Epithelial-Specific TLR4 Knockout Challenges Current Evidence of TLR4 Homeostatic Control of Gut Permeability

Cell-free hemoglobin triggers macrophage cytokine production via TLR4 and MyD88

Cell-free hemoglobin (CFH) is elevated in the airspace of patients with acute respiratory distress syndrome (ARDS) and is sufficient to cause acute lung injury in a murine model. However, the pathways through which CFH causes lung injury are not well understood. Toll-like receptor 4 (TLR4) is a mediator of inflammation after detection of damage- and pathogen-associated molecular patterns. We hypothesized that TLR4 signaling mediates the proinflammatory effects of CFH in the airspace. After intratracheal CFH, BALBc mice deficient in TLR4 had reduced inflammatory cell influx into the airspace [bronchoalveolar lavage (BAL) cell counts, median TLR4 knockout (KO): 0.8 × 104/mL [IQR 0.4-1.2 × 104/mL], wild-type (WT): 3.0 × 104/mL [2.2-4.0 × 104/mL], P < 0.001] and attenuated lung permeability (BAL protein, TLR4KO: 289 µg/mL [236-320], WT: 488 µg/mL [422-536], P < 0.001). These mice also had attenuated production of interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α in the airspace. C57Bl/6 mice lacking TLR4 on myeloid cells only (LysM.Cre+/-TLR4fl/fl) had reduced cytokine production in the airspace after CFH, without attenuation of lung permeability. In vitro studies confirm that WT primary murine alveolar macrophages exposed to CFH (0.01-1 mg/mL) had dose-dependent increases in IL-6, IL-1 β, CXC motif chemokine ligand 1 (CXCL-1), TNF-α, and IL-10 (P < 0.001). Murine MH-S alveolar-like macrophages show TLR4-dependent expression of IL-1β, IL-6, and CXCL-1 in response to CFH. Primary alveolar macrophages from mice lacking TLR4 adaptor proteins myeloid differentiation primary response 88 (MyD88) or TIR-domain-containing adapter-inducing interferon-β (TRIF) revealed that MyD88KO macrophages had 71-96% reduction in CFH-dependent proinflammatory cytokine production (P < 0.001), whereas macrophages from TRIFKO mice had variable changes in cytokine responses. These data demonstrate that myeloid TLR4 signaling through MyD88 is a key regulator of airspace inflammation in response to CFH.NEW & NOTEWORTHY Cell-free hemoglobin (CFH) is elevated in the airspace of most patients with acute respiratory distress syndrome and causes severe inflammation. Here, we identify that CFH contributes to macrophage-induced cytokine production via Toll-like receptor 4 (TLR4) and myeloid differentiation primary response 88 (MyD88) signaling. These data increase our knowledge of the mechanisms through which CFH contributes to lung injury and may inform development of targeted therapeutics to attenuate inflammation.

Toll-like receptor 4 deletion partially protects mice from high fat diet-induced arterial stiffness despite perturbation to the gut microbiota

The present study aimed to determine the effects of toll-like receptor 4 (TLR4) deletion on high fat diet-induced aortic stiffness and gut microbiota alterations. We hypothesized that a high fat diet would result in perturbation of the gut microbiota in both control and TLR4 knockout mice (TLR4-/-), but that the absence of TLR4 signaling would protect mice from downstream vascular consequences of the high fat diet. Male control mice (CON, n=12) and TLR4-/- mice (KO, n=12) were fed either a standard low-fat diet (SD) or a high fat diet (HFD) (60% kcals from fat) for 6 months, after which time measurements of aortic stiffness (via pulse wave velocity [aPWV]) and gut microbiota composition (16S rRNA sequencing) were determined. Compared to the SD, HFD reduced microbial variability, promoted perturbation of the gut microbiota, and increased intestinal permeability in both CON and KO mice, with no effect of genotype. This increased intestinal permeability in HFD mice was accompanied by increases in plasma lipopolysaccharide binding protein (LBP) levels, an indicator of circulating endotoxin (p<0.05 for all comparisons between HFD and SD groups). aPWV was increased in CON+HFD mice (CON+HFD vs CON+SD: 525.4 ± 16.5 cm/sec vs. 455.2 ± 16.5 cm/sec; p<0.05), whereas KO+HFD mice displayed partial protection from HFD-induced arterial stiffening (KO+HFD vs. CON+SD: 488.2 ± 16.6 cm/sec vs. 455.2 ± 16.5 cm/sec; p=0.8) (KO+HFD vs. CON+HFD: 488.2 ± 16.6 cm/sec vs. 525.4 ± 16.5 cm/sec; p=0.1). In summary, TLR4 KO mice are not protected from deleterious alterations in gut microbial composition or intestinal permeability following a HFD, but are partially protected from the downstream arterial stiffening, suggesting that TLR4 signaling is not required for HFD-mediated intestinal disturbances, but is an important determinant of downstream vascular consequences.

Microbial Metabolites as Ligands to Xenobiotic Receptors: Chemical Mimicry as Potential Drugs of the Future

Xenobiotic receptors, such as the pregnane X receptor, regulate multiple host physiologic pathways including xenobiotic metabolism, certain aspects of cellular metabolism, and innate immunity. These ligand-dependent nuclear factors regulate gene expression via genomic recognition of specific promoters and transcriptional activation of the gene. Natural or endogenous ligands are not commonly associated with this class of receptors; however, since these receptors are expressed in a cell-type specific manner in the liver and intestines, there has been significant recent effort to characterize microbially derived metabolites as ligands for these receptors. In general, these metabolites are thought to be weak micromolar affinity ligands. This journal anniversary minireview focuses on recent efforts to derive potentially nontoxic microbial metabolite chemical mimics that could one day be developed as drugs combating xenobiotic receptor-modifying pathophysiology. The review will include our perspective on the field and recommend certain directions for future research. SIGNIFICANCE STATEMENT: Xenobiotic receptors (XRs) regulate host drug metabolism, cellular metabolism, and immunity. Their presence in host intestines allows them to function not only as xenosensors but also as a response to the complex metabolic environment present in the intestines. Specifically, this review focuses on describing microbial metabolite-XR interactions and the translation of these findings toward discovery of novel chemical mimics as potential drugs of the future for diseases such as inflammatory bowel disease.

Intestinal toll-like receptor 4 knockout alters the functional capacity of the gut microbiome following irinotecan treatment

PURPOSE

Irinotecan can cause high levels of diarrhea caused by toxic injury to the gastrointestinal microenvironment. Toll-like receptor 4 (TLR4) and the gut microbiome have previously been implicated in gastrointestinal toxicity and diarrhea; however, the link between these two factors has not been definitively determined. We used a tumor-bearing, intestinal epithelial cell (IEC) TLR4 knockout model (Tlr4^ΔIEC) to assess microbiome changes following irinotecan treatment. We then determined if a fecal microbiota transplant (FMT) between Tlr4^ΔIEC and wild-type (WT) mice altered irinotecan-induced gastrointestinal toxicity.

METHODS

MC-38 colorectal cancer cells were injected into WT and Tlr4^ΔIEC mice. Fecal samples were collected prior to tumor inoculation, prior to irinotecan treatment and at cull. 16S rRNA gene sequencing was used to assess changes in the microbiome. Next, FMT was used to transfer the microbiome phenotype between Tlr4^ΔIEC and WT mice prior to irinotecan treatment. Gastrointestinal toxicity symptoms were assessed.

RESULTS

In study 1, there were no compositional differences in the microbiome between Tlr4^ΔIEC and WT mice at baseline. However, predicted functional capacity of the microbiome was different between WT and Tlr4^ΔIEC at baseline and post-irinotecan. In study 2, Tlr4^ΔIEC mice were protected from grade 3 diarrhea. Additionally, WT mice who did not receive FMT had more colonic damage in the colon compared to controls (P = 0.013). This was not seen in Tlr4^ΔIEC mice or WT mice who received FMT (P > 0.05).

CONCLUSION

Tlr4^ΔIEC and WT had no baseline compositional microbiome differences, but functional differences at baseline and following irinotecan. FMT altered some aspects of irinotecan-induced gastrointestinal toxicity.