Measurement of nonopsonic phagocytic killing by human and mouse phagocytes

Transcriptome-guided metabolic network analysis reveals rearrangements of carbon flux distribution in Neisseria gonorrhoeae during neutrophil co-culture

The ability of bacterial pathogens to metabolically adapt to the environmental conditions of their hosts is critical to both colonization and invasive disease. Infection with Neisseria gonorrhoeae (the gonococcus, Gc) is characterized by the influx of neutrophils [polymorphonuclear leukocytes (PMNs)], which fail to clear the bacteria and make antimicrobial products that can exacerbate tissue damage. The inability of the human host to clear Gc infection is particularly concerning in light of the emergence of strains that are resistant to all clinically recommended antibiotics. Bacterial metabolism represents a promising target for the development of new therapeutics against Gc. Here, we generated a curated genome-scale metabolic network reconstruction (GENRE) of Gc strain FA1090. This GENRE links genetic information to metabolic phenotypes and predicts Gc biomass synthesis and energy consumption. We validated this model with published data and in new results reported here. Contextualization of this model using the transcriptional profile of Gc exposed to PMNs revealed substantial rearrangements of Gc central metabolism and induction of Gc nutrient acquisition strategies for alternate carbon source use. These features enhanced the growth of Gc in the presence of neutrophils. From these results, we conclude that the metabolic interplay between Gc and PMNs helps define infection outcomes. The use of transcriptional profiling and metabolic modeling to reveal new mechanisms by which Gc persists in the presence of PMNs uncovers unique aspects of metabolism in this fastidious bacterium, which could be targeted to block infection and thereby reduce the burden of gonorrhea in the human population. IMPORTANCE The World Health Organization designated Gc as a high-priority pathogen for research and development of new antimicrobials. Bacterial metabolism is a promising target for new antimicrobials, as metabolic enzymes are widely conserved among bacterial strains and are critical for nutrient acquisition and survival within the human host. Here we used genome-scale metabolic modeling to characterize the core metabolic pathways of this fastidious bacterium and to uncover the pathways used by Gc during culture with primary human immune cells. These analyses revealed that Gc relies on different metabolic pathways during co-culture with human neutrophils than in rich media. Conditionally essential genes emerging from these analyses were validated experimentally. These results show that metabolic adaptation in the context of innate immunity is important to Gc pathogenesis. Identifying the metabolic pathways used by Gc during infection can highlight new therapeutic targets for drug-resistant gonorrhea.

Streptococcus suis serotype 2 strains can induce the formation of neutrophil extracellular traps and evade trapping

Streptococcus suis (S. suis) ranks among the five most important porcine pathogens worldwide and occasionally threatens human health, especially in people that come into close contact with pigs or pork products. Streptococcus suis serotype 2 (SS2) is considered to be the most pathogenic and prevalent capsular type. As a first line of immune defense against SS2 infection, neutrophils can eliminate the invader not only by phagocytosis but also by neutrophil extracellular traps (NETs)-mediated killing. SS2 can resist phagocytosis through polysaccharide capsule (CPS), but how this strain evades the effects of NETs remains to be determined. The present study demonstrated that the epidemic strain 05ZY, the highly pathogenic strain P1/7 and the intermediately pathogenic strain A7 could induce the formation of NETs. Furthermore, SS2 strains could successfully resist NETs-mediated killing, and the CPS structure contributed to this resistance by escaping the trapping. Therefore, the CPS structure not only contributed to the SS2 strains' resistance to phagocytosis-mediated killing but also played an essential role in evading NETs trapping and further killing in vitro. This study strengthens our understanding of how S. suis can evade innate immune surveillance and elimination.

The biocontrol fungus Trichoderma stromaticum downregulates respiratory burst and nitric oxide in phagocytes and IFN-gamma and IL-10

Trichoderma stromaticum, a biocontrol agent of the cacao witches' broom pathogen Moniliophthora perniciosa, has been used in Brazil as part of the integrated pest management of cacao. At the present time, little is known about the effects of T. stromaticum on the modulation of in vitro or in vivo immune responses. The present study examined the interaction of T. stromaticum spores with cellular and molecular components of the immune system following intranasal sensitization of mice. Our results showed that T. stromaticum spores prevented the expression and production of inflammatory mediators in macrophages stimulated with interferon (IFN)-γ plus lipopolysaccharide (LPS) and neutrophils stimulated with phorbol myristate 13-acetate (PMA). Quantitative polymerase chain reaction (qPCR) assays revealed that T. stromaticum spores inhibited the expression of dectin-1 and Toll-like-receptor (TLR)2/TLR4. Intranasal injection of BALB/c mice and subsequent challenge with spores of T. stromaticum induced a discrete inflammatory response in the lungs. Interestingly, the spores inhibited local and systemic production of the regulatory IL-10 and proinflammatory IFN-γ cytokines. In addition the spores presented an antiproliferative effect on spleen cells. These findings showed that the biopesticide T. stromaticum may exert immunosuppressive effects in vitro and in vivo.

Resistance of Neisseria gonorrhoeae to non-oxidative killing by adherent human polymorphonuclear leucocytes

Symptomatic infection with Neisseria gonorrhoeae (Gc) is characterized by abundant neutrophil (PMN, polymorphonuclear leucocyte) influx, but PMNs cannot clear initial infection, indicating that Gc possess defences against PMN challenge. In this study, survival of liquid-grown Gc was monitored after synchronous infection of adherent, interleukin 8-treated human PMNs. 40-70% of FA1090 Gc survived 1 h of PMN exposure, after which bacterial numbers increased. Assays with bacterial viability dyes along with soybean lectin to detect extracellular Gc revealed that a subset of both intracellular and extracellular PMN-associated Gc were viable. Gc survival was unaffected in PMNs chemically or genetically deficient for producing reactive oxygen species (ROS). This result held true even for OpaB+ Gc, which stimulate neutrophil ROS production. Catalase- and RecA-deficient Gc, which are more sensitive to ROS in vitro, had no PMN survival defect. recN and ngo1686 mutant Gc also exhibit increased sensitivity to ROS and PMNs, but survival of these mutants was not rescued in ROS-deficient cells. The ngo1686 mutant showed increased sensitivity to extracellular but not intracellular PMN killing. We conclude that Gc are remarkably resistant to PMN killing, killing occurs independently of neutrophil ROS production and Ngo1686 and RecN defend Gc from non-oxidative PMN antimicrobial factors.

Participation of nitric oxide reductase in survival of Pseudomonas aeruginosa in LPS-activated macrophages

Nitric oxide (NO) plays a crucial role in the antimicrobial activity of host defense systems. We investigated the function of Pseudomonas aeruginosa NO reductase as a detoxifying enzyme in phagocytes. We found that the growth of the NO reductase-deficient mutant of P. aeruginosa under a microaerobic condition was inhibited by the exogenous NO. Furthermore, the intracellular survival assay within the NO-producing RAW 264.7 macrophages revealed that the wild-type strain survived longer than the NO reductase-deficient mutant. These results suggest that the P. aeruginosa NO reductase may contribute to the intracellular survival by acting as a counter component against the host's defense systems.

The Bacillus anthracis cholesterol-dependent cytolysin, Anthrolysin O, kills human neutrophils, monocytes and macrophages

BACKGROUND

Bacillus anthracis is an animal and human pathogen whose virulence is characterized by lethal and edema toxin, as well as a poly-glutamic acid capsule. In addition to these well characterized toxins, B. anthracis secretes several proteases and phospholipases, and a newly described toxin of the cholesterol-dependent cytolysin (CDC) family, Anthrolysin O (ALO).

RESULTS

In the present studies we show that recombinant ALO (rALO) or native ALO, secreted by viable B. anthracis, is lethal to human primary polymorphonuclear leukocytes (PMNs), monocytes, monocyte-derived macrophages (MDMs), lymphocytes, THP-1 monocytic human cell line and ME-180, Detroit 562, and A549 epithelial cells by trypan blue exclusion or lactate dehydrogenase (LDH) release viability assays. ALO cytotoxicity is dose and time dependent and susceptibility to ALO-mediated lysis differs between cell types. In addition, the viability of monocytes and hMDMs was assayed in the presence of vegetative Sterne strains 7702 (ALO+), UT231 (ALO-), and a complemented strain expressing ALO, UT231 (pUTE544), and was dependent upon the expression of ALO. Cytotoxicity of rALO is seen as low as 0.070 nM in the absence of serum. All direct cytotoxic activity is inhibited by the addition of cholesterol or serum concentration as low as 10%.

CONCLUSION

The lethality of rALO and native ALO on human monocytes, neutrophils, macrophages and lymphocytes supports the idea that ALO may represent a previously unidentified virulence factor of B. anthracis. The study of other factors produced by B. anthracis, along with the major anthrax toxins, will lead to a better understanding of this bacterium's pathogenesis, as well as provide information for the development of antitoxin vaccines for treating and preventing anthrax.

Cytotoxic necrotizing factor type 1 production by uropathogenic Escherichia coli modulates polymorphonuclear leukocyte function

Many strains of uropathogenic Escherichia coli (UPEC) produce cytotoxic necrotizing factor type 1 (CNF1), a toxin that constitutively activates the Rho GTPases RhoA, Rac1, and Cdc42. We previously showed that CNF1 contributes to the virulence of UPEC in a mouse model of ascending urinary tract infection and a rat model of acute prostatitis and that a striking feature of the histopathology of the mouse bladders and rat prostates infected with CNF1-positive strains is an elevation in levels of polymorphonuclear leukocytes (PMNs). We also found that CNF1 synthesis leads to prolonged survival of UPEC in association with human neutrophils. Here, we tested the hypothesis that CNF1 production by UPEC diminishes the antimicrobial capacity of mouse PMNs by affecting phagocyte function through targeting Rho family GTPases that are critical to phagocytosis and the generation of reactive oxygen species. We found that, as with human neutrophils, CNF1 synthesis provided a survival advantage to UPEC incubated with mouse PMNs. We also observed that CNF1-positive UPEC down-regulated phagocytosis, altered the distribution of the complement receptor CR3 (CD11b/CD18), enhanced the intracellular respiratory burst, and increased levels of Rac2 activation in PMNs. From these results, we conclude that modulation of PMN function by CNF1 facilitates UPEC survival during the acute inflammatory response.

Virulent Burkholderia pseudomallei is more efficient than avirulent Burkholderia thailandensis in invasion of and adherence to cultured human epithelial cells

Burkholderia pseudomallei, a causative agent of melioidosis, is a facultative intracellular gram-negative bacillus that is closely related to its avirulent counterpart, Burkholderia thailandensis. However, pathogenic mechanisms and virulence factors of B. pseudomallei remain elusive. In the present study, we compared the invasiveness, adherence, and replication of B. pseudomallei and B. thailandensis in human respiratory epithelial cells A549. Invasion was determined after 4 h of coculturing using antibiotic protection assay. Adherence was demonstrated by coculturing the cells with fluorescein-labeled bacteria for 1 h and the number of positive cells was analyzed by flow cytometry. The results obtained with this in vitro study demonstrated that compared with its avirulent counterpart, B. pseudomallei is significantly more efficient (P<0.01) in invasion, adherence and inducing cellular damage, as represented by plaque formation.

Defect in neutrophil killing and increased susceptibility to infection with nonpathogenic gram-positive bacteria in peptidoglycan recognition protein-S (PGRP-S)-deficient mice

Insect peptidoglycan recognition protein-S (PGRP-S), a member of a family of innate immunity pattern recognition molecules conserved from insects to mammals, recognizes bacterial cell wall peptidoglycan and activates 2 antimicrobial defense systems, prophenoloxidase cascade and antimicrobial peptides through Toll receptor. We show that mouse PGRP-S is present in neutrophil tertiary granules and that PGRP-S-deficient (PGRP-S-/-) mice have increased susceptibility to intraperitoneal infection with gram-positive bacteria of low pathogenicity but not with more pathogenic gram-positive or gram-negative bacteria. PGRP-S-/- mice have normal inflammatory responses and production of tumor necrosis factor alpha (TNF-alpha) and interleukin 6 (IL-6). Neutrophils from PGRP-S-/- mice have normal phagocytic uptake of bacteria but are defective in intracellular killing and digestion of relatively nonpathogenic gram-positive bacteria. Therefore, mammalian PGRP-S functions in intracellular killing of bacteria. Thus, only bacterial recognition by PGRP-S, but not its effector function, is conserved from insects to mammals.

The capsular polysaccharide of Enterococcus faecalis and its relationship to other polysaccharides in the cell wall

With the goal of identifying and characterizing traits of Enterococcus faecalis that play key roles in human disease, we identified an operon specifying synthesis of a capsular carbohydrate of the type most commonly expressed by clinical isolates. This surface-exposed carbohydrate consists of glycerol phosphate, glucose, and galactose residues, and its biosynthesis is encoded by a determinant that includes 11 ORFs. Insertional inactivation of genes in this pathway yielded mutants with enhanced susceptibility to phagocytic killing in vitro and compromised in the ability to persist in regional lymph nodes in vivo.

A quantitative luminescence assay for measuring cell uptake of aqueous-based microcapsules in vitro

We recently developed a system of microencapsulation consisting of aqueous-based polymers (e.g. alginate) and aqueous amines (e.g. spermine). We found that microencapsulation enhanced virus-specific protective immune responses. In addition, we found that microencapsulation may enhance virus-specific immune responses by selecting for antigen-presenting cells (APC) that are more efficient at processing and presenting viral antigens than those involved after natural infection. To determine the intracellular trafficking patterns and fate of microcapsules within APC, we developed a luminescence assay that permits the determination of specific quantities of proteins introduced into cells by microcapsules. We found that the time-dependent uptake of horseradish peroxidase (HRP)-labeled microcapsules was accurately detected in lysates of peritoneal exudate cells using luminol. The amplitude of HRP-catalyzed chemiluminescence in cell lysates correlated with the capture efficiency and retention kinetics of HRP in three different microcapsule preparations. HRP was most efficiently captured and retained by linking biotinylated HRP to microcapsulses chemically modified at the amine moiety with egg avidin. This preparation yielded more accurate and sensitive quantitation of HRP contained within cells than preparations capturing HRP or HRP-conjugated goat antibody into the microcapsular matrix by ionic interactions.

Loss of resistance to ingestion and phagocytic killing by O(-) and K(-) mutants of a uropathogenic Escherichia coli O75:K5 strain

To determine the importance of the O75 O antigen and the K5 capsular antigen in resistance to phagocytosis and phagocytic killing, we used previously described O75(-) and K5(-) mutants from an O75(+) K5(+) wild-type uropathogenic Escherichia coli strain in phagocytosis assays with polymorphonuclear leukocytes (PMNs) and monocytes. At a 10-to-1 ratio of bacteria to phagocytes and in the presence of 10% serum, the parental strain GR-12 was resistant to both PMNs and monocytes over a 2-h incubation period. The O75(-) and K5(-) mutants were similar in sensitivity to killing by both PMNs and monocytes, decreasing in viability by 80% in the first hour. Yet, a significant difference in killing between the O75(-) and K5(-) mutants was observed in the first 15 min of incubation. The K5(-) mutant decreased in numbers by almost 60%, while the O75(-) mutant increased in numbers similarly to GR-12 in the first 15 min. The difference in killing was found not to be due to the rate of opsonization. To further determine the mechanism of resistance, a fluorescence assay was used to differentiate attached and internalized bacteria. The K5 capsule hindered the association of both the wild-type strain and the O75(-) mutant in the initial incubation time with PMNs. In conclusion, both the K5 capsule and O75 O antigen play crucial roles in resistance to phagocytosis over time.

Expression of sialyltransferase is not required for interaction of Neisseria gonorrhoeae with human epithelial cells and human neutrophils

Sialyltransferase (Stase) in Neisseria gonorrhoeae organisms (gonococci [GC]) transfers sialic acid (N-acetylneuraminic acid [NANA]) from cytidine 5'-monophospho-N-acetylneuraminic acid (CMP-NANA) mainly to the terminal galactose (Gal) residue in the Gal beta-1,4 N-acetylglucosamine (Gal-GlcNAc)-R lipooligosaccharide (LOS) structure. Sialylated GC resist killing by normal human serum, sometimes show reduced invasion of epithelial cells, and have reduced adhesion to and stimulation of human neutrophils. We questioned whether Stase itself modulates the interactions of GC with human epithelial cells and neutrophils in the absence of exogenous CMP-NANA. To that end, we treated strain F62 with ethyl methanesulfonate and grew approximately 175,000 colonies on CMP-NANA plates, and screened them with monoclonal antibody 1B2-1B7 (MAb 1B2). MAb 1B2 is specific for Gal-GlcNAc and reacts only with asialylated GC. We isolated 13 MAb 1B2-reactive mutants, including five null mutants, that had Stase activities ranging from barely detectable to fivefold less than that of wild-type (WT) F62. The LOS phenotype of Stase null mutants was identical to that of WT F62, yet the mutants could not sialylate their LOS when grown with CMP-NANA. The Stase null phenotype was rescuable to Stase+ by transformation with chromosomal DNA from WT F62. Stase null mutants remained serum sensitive even when grown with CMP-NANA. One Stase null mutant, ST94A, adhered to and invaded the human cervical epithelial cell line ME-180 at levels indistinguishable from that of WT F62 in the absence of CMP-NANA. In human neutrophil studies, ST94A stimulated the oxidative burst in and adhered to human neutrophils at levels similar to those of WT F62. ST94A and WT F62 were also phagocytically killed by neutrophils at similar levels. These results indicate that expression of Stase activity is not required for interaction of GC with human cells.

Intracellular survival of Burkholderia pseudomallei

Burkholderia pseudomallei is the causative agent of melioidosis, a disease being increasingly recognized as an important cause of morbidity and mortality in many regions of the world. Several features of melioidosis suggest that B. pseudomallei is a facultative intracellular pathogen. This study was designed to assess the ability of B. pseudomallei to invade and survive in eukaryotic cells. We have shown that B. pseudomallei has the capacity to invade cultured cell lines, including HeLa, CHO, A549, and Vero cells. We have demonstrated intracellular survival of B. pseudomallei in professional phagocytic cells, including rat alveolar macrophages. B pseudomallei was localized inside vacuoles in human monocyte-like U937 cells, a histiocytic lymphoma cell line with phagocytic properties. Additionally, electron microscopic visualization of B. pseudomallei-infected HeLa cells and polymorphonuclear leukocytes confirmed the presence of intracellular bacteria within membrane-bound vacuoles. B. pseudomallei was found to be resistant to the cationic peptide protamine and to purified human defensin HNP-1.