutPro neutrophils have circular or multilobed nuclei, similar to primary bone marrow neutrophils. Differentiation of neutrophils from NeutPro cells leads to increased expression of CD11b, GR1, CD62L, and Ly6G. However, the NeutPro neutrophils expressed lower levels of Ly6G than bone marrow neutrophils. NeutPro neutrophils produced reactive oxygen species at slightly lower levels than bone marrow neutrophils, and the 2 cell types phagocytosed and killed Y. pestis in vitro to a similar degree. To further demonstrate their utility, we used a nonviral method for nuclear delivery of CRISPR/Cas9 guide RNA complexes to delete genes of interest in NeutPro cells. In summary, we have found these cells to be morphologically and functionally equivalent to primary neutrophils and useful for in vitro assays related to studies of bacterial pathogenesis.

Yersiniabactin contributes to overcoming zinc restriction during Yersinia pestis infection of mammalian and insect hosts

Yersinia pestis causes human plague and colonizes both a mammalian host and a flea vector during its transmission cycle. A key barrier to bacterial infection is the host's ability to actively sequester key biometals (e.g., iron, zinc, and manganese) required for bacterial growth. This is referred to as nutritional immunity. Mechanisms to overcome nutritional immunity are essential virulence factors for bacterial pathogens. Y. pestis produces an iron-scavenging siderophore called yersiniabactin (Ybt) that is required to overcome iron-mediated nutritional immunity and cause lethal infection. Recently, Ybt has been shown to bind to zinc, and in the absence of the zinc transporter ZnuABC, Ybt improves Y. pestis growth in zinc-limited medium. These data suggest that, in addition to iron acquisition, Ybt may also contribute to overcoming zinc-mediated nutritional immunity. To test this hypothesis, we used a mouse model defective in iron-mediated nutritional immunity to demonstrate that Ybt contributes to virulence in an iron-independent manner. Furthermore, using a combination of bacterial mutants and mice defective in zinc-mediated nutritional immunity, we identified calprotectin as the primary barrier for Y. pestis to acquire zinc during infection and that Y. pestis uses Ybt to compete with calprotectin for zinc. Finally, we discovered that Y. pestis encounters zinc limitation within the flea midgut, and Ybt contributes to overcoming this limitation. Together, these results demonstrate that Ybt is a bona fide zinc acquisition mechanism used by Y. pestis to surmount zinc limitation during the infection of both the mammalian and insect hosts.