Differential susceptibility of Dectin-1 isoforms to functional inactivation by neutrophil and fungal proteases.

Patients with cystic fibrosis (CF) experience chronic or recurrent bacterial and fungal lung infections. Many patients with CF cannot effectively clear Aspergillus from their lungs. This may result in IgE sensitization and the development of allergic bronchopulmonary aspergillosis, or invasive infections, such as Aspergillus bronchitis. Lung disease in patients with CF is associated with neutrophil-dominated inflammation and elevated levels of the serine protease, neutrophil elastase (NE). Various C-type lectin-like receptors (CLRs), including Dectin-1 and Dectin-2, are involved in the immune response to Aspergillus. Here, we show that purified NE cleaves Dectin-1 in an isoform-specific manner. Bronchoalveolar lavage fluid from patients with CF, which contains high NE activity, induces Dectin-1 cleavage. Similarly, filtrate from a protease-producing strain of Aspergillus fumigatus induces isoform-specific cleavage of Dectin-1. Dectin-1 knockout (KO) cells and NE-treated cells demonstrated reduced phagocytosis of zymosan, a fungal cell wall preparation. In addition, NE cleaves 2 other CLRs, Dectin-2 and Mincle, and fungal-induced cytokine production was reduced in Dectin-1 KO cells, Dectin-2 KO cells, and NE-treated cells. Thus, Dectin-1 and Dectin-2 cleavage by NE and/or A. fumigatus-derived proteases results in an aberrant antifungal immune response that likely contributes to disease pathology in patients with CF.-Griffiths, J. S., Thompson, A., Stott, M., Benny, A., Lewis, N. A., Taylor, P. R., Forton, J., Herrick, S., Orr, S. J., McGreal, E. P. Differential susceptibility of Dectin-1 isoforms to functional inactivation by neutrophil and fungal proteases.

Role of serine proteases in the regulation of interleukin-877 during the development of bronchopulmonary dysplasia in preterm ventilated infants.

RATIONALE: The chemokine interleukin-8 is implicated in the development of bronchopulmonary dysplasia in preterm infants. The 77-amino acid isoform of interleukin-8 (interleukin-877) is a less potent chemoattractant than other shorter isoforms. Although interleukin-877 is abundant in the preterm circulation, its regulation in the preterm lung is unknown. OBJECTIVES: To study expression and processing of pulmonary interleukin-877 in preterm infants who did and did not develop bronchopulmonary dysplasia. METHODS: Total interleukin-8 and interleukin-877 were measured in bronchoalveolar lavage fluid from preterm infants by immunoassay. Neutrophil serine proteases were used to assess processing. Neutrophil chemotaxis assays and degranulation of neutrophil matrix metalloproteinase-9 were used to assess interleukin-8 function. MAIN RESULTS: Peak total interleukin-8 and interleukin-877 concentrations were increased in infants who developed bronchopulmonary dysplasia compared to those who did not. Shorter forms of interleukin-8 predominated in the preterm lung (96.3% No-bronchopulmonary dysplasia vs 97.1% bronchopulmonary dysplasia, p>0.05). Preterm bronchoalveolar lavage fluid significantly converted exogenously added interleukin-877 to shorter isoforms (p<0.001). Conversion was greater in bronchopulmonary dysplasia infants (p<0.05). This conversion was inhibited by α-1 antitrypsin and antithrombin III (p<0.01). Purified neutrophil serine proteases efficiently converted interleukin-877 to shorter isoforms in a time- and dose-dependent fashion; shorter interleukin-8 isoforms were primarily responsible for neutrophil chemotaxis (p<0.001). Conversion by proteinase-3 resulted in significantly increased interleukin-8 activity in vitro (p<0.01). CONCLUSIONS: Shorter, potent, isoforms interleukin-8 predominate in the preterm lung, and are increased in infants developing bronchopulmonary dysplasia, due to conversion of interleukin-877 by neutrophil serine proteases and thrombin. Processing of interleukin-8 provides an attractive therapeutic target to prevent development of bronchopulmonary dysplasia.

Macrophage phenotype is associated with disease severity in preterm infants with chronic lung disease.

BACKGROUND: The etiology of persistent lung inflammation in preterm infants with chronic lung disease of prematurity (CLD) is poorly characterized, hampering efforts to stratify prognosis and treatment. Airway macrophages are important innate immune cells with roles in both the induction and resolution of tissue inflammation. OBJECTIVES: To investigate airway innate immune cellular phenotypes in preterm infants with respiratory distress syndrome (RDS) or CLD. METHODS: Bronchoalveolar lavage (BAL) fluid was obtained from term and preterm infants requiring mechanical ventilation. BAL cells were phenotyped by flow cytometry. RESULTS: Preterm birth was associated with an increase in the proportion of non-classical CD14(+)/CD16(+) monocytes on the day of delivery (58.9 ± 5.8% of total mononuclear cells in preterm vs 33.0 ± 6.1% in term infants, p = 0.02). Infants with RDS were born with significantly more CD36(+) macrophages compared with the CLD group (70.3 ± 5.3% in RDS vs 37.6 ± 8.9% in control, p = 0.02). At day 3, infants born at a low gestational age are more likely to have greater numbers of CD14(+) mononuclear phagocytes in the airway (p = 0.03), but fewer of these cells are functionally polarized as assessed by HLA-DR (p = 0.05) or CD36 (p = 0.05) positivity, suggesting increased recruitment of monocytes or a failure to mature these cells in the lung. CONCLUSIONS: These findings suggest that macrophage polarization may be affected by gestational maturity, that more immature macrophage phenotypes may be associated with the progression of RDS to CLD and that phenotyping mononuclear cells in BAL could predict disease outcome.

Mechanism of neutrophil dysfunction: neutrophil serine proteases cleave and inactivate the C5a receptor.

Neutrophil dysfunction, resulting in inefficient bacterial clearance, is a feature of several serious medical conditions, including cystic fibrosis (CF) and sepsis. Poorly controlled neutrophil serine protease (NSP) activity and complement activation have been implicated in this phenomenon. The capacity for excess NSP secretion and complement activation to influence the expression and function of the important neutrophil-activating receptor C5aR was investigated. Purified NSPs cathepsin G (CG), neutrophil elastase (NE), and proteinase 3 cleaved C5aR to a 26- to 27-kDa membrane-bound fragment, thereby inactivating its C5a-induced signaling ability. In a supernatant transfer assay, NSPs released from neutrophils in response to C5a induced the cleavage of the C5aR on unstimulated cells. Stimulation of myeolomonocytic U937 cells and purified neutrophils with C5a resulted in downregulation of the C5aR on these cells, which, in the case of U937 cells, was largely caused by NSP-mediated cleavage of C5aR, but in the case of neutrophils, intracellular degradation was likely the main mediator in addition to a small role for NSPs. CG and NE in bronchoalveolar lavage fluid from CF patients both contributed to C5aR cleavage. We propose two converging models for C5a- and NSP-mediated neutrophil dysfunction whereby C5aR cleavage is induced by NSPs, secreted in response to: 1) excess C5a generation or other stimuli; or 2) necrosis. The consequent impairment of C5aR activity contributes to suboptimal local neutrophil priming and bacterial clearance. NSP inhibitors with specificity for both CG and NE may aid the treatment of pathologies associated with neutrophil dysfunction including sepsis and CF.

Functional heterogeneity of pulmonary surfactant protein-D in cystic fibrosis.

Pulmonary surfactant protein-D (SP-D) is a soluble collagenous C-type lectin with important anti-microbial and anti-inflammatory properties. Although it is subject to functionally relevant modification by common polymorphisms and unregulated inflammation, the functional status of SP-D in cystic fibrosis (CF) remains unclear. Given the importance of infection and inflammation in CF lung pathology we have undertaken the first systematic analysis of SP-D lectin activity in this population. By ELISA, we found that airway lavage fluid SP-D expression was greater in CF compared to control patients but was reduced in CF patients with infection and correlated negatively with markers of neutrophilic inflammation. In a functional assay, the percentage of SP-D capable of binding zymosan rarely exceeded 60% in CF or control patients and similarly restricted binding activity was observed towards maltose-agarose. SP-D lectin activity also correlated negatively with infection and neutrophilic inflammation but there was little evidence of major proteolytic degradation amongst the non-bound material. SP-D which failed to bind zymosan exhibited features of lower oligomeric form compared to bound material when tested by native gel electrophoresis. Furthermore, when separated by gel chromatography, high and low oligomeric populations of SP-D were observed in CF lavage fluid but only high oligomeric forms exhibited substantial lectin activity towards yeast derived mannan. Our data demonstrate that oligomeric heterogeneity underlies functional diversity amongst SP-D in health and disease and that dynamic regulation of oligomerisation is an important feature of SP-D biology.

Role of interleukin-6, its receptor and soluble gp130 in chronic lung disease of prematurity.

BACKGROUND: Interleukin-6 (IL-6) signalling involves the interplay between IL-6, soluble IL-6 receptor (sIL-6R) and soluble gp130 (sgp130). IL-6 activity is modulated by the soluble receptors to produce both pro- and anti-inflammatory effects in human diseases and animal models. The expression and functional activity of these molecules in lungs of preterm ventilated infants is unknown. OBJECTIVES: We investigated this pathway in preterm infants who were at risk of developing chronic lung disease of prematurity (CLD). METHODS: Cytokines and soluble receptors were measured in bronchoalveolar lavage fluid (BALF) from ventilated preterm infants ≤32 weeks of gestation who did or did not develop CLD. B9 cells, which specifically proliferate to IL-6, were used to assess BALF IL-6 functional activity. RESULTS: Inflammatory cells, IL-8 and monocyte chemotactic protein-1 were increased in BALF from the CLD group when compared to the No CLD group (p < 0.05). BALF IL-6 and sIL-6R were similar in both groups. In contrast, BALF sgp130 and sgp130/sIL-6R were greater in the CLD group when compared to the No CLD group (p = 0.01 and p = 0.02, respectively). However, the increased BALF sgp130 did not appear to modulate the BALF IL-6 functional activity. CONCLUSION: Lung inflammation was observed in the CLD group. Increased BALF sgp130 was noted in the CLD group but it did not appear to modulate the pulmonary IL-6 bioactivity. Further research is needed to investigate the potential modulatory activity of sgp130 in the preterm lung.

Increased prevalence of low oligomeric state surfactant protein D with restricted lectin activity in bronchoalveolar lavage fluid from preterm infants.

BACKGROUND: Surfactant protein D (SP-D) is a soluble oligomeric C-type lectin known to protect against lipopolysaccharide and ventilator-induced lung injury in preterm lambs. Here we assess the expression and functional status of SP-D in bronchoalveolar lavage fluid (BALF) from preterm infants at risk of chronic lung disease (CLD) of prematurity and term controls. This is the first systematic evaluation of SP-D function in any clinical cohort. METHODS: SP-D was quantified in BALF from 28 ventilated preterm infants and five ventilated term infants. SP-D lectin activity was tested in a zymosan binding assay followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and western blot in BALF from the same infants. SP-D lectin activity was also tested towards maltose-agarose and mannan for selected BALF samples. RESULTS: SP-D expression was lower on day 1 in those preterm infants who subsequently developed CLD but increased over the first 5 days of life in term and preterm neonates. The percentage of neonatal SP-D capable of binding zymosan rarely exceeded 50% in any BALF sample and was 3.5 times lower in preterm infants than term infants on day 1 of life. Similar binding defects were observed towards maltose-agarose and mannan. SDS-PAGE analysis revealed that zymosan-bound SP-D was more highly oligomerised (≥12-mers) than unbound SP-D, which was composed primarily of trimers and lower oligomeric forms. CONCLUSIONS: Substantial and functionally relevant variation in the expression and oligomeric distribution of SP-D exists between preterm and term neonatal lung secretions. This has implications for proposed SP-D replacement therapy in this population.

Off to a slow start: under-development of the complement system in term newborns is more substantial following premature birth.

Complement represents a keystone to the innate immune system, with three activation pathways that utilise foreign microbial pattern recognition as well as activation by the host's specific antibodies. However, innate immunity is not synonymous with neonatal immunity. The complement system in healthy term (38-42 weeks gestation) newborns is under-developed and, with only a few exceptions (e.g. C7 and factor D), the circulating complement component concentrations are between 10 and 80% of adult levels. Complement activation is tightly regulated and the circulating regulator levels are also low relative to adults, sometimes at almost undetectable levels (e.g. C4b-binding protein). For premature newborns, these relative deficiencies are even more marked. Newborns are known to be more susceptible to infection, and the importance of complement, not only through its decreased ability to directly lyse bacteria with the common terminal pathway, but also its reduced ability to recruit (chemotaxis) innate and adaptive leukocytes to sites of microbial invasion and reduced ability to enhance phagocytosis (opsonisation) will be discussed. Complement also holds a key role in enhancing and directing refinement of the specific antibody response to pathogens (as an adjuvant) that likely plays a role in the well-known under-performance of the humoral immune response in newborns.

Dynamic expression of IL-6 trans-signalling molecules in the lungs of preterm baboons undergoing mechanical ventilation.

BACKGROUND: Interleukin (IL)-6, when complexed with soluble IL-6 receptor (sIL-6R), has emerged as an important modulator of chemokine expression and leukocyte recruitment during inflammation and in this state can be specifically antagonised by soluble gp130 (sgp130). The expression of these modifiers of IL-6 activity during ventilator-induced inflammation remains poorly understood. OBJECTIVES: To ascertain the expression pattern of IL-6, sIL-6R and sgp130 in response to mechanical ventilation in the preterm neonatal lung and define its relationship to associated markers of inflammation. METHODS: Inflammatory cell recruitment and expression of IL-6, sIL-6R, sgp130, IL-8 and monocyte chemotactic protein-1 (MCP-1) were quantified in tracheal aspirate fluid collected over a 14-day period from preterm (125 days) baboons undergoing mechanical ventilation. RESULTS: Over the period of ventilation, the ratio of agonistic IL-6/sIL-6R increased 4.3-fold between days 3 and 10-11 (p < 0.01) while the ratio of antagonistic sgp130/IL-6 decreased 2.6-fold over the same period (p < 0.05). Over the same period, the relative numbers of neutrophils compared to mononuclear cells shifted from an excess of 1.8 on day 1 to 0.6 on day 14 (p < 0.01). Both IL-8 and MCP-1 were elevated between days 1 and 10-11 of ventilation (p < 0.01). CONCLUSIONS: In the ventilated preterm baboon lung, expression of sIL-6R and dynamic modulation of sgp130 expression appear to modulate the activity and inflammatory potential of IL-6.

Physiological concentration of calcium inhibits elastase-induced cleavage of a functional recombinant fragment of surfactant protein D.

Surfactant protein D (SP-D) plays an important role in lung host defence. SP-D levels have been shown to be depleted in cystic fibrosis (CF) patients. A recombinant fragment of the human SP-D (rfhSP-D) which consist of a hydrophobic neck and a CRD has been shown to be active in vivo and partially reverses the symptoms of the SP-D deficiency in the lungs when administered to SP-D knock-out mice. In this paper we studied the in vitro effect of different proteolytic enzymes commonly found in CF patients lungs, such as neutrophil elastase, cathepsin G and protease 3 as well as Pseudomonas elastase, on rfhSP-D. It was also shown that cleavage was inhibited by physiological concentration of calcium. When Western blot was compared with ELISA, we show that an anti-SP-D ELISA is a not a reliable assay of functional SP-D levels since non-functional fragments of SP-D are also detected. Thus, ELISA cannot be used as a reliable "diagnostic" tool for SP-D deficiency. Finally, we observe that SP-D is not cleaved in control patients but is degraded in about half the samples from cystic fibrosis patients, indicating that degradation of endogenous SP-D, by enzymes present in CF bronchioalveolar lavage fluid (BALF), may lead to deficiency of the protein as seen in CF and therefore rfhSP-D may be a useful future therapy.

Acute lung injury in preterm newborn infants: mechanisms and management.

Respiratory morbidity and mortality remain common in preterm infants. The immature preterm lung is especially prone to injury. This process often starts in-utero due to maternal chorioamnionitis, priming the lung for further injury in response to post-natal ventilation, oxygen and nosocomial infection. Pulmonary inflammation has been strongly implicated in the pathway leading to lung injury in this population of infants. Several therapeutic approaches have been attempted to prevent acute lung injury or to limit its progress. The mechanisms of acute lung injury in preterm infants; their clinical correlates and available therapeutic approaches are reviewed here.

Inactivation of IL-6 and soluble IL-6 receptor by neutrophil derived serine proteases in cystic fibrosis.

The ability of IL-6 to signal via both membrane bound and soluble receptors is thought to explain the capacity of this cytokine to act in both the initiation and resolution of acute inflammatory responses. In cystic fibrosis (CF), poorly resolved neutrophillic inflammation of the lungs is a primary cause of morbidity and mortality. Expression of IL-6 has been reported to be low in CF lung secretions, despite ongoing inflammation, but the status of soluble IL-6 receptor (sIL-6R) in these patients is unknown. We hypothesised that sIL-6R may be an important potentiator of IL-6 activity in CF associated lung disease. IL-6, sIL-6R and sgp130 (a natural antagonist of responses mediated by the sIL-6R) were analysed by ELISA and Western blot in bronchoalveolar lavage fluid (BALF) from 28 paediatric CF patients and nine non-CF controls. Total cell counts in CF were four fold higher compared to controls (median: 1.4 x 10(6) cells/ml v. 0.35 x 10(6) cells/ml in controls) (p<0.001) and the infiltrate was dominated by neutrophils which were elevated by 89 fold (0.62 x 10(6) cells/ml v. 0.007 x 10(6) cells/ml in controls) (p<0.001). Other markers of inflammation such as IL-8 and MCP-1 were elevated 17.5 and 3.8 fold respectively (IL-8; median: 1122 pg/ml v. 64 pg/ml in controls, p<0.01 and MCP-1; median: 692 pg/ml v. 182 pg/ml in controls, p<0.05). IL-6, although present in 23/32 CF BALF specimens compared to 1/9 controls (p<0.01), was weakly expressed (median: 50 pg/ml). Expression of sIL-6R and sgp130 in CF was no different to control patients. We tested whether weak expression of all three molecules was due to degradation by CF BALF. Degradative activity was observed in association with BALF elastase activity and could be specifically blocked by serine protease inhibitors. Degradation of sIL-6R by purified serine proteases (elastase, cathepsin G and proteinase 3) was also observed leading to a loss of trans-signalling activity. Interestingly, sIL-6R was protected from proteolysis by interaction with IL-6. Our data identify and define a novel protease mediated deficiency of IL-6 signalling in the CF lung.

Structural basis of pattern recognition by innate immune molecules.

The importance of the innate immune system as a first line defence against pathogenic challenge has long been recognised. Over the last decade the identity of many of the key molecules mediating innate host defence have been clarified and a model of self/ nonself discrimination by families of pattern recognition receptors (PRRs) has emerged. Although a large amount of information is now available concerning the action of these innate immune molecules at the level of the cell and organism, little is known about the molecular interface between pathogens and innate immune recognition molecules. In this chapter the molecular basis for innate immune discrimination of a wide variety of pathogen derived molecules is discussed in the context of the emerging literature.

The contribution of naturally occurring IgM antibodies, IgM cross-reactivity and complement dependency in murine humoral responses to pneumococcal capsular polysaccharides.

Immunogenicity of 12 capsular polysaccharides (CPS) from Streptococcus pneumoniae did not correlate with pre-existing levels of natural IgM anti-CPS antibodies in mice. Immunization of mice with individual CPS, with the exception of type 14 (the only neutral CPS tested), increased serum IgM that also bound other CPS serotypes independent of structural similarity or commonly known contaminants. Surprisingly only IgM response to type 4 (which has a small immunodominant epitope) was dependent on either complement C3 or complement receptors CD35/CD21. IgG anti-CPS responses were infrequently induced, but critically dependent on complement. Our results have clarified the role of complement in the induction of IgM and IgG anti-CPS antibody responses in mice and have implications for CPS vaccine development.

Carbohydrate-independent recognition of collagens by the macrophage mannose receptor.

Mannose receptor (MR) is the best characterised member of a family of four endocytic molecules that share a common domain structure; a cysteine-rich (CR) domain, a fibronectin-type II (FNII) domain and tandemly arranged C-type lectin-like domains (CTLD, eight in the case of MR). Two distinct lectin activities have been described for MR. The CR domain recognises sulphated carbohydrates while the CTLD mediate binding to mannose, fucose or N-acetylglucosamine. FNII domains are known to be important for collagen binding and this has been studied in the context of two members of the MR family, Endo180 and the phospholipase A2 receptor. Here, we have investigated whether the broad and effective lectin activity mediated by the CR domain and CTLD of MR is favoured to the detriment of FNII-mediated interaction(s). We show that MR is able to bind and internalise collagen in a carbohydrate-independent manner and that MR deficient macrophages have a marked defect in collagen IV and gelatin internalisation. These data have major implications at the molecular level as there are now three distinct ligand-binding sites described for MR. Furthermore our findings extend the range of endogenous ligands recognised by MR, a molecule firmly placed at the interface between homeostasis and immunity.

The carbohydrate-recognition domain of Dectin-2 is a C-type lectin with specificity for high mannose.

We examined the carbohydrate-binding potential of the C-type lectin-like receptor Dectin-2 (Clecf4n). The carbohydrate-recognition domain (CRD) of Dectin-2 exhibited cation-dependent mannose/fucose-like lectin activity, with an IC(50) for mannose of approximately 20 mM compared to an IC(50) of 1.5 mM for the macrophage mannose receptor when assayed by similar methodology. The extracellular domain of Dectin-2 exhibited binding to live Candida albicans and the Saccharomyces-derived particle zymosan. This binding was completely abrogated by cation chelation and was competed by yeast mannans. We compared the lectin activity of Dectin-2 with that of two other C-type lectin receptors (mannose receptor and SIGNR1) known to bind fungal mannans. Both mannose receptor and SIGNR1 were able to bind bacterial capsular polysaccharides derived from Streptococcus pneumoniae, but interestingly they exhibited distinct binding profiles. The Dectin-2 CRD exhibited only weak interactions to some of these capsular polysaccharides, indicative of different structural or affinity requirements for binding, when compared with the other two lectins. Glycan array analysis of the carbohydrate recognition by Dectin-2 indicated specific recognition of high-mannose structures (Man(9)GlcNAc(2)). The differences in the specificity of these three mannose-specific lectins indicate that mannose recognition is mediated by distinct receptors, with unique specificity, that are expressed by discrete subpopulations of cells, and this further highlights the complex nature of carbohydrate recognition by immune cells.

Ligand recognition by antigen-presenting cell C-type lectin receptors.

It is now appreciated that the range of ligands interacting with C-type lectin type receptors on antigen presenting cells includes endogenous self-molecules as well as pathogens and pathogen-derived ligands. Interestingly, not all interactions between these receptors and pathogenic ligands have beneficial outcomes, and it appears that some pathogens have evolved immunoevasive or immunosuppressive activities through receptors such as DC-SIGN. In addition to this, recent data indicate that the well-characterised macrophage mannose receptor is not essential to host defence against fungal pathogens, as previously thought, but has an important role in regulating endogenous glycoprotein clearance. New studies have also demonstrated that different ligand binding and/or sensing receptors collaborate for full and effective immune responses.

Divergent roles for C-type lectins expressed by cells of the innate immune system.

In recent years there has been increasing interest in the diversity and function of carbohydrates present on a range of endogenous mammalian glycoproteins and pathogen surfaces. It is clear that carbohydrate structures are not merely structural components of the molecules which bear them but are, in many instances, a source of information to be decoded by biological systems including the immune system. Macrophages and other antigen presenting cells express a variety of pattern recognition molecules which allow discrimination between self and non-self ligands and are well known for their ability to recognise and internalise foreign antigens. The role of carbohydrates as molecular determinants of self/non-self has been recognised for many years and a family of proteins known as the C-type lectins are implicated as the main players in carbohydrate recognition within the immune system. More recently, C-type lectin receptors which bind ligands other than carbohydrates have been identified, and there are additional receptors which bind both carbohydrate and non-carbohydrate ligands. In this review article we seek to shed light on the varied roles of this family of receptors, particularly those receptors expressed by antigen presenting cells and those with known ligands. We also review more recent data on several members of this family.

Human C1qRp is identical with CD93 and the mNI-11 antigen but does not bind C1q.

It has been suggested that the human C1qRp is a receptor for the complement component C1q; however, there is no direct evidence for an interaction between C1q and C1qRp. In this study, we demonstrate that C1q does not show enhanced binding to C1qRp-transfected cells compared with control cells. Furthermore, a soluble recombinant C1qRp-Fc chimera failed to interact with immobilized C1q. The proposed role of C1qRp in the phagocytic response in vivo is also unsupported in that we demonstrate that this molecule is not expressed by macrophages in a variety of human tissues and the predominant site of expression is on endothelial cells. Studies on the rodent homolog of C1qRp, known as AA4, have suggested that this molecule may function as an intercellular adhesion molecule. Here we show that C1qRp is the Ag recognized by several previously described mAbs, mNI-11 and two anti-CD93 Abs (clones X2 and VIMD2b). Interestingly, mNI-11 (Fab') has been shown to promote monocyte-monocyte and monocyte-endothelial cell adhesive interactions. We produced a recombinant C1qRp-Fc chimera containing the C-type lectin-like domain of C1qRp and found specific binding to vascular endothelial cells in sections of inflamed human tonsil, indicating the presence of a C1qRp ligand at this site. This interaction was Ca(2+) independent and was not blocked by our anti-C1qRp mAb BIIG-4, but was blocked by the proadhesive mAb mNI-11. Collectively, these data indicate that C1qRp is not a receptor for C1q, and they support the emerging role of C1qRp (here renamed CD93) in functions relevant to intercellular adhesion.

Roles of the complement system in human neurodegenerative disorders: pro-inflammatory and tissue remodeling activities.

Complement is an important component of the innate immune response with the capacity to recognize and clear infectious challenges that invade the CNS through a damaged blood brain barrier. For instance, the membrane attack complex is involved in cytotoxic and cytolytic activities while other smaller fragments lead to cell activation (chemotaxis) and phagocytosis of the intruders. It is noteworthy that there is a growing body of evidence that uncontrolled complement biosynthesis and activation in the CNS can contribute to exacerbate the neuronal loss in several neurodegenerative disorders. We provide here an insightful review of the double-edged sword activities of the local innate complement system in the CNS and discuss further the potential therapeutic avenues of delivering complement inhibitors to control brain inflammation.