The role of innate immunity in the host defense against intestinal bacterial pathogens

Pseudomonas aeruginosa: Infections, Animal Modeling, and Therapeutics

Pseudomonas aeruginosa is an important Gram-negative opportunistic pathogen which causes many severe acute and chronic infections with high morbidity, and mortality rates as high as 40%. What makes P. aeruginosa a particularly challenging pathogen is its high intrinsic and acquired resistance to many of the available antibiotics. In this review, we review the important acute and chronic infections caused by this pathogen. We next discuss various animal models which have been developed to evaluate P. aeruginosa pathogenesis and assess therapeutics against this pathogen. Next, we review current treatments (antibiotics and vaccines) and provide an overview of their efficacies and their limitations. Finally, we highlight exciting literature on novel antibiotic-free strategies to control P. aeruginosa infections.

Co-immunomodulatory Activities of Anionic Macromolecules Extracted from Codium fragile with Red Ginseng Extract on Peritoneal Macrophage of Immune-Suppressed Mice

In this study we investigated the immune effects of oral administration of anionic macromolecules extracted from Codium fragile (CFAM) and red ginseng extract mixture on the peritoneal macrophage cells in immune-suppressed mice. Cyclophosphamide (CY) induces the immune-suppressed condition. CY-treated mice were orally fed with different concentrations of CFAM supplemented with red ginseng extract and the peritoneal macrophages collected. CY treatment significantly decreased the immune activities of peritoneal macrophages, compared to the normal mice. The administration of CFAM mixed with red ginseng extract significantly boosted the viability of macrophage cells and nitric oxide production of peritoneal macrophages. Further, the oral administration of CFAM mixed with red ginseng extract up-regulated the expression of iNOS, COX-2, and TLR-4 as well as cytokines such as IL-1β, IL-6, TNF-α, and IFN-γ more than the red ginseng-treated group. This study showed that CFAM enhanced the immune activity of red ginseng extract in the peritoneal macrophage cells of immune-suppressed mice. Furthermore, CFAM might be used as a co-stimulant of red ginseng extract through the regulation of macrophage cells for the enhancement of human health and immunity.

TLR3 absence confers increased survival with improved macrophage activity against pneumonia

Toll-like receptor 3 (TLR3) is a pathogen recognition molecule associated with viral infection with double-stranded RNA (dsRNA) as its ligand. We evaluated the role of TLR3 in bacterial pneumonia using Klebsiella pneumoniae (KP). WT and TLR3-/- mice were subjected to a lethal model of KP. Alveolar macrophage polarization, bactericidal activity, and phagocytic capacity were compared. RNA-sequencing was performed on alveolar macrophages from the WT and TLR3-/- mice. Adoptive transfers of alveolar macrophages from TLR3-/- mice to WT mice with KP were evaluated for survival. Expression of TLR3 in postmortem human lung samples from patients who died from gram-negative pneumonia and pathological grading of pneumonitis was determined. Mortality was significantly lower in TLR3-/-, and survival improved in WT mice following antibody neutralization of TLR3 and with TLR3/dsRNA complex inhibitor. Alveolar macrophages from TLR3-/- mice demonstrated increased bactericidal and phagocytic capacity. RNA-sequencing showed an increased production of chemokines in TLR3-/- mice. Adoptive transfer of alveolar macrophages from the TLR3-/- mice restored the survival in WT mice. Human lung samples demonstrated a good correlation between the grade of pneumonitis and TLR3 expression. These data represent a paradigm shift in understanding the mechanistic role of TLR3 in bacterial pneumonia.

Interactions Between the Gut Microbiota and the Host Innate Immune Response Against Pathogens

The mammalian intestine is colonized by over a trillion microbes that comprise the "gut microbiota," a microbial community which has co-evolved with the host to form a mutually beneficial relationship. Accumulating evidence indicates that the gut microbiota participates in immune system maturation and also plays a central role in host defense against pathogens. Here we review some of the mechanisms employed by the gut microbiota to boost the innate immune response against pathogens present on epithelial mucosal surfaces. Antimicrobial peptide secretion, inflammasome activation and induction of host IL-22, IL-17, and IL-10 production are the most commonly observed strategies employed by the gut microbiota for host anti-pathogen defense. Taken together, the body of evidence suggests that the host gut microbiota can elicit innate immunity against pathogens.

Intestinal epithelial Caspase-8 signaling is essential to prevent necroptosis during Salmonella Typhimurium induced enteritis

Although induction of host cell death is a pivotal step during bacteria-induced gastroenteritis, the molecular regulation remains to be fully characterized. To expand our knowledge, we investigated the role of the central cell death regulator Caspase-8 in response to Salmonella Typhimurium. Here, we uncovered that intestinal salmonellosis was associated with strong upregulation of members of the host cell death machinery in intestinal epithelial cells (IECs) as an early event, suggesting that elimination of infected IECs represents a host defense strategy. Indeed, Casp8^∆IEC mice displayed severe tissue damage and high lethality after infection. Additional deletion of Ripk3 or Mlkl rescued epithelial cell death and lethality of Casp8^∆IEC mice, demonstrating the crucial role of Caspase-8 as a negative regulator of necroptosis. While Casp8^∆IECTnfr1^-/- mice showed improved survival after infection, tissue destruction was similar to Casp8^∆IEC mice, indicating that necroptosis partially depends on TNF-α signaling. Although there was no impairment in antimicrobial peptide secretion during the early phase of infection, functional Caspase-8 seems to be required to control pathogen colonization. Collectively, these results demonstrate that Caspase-8 is essential to prevent Salmonella Typhimurium induced enteritis and to ensure host survival by two different mechanisms: maintenance of intestinal barrier function and restriction of pathogen colonization.

A Type III Effector NleF from EHEC Inhibits Epithelial Inflammatory Cell Death by Targeting Caspase-4

Enterohemorrhagic E. coli (EHEC) is a highly pathogenic bacterial strain capable of inducing severe gastrointestinal disease. Here, we show that EHEC uses the T3SS effector NleF to counteract the host inflammatory response by dampening caspase-4-mediated inflammatory epithelial cell death and by preventing the production of IL-1β. The other two inflammatory caspases, caspase-1 and caspase-5, are not involved in EHEC ΔnleF-induced inflammatory cell death. We found that NleF not only interrupted the heterodimerization of caspase-4-p19 and caspase-4-p10, but also inhibited the interaction of caspase-1 and caspase-4. The last four amino acids of the NleF carboxy terminus are essential in inhibiting caspase-4-dependent inflammatory cell death.

Use of ethanol extracts of Terminalia chebula to prevent periodontal disease induced by dental plaque bacteria

BACKGROUND

The fruit of the Terminalia chebula tree has been widely used for the treatment of various disorders. Its anti-diabetic, anti-mutagenic, anti-oxidant, anti-bacterial, anti-fungal, and anti-viral effects have been studied. Dental plaque bacteria (DPB) are intimately associated with gingivitis and periodontitis. In the quest for materials that will prove useful in the treatment and prevention of periodontal disease, we investigated the preventive effects of an ethanol extract of Terminalia chebula (EETC) on DPB-induced inflammation and bone resorption.

METHODS

The anti-bacterial effect of EETC was analyzed using the disc diffusion method. The anti-inflammatory effect of EETC was determined by molecular biological analysis of the DPB-mediated culture cells. Prevention of osteoclastic bone resorption by EETC was explored using osteoclast formation and pit formation assays.

RESULTS

EETC suppressed the growth of oral bacteria and reduced the induction of inflammatory cytokines and proteases, abolishing the expression of PGE2 and COX-2 and inhibiting matrix damage. By stimulating the DPB-derived lipopolysaccharides, EETC inhibited both osteoclast formation in osteoclast precursors and RANKL expression in osteoblasts, thereby contributing to the prevention of bone resorption.

CONCLUSIONS

EETC may be a beneficial supplement to help prevent DPB-mediated periodontal disease.

Poncirin and its metabolite ponciretin attenuate colitis in mice by inhibiting LPS binding on TLR4 of macrophages and correcting Th17/Treg imbalance

ETHNOPHARMACOLOGICAL RELEVANCE

The fruit of Poncirus trifoliate, which contains poncirin as a main constituent, is frequently used in the traditional Chinese medicine for inflammation, asthma, and infection diseases.

AIM OF THE STUDY

To examine anti-colitic effects of poncirin and ponciretin, a metabolite of poncirin by gut microbiota.

MATERIALS AND METHODS

Colitis was induced in mice by the intrarectal injection of 2,4,6-trinitrobenzenesulfonic acid (TNBS). Inflammatory markers were analyzed by enzyme-linked immunosorbent assay, immunoblotting, quantitative polymerase chain reaction, confocal microscopy, and flow cytometry. Peritoneal macrophages were isolated from mice stimulated with 4% thioglycolate.

RESULTS

Poncirin was metabolized to ponciretin in vitro and in vivo by gut microbiota of mice. Orally administered poncirin and ponciretin suppressed TNBS-induced colitis in mice: these inhibited colon shortening, myeloperoxidase activity, NF-κB activation, and Th17 cell differentiation, but increased occludin, claudin-1, and ZO-1 expressions and Treg cell differentiation. Poncirin and ponciretin suppressed the differentiation of splenocytes into Th17 cells and expression of IL-17 and Foxp3 in vitro, as well as the activation of macrophages stimulated with lipopolysaccharide (LPS) by inhibiting the binding of LPS on TLR4 of macrophages. These increased the differentiation of splenocytes into Treg cells. The ant-inflammatory effect of ponciretin was superior to that of poncirin.

CONCLUSION

Orally administered poncirin is metabolized to ponciretin by gut microbiota and poncirin and ponciretin attenuates colitis by suppressing NF-κB activation through the inhibition of LPS binding on macrophages and correcting Th17/Treg cell imbalance.

Interaction of Salmonella enterica Serovar Typhimurium with Intestinal Organoids Derived from Human Induced Pluripotent Stem Cells

The intestinal mucosa forms the first line of defense against infections mediated by enteric pathogens such as salmonellae. Here we exploited intestinal "organoids" (iHOs) generated from human induced pluripotent stem cells (hIPSCs) to explore the interaction of Salmonella enterica serovar Typhimurium with iHOs. Imaging and RNA sequencing were used to analyze these interactions, and clear changes in transcriptional signatures were detected, including altered patterns of cytokine expression after the exposure of iHOs to bacteria. S. Typhimurium microinjected into the lumen of iHOs was able to invade the epithelial barrier, with many bacteria residing within Salmonella-containing vacuoles. An S. Typhimurium invA mutant defective in the Salmonella pathogenicity island 1 invasion apparatus was less capable of invading the iHO epithelium. Hence, we provide evidence that hIPSC-derived organoids are a promising model of the intestinal epithelium for assessing interactions with enteric pathogens.

Anti-inflammatory effects of ginsenoside Rg1 and its metabolites ginsenoside Rh1 and 20(S)-protopanaxatriol in mice with TNBS-induced colitis

Ginsenoside Rg1, one of the main constituents of Panax ginseng, exhibits anti-inflammatory effect. In a preliminary study, it was observed that ginsenoside Rg1 was metabolized to 20(S)-protopanaxtriol via ginsenosides Rh1 and F1 by gut microbiota. We further investigated the anti-inflammatory effects of ginsenoside Rg1 and its metabolites in vitro and in vivo. Ginsenosides Rg1, Rh1, and 20(S)-protopanaxtriol inhibited the activation of NF-κB activation, phosphorylation of transforming growth factor beta-activated kinase 1 and interleukin (IL)-1 receptor-associated kinase, and expression of tumor necrosis factor-α and IL-1β in lipopolysaccharide (LPS)-stimulated macrophages. They also inhibited the binding of LPS to toll-like receptor 4 on the macrophages. Orally administered ginsenoside Rg1, Rh1, or 20(S)-protopanaxtriol inhibited 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colon shortening, myeloperoxidase activity, and expression of IL-1β, IL-17, and tumor necrosis factor-α in mice with TNBS-induced colitis. They did not only inhibit TNBS-induced NF-κB activation, but also restored TNBS-induced Th17/Treg imbalance. They restored IL-10 and Foxp3 expression. Moreover, they inhibited Th17 cell differentiation in vitro. Of these metabolites, in vitro and in vivo anti-inflammatory effect of 20(S)-protopanaxtriol was the most potent, followed by Rh1. These findings suggest that ginsenoside Rg1 is metabolized to 20(S)-protopanaxtriol via ginsenosides Rh1 and F1 and these metabolites particularly 20(S)-protopanaxtriol, may ameliorate inflammatory disease such as colitis by inhibiting the binding of LPS to TLR4 on macrophages and restoring the Th17/Treg imbalance.

Ursolic acid isolated from the seed of Cornus officinalis ameliorates colitis in mice by inhibiting the binding of lipopolysaccharide to Toll-like receptor 4 on macrophages

Ursolic acid, which was isolated from an ethanol extract of Cornus officinalis seed, potently inhibited nuclear factor κ light-chain enhancer of activated B cells (NF-κB) activation in lipopolysaccharide (LPS)-stimulated peritoneal macrophages. Therefore, we investigated the anti-inflammatory mechanism of ursolic acid in LPS-stimulated macrophages and colitic mice. Ursolic acid inhibited phosphorylation of interleukin 1 receptor-associated kinase (IRAK)1, TAK1, inhibitor of nuclear factor κB kinase subunit β (IKKβ), and IκBα as well as activation of NF-κB and MAPKs in LPS-stimulated macrophages. Ursolic acid suppressed LPS-stimulated interleukin (IL)-1β, IL-6, tumor necrosis factor (TNF)-α, cyclooxygenase (COX)-2, and inducible NO synthetase (iNOS) expression as well as PGE2 and NO levels. Ursolic acid not only inhibited the Alexa Fluor 488-conjugated LPS-mediated shift of macrophages but also reduced the intensity of fluorescent LPS bound to the macrophages transiently transfected with or without MyD88 siRNA. However, ursolic acid did not suppress NF-κB activation in peptidoglycan-stimulated macrophages. Oral administration of ursolic acid significantly inhibited 2,4,6-trinitrobenzenesulfonic acid (TNBS)-induced colon shortening and myeloperoxidase (MPO) activity in mice. Ursolic acid also suppressed TNBS-induced COX-2 and iNOS expression as well as NF-κB activation in colon tissues. Ursolic acid (20 mg/kg) also inhibited TNBS-induced IL-1β, IL-6, TNF-α by 93, 86, and 85%, respectively (p < 0.05). However, ursolic acid reversed TNBS-mediated downregulation of IL-10 expression to 79% of the normal control group (p < 0.05). On the basis of these findings, ursolic acid may ameliorate colitis by regulating NF-κB and MAPK signaling pathways via the inhibition of LPS binding to TLR4 on immune cells.

Ginsenoside Re ameliorates inflammation by inhibiting the binding of lipopolysaccharide to TLR4 on macrophages

Ginseng (the root of Panax ginseng C.A. Meyer, family Araliaceae), which contains protopanaxadiol ginsenoside Rb1 and protopanaxatriol ginsenoside Re as main constituents, is frequently used to treat cancer, inflammation, and stress. In the preliminary study, protopanaxatriol ginsenoside Re inhibited NF-κB activation in lipopolysaccharide (LPS)-stimulated murine peritoneal macrophages. Therefore, we investigated its anti-inflammatory effect in peptidoglycan (PGN)-, LPS-, or tumor necrosis factor-α (TNF-α)-stimulated peritoneal macrophages and, in addition, in LPS-induced systemic inflammation and 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis in mice. Ginsenoside Re inhibited IKK-β phosphorylation and NF-κB activation, as well as the expression of proinflammatory cytokines, TNF-α and IL-1β, in LPS-stimulated peritoneal macrophages, but it did not inhibit them in TNF-α- or PG-stimulated peritoneal macrophages. Ginsenoside Re also inhibited IRAK-1 phosphorylation induced by LPS, as well as IRAK-1 and IRAK-4 degradations in LPS-stimulated peritoneal macrophages. Ginsenoside Re inhibited the binding of Alexa Fluor 488-conjugated LPS to TLR4 on peritoneal macrophages. Furthermore, ginsenoside Re inhibited the binding of LPS to TLR4 on peritoneal macrophages transiently transfected with MyD88 siRNAs. Orally administered ginsenoside Re significantly inhibited the expression of IL-1β and TNF-α on LPS-induced systemic inflammation and TNBS-induced colitis in mice. Ginsenoside Re inhibited colon shortening and myeloperoxidase activity in TNBS-treated mice. Ginsenoside Re reversed the reduced expression of tight-junction-associated proteins ZO-1, claudin-1, and occludin. Ginsenoside Re (20 mg/kg) inhibited the activation of NF-κB in TNBS-treated mice. On the basis of these findings, ginsenoside Re may ameliorate inflammation by inhibiting the binding of LPS to TLR4 on macrophages.

TRIF mobilizes unique primary defense against Gram-negative bacteria in intestinal interface

The gastrointestinal tract is the largest mucosal surface in our body. It houses diverse microorganisms that collectively form the commensal microbial community. The security of this community is kept by host-microbial interactions and is violated by foreign pathogens that induce local as well as systemic pathology. In most cases, gastrointestinal infections are caused by Gram-negative enteropathogens, which trigger host immune responses through the TLR4 signaling pathways. Although TRIF is one of the major pathways downstream of TLR4, very little is known about how the TRIF pathway contributes to intestinal defense against pathogenic infection. Recently, we reported a unique role of TRIF signaling in host response to an enterophathogen Yersinia enterocolitica, which consisted of IFN-β induction from regional macrophages followed by activation of NK cells in the mesenteric lymph nodes. In this addendum, we show distinct roles for TRIF-dependent host response in intestinal vs. systemic infection with Gram-negative enterophathogens.