MyD88-dependent signaling prolongs survival and reduces bacterial burden during pulmonary infection with virulent Francisella tularensis

Identification and functional analysis of myeloid differentiation factor 88 (MyD88) in early development of Haliotis diversicolor

Myeloid differentiation factor 88 (MyD88) is a universal adaptor protein and plays an important role in the signal transduction of Toll like receptors (TLR) family. In this study, the MyD88 gene from the Haliotis diversicolor (hdMyD88) was identified. The full-length cDNA of hdMyD88 has a 1927 base pairs (bp), with an open reading frame of 1314 bp encoding 437 amino acids including a death domain (DD) at the N-terminus and TIR domain at the C-terminus which are typical features of MyD88 family proteins. Three conserved boxes are also found in the hdMyD88, which are similar to MyD88 in vertebrates. The expression levels of hdMyD88 mRNA at different early embryonic developmental stages of abalone were measured by qPCR revealed that their constitutive expression at all developmental stages analyzed with the considerably highest values at 8 cell stage and the lowest level at the trochosphere stage. Additionally, the mRNA expression of hdMyD88 decreased significantly (P < 0.05) after MyD88-dsRNA soak in the stage of trochosphere and veliger than EGFP-dsRNA group and blank control group. Whole embryo in situ hybridization showed that the positive signals of hdMyD88 were in visceral mass of trochophore larvae and veliger larvae. These results indicate hdMyD88 may could respond to pathogenic infection and may play an important role in early innate immunity in the process of abalone larval development.

Identification, functional characterization and expression pattern of myeloid differentiation factor 88 (MyD88) in Nibea albiflora

Myeloid differentiation factor 88 (MyD88), composed of an N-terminal death domain and a C-terminal Toll/interleukin (IL)-IR homology domain, is a key connector protein in the TLR signal transduction pathway. In this study a novel isoform of MyD88 in Nibea albiflora (named as NaMyD88) was identified and functionally characterized (GenBank accession no. MN384261.1). Its complete cDNA sequence was 1672 bp and contained an open reading frame of 879 bp encoding 292 amino acid residues, which was similar to its teleost fish counterparts in the length. The theoretical molecular mass was 33.63 kDa and the isoelectric point was 5.24. BLASTp analysis suggested that the deduced amino acids sequence of NaMyD88 shared high identity to the known MyD88, for instance, 94.77% identity with Collichthys lucidus. Sequence analysis showed that NaMyD88 protein was consistent with MyD88 protein of other species at three conserved domains, N-terminal DD, short middle domain and C-terminal TIR, and the TIR domain contained three highly conserved motifs: Box1, Box2, and Box3. NaMyD88 and red fluorescent protein (Dsred) were fused and expressed in the cytoplasm of the epithelioma papulosum cyprini (EPC cells). The NaTLR9-TIR-EGFP fusion protein, which was obtained in our previous studies, showed green fluorescence and mainly distributed in the cytoplasm. After co-transfection, NaMyD88-Dsred and NaTLR9-TIR-EGFP obviously overlapped and displayed orange-yellow color. The results showed that the homologous MyD88-Dsred could interact with NaTLR9-TIR-EGFP. Based on this result pcMV-NaMyD88-TIR-Myc plasmids and the pcDNA3.1-NaTLR9-TIR-flag were constructed and co-transfected into 293T cells for the immunoprecipitation test. According to Western blot, the protein eluted by Flag-beads could be detected by anti-Flag-tag antibody and anti-Myc tag antibody respectively, while the protein without NaTLR9-TIR could not be found, which further proved that TLR and MyD88 could interact each other. The prokaryotic plasmid of MyD88-TIR domain was constructed, expressed in BL21 (DE3) and purified by Ni-NAT super flow resin conforming to the expected molecular weight of 27 kDa with the corresponding active sites for its conferring protein-protein interaction functions. Real-time fluorescence quantitative PCR showed that NaMyD88 could be expressed in intestine, stomach, liver, kidney, gill, heart and spleen, with the highest in the kidney, and it was up-regulated after being infected with Polyinosinic:polycytidylic acid - Poly (I:C) and Pseudomonas plecoglossicida, which showed that NaMyD88 was involved in the immune response of N.albiflora. These data afforded a basis for understanding the role of NaMyD88 in the TLR signaling pathway of N.albiflora.

Production of IFN-γ by splenic dendritic cells during innate immune responses against Francisella tularensis LVS depends on MyD88, but not TLR2, TLR4, or TLR9

Production of IFN-γ is a key innate immune mechanism that limits replication of intracellular bacteria such as Francisella tularensis (Ft) until adaptive immune responses develop. Previously, we demonstrated that the host cell types responsible for IFN-γ production in response to murine Francisella infection include not only natural killer (NK) and T cells, but also a variety of myeloid cells. However, production of IFN-γ by mouse dendritic cells (DC) is controversial. Here, we directly demonstrated substantial production of IFN-γ by DC, as well as hybrid NK-DC, from LVS-infected wild type C57BL/6 or Rag1 knockout mice. We demonstrated that the numbers of conventional DC producing IFN-γ increased progressively over the course of 8 days of LVS infection. In contrast, the numbers of conventional NK cells producing IFN-γ, which represented about 40% of non-B/T IFN-γ-producing cells, peaked at day 4 after LVS infection and declined thereafter. This pattern was similar to that of hybrid NK-DC. To further confirm IFN-γ production by infected cells, DC and neutrophils were sorted from naïve and LVS-infected mice and analyzed for gene expression. Quantification of LVS by PCR revealed the presence of Ft DNA not only in macrophages, but also in highly purified, IFN-γ producing DC and neutrophils. Finally, production of IFN-γ by infected DC was confirmed by immunohistochemistry and confocal microscopy. Notably, IFN-γ production patterns similar to those in wild type mice were observed in cells derived from LVS-infected TLR2, TLR4, and TLR2xTLR9 knockout (KO) mice, but not from MyD88 KO mice. Taken together, these studies demonstrate the pivotal roles of DC and MyD88 in IFN-γ production and in initiating innate immune responses to this intracellular bacterium.

A predicted Francisella tularensis DXD-motif glycosyltransferase blocks immune activation

Pathogens enhance their survival during infections by manipulating host defenses. Francisella tularensis evades innate immune responses, which we have found to be dependent on an understudied gene ybeX (FTL_0883/FTT_0615c). To understand the function of YbeX, we sought protein interactors in F. tularensis subsp. holarctica live vaccine strain (LVS). An unstudied Francisella protein co-immunoprecipitated with recombinant YbeX, which is a predicted glycosyltransferase with a DXD-motif. There are up to four genomic copies of this gene with identical sequence in strains of F. tularensis pathogenic to humans, despite ongoing genome decay. Disruption mutations were generated by intron insertion into all three copies of this glycosyltransferase domain containing gene in LVS, gdcA1-3. The resulting strains stimulated more cytokines from macrophages in vitro than wild-type LVS and were attenuated in two in vivo infection models. GdcA was released from LVS during culture and was sufficient to block NF-κB activation when expressed in eukaryotic cells. When co-expressed in zebrafish, GdcA and YbeX were synergistically lethal to embryo development. Glycosyltransferases with DXD-motifs are found in a variety of pathogens including NleB, an Escherichia coli type-III secretion system effector that inhibits NF-κB by antagonizing death receptor signaling. To our knowledge, GdcA is the first DXD-motif glycosyltransferase that inhibits NF-κB in immune cells. Together, these findings suggest DXD-motif glycosyltransferases may be a conserved virulence mechanism used by pathogenic bacteria to remodel host defenses.

Innate Immune Recognition: An Issue More Complex Than Expected

Primary interaction of an intracellular bacterium with its host cell is initiated by activation of multiple signaling pathways in response to bacterium recognition itself or as cellular responses to stress induced by the bacterium. The leading molecules in these processes are cell surface membrane receptors as well as cytosolic pattern recognition receptors recognizing pathogen-associated molecular patterns or damage-associated molecular patterns induced by the invading bacterium. In this review, we demonstrate possible sequences of events leading to recognition of Francisella tularensis, present findings on known mechanisms for manipulating cell responses to protect Francisella from being killed, and discuss newly published data from the perspective of early stages of host-pathogen interaction.

Interleukin 1α (IL-1α) Promotes Pathogenic Immature Myeloid Cells and IL-1β Favors Protective Mature Myeloid Cells During Acute Lung Infection

Bacterial pneumonia is a common risk factor for acute lung injury and sepsis-mediated death, but the mechanisms underlying the overt inflammation and accompanying pathology are unclear. Infiltration of immature myeloid cells and necrotizing inflammation mediate severe pathology and death during pulmonary infection with Francisella tularensis. However, eliciting mature myeloid cells provides protection. Yet, the host factors responsible for this pathologic immature myeloid cell response are unknown. Here, we report that while the influx of both mature and immature myeloid cells is strictly MyD88 dependent, the interleukin 1 (IL-1) receptor mediates an important dual function via its ligands IL-1α and IL-1β. Although IL-1β favors the appearance of bacteria-clearing mature myeloid cells, IL-1α contributes to lung infiltration by ineffective and pathologic immature myeloid cells. Finally, IL-1α and IL-1β are not the sole factors involved, but myeloid cell responses during acute pneumonia were largely unaffected by lung levels of interleukin 10, interleukin 17, CXCL1, granulocyte colony-stimulating factor, and granulocyte-macrophage colony-stimulating factor.

Inflammasomes as therapeutic targets for Alzheimer's disease

Alzheimer's disease is the most common form of progressive dementia, typified initially by short term memory deficits which develop into a dramatic global cognitive decline. The classical hall marks of Alzheimer's disease include the accumulation of amyloid oligomers and fibrils, and the intracellular formation of neurofibrillary tangles of hyperphosphorylated tau. It is now clear that inflammation also plays a central role in the pathogenesis of the disease through a number of neurotoxic mechanisms. Microglia are the key immune regulators of the CNS which detect amyloidopathy through cell surface and cytosolic pattern recognition receptors (PRRs) and respond by initiating inflammation through the secretion of cytokines such as interleukin-1β (IL-1β). Inflammasomes, which regulate IL-1β release, are formed following activation of cytosolic PRRs, and using genetic and pharmacological approaches, NLRP3 and NLRP1 inflammasomes have been found to be integral in pathogenic neuroinflammation in animal models of Alzheimer's disease. Therefore, the inflammasomes are very promising novel pharmacological targets which merit further research in the continued endeavor for efficacious therapeutics for Alzheimer's disease.

Molecular identification and functional analysis of two variants of myeloid differentiation factor 88 (MyD88) from disk abalone (Haliotis discus discus)

Myeloid differentiation factor 88 (MyD88) is a crucial adaptor protein of the Toll-like receptor (TLR)- and interleukin 1 receptor-mediated signaling pathways and is involved in a diverse array of inflammatory responses via NF-κB activation. In the present study, two MyD88 variants were identified from disk abalone (Haliotis discus discus) and designated AbMyD88-2 and AbMyD88-X. The deduced AbMyD88-2 and AbMyD88-X comprised 433 and 354 amino acids with predicted molecular masses of 48.85 kDa and 40.17 kDa, respectively. AbMyD88-2 and AbMyD88-X possessed typical MyD88 domain structural features including an N-terminal death domain (DD) and C-terminal toll interleukin 1 receptor (TIR) domain similar to those in mammals. Expression analysis of AbMyD88-2 and AbMyD88-X mRNA at different early embryonic developmental stages of abalone by qPCR revealed that their constitutive expression at all developmental stages analyzed with the considerably higher values at the 16-cell (AbMyD88-2) and morula stages (AbMyD88-X). In unchallenged disk abalones, AbMyD88-2 was highly expressed in muscles, while AbMyD88-X mRNA was predominantly transcribed in hemocytes. Moreover, AbMyD88-2 and AbMyD88-X mRNA were differentially modulated in abalone hemocytes after a challenge with live bacteria (Vibrio parahaemolyticus, Listeria monocytogenes), virus (viral hemorrhagic septicemia virus), and pathogen-associated molecular patterns (lipopolysaccharides and Poly I:C). Overexpression of AbMyD88-2 and AbMyD88-X in HEK293T cells induced the activation of the NF-κB promoter. AbMyD88-2 and AbMyD88-X involvement in inflammatory responses was characterized by their overexpression in RAW264.7 murine macrophage cells. These results revealed comparatively higher NO (Nitric oxide) production, induction of inflammatory mediator genes (iNOS and COX2), and proinflammatory genes (IL1β, IL6 and TNFα) expression in abalone MyD88s-overexpressing cells than in mock control in the presence or absence of LPS stimulation. Altogether, these results suggest that existence of a MyD88-dependent like signaling pathway in disk abalone and that both AbMyD88-2 and AbMyD88-X might be involved in innate immune and inflammatory responses.

Innate Immune Recognition: Implications for the Interaction of Francisella tularensis with the Host Immune System

The intracellular bacterial pathogen Francisella tularensis causes serious infectious disease in humans and animals. Moreover, F. tularensis, a highly infectious pathogen, poses a major concern for the public as a bacterium classified under Category A of bioterrorism agents. Unfortunately, research has so far failed to develop effective vaccines, due in part to the fact that the pathogenesis of intracellular bacteria is not fully understood and in part to gaps in our understanding of innate immune recognition processes leading to the induction of adaptive immune response. Recent evidence supports the concept that immune response to external stimuli in the form of bacteria is guided by the primary interaction of the bacterium with the host cell. Based on data from different Francisella models, we present here the basic paradigms of the emerging innate immune recognition concept. According to this concept, the type of cell and its receptor(s) that initially interact with the target constitute the first signaling window; the signals produced in the course of primary interaction of the target with a reacting cell act in a paracrine manner; and the innate immune recognition process as a whole consists in a series of signaling windows modulating adaptive immune response. Finally, the host, in the strict sense, is the interacting cell.

Hematopoietic MyD88 and IL-18 are essential for IFN-γ-dependent restriction of type A Francisella tularensis infection

Francisella tularensis is a highly infectious intracellular bacterium that causes the potentially fatal disease tularemia. We used mice with conditional MyD88 deficiencies to investigate cellular and molecular mechanisms by which MyD88 restricts type A F. tularensis infection. F. tularensis-induced weight loss was predominately dependent on MyD88 signaling in nonhematopoietic cells. In contrast, MyD88 signaling in hematopoietic cells, but not in myeloid and dendritic cells, was essential for control of F. tularensis infection in tissue. Myeloid and dendritic cell MyD88 deficiency also did not markedly impair cytokine production during infection. Although the production of IL-12 or -18 was not significantly reduced in hematopoietic MyD88-deficient mice, IFN-γ production was abolished in these animals. In addition, neutralization studies revealed that control of F. tularensis infection mediated by hematopoietic MyD88 was entirely dependent on IFN-γ. Although IL-18 production was not significantly affected by MyD88 deficiency, IL-18 was essential for IFN-γ production and restricted bacterial replication in an IFN-γ-dependent manner. Caspase-1 was also found to be partially necessary for the production of IL-18 and IFN-γ and for control of F. tularensis replication. Our collective data show that the response of leukocytes to caspase-1-dependent IL-18 via MyD88 is critical, whereas MyD88 signaling in myeloid and dendritic cells is dispensable for IFN-γ-dependent control of type A F. tularensis infection.

Pam2CSK4 and Pam3CSK4 induce iNOS expression via TBK1 and MyD88 molecules in mouse macrophage cell line RAW264.7

OBJECTIVE

The aim of this study was to investigate the involvement of TLR adaptor molecules, such as TRIF, MyD88, and TBK1 in the induction of iNOS and nitric oxide (NO) production in Pam2CSK4 and Pam3CSK4-treated mouse macrophages.

METHOD

Mouse macrophage cell line (RAW264.7) was transfected with trif, myd88, and tbk1 siRNAs before stimulated with Pam2CSK4 and Pam3CSK4. The iNOS gene and protein expression were determined by RT-PCR and immunoblotting, respectively. The NO production was determined by Griess reaction assay.

RESULTS

The results showed that the induction of iNOS expression and NO production by Pam2CSK4 and Pam3CSK4 were diminished in tbk1 and myd88-depleted mouse macrophages but not trif-depleted cells.

CONCLUSION

These results suggested that the TBK1 and MyD88 molecules were essential for the induction of iNOS expression and NO production by both Pam2CSK4 and Pam3CSK4 via TLR2 signaling.

Differential expression of microRNAs in Francisella tularensis-infected human macrophages: miR-155-dependent downregulation of MyD88 inhibits the inflammatory response

Francisella tularensis is a Gram-negative, facultative intracellular pathogen that replicates in the cytosol of macrophages and is the causative agent of the potentially fatal disease tularemia. A characteristic feature of F. tularensis is its limited proinflammatory capacity, but the mechanisms that underlie the diminished host response to this organism are only partially defined. Recently, microRNAs have emerged as important regulators of immunity and inflammation. In the present study we investigated the microRNA response of primary human monocyte-derived macrophages (MDMs) to F. tularensis and identified 10 microRNAs that were significantly differentially expressed after infection with the live vaccine strain (LVS), as judged by Taqman Low Density Array profiling. Among the microRNAs identified, miR-155 is of particular interest as its established direct targets include components of the Toll-like receptor (TLR) pathway, which is essential for innate defense and proinflammatory cytokine production. Additional studies demonstrated that miR-155 acted by translational repression to downregulate the TLR adapter protein MyD88 and the inositol 5′-phosphatase SHIP-1 in MDMs infected with F. tularensis LVS or the fully virulent strain Schu S4. Kinetic analyses indicated that miR-155 increased progressively 3-18 hours after infection with LVS or Schu S4, and target proteins disappeared after 12–18 hours. Dynamic modulation of MyD88 and SHIP-1 was confirmed using specific pre-miRs and anti-miRs to increase and decrease miR-155 levels, respectively. Of note, miR-155 did not contribute to the attenuated cytokine response triggered by F. tularensis phagocytosis. Instead, this microRNA was required for the ability of LVS-infected cells to inhibit endotoxin-stimulated TNFα secretion 18–24 hours after infection. Thus, our data are consistent with the ability of miR-155 to act as a global negative regulator of the inflammatory response in F. tularensis-infected human macrophages.