The antibacterial properties of secreted phospholipases A2: a major physiological role for the group IIA enzyme that depends on the very high pI of the enzyme to allow penetration of the bacterial cell wall

Epithelial-immune cell crosstalk for intestinal barrier homeostasis

The intestinal barrier is mainly formed by a monolayer of epithelial cells, which forms a physical barrier to protect the gut tissues from external insults and provides a microenvironment for commensal bacteria to colonize while ensuring immune tolerance. Moreover, various immune cells are known to significantly contribute to intestinal barrier function by either directly interacting with epithelial cells or by producing immune mediators. Fulfilling this function of the gut barrier for mucosal homeostasis requires not only the intrinsic regulation of intestinal epithelial cells (IECs) but also constant communication with immune cells and gut microbes. The reciprocal interactions between IECs and immune cells modulate mucosal barrier integrity. Dysregulation of barrier function could lead to dysbiosis, inflammation, and tumorigenesis. In this overview, we provide an update on the characteristics and functions of IECs, and how they integrate their functions with tissue immune cells and gut microbiota to establish gut homeostasis.

Re-visiting humoral constitutive antibacterial heterogeneity in bloodstream infections

Although cellular immunity has garnered much attention in the era of single-cell technologies, humoral innate immunity has receded in priority due to its presumed limited roles. Hence, despite the long-recognised bactericidal activity of serum-a functional characteristic of constitutive humoral immunity-much remains unclear regarding mechanisms underlying its inter-individual heterogeneity and clinical implications in bloodstream infections. Recent work suggests that the immediate antimicrobial effect of humoral innate immunity contributes to suppression of the excessive inflammatory responses to infection by reducing the amount of pathogen-associated molecular patterns. In this Personal View, we propose the need to re-explore factors underlying the inter-individual heterogeneity in serum antibacterial competence as a new approach to better understand humoral innate immunity and revisit the clinical use of measuring serum antibacterial activity in the management of bacterial bloodstream infections. Given the current emphasis on subtyping sepsis, a serum bactericidal assay might prove useful in defining a distinct sepsis endotype, to enable more personalised management.

IL32 downregulation lowers triglycerides and type I collagen in di-lineage human primary liver organoids

Steatotic liver disease (SLD) prevails as the most common chronic liver disease yet lack approved treatments due to incomplete understanding of pathogenesis. Recently, elevated hepatic and circulating interleukin 32 (IL-32) levels were found in individuals with severe SLD. However, the mechanistic link between IL-32 and intracellular triglyceride metabolism remains to be elucidated. We demonstrate in vitro that incubation with IL-32β protein leads to an increase in intracellular triglyceride synthesis, while downregulation of IL32 by small interfering RNA leads to lower triglyceride synthesis and secretion in organoids from human primary hepatocytes. This reduction requires the upregulation of Phospholipase A2 group IIA (PLA2G2A). Furthermore, downregulation of IL32 results in lower intracellular type I collagen levels in di-lineage human primary hepatic organoids. Finally, we identify a genetic variant of IL32 (rs76580947) associated with lower circulating IL-32 and protection against SLD measured by non-invasive tests. These data suggest that IL32 downregulation may be beneficial against SLD.

Could anionic LDL be a ligand for RAGE and TREM2 in addition to LOX-1 and thus exacerbate lung disease and dementia?

We recently highlighted the potential of protein glycation to generate anionic (electronegative) surfaces. We hypothesised that these anionic proteins are perceived by the innate immune system as arising from infection or damaged cell components, producing an inflammatory response within the lung involving the receptor RAGE. We now review two other pathologies linked to the innate immune response, cardiovascular disease and dementia that involve receptors LOX-1 and TREM2 respectively. Remarkable similarities in properties between RAGE, LOX-1 and TREM2 suggest that electronegative LDL may act as a pathogenic anionic ligand for all three receptors and exacerbate lung inflammation and dementia.

Modulation of immunity by the secreted phospholipase A2 family

Among the phospholipase A2 (PLA2 ) superfamily, which typically catalyzes the sn-2 hydrolysis of phospholipids to yield fatty acids and lysophospholipids, the secreted PLA2 (sPLA2 ) family contains 11 isoforms in mammals. Individual sPLA2 s have unique enzymatic specificity toward fatty acids and polar heads of phospholipid substrates and display distinct tissue/cellular distributions, suggesting their distinct physiological functions. Recent studies using knockout and/or transgenic mice for a full set of sPLA2 s have revealed their roles in modulation of immunity and related disorders. Application of mass spectrometric lipidomics to these mice has enabled to identify target substrates and products of individual sPLA2 s in given tissue microenvironments. sPLA2 s hydrolyze not only phospholipids in the plasma membrane of activated, damaged or dying mammalian cells, but also extracellular phospholipids such as those in extracellular vesicles, microbe membranes, lipoproteins, surfactants, and dietary phospholipids, thereby exacerbating or ameliorating various diseases. The actions of sPLA2 s are dependent on, or independent of, the generation of fatty acid- or lysophospholipid-derived lipid mediators according to the pathophysiological contexts. In this review, we make an overview of our current understanding of the roles of individual sPLA2 s in various immune responses and associated diseases.

Alternative Targets for sPLA2 Activity: Role of Membrane-Enzyme Interactions

The secreted phospholipases A2 (sPLA2s) play important roles both physiologically and pathologically, with their expression increasing significantly in diseases such as sepsis, inflammation, different cancers, glaucoma, obesity, Alzheimer's disease and even COVID-19. The fact has led to a large-scale search for inhibitors of these enzymes. In total, several dozen promising molecules have been proposed, but not a single one has successfully passed clinical trials. The failures in clinical studies motivated in-depth fundamental studies of PLA2s. Here we review alternative ways to control sPLA2 activity, outside its catalytic site. The concept can be realized by preventing sPLA2 from attaching to the membrane surface; by binding to an external protein which blocks sPLA2 hydrolytic activity; by preventing sPLA2 from orienting properly on the membrane surface; and by preventing substrate binding to the enzyme, keeping the catalytic site unaltered. Evidence in the literature is summarized in the review with the aim to serve as a starting point for new types of sPLA2 inhibitors.

Biochemical, Kinetic and Biological Properties of Group V Phospholipase A2 from Dromedary

Secretory group V phospholipase A2 (PLA₂-V) is known to be involved in inflammatory processes in cellular studies, nevertheless, the biochemical and the enzymatic characteristics of this important enzyme have been unclear yet. We reported, as a first step towards understanding the biochemical properties, catalytic characteristics, antimicrobial and cytotoxic effects of this PLA₂, the production of PLA₂-V from dromedary. The obtained DrPLA₂-V has an absolute requirement for Ca²⁺ and NaTDC for enzymatic activity with an optimum pH of 9 and temperature of 45 °C with phosphatidylethanolamine as a substrate. Kinetic parameters showed that K_cat/Km_app is 2.6 ± 0.02 mM⁻¹ s⁻¹. The enzyme was found to display potent Gram-positive bactericidal activity (with IC50 values of about 5 µg/mL) and antifungal activity (with IC50 values of about 25 µg/mL)in vitro. However, the purified enzyme did not display a cytotoxic effect against cancer cells.

Intestinal homeostasis and inflammation: gut microbiota at the crossroads of pancreas-intestinal barrier axis

The pancreas contains exocrine glands, which release enzymes (e.g., amylase, trypsin, and lipase) that are important for digestion and islets, which produce hormones. Digestive enzymes and hormones are secreted from the pancreas into the duodenum and bloodstream, respectively. Growing evidence suggests that the roles of the pancreas extend to not only the secretion of digestive enzymes and hormones but also to the regulation of intestinal homeostasis and inflammation (e.g., mucosal defense to pathogens and pathobionts). Organ crosstalk between the pancreas and intestine is linked to a range of physiological, immunological, and pathological activities, such as the regulation of the gut microbiota by the pancreatic proteins and lipids, the retroaction of the gut microbiota on the pancreas, the relationship between inflammatory bowel disease, and pancreatic diseases. We herein discuss the current understanding of the pancreas–intestinal barrier axis and the control of commensal bacteria in intestinal inflammation.

Age, obesity and hyperglycaemia: Activation of innate immunity initiates a series of molecular interactions involving anionic surfaces leading to COVID-19 morbidity and mortality

Obesity and type 2 diabetes are major factors in COVID-19 causing a progression to excessive morbidity and mortality. An important characteristic of these conditions is poor glycaemic control leading to inappropriate chemical reactions and the production of glycated proteins in which positively charged lysine and arginine residues are neutralised. We propose that this protein glycation primes the inflammatory system as the presence of aspartate and glutamate residues in any glycated zwitterionic protein will thus increase its anionic characteristics. As a result, these macromolecules will be recognised by the innate immune system and identified as originating from infection or cell damage (sterile inflammation). Many proteins in the body exist to non-specifically target these anionic macromolecules and rely heavily on positively charged (cationic) binding-sites to produce a relatively non-specific interaction as the first step in the body’s response. Proteins involved in this innate immunity are collectively referred to as damage associated molecular pattern molecules or pathogen associated molecular pattern molecules. A crucial player in this process is RAGE (Receptor for Advanced Glycation End products). RAGE plays a central role in the inflammatory response and on ligand binding stimulates many aspects of inflammation including the production of the key inflammatory mediator NF-κB, and the subsequent production of inflammatory cytokines. This process has the potential to show a positive feedback loop resulting in a dramatic response within the tissue. We propose that protein glycation primes the inflammatory system by generating negatively charged surfaces so that when a SARS-Cov-2 infection occurs within the lung the further release of negatively-charged macromolecules due to cell damage results in a potentially catastrophic inflammatory response resulting in the cytokine storm associated with COVID-19 morbidity and mortality. That part of the population who do not suffer from inflammatory priming (Phase 1), such as the young and the non-obese, should not be subjected to the catastrophic inflammatory response seen in others (Phase 2). This hypothesis further highlights the need for improved dietary intake to minimise the inflammatory priming resulting from poor glycaemic control.

Outer membrane permeabilization by the membrane attack complex sensitizes Gram-negative bacteria to antimicrobial proteins in serum and phagocytes

Infections with Gram-negative bacteria form an increasing risk for human health due to antibiotic resistance. Our immune system contains various antimicrobial proteins that can degrade the bacterial cell envelope. However, many of these proteins do not function on Gram-negative bacteria, because the impermeable outer membrane of these bacteria prevents such components from reaching their targets. Here we show that complement-dependent formation of Membrane Attack Complex (MAC) pores permeabilizes this barrier, allowing antimicrobial proteins to cross the outer membrane and exert their antimicrobial function. Specifically, we demonstrate that MAC-dependent outer membrane damage enables human lysozyme to degrade the cell wall of E. coli. Using flow cytometry and confocal microscopy, we show that the combination of MAC pores and lysozyme triggers effective E. coli cell wall degradation in human serum, thereby altering the bacterial cell morphology from rod-shaped to spherical. Completely assembled MAC pores are required to sensitize E. coli to the antimicrobial actions of lysozyme and other immune factors, such as Human Group IIA-secreted Phospholipase A2. Next to these effects in a serum environment, we observed that the MAC also sensitizes E. coli to more efficient degradation and killing inside human neutrophils. Altogether, this study serves as a proof of principle on how different players of the human immune system can work together to degrade the complex cell envelope of Gram-negative bacteria. This knowledge may facilitate the development of new antimicrobials that could stimulate or work synergistically with the immune system.

In this paper we identified how different players of the human immune system cooperate to degrade the complex cell envelope of Gram-negative bacteria. The outer membrane of Gram-negative bacteria forms an impermeable barrier for various antimicrobial proteins of the immune system. Here we show that complement-dependent Membrane Attack Complex (MAC) formation permeabilizes this barrier, allowing otherwise impermeable antimicrobial proteins to reach their targets underneath the outer membrane. Specifically, we show that outer membrane damage by the MAC allows lysozyme to degrade the peptidoglycan layer, and secreted phospholipase A₂-IIA to hydrolyze the bacterial inner membrane. MAC formation also sensitizes Gram-negative bacteria to more efficient degradation and killing inside human neutrophils. Altogether, this knowledge may guide the development of new antimicrobial strategies to treat infections caused by Gram-negative bacteria.

A Potential Role of Phospholipase 2 Group IIA (PLA2-IIA) in P. gingivalis-Induced Oral Dysbiosis

Porphyromonas gingivalis is an oral pathogen with the ability to induce oral dysbiosis and periodontal disease. Nevertheless, the mechanisms by which P. gingivalis could abrogate the host-microbe symbiotic relationship leading to oral dysbiosis remain unclear. We have recently demonstrated that P. gingivalis specifically increased the antimicrobial properties of oral epithelial cells, through a strong induction of the expression of PLA₂-IIA in a mechanism that involves activation of the Notch-1 receptor. Moreover, gingival expression of PLA₂-IIA was significantly increased during initiation and progression of periodontal disease in non-human primates and interestingly, those PLA₂-IIA expression changes were concurrent with oral dysbiosis. In this chapter, we present an innovative hypothesis of a potential mechanism involved in P. gingivalis-induced oral dysbiosis and inflammation based on our previous observations and a robust body of literature that supports the antimicrobial and proinflammatory properties of PLA₂-IIA as well as its role in other chronic inflammatory diseases.

Molecular cloning and analysis of Suncus murinus group IIA secretary phospholipase A2 expression

The intestinal epithelial monolayer forms a mucosal barrier between the gut microbes and the host tissue. The mucosal barrier is composed of mucins and antimicrobial peptides and proteins (AMPs). Several animal studies have reported that Paneth cells, which occupy the base of intestinal crypts, play an important role in the intestinal innate immunity by producing AMPs, such as lysozyme, Reg3 lectins, α-defensins, and group IIA secretory phospholipase A2 (GIIA sPLA2). The house musk shrew (Suncus murinus) has only a few intestinal commensal bacteria and is reported to lack Paneth cells in the intestine. Although the expression of lysozyme was reported in the suncus intestine, the expression of other AMPs has not yet been reported. Therefore, the current study was focused on GIIA sPLA2 expression in Suncus murinus. GIIA sPLA2 mRNA was found to be most abundant in the spleen and also highly expressed in the intestine. Cells expressing GIIA sPLA2 mRNA were distributed not only in the crypt, but also in the villi. In addition, intragastric injection of lipopolysaccharide increased GIIA sPLA2 expression in the small intestine of suncus. These results suggest that suncus may host unique AMP-secreting cells in the intestine.

Streptococcal Lancefield polysaccharides are critical cell wall determinants for human Group IIA secreted phospholipase A2 to exert its bactericidal effects

Human Group IIA secreted phospholipase A2 (hGIIA) is an acute phase protein with bactericidal activity against Gram-positive bacteria. Infection models in hGIIA transgenic mice have suggested the importance of hGIIA as an innate defense mechanism against the human pathogens Group A Streptococcus (GAS) and Group B Streptococcus (GBS). Compared to other Gram-positive bacteria, GAS is remarkably resistant to hGIIA activity. To identify GAS resistance mechanisms, we exposed a highly saturated GAS M1 transposon library to recombinant hGIIA and compared relative mutant abundance with library input through transposon-sequencing (Tn-seq). Based on transposon prevalence in the output library, we identified nine genes, including dltA and lytR, conferring increased hGIIA susceptibility. In addition, seven genes conferred increased hGIIA resistance, which included two genes, gacH and gacI that are located within the Group A Carbohydrate (GAC) gene cluster. Using GAS 5448 wild-type and the isogenic gacI mutant and gacI-complemented strains, we demonstrate that loss of the GAC N-acetylglucosamine (GlcNAc) side chain in the ΔgacI mutant increases hGIIA resistance approximately 10-fold, a phenotype that is conserved across different GAS serotypes. Increased resistance is associated with delayed penetration of hGIIA through the cell wall. Correspondingly, loss of the Lancefield Group B Carbohydrate (GBC) rendered GBS significantly more resistant to hGIIA-mediated killing. This suggests that the streptococcal Lancefield antigens, which are critical determinants for streptococcal physiology and virulence, are required for the bactericidal enzyme hGIIA to exert its bactericidal function.

Antibacterial activities of plasma from the common (Chelydra serpentina) and alligator snapping turtle (Macrochelys temminckii)

Innate immunity provides a fast-acting and nonspecific defense against microbial infection, and appears to have particular importance in the immune response of ectothermic vertebrates. Chelonians are a globally distributed and diverse group, yet little is known about their basic immune function. The chelonian family Chelydridae is made up of two genera (Chelydra and Macrochelys), represented in our study by the widespread common snapping turtle ( Chelydra serpentina; CST) and the southeast USA endemic alligator snapping turtle ( Macrochelys temminckii; AST). Our goal was to quantify the innate immune response of the family Chelydridae, using the antibacterial activity of plasma as a measure of immune function. Our results show that the plasma of both species has strong antibacterial properties, but CST plasma kills a higher percentage of bacteria than AST plasma. In addition, while both species showed the highest antibacterial activity at 25 to 30°C, CST plasma retained its antibacterial properties at lower and higher temperatures than AST plasma. Our results indicate that, like many ectotherms, Chelydridae have a relatively strong innate immune response. The stronger, more robust immune response of CSTs compared with ASTs is likely correlated to the differences in geographic ranges but may also have implications for each species' tolerance to anthropogenic habitat degradation and global climate change.

Roles of secreted phospholipase A2 group IIA in inflammation and host defense

Among all members of the secreted phospholipase A₂ (sPLA₂) family, group IIA sPLA₂ (sPLA₂-IIA) is possibly the most studied enzyme. Since its discovery, many names have been associated with sPLA₂-IIA, such as "non-pancreatic", "synovial", "platelet-type", "inflammatory", and "bactericidal" sPLA_2. Whereas the different designations indicate comprehensive functions or sources proposed for this enzyme, the identification of the precise roles of sPLA₂-IIA has remained a challenge. This can be attributed to: the expression of the enzyme by various cells of different lineages, its limited activity towards the membranes of immune cells despite its expression following common inflammatory stimuli, its ability to interact with certain proteins independently of its catalytic activity, and its absence from multiple commonly used mouse models. Nevertheless, elevated levels of the enzyme during inflammatory processes and associated consistent release of arachidonic acid from the membrane of extracellular vesicles suggest that sPLA₂-IIA may contribute to inflammation by using endogenous substrates in the extracellular milieu. Moreover, the remarkable potency of sPLA₂-IIA towards bacterial membranes and its induced expression during the course of infections point to a role for this enzyme in the defense of the host against invading pathogens. In this review, we present current knowledge related to mammalian sPLA₂-IIA and its roles in sterile inflammation and host defense.

Secreted PLA2 group X orchestrates innate and adaptive immune responses to inhaled allergen

Phospholipase A2 (PLA2) enzymes regulate the formation of eicosanoids and lysophospholipids that contribute to allergic airway inflammation. Secreted PLA2 group X (sPLA2-X) was recently found to be increased in the airways of asthmatics and is highly expressed in airway epithelial cells and macrophages. In the current study, we show that allergen exposure increases sPLA2-X in humans and in mice, and that global deletion of Pla2g10 results in a marked reduction in airway hyperresponsiveness (AHR), eosinophil and T cell trafficking to the airways, airway occlusion, generation of type-2 cytokines by antigen-stimulated leukocytes, and antigen-specific immunoglobulins. Further, we found that Pla2g10-/- mice had reduced IL-33 levels in BALF, fewer type-2 innate lymphoid cells (ILC2s) in the lung, less IL-33-induced IL-13 expression in mast cells, and a marked reduction in both the number of newly recruited macrophages and the M2 polarization of these macrophages in the lung. These results indicate that sPLA2-X serves as a central regulator of both innate and adaptive immune response to proteolytic allergen.

Proteomic analysis of human lacrimal and tear fluid in dry eye disease

To understand the pathophysiology of dry eye disease (DED), it is necessary to characterize proteins in the ocular surface fluids, including tear fluid (TF) and lacrimal fluid (LF). There have been several reports of TF proteomes, but few proteomic studies have examined LF secreted from the lacrimal gland (LG). Therefore, we characterized the proteins constituting TF and LF by liquid chromatography mass spectrometry. TF and LF were collected from patients with non-Sjögren syndrome DED and from healthy subjects. Through protein profiling and label-free quantification, 1165 proteins from TF and 1448 from LF were identified. In total, 849 proteins were present in both TF and LF. Next, candidate biomarkers were verified using the multiple reaction monitoring assay in both TF and LF of 17 DED patients and 17 healthy controls. As a result, 16 marker proteins were identified (fold-change > 1.5, p-value < 0.05), of which 3 were upregulated in TF and 8 up- and 5 down-regulated in LF. In conclusion, this study revealed novel DED markers originating from the LG and tears by in-depth proteomic analysis and comparison of TF and LF proteins.

Distinguishing septic from normal donors by detection of sPLA2-IIA from human plasma using a microsieve-based immunoassay

Bloodstream infections that progress to septic shock are responsible for hundreds of thousands of deaths each year, and are associated with significant healthcare costs. Recent studies have shown that a member of the secreted phospholipase protein family, termed sPLA2-IIA, may play a role during the innate immune response to bacterial infections, and is elevated in the plasma of septic patients. In this report, the feasibility of a simple microsieve-based sPLA2-IIA detection immunoassay was explored. Microsieves containing 5μm pores were covalently coupled with a sPLA2-IIA-specific monoclonal antibody at 0.1, 1.0, and 10μg/mL and then assayed with plasma-based positive and negative controls to determine the optimal coating concentration. Recombinant sPLA2-IIA was then serially diluted to a final concentration of 200, 100, 50, 25, 12.5, and 6.25ng/mL and tested alongside a non-spiked sample to estimate the detection limit of the prototype assay. Recombinant sPLA2-IIA was also spiked into serum, EDTA-plasma, and Lithium-Heparin plasma, in an effort to evaluate assay performance when analyzing these sample matrices. The preliminary limit of detection studies suggests that the microsieve assay is able to distinguish approximately 6-12ng/mL of sPLA2-IIA from a non-spiked sample. When compared to an immunoassay diluent, the microsieve assay also yielded acceptable percent recoveries for each of the three sample matrices spiked with clinically significant levels of sPLA2-IIA. The sPLA2-IIA microsieve assay prototype also clearly distinguished five samples from septic patients from five normal donor samples, and the results were in good agreement with a comparator ELISA test system (R²=0.9347).

Transcriptomic Analysis of Persistent Infection with Foot-and-Mouth Disease Virus in Cattle Suggests Impairment of Apoptosis and Cell-Mediated Immunity in the Nasopharynx

In order to investigate the mechanisms of persistent foot-and-mouth disease virus (FMDV) infection in cattle, transcriptome alterations associated with the FMDV carrier state were characterized using a bovine whole-transcriptome microarray. Eighteen cattle (8 vaccinated with a recombinant FMDV A vaccine, 10 non-vaccinated) were challenged with FMDV A24 Cruzeiro, and the gene expression profiles of nasopharyngeal tissues collected between 21 and 35 days after challenge were compared between 11 persistently infected carriers and 7 non-carriers. Carriers and non-carriers were further compared to 2 naïve animals that had been neither vaccinated nor challenged. At a controlled false-discovery rate of 10% and a minimum difference in expression of 50%, 648 genes were differentially expressed between FMDV carriers and non-carriers, and most (467) had higher expression in carriers. Among these, genes associated with cellular proliferation and the immune response-such as chemokines, cytokines and genes regulating T and B cells-were significantly overrepresented. Differential gene expression was significantly correlated between non-vaccinated and vaccinated animals (biological correlation +0.97), indicating a similar transcriptome profile across these groups. Genes related to prostaglandin E2 production and the induction of regulatory T cells were overexpressed in carriers. In contrast, tissues from non-carrier animals expressed higher levels of complement regulators and pro-apoptotic genes that could promote virus clearance. Based on these findings, we propose a working hypothesis for FMDV persistence in nasopharyngeal tissues of cattle, in which the virus may be maintained by an impairment of apoptosis and the local suppression of cell-mediated antiviral immunity by inducible regulatory T cells.

Antimicrobial activity of apitoxin, melittin and phospholipase A₂ of honey bee (Apis mellifera) venom against oral pathogens

In this work, we used the Minimum Inhibitory Concentration (MIC) technique to evaluate the antibacterial potential of the apitoxin produced by Apis mellifera bees against the causative agents of tooth decay. Apitoxin was assayed in natura and in the commercially available form. The antibacterial actions of the main components of this apitoxin, phospholipase A2, and melittin were also assessed, alone and in combination. The following bacteria were tested: Streptococcus salivarius, S. sobrinus, S. mutans, S. mitis, S. sanguinis, Lactobacillus casei, and Enterococcus faecalis. The MIC results obtained for the commercially available apitoxin and for the apitoxin in natura were close and lay between 20 and 40 µg / mL, which indicated good antibacterial activity. Melittin was the most active component in apitoxin; it displayed very promising MIC values, from 4 to 40 µg / mL. Phospholipase A2 presented MIC values higher than 400 µg / mL. Association of mellitin with phospholipase A2 yielded MIC values ranging between 6 and 80 µg / mL. Considering that tooth decay affects people's health, apitoxin and its component melittin have potential application against oral pathogens.

Molecular determinants of bacterial sensitivity and resistance to mammalian Group IIA phospholipase A2

Group IIA secretory phospholipase A2 (sPLA(2)-IIA) of mammalian species is unique among the many structurally and functionally related mammalian sPLA(2) in their high net positive charge and potent (nM) antibacterial activity. Toward the Gram-positive bacteria tested thus far, the global cationic properties of sPLA(2)-IIA are necessary for optimal binding to intact bacteria and penetration of the multi-layered thick cell wall, but not for the degradation of membrane phospholipids that is essential for bacterial killing. Various Gram-positive bacterial species can differ as much as 1000-fold in sPLA(2)-IIA sensitivity despite similar intrinsic enzymatic activity of sPLA(2)-IIA toward the membrane phospholipids of various bacteria. d-alanylation of wall- and lipo-teichoic acids in Staphylococcus aureus and sortase function in Streptococcus pyogenes increase bacterial resistance to sPLA(2)-IIA by up to 100-fold apparently by affecting translocation of bound sPLA(2)-IIA to the cell membrane. Action of the sPLA(2)-IIA and other related sPLA(2) against Gram-negative bacteria is more dependent on cationic properties of the enzyme near the amino-terminus of the protein and collaboration with other host defense proteins that produce alterations of the unique Gram-negative bacterial outer membrane that normally represents a barrier to sPLA(2)-IIA action. This article is part of a Special Issue entitled: Bacterial Resistance to Antimicrobial Peptides.

The enteric nervous system neuropeptide, bombesin, reverses innate immune impairments during parenteral nutrition

BACKGROUND

Lack of enteral stimulation during parenteral nutrition (PN) impairs mucosal immunity. Bombesin (BBS), a gastrin-releasing peptide analogue, reverses PN-induced defects in acquired immunity. Paneth cells produce antimicrobial peptides (AMPs) of innate immunity for release after cholinergic stimulation.

OBJECTIVE

Determine if BBS restores AMPs and bactericidal function during PN.

METHODS

Intravenously cannulated male ICR mice were randomized to Chow, PN, or PN+BBS (15 μg 3 times daily, n = 7 per group) for 5 days. Ileum was analyzed for AMPs (Protein: sPLA2 by fluorescence, lysozyme and RegIII-γ by western andcryptdin-4 by ELISA; mRNA: all by RT-PCR). Cholinergic stimulated (100 μM bethanechol) ileal specimens assessed Pseudomonas bactericidal activity. Ileum (Chow: n = 7; PN: n = 9; PN+BBS: n = 8) was assessed for Escherichia coli invasion in ex-vivo culture.

RESULTS

PN significantly decreased most AMPs versus Chow while BBS maintained Chow levels (sPLA2: Chow: 107 + 14*, PN: 44.6 + 7.2, PN+BBS: 78.7 + 13.4* Fl/min/μL/total protein; Lysozyme: Chow: 63.9 + 11.9*, PN: 26.8 + 6.2; PN+BBS: 64.9 + 13.8* lysozyme/total protein; RegIII-γ: Chow: 51.5 + 10.0*, PN: 20.4 + 4.3, PN+BBS: 31.0 + 8.4 RegIII-γ/total protein; Cryptdin-4: Chow: 18.4 + 1.5*, PN: 12.7 + 1.6, PN+BBS: 26.1 + 2.4*† pg/mg [all *P < 0.05 vs PN and †P < 0.05 vs Chow]). Functionally, BBS prevented PN loss of bactericidal activity after cholinergic stimulation (Chow: 25.3 + 3.6*, PN: 13.0 + 3.2; PN+BBS: 27.0 + 4.7* percent bacterial killing, *P < 0.05 vs PN). BBS reduced bacterial invasion in unstimulated tissue barely missing significance (P = 0.06).

CONCLUSIONS

The enteric nervous system (ENS) controls AMP levels in Paneth cells during PN but mucosal protection by innate immunity requires both ENS and parasympathetic stimulation.

Antimicrobial defense of the intestine

The mammalian gastrointestinal tract is home to a dense community of resident bacteria and is also exposed to microorganisms from the external environment. The epithelial surface of the intestine plays a critical role in host protection by producing a diverse repertoire of antimicrobial proteins that directly kill or hinder the growth of microorganisms. Here we discuss the general principles that govern the mechanisms of action of epithelial antimicrobial proteins, regulation of antimicrobial protein expression and activity, and in vivo functions of intestinal antimicrobial proteins. We also consider how altered antimicrobial protein expression and function can contribute to disease and how these endogenous antibiotics might be harnessed for the benefit of human health.

Pregnancy Recognition and Implantation of the Conceptus in the Mare

Few, if any, biological processes are as diverse among domestic species as establishment of early pregnancy, in particular maternal recognition of pregnancy. Following fertilization and initial development in the mare oviduct, selective transport of the embryo through the uterotubal junction driven by embryo-derived PGE2 occurs. Upon arrival in the uterus, an acellular glycoprotein capsule is formed that covers the embryo, blastocyst, and conceptus (embryo and associated extraembryonic membranes) between the second and third weeks of pregnancy. Between Days 9 and 15/16 of pregnancy, the conceptus undergoes an extended phase of mobility. Conceptus mobility is driven by conceptus-derived PGF2α and PGE2 that stimulate uterine contractions which in turn propel migration of the conceptus within the uterine lumen. Cessation of conceptus mobility is referred to as fixation and appears to be attributable to increasing size of the conceptus, preferential thickening of the endometrium near the mesometrial attachment referred to as encroachment, and a reduction in sialic acid content of the capsule. During maternal recognition of pregnancy, endometrial PGF2α release is attenuated, a consequence of reduced expression of key enzymes involved in prostaglandin production. Oxytocin responsiveness is altered during early pregnancy, and reduced expression of the oxytocin receptor appears to be regulated at the posttranscriptional level rather than the transcriptional level. Prostaglandin release is attenuated temporarily only during early pregnancy; during the third week of pregnancy, the endometrium resumes the ability to secrete PGF2α. The equine conceptus initiates steroidogenesis as early as Day 6 and synthesizes estrogens, androgens, and progesterone. Estrogens are metabolized locally, presumably regulating their bioavailability and actions. Results of experiments attempting to prove that conceptus-derived estrogens are responsible for extension of corpus luteum function have been inconclusive. By the fourth week of pregnancy, the chorionic girdle becomes visible on the trophoblast. Subsequent invasion of chorionic girdle cells leads to formation of endometrial cups which secrete equine chorionic gonadotropin. Equine chorionic gonadotropin has luteinizing hormone functions in the mare, causing luteinization of follicles resulting in the formation of secondary corpora lutea essential to production of progesterone and maintenance of pregnancy.

SIM2 maintains innate host defense of the small intestine

The single-minded 2 (SIM2) protein is a basic helix-loop-helix transcription factor regulating central nervous system (CNS) development in Drosophila. In humans, SIM2 is located within the Down syndrome critical region on chromosome 21 and may be involved in the development of mental retardation phenotype in Down syndrome. In this study, knockout of SIM2 expression in mice resulted in a gas distention phenotype in the gastrointestinal tract. We found that SIM2 is required for the expression of all cryptdins and numerous other antimicrobial peptides (AMPs) expressed in the small intestine. The mechanism underlying how SIM2 controls AMP expression involves both direct and indirect regulations. For the cryptdin genes, SIM2 regulates their expression by modulating transcription factor 7-like 2, a crucial regulator in the Wnt/β-catenin signaling pathway, while for other AMP genes, such as RegIIIγ, SIM2 directly activates their promoter activity. Our results establish that SIM2 is a crucial regulator in controlling expression of intestinal AMPs to maintain intestinal innate immunity against microbes.

Anti-bactericidal properties of stingray Dasyatis pastinaca groups V, IIA, and IB phospholipases A2: a comparative study

Group IIA secreted phospholipase A2 (group IIA sPLA2) is known to display potent Gram-positive bactericidal activity in vitro and in vivo. We have analyzed the bactericidal activity of the full set of native stingray and dromedary groups V, IIA, and IB sPLA2s on several Gram-positive and Gram-negative strains. The rank order potency among both marine and mammal sPLA2s against Gram-positive bacteria is group IIA > V > IB, whereas Gram-negative bacteria exhibited a much higher resistance. There is a synergic action of the sPLA2 with lysozyme when added to the bacteria culture prior to sPLA2.The bactericidal efficiency of groups V and IIA sPLA2s was shown to be dependent upon the presence of calcium ions and to a less extent Mg(2+) ions and then a correlation could be made to its hydrolytic activity of membrane phospholipids. Importantly, we showed that stingray and dromedary groups V, IIA, and IB sPLA2s present no cytotoxicity after their incubation with MDA-MB-231cells. stingray groups V and IIA sPLA2s, like mammal ones, may be considered as future therapeutic agents against bacterial infections.

Intestinal dysbiosis and bacterial enteroinvasion in a murine model of Hirschsprung's disease

BACKGROUND/PURPOSE

Hirschsprung's disease (HSCR), characterized by the absence of ganglia in the distal colon, results in functional obstruction. Despite surgical resection of the aganglionic segment, around 40% of patients suffer recurrent life threatening Hirschsprung's-associated enterocolitis (HAEC). The aim of this study was to investigate whether gut microbiota and intestinal immunity changes contribute to the HAEC risk in an HSCR model.

METHODS

Mice with neural crest conditional deletion of Endothelin receptor B (EdnrB) and their littermate controls were used (EdnrB-null and EdnrB-het). Bacterial DNA was prepared from cecal contents of P16-18 and P21-24 animals and pyrosequencing employed for microbiome analysis. Ileal tissue was isolated and secretory phospholipase A2 (sPLA2) expression and activity determined. Enteroinvasion of Escherichia coli into ileal explants was measured using an ex vivo organ culture system.

RESULTS

EdnrB-het and EdnrB-nulls displayed similar flora, sPLA2 expression and activity at P16-18. However, by P21-24, EdnrB-hets demonstrated increased Lactobacillus and decreased Bacteroides and Clostridium, while EdnrB-nulls exhibited reciprocal changes. EdnrB-nulls also showed reduced sPLA2 expression and luminal activity at this stage. Functionally, EdnrB-nulls were more susceptible to enteroinvasion with E. coli ex vivo and released less sPLA2 than EdnrB-hets.

CONCLUSIONS

Initially, EdnrB-het and EdnrB-nulls contain similar cecal flora but then undergo reciprocal changes. EdnrB-nulls display dysbiosis, demonstrate impaired mucosal defense, decreased luminal sPLA2 and increased enteroinvasion of E. coli just prior to robust colonic inflammation and death. These findings suggest a role for the intestinal microbiome in the development of HAEC.

Longitudinal changes in tear fluid lipidome brought about by eyelid-warming treatment in a cohort of meibomian gland dysfunction

Meibomian gland dysfunction (MGD) is a leading cause of evaporative dry eye and ocular discomfort characterized by an unstable tear film principally attributed to afflicted delivery of lipids to the ocular surface. Herein, we elucidated longitudinal tear lipid alterations associated with disease alleviation and symptom improvement in a cohort of MGD patients undergoing eyelid-warming treatment for 12 weeks. Remarkably, eyelid-warming resulted in stark reductions in lysophospholipids (P < 0.001 for lyso-plasmalogen phosphatidylethanolamine, lysophosphatidylcholine, and lysophosphatidylinositol), as well as numerous PUFA-containing diacylglyceride species in tears, accompanied by significant increases in several PUFA-containing phospholipids. These changes in tear lipidomes suggest that eyelid-warming leads to diminished activity of tear phospholipases that preferentially target PUFA-containing phospholipids. In addition, treatment led to appreciable increases (P < 0.001) in O-acyl-ω-hydroxy-FAs (OAHFAs), which are lipid amphiphiles critical to the maintenance of tear film stability. Longitudinal changes in the tear lipids aforementioned also significantly (P < 0.05) correlated with reduced rate of ocular evaporation and improvement in ocular symptoms. The foregoing data thus indicate that excess ocular surface phospholipase activity detrimental to tear film stability could be alleviated by eyelid warming alone without application of steroids and identify tear OAHFAs as suitable markers to monitor treatment response in MGD.

IL-25 improves luminal innate immunity and barrier function during parenteral nutrition

BACKGROUND

Parenteral nutrition (PN) increases risks of infections in critically injured patients. Recently, PN was shown to reduce intestine luminal levels of the Paneth cell antimicrobial molecule secretory phospholipase A2 (sPLA2) and the goblet cell glycoprotein mucin2 (MUC2). These molecules are critical factors for innate mucosal immunity and provide barrier protection. Interleukin-4 (IL-4) and IL-13 regulate sPLA2 and MUC2 production through the IL-13 receptor. Because IL-25 stimulates IL-4 and IL-13 release and PN reduces luminal sPLA2 and MUC2, we hypothesized that adding IL-25 to PN would restore these innate immune factors and maintain barrier function.

METHODS

Two days after venous cannulation, male ICR (Institute of Cancer Research) mice were randomized to receive chow (n = 12), PN (n = 9), or PN + 0.7 μg of exogenous IL-25 (n = 11) daily for 5 days. Small-intestine wash fluid (SIWF) was collected for analysis of sPLA2 activity, MUC2 density, and luminal levels of IL-4 and IL-13. Small-intestinal tissue was harvested for analysis of tissue sPLA2 activity or immediate use in an ex-vivo intestinal segment culture (EVISC) to assess susceptibility of the tissue segments to enteroinvasive Escherichia coli.

RESULTS

PN reduced luminal sPLA2 (P < 0.0001) and MUC2 (P <0.002) compared with chow, whereas the addition of IL-25 to PN increased luminal sPLA2 (P < 0.0001) and MUC2 (P < 0.02) compared with PN. Tissue IL-4 and IL-13 decreased with PN compared with chow (IL-4: P < 0.0001, IL-13: P < 0.002), whereas IL-25 increased both cytokines compared with PN (IL-4: P < 0.03, IL-13: P < 0.02). Tissue levels of sPLA2 were significantly decreased with PN compared with chow, whereas IL-25 significantly increased tissue sPLA2 levels compared with PN alone. Functionally, more bacteria invaded the PN-treated tissue compared with chow (P < 0.01), and the addition of IL-25 to PN decreased enteroinvasion to chow levels (P < 0.01).

CONCLUSIONS

PN impairs innate mucosal immunity by suppressing luminal sPLA2 activity and MUC2 density compared with chow. PN also increases bacterial invasion in ex-vivo tissue. Administration of exogenous IL-25 reverses this dysfunction and increases luminal sPLA2 and MUC2. PN tissue treated with IL-25 was significantly more resistant to bacterial invasion than with PN alone, suggesting that IL-25-induced effects augment the barrier defense mechanisms.

Secreted phospholipase A2 is increased in meconium-stained amniotic fluid of term gestations: potential implications for the genesis of meconium aspiration syndrome

BACKGROUND

Meconium-stained amniotic fluid (MSAF) represents the passage of fetal colonic content into the amniotic cavity. Meconium aspiration syndrome (MAS) is a complication that occurs in a subset of infants with MSAF. Secreted phospholipase A2 (sPLA2) is detected in meconium and is implicated in the development of MAS. The purpose of this study was to determine if sPLA2 concentrations are increased in the amniotic fluid of women in spontaneous labor at term with MSAF.

MATERIALS AND METHODS

This was a cross-sectional study of patients in spontaneous term labor who underwent amniocentesis (n = 101). The patients were divided into two study groups: (1) MSAF (n = 61) and (2) clear fluid (n = 40). The presence of bacteria and endotoxin as well as interleukin-6 (IL-6) and sPLA2 concentrations in the amniotic fluid were determined. Statistical analyses were performed to test for normality and bivariate analysis. The Spearman correlation coefficient was used to study the relationship between sPLA2 and IL-6 concentrations in the amniotic fluid.

RESULTS

Patients with MSAF have a higher median sPLA2 concentration (ng/mL) in amniotic fluid than those with clear fluid [1.7 (0.98-2.89) versus 0.3 (0-0.6), p < 0.001]. Among patients with MSAF, those with either microbial invasion of the amniotic cavity (MIAC, defined as presence of bacteria in the amniotic cavity), or bacterial endotoxin had a significantly higher median sPLA2 concentration (ng/mL) in amniotic fluid than those without MIAC or endotoxin [2.4 (1.7-6.0) versus 1.7 (1.3-2.5), p < 0.05]. There was a positive correlation between sPLA2 and IL-6 concentrations in the amniotic fluid (Spearman Rho = 0.3, p < 0.05).

CONCLUSION

MSAF that contains bacteria or endotoxin has a higher concentration of sPLA2, and this may contribute to induce lung inflammation when meconium is aspirated before birth.

Parenteral nutrition increases susceptibility of ileum to invasion by E coli

BACKGROUND

Parenteral nutrition (PN), with the lack of enteral feeding, compromises mucosal immune function and increases the risk of infections. We developed an ex vivo intestinal segment culture (EVISC) model to study the ex vivo effects of PN on susceptibility of the ileum to invasion by extra-intestinal pathogenic Escherichia coli (ExPEC) and on ileal secretion of antimicrobial secretory phospholipase A2 (sPLA2) in response to the pathogen.

MATERIALS AND METHODS

Study 1: Using mouse (n = 7) ileal tissue, we examined the effects of ileal region (proximal versus distal) and varying ExPEC inoculum concentrations on ex vivo susceptibility to ExPEC invasion and sPLA2 secretion. Study 2: Ten mice were randomized to oral chow or intravenous PN feeding for 5 d (n = 5/group). Using the EVISC model, we compared the susceptibility of ileal tissue to invasion by ExPEC and sPLA2 secretion in response to the pathogen.

RESULTS

Study 1: The proximal ileum was more susceptible to invasion (P < 0.0001) and secreted lower amounts of sPLA2 (P = 0.0002) than the distal ileum. Study 2: Ileal tissue from PN-fed animals was more susceptible (approximately 4-fold, P = 0.018) to invasion than those from chow-fed animals. Ileal tissue from PN-fed animals secreted less sPLA2 (P < 0.02) than those from chow-fed animals.

CONCLUSIONS

The data illustrate EVISC as a reproducible model for studying host-pathogen interactions and the effects of diet on susceptibility to infections. Specifically, the findings support our hypothesis that PN with the lack of enteral feeding decreases mucosal responsiveness to pathogen exposure and provides a plausible mechanism by which PN is associated with increased risk of infectious complication.

Secretory phospholipase A2 in dromedary tears: a host defense against staphylococci and other gram-positive bacteria

The best known physiologic function of secreted phospholipase A2 (sPLA2) group IIA (sPLA2-IIA) is defense against bacterial infection through hydrolytic degradation of bacterial membrane phospholipids. In fact, sPLA2-IIA effectively kills Gram-positive bacteria and to a lesser extent Gram-negative bacteria and is considered a major component of the eye's innate immune defense system. The antibacterial properties of sPLA2 have been demonstrated in rabbit and human tears. In this report, we have analyzed the bactericidal activity of dromedary tears and the subsequently purified sPLA2 on several Gram-positive bacteria. Our results showed that the sPLA2 displays a potent bactericidal activity against all the tested bacteria particularly against the Staphylococcus strains when tested in the ionic environment of tears. There is a synergic action of the sPLA2 with lysozyme when added to the bacteria culture prior to sPLA2. Interestingly, lysozyme purified from dromedary tears showed a significant bactericidal activity against Listeria monocytogene and Staphylococcus epidermidis, whereas the one purified from human tears displayed no activity against these two strains. We have also demonstrated that Ca(2+) is crucial for the activity of dromedary tear sPLA2 and to a less extent Mg(2+) ions. Given the presence of sPLA2 in tears and intestinal secretions, this enzyme may play a substantial role in innate mucosal and systemic bactericidal defenses against Gram-positive bacteria.

Antibacterial properties of intestinal phospholipase A2 from the common stingray Dasyatis pastinaca

Stingray phospholipase A(2) group IIA (SPLA(2)-IIA) was recently isolated and purified to homogeneity from the intestine of the common stingray Dasyatis pastinaca, suggesting that this enzyme plays an important role in systemic bactericidal defense. The present study showed that SPLA(2)-IIA was highly bactericidal against Gram-positive bacteria with inhibition zones and minimal inhibitory concentration values in the range of 13-25 mm and 2-8 μg/ml, respectively, whereas Gram-negative bacteria exhibited a much higher resistance. The bactericidal efficiency of SPLA(2)-IIA was shown to be unaffected by high protein and salt concentrations, but dependent upon the presence of calcium ions, and then correlated to the hydrolytic activity of membrane phospholipids. Importantly, we showed that stingray phospholipase A(2) group IIA presents no cytotoxicity after its incubation with MDA-MB-231 cells. SPLA(2)-IIA may be considered as a future therapeutic agent against bacterial infections.

Parenteral nutrition suppresses the bactericidal response of the small intestine

BACKGROUND

Parenteral nutrition (PN) increases infectious risk in critically ill patients compared with enteral feeding. Previously, we demonstrated that PN feeding suppresses the concentration of the Paneth cell antimicrobial protein secretory phospholipase A2 (sPLA2) in the gut lumen. sPLA2 and other Paneth cell proteins are released in response to bacterial components, such as lipopolysaccharide (LPS), and they modulate the intestinal microbiome. Because the Paneth cell protein sPLA2 was suppressed with PN feeding, we hypothesized PN would diminish the responsiveness of the small bowel to LPS through reduced secretions and as a result exhibit less bactericidal activity.

METHODS

The distal ileum was harvested from Institute of Cancer Research mice, washed, and randomized for incubation with LPS (0, 1, or 10 μg/mL). Culture supernatant was collected and sPLA2 activity was measured. Bactericidal activity of the ileum segment secretions was assessed against Pseudomonas aeruginosa with and without an sPLA2 inhibitor at 2 concentrations, 100 nmol/L and 1 μmol/L. Institute of Cancer Research mice were randomized to chow or PN for 5 days. Tissue was collected for immunohistochemistry (IHC) and ileal segments were incubated with LPS (0 or 10 μg/mL). sPLA2 activity and bactericidal activity were measured in secretions from ileal segments.

RESULTS

Ileal segments responded to 10 μg/mL LPS with significantly greater sPLA2 activity and bactericidal activity. The bactericidal activity of secretions from LPS stimulated tissue was suppressed 50% and 70%, respectively, with the addition of the sPLA2-inhibitor. Chow displayed greater sPLA2 in the Paneth cell granules and secreted higher levels of sPLA2 than PN before and after LPS. Accordingly, media collected from chow was more bactericidal than PN. IHC confirmed a reduction in Paneth cell granules after PN.

CONCLUSION

This work demonstrates that ileal segments secrete bactericidal secretions after LPS exposure and the inhibition of the Paneth cell antimicrobial protein sPLA2 significantly diminishes this. PN feeding resulted in suppressed secretion of the sPLA2 and resulted in increased bacterial survival. This demonstrates that PN significantly impairs the innate immune response by suppressing Paneth cell function.

Lactoferrin: an alternative view of its role in human biological fluids

Lactoferrin is a major component of biologically important mucosal fluids and of the specific granules of neutrophils. Understanding its biological function is essential for understanding neutrophil- and mucosal-mediated immunity. In this review, we reevaluate the in vivo functions of human lactoferrin (hLF) emphasizing in vivo studies and in vitro studies performed in biologically relevant fluids. We discuss the evidence in the literature that supports (or does not support) proposed roles for hLF in mucosal immunity and in neutrophil function. We argue that the current literature supports a microbiostatic role, but not a microbicidal role, for hLF in vivo. The literature also supports a role for hLF in inhibiting colonization and infection of epithelial surfaces by microorganisms and in protecting tissues from neutrophil-mediated damage. Using this information, we briefly discuss hLF in the context of the complex biological fluids in which it is found.

Route and type of nutrition and surgical stress influence secretory phospholipase A2 secretion of the murine small intestine

BACKGROUND

The function of secretory phospholipase A2 (sPLA2) is site dependent. In tissue, sPLA2 regulates eicosanoid production; in circulation, sPLA2 primes neutrophils; and in the intestinal lumen, sPLA2 provides innate bactericidal immunity as a defensin-related protein. Since parenteral nutrition (PN) primes leukocytes while suppressing intraluminal mucosal immunity, the authors hypothesized that (1) PN would diminish luminal sPLA2 activity but increase activity in intestinal tissue and serum and (2) stress would accentuate these changes.

METHODS

Mice received chow, a complex enteral diet (CED), intragastric PN (IG-PN), or PN in experiment 1 and chow, chow+stress, PN, or PN+stress in experiment 2.

RESULTS

In experiment 1, luminal sPLA2 activity was greatest in chow and decreased in CED, IG-PN, and PN, with PN lower than CED and IG-PN. Compared to that after chow, serum sPLA2 activity dropped after CED, IG-PN, and PN. Serum sPLA2 was higher in portal than systemic serum. In experiment 2, PN lowered luminal sPLA2 activity vs chow. Stress lowered luminal sPLA2 activity in chow, without change in PN. Following stress, luminal immunoglobulin A increased in chow but not PN. Serum sPLA2 activity increased in PN.

CONCLUSIONS

PN attenuates sPLA2 activity in intestinal fluid, consistent with suppressed innate mucosal defense. Stress suppresses luminal fluid sPLA2 activity in chow but not the immunoglobulin A response; PN impairs both. Stress significantly elevates serum sPLA2 in PN-fed mice, consistent with known increased neutrophil priming with PN. PN reduces innate bactericidal immunity of the gut but upregulates serum proinflammatory products poststress.

Investigation into the role of phosphatidylserine in modifying the susceptibility of human lymphocytes to secretory phospholipase A(2) using cells deficient in the expression of scramblase

Normal human lymphocytes resisted the hydrolytic action of secretory phospholipase A(2) but became susceptible to the enzyme following treatment with a calcium ionophore, ionomycin. To test the hypothesis that this susceptibility requires exposure of the anionic lipid phosphatidylserine on the external face of the cell membrane, experiments were repeated with a human Burkitt's lymphoma cell line (Raji cells). In contrast to normal lymphocytes or S49 mouse lymphoma cells, most of the Raji cells (83%) did not translocate phosphatidylserine to the cell surface upon treatment with ionomycin. Those few that did display exposed phosphatidylserine were hydrolyzed immediately upon addition of phospholipase A(2). Interestingly, the remaining cells were also completely susceptible to the enzyme but were hydrolyzed at a slower rate and after a latency of about 100s. In contradistinction to the defect in phosphatidylserine translocation, Raji cells did display other physical membrane changes upon ionomycin treatment that may be relevant to hydrolysis by phospholipase A(2). These changes were detected by merocyanine 540 and trimethylammonium diphenylhexatriene fluorescence and were common among normal lymphocytes, S49 cells, and Raji cells. The levels of these latter effects corresponded well with the relative rates of hydrolysis among the three cell lines. These results suggested that while phosphatidylserine enhances the rate of cell membrane hydrolysis by secretory phospholipase A(2), it is not an absolute requirement. Other physical properties such as membrane order contribute to the level of membrane susceptibility to the enzyme independent of phosphatidylserine.

Characterization of suramin binding sites on the human group IIA secreted phospholipase A2 by site-directed mutagenesis and molecular dynamics simulation

Suramin is a polysulphonated naphthylurea with inhibitory activity against the human secreted group IIA phospholipase A(2) (hsPLA2GIIA), and we have investigated suramin binding to recombinant hsPLA2GIIA using site-directed mutagenesis and molecular dynamics (MD) simulations. The changes in suramin binding affinity of 13 cationic residue mutants of the hsPLA2GIIA was strongly correlated with alterations in the inhibition of membrane damaging activity of the protein. Suramin binding to hsPLA2GIIA was also studied by MD simulations, which demonstrated that altered intermolecular potential energy of the suramin/mutant complexes was a reliable indicator of affinity change. Although residues in the C-terminal region play a major role in the stabilization of the hsPLA2GIIA/suramin complex, attractive and repulsive hydrophobic and electrostatic interactions with residues throughout the protein together with the adoption of a bent suramin conformation, all contribute to the stability of the complex. Analysis of the hsPLA2GIIA/suramin interactions allows the prediction of the properties of suramin analogues with improved binding and higher affinities which may be candidates for novel phospholipase A(2) inhibitors.

Characterization of serum phospholipase a(2) activity in three diverse species of west african crocodiles

Secretory phospholipase A(2), an enzyme that exhibits substantial immunological activity, was measured in the serum of three species of diverse West African crocodiles. Incubation of different volumes of crocodile serum with bacteria labeled with a fluorescent fatty acid in the sn-2 position of membrane lipids resulted in a volume-dependent liberation of fluorescent probe. Serum from the Nile crocodile (Crocodylus niloticus) exhibited slightly higher activity than that of the slender-snouted crocodile (Mecistops cataphractus) and the African dwarf crocodile (Osteolaemus tetraspis). Product formation was inhibited by BPB, a specific PLA(2) inhibitor, confirming that the activity was a direct result of the presence of serum PLA(2). Kinetic analysis showed that C. niloticus serum produced product more rapidly than M. cataphractus or O. tetraspis. Serum from all three species exhibited temperature-dependent PLA(2) activities but with slightly different thermal profiles. All three crocodilian species showed high levels of activity against eight different species of bacteria.

A novel bacterial resistance mechanism against human group IIA-secreted phospholipase A2: role of Streptococcus pyogenes sortase A

Human group IIA-secreted phospholipase A(2) (sPLA(2)-IIA) is a bactericidal molecule important for the innate immune defense against Gram-positive bacteria. In this study, we analyzed its role in the host defense against Streptococcus pyogenes, a major human pathogen, and demonstrated that this bacterium has evolved a previously unidentified mechanism to resist killing by sPLA(2)-IIA. Analysis of a set of clinical isolates demonstrated that an ~500-fold higher concentration of sPLA(2)-IIA was required to kill S. pyogenes compared with strains of the group B Streptococcus, which previously were shown to be sensitive to sPLA(2)-IIA, indicating that S. pyogenes exhibits a high degree of resistance to sPLA(2)-IIA. We found that an S. pyogenes mutant lacking sortase A, a transpeptidase responsible for anchoring LPXTG proteins to the cell wall in Gram-positive bacteria, was significantly more sensitive (~30-fold) to sPLA(2)-IIA compared with the parental strain, indicating that one or more LPXTG surface proteins protect S. pyogenes against sPLA(2)-IIA. Importantly, using transgenic mice expressing human sPLA(2)-IIA, we showed that the sortase A-mediated sPLA(2)-IIA resistance mechanism in S. pyogenes also occurs in vivo. Moreover, in this mouse model, we also showed that human sPLA(2)-IIA is important for the defense against lethal S. pyogenes infection. Thus, we demonstrated a novel mechanism by which a pathogenic bacterium can evade the bactericidal action of sPLA(2)-IIA and we showed that sPLA(2)-IIA contributes to the host defense against S. pyogenes infection.

Active site mutants of human secreted Group IIA Phospholipase A2 lacking hydrolytic activity retain their bactericidal effect

The Human Secreted Group IIA Phospholipase A(2) (hsPLA2GIIA) presents potent bactericidal activity, and is considered to contribute to the acute-phase immune response. Hydrolysis of inner membrane phospholipids is suggested to underlie the bactericidal activity, and we have evaluated this proposal by comparing catalytic activity with bactericidal and liposome membrane damaging effects of the G30S, H48Q and D49K hsPLA2GIIA mutants. All mutants showed severely impaired hydrolytic activities against mixed DOPC:DOPG liposome membranes, however the bactericidal effect against Micrococcus luteus was less affected, with 50% killing at concentrations of 1, 3, 7 and 9 μg/mL for the wild-type, D49K, H48Q and G30S mutants respectively. Furthermore, all proteins showed Ca(2+)-independent damaging activity against liposome membranes demonstrating that in addition to the hydrolysis-dependent membrane damage, the hsPLA2GIIA presents a mechanism for permeabilization of phospholipid bilayers that is independent of catalytic activity, which may play a role in the bactericidal function of the protein.

Role of net charge on catalytic domain and influence of cell wall binding domain on bactericidal activity, specificity, and host range of phage lysins

The recombinant lysins of lytic phages, when applied externally to Gram-positive bacteria, can be efficient bactericidal agents, typically retaining high specificity. Their development as novel antibacterial agents offers many potential advantages over conventional antibiotics. Protein engineering could exploit this potential further by generating novel lysins fit for distinct target populations and environments. However, access to the peptidoglycan layer is controlled by a variety of secondary cell wall polymers, chemical modifications, and (in some cases) S-layers and capsules. Classical lysins require a cell wall-binding domain (CBD) that targets the catalytic domain to the peptidoglycan layer via binding to a secondary cell wall polymer component. The cell walls of Gram-positive bacteria generally have a negative charge, and we noticed a correlation between (positive) charge on the catalytic domain and bacteriolytic activity in the absence of the CBD (nonclassical behavior). We investigated a physical basis for this correlation by comparing the structures and activities of pairs of lysins where the lytic activity of one of each pair was CBD-independent. We found that by engineering a reversal of sign of the net charge of the catalytic domain, we could either eliminate or create CBD dependence. We also provide evidence that the S-layer of Bacillus anthracis acts as a molecular sieve that is chiefly size-dependent, favoring catalytic domains over full-length lysins. Our work suggests a number of facile approaches for fine-tuning lysin activity, either to enhance or reduce specificity/host range and/or bactericidal potential, as required.