Y. enterocolitica translocated Yops impair stimulation of T-cells by antigen presenting cells

Subversion of mouse dendritic cell subset function by bacterial pathogens

Dendritic cells (DCs) play an important role as sentinels of the immune system in initiating and controlling the quality of adaptive immune responses. Located at entry points of the host they can sense and alert the body from dangers such as infection by pathogenic bacteria. Considering their strategic localization it is not surprising that DCs have evolved in a series of DC subtypes, which are well adapted to their microenvironment. Nowadays, the advent of the identification of specific DC subtypes has opened the way for the study of pathogen-DCs interactions and the involved mechanisms of these interactions. Due to key aspect of DCs, several bacterial pathogens have taken advantage of these cells and developed mechanisms to subvert DC function and thereby evade the immune system. This review brings recent insights into DC-pathogenic bacteria cross-talk using the mouse model of infection with an emphasis on DC subtypes.

Immune response to diphtheria toxin-mediated depletion complicates the use of the CD11c-DTR(tg) model for studies of bacterial gastrointestinal infections

Dendritic cells play an important role in the immune response against pathogens, as they are responsible for the activation and control of both innate and adaptive immune responses. The CD11c-DTR(tg) model, which allows transient elimination of dendritic cells by diphtheria toxin-treatment (DTx), has been extensively used to study the importance of this immune cell during steady-state and infection conditions in mice. Infecting dendritic cell-depleted mice orally with Yersinia pseudotuberculosis results in a markedly reduced level of infection compared with infection of non-depleted mice. We show here that it is not the lack of dendritic cells per se that is responsible for the reduced infection efficiency, instead it is an immune response induced by the DTx-treatment that prevents the bacteria from establishing colonization in Peyer's patches. The DTx-induced depletion initiates an immune response, with elevated serum levels of keratinocyte-derived cytokine (KC) and recruitment of polymorphonuclear neutrophils to dendritic cell-containing organs, such as Peyer's patches. Since the window for having an animal depleted of dendritic cells is limited in time for this model, the DTx-mediated effect on the immune system complicates the use of this model in studies of early events during bacterial infections.

Susceptibility to Yersinia pseudotuberculosis infection is linked to the pattern of macrophage activation

T helper 1 cells play a crucial role in the clearance of Yersinia pseudotuberculosis infection. By producing cytokines and presenting antigens to T cells, activated macrophages can orientate the adaptive immune response. The pathway used by macrophages to metabolize arginine has been employed as an important parameter to discriminate their activation state. In this study, the pattern of macrophage activation in Y. pseudotuberculosis-infected BALB/c (Yersinia-susceptible) and C57BL/6 (Yersinia-resistant) mice and their immunostimulatory capacity were analysed. In the early phase of infection, macrophages obtained from C57BL/6 mice produced higher levels of NO, lower arginase activity, and larger amounts of IL-12 and TNF-alpha than macrophages from BALB/c mice. On the other hand, macrophages derived from BALB/c mice produced higher levels of IL-10 and TGF-beta than C57BL/6 mice. The Y. pseudotuberculosis infection leads to a fall in the macrophage immunostimulatory capacity of both strains of mice, with T-cell proliferation significantly reduced 12 h after infection. Moreover, we observed in the supernatant of co-culture of macrophages from infected mice with T lymphocytes from heat-killed Yersinia-immunized mice lower IFN-gamma production by cells from BALB/c mice than by C57BL/6 mice, and IL-4 was produced only by BALB/c mice on the first- and third-day post-infection. These results suggest that the pattern of macrophage activation is associated with susceptibility and resistance to Y. pseudotuberculosis infection in BALB/c and C57BL/6 mice.

Genome rearrangements, deletions, and amplifications in the natural population of Bartonella henselae

Cats are the natural host for Bartonella henselae, an opportunistic human pathogen and the agent of cat scratch disease. Here, we have analyzed the natural variation in gene content and genome structure of 38 Bartonella henselae strains isolated from cats and humans by comparative genome hybridizations to microarrays and probe hybridizations to pulsed-field gel electrophoresis (PFGE) blots. The variation in gene content was modest and confined to the prophage and the genomic islands, whereas the PFGE analyses indicated extensive rearrangements across the terminus of replication with breakpoints in areas of the genomic islands. We observed no difference in gene content or structure between feline and human strains. Rather, the results suggest multiple sources of human infection from feline B. henselae strains of diverse genotypes. Additionally, the microarray hybridizations revealed DNA amplification in some strains in the so-called chromosome II-like region. The amplified segments were centered at a position corresponding to a putative phage replication initiation site and increased in size with the duration of cultivation. We hypothesize that the variable gene pool in the B. henselae population plays an important role in the establishment of long-term persistent infection in the natural host by promoting antigenic variation and escape from the host immune response.

Discordance in the effects of Yersinia pestis on the dendritic cell functions manifested by induction of maturation and paralysis of migration

The encounter between invading microorganisms and dendritic cells (DC) triggers a series of events which include uptake and degradation of the microorganism, induction of a maturation process, and enhancement of DC migration to the draining lymph nodes. Various pathogens have developed strategies to counteract these events as a measure to evade the host defense. In the present study we found that interaction of the Yersinia pestis EV76 strain with DC has no effect on cell viability and is characterized by compliance with effective maturation, which is manifested by surface display of major histocompatibility complex class II, of costimulatory markers, and of the chemokine receptor CCR7. This is in contrast to maturation inhibition and cell death induction exerted by the related species Yersinia enterocolitica WA O:8. Y. pestis interactions with DC were found, however, to impair functions related to cytoskeleton rearrangement. DC pulsed with Y. pestis failed to adhere to solid surfaces and to migrate toward the chemokine CCL19 in an in vitro transmembrane assay. Both effects were dependent on the presence of the pCD1 virulence plasmid and on a bacterial growth shift to 37 degrees C prior to infection. Moreover, while instillation of a pCD1-cured Y. pestis strain into mouse airways triggered effective transport of alveolar DC to the mediastinal lymph node, instillation of Y. pestis harboring the plasmid failed to do so. Taken together, these results suggest that virulence plasmid-dependent impairment of DC migration is the major mechanism utilized by Y. pestis to subvert DC function.