Myeloid differentiation factor-88 (MyD88) is essential for control of primary in vivo Francisella tularensis LVS infection, but not for control of intra-macrophage bacterial replication

Subversion of complement activation at the bacterial surface promotes serum resistance and opsonophagocytosis of Francisella tularensis

Francisella tularensis (Ft) is resistant to serum but requires complement factor C3-derived opsonins for uptake by phagocytic cells and subsequent intracellular growth. In this study, we show that C3 fragments, deposited on Ft, are detected by anti-C3d and -iC3b mAb and that the classical and the alternative pathways are involved in this event. This was demonstrated using C2-depleted sera and specific inhibitors of the classical-versus-alternative pathways of complement activation. Further, we demonstrate that factor C4b, which is crucial for the classical pathway, is deposited on the surface of Ft. In contrast, the C5b-C9 membrane attack complex (MAC) is not assembled on the surface of Ft, which may explain its resistance to complement killing. Deposition of C3 opsonins leads to enhanced phagocytosis by human immature dendritic cells (DC), which leads to intracellular survival, growth, and DC death. Finally, we show that factor H (fH) can bind to the surface of Ft. We believe our data suggest that important virulence factors for Ft are its ability to bind fH and inactivate C3b to iC3b, which culminates in opsonin-induced uptake for subsequent intracellular growth. C3b inactivation also leads to inefficient MAC assembly, which contributes to the ability of this bacterium to resist complement lysis.

Identification of immunologic and pathologic parameters of death versus survival in respiratory tularemia

Francisella tularensis can cause severe disseminated disease after respiratory infection. The identification of factors involved in mortality or recovery following induction of tularemia in the mouse will improve our understanding of the natural history of this disease and facilitate future evaluation of vaccine candidate preparations. BALB/c mice were infected intranasally with the live vaccine strain (LVS) of F. tularensis subsp. holarctica and euthanized at different stages of disease to analyze the induction of immune molecules, gross anatomical features of organs, bacterial burdens, and progression of the histopathological changes in lung and spleen. Tissue-specific interleukin-6 (IL-6), macrophage inflammatory protein 2, and monocyte chemotactic protein 1 were immune markers of mortality, while anti-LVS immunoglobulin M and IL-1beta were associated with survival. Moribund mice had enlarged spleens and lungs, while surviving mice had even more prominent splenomegaly and normal-appearing lungs. Histopathology of the spleens of severely ill mice was characterized by disrupted lymphoid follicles and fragmented nuclei, while the spleens of survivors appeared healthy but with increased numbers of megakaryocytes and erythrocytes. Histopathology of the lungs of severely ill mice indicated severe pneumonia. Lungs of survivors at early time points showed increased inflammation, while at late times they appeared healthy with peribronchial lymphoid aggregates. Our results suggest that host immune factors are able to affect bacterial dissemination after respiratory tularemia, provide new insights regarding the pathological characteristics of pulmonary tularemia leading to systemic disease, and potentially identify immune markers associated with recovery from the disease.

Toll-like receptor 2 controls the gamma interferon response to Francisella tularensis by mouse liver lymphocytes

The production of gamma interferon (IFN-gamma) is a key step in the protective innate immune response to Francisella tularensis. Natural killer cells and T cells in the liver are important sources of this cytokine during primary F. tularensis infections, and interleukin-12 (IL-12) appears to be an essential coactivating cytokine for hepatic IFN-gamma expression. The present study was undertaken to determine whether or not macrophages (Mphi) or dendritic cells (DC) provide coactivating signals for the liver IFN-gamma response in vitro, whether IL-12 mediates these effects, and whether Toll-like receptor (TLR) signaling is essential to induce this costimulatory activity. Both bone marrow-derived Mphi and DC significantly augmented the IFN-gamma response of F. tularensis-challenged liver lymphocytes in vitro. While both cell types produced IL-12p40 in response to F. tularensis challenge, only DC secreted large quantities of IL-12p70. DC from both IL-12p35-deficient and TLR2-deficient mice failed to produce IL-12p70 and did not costimulate liver lymphocytes for IFN-gamma production in response to viable F. tularensis organisms. Conversely, liver lymphocytes from TLR2-deficient mice cocultured with wild-type accessory cells produced IFN-gamma at levels comparable to those for wild-type hepatic lymphocytes. These findings indicate that TLR2 controls hepatic lymphocyte IFN-gamma responses to F. tularensis by regulating DC IL-12 production. While Mphi also coinduced hepatic IFN-gamma production in response to F. tularensis, they did so in a fashion less dependent on TLR2.

Activation of the inflammasome upon Francisella tularensis infection: interplay of innate immune pathways and virulence factors

Tularaemia is a zoonotic disease caused by the facultative intracellular bacterium Francisella tularensis. The virulence of this pathogen depends on its ability to escape into the cytosol of host cells. Pathogens are detected by the innate immune system's pattern recognition receptors which are activated in response to conserved microbial molecules (pathogen-associated molecular patterns). Cytosolic bacteria are sensed intracellularly, often leading to activation of the cysteine protease caspase-1 within a multimolecular complex called the inflammasome. Caspase-1 activation leads to both host cell death and release of pro-inflammatory cytokines in a process called pyroptosis. Here we review the pathway leading to, and the consequences of, inflammasome activation upon F. tularensis infection both in vitro and in vivo. Finally, we discuss recent data on how other innate immune pathways and F. tularensis virulence factors control the activation of the inflammasome during infection.

Toll-like receptor 2-mediated signaling requirements for Francisella tularensis live vaccine strain infection of murine macrophages

Francisella tularensis, an aerobic, non-spore-forming, gram-negative coccobacillus, is the causative agent of tularemia. We reported previously that F. tularensis live vaccine strain (LVS) elicited strong, dose-dependent NF-kappaB reporter activity in Toll-like receptor 2 (TLR2)-expressing HEK293T cells and proinflammatory gene expression in primary murine macrophages. Herein, we report that F. tularensis LVS-induced murine macrophage proinflammatory cytokine gene and protein expression are overwhelmingly TLR2 dependent, as evidenced by the abrogated responses of TLR2(-/-) macrophages. F. tularensis LVS infection also increased expression of TLR2 both in vitro, in mouse macrophages, and in vivo, in livers from F. tularensis LVS-infected mice. Colocalization of intracellular F. tularensis LVS, TLR2, and MyD88 was visualized by confocal microscopy. Signaling was abrogated if the F. tularensis LVS organisms were heat or formalin killed or treated with chloramphenicol, indicating that the TLR2 agonist activity is dependent on new bacterial protein synthesis. F. tularensis LVS replicates in macrophages; however, bacterial replication was not required for TLR2 signaling because LVSDeltaguaA, an F. tularensis LVS guanine auxotroph that fails to replicate in the absence of exogenous guanine, activated NF-kappaB in TLR2-transfected HEK293T cells and induced cytokine expression in wild-type macrophages comparably to wild-type F. tularensis LVS. Collectively, these data indicate that the primary macrophage response to F. tularensis LVS is overwhelmingly TLR2 dependent, requires de novo bacterial protein synthesis, and is independent of intracellular F. tularensis replication.

Role of primary human alveolar epithelial cells in host defense against Francisella tularensis infection

Francisella tularensis, an intracellular pathogen, is highly virulent when inhaled. Alveolar epithelial type I (ATI) and type II (ATII) cells line the majority of the alveolar surface and respond to inhaled pathogenic bacteria via cytokine secretion. We hypothesized that these cells contribute to the lung innate immune response to F. tularensis. Results demonstrated that the live vaccine strain (LVS) contacted ATI and ATII cells by 2 h following intranasal inoculation of mice. In culture, primary human ATI or ATII cells, grown on transwell filters, were stimulated on the apical (AP) surface with virulent F. tularensis Schu 4 or LVS. Basolateral (BL) conditioned medium (CM), collected 6 and 24 h later, was added to the BL surfaces of transwell cultures of primary human pulmonary microvasculature endothelial cells (HPMEC) prior to the addition of polymorphonuclear leukocytes (PMNs) or dendritic cells (DCs) to the AP surface. HPMEC responded to S4- or LVS-stimulated ATII, but not ATI, CM as evidenced by PMN and DC migration. Analysis of the AP and BL ATII CM revealed that both F. tularensis strains induced various levels of a variety of cytokines via NF-kappaB activation. ATII cells pretreated with an NF-kappaB inhibitor prior to F. tularensis stimulation substantially decreased interleukin-8 secretion, which did not occur through Toll-like receptor 2, 2/6, 4, or 5 stimulation. These data indicate a crucial role for ATII cells in the innate immune response to F. tularensis.

Toll-like receptor 9 acts at an early stage in host defence against pneumococcal infection

Toll-like receptor 9 (TLR9) induces an inflammatory response by recognition of unmethylated CpG dinucleotides, mainly present in prokaryotic DNA. So far, TLR9-deficient mice have been shown to be more sensitive than wild-type mice to viral, but not to bacterial infections. Here, we show that mice deficient in TLR9 but not in TLR1, TLR2, TLR4 and TLR6 or IL-1R/IL-18R are more susceptible to a respiratory tract bacterial infection caused by Streptococcus pneumoniae. Intranasal challenge studies revealed that TLR9 plays a protective role in the lungs at an early stage of infection prior to the entry of circulating inflammatory cells. Alveolar as well as bone marrow-derived macrophages deficient in either TLR9 or the myeloid adaptor differentiation protein MyD88 were impaired in pneumococcal uptake and in pneumococcal killing. Our data suggest that in the airways, pneumococcal infection triggers a TLR9 and MyD88-dependent activation of phagocytic activity from resident macrophages leading to an early clearance of bacteria from the lower respiratory tract.

Immunologic consequences of Francisella tularensis live vaccine strain infection: role of the innate immune response in infection and immunity

Francisella tularensis (Ft), a Gram-negative intracellular bacterium, is the etiologic agent of tularemia. Although attenuated for humans, i.p. infection of mice with <10 Ft live vaccine strain (LVS) organisms causes lethal infection that resembles human tularemia, whereas the LD50 for an intradermal infection is >10(6) organisms. To examine the immunological consequences of Ft LVS infection on the innate immune response, the inflammatory responses of mice infected i.p. or intradermally were compared. Mice infected i.p. displayed greater bacterial burden and increased expression of proinflammatory genes, particularly in the liver. In contrast to most LPS, highly purified Ft LVS LPS (10 microg/ml) was found to be only minimally stimulatory in primary murine macrophages and in HEK293T cells transiently transfected with TLR4/MD-2/CD14, whereas live Ft LVS bacteria were highly stimulatory for macrophages and TLR2-expressing HEK293T cells. Despite the poor stimulatory activity of Ft LVS LPS in vitro, administration of 100 ng of Ft LVS LPS 2 days before Ft LVS challenge severely limited both bacterial burden and cytokine mRNA and protein expression in the absence of detectable Ab at the time of bacterial challenge, yet these mice developed a robust IgM Ab response within 2 days of infection and survived. These data suggest that prior administration of Ft LVS LPS protects the host by diminishing bacterial burden and blunting an otherwise overwhelming inflammatory response, while priming the adaptive immune response for development of a strong Ab response.