Browsing by Subject "Yersinia pestis"

Lymph nodes are organs of efficiency. Once activated, they essentially function to optimize and accelerate the production of the adaptive immune response, which has the potential to determine survival of the host during an initial infection and protect against repeated infections, should specific and appropriate immunological memory be sufficiently induced. We now have an understanding of the fundamental structure of lymph nodes and many of the interactions that occur within them throughout this process. Yet, lymph nodes are dynamic and malleable organs and much remains to be investigated with regards to their responses to various types of challenges. In this work, we examined multiple inflammatory scenarios and sought to understand the complex ways that lymph nodes can be externally targeted to impact immunity. First, we outline a novel mechanism of cellular communication, where cytokine messages from the periphery are delivered to draining lymph nodes during inflammation. These signals are sent as particles, released by mast cells, and demonstrate the ability of the infected tissue to communicate to lymph nodes and shape their responses. Based on these interactions, we also explored the ability to therapeutically or prophylactically modulate lymph node function, using bioengineered particles based on mast cell granules, containing encapsulated cytokines. When we used these particles as a vaccine adjuvant, we were able to polarize adaptive immune responses, such as to promote a Th1 phenotype, or enhance a specific attribute of the immune response, such as the production of high avidity antibodies. We then explore three examples of lymph node-targeting pathogens: Salmonella typhimurium, Yersinia pestis and Dengue virus. Each of these pathogens has a well-characterized lifecycle including colonization of draining lymph node tissue. In the case of S. typhimurim, we report that the virulence this pathogen depends on a specific shut down of the chemotactic signals in the lymph node that are required to maintain appropriate cellular localization within it. Our results demonstrate that these architecture changes allow S. typhimurim to target the adaptive immune process in lymph nodes and contribute to its spread in vivo and lethality to the host. With Y. pestis, similar targeting of cellular trafficking pathways occurs through the modulation of chemokine expression. Y. pestis appears to use the host's cellular trafficking pathways to spread to lymph nodes in two distinct waves, first exploiting dendritic cell movement to lymph nodes and then enhancing monocyte chemoattractants to replicate within monocytes in draining lymph nodes. These processes also promote bacterial spread in vivo and we further demonstrate that blocking monocyte chemotaxis can prolong the host's survival. In the third example of pathogen challenge, we report for the first time that mast cells can contribute functionally to immunosurveillance for viral pathogen, here, promoting cellular trafficking of innate immune cells, including NK cells, and limiting the spread of virus to draining lymph nodes. For each of these three examples of lymph node targeting by microbial pathogens, we provide data that modulation of cellular trafficking to and within lymph nodes can drastically influence the nature of the adaptive immune response and, therefore, the appropriateness of that response for meeting a unique infectious challenge. Cumulatively this work highlights that a balance exists between host and pathogen-driven modulation of lymph nodes, a key aspect of which is movement of cells within and into this organ. Cytokine and chemokine pathways are an area of vulnerability for the host when faced with host-adapted pathogens, yet the lymph node's underlying plasticity and the observation that slight modulations can be beneficial or detrimental to immunity also suggests the targeting of these pathways with therapeutic intentions and during vaccine design.

The key components of innate immune defense to pathogens are various migratory as well as tissue resident innate immune cells, however, their interactions with pathogens as well as their immune-orchestrating roles are often poorly understood. While immune cells encounter pathogens at barrier sites and mount the first line of defense, pathogens are well adapted to bypass, inactivate and even exploit the functions of these cells. Better understanding of the interactions between pathogens and innate immune cells can teach us how pathogens avoid or exploit immune cells and how to overcome these mechanisms of pathogenesis by therapeutic interventions. In this work, we examined two scenarios of pathogen invasion and sought to understand the complex ways of external targeting of innate immunocytes that can either benefit the pathogen or the host.

First, we studied the migratory innate immunocytes in draining lymph nodes upon entry of Yersinia pestis via the skin and identified how this plague-causing bacterium coopted host cell death pathways of infiltrated mononuclear phagocytes. By employing time-lapse microscopy and flow cytometry, we demonstrated that within the confines of infected lymph nodes, bacteria-triggered necroptotic cell death resulted in the release of intracellular bacteria into the extracellular environment and attracted neighboring phagocytic cells, promoting their infection by these recently released bacteria. This expansion of bacteria-bearing immune cells which eventually migrate to secondary lymph nodes, enables large numbers of Y. pestis to disseminate from one node to the next via the lymphatic system. We show this mechanism of dissemination being essential for the transition of plague from a bubonic to septicemic stage and demonstrate immunotherapeutic potential of necroptosis inhibitors.

Next, we focused on mast cells, a resident innate immunocyte in the context of skin infection by Staphylococcus aureus. We showed that connective tissue mast cells promoted recruitment of neutrophils at the early stage and CD301b+ dendritic cells at the later stages of infection, which played critical roles in infection control and repair, respectively. We further demonstrated that exogenous activation of skin mast cells via a mast cell-specific G protein-coupled receptor controlled infection as well as enhanced mobilization of dendritic cells to draining lymph nodes in a mast-cell dependent manner and protected mice from re-infection. Therefore, selective activation of mast cells appears to orchestrate immunomodulation integrating both the innate and adaptive immune arms.

These studies reveal the yin and yang of innate immune cells in two very different infectious settings. They emphasize how different strategies to target these cells at the immune checkpoints can be beneficial for host-directed therapy against bacterial infections.