Role of Yersinia pseudotuberculosis outer proteins (Yops) in murine humoral immune response

Role of the Yersinia pseudotuberculosis Virulence Plasmid in Pathogen-Phagocyte Interactions in Mesenteric Lymph Nodes

Yersinia pseudotuberculosis is an Enterobacteriaceae family member that is commonly transmitted by the fecal-oral route to cause infections. From the small intestine, Y. pseudotuberculosis can invade through Peyer's patches and lymph vessels to infect the mesenteric lymph nodes (MLNs). Infection of MLNs by Y. pseudotuberculosis results in the clinical presentation of mesenteric lymphadenitis. MLNs are important for immune responses to intestinal pathogens and microbiota in addition to their clinical relevance to Y. pseudotuberculosis infections. A characteristic of Y. pseudotuberculosis infection in MLNs is the formation of pyogranulomas. Pyogranulomas are composed of neutrophils, inflammatory monocytes, and lymphocytes surrounding extracellular microcolonies of Y. pseudotuberculosis. Key elements of the complex pathogen-host interaction in MLNs have been identified using mouse infection models. Y. pseudotuberculosis requires the virulence plasmid pYV to induce the formation of pyogranulomas in MLNs. The YadA adhesin and the Ysc-Yop type III secretion system (T3SS) are encoded on pYV. YadA mediates bacterial binding to host receptors, which engages the T3SS to preferentially translocate seven Yop effectors into phagocytes. The effectors promote pathogenesis by blocking innate immune defenses such as superoxide production, degranulation, and inflammasome activation, resulting in survival and growth of Y. pseudotuberculosis. On the other hand, certain effectors can trigger immune defenses in phagocytes. For example, YopJ triggers activation of caspase-8 and an apoptotic cell death response in monocytes within pyogranulomas that limits dissemination of Y. pseudotuberculosis from MLNs to the bloodstream. YopE can be processed as an antigen by phagocytes in MLNs, resulting in T and B cell responses to Y. pseudotuberculosis. Immune responses to Y. pseudotuberculosis in MLNs can also be detrimental to the host in the form of chronic lymphadenopathy. This review focuses on interactions between Y. pseudotuberculosis and phagocytes mediated by pYV that concurrently promote pathogenesis and host defense in MLNs. We propose that MLN pyogranulomas are immunological arenas in which opposing pYV-driven forces determine the outcome of infection in favor of the pathogen or host.

YopJ-promoted cytotoxicity and systemic colonization are associated with high levels of murine interleukin-18, gamma interferon, and neutrophils in a live vaccine model of Yersinia pseudotuberculosis infection

Several Yersinia species have been utilized as live attenuated vaccines to prime protective immunity against yersiniae and other pathogens. A type III secretion system effector known as YopJ in Y. pseudotuberculosis and Y. pestis and YopP in Y. enterocolitica has been shown to regulate host immune responses to live Yersinia vaccines. YopJ/P kills macrophages and dendritic cells, reduces their production of tumor necrosis factor alpha (TNF-alpha) and interleukin-12 (IL-12), and promotes systemic colonization in mouse models of intestinal Yersinia infection. Furthermore, YopP activity decreases antigen presentation by dendritic cells, and a yopP mutant of a live Y. enterocolitica carrier vaccine elicited effective priming of CD8 T cells to a heterologous antigen in mice. These results suggest that YopJ/P activity suppresses both innate and adaptive immune responses to live Yersinia vaccines. Here, a sublethal intragastric mouse infection model using wild-type and catalytically inactive yopJ mutant strains of Y. pseudotuberculosis was developed to further investigate how YopJ action impacts innate and adaptive immune responses to a live vaccine. Surprisingly, YopJ-promoted cytotoxicity and systemic colonization were associated with significant increases in neutrophils in spleens and the proinflammatory cytokines IL-18 and gamma interferon (IFN-gamma) in serum samples of mice vaccinated with Y. pseudotuberculosis. Secretion of IL-18 accompanied YopJ-mediated killing of macrophages infected ex vivo with Y. pseudotuberculosis, suggesting a mechanism by which this effector directly increases proinflammatory cytokine levels in vivo. Mice vaccinated with the wild-type strain or the yopJ mutant produced similar levels of antibodies to Y. pseudotuberculosis antigens and were equally resistant to lethal intravenous challenge with Y. pestis. The findings indicate that a proinflammatory, rather than anti-inflammatory, process accompanies YopJ-promoted cytotoxicity, leading to increased systemic colonization by Y. pseudotuberculosis and potentially enhancing adaptive immunity to a live vaccine.