Characterization of bacterial DNA binding to human neutrophil surface

Interactions of Neutrophils with the Polymeric Molecular Components of the Biofilm Matrix in the Context of Implant-Associated Bone and Joint Infections

In the presence of orthopedic implants, opportunistic pathogens can easily colonize the biomaterial surfaces, forming protective biofilms. Life in biofilm is a central pathogenetic mechanism enabling bacteria to elude the host immune response and survive conventional medical treatments. The formation of mature biofilms is universally recognized as the main cause of septic prosthetic failures. Neutrophils are the first leukocytes to be recruited at the site of infection. They are highly efficient in detecting and killing planktonic bacteria. However, the interactions of these fundamental effector cells of the immune system with the biofilm matrix, which is the true interface of a biofilm with the host cells, have only recently started to be unveiled and are still to be fully understood. Biofilm matrix macromolecules consist of exopolysaccharides, proteins, lipids, teichoic acids, and the most recently described extracellular DNA. The latter can also be stolen from neutrophil extracellular traps (NETs) by bacteria, who use it to strengthen their biofilms. This paper aims to review the specific interactions that neutrophils develop when they physically encounter the matrix of a biofilm and come to interact with its polymeric molecular components.

Extracellular DNA (eDNA). A Major Ubiquitous Element of the Bacterial Biofilm Architecture

After the first ancient studies on microbial slime (the name by which the biofilm matrix was initially indicated), multitudes of studies on the morphology, composition and physiology of biofilms have arisen. The emergence of the role that biofilms play in the pathogenesis of recalcitrant and persistent clinical infections, such as periprosthetic orthopedic infections, has reinforced scientific interest. Extracellular DNA (eDNA) is a recently uncovered component that is proving to be almost omnipresent in the extracellular polymeric substance (EPS) of biofilm. This macromolecule is eliciting unprecedented consideration for the critical impact on the pathogenesis of chronic clinical infections. After a systematic review of the literature, an updated description of eDNA in biofilms is presented, with a special focus on the latest findings regarding its fundamental structural role and the contribution it makes to the complex architecture of bacterial biofilms through interactions with a variety of other molecular components of the biofilm matrix.

Immune Responses to Pseudomonas aeruginosa Biofilm Infections

Pseudomonas aeruginosa is a key pathogen of chronic infections in the lungs of cystic fibrosis patients and in patients suffering from chronic wounds of diverse etiology. In these infections the bacteria congregate in biofilms and cannot be eradicated by standard antibiotic treatment or host immune responses. The persistent biofilms induce a hyper inflammatory state that results in collateral damage of the adjacent host tissue. The host fails to eradicate the biofilm infection, resulting in hindered remodeling and healing. In the present review we describe our current understanding of innate and adaptive immune responses elicited by P. aeruginosa biofilms in cystic fibrosis lung infections and chronic wounds. This includes the mechanisms that are involved in the activation of the immune responses, as well as the effector functions, the antimicrobial components and the associated tissue destruction. The mechanisms by which the biofilms evade immune responses, and potential treatment targets of the immune response are also discussed.

Primed PMNs in healthy mouse and human circulation are first responders during acute inflammation

Polymorphonuclear neutrophils (PMNs) are the most abundant circulating leukocytes, and the first cells recruited to sites of tissue inflammation. Using a fixation method to preserve native CD marker expression prior to immunophenotyping, we identified a distinct population of "primed for recruitment" PMNs in healthy mouse and human blood that has high expression of adhesion and activation markers compared with the bulk resting-state PMNs. In response to acute tissue inflammation, primed PMNs (pPMNs) were rapidly depleted from the circulation and recruited to the tissue. One hour after acute peritoneal insult, pPMNs became the dominant PMN population in bone marrow (BM) and blood, returning to baseline levels with resolution of inflammation. PMN priming was induced by the granulopoietic factors granulocyte-macrophage-colony-stimulating factor (GM-CSF) and granulocyte-colony-stimulating factor (G-CSF). High levels of pPMNs were observed in neutropenic mice and in pediatric neutropenic patients who were resistant to infection, highlighting an important role of this population in innate immune function.

Release mechanisms and molecular interactions of Pseudomonas aeruginosa extracellular DNA

Pseudomonas aeruginosa infection is a significant threat for clinicians. Increasing incidents of resistant biofilm infection result in high mortality rates worldwide. There is a considerable current interest in the field of extracellular DNA (eDNA)-mediated P. aeruginosa biofilm formation. eDNA acts as a glue to make biofilm more stable. This review focuses on the diverse mechanisms and factors, which enhance the eDNA release into the extracellular milieu. Furthermore, eDNA-mediated molecular interactions within the biofilm are emphasized. In addition, drug resistance mechanisms due to the versatility of eDNA are discussed. Spatial physiological diversity is expected due to different metabolic activity of bacterial subpopulation present in P. aeruginosa biofilm layers. In P. aeruginosa, eDNA release is accomplished by cell lysis and OMVs (outer membrane vesicles). eDNA release is a spontaneous and multifactorial process, which may be accomplished by PQS, pyocyanin, and lambda prophage induction. Hydrogen peroxide and pyocin trigger cell death, which may facilitate eDNA release. Lung mucosa of cystic fibrosis patients is enriched with eDNA, which acidifies biofilm and develops P. aeruginosa resistance to aminoglycosides. Further studies on spatial and molecular characterization of bacterial subpopulation in biofilm will shed light on eDNA-biofilm interaction more precisely.Key Points• Extracellular DNA (eDNA) is a key component of Pseudomonas aeruginosa biofilm.• P. aeruginosa eDNA acts as a glue to make biofilm more stronger.• Bacterial cell death or lysis may be the potential way to release P. aeruginosa eDNA into extracellular milieu.• P. aeruginosa eDNA contributes to develop resistance to antimicrobials.

Unveiling and Characterizing Early Bilateral Interactions between Biofilm and the Mouse Innate Immune System

A very substantial progress has been made in our understanding of infectious diseases caused by invasive bacteria. Under their planktonic forms, bacteria transiently reside in the otherwise sterile mammal body tissues, as the physiological inflammation insures both their clearance and repair of any tissue damage. Yet, the bacteria prone to experience planktonic to biofilm developmental transition still need to be studied. Of note, sessile bacteria not only persist but also concur preventing the effectors and regulators of the physiological inflammation to operate. Thus, it is urgent to design biologically sound experimental approaches aimed to extract, at the earliest stage, immune signatures of mono-bacteria planktonic to biofilm developmental transition in vivo and ex vivo. Indeed, the transition is often the first event to which succeeds the “chronicization” process whereby classical bacteria-targeting therapies are no more efficacious. An in vivo model of micro-injection of Staphylococcus aureus planktonic or biofilm cells in the ear pinna dermis of laboratory transgenic mice with fluorescent immune cells is proposed. It allows visualizing, in real time, the range of the early interactions between the S. aureus and myeloid cell subsets- the resident macrophages and dendritic cells, the recruited neutrophil granulocytes/polymorphonuclear neutrophils, monocytes otherwise known to differentiate as macrophages or dendritic cells. One main objective is to extract contrasting immune signatures of the modulation of the physiological inflammation with respect to the two bacterial lifestyles.

B. abortus RNA is the component involved in the down-modulation of MHC-I expression on human monocytes via TLR8 and the EGFR pathway

Despite eliciting a potent CD8⁺ T cell response, Brucella abortus is able to persist and establish a chronic infection inside its host. We have previously reported that the infection of human monocytes/macrophages with B. abortus inhibits the IFN-γ-induced MHC-I cell surface expression down-modulating cytotoxic CD8⁺ T cell responses. MHC-I down-modulation depends on bacterial viability and results from the capacity of B. abortus to retain the MHC-I molecules within the Golgi apparatus. Furthermore, we recently demonstrated that epidermal growth factor receptor (EGFR) pathway is involved in this phenomenon and that this is an early event during infection. However, the components and mechanisms whereby B. abortus is able to down-modulate MHC-I remained to be elucidated. In this study we demonstrated that the down-modulation of MHC-I expression is not mediated by well-known Brucella virulence factors but instead by B. abortus RNA, a PAMP associated to viability (vita-PAMP). Surprisingly, completely degraded RNA was also able to inhibit MHC-I expression to the same extent as intact RNA. Accordingly, B. abortus RNA and its degradation products were able to mimic the MHC-I intracellular retention within the Golgi apparatus observed upon infection. We further demonstrated that TLR8, a single-stranded RNA and RNA degradation products sensor, was involved in MHC-I inhibition. On the other hand, neutralization of the EGFR reversed the MHC-I inhibition, suggesting a connection between the TLR8 and EGFR pathways. Finally, B. abortus RNA-treated macrophages display diminished capacity of antigen presentation to CD8⁺ T cells. Overall, our results indicate that the vita-PAMP RNA as well as its degradation products constitute novel virulence factors whereby B. abortus, by a TLR8-dependent mechanism and through the EGFR pathway, inhibits the IFN-γ-induced MHC-I surface expression on human monocytes/macrophages. Thus, bacteria can hide within infected cells and avoid the immunological surveillance of cytotoxic CD8⁺ T cells.

Brucella abortus is one of the intracellular bacterial species that cause brucellosis, a zoonotic worldwide disease. An intricate enigma of Brucella immunity is its long-term persistence inside host despite a vigorous and specific immune response. Our study describes a novel immune evasion strategy exploited by B. abortus: the down-modulation of the IFN-γ-induced expression of the molecules responsible for antigen presentation to CD8⁺ T cells on the surface of monocytes, Major Histocompatibility Complex Class I (MHC-I) molecules. We found that the bacterial component responsible for this phenomenon is its RNA, a component associated with bacterial viability itself. Specifically, we demonstrated that not only does intact RNA down-modulate MHC-I but also the RNA degradation products. Bacterial RNA is sensed by pattern recognition receptors (PRRs), among which TLRs family has gained more attention. In this study, we demonstrated that the receptor involved in this phenomenon is TLR8, a single-stranded RNA and RNA degradation products sensor. By means of this mechanism, Brucella impairs antigen presentation to CD8⁺ T cells, hiding within infected cells and avoiding the immunological surveillance of cytotoxic cells. On balance, these results provide new evidence to understand how B. abortus can survive inside the host and persist chronically.

Biofilms and host response - helpful or harmful

Biofilm infections are one of the modern medical world's greatest challenges. Probably, all non-obligate intracellular bacteria and fungi can establish biofilms. In addition, there are numerous biofilm-related infections, both foreign body-related and non-foreign body-related. Although biofilm infections can present in numerous ways, one common feature is involvement of the host response with significant impact on the course. A special characteristic is the synergy of the innate and the acquired immune responses for the induced pathology. Here, we review the impact of the host response for the course of biofilm infections, with special focus on cystic fibrosis, chronic wounds and infective endocarditis.

Vasoactive Intestinal Peptide modulates trophoblast-derived cell line function and interaction with phagocytic cells through autocrine pathways

Trophoblast cells migrate and invade the decidual stroma in a tightly regulated process to maintain immune homeostasis at the maternal-placental interface during the first weeks of pregnancy. Locally synthesized factors modulate trophoblast cell function and their interaction with maternal leukocytes to promote the silent clearance of apoptotic cells. The vasoactive intestinal peptide (VIP) is a pleiotropic polypeptide with trophic and anti-inflammatory effects in murine pregnancy models. We explored the effect of VIP on two human first trimester trophoblast cell lines, particularly on their migration, invasiveness and interaction with phagocytic cells, and the signalling and regulatory pathways involved. We found that VIP enhanced trophoblast cell migration and invasion through the activation of high affinity VPAC receptors and PKA-CRE signalling pathways. VIP knocked-down trophoblast cells showed reduced migration in basal and leukemic inhibitor factor (LIF)-elicited conditions. In parallel, VIP-silenced trophoblast cells failed to induce the phagocytosis of apoptotic bodies and the expression of immunosuppressant markers by human monocytes. Our results suggest that VIP-mediated autocrine pathways regulate trophoblast cell function and contribute to immune homeostasis maintenance at placentation and may provide new clues for therapeutic intervention in pregnancies complicated by defective deep placentation.

Trophoblast cells primed with vasoactive intestinal peptide enhance monocyte migration and apoptotic cell clearance through αvβ3 integrin portal formation in a model of maternal-placental interaction

STUDY HYPOTHESIS

Is apoptotic cell phagocytosis by monocytes modulated by pathways elicited by vasoactive intestinal peptide (VIP) action on trophoblast?

STUDY FINDING

Targeting trophoblast cells with VIP induces monocyte migration, polarization to anti-inflammatory phenotypes and apoptotic trophoblast cell clearance which involves increased αvβ3 integrin expression on phagocytic cells and binding to thrombospondin 1.

WHAT IS KNOWN ALREADY

Monocytes recruited to the maternal-placental interface interact with trophoblast cells and differentiate to alternatively activated macrophages involved in the silent clearance of apoptotic cells. Vasoactive intestinal peptide (VIP) is an immunomodulatory polypeptide synthesized at the human placenta that can target both trophoblast cells and monocytes/macrophages. Integrin αvβ3 and thrombospondin 1 are involved in the formation of a phagocytic portal for the immunosuppressant clearance of apoptotic cells.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

This is a laboratory-based study studying monocytes isolated from peripheral blood of healthy women (n = 33) and their interaction in vitro with first trimester trophoblast cell lines. Peripheral blood monocytes were isolated from healthy volunteers by Percoll gradient and tested in co-culture settings with first trimester trophoblast cell lines (Swan 71 and HTR8) or with trophoblast cell conditioned media obtained in the presence or absence of 10 or 100 nM VIP. The effect of VIP-conditioned media on monocyte migration was assessed through transwell systems and monocyte/macrophage phenotype was determined by flow cytometry. Phagocytosis of apoptotic cells and the mechanisms involved in phagocytic portal formation were assessed by flow cytometry, confocal microscopy, immunological blockade and RT-PCR.

MAIN RESULTS AND THE ROLE OF CHANCE

Exposing cells to 100 nM VIP increased the migration of monocytes toward trophoblast cell conditioned media (VIP conditioned medium) (P < 0.05 versus conditioned media from cells not exposed to VIP) and contributed to the monocytes acquiring an anti-inflammatory profile with increased CD39 and IL-10 expression (P < 0.05). Phagocytosis of apoptotic trophoblast cells by monocytes and monocyte-differentiated macrophages was increased by VIP conditioned medium (P < 0.05 versus media conditioned in the absence of VIP or direct addition of 100 nM VIP). The boosting effect of VIP conditioned medium on phagocytosis involved increased expression and re-localization of αvβ3 integrin on phagocytic cells along with enhanced expression of thrombospondin 1 on trophoblast cells.

LIMITATIONS, REASONS FOR CAUTION

The conclusions are based on in vitro experiments with monocytes drawn from peripheral blood of healthy individuals and trophoblast cell lines and we were unable to ascertain that these mechanisms operate similarly in vivo. We cannot rule out a differential behavior of either trophoblast cells targeted in vivo with VIP, or primary cultures of first trimester trophoblast cells assayed in vitro.

WIDER IMPLICATIONS OF THE FINDINGS

The results presented provide new clues for immune and trophoblast cell pharmacological targeting in pregnancy complications of immunopathologic nature.

STUDY FUNDING/COMPETING INTERESTS

This work was funded by the National Agency of Sciences and Technology ANPCyT (PICT 2011-0144), National Research Council CONICET (PIP 602/2012) and University of Buenos Aires (UBACyT 20020130100040BA) to C.P.L. The authors have no conflicts of interest to disclose.

Immunomodulatory oligonucleotides inhibit neutrophil migration by decreasing the surface expression of interleukin-8 and leukotriene B4 receptors

Neutrophils play important roles in many inflammatory diseases. The migration of neutrophils to the inflammatory site is tightly regulated by specific chemokines, of which interleukin-8 (IL-8) and leukotriene B4 (LTB4 ) constitute key mediators by binding to the surface receptors CXCR1/2 and BLT1, respectively. Oligonucleotides (ODN) containing CpG motifs mediate potent immunomodulatory effects through binding to Toll-like receptor 9. So far, knowledge on how ODN can affect neutrophil migration during inflammation is lacking. This study demonstrates that several novel CpG ODN significantly down-regulate the surface expression of CXCR1/2 and BLT1. In addition, the ODN significantly blocked IL-8-induced and LTB4 -induced neutrophil migration in vitro, as well as leucocyte migration in vivo demonstrated in mice by intravital microscopy and in a model of airway inflammation. The down-regulation of CXCR1 is rapid, occurring 15 min after ODN stimulation, and can be mediated through an endosomally independent mechanism. Inhibition of the IL-8 and LTB4 pathways may provide new opportunities of therapeutic intervention using ODN to reduce neutrophil infiltration during inflammation.

Dynamic interactions of neutrophils and biofilms

BACKGROUND

The majority of microbial infections in humans are biofilm-associated and difficult to treat, as biofilms are highly resistant to antimicrobial agents and protect themselves from external threats in various ways. Biofilms are tenaciously attached to surfaces and impede the ability of host defense molecules and cells to penetrate them. On the other hand, some biofilms are beneficial for the host and contain protective microorganisms. Microbes in biofilms express pathogen-associated molecular patterns and epitopes that can be recognized by innate immune cells and opsonins, leading to activation of neutrophils and other leukocytes. Neutrophils are part of the first line of defense and have multiple antimicrobial strategies allowing them to attack pathogenic biofilms.

OBJECTIVE/DESIGN

In this paper, interaction modes of neutrophils with biofilms are reviewed. Antimicrobial strategies of neutrophils and the counteractions of the biofilm communities, with special attention to oral biofilms, are presented. Moreover, possible adverse effects of neutrophil activity and their biofilm-promoting side effects are discussed.

RESULTS/CONCLUSION

Biofilms are partially, but not entirely, protected against neutrophil assault, which include the processes of phagocytosis, degranulation, and formation of neutrophil extracellular traps. However, virulence factors of microorganisms, microbial composition, and properties of the extracellular matrix determine whether a biofilm and subsequent microbial spread can be controlled by neutrophils and other host defense factors. Besides, neutrophils may inadvertently contribute to the physical and ecological stability of biofilms by promoting selection of more resistant strains. Moreover, neutrophil enzymes can degrade collagen and other proteins and, as a result, cause harm to the host tissues. These parameters could be crucial factors in the onset of periodontal inflammation and the subsequent tissue breakdown.

TLR9 and NF-κB are partially involved in activation of human neutrophils by Helicobacter pylori and its purified DNA

Helicobacter pylori infection represents one of the most common bacterial infections worldwide. The inflammatory response to this bacterium involves a large influx of neutrophils to the lamina propria of the gastric mucosa. However, little is known about the receptors and molecular mechanisms involved in activation of these neutrophils. In this study, we aimed to determine the role of toll-like receptor 9 (TLR9) in the response of human neutrophils to H. pylori and purified H. pylori DNA (Hp-DNA). Neutrophils were isolated from the blood of adult volunteers and challenged with either H. pylori or Hp-DNA. We found that both, H. pylori and Hp-DNA induced increased expression and release of IL-8. Furthermore, we showed that TLR9 is involved in the induction of IL-8 production by H. pylori and Hp-DNA. IL-8 production induced by H. pylori but not by Hp-DNA was partially mediated by NF-κB. In conclusion, this study showed for first time that both, H. pylori and Hp-DNA activate TLR9 and induce a different inflammatory response that leads to activation of neutrophils.

Primary blood neutrophils express a functional cell surface Toll-like receptor 9

Polymorphonuclear leukocytes (PMNs) represent one of the first lines of defense against pathogens. TLR9 is normally expressed in endosomes/lysosomes where it is activated by pathogen-derived DNA. Here we show that freshly isolated human and mouse primary PMNs express TLR9 at the cell surface ex vivo. Moreover, surface TLR9 expression is upregulated upon activation of PMNs with different stimuli and not only TLR9 agonists. Importantly, surface TLR9 is processed, active, and functional. TLR9 ligands, oligo-nucleotides containing unmethylated CpG motifs, indeed bind to surface TLR9 and binding was strongly observed at the cell surface of human cells expressing surface TLR9 and at the surface of WT but not TLR9-deficient mouse PMNs. Finally, CpG oligonucleotides cross-linked onto a solid phase and having no access to intracellular TLR9 are able to trigger cell surface TLR9 and induce neutrophil activation, even when endosomal acidification is inhibited. This is the first demonstration of a functional TLR9 expressed at the cell surface of human primary cells. This pathway may be triggered when pathogen-derived TLR9 ligands cannot reach the endosome, offering a rescue mechanism for neutrophil activation.

DNA from Porphyromonas gingivalis and Tannerella forsythia induce cytokine production in human monocytic cell lines

Toll-like receptor 9 (TLR9) expression is increased in periodontally diseased tissues compared with healthy sites indicating a possible role of TLR9 and its ligand, bacterial DNA (bDNA), in periodontal disease pathology. Here, we determine the immunostimulatory effects of periodontal bDNA in human monocytic cells (THP-1). THP-1 cells were stimulated with DNA of two putative periodontal pathogens: Porphyromonas gingivalis and Tannerella forsythia. The role of TLR9 in periodontal bDNA-initiated cytokine production was determined either by blocking TLR9 signaling in THP-1 cells with chloroquine or by measuring IL-8 production and nuclear factor-kappaB (NF-kappaB) activation in HEK293 cells stably transfected with human TLR9. Cytokine production (IL-1beta, IL-6, and TNF-alpha) was increased significantly in bDNA-stimulated cells compared with controls. Chloroquine treatment of THP-1 cells decreased cytokine production, suggesting that TLR9-mediated signaling pathways are operant in the recognition of DNA from periodontal pathogens. Compared with native HEK293 cells, TLR9-transfected cells demonstrated significantly increased IL-8 production (P < 0.001) and NF-kappaB activation in response to bDNA, further confirming the role of TLR9 in periodontal bDNA recognition. The results of PCR arrays demonstrated upregulation of proinflammatory cytokine and NF-kappaB genes in response to periodontal bDNA in THP-1 cells, suggesting that cytokine induction is through NF-kappaB activation. Hence, immune responses triggered by periodontal bacterial nucleic acids may contribute to periodontal disease pathology by inducing proinflammatory cytokine production through the TLR9 signaling pathway.

The immune system vs. Pseudomonas aeruginosa biofilms

Ilya Metchnikoff and Paul Ehrlich were awarded the Nobel price in 1908. Since then, numerous studies have unraveled a multitude of mechanistically different immune responses to intruding microorganisms. However, in the vast majority of these studies, the underlying infectious agents have appeared in the planktonic state. Accordingly, much less is known about the immune responses to the presence of biofilm-based infections (which is probably also due to the relatively short period of time in which the immune response to biofilms has been studied). Nevertheless, more recent in vivo and in vitro studies have revealed both innate as well as adaptive immune responses to biofilms. On the other hand, measures launched by biofilm bacteria to achieve protection against the various immune responses have also been demonstrated. Whether particular immune responses to biofilm infections exist remains to be firmly established. However, because biofilm infections are often persistent (or chronic), an odd situation appears with the simultaneous activation of both arms of the host immune response, neither of which can eliminate the biofilm pathogen, but instead, in synergy, causes collateral tissue damage. Although the present review on the immune system vs. biofilm bacteria is focused on Pseudomonas aeruginosa (mainly because this is the most thoroughly studied), many of the same mechanisms are also seen with biofilm infections generated by other microorganisms.

The investigation of toll-like receptor 3, 9 and 10 gene polymorphisms in Turkish rheumatoid arthritis patients

Toll-like receptors (TLRs) play an important role in the induction and regulation of the innate immune system or adaptive immune responses. Genetic variations within human TLRs have been reported to be associated with a range of immune-related diseases. This study was conducted to investigate the frequencies of TLR3 rs3775290, TLR9 rs187084, and TLR10 rs4129009 polymorphisms and to detect between polymorphisms and autoantibody positive as RF, collagen type II, anti-RNP, and anti-CCP in patient group. We performed a case-control study of 100 rheumatoid arthritis (RA) cases and 100 healthy controls matched on age, sex, and residence. All polymorphisms in TLRs were determined by polymerase chain reaction-based restriction fragment length polymorphism. Serum autoantibody level was measured using quantitative ELISA. SNPs were genotyped in all samples. Our results showed that TT genotype for SNP 1237 T/C increased the RA risk significantly (p < 0.05). No statistically significant differences were found in the TLR3 and TLR10 genotypes or allele distribution between RA patients and control individuals. No associations were noted with autoantibody production and TLR3, TLR9, and TLR10 polymorphisms genotypes (p > 0.05). Our study suggests that a single nucleotide polymorphism (rs187084) in TLR9 gene may be a susceptibility factor for RA in Turkish population. Further studies are required to explore the role of TLRs gene polymorphisms in the risk of RA, especially in ethnically different populations to confirm our results.

Extracellular DNA: a major proinflammatory component of Pseudomonas aeruginosa biofilms

We previously demonstrated that extracellular bacterial DNA activates neutrophils through a CpG- and TLR9-independent mechanism. Biofilms are microbial communities enclosed in a polymeric matrix that play a critical role in the pathogenesis of many infectious diseases. Because extracellular DNA is a key component of biofilms of different bacterial species, the aim of this study was to determine whether it plays a role in the ability of biofilms to induce human neutrophil activation. We found that degradation of matrix extracellular DNA with DNase I markedly reduced the capacity of Pseudomonas aeruginosa biofilms to induce the release of the neutrophil proinflammatory cytokines IL-8 and IL-1beta (>75%); reduced the upregulation of neutrophil activation markers CD18, CD11b, and CD66b (p < 0.001); reduced the number of bacteria phagocytosed per neutrophil contacting the biofilm; and reduced the production of neutrophil extracellular traps. Consistent with these findings, we found that biofilms formed by the lasI rhlI P. aeruginosa mutant strain, exhibiting a very low content of matrix extracellular DNA, displayed a lower capacity to stimulate the release of proinflammatory cytokines by neutrophils, which was not decreased further by DNase I treatment. Together, our findings support that matrix extracellular DNA is a major proinflammatory component of P. aeruginosa biofilms.