Kevin P. Mollen

Emerging paradigm: toll-like receptor 4-sentinel for the detection of tissue damage.

ABSTRACT The systemic inflammatory response syndrome initiated by infection shares many features in common with the trauma-induced systemic response. The toll-like receptors (TLRs) stand at the interface of innate immune activation in the settings of both infection and sterile injury by responding to a variety of microbial and endogenous ligands alike. Recently, a body of literature has evolved describing a key role for TLRs in acute injury using rodent models of hemorrhagic shock, ischemia and reperfusion, tissue trauma and wound repair, and various toxic exposures. This review will detail the observations implicating a TLR family member, TLR4, as a key component of the initial injury response.

Nitrite Potently Inhibits Hypoxic and Inflammatory Pulmonary Arterial Hypertension and Smooth Muscle Proliferation via Xanthine Oxidoreductase–Dependent Nitric Oxide Generation

Background— Pulmonary arterial hypertension is a progressive proliferative vasculopathy of the small pulmonary arteries that is characterized by a primary failure of the endothelial nitric oxide and prostacyclin vasodilator pathways, coupled with dysregulated cellular proliferation. We have recently discovered that the endogenous anion salt nitrite is converted to nitric oxide in the setting of physiological and pathological hypoxia. Considering the fact that nitric oxide exhibits vasoprotective properties, we examined the effects of nitrite on experimental pulmonary arterial hypertension. Methods and Results— We exposed mice and rats with hypoxia or monocrotaline-induced pulmonary arterial hypertension to low doses of nebulized nitrite (1.5 mg/min) 1 or 3 times a week. This dose minimally increased plasma and lung nitrite levels yet completely prevented or reversed pulmonary arterial hypertension and pathological right ventricular hypertrophy and failure. In vitro and in vivo studies revealed that nitrite in the lung was metabolized directly to nitric oxide in a process significantly enhanced under hypoxia and found to be dependent on the enzymatic action of xanthine oxidoreductase. Additionally, physiological levels of nitrite inhibited hypoxia-induced proliferation of cultured pulmonary artery smooth muscle cells via the nitric oxide–dependent induction of the cyclin-dependent kinase inhibitor p21Waf1/Cip1. The therapeutic effect of nitrite on hypoxia-induced pulmonary hypertension was significantly reduced in the p21-knockout mouse; however, nitrite still reduced pressures and right ventricular pathological remodeling, indicating the existence of p21-independent effects as well. Conclusion— These studies reveal a potent effect of inhaled nitrite that limits pathological pulmonary arterial hypertrophy and cellular proliferation in the setting of experimental pulmonary arterial hypertension.

Early events in the recognition of danger signals after tissue injury

The systemic inflammatory response observed in the setting of overwhelming infection bears striking similarities to that observed in the setting of severe traumatic injury from a clinical and physiologic standpoint. Recent observations have demonstrated that these disparate clinical entities share common mediators on a molecular level. TLRs, specifically TLR4, and the endogenous molecule high‐mobility group box 1 are among the mediators that are known to play a role in inflammation in the setting of sepsis. Evidence is accumulating that demonstrates that these mediators also play a role in the host response to tissue injury. Here, we highlight findings from the 7th World Conference on Trauma, Shock, Inflammation and Sepsis in Munich, Germany, in the context of this growing body of literature.

Signaling of high mobility group box 1 (HMGB1) through toll-like receptor 4 in macrophages requires CD14.

High mobility group box 1 (HMGB1) is a DNA-binding protein that possesses cytokinelike, proinflammatory properties when released extracellularly in the C23–C45 disulfide form. HMGB1 also plays a key role as a mediator of acute and chronic inflammation in models of sterile injury. Although HMGB1 interacts with multiple pattern recognition receptors (PRRs), many of its effects in injury models occur through an interaction with toll-like receptor 4 (TLR4). HMGB1 interacts directly with the TLR4/myeloid differentiation protein 2 (MD2) complex, although the nature of this interaction remains unclear. We demonstrate that optimal HMGB1-dependent TLR4 activation in vitro requires the coreceptor CD14. TLR4 and MD2 are recruited into CD14-containing lipid rafts of RAW264.7 macrophages after stimulation with HMGB1, and TLR4 interacts closely with the lipid raft protein GM1. Furthermore, we show that HMGB1 stimulates tumor necrosis factor (TNF)-α release in WT but not in TLR4−/−, CD14−/−, TIR domain-containing adapter-inducing interferon-β (TRIF)−/− or myeloid differentiation primary response protein 88 (MyD88)−/− macrophages. HMGB1 induces the release of monocyte chemotactic protein 1 (MCP-1), interferon gamma-induced protein 10 (IP-10) and macrophage inflammatory protein 1α (MIP-1α) in a TLR4- and CD14-dependent manner. Thus, efficient recognition of HMGB1 by the TLR4/MD2 complex requires CD14.

Mechanisms of Toll-like receptor 4 (TLR4)-mediated inflammation after cold ischemia/reperfusion in the heart.

Background. Toll-Like Receptor 4 (TLR4) signaling mediates early inflammation after cold ischemia-reperfusion (I/R). We hypothesized that the TLR4 coreceptor CD14, the intracellular adaptor proteins myeloid differentiation factor 88 (MyD88) and TIR domain-containing-adaptor inducing IFN&bgr; (TRIF) would be required for cold I/R induced inflammation. High mobility group box 1 (HMGB1) is a putative endogenous activator of TLR4. Therefore, we also assessed the contribution of HMGB1 in cold I/R induced inflammation. Methods. Syngeneic heart transplants were performed in mice deficient in CD14, MyD88, TRIF, or wild-type mice. In other experiments, anti-HMGB1 neutralizing antibody or control IgG was administered at reperfusion. Donor hearts were subjected to 2 hr of cold ischemia and retrieved after 3 hr of reperfusion. Results. After cold I/R, grafts revealed striking translocation of HMGB1 out of the nucleus in cardiac myocytes. Administration of an anti-HMGB1 neutralizing antibody resulted in reduced systemic interleukin (IL)-6, tumor necrosis factor alpha (TNF&agr;), and intercellular adhesion molecule-1 (ICAM-1) messenger RNA (mRNA) levels (P≤0.05). Compared with controls, CD14 knock-out (KO) mice exhibited significantly lower (P≤0.05) systemic IL-6 and JE/monocyte chemotractant protein-1 levels after cold I/R. Intragraft TNF&agr; and IL-1&bgr; mRNA levels were also significantly lower (P≤0.05) in CD14 KO grafts. MyD88 KO mice exhibited significantly lower (P≤0.05) systemic IL-6 levels compared with control mice after cold I/R. Intragraft TNF&agr;, IL-6, and ICAM-1 mRNA levels were also significantly lower (P≤0.05) in MyD88 KO grafts. Significantly lower levels (P≤0.05) of serum IL-6, monocyte chemotractant protein-1 as well as intragraft TNF&agr;, IL-6, IL-1&bgr;, and ICAM-1 were observed after cold I/R in TRIF deficient animals compared with controls. Conclusions. CD14, MyD88, TRIF, and HMGB1 contribute to the inflammatory response that occurs after cold I/R. These results provide insight into the mechanisms of TLR4-mediated inflammation after cold I/R.

Systemic inflammation and end organ damage following trauma involves functional TLR4 signaling in both bone marrow-derived cells and parenchymal cells

Endogenous damage‐associated molecular pattern (DAMP) molecules are released from cells during traumatic injury, allowing them to interact with pattern recognition receptors such as the toll‐like receptors (TLRs) on other cells and subsequently, to stimulate inflammatory signaling. TLR4, in particular, plays a key role in systemic and remote organ responses to hemorrhagic shock (HS) and peripheral tissue injury in the form of bilateral femur fracture. TLR4 chimeric mice were generated to investigate the cell lineage in which functional TLR4 is needed to initiate the injury response to trauma. Chimeric mice were generated by adoptive bone marrow (BM) transfer, whereby donor marrow was given to an irradiated host using reciprocal combinations of TLR4 wild‐type (WT; C3H/HeOuJ) and TLR4 mutant (Mu; C3H/HeJ) mice. After a period of engraftment, chimeric mice were then subjected to HS or bilateral femur fracture. Control groups, including TLR4‐WT mice receiving WT BM and TLR4‐Mu mice receiving Mu BM, responded to injury in a similar pattern to unaltered HeOuJ and HeJ mice, and protection was afforded to those mice lacking functional TLR4. In contrast, TLR4‐WT mice receiving Mu BM and TLR4‐Mu mice receiving WT BM demonstrated intermediate inflammatory and cellular damage profiles. These data demonstrate that functional TLR4 is required in BM‐derived cells and parenchymal cells for an optimal inflammatory response to trauma.

Toll-like receptor 4 mediates the early inflammatory response after cold ischemia/reperfusion.

Background. Ischemia/reperfusion (I/R) injury leads to graft dysfunction and may contribute to alloimmune responses posttransplantation. The molecular mechanisms of cold I/R injury are only partially characterized but may involve toll-like receptor (TLR)-4 activation by endogenous ligands. We tested the hypothesis that TLR4 mediates the early inflammatory response in the setting of cold I/R in a murine cardiac transplant model. Methods. Syngeneic heart transplants were performed in mutant mice deficient in TLR4 signaling (C3H/HeJ) and wild-type mice (C3H/HeOuJ). Transplants were also performed between the strains (mutant hearts into wild-type recipients and the converse). Donor hearts were subjected to 2 hr of cold ischemia. The grafts were retrieved at 3 and 24 hr after reperfusion. Serum samples were collected for cytokine analysis. Reverse-transcription polymerase chain reaction and histologic analysis were used to assess intra-graft inflammation. Results. After transplant, serum tumor necrosis factor (TNF), interleukin (IL)-6, JE/monocyte chemotractant protein (MCP)-1, IL-1&bgr;, and troponin I levels, as well as intragraft TNF, IL-1&bgr;, IL-6, early growth response (EGR)-1, intercellular adhesion molecule (ICAM)-1, and inducible nitric oxide synthase (iNOS) mRNA levels, were significantly lower in the mutant→mutant group compared to the wild-type→wild-type group (P≤0.05). Intermediate levels of serum IL-6, JE/MCP-1, as well as intragraft TNF, IL-1&bgr;, IL-6, and ICAM-1 mRNA were observed after transplants in the mutant→wild-type and wild-type→mutant groups. Immunohistochemistry revealed less myocardial nuclear factor-&kgr;B nuclear translocation at and less neutrophil infiltration in the mutant→mutant group compared to the wild-type→wild-type group. Conclusions. These findings demonstrate that TLR4 signaling is central to both the systemic and intragraft inflammatory responses that occur after cold I/R in the setting of organ transplantation and that TLR4 signaling on both donor and recipient cells contributes to this response.

PATTERNS OF CYTOKINE RELEASE AND EVOLUTION OF REMOTE ORGAN DYSFUNCTION AFTER BILATERAL FEMUR FRACTURE

ABSTRACT The interaction between the complex pattern of cytokine release and remote organ dysfunction after trauma is incompletely understood. The aim of this study was to investigate the pattern of cytokine release and its association with the evolution of remote organ dysfunction after bilateral femur fracture. Male C57/BL6 mice were euthanized at six different time points (1-6 h) after bilateral femur fracture. Serum cytokine concentrations were measured with the Luminex multiplexing platform, and serum alanine aminotransferase levels were measured with the Vitros 950 Chemistry System. Hepatic and pulmonary myeloperoxidase activity was determined with an enzyme-linked immunosorbent assay kit. Permeability changes of the lung were assessed via bronchoalveolar lavage, and those of the liver via assessment of edema formation. Serum TNF-&agr; was unchanged in the fracture group throughout the experiment. Serum IL-6 and keratinocyte levels peaked at 5 h postinjury, whereas IL-10 levels peaked at 2 and 6 h. A brief IL-1&bgr; peak was observed at 3 h after fracture. Hepatic and pulmonary myeloperoxidase activity was significantly elevated within 1 h after trauma. Hepatic permeability was significantly increased within 2 h, and pulmonary permeability was significantly increased within 6 h after injury. Serum alanine aminotransferase levels peaked at 3 and 5 h postinjury. The pattern of serum IL-6, keratinocyte, IL-10, and IL-1&bgr; release was dynamic, whereas no significant elevations in TNF-&agr; were observed. The early hepatic and pulmonary infiltration of polymorphonuclear cells occurred in the absence of significantly elevated serum cytokine levels, suggesting that either early minor changes with an unbalance in inflammatory mediators or locally produced cytokines may initiate this process.

Toll Like Receptor 4-mediated Endoplasmic Reticulum Stress in Intestinal Crypts Induces Necrotizing Enterocolitis

Background: Cellular cues that regulate intestinal stem cell (ISC) apoptosis are unknown. Results: Toll-like-receptor 4 (TLR4) activation on ISCs induces endoplasmic reticulum (ER) stress, leading to ISC apoptosis and necrotizing enterocolitis (NEC). Conclusion: TLR4-induced ER stress in ISCs leads to apoptosis and NEC. Significance: This is the first study revealing that ER stress in ISCs via immune receptors induces NEC. The cellular cues that regulate the apoptosis of intestinal stem cells (ISCs) remain incompletely understood, yet may play a role in diseases characterized by ISC loss including necrotizing enterocolitis (NEC). Toll-like receptor-4 (TLR4) was recently found to be expressed on ISCs, where its activation leads to ISC apoptosis through mechanisms that remain incompletely explained. We now hypothesize that TLR4 induces endoplasmic reticulum (ER) stress within ISCs, leading to their apoptosis in NEC pathogenesis, and that high ER stress within the premature intestine predisposes to NEC development. Using transgenic mice and cultured enteroids, we now demonstrate that TLR4 induces ER stress within Lgr5 (leucine-rich repeat-containing G-protein-coupled receptor 5)-positive ISCs, resulting in crypt apoptosis. TLR4 signaling within crypts was required, because crypt ER stress and apoptosis occurred in TLR4ΔIEC-OVER mice expressing TLR4 only within intestinal crypts and epithelium, but not TLR4ΔIEC mice lacking intestinal TLR4. TLR4-mediated ER stress and apoptosis of ISCs required PERK (protein kinase-related PKR-like ER kinase), CHOP (C/EBP homologous protein), and MyD88 (myeloid differentiation primary response gene 88), but not ATF6 (activating transcription factor 6) or XBP1 (X-box-binding protein 1). Human and mouse NEC showed high crypt ER stress and apoptosis, whereas genetic inhibition of PERK or CHOP attenuated ER stress, crypt apoptosis, and NEC severity. Strikingly, using intragastric delivery into fetal mouse intestine, prevention of ER stress reduced TLR4-mediated ISC apoptosis and mucosal disruption. These findings identify a novel link between TLR4-induced ER stress and ISC apoptosis in NEC pathogenesis and suggest that increased ER stress within the premature bowel predisposes to NEC development.

Mitochondrial dysfunction in inflammatory bowel disease.

Inflammatory Bowel Disease (IBD) represents a group of idiopathic disorders characterized by chronic or recurring inflammation of the gastrointestinal tract. While the exact etiology of disease is unknown, IBD is recognized to be a complex, multifactorial disease that results from an intricate interplay of genetic predisposition, an altered immune response, changes in the intestinal microbiota, and environmental factors. Together, these contribute to a destruction of the intestinal epithelial barrier, increased gut permeability, and an influx of immune cells. Given that most cellular functions as well as maintenance of the epithelial barrier is energy-dependent, it is logical to assume that mitochondrial dysfunction may play a key role in both the onset and recurrence of disease. Indeed several studies have demonstrated evidence of mitochondrial stress and alterations in mitochondrial function within the intestinal epithelium of patients with IBD and mice undergoing experimental colitis. Although the hallmarks of mitochondrial dysfunction, including oxidative stress and impaired ATP production are known to be evident in the intestines of patients with IBD, it is as yet unclear whether these processes occur as a cause of consequence of disease. We provide a current review of mitochondrial function in the setting of intestinal inflammation during IBD.