Organ-specific role of MyD88 for gene regulation during polymicrobial peritonitis

CASP-Model Sepsis Triggers Systemic Innate Immune Responses Revealed by the Systems-Level Signaling Pathways

Colon ascendens stent peritonitis (CASP) surgery induces a leakage of intestinal contents which may cause polymicrobial sepsis related to post-operative failure of remote multi-organs (including kidney, liver, lung and heart) and possible death from systemic syndromes. Mechanisms underlying such phenomena remain unclear. This article aims to elucidate the mechanisms underlying the CASP-model sepsis by analyzing real-world GEO data (GSE24327_A, B and C) generated from mice spleen 12 hours after a CASP-surgery in septic MyD88-deficient and wildtype mice, compared with untreated wildtype mice. Firstly, we identify and characterize 21 KO MyD88-associated signaling pathways, on which true key regulators (including ligands, receptors, adaptors, transducers, transcriptional factors and cytokines) are marked, which were coordinately, significantly, and differentially expressed at the systems-level, thus providing massive potential biomarkers that warrant experimental validations in the future. Secondly, we observe the full range of polymicrobial (viral, bacterial, and parasitic) sepsis triggered by the CASP-surgery by comparing the coordinated up- or down-regulations of true regulators among the experimental treatments born by the three data under study. Finally, we discuss the observed phenomena of “systemic syndrome”, “cytokine storm” and “KO MyD88 attenuation”, as well as the proposed hypothesis of “spleen-mediated immune-cell infiltration”. Together, our results provide novel insights into a better understanding of innate immune responses triggered by the CASP-model sepsis in both wildtype and MyD88-deficient mice at the systems-level in a broader vision. This may serve as a model for humans and ultimately guide formulating the research paradigms and composite strategies for the early diagnosis and prevention of sepsis.

Alterations in Toll-Like Receptor 7 and -9 mRNA Levels in Lungs after Ovariohysterectomy in a Pyometra Mouse Model

BACKGROUND

Pyometra (P) leads to sepsis and multiple organ dysfunction syndrome. Toll-like receptors (TLRs) recognize pathogens which can cause P. The aim of this study was to investigate TLR-7 and -9 via the MYD88 pathway and the nuclear factor kappa B (NFκB) response in the uterus of a P mouse model before and after ovariohysterectomy (RP) as well as potential lung injury.

MATERIALS AND METHODS

200 female C57BL/6J mice were randomly divided into groups (N = 10/subgroup; sham 1, 2, 3, 7; P1, 2, 3, 7; 1RP1, 2, 3, 7; 2RP1, 2, 3, 7; 3RP1, 2, 3, 7) according to the day of euthanasia. Pathogens were administrated in the groups P and RP in order to induce P.

RESULTS

Alterations in blood chemistry, histopathology, and RT-qPCT analysis before (P) and after RP were observed. Significant correlations were also found between MYD88, NFκB, and TLR9 in P and RP groups in the lungs and in RP groups in the uterus, suggesting that the immune system responded via the TLR9-MYD88 pathway.

CONCLUSIONS

This is the first report of immunohistochemical TLR-7 and -9 localization and of TLR-7, -9, MYD88, and NFκB mRNA expression in the uterus causing lung injury in a P mouse model.

Role of NLRP3 inflammasome in liver disease

Inflammasomes have become an important natural sensor of host immunity, and can protect various organs against pathogenic infections, metabolic syndromes, cellular stress and cancer metastasis. Inflammasomes are intracellular multi-protein complexes found in both parenchymal and non-parenchymal cells, stimulating the initiation of caspase-1 and interleukin (IL)-1β and IL-18 in response to cell danger signals. Inflammasomes induce cell death mechanisms. The potential role of NOD-like receptor protein 3 (NLRP3) inflammasome in alcoholic and non-alcoholic steatohepatitis, hepatitis, nanoparticle-induced liver injury and other liver diseases has recently attracted widespread attention from clinicians and researchers. In this review we summarize the role played by the NLRP3 inflammasome in molecular and pathophysiological mechanisms in the pathogenesis and progression of liver disease. This article aims to establish that targeting the NLRP3 inflammasome and other inflammasome components may make a significant therapeutic approach to the treatment of liver disease.

Dissociation of TRIF bias and adjuvanticity

Toll-like receptors (TLRs), a family of "pattern recognition receptors," bind microbial and host-derived molecules, leading to intracellular signaling and proinflammatory gene expression. TLR4 is unique in that ligand-mediated activation requires the co-receptor myeloid differentiation 2 (MD2) to initiate two signaling cascades: the MyD88-dependent pathway is initiated at the cell membrane, and elicits rapid MAP kinase and NF-κB activation, while the TIR-domain containing adaptor inducing interferon-β (TRIF)-dependent pathway is initiated from TLR4-containing endosomes and results in IRF3 activation. Previous studies associated inflammation with the MyD88 pathway and adjuvanticity with the TRIF pathway. Gram-negative lipopolysaccharide (LPS) is a potent TLR4 agonist, and structurally related molecules signal through TLR4 to differing extents. Herein, we compared monophosphoryl lipid A (sMPL) and E6020, two synthetic, non-toxic LPS lipid A analogs used as vaccine adjuvants, for their capacities to activate TLR4-mediated innate immune responses and to enhance antibody production. In mouse macrophages, high dose sMPL activates MyD88-dependent signaling equivalently to E6020, while E6020 exhibits significantly more activation of the TRIF pathway (a "TRIF bias") than sMPL. Eritoran, a TLR4/MD2 antagonist, competitively inhibited sMPL more strongly than E6020. Despite these differences, sMPL and E6020 adjuvants enhanced antibody responses to comparable extents, with balanced immunoglobulin (Ig) isotypes in two immunization models. These data indicate that a TRIF bias is not necessarily predictive of superior adjuvanticity.

Differential role of MyD88 signaling in Streptococcus suis serotype 2-induced systemic and central nervous system diseases

Streptococcus suis serotype 2 is an important porcine bacterial pathogen and a zoonotic agent responsible for sudden death, septic shock and meningitis, with exacerbated inflammation being a hallmark of the systemic and central nervous system (CNS) infections. However, S. suis serotype 2 strains are genetically and phenotypically heterogeneous, being composed of a multitude of sequence types (STs) whose virulence greatly varies. Yet, most studies have used 'classical' virulent Eurasian ST1 or ST7 strains, even though ST25 and ST28 strains account for most isolates in North America. While recognition of S. suis by innate immune cells has been associated with the myeloid differentiation primary response 88 (MyD88)-dependent Toll-like receptor (TLR) pathway in vitro, particularly surface-associated TLR2, little information is available regarding its role in vivo. This study demonstrates for the first time a differential role of MyD88 signaling in S. suis-induced systemic and CNS diseases, regardless of strain background diversity. The MyD88-dependent pathway is critical for the development of systemic disease via its role in inflammation, which subsequently controls bacterial burden. However, and differently from what has been described in vitro, TLR2 and TLR4 individually do not contribute to systemic disease, suggesting possible compensation in their absence and/or a collaborative role with other MyD88-dependent TLRs. On the other hand, CNS disease does not necessarily require MyD88 signaling and, consequently, neither TLR2 nor TLR4, suggesting a partial implication of other pathways. Finally, regardless of its notable heterogeneity, recognition of S. suis serotype 2 appears to be similar, indicating that recognized components are conserved motifs.

Hematopoietic but not endothelial cell MyD88 contributes to host defense during gram-negative pneumonia derived sepsis

Klebsiella pneumoniae is an important cause of sepsis. The common Toll-like receptor adapter myeloid differentiation primary response gene (MyD)88 is crucial for host defense against Klebsiella. Here we investigated the role of MyD88 in myeloid and endothelial cells during Klebsiella pneumosepsis. Mice deficient for MyD88 in myeloid (LysM-Myd88(-/-)) and myeloid plus endothelial (Tie2-Myd88(-/-)) cells showed enhanced lethality and bacterial growth. Tie2-Myd88(-/-) mice reconstituted with control bone marrow, representing mice with a selective MyD88 deficiency in endothelial cells, showed an unremarkable antibacterial defense. Myeloid or endothelial cell MyD88 deficiency did not impact on lung pathology or distant organ injury during late stage sepsis, while LysM-Myd88(-/-) mice demonstrated a strongly attenuated inflammatory response in the airways early after infection. These data suggest that myeloid but not endothelial MyD88 is important for host defense during gram-negative pneumonia derived sepsis.

Functional analysis of a zebrafish myd88 mutant identifies key transcriptional components of the innate immune system

Toll-like receptors (TLRs) are an important class of pattern recognition receptors (PRRs) that recognize microbial and danger signals. Their downstream signaling upon ligand binding is vital for initiation of the innate immune response. In human and mammalian models, myeloid differentiation factor 88 (MYD88) is known for its central role as an adaptor molecule in interleukin 1 receptor (IL-1R) and TLR signaling. The zebrafish is increasingly used as a complementary model system for disease research and drug screening. Here, we describe a zebrafish line with a truncated version of MyD88 as the first zebrafish mutant for a TLR signaling component. We show that this immune-compromised mutant has a lower survival rate under standard rearing conditions and is more susceptible to challenge with the acute bacterial pathogens Edwardsiella tarda and Salmonella typhimurium. Microarray and quantitative PCR analysis revealed that expression of genes for transcription factors central to innate immunity (including NF-ĸB and AP-1) and the pro-inflammatory cytokine Il1b, is dependent on MyD88 signaling during these bacterial infections. Nevertheless, expression of immune genes independent of MyD88 in the myd88 mutant line was sufficient to limit growth of an attenuated S. typhimurium strain. In the case of infection with the chronic bacterial pathogen Mycobacterium marinum, we show that MyD88 signaling has an important protective role during early pathogenesis. During mycobacterial infection, the myd88 mutant shows accelerated formation of granuloma-like aggregates and increased bacterial burden, with associated lower induction of genes central to innate immunity. This zebrafish myd88 mutant will be a valuable tool for further study of the role of IL1R and TLR signaling in the innate immunity processes underlying infectious diseases, inflammatory disorders and cancer.

IRF3 contributes to sepsis pathogenesis in the mouse cecal ligation and puncture model

Much remains to be learned regarding which components of the innate immune response are protective versus detrimental during sepsis. Prior reports demonstrated that TLR9 and MyD88 play key roles in the CLP mouse model of sepsis; however, the role of additional PRRs and their signaling intermediates remains to be explored. In a prior report, we demonstrated that the signal adaptor IRF3 contributes to the systemic inflammatory response to liposome:DNA. We hypothesized that IRF3 might likewise promote sepsis in the CLP model. Here, we present results demonstrating that IRF3-KO mice have reduced disease score, mortality, hypothermia, and bacterial load following CLP versus WT counterparts. This is paired with reduced levels of systemic inflammatory mediators in IRF3-KO mice that undergo CLP. We demonstrate that peritoneal cells from WT CLP mice produce more cytokines than IRF3-KO counterparts on a per-cell basis; however, there are more cells in the peritoneum of IRF3-KO CLP mice. Finally, we show that IRF3 is activated in macrophages cultured with live or sonicated commensal bacteria. These results demonstrate that IRF3 plays a detrimental role in this mouse model of sepsis.

The essential adaptors of innate immune signaling

Microbial components and the endogenous molecules released from damaged cells can stimulate germ-line-encoded pattern recognition receptors (PRRs) to transduce signals to the hub of the innate immune signaling network-the adaptor proteins MyD88/TRIF/MAVS/STING/Caspase-1, where integrated signals relay to the relevant transcription factors IRF3/IRF7/NF-κB/ AP-1 and the signal transducer and activator of transcription 6 (STAT6) to trigger the expression of type I interferons and inflammatory cytokines or the assembly of inflammasomes. Most pleiotropic cytokines are secreted and bind to specific receptors, activating the signaling pathways including JAK-STAT for the proliferation, differentiation and functional capacity of immune cells. This review focuses on several critical adaptors in innate immune signaling cascades and recent progress in their molecular mechanisms.

Cutting edge: Divergent cell-specific functions of MyD88 for inflammatory responses and organ injury in septic peritonitis

Although global MyD88 deficiency attenuates lethal inflammation in sepsis, cell-specific functions of MyD88 remain largely unknown. Using mice with selective expression of MyD88 in myeloid cells (Myd88(MYEL)), we show that, during polymicrobial septic peritonitis, both myeloid and nonmyeloid cells contribute to systemic inflammation, whereas myeloid cell MyD88 was sufficient to fully establish the peritoneal cytokine response. Importantly, Myd88(MYEL) mice developed markedly aggravated liver injury that was linked to impaired upregulation of cellular inhibitor of apoptosis protein 2 and an excessive production of TNF-α. Upregulation of inducible cAMP early repressor (ICER), a known transcriptional repressor of the Tnfa gene, was impaired in Myd88(MYEL) mice. Moreover, Myd88(MYEL) mice showed enhanced transcription of the Tnfa gene and an excessive production of CCL3, which is also negatively regulated by ICER, but they had normal levels of CXCL1, which is expressed in an ICER-independent manner. Together, these findings suggest a novel protective role for nonmyeloid cell MyD88 in attenuating liver injury during septic peritonitis.

Polymicrobial periodontal pathogen transcriptomes in calvarial bone and soft tissue

Porphyromonas gingivalis, Treponema denticola, and Tannerella forsythia are consistently associated with adult periodontitis. This study sought to document the host transcriptome to a P. gingivalis, T. denticola, and T.forsythia challenge as a polymicrobial infection using a murine calvarial model of acute inflammation and bone resorption. Mice were infected with P. gingivalis, T. denticola, and T. forsythia over the calvaria, after which the soft tissues and calvarial bones were excised. A Murine GeneChip(®) array analysis of transcript profiles showed that 6997 genes were differentially expressed in calvarial bones (P < 0.05) and 1544 genes were differentially transcribed in the inflamed tissues after the polymicrobial infection. Of these genes, 4476 and 1035 genes in the infected bone and tissues were differentially expressed by upregulation. Biological pathways significantly impacted by the polymicrobial infection in calvarial bone included leukocyte transendothelial migration (LTM), cell adhesion molecules, adherens junction, major histocompatibility complex antigen, extracellular matrix-receptor interaction, and antigen processing and presentation resulting in inflammatory/cytokine/chemokine transcripts stimulation in bone and soft tissue. Intense inflammation and increased activated osteoclasts were observed in calvarias compared with sham-infected controls. Quantitative real-time RT-PCR analysis confirmed that the mRNA level of selected genes corresponded with the microarray expression. The polymicrobial infection regulated several LTM and extracellular membrane pathway genes in a manner distinct from mono-infection with P. gingivalis, T. denticola, or T. forsythia. To our knowledge, this is the first definition of the polymicrobially induced transcriptome in calvarial bone and soft tissue in response to periodontal pathogens.

Improved host defense against septic peritonitis in mice lacking MyD88 and TRIF is linked to a normal interferon response

The signaling adapters MyD88 and TRIF are engaged by TLRs and/or receptors of the IL-1 family and are considered important for innate immune responses that combat bacterial infections. Here, the consequences of a combined MyD88 and TRIF deficiency for the innate immune response against severe septic peritonitis was examined. We demonstrate that Myd88(-/-);Trif(Lps2/Lps2) mice had markedly reduced bacterial numbers in the peritoneal cavity and peripheral blood, indicating that bacterial clearance in this model is improved in the absence of MyD88/TRIF signals. Survival of Myd88(-/-); Trif(Lps2/Lps2) mice was improved significantly. The lack of MyD88/TRIF signaling prevented the excessive production of inflammatory cytokines and of IL-10. In contrast, Ifng mRNA was expressed at WT levels, and induction of Ifnb mRNA was reduced only by one-half. Consistent with these findings, numerous IFN-regulated genes, including p47 and p65 GTPases, as well as CXCL10, were expressed in a MyD88/TRIF-independent manner. In support of the in vivo data, Myd88(-/-); Trif(Lps2/Lps2) macrophages stimulated with live intestinal bacteria produced normal amounts of CXCL10. The production of p47 GTPases and CXCL10 in septic peritonitis was found to be dependent on the presence of IFNAR1, but not IFN-γ, indicating a normal induction of the type I IFN response in Myd88(-/-);Trif(Lps2/Lps2) mice, despite attenuated IFN-β production. Together, these results provide evidence that in severe septic peritonitis, the absence of MyD88 and TRIF balances the innate immune response in a favorable manner by attenuating deleterious responses such as excessive cytokine release, while leaving intact protective IFN responses.

Increased inflammation and lethality of Dusp1-/- mice in polymicrobial peritonitis models

The mitogen-activated protein kinase phosphatase Dusp1 (also known as MKP-1) is essential for control of the inflammatory response to systemic challenge with the lipopolysaccharide of Gram-negative bacteria. Here, we have investigated the consequences of Dusp1-deficiency in colon ascendens stent peritonitis (CASP) and caecal ligation and puncture (CLP), two mouse models of septic peritonitis. Following CASP, Dusp1(-/-) mice had increased serum levels of CCL4, interleukin-10 (IL-10) and IL-6, with differences from wild-type mice being dependent on severity of sepsis. These cytokines, along with inducible nitric oxide synthase messenger RNA, were also expressed at higher levels in spleen and liver. Similar over-production of these cytokines was detected in the CLP model, with even larger differences from wild-type mice. Despite the increased inflammatory response, bacterial clearance was impaired in Dusp1(-/-) mice subjected to CASP and CLP. Dusp1(-/-) mice suffered increased lethality in both peritonitis models. Together our data indicate that exaggerated inflammatory responses to gut bacteria introduced into the peritoneum in the absence of Dusp1 do not help to control bacterial replication but are detrimental for the host.

Effects of Muclin (Dmbt1) deficiency on the gastrointestinal system

The Dmbt1 gene encodes alternatively spliced glycoproteins that are either membrane-associated or secreted epithelial products. Functions proposed for Dmbt1 include it being a tumor suppressor, having roles in innate immune defense and inflammation, and being a Golgi-sorting receptor in the exocrine pancreas. The heavily sulfated membrane glycoprotein mucin-like glycoprotein (Muclin) is a Dmbt1 product that is strongly expressed in organs of the gastrointestinal (GI) system. To explore Muclin's functions in the GI system, the Dmbt1 gene was targeted to produce Muclin-deficient mice. Muclin-deficient mice have normal body weight gain and are fertile. The Muclin-deficient mice did not develop GI tumors, even when crossed with mice lacking the known tumor suppressor p53. When colitis was induced by dextran sulfate sodium, there was no significant difference in disease severity in Muclin-deficient mice. Also, when acute pancreatitis was induced with supraphysiological caerulein, there was no difference in disease severity in the Muclin-deficient mice. Exocrine pancreatic function was impaired, as measured by attenuated neurohormonal-stimulated amylase release from Muclin-deficient acinar cells. Also, by [(35)S]Met/Cys pulse-chase analysis, traffic of newly synthesized protein to the stimulus-releasable pool was significantly retarded in Muclin-deficient cells compared with wild type. Thus Muclin deficiency impairs trafficking of regulated proteins to a stimulus-releasable pool in the exocrine pancreas.

MyD88 functions as a negative regulator of TLR3/TRIF-induced corneal inflammation by inhibiting activation of c-Jun N-terminal kinase

The adaptor molecule MyD88 is necessary for responses to all Toll-like receptors except TLR3 and a subset of TLR4 signaling events, which are mediated by the adaptor molecule TRIF. To determine the role of TRIF in host inflammatory responses, corneal epithelium of C57BL/6, TLR3(-/-), TRIF(-/-), and MyD88(-/-) mice was abraded and stimulated with the synthetic TLR3 ligand poly(I:C). We found that poly(I:C) induced a pronounced cellular infiltration into the corneal stroma, which was TLR3- and TRIF-dependent. Unexpectedly, the inflammatory response was exacerbated in MyD88(-/-) mice, with enhanced neutrophil and F4/80(+) cell infiltration into the corneal stroma and elevated corneal haze, which is an indicator of loss of corneal transparency. To determine whether MyD88-dependent inhibition of TLR3/TRIF responses is a general phenomenon, we examined cytokine production by MyD88(-/-) bone marrow-derived macrophages; however, no significant difference was observed between MyD88(+/+) or MyD88(-/-) macrophages. In contrast, human corneal epithelial cells (HCECs) transfected with MyD88 small interfering RNA had significantly increased (2.5-fold) CCL5/RANTES production compared with control HCECs, demonstrating a negative regulatory role for MyD88 in TLR3/TRIF responses in these cells. Finally, knockdown of MyD88 in HCECs resulted in increased phosphorylation of c-Jun N-terminal kinase (JNK), but not p38, IRF-3, or NF-kappaB. Consistent with this finding, the JNK inhibitor SP600125, but not p38 inhibitor SB203580, ablated this response. Taken together, these findings demonstrate a novel JNK-dependent inhibitory role for MyD88 in the TLR3/TRIF activation pathway.

Transcriptome profiling of the small intestinal epithelium in germfree versus conventional piglets

BACKGROUND

To gain insight into host-microbe interactions in a piglet model, a functional genomics approach was used to address the working hypothesis that transcriptionally regulated genes associated with promoting epithelial barrier function are activated as a defensive response to the intestinal microbiota. Cesarean-derived germfree (GF) newborn piglets were colonized with adult swine feces, and villus and crypt epithelial cell transcriptomes from colonized and GF neonatal piglets were compared using laser-capture microdissection and high-density porcine oligonucleotide microarray technology.

RESULTS

Consistent with our hypothesis, resident microbiota induced the expression of genes contributing to intestinal epithelial cell turnover, mucus biosynthesis, and priming of the immune system. Furthermore, differential expression of genes associated with antigen presentation (pan SLA class I, B2M, TAP1 and TAPBP) demonstrated that microbiota induced immune responses using a distinct regulatory mechanism common for these genes. Specifically, gene network analysis revealed that microbial colonization activated both type I (IFNAR) and type II (IFNGR) interferon receptor mediated signaling cascades leading to enhanced expression of signal transducer and activator of transcription 1 (STAT1), STAT2 and IFN regulatory factor 7 (IRF7) transcription factors and the induction of IFN-inducible genes as a reflection of intestinal epithelial inflammation. In addition, activated RNA expression of NF-kappa-B inhibitor alpha (NFkappaBIA; a.k.a I-kappa-B-alpha, IKBalpha) and toll interacting protein (TOLLIP), both inhibitors of inflammation, along with downregulated expression of the immunoregulatory transcription factor GATA binding protein-1 (GATA1) is consistent with the maintenance of intestinal homeostasis.

CONCLUSION

This study supports the concept that the intestinal epithelium has evolved to maintain a physiological state of inflammation with respect to continuous microbial exposure, which serves to sustain a tight intestinal barrier while preventing overt inflammatory responses that would compromise barrier function.