Relative roles of complement factor 3 and mannose-binding lectin in host defense against infection

Cellular uptake and in vivo distribution of mesenchymal-stem-cell-derived extracellular vesicles are protein corona dependent

Extracellular vesicles (EVs) derived from mesenchymal stem cells are promising nanotherapeutics in liver diseases due to their regenerative and immunomodulatory properties. Nevertheless, a concern has been raised regarding the rapid clearance of exogenous EVs by phagocytic cells. Here we explore the impact of protein corona on EVs derived from two culturing conditions in which specific proteins acquired from media were simultaneously adsorbed on the EV surface. Additionally, by incubating EVs with serum, simulating protein corona formation upon systemic delivery, further resolved protein corona-EV complex patterns were investigated. Our findings reveal the potential influences of corona composition on EVs under in vitro conditions and their in vivo kinetics. Our data suggest that bound albumin creates an EV signature that can retarget EVs from hepatic macrophages. This results in markedly improved cellular uptake by hepatocytes, liver sinusoidal endothelial cells and hepatic stellate cells. This phenomenon can be applied as a camouflage strategy by precoating EVs with albumin to fabricate the albumin-enriched protein corona-EV complex, enhancing non-phagocytic uptake in the liver. This work addresses a critical challenge facing intravenously administered EVs for liver therapy by tailoring the protein corona-EV complex for liver cell targeting and immune evasion.

Reg4 and complement factor D prevent the overgrowth of E. coli in the mouse gut

The expansion of Enterobacteriaceae, such as E. coli is a main characteristic of gut inflammation and is related to multiple human diseases. However, how to control these E. coli overgrowth is not well understood. Here, we demonstrate that gut complement factor D (CFD) plays an important role in eliminating E. coli. Increased E. coli, which could stimulate inflammatory macrophages to induce colitis, were found in the gut of CFD deficient mice. We also showed that gut Reg4, which is expressed in gut epithelial cells, stimulated complement-mediated attack complexes to eliminate E. coli. Reg4 deficient mice also had increased E. coli. The dominant E. coli were isolated from colitis tissues of mice and found to be sensitive to both CFD- and Reg4-mediated attack complexes. Thus, gut Reg4- and CFD-mediated membrane attack complexes may maintain gut homeostasis by killing inflammatory E. coli.

Qi et al. show that gut complement factor D (CFD) plays an important role in eliminating Escherichia coli, using gut specific CFD null mice. They find that E. coli isolated from inflamed guts are sensitive to both CFD and Reg4-mediated attack complexes. This study provides insights into how the level of E. Coli is kept low in the gut to prevent its inflammation.

Low circulating mannan-binding lectin levels correlate with increased frequency and severity of febrile episodes in myeloma patients who undergo ASCT and do not receive antibiotic prophylaxis

Patients with multiple myeloma (MM) who undergo autologous stem cell transplantation (ASCT) are susceptible to severe infections. Low levels of circulating mannan-binding lectin (MBL) are associated with increased risk of infection. In this prospective study, we evaluated 100 patients who underwent ASCT regarding the effect of MBL on the incidence and severity of febrile episodes. Seventeen patients had MBL levels <500 ng/mL (11 received antibiotic prophylaxis and 6 did not). Although there was no statistical difference regarding the development of febrile episodes between patients with low and normal MBL, among 17 patients with low MBL levels, six out of eleven patients who received antibiotic prophylaxis developed a febrile episode compared with six out of six patients who did not receive antibiotic prophylaxis and developed a febrile episode. Patients with low MBL levels who responded less frequently to first line antibiotic therapy required more frequent administration of a second more advanced line of antibiotics, independently of receiving or not prophylaxis, and required prolonged hospitalization. In the univariate analysis low MBL associated with shorter OS. Our results suggest that patient with low MBL levels should receive antibiotic prophylaxis to reduce the number of febrile episodes and raise the issue of MBL replacement for these patients.

Mannose-binding lectin and l-ficolin polymorphisms in patients with community-acquired pneumonia caused by intracellular pathogens

Community-acquired pneumonia (CAP) is the leading infectious disease requiring hospitalization in the western world. Genetic variability affecting the host response to infection may play a role in susceptibility and outcome in patients with CAP. Mannose-binding lectin (MBL) and l-ficolin (l-FCN) are two important activators of the complement system and they can enhance phagocytosis by opsonization. In a prospective cohort of 505 Dutch patients with CAP and 227 control participants we studied whether polymorphisms in the MBL (MBL2) and FCN (FCN2) genes influenced susceptibility and outcome. No difference in frequency of these genotypes was found between patients with CAP in general and controls. However, the +6424G>T single nucleotide polymorphism (SNP) in FCN2 was more common in patients with a Coxiella burnetii pneumonia (P = 0·014). Moreover, the haplotypes coding for the highest MBL serum levels (YA/YA and YA/XA) predisposed to atypical pneumonia (C. burnetii, Legionella or Chlamydia species or Mycoplasma pneumoniae) compared with controls (P = 0·016). Furthermore, patients with these haplotypes were more often bacteraemic (P = 0·019). It can therefore be concluded that MBL2 and FCN2 polymorphisms are not major risk factors for CAP in general, but that the +6424G>T SNP in the FCN2 gene predisposes to C. burnetii pneumonia. In addition, patients with genotypes corresponding with high serum MBL levels are at risk for atypical pneumonia, possibly caused by enhanced phagocytosis, thereby promoting cell entry of these intracellular bacteria.

Deficiency of the Complement Component 3 but Not Factor B Aggravates Staphylococcus aureus Septic Arthritis in Mice

The complement system plays an essential role in the innate immune response and protection against bacterial infections. However, detailed knowledge regarding the role of complement in Staphylococcus aureus septic arthritis is still largely missing. In this study, we elucidated the roles of selected complement proteins in S. aureus septic arthritis. Mice lacking the complement component 3 (C3(-/-)), complement factor B (fB(-/-)), and receptor for C3-derived anaphylatoxin C3a (C3aR(-/-)) and wild-type (WT) control mice were intravenously or intra-articularly inoculated with S. aureus strain Newman. The clinical course of septic arthritis, as well as histopathological and radiological changes in joints, was assessed. After intravenous inoculation, arthritis severity and frequency were significantly higher in C3(-/-)mice than in WT controls, whereas fB(-/-)mice displayed intermediate arthritis severity and frequency. This was in accordance with both histopathological and radiological findings. C3, but not fB, deficiency was associated with greater weight loss, more frequent kidney abscesses, and higher bacterial burden in kidneys. S. aureus opsonized with C3(-/-)sera displayed decreased uptake by mouse peritoneal macrophages compared with bacteria opsonized with WT or fB(-/-)sera. C3aR deficiency had no effect on the course of hematogenous S. aureus septic arthritis. We conclude that C3 deficiency increases susceptibility to hematogenous S. aureus septic arthritis and impairs host bacterial clearance, conceivably due to diminished opsonization and phagocytosis of S. aureus.

Lessons learned from mice deficient in lectin complement pathway molecules

The lectin pathway of the complement system is initiated when the pattern-recognition molecules, mannose-binding lectin (MBL), ficolins or collectin-11, bind to invading pathogens or damaged host cells. This leads to activation of MBL/ficolin/collectin-11 associated serine proteases (MASPs), which in turn activate downstream complement components, ultimately leading to elimination of the pathogen. Mice deficient in the key molecules of lectin pathway of complement have been generated in order to build knowledge of the molecular mechanisms of the lectin pathway in health and disease. Despite differences in the genetic arrangements of murine and human orthologues of lectin pathway molecules, the knockout mice have proven to be valuable models to explore the effect of deficiency states in humans. In addition, new insight and unexpected findings on the diverse roles of lectin pathway molecules in complement activation, pathogen infection, coagulation, host tissue injury and developmental biology have been revealed by in vivo investigations. This review provides an overview of the mice deficient in lectin pathway molecules and highlights some of the most important findings that have resulted from studies of these.

Association of mannose-binding lectin 2 gene polymorphisms with persistent Staphylococcus aureus bacteremia

Objectives

Mannose-binding lectin (MBL) is an important component of innate immunity. Structural and promoter polymorphisms in the MBL2 gene that are responsible for low MBL levels are associated with susceptibility to infectious diseases. The objective of this study was to investigate the association of serum MBL levels and MBL2 polymorphisms with persistent Staphylococcus aureus bacteremia (SAB) in adult Korean patients.

Methods

We conducted a case-control study nested in a prospective cohort of patients with SAB. The study compared 41 patients with persistent bacteremia (≥7 days) and 46 patients with resolving bacteremia (<3 days). In each subject, we genotyped six single-nucleotide polymorphisms in the promoter region (alleles H/L, X/Y, and P/Q) and exon 1 (alleles A/B, A/C, and A/D) of the MBL2 gene and measured serum MBL concentrations. We also compared MBL2 genotypes between SAB patients and healthy people.

Results

Patients with persistent bacteremia were significantly more likely to have low/deficient MBL-producing genotypes and resultant low serum MBL levels, than were patients with resolving bacteremia (P = 0.019 and P = 0.012, respectively). Independent risk factors for persistent bacteremia were metastatic infection (adjusted odds ratio [aOR], 34.7; 95% confidence interval [CI], 12.83–196.37; P = 0.003), methicillin resistance (aOR, 4.10; 95% CI, 3.19–29.57; P = 0.025), and low/deficient MBL-producing genotypes (aOR, 7.64; 95% CI, 4.12–63.39; P = 0.003). Such genotypes were significantly more common in patients with persistent bacteremia than in healthy people (OR, 2.09; 95% CI, 1.03–4.26; P = 0.040).

Conclusions

This is the first demonstration of an association of low MBL levels and MBL2 polymorphisms responsible for low or deficient MBL levels with persistent SAB. A combination of factors, including clinical and microbiological characteristics and host defense factors such as MBL levels, may together contribute to the development of persistent SAB.

CD137 expressed on neutrophils plays dual roles in antibacterial responses against Gram-positive and Gram-negative bacterial infections

Severe sepsis and septic shock caused mainly by bacterial infections are life-threatening conditions that urge the development of novel therapies. However, host responses to and pathophysiology of sepsis have not been clearly understood, which remains a major obstacle for the development of effective therapeutics. Recently, we have shown that stimulation of a costimulatory molecule, CD137, enhanced survival of mice infected with the Gram-positive (G(+)) intracellular bacterium Listeria monocytogenes but decreased survival in a polymicrobial sepsis model. Herein, we report that CD137 deficiency or blocking of CD137 signaling decreased antibacterial responses of mice infected with G(+) bacteria (Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecalis) but increased these responses in mice infected with Gram-negative (G(-)) bacteria (Escherichia coli, Pseudomonas aeruginosa, and Salmonella enterica serovar Typhimurium). Consistent with these findings, stimulation of CD137 by administration of agonistic antibody enhanced responses against G(+) bacteria, whereas it decreased these responses against G(-) bacteria. Neutrophils were responsible for CD137-mediated opposite roles in control of G(+) and G(-) bacterial infections. Stimulation of CD137 enhanced activities of neutrophils against S. aureus but decreased these activities against E. coli, while CD137 blocking produced opposite results with the stimulation of CD137 in vivo and in vitro. Furthermore, we found that combined signaling of CD137 and Toll-like receptor 2 (TLR2) induced synergistic production of tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) by neutrophils, but combined signaling of CD137 and TLR4 did not. Our data strongly suggest that CD137 may play a dual role in sepsis in association with TLRs.

Mannose-binding lectin regulates host resistance and pathology during experimental infection with Trypanosoma cruzi

Mannose-binding lectin (MBL) is a humoral pattern-recognition molecule important for host defense. Although recent genetic studies suggest an involvement of MBL/MASP2-associated pathways in Chagas’ disease, it is currently unknown whether MBL plays a role in host resistance to the intracellular protozoan Trypanosoma cruzi, the causative agent of Chagas’ disease. In this study we employed MBL^−/− mice to assess the role of MBL in resistance to experimental infection with T. cruzi. T. cruzi infection enhanced tissue expression of MBL both at the mRNA and protein level. Similarly, symptomatic acute Chagas’ disease patients displayed increased serum concentrations of MBL compared to patients with indeterminate, asymptomatic forms of the disease. Furthermore, increased parasite loads in the blood and/or tissue were observed in MBL^−/− mice compared to WT controls. This was associated with reduced systemic levels of IL-12/23p40 in MBL^−/− mice. Importantly, MBL^−/− mice infected with a cardiotropic strain of T. cruzi displayed increased myocarditis and cardiac fibrosis compared to WT controls. The latter was accompanied by elevated hydroxyproline content and mRNA levels of collagen-1 and -6 in the heart. These observations point to a previously unappreciated role for MBL in regulating host resistance and cardiac inflammation during infection with a major human pathogen.

Mannose-binding lectin and the balance between immune protection and complication

The innate immune system is evolutionarily ancient and biologically primitive. Historically, it was first identified as an element of the immune system that provides the first-line response to pathogens, and increasingly it is recognized for its central housekeeping role and its essential functions in tissue homeostasis, including coagulation and inflammation, among others. A pivotal link between the innate immune system and other functions is mannose-binding lectin (MBL), a pattern recognition molecule. Multiple studies have demonstrated that MBL deficiency increases susceptibility to infection, and the mechanisms associated with this susceptibility to infection include reduced opsonophagocytic killing and reduced activation of the lectin complement pathway. Results from our laboratory have demonstrated that MBL and MBL-associated serine protease (MASP)-1/3 together mediate coagulation factor-like activities, including thrombin-like activity. MBL and/or MASP-1/3-deficient hosts demonstrate in vivo evidence that MBL and MASP-1/3 are involved with hemostasis following injury. Staphylococcus aureus-infected MBL null mice developed disseminated intravascular coagulation, which was associated with elevated blood IL-6 levels (but not TNF-α) and systemic inflammatory responses. Infected MBL null mice also develop liver injury. These findings suggest that MBL deficiency may manifest as disseminated intravascular coagulation and organ failure with infection. Beginning from these observations, this review focuses on the interaction of innate immunity and other homeostatic systems, the derangement of which may lead to complications in infection and other inflammatory states.

Staphylococcus aureus virulence is enhanced by secreted factors that block innate immune defenses

Staphylococcus aureus is a leading human pathogen that causes a large variety of diseases. In vitro studies have shown that S. aureus secretes several small proteins that block specific elements of the host innate immune system, but their role in bacterial pathogenicity is unknown. For instance, the extracellular complement-binding protein (Ecb) impairs complement activation by binding to the C3d domain of C3. Its homolog, the extracellular fibrinogen-binding protein (Efb), is known to block both complement activation and neutrophil adhesion to fibrinogen. Here, we show that targeted inactivation of the genes encoding Ecb and Efb strongly attenuates S. aureus virulence in a murine infection model: mice experienced significantly higher mortality rates upon intravenous infection with wild-type bacteria (79%) than with an isogenic ΔEcbΔEfb mutant (21%). In addition, Ecb and Efb are both required for staphylococcal persistence in host tissues and abscess formation in the kidneys (27% for wild-type vs. 7% for the ΔEcbΔEfb mutant). During staphylococcal pneumonia, Ecb and Efb together promote bacterial survival in the lungs (p = 0.03) and block neutrophil influx into the lungs. Thus, Ecb and Efb are essential to S. aureus virulence in vivo and could be attractive targets in future vaccine development efforts.

The role of nanometer-scaled ligand patterns in polyvalent binding by large mannan-binding lectin oligomers

Mannan-binding lectin (MBL) is an important protein of the innate immune system and protects the body against infection through opsonization and activation of the complement system on surfaces with an appropriate presentation of carbohydrate ligands. The quaternary structure of human MBL is built from oligomerization of structural units into polydisperse complexes typically with three to eight structural units, each containing three lectin domains. Insight into the connection between the structure and ligand-binding properties of these oligomers has been lacking. In this article, we present an analysis of the binding to neoglycoprotein-coated surfaces by size-fractionated human MBL oligomers studied with small-angle x-ray scattering and surface plasmon resonance spectroscopy. The MBL oligomers bound to these surfaces mainly in two modes, with dissociation constants in the micro to nanomolar order. The binding kinetics were markedly influenced by both the density of ligands and the number of ligand-binding domains in the oligomers. These findings demonstrated that the MBL-binding kinetics are critically dependent on structural characteristics on the nanometer scale, both with regard to the dimensions of the oligomer, as well as the ligand presentation on surfaces. Therefore, our work suggested that the surface binding of MBL involves recognition of patterns with dimensions on the order of 10-20 nm. The recent understanding that the surfaces of many microbes are organized with structural features on the nanometer scale suggests that these properties of MBL ligand recognition potentially constitute an important part of the pattern-recognition ability of these polyvalent oligomers.

Mannose-binding lectin (MBL2) and ficolin-2 (FCN2) polymorphisms in patients on peritoneal dialysis with staphylococcal peritonitis

BACKGROUND

Mannose-binding lectin (MBL) and ficolin-2 (FCN) are activators of the lectin pathway of complement and act as primary defences against infection. Single-nucleotide polymorphisms (SNPs) in the MBL2 and FCN2 genes influence the functionality of the proteins. Both proteins are capable of binding staphylococci, which are pathogens that frequently cause peritonitis in patients on continuous ambulatory peritoneal dialysis (CAPD). We studied the role of polymorphisms in the MBL2 and FCN2 genes as a risk factor for developing CAPD peritonitis caused by staphylococci.

METHODS

We analysed SNPs in the MBL2 and FCN2 genes in 40 CAPD patients with staphylococcal peritonitis and in 65 CAPD patients without any history of peritonitis. Additionally, we analysed the prevalence of exit site infections and nasal Staphylococcus aureus carriage in both groups.

RESULTS

The + 6359C > T SNP leading to the Thr236Met amino acid alteration in the FCN2 gene, associated with decreased substrate binding, was significantly more prevalent in CAPD patients with a history of staphylococcal peritonitis compared with patients on CAPD without a history of peritonitis (P = 0.037). No difference was found in MBL2 genotypes between the two groups. In CAPD patients with a history of staphylococcal peritonitis, exit site infection with S. aureus was also more prevalent (P < 0.01), while S. aureus carriage was not (P = 0.073).

CONCLUSIONS

In addition to known risk factors such as exit site infection, the + 6359C > T SNP in the FCN2 gene might be a risk factor for staphylococcal peritonitis in CAPD patients due to decreased binding of FCN to staphylococci.

Distinct different contributions of the alternative and classical complement activation pathway for the innate host response during sepsis

Complement activation represents a crucial innate defense mechanism to invading microorganisms, but there is an eminent lack of understanding of the separate contribution of the different complement activation pathways to the host response during sepsis. We therefore investigated different innate host immune responses during cecal ligation and puncture (CLP)-induced sepsis in mice lacking either the alternative (fD(-/-)) or classical (C1q(-/-)) complement activation pathway. Both knockout mice strains showed a significantly reduced survival and increased organ dysfunction when compared with control mice. Surprisingly, fD(-/-) mice demonstrated a compensated bacterial clearance capacity as control mice at 6 h post CLP, whereas C1q(-/-) mice were already overwhelmed by bacterial growth at this time point. Interestingly, at 24 h after CLP, fD(-/-) mice failed to clear bacteria in a way comparable to control mice. However, both knockout mice strains showed compromised C3 cleavage during sepsis. Investigating potential causes for this discrepancy, we were able to demonstrate that despite normal bacterial clearance capacity early during the onset of sepsis, fD(-/-) mice displayed increased inflammatory cytokine generation and neutrophil recruitment into lungs and blood when compared with both control- and C1q(-/-) mice, indicating a potential loss of control over these immune responses. Further in vitro experiments revealed a strongly increased Nf-κB activation capacity in isolated neutrophils from fD(-/-) mice, supporting this hypothesis. Our results provide evidence for the new concept that the alternative complement activation pathway exerts a distinctly different contribution to the innate host response during sepsis when compared with the classical pathway.

Mannose-binding lectin and its associated proteases (MASPs) mediate coagulation and its deficiency is a risk factor in developing complications from infection, including disseminated intravascular coagulation

The first line of host defense is the innate immune system that includes coagulation factors and pattern recognition molecules, one of which is mannose-binding lectin (MBL). Previous studies have demonstrated that MBL deficiency increases susceptibility to infection. Several mechanisms are associated with increased susceptibility to infection, including reduced opsonophagocytic killing and reduced lectin complement pathway activation. In this study, we demonstrate that MBL and MBL-associated serine protease (MASP)-1/3 together mediate coagulation factor-like activities, including thrombin-like activity. MBL and/or MASP-1/3 deficient hosts demonstrate in vivo evidence that MBL and MASP-1/3 are involved with hemostasis following injury. Staphylococcus aureus infected MBL null mice developed disseminated intravascular coagulation (DIC), which was associated with elevated blood IL-6 levels (but not TNF-α and multi-organ inflammatory responses). Infected MBL null mice also develop liver injury. These findings suggest that MBL deficiency may manifest into DIC and organ failure during infectious diseases.

Staphylococcal superantigen-like protein 10 (SSL10) binds to human immunoglobulin G (IgG) and inhibits complement activation via the classical pathway

Staphylococcal superantigen-like (SSL) proteins are a family of exoproteins that share structural similarity with staphylococcal superantigens but exhibit no superantigenic activity. It was previously reported that two members (SSL5 and SSL7) bound to serum components and cell adhesion molecules involved in host immune response; however, the other family members have not been functionally characterized. In this study, we attempted to isolate SSL10-binding proteins from human serum and found that recombinant His-tagged SSL10 bound two major polypeptides of approximately 50 and approximately 25 kDa after affinity purification and SDS-polyacrylamide gel electrophoresis. These polypeptides were identified as heavy and light chains of human IgG by peptide mass fingerprinting analysis. The specific interaction between recombinant SSL10 and human IgG was confirmed by far Western blot analysis using immobilized SSL10 and pull-down analysis using SSL10-conjugated Sepharose. Surface plasmon resonance analysis revealed that the dissociation equilibrium constant for the interaction between human IgG and recombinant SSL10 was estimated to be 220 nM. We also found that recombinant SSL10 inhibited the binding of complement component C1q to IgG-Sepharose and hemolysis of IgG-sensitized sheep erythrocytes via the classical complement activation pathway. These results suggest that SSL10 may play a role in the evasion of Staphylococcus aureus from the host immune system via interfering complement activation.

Mannose-binding lectin and innate immunity

Innate immunity is the earliest response to invading microbes and acts to contain infection in the first minutes to hours of challenge. Unlike adaptive immunity that relies upon clonal expansion of cells that emerge days after antigenic challenge, the innate immune response is immediate. Soluble mediators, including complement components and the mannose binding lectin (MBL) make an important contribution to innate immune protection and work along with epithelial barriers, cellular defenses such as phagocytosis, and pattern-recognition receptors that trigger pro-inflammatory signaling cascades. These four aspects of the innate immune system act in concert to protect from pathogen invasion. Our work has focused on understanding the protection provided by this complex defense system and, as discussed in this review, the particular contribution of soluble mediators such as MBL and phagocytic cells. Over the past two decades both human epidemiological data and mouse models have indicated that MBL plays a critical role in innate immune protection against a number of pathogens. As demonstrated by our recent in vitro work, we show that MBL and the innate immune signaling triggered by the canonical pattern-recognition receptors (PRRs), the Toll-like receptors (TLRs), are linked by their spatial localization to the phagosome. These observations demonstrated a novel role for MBL as a TLR co-receptor and establishes a new paradigm for the role of opsonins, which we propose to function not only to increase microbial uptake but also to spatially coordinate, amplify, and synchronize innate immune defenses mechanism. In this review we discuss both the attributes of MBL that make it a unique soluble pattern recognition molecule and also highlight its broader role in coordinating innate immune activation.

Deficiency of mannose-binding lectin greatly increases antibody response in a mouse model of vaccination

Mannose-binding lectin (MBL), a pattern recognition innate immune molecule, selectively binds distinct chemical patterns, including carbohydrates expressed on Group B streptococcus (GBS). MBL interacts with IgM, resulting in the activation of MBL-associated serine proteases (MASPs), thus is initiating a lectin complement pathway. Complement proteins and IgM modulate production of antigen specific antibody. In this study, we investigated the relative effect of MBL in antibody response against tetanus toxoid-conjugated GBS polysaccharide vaccines (GBS PS-TT) by comparing wild type and null mice for MBL, complement 3 (C3), IgM, MBL/C3, and MBL/IgM. We found that GBS PS specific IgG response was upregulated in MBL deficient mice following immunization with GBS PS-TT but not GBS PS. B1 cells were expanded in peritonium but not in spleen of MBL null mice. The mechanisms of heightened IgG response in MBL null mice were related to C3, and share the same pathway with IgM.

Staphylococcus aureus clumping factor A binds to complement regulator factor I and increases factor I cleavage of C3b

The human complement system plays an important role in the control of Staphylococcus aureus infection. For instance, we previously demonstrated that the central complement component deposited on the organism's surface, C3b, can be cleaved by the host complement control protein, factor I, resulting in diminished phagocytosis of S. aureus. In the present study, we have identified clumping factor A (ClfA) from cell wall proteins of S. aureus as a specific protein bound by factor I. Recombinant ClfA (rClfA) containing the full-length A region (peptides 40-559) also bound factor I. We identified an 50-kDa fragment of ClfA that is shed by S. aureus into growth medium. The shed ClfA fragment was derived from the A region of ClfA and bound factor I. rClfA and the shed ClfA fragment increased factor I cleavage of C3b into inactive C3b. Our findings describe a new S. aureus mechanism for modification of host complement activities.

Mannose-binding lectin haplotypes influence Brucella abortus infection in the water buffalo (Bubalus bubalis)

A case-control study established that the haplotype pair HYA/HYA at the MBL (mannose binding lectin) locus of water buffalo is associated with resistance to Brucella abortus infection (P < 10(-7)) and the haplotype pairs LYD/LYD with susceptibility to the same pathogen (P < 10(-7)). The subjects included in the present study were tested twice-at a 1-month interval-for the presence of anti-B. abortus antibodies in the serum by agglutination, complement fixation and flow cytometry. Cases (335 subjects) included animals consistently positive to all these tests; controls (335 subjects) comprised animals exposed yet negative by the same tests. The serum from genetically resistant subjects displayed in vitro significantly higher antibacterial activity compared to the serum from genetically susceptible subjects, lending biological significance to the results from the association study. Inhibition of the antibacterial activity following heat treatment of the serum, addition of specific MBL inhibitors (EDTA, mannose, N-acetyl-D: -glucosamine) or anti-human MBL antiserum provide convincing evidence that the antibacterial activity present in the serum results from the interaction between MBL and B. abortus. A replication study (comprising 100 cases and 100 controls) confirmed the results from the original study.

Bridging effect of recombinant human mannose-binding lectin in macrophage phagocytosis of Escherichia coli

Mannose-binding lectin (MBL) exists in the serum as a complex with MBL-associated serine protease (MASP). A recent paper described how MASP-free recombinant rat MBL stimulates the phagocytosis of Escherichia coli and Staphylococcus aureus by rat Kupffer cells through an increase in the level of a phagocytosis receptor. We have examined the effect of human MBL on the phagocytic action of human macrophages. Purified recombinant human MBL stimulated the phagocytosis of E. coli by THP-1 macrophages, leaving that of latex beads, apoptotic human cells, zymosan particles or S. aureus unchanged. This stimulatory effect was observed when either phagocytes or targets were preincubated with MBL. Furthermore, MBL bound to THP-1 macrophages as well as to E. coli, but not to S. aureus, through lipid A. These results indicated that human MBL in the absence of MASP stimulates macrophage phagocytosis of E. coli by bridging targets and phagocytes.

Mannose-binding lectin enhances Toll-like receptors 2 and 6 signaling from the phagosome

Innate immunity is the first-line defense against pathogens and relies on phagocytes, soluble components, and cell-surface and cytosolic pattern recognition receptors. Despite using hard-wired receptors and signaling pathways, the innate immune response demonstrates surprising specificity to different pathogens. We determined how combinatorial use of innate immune defense mechanisms defines the response. We describe a novel cooperation between a soluble component of the innate immune system, the mannose-binding lectin, and Toll-like receptor 2 that both specifies and amplifies the host response to Staphylococcus aureus. Furthermore, we demonstrate that this cooperation occurs within the phagosome, emphasizing the importance of engulfment in providing the appropriate cellular environment to facilitate the synergy between these defense pathways.

The Role of TLR2 In Vivo following Challenge withStaphylococcus aureusand Prototypic Ligands

Based on a wealth of in vitro macrophage studies, immunity to Staphylococcus aureus cell wall-derived peptidoglycan (PGN) and lipoteichoic acid has been attributed to TLR2. We investigated whether the in vitro paradigm of TLR2 dominance would hold true in vivo. Using an experimental peritonitis model, we challenged mice with PGN or lipoteichoic acid and found that only PGN resulted in significant leukocyte (primarily neutrophil) accumulation in the peritoneum at 4 h. PGN-mediated leukocyte recruitment was P-/E-selectin dependent but only partially TLR2 dependent, and also involved the C5aR. Concomitant inhibition of TLR2 and C5aR resulted in a further reduction in PGN-induced peritonitis. Peritoneal neutrophilia was partially mast cell dependent; however, the defect could not be reconstituted with TLR2(-/-) or C5aR(-/-) mast cells. Interestingly, macrophage-deficient mice did not have defective neutrophil recruitment. By 24 h, the response to PGN involved primarily monocytes and was TLR2 and C5aR independent. Finally, we challenged mice with live S. aureus and found a similar degree of TLR2 involvement in leukocyte recruitment to that observed with PGN. Most importantly, bacterial clearance from the spleen and peritoneum was not altered in TLR2(-/-) mice vs wild-type mice. Morbidity was only significantly increased in S. aureus-infected mice treated with a blocking Fab against C5aR. Taken together, these studies indicate that in vivo responses to prototypic TLR2 ligands do not necessarily recapitulate the absolute necessity for TLR2 observed in vitro, and additional receptors contribute, in a significant manner, to PGN and S. aureus-mediated immune responses.