Molecular and biological characterization of pyocyanin from clinical and environmental Pseudomonas aeruginosa.

BACKGROUND: Pyocyanin is a secondary metabolite secreted by P. aeruginosa. It is a redox-active blue/green phenazine pigment that has various beneficial applications. The present study aims at screening the production of pyocyanin among clinical and environmental P. aeruginosa isolates in Dakahlya governorate, Egypt. Thereafter, large-scale production, purification, structure elucidation, and assessment of the biological activity of the highest pyocyanin producers were targeted. RESULTS: Pyocyanin from the highest clinical (PsC05) and environmental (PsE02) producers were subjected to large-scale production, followed by purification using silica gel column. Pyocyanin was characterized using TLC, UV-Vis, 1 H NMR, and FTIR spectroscopy to confirm its structure and purity. Purified pyocyanin showed remarkable antimicrobial efficacy against all tested food-borne pathogens, MDR/XDR clinically isolated bacteria and C. albicans. Furthermore, it showed a substantial effect on biofilm inhibition and eradication of pre-formed biofilm against strong biofilm producing bacterial pathogens. However, it had limited antibiofilm activity against C. albicans. Pyocyanin from PsC05 had higher antioxidant and radicals scavenging activity than that from PsE02 as determined by FRAP, DPPH, and ABTS assays. Likewise, pyocyanin from PsC05 was more active against tested cancer cell lines, especially human Breast Cancer (MCF-7) and Colorectal Carcinoma (HCT-116), than that from PsE02. More importantly, it showed minimal cytotoxicity to normal cells. CONCLUSIONS: P. aeruginosa clinical and environmental isolates produce pyocyanin pigment in varying amounts. Pyocyanin exhibits substantial anti-bacterial, and anti-fungal activity; thus, enhancing its medical applicability. It could be used to inhibit and/or eradicate biofilm from the surfaces of medical devices which is a chief source of nosocomial infections. Its antioxidant along with cytotoxic activity against cancer cell lines, make it a promising contender for use as a substitute for synthetic agents in cancer treatment.

The linkage between prevalence of integron I and reduced susceptibility to biocides in MDR Klebsiella pneumoniae isolated from neonates.

Background and Objectives: Klebsiella pneumoniae causes challenging nosocomial fatal infections including neonatal sepsis. Our study aims at clarifying the contribution of integrons in the observed reduced susceptibility of multidrug-resistant (MDR) K. pneumoniae isolated from septicemic neonates to the clinically used antimicrobial agents and biocides. Materials and Methods: Eighty-six K. pneumoniae isolates were collected from Mansoura University Children's Hospital from septicemic neonates. Isolates were subjected to antibiotic and biocide susceptibility using disk diffusion and the agar dilution method, respectively. The distribution of different classes of integrons was screened in the isolates by PCR. Detected inegron I was sequenced in selected isolates. Results: Fifty-seven isolates (66.27%) were MDR. In the MDR isolates, class I integron was detected in 23 (40.3%), integron III was detected in 20 (35%), whereas integron II could not be detected. Sequencing results of integron I from MDR K. pneumoniae isolates revealed that only aminoglycoside and folate synthesis inhibitors gene cassettes were detected, while the rest of the resistance genes were not associated with integron I. Conclusion: The presence of integron I in MDR K. pneumoniae tested isolates may contribute only to some biocide resistance, however, it does not seem to be the only contributor in multiple drug resistance.

A novel pentavalent vaccine candidate completely protects against Acinetobacter baumannii in a mouse model of peritonitis.

Acinetobacter baumannii is considered as one of the most virulent and infectious organisms that have an increased ability to both evade host immune response and resist various classes of antibiotics, leading to life-threatening infections. Multiple virulence factors have been implicated in the high prevalence rate of A. baumannii in hospitalized and immunocompromised patients. Moreover, improper use of antibiotics has led to the emergence of extensive drug-resistant strains that urgently require alternative strategies to control this superbug. Unfortunately, the availability of a licensed vaccine against A. baumannii infections is still challenged by the vast diversity among A. baumannii strains. Here, we report the development of a novel pentavalent vaccine candidate composed of two recombinant proteins (Wza and YiaD) and a pool of capsular polysaccharides isolated from 3 clinical isolates. We tested this new vaccine in vivo in a mouse model of peritonitis against the standard strain ATCC 19606 in addition to 3 clinical isolates of A. baumannii. Immunization with this vaccine completely protected the challenged mice with 100% survival rate in the case of all the tested bacteria. Further clinical studies are urgently needed to evaluate the efficacy and safety of this proprietary vaccine to protect patients from A. baumannii lethal infections. KEY POINTS: • Recombinant proteins pool (Wza and YiaD) immunization led to a synergistic immune response. • Capsular polysaccharides pool induced up to 90% protection of tested clinical isolates. • The pentavalent pool showed superiority with 100% survival of immunized mice.

A small fragment of factor B as a potential inhibitor of complement alternative pathway activity.

BACKGROUND: The complement system is a key player in innate immunity and a modulator of the adaptive immune system. Among the three pathways of complement, the alternative pathway (AP) accounts for most of the complement activation. Factor B (FB) is a major protease of the AP, making it a promising target to inhibit the AP activity in conditions of uncontrolled complement activation. METHODS: Based on the data obtained from sequence analysis and conformational changes associated with FB, we expressed and purified a recombinant FB fragment (FBfr). We tested the inhibitory activity of the protein against the AP by in vitro assays. RESULTS: FBfr protein was proven to inhibit the complement AP activity when tested by C3b deposition assay and rabbit erythrocyte hemolytic assay. CONCLUSION: Our recombinant FBfr was able to compete with the native human FB, which allowed it to inhibit the AP activity. This novel compound is a good candidate for further characterization and testing to be used in complement diagnostic tests and as a drug lead in the field of complement therapeutics.

Dasatinib mitigates renal fibrosis in a rat model of UUO via inhibition of Src/STAT-3/NF-κB signaling.

This research aimed to investigate the reno-protective impact of the tyrosine kinase inhibitor dasatinib (DAS) against renal fibrosis induced by unilateral ureteral obstruction (UUO) in rats. DAS administration improved renal function and mitigated renal oxidative stress with paralleled reduction in the ligated kidney mass index, significant retraction in renal histopathological alterations and suppression of renal interstitial fibrosis. Nevertheless, DAS administration attenuated renal expression of phosphorylated Src (p-Src), Abelson (c-Abl) tyrosine kinases, nuclear factor-kappaB (NF-κB) p65, and phosphorylated signal transducer and activator of transcription-3 (p-STAT-3)/STAT-3 with paralleled reduction in renal contents of tumor necrosis factor-α (TNF-α), interleukin-1ß (IL-1ß), and monocyte chemoattractant protein-1 (MCP-1). DAS diminished interstitial macrophage infiltration and decreased renal profibrotic transforming growth factor-ß1 (TGF-ß1) levels and suppressed interstitial expression of renal α-smooth muscle actin (α-SMA) and fibronectin. Collectively, DAS slowed the progression of renal interstitial fibrosis, possibly via attenuating renal oxidative stress, impairing Src/STAT-3/NF-κB signaling, and reducing renal inflammation.

Occult hepatitis B infection among blood donors from Yaoundé, Cameroon.

BACKGROUND: In Cameroon, the prevention of hepatitis B virus (HBV) transmission by blood transfusion is still only based on hepatitis B surface antigen (HBsAg) screening. However, occult HBV infection (OBI) characterised by the absence of detectable HBsAg and low level of viral DNA remains a potential threat for blood safety. The prevalence of OBI was investigated in blood donors from Yaoundé to provide evidence-based recommendations to improve HBV blood safety. MATERIAL AND METHODS: Blood donations from August 1st, 2016 to March 31st, 2017 were routinely screened for HBV, human immunodeficiency virus (HIV), and hepatitis C virus (HCV) infections (Murex HBsAg Version 3, Murex HIV Ag/Ab Combination, and Murex HCV Ag/Ab Combination [DiaSorin]). Additional HBV investigations were performed, including hepatitis B core antibody ([HBc] Monolisa Anti-HBc PLUS; BIO-RAD) and HBV DNA tested in minipools of two samples using the quantitative Cobas Taqman HBV assay (Roche; LoQ: 6 IU/mL) and HBV DNA genotyping by sequencing. RESULTS: Of 1,162 donations analysed, 91 (7.8%) were reactive for HBsAg. All of them were also anti-HBc positive. Among the 1,071 HBsAg negative samples, 522 (48.7%) were reactive for anti-HBc. Six (0.56% of all donations) samples fulfilled the consensus definition of OBI and showed low HBV DNA loads (all <6 IU/mL). Following nested polymerase chain reaction amplifications, HBV DNA sequences were obtained for 4 of these samples (1 nearly whole genome [3123 nt], 2 Pre-S/S regions [1,356 nt], and 1 S region [445 nt]). Phylogenetic analysis identified genotype E in all samples. DISCUSSION: Around 1 in 100 Cameroonian blood donors screened who resulted HBsAg negative and anti-HBc positive carried occult HBV infection. HBsAg alone for screening prospective donors is not sufficient to eliminate the risk of HBV transfusion transmission in Cameroon, and because anti-HBc screening does not seem to be feasible without compromising blood supply, implementation of HBV nucleic acid testing could be considered when possible.

Absence of the Lectin Activation Pathway of Complement Ameliorates Proteinuria-Induced Renal Injury.

Proteinuria is an adverse prognostic feature in renal diseases. In proteinuric nephropathies, filtered proteins exert an injurious effect on the renal tubulointerstitium, resulting in inflammation and fibrosis. In the present study, we assessed to what extent complement activation via the lectin pathway may contribute to renal injury in response to proteinuria-related stress in proximal tubular cells. We used the well-established mouse model of protein overload proteinuria (POP) to assess the effect of lectin pathway inhibition on renal injury and fibrotic changes characteristic of proteinuric nephropathy. To this end, we compared experimental outcomes in wild type mice with MASP-2-deficient mice or wild type mice treated with MASP-2 inhibitor to block lectin pathway functional activity. Multiple markers of renal injury were assessed including renal function, proteinuria, macrophage infiltration, and cytokine release profiles. Both MASP-2-deficient and MASP-2 inhibitor-treated wild type mice exhibited renoprotection from proteinuria with significantly less tubulointerstitial injury when compared to isotype control antibody treated mice. This indicates that therapeutic targeting of MASP-2 in proteinuric nephropathies may offer a useful strategy in the clinical management of proteinuria associated pathologies in a variety of different underlying renal diseases.

Antioxidant and anti-inflammatory activities of berberine attenuate hepatic fibrosis induced by thioacetamide injection in rats.

Berberine (BBR) is an isoquinoline alkaloid extracted from the roots, rhizomes and stems of coptis. Liver fibrosis is a worldwide health problem with no established therapy until now. The aim of our study is to investigate the efficacy of BBR on hepatic fibrosis induced in rats and to uncover other mechanisms. Rats were injected with thioacetamide (TAA) (200 mg/kg, i.p) twice per week for 6 weeks to induce fibrosis. Treated groups were gavaged with BBR (50 mg/kg/day, p.o) simultaneously with TAA injection. Hepatic antioxidant enzymes (catalase, SOD, GPx) were assessed in hepatic homogenate. Their activities were attenuated by TAA injection and elevated by BBR administration. Additionally, serum IL-6 and mRNA levels of IL-1ß, IL-6, IL-10 and IFN-γ were evaluated as inflammatory markers. Our results showed that BBR suppressed the inflammation induced by TAA injection. Tissue expression of α-SMA (marker of activated HSCs), TGF-ß1 and fibronectin were measured by immunohistochemistry as well as mRNA expressions of TGF-ß1 and fibronectin were quantified as fibrotic markers. The collagen deposition in hepatic tissues was assessed by Masson's trichome staining. BBR significantly alleviated TGF-ß1 production, decreased collagen and fibronectin deposition and consequently attenuated hepatic fibrogenesis. Akt pathway controls cell survival, proliferation, migration and adhesion. The relative phosphorylation of Akt was determined in hepatic homogenates that was increased with TAA injection and decreased by BBR treatment. Inhibition of Akt pathway has been linked to the intrinsic pathway of apoptosis. Caspase-3, caspase-9, Bcl-2 and Bax were quantified as apoptotic markers using qPCR and also caspase-3 by immunohistochemistry. BBR-treated rats showed an increase in the expression of apoptotic markers. Moreover, BBR-treated rats showed restoration of normal liver lobular architecture as shown by H&E staining. In conclusion, BBR is a potential therapeutic candidate for liver fibrosis owing to its antioxidant and anti-inflammatory activities.

Immunization with outer membrane proteins (OprF and OprI) and flagellin B protects mice from pulmonary infection with mucoid and nonmucoid Pseudomonas aeruginosa.

BACKGROUND: Pseudomonas aeruginosa is a Gram-negative opportunistic bacterium, which considered as a common cause of nosocomial infection and life-threatening complications in immunocompromized and cystic fibrosis patients. Here, we evaluate the protective effect of recombinant vaccines composed of outer membrane proteins OprF and OprI alone or in combination with flagellin B against mucoid and nonmucoid pseudomonas infection. METHODS: BALB/C mice were immunized subcutaneous using OprF and OprI with or without flagellin B and antibody titers were determined. Serum bactericidal and opsonophagocytosis activities of immunized and control sera were estimated against mucoid and nonmucoid pseudomonas strains. Lung tissue sections from immunized and nonimmunized mice were analyzed and the levels of peripheral neutrophils infiltration into the lung and tissue inflammation were scored. RESULTS: Subcutaneous immunization using OprF and OprI with or without flagellin B elicited higher antibody titers against OprF, OprI, and flagellin B. The produced antibodies successfully opsonized both mucoid and nonmucoid strains with subsequent activation of the terminal pathway of complement that enhances killing of nonmucoid strains via complement-mediated lysis. Furthermore, opsonized mucoid and nonmucoid strains showed enhanced opsonophagocytosis via human peripheral neutrophils, a mechanism that kills P. aeruginosa when complement mediated lysis is not effective especially with mucoid strains. Immunized mice also showed a significant prolonged survival time, lower bacteremia, and reduced lung damage when compared with control nonimmunized mice. CONCLUSION: Our data showed that mice immunized with OprF/OprI or OprF/OprI and flagellin B are significantly protected from infection caused by mucoid and nonmucoid strains of P. aeruginosa.

Naringin attenuates thioacetamide-induced liver fibrosis in rats through modulation of the PI3K/Akt pathway.

AIMS: Naringin (NR) is a flavanone glycoside extracted from grapefruits and citrus fruits. The aim of this study is to investigate the antifibrotic efficacy of NR in thioacetamide (TAA)-induced hepatic fibrosis in rats through evaluating NR effect on the PI3K/Akt pathway. MAIN METHODS: Hepatic fibrosis was induced in rats by intraperitoneal injection of TAA (200mg/kg) twice per week for 6weeks. Simultaneously, NR (40mg/kg/day, p.o.) was given along with TAA injection. The ratio of P-Akt/Akt was assessed in hepatic homogenate as well as antioxidant enzymes (catalase, superoxide dismutase (SOD), glutathione peroxidase (GPx)) and lipid peroxidation marker, malondialdehyde (MDA). Serum level of interleukin (IL)-6 were measured using ELISA. Hepatic tissues were examined histopathologically using hematoxylin and eosin (H&E) and Masson trichome staining. Tissue expression of alpha smooth muscle actin (α-SMA), transforming growth factor ß1 (TGF-ß1), caspase-3 and fibronectin were scored immunohistochemically. Finally, the mRNA level of cytokine genes (IL-1ß, IL-6, IL-10, interferon gamma (IFN-γ)), caspase-3, TGF-ß1 and fibronectin were quantified using qPCR. KEY FINDINGS: NR significantly suppressed Akt phosphorylation associated with increased number of caspase-3 positive cells especially in the fibrotic areas. Liver tissues of treated rats showed restoration of normal liver histology and decrease in collagen and fibronectin deposition. Furthermore, NR treatment ameliorated oxidative stress and inflammatory cytokine production. SIGNIFICANCE: NR alleviated experimental liver fibrosis through inhibition of PI3K/Akt pathway beside its anti-inflammatory and antioxidant effects. Therefore, NR is a promising therapeutic candidate for hepatic fibrosis.

Hepatoprotective effect of hesperidin in hepatocellular carcinoma: Involvement of Wnt signaling pathways.

AIMS: Wnt3a and Wnt5a are ligands orchestrating the canonical and non-canonical pathways, respectively, with involvement in hepatocellular carcinoma (HCC). Hesperidin (HP) is a natural product found in citrus fruits and reputed for its antitumor activity. The present study aims to investigate the potential hepatoprotective effect of HP against thioacetamide (TAA)-induced HCC focusing on its potential role on Wnt3a and Wnt5a signaling pathways. MAIN METHODS: Forty rats were equally divided into groups; normal control, HP control (receiving HP, 150mg/kg/day), HCC (receiving TAA, 200mg/kg twice weekly for 14weeks) and HP-HCC (receiving HP and TAA). Gene expressions of Wnt3a, Wnt5a, ß-catenin and Cyclin D1 were assessed by qPCR, while their protein levels, along with active caspase-3 level, were quantified by ELISA and immunohistochemistry. Liver functions, oxidative stress parameters and myeloperoxidase activity were measured. MTT assay of hepG2 cells treated with recombinant Wnt3a (10ng/ml) in presence or absence of HP (100µM) was performed. KEY FINDINGS: HCC group exhibited a significant increase in Wnt3a, ß-catenin, Cyclin D1 and Wnt5a gene expressions, as well as, their protein levels. HP significantly prevented TAA-activated Wnt3a/ß-catenin and Wnt5a pathways. Moreover, HP exerted hepatoprotective effect by significantly improving the oxidative imbalance, inflammation and liver function parameters, serum ALT, AST activities, and albumin level. SIGNIFICANCE: Our study is the first to report the possible role of Wnt3a/ß-catenin and Wnt5a pathways in TAA-induced early HCC model in rats. HP has a prophylactic effect against hepatocarcinogenesis via preventing the induction of both canonical and non-canonical Wnt pathways.

Complement-Coagulation Cross-Talk: A Potential Mediator of the Physiological Activation of Complement by Low pH.

The complement system is a major constituent of the innate immune system. It not only bridges innate and adaptive arms of the immune system but also links the immune system with the coagulation system. Current understanding of the role of complement has extended far beyond fighting of infections, and now encompasses maintenance of homeostasis, tissue regeneration, and pathophysiology of multiple diseases. It has been known for many years that complement activation is strongly pH sensitive, but only relatively recently has the physiological significance of this been appreciated. Most complement assays are carried out at the physiological pH 7.4. However, pH in some extracellular compartments, for example, renal tubular fluid in parts of the tubule, and extracellular fluid at inflammation loci, is sufficiently acidic to activate complement. The exact molecular mechanism of this activation is still unclear, but possible cross-talk between the contact system (intrinsic pathway) and complement may exist at low pH with subsequent complement activation. The current article reviews the published data on the effect of pH on the contact system and complement activity, the nature of the pH sensor molecules, and the clinical implications of these effects. Of particular interest is chronic kidney disease (CKD) accompanied by metabolic acidosis, in which therapeutic alkalinization of urine has been shown significantly to reduce tubular complement activation products, an effect, which may have important implications for slowing progression of CKD.

Low-dose recombinant properdin provides substantial protection against Streptococcus pneumoniae and Neisseria meningitidis infection.

Modern medicine has established three central antimicrobial therapeutic concepts: vaccination, antibiotics, and, recently, the use of active immunotherapy to enhance the immune response toward specific pathogens. The efficacy of vaccination and antibiotics is limited by the emergence of new pathogen strains and the increased incidence of antibiotic resistance. To date, immunotherapy development has focused mainly on cytokines. Here we report the successful therapeutic application of a complement component, a recombinant form of properdin (Pn), with significantly higher activity than native properdin, which promotes complement activation via the alternative pathway, affording protection against N. menigitidis and S. pneumoniae. In a mouse model of infection, we challenged C57BL/6 WT mice with N. menigitidis B-MC58 6 h after i.p. administration of Pn (100 µg/mouse) or buffer alone. Twelve hours later, all control mice showed clear symptoms of infectious disease while the Pn treated group looked healthy. After 16 hours, all control mice developed sepsis and had to be culled, while only 10% of Pn treated mice presented with sepsis and recoverable levels of live Meningococci. In a parallel experiment, mice were challenged intranasally with a lethal dose of S. pneumoniae D39. Mice that received a single i.p. dose of Pn at the time of infection showed no signs of bacteremia at 12 h postinfection and had prolonged survival times compared with the saline-treated control group (P < 0.0001). Our findings show a significant therapeutic benefit of Pn administration and suggest that its antimicrobial activity could open new avenues for fighting infections caused by multidrug-resistant neisserial or streptococcal strains.

Trypanosoma cruzi calreticulin inhibits the complement lectin pathway activation by direct interaction with L-Ficolin.

Trypanosoma cruzi, the agent of Chagas' disease, the sixth neglected tropical disease worldwide, infects 10-12 million people in Latin America. Differently from T. cruzi epimastigotes, trypomastigotes are complement-resistant and infective. CRPs, T-DAF, sialic acid and lipases explain at least part of this resistance. In vitro, T. cruzi calreticulin (TcCRT), a chaperone molecule that translocates from the ER to the parasite surface: (a) Inhibits the human classical complement activation, by interacting with C1, (b) As a consequence, an increase in infectivity is evident and, (c) It inhibits angiogenesis and tumor growth. We report here that TcCRT also binds to the L-Ficolin collagenous portion, thus inhibiting approximately between 35 and 64% of the human complement lectin pathway activation, initiated by L-Ficolin, a property not shared by H-Ficolin. While L-Ficolin binds to 60% of trypomastigotes and to 24% of epimastigotes, 50% of the former and 4% of the latter display TcCRT on their surfaces. Altogether, these data indicate that TcCRT is a parasite inhibitory receptor for Ficolins. The resulting evasive activities, together with the TcCRT capacity to inhibit C1, with a concomitant increase in infectivity, may represent T. cruzi strategies to inhibit important arms of the innate immune response.

Human L-ficolin, a recognition molecule of the lectin activation pathway of complement, activates complement by binding to pneumolysin, the major toxin of Streptococcus pneumoniae.

The complement system is an essential component of the immune response, providing a critical line of defense against different pathogens including S. pneumoniae. Complement is activated via three distinct pathways: the classical (CP), the alternative (AP) and the lectin pathway (LP). The role of Pneumolysin (PLY), a bacterial toxin released by S. pneumoniae, in triggering complement activation has been studied in vitro. Our results demonstrate that in both human and mouse sera complement was activated via the CP, initiated by direct binding of even non-specific IgM and IgG3 to PLY. Absence of CP activity in C1q(-/-) mouse serum completely abolished any C3 deposition. However, C1q depleted human serum strongly opsonized PLY through abundant deposition of C3 activation products, indicating that the LP may have a vital role in activating the human complement system on PLY. We identified that human L-ficolin is the critical LP recognition molecule that drives LP activation on PLY, while all of the murine LP recognition components fail to bind and activate complement on PLY. This work elucidates the detailed interactions between PLY and complement and shows for the first time a specific role of the LP in PLY-mediated complement activation in human serum.

Role of ficolin-A and lectin complement pathway in the innate defense against pathogenic Aspergillus species.

Aspergillus species are saprophytic molds causing life-threatening invasive fungal infections in the immunocompromised host. Innate immune recognition, in particular, the mechanisms of opsonization and complement activation, has been reported to be an integral part of the defense against fungi. We have shown that the complement component ficolin-A significantly binds to Aspergillus conidia and hyphae in a concentration-dependent manner and was inhibited by N-acetylglucosamine and N-acetylgalactosamine. Calcium-independent binding to Aspergillus fumigatus and A. terreus was observed, but binding to A. flavus and A. niger was calcium dependent. Ficolin-A binding to conidia was increased under low-pH conditions, and opsonization led to enhanced binding of conidia to A549 airway epithelial cells. In investigations of the lectin pathway of complement activation, ficolin-A-opsonized conidia did not lead to lectin pathway-specific C4 deposition. In contrast, the collectin mannose binding lectin C (MBL-C) but not MBL-A led to efficient lectin pathway activation on A. fumigatus in the absence of ficolin-A. In addition, ficolin-A opsonization led to a modulation of the proinflammatory cytokine interleukin-8. We conclude that ficolin-A may play an important role in the innate defense against Aspergillus by opsonizing conidia, immobilizing this fungus through enhanced adherence to epithelial cells and modulation of inflammation. However, it appears that other immune pattern recognition molecules, i.e., those of the collectin MBL-C, are involved in the Aspergillus-lectin complement pathway activation rather than ficolin-A.

Absence of the lectin activation pathway of complement does not increase susceptibility to Pseudomonas aeruginosa infections.

Pseudomonas aeruginosa remains one of the major clinical pathogens that burden immuno-compromised patients and patients with cystic fibrosis. The present study aimed to define the role of the lectin pathway of complement in the immune-defence against P. aeruginosa in a mouse model of invasive pneumonia. Using in vitro assays specific for each of the three complement pathways, we demonstrate that some strains of P. aeruginosa bind lectin pathway recognition sub-components and initiate complement activation in a lectin pathway-specific mode. All of the tested strains activated complement via classical and alternative pathways. We assessed the importance of lectin pathway activation in fighting P. aeruginosa infections by testing a lectin pathway activating strain in a mouse model of intra-nasal infection. MASP-2 (mannan binding lectin associated serine protease-2) deficient mice, which have no lectin pathway activity, had no significant survival disadvantage compared to wild type littermates (72.7% and 81.8% survival, respectively, p=0.48). Likewise, no difference in opsonising activity was seen between MASP-2 sufficient and MASP-2 deficient mouse sera. Moreover, cytokine expression profiles in the lungs of WT mice and MASP-2-/- mice were similar throughout the course of P. aeruginosa infection. We conclude that the lectin pathway does not play an essential role in fighting P. aeruginosa infection in mice.