Peptidoglycan recognition proteins: a novel family of four human innate immunity pattern recognition molecules

The danger is growing! A new paradigm for immune system activation and peripheral tolerance

Successful immune defense is a complex balancing act. In order to protect a host against invasion by harmful pathogens, an immune response must be rapid and vigorous, and must eliminate foreign invaders before their populations grow beyond control. That same immune response, however, must be selective enough to recognize and ignore commensal bacteria, environmental antigens and host tissue itself. How the immune system makes the crucial decision whether or not to attack a particular antigen has been a long-standing question central to the study of immunology. Here we show that the structure of the signaling network between regulatory T-cells and type 17 helper T-cells allows the immune system to selectively attack pathogens based on whether or not the pathogens represent a growing, and thus dangerous population. We term this mechanism for immune system activation the 'Growth Detection Paradigm', because it offers an entirely new explanation for immune system regulation and peripheral tolerance.

Profiling carbohydrate-receptor interaction with recombinant innate immunity receptor-Fc fusion proteins

The recognition of bacteria, viruses, fungi, and other microbes is controlled by host immune cells, which are equipped with many innate immunity receptors, such as Toll-like receptors, C-type lectin receptors, and immunoglobulin-like receptors. Our studies indicate that the immune modulating properties of many herbal drugs, for instance, the medicinal fungus Reishi (Ganoderma lucidum) and Cordyceps sinensis, could be attributed to their polysaccharide components. These polysaccharides specifically interact with and activate surface receptors involved in innate immunity. However, due to the complexity of polysaccharides and their various sources from medicinal fungi, quantitative analysis of medicinal polysaccharide extracts with regard to their functions represents a major challenge. To profile carbohydrate-immune receptor interactions, the extracellular domains of 17 receptors were cloned as Fc-fusion proteins, such that their interactions with immobilized polysaccharides could be probed in an enzyme-linked immunosorbent assay. The results show that several innate immune receptors, including Dectin-1, DC-SIGN, Langerin, Kupffer cell receptor, macrophage mannose receptor, TLR2, and TLR4, interact with the polysaccharide extracts from G. lucidum (GLPS). This analysis revealed distinct polysaccharide profiles from different sources of medicinal fungi, and the innate immune receptor-based enzyme-linked immunosorbent assay described here can serve as a high-throughput profiling method for the characterization and quality control of medicinal polysaccharides. It also provides a means to dissect the molecular mechanism of medicinal polysaccharide-induced immunomodulation events.

Toll-like receptors mediate induction of peptidoglycan recognition proteins in human corneal epithelial cells

Human peptidoglycan recognition proteins (PGLYRPs) are a novel family of pattern recognition receptors, and also act as anti-bacterial proteins. This study was to explore the toll-like receptor (TLR)-mediated regulation of PGLYRPs in human corneal epithelial cells (HCECs). Fresh human donor corneoscleral tissues were used to prepare cryosections. Primary HCECs, established from limbal explants, were treated with microbial ligands to TLRs 1-9 for 4-48 h, with or without pretreatment of TLR antibodies, NFkB inhibitor, or siRNA transfection. The mRNA of PGLYRPs was evaluated by RT and real-time PCR, and their proteins and NFkB activation were determined by immunostaining and Western blot. The nuclear IRF3 activity was quantified using an ELISA-based TransAM kit. PGLYRP-2, -3 and -4 were found to be expressed by human corneal epithelium while PGLYRP-1 was not detected. In primary HCEC cultures, PGLYRP-3 and -4 were constitutively expressed while PGLYRP-2 was largely inducible. PGLYRP-2 was induced by bacterial components, Pam3CSK4, PGN, flagellin and FSL-1, ligands for TLR2/1, 2, 5 and 2/6, respectively. Interestingly, PGLYRP-2 was strongest stimulated by polyI:C representing viral dsRNA. TLR3 antibody or NFkB inhibitor blocked IRF3 and NFkB p65 activation as well as polyI:C-stimulated PGLYRP-2. RNA interference indicates that the polyI:C-induced PGLYRP-2 was dramatically blocked in the cells transfected with siRNA-TRIF but neither siRNA-MyD88 nor the negative control siRNA-F. These findings suggest that human corneal epithelium may response to viral or bacterial infection by producing PGLYRPs through TLRs, and the induction of PGLYRP-2 by dsRNA was through TLR3-TRIF-IRF3-NFkB signaling pathways.

A novel antimicrobial peptidoglycan recognition protein in the cornea

PURPOSE

In an earlier gene expression study, the authors identified a novel antimicrobial gene, Peptidoglycan recognition protein 1 (Pglyrp1), in the mouse cornea. Here the expression of the Pglyrp1 transcript and the encoded protein, PGLYRP1, in the cornea was investigated. The role of PGLYRP1 in the cornea was further investigated using wild-type and Pglyrp1-deficient mice. This is the first report of this antimicrobial protein in the cornea.

METHODS

PGLYRP1 was detected in the cornea and was further localized to the epithelium by immunohistology, confocal microscopy, immunoblotting, and real-time PCR. The role of PGLYRP1 in the cornea was investigated by comparing the response of wild-type and Pglyrp1(-/-) mice to corneal epithelial wounds and Pseudomonas aeruginosa-mediated corneal infections. The antibacterial effects of corneal PGLYRP1 were assayed by measuring bacterial growth in vitro, in the presence of wild-type corneal epithelial extracts, before and after antibody-mediated blocking of PGLYRP1.

RESULTS

PGLYRP1 is expressed at high levels in the mouse corneal epithelium. PGLYRP1 was localized to the mouse corneal epithelium and the human corneal epithelium. The Pglyrp1(-/-) mouse shows delayed healing and poor clearing of bacterial keratitis; in vitro its epithelial protein extract shows reduced bacteriostatic activity compared with wild-type mice.

CONCLUSIONS

PGLYRP1 is a novel antimicrobial protein of the corneal epithelium and protects the ocular surface from bacterial infections.

Novel peptide therapeutics for treatment of infections

As antibiotic resistance increases worldwide, there is an increasing pressure to develop novel classes of antimicrobial compounds to fight infectious disease. Peptide therapeutics represent a novel class of therapeutic agents. Some, such as cationic antimicrobial peptides and peptidoglycan recognition proteins, have been identified from studies of innate immune effector mechanisms, while others are completely novel compounds generated in biological systems. Currently, only selected cationic antimicrobial peptides have been licensed, and only for topical applications. However, research using new approaches to identify novel antimicrobial peptide therapeutics, and new approaches to delivery and improving stability, will result in an increased range of peptide therapeutics available in the clinic for broader applications.

Psoriasis and streptococci: the natural selection of psoriasis revisited

We have previously postulated that surviving invasive streptococcal infections may have been a factor in psoriasis becoming a common skin disease in some parts of the world. Many of the candidate genes linked to psoriasis are associated with the acquired or innate immune system, which are also important in host defence to invasive streptococcal infections. High rates of positive streptococcal throat swabs among patients with chronic plaque psoriasis suggest that they are efficient at internalizing/carrying beta-haemolytic streptococci. Internalization of streptococci in the throat is dependent upon the transforming growth factor (TGF)-beta/fibronectin/alpha 5 beta 1 integrin pathway. The immune cell Th17 and its related cytokine network are important in mucosal defence, being very effective against extracellular microbes but having little effect on intracellular organisms. The TGF-beta/fibronectin/alpha 5 beta 1 integrin pathway and the Th17 cell network also appear to be operative in psoriasis, animal models of both TGF-beta and alpha 5 beta 1 cutaneous overexpression being associated with characteristic psoriasis lesions. We postulate that some of the genotypic/phenotypic changes in different immunological pathways in psoriasis, including the acquired T-cell response, the innate immune response, the TGF-beta/fibronectin/alpha 5 beta 1 integrin pathway and the Th17 cell system, confer protection against mortality during epidemics of invasive streptococcal infections, heightened efficiency in internalizing and allowing carriage of streptococci as well as predisposition to the development of psoriasis.

Host defense and pathogenesis in Staphylococcus aureus infections

Staphylococcus aureus is the most abundant cause of bacterial infections in the United States. As such, the pathogen has devised means to circumvent destruction by the innate immune system. Neutrophils are a critical component of innate immunity and the primary cellular defense against S aureus infections. This article reviews human neutrophil function in the context of S aureus virulence mechanisms and provides an overview of community-associated methicillin-resistant S aureus pathogenicity.

Porcine host defense peptides: expanding repertoire and functions

Host defense peptides (HDPs) are a large group of innate immune effectors that are also termed antimicrobial peptides. Because of the rapid progress that has been made in completing several animal genomes, many HDPs have been systemically defined using bioinformatic analysis and partially characterized using reverse genomic approaches. In pigs, about 30 HDPs have been identified and partially characterized relative to structure and function. Antimicrobial activity of porcine HDPs has been extensively evaluated against a broad spectrum of microorganisms in vitro and evaluated for their protective role in vivo. Increasing evidence indicates that HDPs are functionally differentiated during posttranslational and postsecretory processing, and that the structural units for antimicrobial and immunoregulatory functions are separate. These findings suggest promising new avenues for therapeutic drug design based on HDPs, including porcine HDPs. This review summarizes and discusses advances in porcine HDPs research during the last decade with an emphasis on the rapidly expanding profiles and biological functions.

PGLYRP-2 and Nod2 are both required for peptidoglycan-induced arthritis and local inflammation

Peptidoglycan recognition proteins (PGRPs) are structurally conserved from insects to mammals. Insect PGRPs have diverse host-defense functions. Mammalian PGRPs PGLYRP-1, PGLYRP-3, and PGLYRP-4 have bactericidal activity, while PGLYRP-2 has amidase activity. To extend the known functions of mammalian PGRPs, we examined whether they have immunomodulating activities in peptidoglycan-induced arthritis in mice. We demonstrate that PGLYRP-2 and Nod2 are both required for arthritis in this model. The sequence of events in peptidoglycan-induced arthritis is activation of Nod2, local expression of PGLYRP-2, chemokine production, and recruitment of neutrophils into the limbs, which induces acute arthritis. Only PGLYRP-2 among the four mammalian PGRPs displays this proinflammatory function, and PGLYRP-1 is anti-inflammatory. Toll-like receptor 4 (TLR4) and MyD88 are required for maturation of neutrophils before peptidoglycan challenge. Our results demonstrate that PGRPs, Nod2, and TLR4, representing three different types of pattern-recognition molecules, play interdependent in vivo roles in local inflammation.

Lessons from the fly: pattern recognition in Drosophila melanogaster

Drosophila have a variety of innate immune strategies for defending itself from infection, including humoral and cell mediated responses to invading microorganisms. At the front lines of these responses, are a diverse group of pattern recognition receptors that recognize pathogen associated molecular patterns. These patterns include bacterial lipopolysaccharides, peptidoglycans, and fungal beta-1,3 glucans. Some of the receptors catalytically modify the pathogenic determinant, but all are responsible for directly facilitating a signaling event that results in an immune response. Some of these events require multiple pattern recognition receptors acting sequentially to activate a pathway. In some cases, a signaling pathway may be activated by a variety of different pathogens, through parallel receptors detecting different pathogenic determinants. In this chapter, we review what is known about pattern recognition receptors in Drosophila, and how those lessons may be applied towards a broader understanding of immunity.

Cellular and molecular mechanisms in atopic dermatitis

Atopic dermatitis (AD) is a pruritic inflammatory skin disease associated with a personal or family history of allergy. The prevalence of AD is on the rise and estimated at approximately 17% in the USA. The fundamental lesion in AD is a defective skin barrier that results in dry itchy skin, and is aggravated by mechanical injury inflicted by scratching. This allows entry of antigens via the skin and creates a milieu that shapes the immune response to these antigens. This review discusses recent advances in our understanding of the abnormal skin barrier in AD, namely abnormalities in epidermal structural proteins, such as filaggrin, mutated in approximately 15% of patients with AD, epidermal lipids, and epidermal proteases and protease inhibitors. The review also dissects, based on information from mouse models of AD, the contributions of the innate and adaptive immune system to the pathogenesis of AD, including the effect of mechanical skin injury on the polarization of skin dendritic cells, mediated by keratinocyte-derived cytokines such as thymic stromal lymphopoietin (TSLP), IL-6, and IL-1, that results in a Th2-dominated immune response with a Th17 component in acute AD skin lesions and the progressive conversion to a Th1-dominated response in chronic AD skin lesions. Finally, we discuss the mechanisms of susceptibility of AD skin lesions to microbial infections and the role of microbial products in exacerbating skin inflammation in AD. Based on this information, we discuss current and future therapy of this common disease.

Bacillus anthracis peptidoglycan stimulates an inflammatory response in monocytes through the p38 mitogen-activated protein kinase pathway

We hypothesized that the peptidoglycan component of B. anthracis may play a critical role in morbidity and mortality associated with inhalation anthrax. To explore this issue, we purified the peptidoglycan component of the bacterial cell wall and studied the response of human peripheral blood cells. The purified B. anthracis peptidoglycan was free of non-covalently bound protein but contained a complex set of amino acids probably arising from the stem peptide. The peptidoglycan contained a polysaccharide that was removed by mild acid treatment, and the biological activity remained with the peptidoglycan and not the polysaccharide. The biological activity of the peptidoglycan was sensitive to lysozyme but not other hydrolytic enzymes, showing that the activity resides in the peptidoglycan component and not bacterial DNA, RNA or protein. B. anthracis peptidoglycan stimulated monocytes to produce primarily TNFα; neutrophils and lymphocytes did not respond. Peptidoglycan stimulated monocyte p38 mitogen-activated protein kinase and p38 activity was required for TNFα production by the cells. We conclude that peptidoglycan in B. anthracis is biologically active, that it stimulates a proinflammatory response in monocytes, and uses the p38 kinase signal transduction pathway to do so. Given the high bacterial burden in pulmonary anthrax, these findings suggest that the inflammatory events associated with peptidoglycan may play an important role in anthrax pathogenesis.

Replication of a genome-wide case-control study of esophageal squamous cell carcinoma

In a previous pilot case-control study of individuals diagnosed with esophageal squamous cell carcinoma (ESCC) and matched controls from a high-risk area in China, we identified 38 single nucleotide polymorphisms (SNPs) associated with ESCC located in or near one of 33 genes. In our study, we attempted to replicate the results of these 38 gene-related SNPs in a new sample of 300 ESCC cases and 300 matched controls from the same study conducted in Shanxi Province, China. Among 36 evaluable SNPs, 4 were significant in one or more analyses, including SNPs located in EPHB1, PGLYRP2, PIK3C3 and SLC9A9, although the odds ratios (ORs) for these genotypes were modest. Associations were found with EPHB1/rs1515366 (OR 0.92, 95% CI 0.86-0.99; p = 0.019), PIK3C3/rs52911 (OR 0.93, 95% CI 0.88-0.99; p = 0.02) and PGLYRP2/rs959117 (OR 0.93, 95% CI, 0.86-1.01; p = 0.061) in general linear models (additive mode); and the genotype distribution differed between cases and controls for SLC9A9/rs956062 (p = 0.024). To examine these 4 genes in more detail, 40 HapMap-based tag SNPs from these 4 genes were evaluated in the same subjects and 7 additional SNPs associated with ESCC were identified. Further confirmation of these findings in other populations and other studies are needed to determine if the signals from these SNPs are indirectly associated due to linkage disequilibrium, or are directly related to biologic function and the development of ESCC.

The Drosophila peptidoglycan recognition protein PGRP-LF blocks PGRP-LC and IMD/JNK pathway activation

Eukaryotic peptidoglycan recognition proteins (PGRPs) are related to bacterial amidases. In Drosophila, PGRPs bind peptidoglycan and function as central sensors and regulators of the innate immune response. PGRP-LC/PGRP-LE constitute the receptor complex in the immune deficiency (IMD) pathway, which is an innate immune cascade triggered upon Gram-negative bacterial infection. Here, we present the functional analysis of the nonamidase, membrane-associated PGRP-LF. We show that PGRP-LF acts as a specific negative regulator of the IMD pathway. Reduction of PGRP-LF levels, in the absence of infection, is sufficient to trigger IMD pathway activation. Furthermore, normal development is impaired in the absence of functional PGRP-LF, a phenotype mediated by the JNK pathway. Thus, PGRP-LF prevents constitutive activation of both the JNK and the IMD pathways. We propose a model in which PGRP-LF keeps the Drosophila IMD pathway silent by sequestering circulating peptidoglycan.

Innate immune response in insects: recognition of bacterial peptidoglycan and amplification of its recognition signal

The major cell wall components of bacteria are lipopolysaccharide, peptidoglycan, and teichoic acid. These molecules are known to trigger strong innate immune responses in the host. The molecular mechanisms by which the host recognizes the peptidoglycan of Gram-positive bacteria and amplifies this peptidoglycan recognition signals to mount an immune response remain largely unclear. Recent, elegant genetic and biochemical studies are revealing details of the molecular recognition mechanism and the signalling pathways triggered by bacterial peptidoglycan. Here we review recent progress in elucidating the molecular details of peptidoglycan recognition and its signalling pathways in insects. We also attempt to evaluate the importance of this issue for understanding innate immunity.

Toll-like receptor pathway gene expression is associated with human immunodeficiency virus-associated neurodegeneration

The innate immune system is a significant component of the brain's defense against infection, especially as the blood-brain barrier restricts access of the members of the adaptive immune system, such as T and B cells. The innate immune system includes Toll-like receptors (TLRs) that recognize pathogen-associated molecular patterns. Within the central nervous system, they are expressed on glial cells and their expression can be modulated by pathological states. Although their function is to recognize foreign pathogens and stimulate a protective immune response through the production of cytokines and interferons, there is emerging evidence that activation of these receptors can result in neurodegeneration. In the current study, the authors assessed the expression of TLR-related genes, using a customized Superarray gene chip, and correlated the expression findings with indices of neurodegeneration. We found that, using a stringent threshold for statistical significance to overcome the potential problem of multiple statistical testing, there were significant correlations between the expression of nine TLR-related genes and reduction in dendritic and synaptic staining. Two of these genes, TLR4 and SIGIRR, were validated by quantitative real-time polymerase chain reaction. Additionally, the authors demonstrated in vitro at the protein level that human primary astrocytes exposed to the toxic human immunodeficiency virus (HIV) envelope protein gp120 had a significant increase in TLR4 protein expression. In conclusion, these findings indicate that TLR-related gene expression may contribute to the development of HIV-related neurodegeneration.

Sepsis: from bench to bedside

Sepsis is a syndrome related to severe infections. It is defined as the systemic host response to microorganisms in previously sterile tissues and is characterized by end-organ dysfunction away from the primary site of infection. The normal host response to infection is complex and aims to identify and control pathogen invasion, as well as to start immediate tissue repair. Both the cellular and humoral immune systems are activated, giving rise to both anti-inflammatory and proinflammatory responses. The chain of events that leads to sepsis is derived from the exacerbation of these mechanisms, promoting massive liberation of mediators and the progression of multiple organ dysfunction. Despite increasing knowledge about the pathophysiological pathways and processes involved in sepsis, morbidity and mortality remain unacceptably high. A large number of immunomodulatory agents have been studied in experimental and clinical settings in an attempt to find an efficacious anti-inflammatory drug that reduces mortality. Even though preclinical results had been promising, the vast majority of these trials actually showed little success in reducing the overwhelmingly high mortality rate of septic shock patients as compared with that of other critically ill intensive care unit patients. Clinical management usually begins with prompt recognition, determination of the probable infection site, early administration of antibiotics, and resuscitation protocols based on "early-goal" directed therapy. In this review, we address the research efforts that have been targeting risk factor identification, including genetics, pathophysiological mechanisms and strategies to recognize and treat these patients as early as possible.

Three peptidoglycan recognition protein (PGRP) genes encoding potential amidase from eri-silkworm, Samia cynthia ricini

Three cDNA clones encoding peptidoglycan recognition proteins (PGRP-B, -C and -D) were isolated from larval fat body of immunized Samia cynthia ricini. The deduced amino acid sequences show high homology to each other and also to Drosophila PGRP-LB, but rather lower homology to all of the known lepidopteran PGRPs including Samia PGRP-A, a receptor-type PGRP. The three PGRPs conserve the five amino acid residues which form the catalytic site of N-acetylmuramoyl L-alanine amidase as in Drosophila LB. The PGRP-C and -D genes were silent in naive larvae, but strongly induced in fat body by an injection of peptidoglycan. PGRP-B gene, in contrast, constitutively expressed at high levels in naive midgut, and the gene was weakly induced in fat body after injection of peptidoglycan.

Molecular cloning and characterization of a short type peptidoglycan recognition protein (CfPGRP-S1) cDNA from Zhikong scallop Chlamys farreri

Peptidoglycan recognition protein (PGRP) specifically binds to peptidoglycan and plays a crucial role in the innate immune responses as a pattern recognition receptor (PRR). The cDNA of a short type PGRP was cloned from scallop Chlamys farreri (named CfPGRP-S1) by homology cloning with degenerate primers, and confirmed by virtual Northern blots. The full length of CfPGRP-S1 cDNA was 1073 bp in length, including a 5' untranslated region (UTR) of 59 bp, a 3' UTR of 255 bp, and an open reading frame (ORF) of 759 bp encoding a polypeptide of 252 amino acids with an estimated molecular mass of 27.88 kDa and a predicted isoelectric point of 8.69. BLAST analysis revealed that CfPGRP-S1 shared high identities with other known PGRPs. A conserved PGRP domain and three zinc-binding sites were present at its C-terminus. The temporal expression of CfPGRP-S1 gene in healthy, Vibrio anguillarum-challenged and Micrococcus lysodeikticus-challenged scallops was measured by RT-PCR analysis. The expression of CfPGRP-S1 was upregulated initially in the first 12 h or 24 h either by M. lysodeikticus or V. anguillarum challenge and reached the maximum level at 24 h or 36 h, then dropped progressively, and recovered to the original level as the stimulation decreased at 72 h. There was no significant difference between V. anguillarum and M. lysodeikticus challenge. The results indicated that the CfPGRP-S1 was a constitutive and inducible acute-phase protein which was involved in the immune response against bacterial infection.

Zebrafish peptidoglycan recognition proteins are bactericidal amidases essential for defense against bacterial infections

Peptidoglycan recognition proteins (PGRPs) are structurally conserved through evolution, but their functions in innate immunity are different in invertebrates and vertebrates. We asked what the functions of PGRPs in fish are and whether they are indispensable for defense against infection because fish are the first vertebrates that developed adaptive immunity, but they still rely solely on innate immunity during early development of embryos. We identified and cloned three zebrafish PGRPs and showed that they are highly expressed in eggs, developing embryos, and adult tissues that contact external environment. Zebrafish PGRPs have both peptidoglycan-lytic amidase activity and broad-spectrum bactericidal activity, which is a unique feature. Furthermore, we demonstrated that in the developing zebrafish embryo, one of these PGRPs is essential for defense and survival during bacterial infections. These data demonstrate an absolute requirement for innate immunity in defense against infections in fish embryos and for a PGRP protein for survival in vertebrates.

Peptidoglycan recognition protein (PGRP) from eri-silkworm, Samia cynthia ricini; protein purification and induction of the gene expression

Peptidoglycan recognition protein (PGRP) was isolated from immunized hemolymph of the wild silkworm, Samia cynthia ricini, detecting the biding activity with (125)I-labeled peptidoglycan (PGN). The binding specificity of PGRP was tested by competitive inhibition of the binding to (125)I-labeled-PGN by a large excess amount of non-labeled-PGN or other glucans. The binding to labeled uncross-linked Lys-type PGN from Micrococcus luteus was strongly inhibited by non-labeled-PGN of the same structure and meso-diaminopimelic acid (DAP)-type cross-linked PGN from Bacillus cell wall, but only a little by cross-linked PGN from M. luteus cell wall. The PGRP cDNA encodes a 193 amino acid open reading frame. The deduced amino acid sequence had 62 to 91% identities to known lepidopteran PGRPs, but less than 40% to Drosophila PGRPs. The PGRP gene constitutively expressed at a low level in naive fat body, and strongly induced by an injection of DAP-type cross-linked and Lys-type uncross-linked PGNs, but only weakly by Lys-type cross-linked PGN from M. luteus. The silkworm possibly distinguish between PGNs based on the structure of cross-linking peptide, but has less if any preference for the diamino acid residue of the stem peptide.

Early events in innate immunity in the recognition of microbial pathogens

Innate immunity is characterised by a rapid action of host effector molecules and leukocytes aimed at limiting the multiplication of invading microbial organisms and destroying them. The recognition and destruction of microorganisms involves humoral factors (e.g., the complement system and natural antibodies) and different cell types (e.g., phagocytic cells, mast cells, natural killer cells). Microbial detection by cells involves germ line-encoded pattern-recognition receptors such as Toll-like receptors and nucleotide-binding oligomerization domain-like receptors. Cellular activation by pathogens leads to the release of antimicrobial peptides (e.g., defensins and peptidoglycan recognition proteins) and cytokines that orchestrate the anti-infectious response. Cytokines enhance phagocytosis and leukocyte microbicidal activity, allow cellular recruitment into the infectious focus, boost hematopoiesis, induce fever and lead to the production of acute phase proteins.

Cytotoxic T lymphocytes carrying a pattern recognition protein Tag7 can detect evasive, HLA-negative but Hsp70-exposing tumor cells, thereby ensuring FasL/Fas-mediated contact killing

Within the broad problem of host immune surveillance versus tumor immune evasion, a most intriguing question is how the cellular immunity can cope with cancerous cells that have gotten rid of the classical antigen-presenting machinery. One such option stems from (1) the fact that HLA loss is often attended with expression of Hsp70 on the tumor cell surface, and (2) our findings that human lymphocytes express a protein Tag7 (also known as PGRP-S) capable of tight and specific interaction with cognate Hsp70. Here we show that a subpopulation of human CD4(+)CD25(+) lymphocytes, obtained either in culture as lymphokine-activated killers or directly from healthy donors, carry Tag7 and FasL on their surface and can indeed kill the HLA-negative tumor-derived cells K562 and MOLT-4 that expose Hsp70 and Fas. The primary binding of lymphocyte Tag7 to target-cell Hsp70 is very specific (eg, it is blocked by preincubating either cell with minimal peptides from the "partner" protein), and secures cell contact indispensable for subsequent FasL/Fas-triggered apoptosis. Unrelated to natural killer cell action or the putative role of Hsp as an antigen-presenting substitute, this novel mechanism is rather a backup analog of orthodox (CD8(+)) target recognition (Tag7 acting as built-in T-cell receptor and Hsp70 itself as ligand).

Human peptidoglycan recognition proteins require zinc to kill both gram-positive and gram-negative bacteria and are synergistic with antibacterial peptides

Mammals have four peptidoglycan recognition proteins (PGRPs or PGLYRPs), which are secreted innate immunity pattern recognition molecules with effector functions. In this study, we demonstrate that human PGLYRP-1, PGLYRP-3, PGLYRP-4, and PGLYRP-3:4 have Zn(2+)-dependent bactericidal activity against both Gram-positive and Gram-negative bacteria at physiologic Zn(2+) concentrations found in serum, sweat, saliva, and other body fluids. The requirement for Zn(2+) can only be partially replaced by Ca(2+) for killing of Gram-positive bacteria but not for killing of Gram-negative bacteria. The bactericidal activity of PGLYRPs is salt insensitive and requires N-glycosylation of PGLYRPs. The LD(99) of PGLYRPs for Gram-positive and Gram-negative bacteria is 0.3-1.7 muM, and killing of bacteria by PGLYRPs, in contrast to killing by antibacterial peptides, does not involve permeabilization of cytoplasmic membrane. PGLYRPs and antibacterial peptides (phospholipase A(2), alpha- and beta-defensins, and bactericidal permeability-increasing protein), at subbactericidal concentrations, synergistically kill Gram-positive and Gram-negative bacteria. These results demonstrate that PGLYRPs are a novel class of recognition and effector molecules with broad Zn(2+)-dependent bactericidal activity against both Gram-positive and Gram-negative bacteria that are synergistic with antibacterial peptides.

Various human epithelial cells express functional Toll-like receptors, NOD1 and NOD2 to produce anti-microbial peptides, but not proinflammatory cytokines

Epithelial cells may form the first barrier of defense against bacteria in human tissues. We recently revealed that oral epithelial cells generated anti-bacterial factors, such as peptidoglycan recognition proteins (PGRPs) and beta-defensin 2, but not proinflammatory cytokines, such as interleukin-8 (IL-8), upon stimulation with bacterial cell-surface components. In this study, we found clear expressions of Toll-like receptor (TLR)2, TLR3, TLR4, TLR7, NOD1 and NOD2 in oral, tongue, salivary gland, pharyngeal, esophageal, intestinal, cervical, breast, lung, and kidney epithelial cells. However, tongue, salivary gland, pharyngeal, esophageal, intestinal, cervical, breast, lung, and kidney epithelial cells, as well as oral epithelial cells, did not secrete IL-6, IL-8 or monocyte chemoattractant protein-1 in response to chemically synthesized TLR and NOD agonists mimicking microbial components: TLR2 agonistic lipopeptide (Pam3CSSNA), TLR3 agonistic Poly I:C, TLR4 agonistic lipid A (LA-15-PP), TLR7 agonistic single stranded RNA (ssPoly U), NOD1 agonistic iE-DAP (gamma-D-glumtamyl-meso-diaminopimelic acid), and NOD2 agonistic muramyldipeptide (MDP). Although PGRPs on oral epithelial cells were significantly up-regulated upon stimulation with these synthetic components, PGRPs on pharyngeal epithelial cells were only slightly up-regulated, and PGRPs on esophageal, intestinal and cervical epithelial cells were not up-regulated upon stimulation with the components. In contrast, stimulation with synthetic TLRs and NODs ligands induced beta-defensin 2 generation in all epithelial cells examined. These findings indicate that TLR and NOD in various epithelial cells are functional receptors that induce anti-bacterial responses in general without being accompanied by inflammatory responses.

Peptidoglycan recognition proteins: pleiotropic sensors and effectors of antimicrobial defences

Peptidoglycan recognition proteins (PGRPs) are innate immunity molecules that are present in most invertebrate and vertebrate animals. All PGRPs function in antimicrobial defence and are homologous to the prokaryotic peptidoglycan-lytic type 2 amidases. However, only some PGRPs have the catalytic activity that protects the host from excessive inflammation, and most PGRPs have diversified to carry out other host-defence functions. Insect and mammalian PGRPs defend host cells against infection through very different mechanisms. Insect PGRPs activate signal transduction pathways in host cells or trigger proteolytic cascades in the haemolymph, both of which generate antimicrobial effectors. By contrast, mammalian PGRPs are directly bactericidal. Here, we review these contrasting modes of action.

Peptidoglycan recognition proteins of the innate immune system

Peptidoglycan (PGN) is the major component of bacterial cell walls and one of the main microbial products recognized by the innate immune system. PGN recognition is mediated by several families of pattern recognition molecules, including Toll-like receptors, nucleotide-binding oligomerization domain-containing proteins, and peptidoglycan recognition proteins (PGRPs). However, only the interaction of PGN with PGRPs, which are highly conserved from insects to mammals, has so far been characterized at the molecular level. Here, we describe recent structural studies of PGRPs that reveal the basis for PGN recognition and provide insights into the signal transduction and antibacterial activities of these innate immune proteins.

Peptidoglycan detection by mammals and flies

Peptidoglycan is an essential component of bacteria. The host exploits the peptidoglycan particular composition and uniqueness to bacteria for specific bacterial recognition. Insects and mammals accomplish this via receptors such as PGRP and Nod proteins.

Peptidoglycan and mannose-based molecular patterns trigger the arachidonic acid cascade in human polymorphonuclear leukocytes

The release of arachidonic acid (AA) in response to microorganism-derived products acting on pattern recognition receptors (PRR) was assayed in human polymorphonuclear leukocytes (PMN). Peptidoglycan (PGN) and mannan were found to be strong inducers of AA metabolism, as they produced the release of AA at a similar extent to that produced by agonists of pathophysiological relevance such as complement-coated zymosan particles and IgG immune complexes. In sharp contrast, lipoteichoic acid, LPS, muramyldipeptide, and the bacterial lipoprotein mimic palmitoyl-3-cysteine-serine-lysine-4 failed to do so. Leukotriene B4 and PGE2 were synthesized in response to mannan and PGN, thus suggesting that the lipoxygenase and the cyclooxygenase routes are operative in human PMN in response to pathogen-associated molecular patterns (PAMP). Analysis of the lipid extracts of supernatants and cell pellets as well as pharmacological studies with the calpain inhibitor calpeptin and the cytosolic phospholipase A2 (PLA2) inhibitor pyrrolidine-1 showed the dependence of AA release on cytosolic PLA2-catalyzed reactions. The effect of PGN was not inhibited by previous treatment with anti-TLR2 mAb, thus suggesting a nonarchetypal involvement of the TLR2 signaling route and/or participation of other receptors. Because of the abundance of mannose-based and PGN-containing PAMP in fungi and bacteria and the wide array of PRR in human PMN, these finding disclose a role of prime importance for PAMP and PRR in AA metabolism in the inflammatory response mediated by PMN.

The peptidoglycan recognition proteins (PGRPs)

Peptidoglycan recognition proteins (PGRPs) are found in insects, mollusks, echinoderms, and vertebrates, and they protect animals against infections. The four mammalian family members are either bactericidal proteins or amidases that hydrolyze bacterial peptidoglycan.

Peptidoglycan recognition proteins (PGRPs) are innate immunity molecules present in insects, mollusks, echinoderms, and vertebrates, but not in nematodes or plants. PGRPs have at least one carboxy-terminal PGRP domain (approximately 165 amino acids long), which is homologous to bacteriophage and bacterial type 2 amidases. Insects have up to 19 PGRPs, classified into short (S) and long (L) forms. The short forms are present in the hemolymph, cuticle, and fat-body cells, and sometimes in epidermal cells in the gut and hemocytes, whereas the long forms are mainly expressed in hemocytes. The expression of insect PGRPs is often upregulated by exposure to bacteria. Insect PGRPs activate the Toll or immune deficiency (Imd) signal transduction pathways or induce proteolytic cascades that generate antimicrobial products, induce phagocytosis, hydrolyze peptidoglycan, and protect insects against infections. Mammals have four PGRPs, which are secreted; it is not clear whether any are directly orthologous to the insect PGRPs. One mammalian PGRP, PGLYRP-2, is an N-acetylmuramoyl-L-alanine amidase that hydrolyzes bacterial peptidoglycan and reduces its proinflammatory activity; PGLYRP-2 is secreted from the liver into the blood and is also induced by bacteria in epithelial cells. The three remaining mammalian PGRPs are bactericidal proteins that are secreted as disulfide-linked homo- and hetero-dimers. PGLYRP-1 is expressed primarily in polymorphonuclear leukocyte granules and PGLYRP-3 and PGLYRP-4 are expressed in the skin, eyes, salivary glands, throat, tongue, esophagus, stomach, and intestine. These three proteins kill bacteria by interacting with cell wall peptidoglycan, rather than permeabilizing bacterial membranes as other antibacterial peptides do. Direct bactericidal activity of these PGRPs either evolved in the vertebrate (or mammalian) lineage or is yet to be discovered in insects.

Molecular cloning and mRNA expression of peptidoglycan recognition protein (PGRP) gene in bay scallop (Argopecten irradians, Lamarck 1819)

Peptidoglycan recognition proteins (PGRPs) are a type of pattern recognition molecules (PRM) that recognize the unique cell wall component peptidoglycan (PGN) of bacteria and are involved in innate immunity. The first bivalve PGRP cDNA sequence was cloned from bay scallop Argopecten irradians by expressed sequence tag (EST) and PCR technique. The full-length cDNA of bay scallop PGRP (designated AiPGRP) gene contained 1018bp with a 615-bp open reading frame that encoded a polypeptide of 205 amino acids. The predicted amino acid sequence of AiPGRP shared high identity with PGRP in other organisms, such as PGRP precursor in Trichoplusia ni and PGRP SC2 in Drosophila melanogaster. A quantitative reverse transcriptase Real-Time PCR (qRT-PCR) assay was developed to assess the mRNA expression of AiPGRP in different tissues and the temporal expression of AiPGRP in the mixed primary cultured hemocytes challenged by microbial components lipopolyssacharide (LPS) from Escherichia coli and PGN from Micrococcus luteus. Higher-level mRNA expression of AiPGRP was detected in the tissues of hemocytes, gonad and kidney. The expression of AiPGRP in the mixed primary cultured hemocytes was up regulated after stimulated by PGN, while LPS from E. coli did not induce AiPGRP expression. The results indicated that AiPGRP was a constitutive and inducible expressed protein that was mainly induced by PGN and could be involved in scallop immune response against Gram-positive bacteria infection.

Peptidoglycan: a major aetiological factor for psoriasis?

Peptidoglycan (PG), a major cell-wall component of Gram-positive bacteria, has been detected within antigen-presenting cells in various inflammatory conditions, including psoriasis. The additional presence of T-helper 1 cells specific for streptococcal or staphylococcal PG in psoriasis skin lesions implicates PG as an important T-cell stimulator for the disease. PG is a major target for the innate immune system, and associations between genetic polymorphisms of recognition receptors for PG and various auto-inflammatory diseases have been identified. The location of these genes within four linkage sites for psoriasis raises the possibility that an altered innate recognition of PG might contribute to the enhanced T-cell response to the bacterial antigen. These observations suggest that PG is a major aetiological factor for psoriasis and emphasize the importance of PG in bacterial-infection-induced inflammatory disease.

Transcriptosome and serum cytokine profiling of an atypical case of myelodysplastic syndrome with progression to acute myelogenous leukemia

A Native American-Indian female presenting with anemia and thrombocytosis was diagnosed with myelodysplastic syndrome (MDS, refractory anemia). Over the course of 5 years she developed cytopenias and periods of leukocytosis with normal bone marrow (BM) blast counts, features of an unclassifiable MDS/MPS syndrome. The patient ultimately progressed to acute myelogenous leukemia (AML, FAB M2) and had a normal karyotype throughout her course. The episodes of leukocytosis were associated with infectious complications. Transformation to AML was characterized by a BM blast percentage of 49%. Peripheral blood and BM samples were obtained for serum protein analysis and gene expression profiling (GEP) to elucidate her disease process. An ELISA assay of the serum analyzed approximately 80 cytokines, which demonstrated that hepatocyte growth factor/scatter factor and insulin-like growth factor binding protein 1 were markedly elevated compared to normal. GEP demonstrated a unique "tumor molecular profile," which included overexpression of oncogenes (HOXA9, N-MYC, KOC1), proliferative genes (PAWR, DLG5, AKR1C3), invasion/metastatic genes (FN1, N-CAM-1, ITGB5), pro-angiogenesis genes (c-Kit), and down regulation of tumor suppressor genes (SUI1, BARD1) and anti-apoptotic genes (PGLYRP, SERPINB2, MPO). Hence, a biomics approach has provided insight into elucidating disease mechanisms, molecular prognostic factors, and discovery of novel targets for therapeutic intervention.

Mycobacterium tuberculosis and Mycobacterium avium inhibit IFN- gamma -induced gene expression by TLR2-dependent and independent pathways

Mycobacteria-infected macrophages are poor responders to interferon-gamma (IFN-gamma), resulting in decreased expression of IFN-gamma-induced genes. In the present study, we examined the inhibition of IFN-gamma-induced gene expression by Mycobacterium tuberculosis and four different Mycobacterium avium strains in mouse RAW264.7 macrophages. Gamma-irradiated M. tuberculosis inhibited mRNA expression of a panel of six different IFN- gamma-induced genes. All four of the M. avium strains completely inhibited IFN-gamma-induced expression of MHC class II Aalpha and Ebeta mRNA. However, the Mac101 strain, which is serovar 1, inhibited IFN-gamma induction of IFN regulatory factor-1 (IRF-1) and guanylate-binding protein-1 (GBP-1) mRNA to a greater extent than the other M. avium strains, which are serovar 2. In this study, we also show that mycobacteria inhibit gene expression by both toll-like receptor 2 (TLR2)-dependent and independent pathways. The inhibition of IFN-gamma-induced gene expression by M. avium was reduced but not completely blocked in macrophages from TLR2(/) mice. IFN-gamma-induced gene expression was also inhibited by mycobacteria in RAW264.7 cells expressing dominantnegative TLR2 or myeloid differentiation factor 88 (MyD88), further indicating the existence of a pathway independent of TLR2 and MyD88. These data suggest that mycobacteria inhibit IFN-gamma-induced gene expression by multiple pathways involving both TLR2 and non-TLR receptors.

Recognition and elimination of diversified pathogens in insect defense systems

The elimination of infectious non-self by the host defense systems of multicellular organisms requires a variety of recognition and effector molecules. The diversity is generated in somatic cells or encoded in the germ-line. In adaptive immunity in jawed vertebrates, the diversity of immunoglobulins and antigen receptors is generated by gene rearrangements in somatic cells. In innate immunity, various effector molecules and pattern recognition receptors, such as antimicrobial peptides and peptidoglycan recognition proteins, are encoded in the germ-line of multicellular organisms, including insects and jawed vertebrates. In the present review, we discuss how insect host defense systems recognize and eliminate a multitude of microbes via germ-line-encoded molecules, including recent findings that a Drosophila member of the immunoglobulin superfamily is extensively diversified by alternative splicing in somatic immune cells and participates in the elimination of bacteria.

Pattern recognition receptors: a contemporary view on liver diseases

Pattern recognition receptors (PRRs) function as sensors of microbial danger signals enabling the vertebrate host to initiate an immune response. PRRs are present not only in immune cells but also in liver parenchymal cells and the complexity of the cell populations provide unique aspects to pathogen recognition and tissue damage in the liver. This review discusses the role of different PRRs in pathogen recognition in the liver, and focuses on the role of PRRs in hepatic inflammation, cholestasis, ischemia, repair and fibrosis. PRRs as novel therapeutic targets are evaluated.

Meso-Diaminopimelic Acid andMeso-Lanthionine, Amino Acids Specific to Bacterial Peptidoglycans, Activate Human Epithelial Cells through NOD1

Peptidoglycans (PGNs) are ubiquitous constituents of bacterial cell walls and exhibit various immunobiological activities. Two types of minimum essential PGN structures for immunobiological activities were chemically synthesized and designated as muramyldipeptide; N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) and gamma-D-glutamyl-meso-diaminopimelic acid (iE-DAP), which are common constituents of both Gram-positive and Gram-negative bacteria, as well as most Gram-negative and some Gram-positive bacteria, respectively. Recently, intracellular receptors for MDP and iE-DAP have been demonstrated to be nucleotide-binding oligomerization domain (NOD)1 and NOD2, respectively. In this study, we demonstrated that chemically synthesized meso-DAP itself activated human epithelial cells from various tissues, through NOD1 to generate antibacterial factors, PGN recognition proteins and beta-defensin 2, and cytokines in specified cases, although the activities of meso-DAP were generally weaker than those of known NOD agonists. However, stereoisomers of meso-DAP, LL-DAP, and DD-DAP were only slightly activated or remained inactive, respectively. Synthetic meso-lanthionine, which is another diamino-type amino acid specific to PGN of the specified Gram-negative bacteria, was also recognized by NOD1. In human monocytic cells, in the presence of cytochalasin D meso-DAP induced slightly but significantly increased production of cytokines, although the cells did not respond to meso-DAP in the absent of cytochalasin D. Our findings suggest that NOD1 is a special sentinel molecule, especially in the epithelial barrier, allowing the intracellular detection of bacteria through recognizing meso-DAP or comparable moiety of PGN from specified bacteria in cooperation with NOD2, thereby playing a key role in innate immunity.

Mammalian PGRPs: novel antibacterial proteins

Peptidoglycan recognition proteins (PGRPs) are innate immunity molecules conserved from insects to mammals. Insects have up to 19 PGRPs, which activate Toll or Imd signal transduction pathways or induce proteolytic cascades that generate antimicrobial products, induce phagocytosis, hydrolyse peptidoglycan, and protect insects against infections. Mammals have four PGRPs, which were hypothesized to function as signal-transducing pattern recognition receptors. However, all mammalian PGRPs are secreted, usually as disulphide-linked homo- and heterodimers. One mammalian PGRP, PGLYRP-2, is an N-acetylmuramoyl-L-alanine amidase that hydrolyses bacterial peptidoglycan and reduces its proinflammatory activity. PGLYRP-2 is secreted from liver into blood, and is also induced by bacteria in epithelial cells. The three remaining mammalian PGRPs are bactericidal or bacteriostatic proteins. PGLYRP-1 is expressed primarily in the granules of polymorphonuclear leucocytes (PMNs) , and PGLYRP-3 and PGLYRP-4 are expressed in the skin, eyes, salivary glands, throat, tongue, esophagus, stomach and intestine, and protect the host against infections. They kill bacteria by interacting with their cell wall peptidoglycan, rather than permeabilizing their membranes. These PGRPs therefore are a new class of bactericidal and bacteriostatic proteins that have different structure, mechanism of action, and expression pattern from currently known vertebrate antimicrobial peptides. Direct bactericidal activity of these PGRPs either evolved in vertebrates or mammals, or it is yet to be discovered in insects.

How to kill a mocking bug?

All metazoans have evolved means to protect themselves from threats present in the environment: injuries, viruses, fungi, bacteria and other parasites. Insect protection includes innate physical barriers and both cellular and humoral responses. The insect innate immune response, best characterized in Drosophila melanogaster, is a rapid broad response, triggered by pathogen-associated molecular patterns (PAMPs) recognition, which produces a limited range of effectors that does not alter upon continued pathogen exposure and lacks immunological memory. The Drosophila response, particularly its humoral response, has been investigated by both low and high-throughput methods. Three signalling pathways conserved between insects and mammals have been implicated in this response: Toll (equivalent to mammalian TLR), Imd (equivalent to TNFalpha) and Hop (equivalent to JAK/STAT). This review provides an entry point to the insect immune system literature outlining the main themes in D. melanogaster bacterial pathogen detection and humoral and cellular immune responses. The Drosophila immune response is compared with other insects and the mammalian immune system.

Recognition of pathogens and activation of immune responses in Drosophila and horseshoe crab innate immunity

In innate immunity, pattern recognition receptors discriminate between self- and infectious non-self-matter. Mammalian homologs of the Drosophila Toll protein, which are collectively referred to as Toll-like receptors (TLRs), recognize pathogen-associated molecular patterns (PAMPs), including lipopolysaccharides (LPS) and lipoproteins, whereas the Drosophila Toll protein does not act as a PAMP receptor, but rather binds to Spätzle, an endogenous peptide. In Drosophila, innate immune surveillance is mediated by members of the peptidoglycan recognition protein (PGRP) family, which recognize diverse bacteria-derived peptidoglycans and initiate appropriate immune reactions including the release of antimicrobial peptides and the activation of the prophenoloxidase cascade, the latter effecting localized wound healing, melanization, and microbial phagocytosis. In the horseshoe crab, LPS induces hemocyte exocytotic degranulation, resulting in the secretion of various defense molecules, such as coagulation factors, antimicrobial peptides, and lectins. Recent studies have demonstrated that the zymogen form of the serine protease factor C, a major granular component of hemocyte, also exists on the hemocyte surface and functions as a biosensor for LPS. The proteolytic activity of activated factor C initiates hemocyte exocytosis via a G protein mediated signal transduction pathway. Furthermore, it has become clear that an endogenous mechanism for the feedback amplification of the innate immune response exists and is dependent upon a granular component of the horseshoe crab hemocyte.

Differential expression of peptidoglycan recognition protein 2 in the skin and liver requires different transcription factors

Human peptidoglycan recognition protein 2 (PGLYRP2) is an N-acetylmuramoyl-L-alanine amidase that hydrolyzes bacterial peptidoglycan and is differentially expressed in the two major organs in the human body, liver and skin. PGLYRP2 has a high constitutive expression in the liver but is not expressed in healthy human skin. PGLYRP2 mRNA is also not expressed in cultured human keratinocytes but is highly induced upon exposure to bacteria. In this study we identified the transcription start site for pglyrp2 and demonstrated that the differential expression of PGLYRP2 in hepatocytes and keratinocytes is regulated by different transcription factors whose binding sequences are located in different regions of the pglyrp2 promoter. Induction of pglyrp2 in keratinocytes is regulated by sequences in the distal region of the promoter and requires transcription factors NF-kappaB and Sp1, whereas constitutive expression of pglyrp2 in a hepatocyte cell line is regulated by sequences in the proximal region of the promoter and requires transcription factors c-Jun and ATF2. Regulation of constitutive and inducible expression of pglyrp2 is important for systemic and local innate immune responses to bacterial infections.

Mammalian peptidoglycan recognition protein TagL inhibits Listeria monocytogenes invasion into epithelial cells

Peptidoglycan recognition proteins are a family of evolutionary conserved proteins that play a basic role in the innate immunity of insects, but their role in the immunity of mammals remains unclear. To elucidate its functions, a mouse member of the peptidoglycan recognition proteins family, TagL, was stably expressed in colon adenocarcinoma HT29 cells, and its effect on the invasion and intracellular growth of the enteroinvasive pathogenic bacterium Listeria monocytogenes was assessed. The expression of TagL substantially impaired bacterial invasion and early intracellular growth. The observed effects were partly caused by a loss of viability by intraphagosomal bacteria. Efficient phagosome escaping but not efficient invasion helped bacteria to overplay TagL.

Peptidoglycan Recognition Proteins Are a New Class of Human Bactericidal Proteins

Skin and mucous membranes come in contact with external environment and protect tissues from infections by producing antimicrobial peptides. We report that human peptidoglycan recognition proteins 3 and 4 (PGLYRP3 and PGLYRP4) are secreted as 89-115-kDa disulfide-linked homo- and heterodimers and are bactericidal against several pathogenic and nonpathogenic transient, but not normal flora, Gram-positive bacteria. PGLYRP3 and PGLYRP4 are also bacteriostatic toward all other tested bacteria, which include Gram-negative bacteria and normal flora Gram-positive bacteria. PGLYRP3 and PGLYRP4 are also active in vivo and protect mice against experimental lung infection. In contrast to antimicrobial peptides, PGLYRPs kill bacteria by interacting with their cell wall peptidoglycan, rather than permeabilizing their membranes. PGLYRP3 and PGLYRP4 are expressed in the skin, eyes, salivary glands, throat, tongue, esophagus, stomach, and intestine. Thus, we have identified the function of mammalian PGLYRP3 and PGLYRP4, and show that they are a new class of bactericidal and bacteriostatic proteins that have different structures, mechanism of actions, and expression patterns than antimicrobial peptides.

Downregulation of the Drosophila immune response by peptidoglycan-recognition proteins SC1 and SC2

Peptidoglycan-recognition proteins (PGRPs) are evolutionarily conserved molecules that are structurally related to bacterial amidases. Several Drosophila PGRPs have lost this enzymatic activity and serve as microbe sensors through peptidoglycan recognition. Other PGRP family members, such as Drosophila PGRP-SC1 or mammalian PGRP-L, have conserved the amidase function and are able to cleave peptidoglycan in vitro. However, the contribution of these amidase PGRPs to host defense in vivo has remained elusive so far. Using an RNA-interference approach, we addressed the function of two PGRPs with amidase activity in the Drosophila immune response. We observed that PGRP-SC1/2–depleted flies present a specific over-activation of the IMD (immune deficiency) signaling pathway after bacterial challenge. Our data suggest that these proteins act in the larval gut to prevent activation of this pathway following bacterial ingestion. We further show that a strict control of IMD-pathway activation is essential to prevent bacteria-induced developmental defects and larval death.

It has long been known that the mammalian immune response needs to be kept under tight control. Responses that are delayed or of insufficient vigor can lead to a failure to control infection. However, excessive or inappropriate inflammation can be harmful or event fatal. Using the fruit fly as a model, evidence is presented that such an immuno-modulation is also essential in invertebrates and is mediated by peptidoglycan-recognition proteins (PGRPs). PGRPs are evolutionarily conserved molecules derived from enzymes that cleave bacterial peptidoglycan. It has been shown previously that some PGRPs have lost this enzymatic activity and function as sensors of bacteria upstream of the Drosophila immune pathways. The contribution of PGRPs which have maintained enzymatic activity to host defense has remained elusive so far. Here, the authors investigate in vivo data on the role of Drosophila PGRPs with enzymatic activity. Their results suggest that these proteins are required in the larval gut to negatively regulate the immune response, thus preventing bacterially induced developmental defects and death.

Structural basis for preferential recognition of diaminopimelic acid-type peptidoglycan by a subset of peptidoglycan recognition proteins

Drosophila peptidoglycan recognition protein (PGRP)-LCx and -LCa are receptors that preferentially recognize meso-diaminopimelic acid (DAP)-type peptidoglycan (PGN) present in Gram-negative bacteria over lysine-type PGN of gram-positive bacteria and initiate the IMD signaling pathway, whereas PGRP-LE plays a synergistic role in this process of innate immune defense. How these receptors can distinguish the two types of PGN remains unclear. Here the structure of the PGRP domain of Drosophila PGRP-LE in complex with tracheal cytotoxin (TCT), the monomeric DAP-type PGN, reveals a buried ionic interaction between the unique carboxyl group of DAP and a previously unrecognized arginine residue. This arginine is conserved in the known DAP-type PGN-interacting PGRPs and contributes significantly to the affinity of the protein for the ligand. Unexpectedly, TCT induces infinite head-to-tail dimerization of PGRP-LE, in which the disaccharide moiety, but not the peptide stem, of TCT is positioned at the dimer interface. A sequence comparison suggests that TCT induces heterodimerization of the ectodomains of PGRP-LCx and -LCa in a closely analogous manner to prime the IMD signaling pathway, except that the heterodimer formation is nonperpetuating.

Chemically synthesized pathogen-associated molecular patterns increase the expression of peptidoglycan recognition proteins via toll-like receptors, NOD1 and NOD2 in human oral epithelial cells

Peptidoglycan recognition proteins (PGRPs), a novel family of pattern recognition molecules (PRMs) in innate immunity conserved from insects to mammals, recognize bacterial cell wall peptidoglycan (PGN) and are suggested to act as anti-bacterial factors. In humans, four kinds of PGRPs (PGRP-L, -Ialpha, -Ibeta and -S) have been cloned and all four human PGRPs bind PGN. In this study, we examined the possible regulation of the expression of PGRPs in oral epithelial cells upon stimulation with chemically synthesized pathogen-associated molecular patterns (PAMPs) in bacterial cell surface components: Escherichia coli-type tryacyl lipopeptide (Pam3CSSNA), E. coli-type lipid A (LA-15-PP), diaminopimelic acid containing desmuramyl peptide (gamma-D-glutamyl-meso-DAP; iE-DAP), and muramyldipeptide (MDP). These synthetic PAMPs markedly upregulated the mRNA expression of the four PGRPs and cell surface expression of PGRP-Ialpha and -Ibeta, but did not induce either mRNA expression or secretion of inflammatory cytokines, in oral epithelial cells. Suppression of the expression of Toll-like receptor (TLR)2, TLR4, nucleotide-binding oligomerization domain (NOD)1 and NOD2 by RNA interference specifically inhibited the upregulation of PGRP mRNA expression induced by Pam3CSSNA, LA-15-PP, iE-DAP and MDP respectively. These PAMPs definitely activated nuclear factor (NF)-kappaB in the epithelial cells, and suppression of NF-kappaB activation clearly prevented the induction of PGRP mRNA expression induced by these PAMPs in the cells. These findings suggested that bacterial PAMPs induced the expression of PGRPs, but not proinflammatory cytokines, in oral epithelial cells, and the PGRPs might be involved in host defence against bacterial invasion without accompanying inflammatory responses.