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Pharmacology and therapeutic potential of pattern recognition receptors

Pharmacologists have used pathogen-associated molecular patterns (PAMPs), such as lipopolysaccharide (LPS) for decades as a stimulus for studying mediators involved in inflammation and for the screening of anti-inflammatory compounds. However, in the view of immunologists, LPS was too non-specific for studying the mechanisms of immune signalling in infection and inflammation, as no receptors had been identified. This changed in the late 1990s with the discovery of the Toll-like receptors. These 'pattern recognition receptors' (PRRs) were able to recognise highly conserved sequences, the so called pathogen associated molecular patterns (PAMPs) present in or on pathogens. This specificity of particular PAMPs and their newly defined receptors provided a common ground between pharmacologists and immunologists for the study of inflammation. PRRs also recognise endogenous agonists, the so called danger-associated molecular patterns (DAMPs), which can result in sterile inflammation. The signalling pathways and ligands of many PRRs have now been characterised and there is no doubt that this rich vein of research will aid the discovery of new therapeutics for infectious conditions and chronic inflammatory disease.

In vitro analysis of acetalated dextran microparticles as a potent delivery platform for vaccine adjuvants

Toll-like receptor (TLR) agonists induce potent innate immune responses and can be used in the development of novel vaccine adjuvants. However, access to TLRs can be challenging as exemplified by TLR 7, which is located intracellularly in endosomal compartments. To increase recognition and subsequent stimulatory effects of TLR 7, imiquimod was encapsulated in acetalated dextran (Ac-DEX) microparticles. Ac-DEX, a water-insoluble and biocompatible polymer, is relatively stable at pH 7.4, but degrades rapidly under acidic conditions, such as those found in lysosomal vesicles. To determine the immunostimulatory capacity of encapsulated imiquimod, we compared the efficacy of free versus encapsulated imiquimod in activating RAW 264.7 macrophages, MH-S macrophages, and bone marrow derived dendritic cells. Encapsulated imiquimod significantly increased IL-1 beta, IL-6, and TNF-alpha cytokine expression in macrophages relative to the free drug. Furthermore, significant increases were observed in classic macrophage activation markers (iNOS, PD1-L1, and NO) after treatment with encapsulated imiquimod over the free drug. Also, bone marrow derived dendritic cells produced significantly higher levels of IL-1 beta, IL-6, IL-12p70, and MIP-1 alpha as compared to their counterparts receiving free imiquimod. These results suggest that encapsulation of TLR ligands within Ac-DEX microparticles results in increased immunostimulation and potentially better protection from disease when used in conjunction with vaccine formulations.

Sepsis: links between pathogen sensing and organ damage

The host's inflammatory response to sepsis can be divided into two phases, the initial detection and response to the pathogen initiated by the innate immune response, and the persistent inflammatory state characterized by multiple organ dysfunction syndrome (MODS). New therapies aimed at pathogen recognition receptors (PRRs) particularly the TLRs and the NOD-like receptors offer hope to suppress the initial inflammatory response in early sepsis and to bolster this response in late sepsis. The persistence of MODS after the initial inflammatory surge can also be a determining factor to host survival. MODS is due to the cellular damage and death induced by sepsis. The mechanism of this cell death depends in part upon mitochondrial dysfunction. Damaged mitochondria have increased membrane permeability prompting their autophagic removal if few mitochondria are involved but apoptotic cell death may occur if the mitochondrial losses are more extensive. In addition. severe loss of mitochondria results in low cell energy stores, necrotic cell death, and increased inflammation driven by the release of cell components such as HMGB1. Therapies, which aim at improving cellular energy reserves such as the promotion of mitochondrial biogenesis by insulin, may have a role in future sepsis therapies. Finally, both the inflammatory responses and the susceptibility to organ failure may be modulated by nutritional status and micronutrients, such as zinc, Therapies aimed at micronutrient repletion may further augment approaches targeting PRR function and mitochondrial viability.

Genetics of the innate immune response in inflammatory bowel disease

The discovery of nucleotide-binding oligomerization domain 2/caspase recruitment domain-containing protein 15 (NOD2/CARD15) as the first susceptibility gene in Crohn's disease (CD) has shifted the focus of research into the pathogenesis of inflammatory bowel disease (IBD) firmly to the innate immune response and the integrity of the epithelial barrier. The subsequent implication in IBD of variant alleles of OCTN, DLG5, MDR1, and TLRs has provided further support for a new, more complex model of innate immunity function in the gastrointestinal tract. In this review, we examine the recent advances in our understanding of the influence of genetics of the innate immune response on IBD. We will focus on germline variation of genes encoding pathogen-recognition receptors, proteins involved in epithelial homeostasis and secreted antimicrobial proteins.

Selective NOD1 agonists cause shock and organ injury/dysfunction in vivo

RATIONALE

NLRs (nucleotide oligomerisation domain [NOD] proteins containing a leucine-rich repeat) are cytosolic pattern recognition receptors. NOD1 senses diaminopimelic acid-containing peptidoglycan present in gram-negative bacteria, whereas NOD2 senses the muramyl dipeptide (MDP) present in most organisms. Bacteria are the most common cause of septic shock, which is characterized clinically by hypotension resistant to vasopressor agents. In animal models, gram-negative septic shock is mimicked by lipopolysaccharide (LPS), which signals through Toll-like receptor 4 (TLR4) and its adaptor MyD88. The role of NLRs in the pathophysiology of septic shock is not known.

OBJECTIVES

To compare the effects of selective NOD1 agonists with LPS in vivo.

METHODS

Vascular smooth muscle cells or whole aortas from wild-type or genetically modified mice were stimulated in vitro with agonists of NOD1 (FK565) or NOD2 (MDP). Vasoconstriction was measured using wire myography. Nitric oxide (NO) formation was measured using Griess reaction and NO synthase-II protein by Western blotting. In vivo, blood pressure, heart rate, and urine output were measured in sham-, LPS-, or FK565-treated animals. Biomarkers of end-organ injury, coagulation activation, NO, and cytokines were measured in plasma.

MAIN RESULTS

FK565, but not MDP, induced NO synthase-II protein/activity in vascular smooth muscle and vascular hyporeactivity to pressor agents. FK565 had no effect on vessels from NOD1(-/-) mice, but was active in vessels from TLR4(-/-), TLR2(-/-), or MyD88(-/-) mice. FK565 induced hypotension, increased heart rate, and caused multiple (renal, liver) injury and dysfunction in vivo.

CONCLUSIONS

Activation of NOD1 induces shock and multiple organ injury/dysfunction.