Complement defense mechanisms

Artificial surface labelling of Escherichia coli with StrepTagII antigen to study how monoclonal antibodies drive complement-mediated killing

Antibodies play a key role in the immune defence against Gram-negative bacteria. After binding to bacterial surface antigens, IgG and IgM can activate the complement system and trigger formation of lytic membrane attack complex (MAC) pores. Molecular studies to compare functional activity of antibodies on bacteria are hampered by the limited availability of well-defined antibodies against bacterial surface antigens. Therefore, we genetically engineered E. coli by expressing the StrepTagII antigen into outer membrane protein X (OmpX) and validated that these engineered bacteria were recognised by anti-StrepTagII antibodies. We then combined this antigen-antibody system with a purified complement assay to avoid interference of serum components and directly compare MAC-mediated bacterial killing via IgG1 and pentameric IgM. While both IgG1 and IgM could induce MAC-mediated killing, we show that IgM has an increased capacity to induce complement-mediated killing of E. coli compared to IgG1. While Fc mutations that enhance IgG clustering after target binding could not improve MAC formation, mutations that cause formation of pre-assembled IgG hexamers enhanced the complement activating capacity of IgG1. Altogether, we here present a system to study antibody-dependent complement activation on E. coli and show IgM's enhanced capacity over IgG to induce complement-mediated lysis of E. coli.

Intestinal Immune Development Is Accompanied by Temporal Deviation in Microbiota Composition of Newly Hatched Pigeon Squabs

Identifying the interaction between intestinal mucosal immune system development and commensal microbiota colonization in neonates is of paramount importance for understanding how early life events affect resistance to disease later in life. However, knowledge about this interaction during the early posthatch development period in altrices is limited. To fill this gap, samples of intestinal content and tissue were collected from newly hatched pigeon squabs at four time points (days 0, 7, 14, and 21) for microbial community analysis and genome-wide transcriptome profiling, respectively. We show that the first week after hatching seems to be the critical window for ileal microbiota colonization and that a potentially stable microbiota has not yet been well established at 21 days of age. Regional transcriptome differences revealed that the jejunum rather than the ileum plays a crucial role in immunity at both the innate and adaptive levels. In the ileum, temporal deviation in innate immune-related genes mainly occurs in the first week of life and is accompanied by a temporal change in microbiota composition, indicating that the ileal innate mucosal immune system development regulated by microbial colonization occurs mainly in this period. Furthermore, we provide evidence that colonization by Escherichia and Lactobacillus within the first week of life is likely one of the causative factors for the induction of proinflammatory cytokine expression in the ileum. We also demonstrate that cellular adaptive immune responses mediated by Th17 cells following commensal-induced proinflammatory cytokine production in the ileum begin as early as the first week posthatch, but this cellular immunity seems to be less effective in terms of maintaining the inflammatory response balance. Because the induction of high levels of mucosal secretory IgA (SIgA) seems to take approximately 3 weeks, we favor the idea that humoral adaptive immunity might be less active, at least, during the first 2 weeks of life. Our data may help to explain the phenomenon of the occurrence of intestinal infections mainly in the ileum of pigeon squabs during the early posthatch period. IMPORTANCE The pigeon (Columba livia), an altricial bird, is one of the most economically important farmed poultry for table purposes. Identifying the interaction between intestinal mucosal immune system development and commensal microbiota colonization in neonates is of paramount importance for understanding how early life events affect resistance to disease and potential productivity later in life. However, knowledge about this interaction during the early posthatch development period in altricial birds is limited. The study described herein is the first to try to provide insights into this interaction. Our data provide evidence on the mutual relationship between intestinal mucosal immune system development and commensal microbiota colonization in pigeon squabs and may help to explain the phenomenon of the occurrence of intestinal infections mainly in the ileum of pigeon squabs during the early posthatch period.

Binding of raloxifene to human complement fragment 5a (hC5a): a perspective on cytokine storm and COVID19

Human C5a (hC5a), one of the pro-inflammatory glycoproteins of the complement system is known to undergo production hyperdrive in response to stress and infection. hC5a has been associated with the pathogenesis of many chronic and acute diseases, due to its proven ability in triggering the 'cytokine storm', by binding to its cognate receptor C5aR, expressed in myriad of tissues. Given the pleiotropic downstream function of hC5a, it is logical to consider the hC5a or its precursors as potential drug targets, and thus, we have been rationally pursuing the idea of neutralizing the harmful effect of excessive hC5a, by implementing the repurposing strategies for FDA-approved drugs. Indeed, the proof of principle biophysical studies published recently is encouraging, which strongly supports the potential of this strategy. Considering BSA-carprofen as a reference model system, the current study further explores the inherent conformational plasticity of hC5a and its effect in accommodating more than one drug molecule cooperatively at multiple sites. The data generated by recruiting a battery of experimental and computational biology techniques strongly suggest that hC5a can sequentially accommodate more than one raloxifene molecule with an estimated Ki ∼ 0.5 µM and Ki ∼ 3.58 µM on its surface at non-analogous sites. The study hints at exploration of polypharmacology approach, as a new avenue for discovering synergistic drug molecule pairs, or drug molecules with 'broad-range' binding affinity for targeting the different 'hot spots' on hC5a, as an alternative combination therapy for possible management of the 'cytokine storm'-related inflammatory diseases, like COVID19.Communicated by Ramaswamy H. Sarma.

C1q binding to surface-bound IgG is stabilized by C1r2s2 proteases

Antibody-dependent complement activation plays a major role in various pathophysiological processes in our body, including infection, inflammation, autoimmunity, and transplant rejection. In order to activate complement, antibodies should bind to target cells and recruit complement component C1. C1 is a large, multimolecular complex that consists of the antibody recognition protein C1q and a heterotetramer of proteases (C1r₂s₂). Although it is believed that interactions between C1 and IgGs are solely mediated by C1q, we here show that C1r₂s₂ proteases affect the capacity of C1q to form an avid complex with surface-bound IgG molecules. Furthermore, we demonstrate that C1q–IgG stability is influenced by IgG oligomerization and that promoting IgG oligomerization improves phagocytosis of the pathogenic bacterium Staphylococcus aureus.

Complement is an important effector mechanism for antibody-mediated clearance of infections and tumor cells. Upon binding to target cells, the antibody’s constant (Fc) domain recruits complement component C1 to initiate a proteolytic cascade that generates lytic pores and stimulates phagocytosis. The C1 complex (C1qr₂s₂) consists of the large recognition protein C1q and a heterotetramer of proteases C1r and C1s (C1r₂s₂). While interactions between C1 and IgG-Fc are believed to be mediated by the globular heads of C1q, we here find that C1r₂s₂ proteases affect the capacity of C1q to form an avid complex with surface-bound IgG molecules (on various 2,4-dinitrophenol [DNP]-coated surfaces and pathogenic Staphylococcus aureus). The extent to which C1r₂s₂ contributes to C1q–IgG stability strongly differs between human IgG subclasses. Using antibody engineering of monoclonal IgG, we reveal that hexamer-enhancing mutations improve C1q–IgG stability, both in the absence and presence of C1r₂s₂. In addition, hexamer-enhanced IgGs targeting S. aureus mediate improved complement-dependent phagocytosis by human neutrophils. Altogether, these molecular insights into complement binding to surface-bound IgGs could be important for optimal design of antibody therapies.

Pulp capping materials modulate the balance between inflammation and regeneration

The interrelations between inflammation and regeneration are of particular significance within the dental pulp tissue inextensible environment. Recent data have demonstrated the pulp capacity to respond to insults by initiating an inflammatory reaction and dentin pulp regeneration. Different study models have been developed in vitro and in vivo to investigate the initial steps of pulp inflammation and regeneration. These include endothelial cell interaction with inflammatory cells, stem cell interaction with pulp fibroblasts, migration chambers to study cell recruitment and entire human tooth culture model. Using these models, the pulp has been shown to possess an inherent anti-inflammatory potential and a high regeneration capacity in all teeth and at all ages. The same models were used to investigate the effects of tricalcium silicate-based pulp capping materials, which were found to modulate the pulp anti-inflammatory potential and regeneration capacity. Among these, resin-containing materials such as TheraCal^® shift the pulp response towards the inflammatory reaction while altering the regeneration process. On the opposite, resin-free materials such as Biodentine™ have an anti-inflammatory potential and induce the pulp regeneration capacity. This knowledge contradicts the new tendency of developing resin-based calcium silicate hybrid materials for direct pulp capping. Additionally, it would allow investigating the modulatory effects of newly released pulp capping materials on the balance between tissue inflammation and regeneration. It would also set the basis for developing future capping materials targeting these processes.

Effect of complement component 5 polymorphisms on mastitis resistance in Egyptian buffalo and cattle

Mastitis is one of the costliest diseases affecting the world's dairy industry. The important contribution of complement Component 5 (C5) to phagocytosis, which plays a major role in the defence of the bovine mammary gland against infection, makes this component of innate immunity a potential contributor in defending udder against mastitis. The objectives of this study were to sequence and analyse the whole coding region of the C5 gene in Egyptian buffalo and cattle, to detect any nucleotide variations (polymorphisms) and to investigate their associations with milk somatic cell score (SCS) as an indicator of mastitis in dairy animals. We sequenced a buffalo C5 cDNA fragment of 5336 bp (KP221293) and a cattle C5 cDNA fragment of 5303 bp (KP221294), which included the whole coding region and 3-UTR. Buffalo and cattle C5 cDNA shared sequence identity of 99%. The predicted complement C5 proteins consist of 1677 amino acid residues in both animals, one amino acid less than in humans and three amino acids more than in mouse C5 protein. Comparing cDNA sequences of different animals revealed nine novel SNPs in buffalo and seven SNPs in cattle, with two of them being novel. The association analysis revealed that five SNPs in buffalo are highly associated with SCS; indicating the contribution of complement C5 variants in buffalo mastitis resistance. No significant associations were detected between C5 variants and SCS in cattle. This is the first report about C5 variants in buffalo and its association with SCS.

Bovine milk RNases modulate pro-inflammatory responses induced by nucleic acids in cultured immune and epithelial cells

Activation of innate immune receptors by exogenous substances is crucial for the detection of microbial pathogens and a subsequent inflammatory response. The inflammatory response to microbial lipopolysaccharide via Toll-like receptor 4 (TLR4) is facilitated by soluble accessory proteins, but the role of such proteins in the activation of other pathogen recognition receptors for microbial nucleic acid is not well understood. Here we demonstrate that RNase4 and RNase5 purified from bovine milk bind to Salmonella typhimurium DNA and stimulate pro-inflammatory responses induced by nucleic acid mimetics and S. typhimurium DNA in an established mouse macrophage cell culture model, RAW264.7, as well as in primary bovine mammary epithelial cells. RNase4 and 5 also modulated pro-inflammatory signalling in response to nucleic acids in bovine peripheral blood mononuclear cells, although producing a distinct response. These results support a role for RNase4 and RNase5 in mediating inflammatory signals in both immune and epithelial cells, involving mechanisms that are cell-type specific.

Transcriptome analysis reveals regional and temporal differences in mucosal immune system development in the small intestine of neonatal calves

Background

Postnatal development of the mammalian mucosal immune system is crucial for responding to the rapid colonization by commensal bacteria and possible exposure to pathogens. This study analyzed expression patterns for mRNAs and their relationship with microRNAs (miRNAs) in the bovine small intestine during the critical neonatal period (0 to 42 days). This analysis revealed molecular mechanisms regulating the postnatal development of the intestinal mucosal immune system.

Results

Small intestine samples (jejunum and ileum) were collected from newborn male, Holstein calves immediately post-partum (n = 3) and at 7 (n = 5), 21 (n = 5), and 42 (n = 5) days of age and the transcriptomes were profiled using RNA-Seq. When analyzing all time points collectively, greater expression of genes encoding the complement functional pathway, as well as lower expression of genes encoding Toll-like receptors and NOD-like receptors were observed in the jejunum when compared to the ileum. In addition, significant changes in the expression of immune-related genes were detected within the first week post-partum in both jejunum and ileum. For example, increased expression of genes encoding tight junction proteins (claudin 1, claudin 4 and occludin), an antimicrobial peptide (Regenerating Islet-Derived 3-γ), NOD-like receptors (NACHT, LRR and PYD domain-containing protein 3), regulatory T cell marker (forkhead box P3), and both anti-inflammatory (interleukin 10) and pro-inflammatory (interleukin 8) cytokines was observed throughout the small intestine of 7-day-old calves when compared to newborn calves. Moreover, the expression of mucosal immune-related genes were either positively or negatively correlated with total bacterial population depending on both intestinal region and age. The integrated analysis of miRNAs and mRNAs supported the conclusion that miRNAs may regulate temporal changes in the expression of genes encoding tight junction proteins (miR-335), cytokines (miR-335) and bacterial recognition (miR-100) during the first week of small intestine development.

Conclusion

The rapid development of transcriptional differences between jejunum and ileum reveal that these two intestinal regions make distinct contributions to the intestinal mucosal immune system during the early neonatal period. In addition, transcriptome analysis indicates that the first week after birth is a very dynamic developmental period for the intestinal mucosal immune system and these changes may be regulated by both miRNAs and microbial colonization. Findings from this study indicate that a detailed analysis of both the abundance and diversity of the colonizing microbiome may be necessary to understand factors regulating the rapid development of the mucosal immune system during the first week of life.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2957-y) contains supplementary material, which is available to authorized users.

Can Pulp Fibroblasts Kill Cariogenic Bacteria? Role of Complement Activation

Complement system activation has been shown to be involved in inflammation and regeneration processes that can be observed within the dental pulp after moderate carious decay. Studies simulating carious injuries in vitro have shown that when human pulp fibroblasts are stimulated by lipoteichoic acid (LTA), they synthetize all complement components. Complement activation leads to the formation of the membrane attack complex (MAC), which is known for its bacterial lytic effect. This work was designed to find out whether human pulp fibroblasts can kill Streptococcus mutans and Streptococcus sanguinis via complement activation. First, histological staining of carious tooth sections showed that the presence of S. mutans correlated with an intense MAC staining. Next, to simulate bacterial infection in vitro, human pulp fibroblasts were incubated in serum-free medium with LTA. Quantification by an enzymatic assay showed a significant increase of MAC formation on bacteria grown in this LTA-conditioned medium. To determine whether the MAC produced by pulp fibroblasts was functional, bacteria sensitivity to LTA-conditioned medium was evaluated using agar well diffusion assay and succinyl dehydrogenase (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide [MTT]) assay. Both assays showed that S. mutans and S. sanguinis were sensitive to LTA-conditioned medium. Finally, to evaluate whether MAC formation on cariogenic bacteria, by pulp fibroblasts, can be directly induced by the presence of these bacteria, a specific coculture model of human pulp fibroblasts and bacteria was developed. Immunofluorescence revealed an intense MAC labeling on bacteria after direct contact with pulp fibroblasts. The observed MAC formation and its lethal effects were significantly reduced when CD59, an inhibitor of MAC formation, was added. Our findings demonstrate that the MAC produced by LTA-stimulated pulp fibroblasts is functional and can kill S. mutans and S. sanguinis. Taken together, these data clearly highlight the function of pulp fibroblasts in destroying cariogenic bacteria.

Enhancement of neutrophil autophagy by an IVIG preparation against multidrug-resistant bacteria as well as drug-sensitive strains

Autophagy occurs in human neutrophils after the phagocytosis of multidrug-resistant bacteria and drug-sensitive strains, including Escherichia coli and Pseudomonas aeruginosa. The present study detected autophagy by immunoblot analysis of LC3B conversion, by confocal scanning microscopic examination of LC3B aggregate formation and by transmission electron microscopic examination of bacteria-containing autophagosomes. Patients with severe bacterial infections are often treated with IVIG alongside antimicrobial agents. Here, we showed that IVIG induced neutrophil-mediated phagocytosis of multidrug-resistant strains. Compared with untreated neutrophils, neutrophils exposed to IVIG showed increased levels of bacterial cell killing, phagocytosis, O(2)(-) release, MPO release, and NET formation. IVIG also increased autophagy in these cells. Inhibiting the late phase of autophagy (fusion of lysosomes with autophagosomes) with bafilomycin A1-reduced, neutrophil-mediated bactericidal activity. These findings indicate that autophagy plays a critical role in the bactericidal activity mediated by human neutrophils. Furthermore, the autophagosomes within the neutrophils contained bacteria only and their organelles only, or both bacteria and their organelles, a previously undocumented observation. Taken together, these results suggest that the contents of neutrophil autophagosomes may be derived from specific autophagic systems, which provide the neutrophil with an advantage. Thus, IVIG promotes the neutrophil-mediated killing of multidrug-resistant bacteria as well as drug-sensitive strains.

LPS Induces Pulp Progenitor Cell Recruitment via Complement Activation

Complement system, a major component of the natural immunity, has been recently identified as an important mediator of the dentin-pulp regeneration process through STRO-1 pulp cell recruitment by the C5a active fragment. Moreover, it has been shown recently that under stimulation with lipoteichoic acid, a complex component of the Gram-positive bacteria cell wall, human pulp fibroblasts are able to synthesize all proteins required for complement activation. However, Gram-negative bacteria, which are also involved in tooth decay, are known as powerful activators of complement system and inflammation. Here, we investigated the role of Gram-negative bacteria-induced complement activation on the pulp progenitor cell recruitment using lipopolysaccharide (LPS), a major component of all Gram-negative bacteria. Our results show that incubating pulp fibroblasts with LPS induced membrane attack complex formation and C5a release in serum-free fibroblast cultures. The produced C5a binds to the pulp progenitor cells’ membrane and induces their migration toward the LPS stimulation chamber, as revealed by the dynamic transwell migration assays. The inhibition of this migration by the C5aR-specific antagonist W54011 indicates that the pulp progenitor migration is mediated by the interaction between C5a and C5aR. Our findings demonstrate, for the first time, a direct interaction between the recruitment of progenitor pulp cells and the activation of complement system generated by pulp fibroblast stimulation with LPS.

The protective role of CD59 and pathogenic role of complement in hepatic ischemia and reperfusion injury

Hepatic ischemia-reperfusion injury (IRI) is a major factor influencing graft outcome in liver transplantation, but its mechanism is not well defined. Although complement, including the membrane attack complex (MAC), a terminal product of complement activation, is thought to be involved in the multiple reactions subsequent to the ischemia-reperfusion (IR) process, the role of MAC in the pathogenesis of hepatic IRI requires further investigation. We used a warm ischemia-reperfusion injury model in mice and a syngeneic orthotopic liver transplantation model in rats to define the role of complement, including MAC, in hepatic IR. CD59-deficient mice had more severe liver dysfunction, evidenced by increased aspartate aminotransferase levels and increased injury of liver parenchymal and nonparenchymal cells than did CD59-sufficient mice during warm hepatic IR. Furthermore, complement depletion in CD59-deficient mice by pretreatment with cobra venom factor (CVF) or the genetic introduction of C3 deficiency partially protected against development of the severe liver dysfunction that occurred in CD59-deficient mice. Severity of liver dysfunction correlated with MAC deposition, apoptotic cells, and increased inflammatory mediators such as tumor necrosis factor α. Moreover, depletion of complement with CVF in orthotopic liver transplantation recipient rats attenuated IRI of the donor livers. Taken together, these results highlight the protective role of CD59 and pathogenic role of complement, including MAC, in the pathogenesis of hepatic IRI.

Bordetella pertussis autotransporter Vag8 binds human C1 esterase inhibitor and confers serum resistance

Bordetella pertussis employs numerous strategies to evade the immune system, including the ability to resist killing via complement. Previously we have shown that B. pertussis binds a complement regulatory protein, C1 esterase inhibitor (C1inh) to its surface in a Bvg-regulated manner (i.e. during its virulence phase), but the B. pertussis factor was not identified. Here we set out to identify the B. pertussis C1inh-binding factor. Using a serum overlay assay, we found that this factor migrates at approximately 100 kDa on an SDS-PAGE gel. To identify this factor, we isolated proteins of approximately 100 kDa from wild type strain BP338 and from BP347, an isogenic Bvg mutant that does not bind C1inh. Using mass spectrometry and bioinformatics, we identified the autotransporter protein Vag8 as the putative C1inh binding protein. To prove that Vag8 binds C1inh, vag8 was disrupted in two different B. pertussis strains, namely BP338 and 18-323, and the mutants were tested for their ability to bind C1inh in a surface-binding assay. Neither mutant strain was capable of binding C1inh, whereas a complemented strain successfully bound C1inh. In addition, the passenger domain of Vag8 was expressed and purified as a histidine-tagged fusion protein and tested for C1inh-binding in an ELISA assay. Whereas the purified Vag8 passenger bound C1inh, the passenger domain of BrkA (a related autotransporter protein) failed to do so. Finally, serum assays were conducted to compare wild type and vag8 mutants. We determined that vag8 mutants from both strains were more susceptible to killing compared to their isogenic wild type counterparts. In conclusion, we have discovered a novel role for the previously uncharacterized protein Vag8 in the immune evasion of B. pertussis. Vag8 binds C1inh to the surface of the bacterium and confers serum resistance.

Antibodies protect against intracellular bacteria by Fc receptor-mediated lysosomal targeting

The protective effect of antibodies (Abs) is generally attributed to neutralization or complement activation. Using Legionella pneumophila and Mycobacterium bovis bacillus Calmette-Guérin as a model, we discovered an additional mechanism of Ab-mediated protection effective against intracellular pathogens that normally evade lysosomal fusion. We show that Fc receptor (FcR) engagement by Abs, which can be temporally and spatially separated from bacterial infection, renders the host cell nonpermissive for bacterial replication and targets the pathogens to lysosomes. This process is strictly dependent on kinases involved in FcR signaling but not on host cell protein synthesis or protease activation. Based on these findings, we propose a mechanism whereby Abs and FcR engagement subverts the strategies by which intracellular bacterial pathogens evade lysosomal degradation.

Molecular and expression analysis of complement component C5 in the nurse shark (Ginglymostoma cirratum) and its predicted functional role

We present the complete cDNA sequence of shark (Ginglymostoma cirratum) pro-C5 and its molecular characterization with a descriptive analysis of the structural elements necessary for its potential functional role as a potent mediator of inflammation (fragment C5a) and initiator molecule (fragment C5b) for the assembly of the membrane attack complex (MAC) upon activation by C5 convertase. In mammals the three complement activation cascades, the classical, alternative and lectin pathways, converge at the activation of C3, a pivotal complement protein. It is, however, the subsequent activation of the next complement component, C5, which is the focal point at which the initiation of the terminal lytic pathway takes place and involves the stepwise assembly of the MAC. The effector cytolytic function of complement occurs with the insertion of MAC into target membranes causing dough-nut like holes and cell leakage. The lytic activity of shark complement results in structurally similar holes in target membranes suggesting the assembly of a shark MAC that likely involves a functional analogue of C5. The composition of shark MAC remains unresolved and to date conclusive evidence has been lacking for shark C5. The gene has not been cloned nor has the serum protein been characterized for any elasmobranch species. This report is the first to confirm the presence of C5 homologue in the shark. GcC5 is remarkably similar to human C5 in overall structure and domain arrangement. The GcC5 cDNA measured 5160-bp with 5' and 3' UTRs of 35 bp and 79 bp, respectively. Structural analysis of the derived protein sequence predicts a molecule that is a two-chain structure which lacks a thiolester bond and contains a C5 convertase cleavage site indicating that activation will generate two peptides, akin to C5b and C5a. The putative GcC5 molecule also contains the C-terminal C345C/Netrin module that characterizes C3, C4 and C5. Multiple alignment of deduced amino acid sequences shows that GcC5 shares more amino acid identities/similarities with mammals than that with bony fish. We conclude that at the time of emergence of sharks the elaborate mosaic structure of C5 had already evolved.

Decay accelerating factor can control T cell differentiation into IFN-gamma-producing effector cells via regulating local C5a-induced IL-12 production

A newly recognized link between the complement system and adaptive immunity is that decay accelerating factor (DAF), a cell surface C3/C5 convertase regulator, exerts control over T cell responses. Extending these results, we show that cultures of Marilyn TCR-transgenic T cells stimulated with DAF-deficient (Daf1(-/-)) APCs produce significantly more IL-12, C5a, and IFN-gamma compared with cultures containing wild-type APCs. DAF-regulated IL-12 production and subsequent T cell differentiation into IFN-gamma-producing effectors was prevented by the deficiency of either C3 or C5a receptor (C5aR) in the APC, demonstrating a link between DAF, local complement activation, IL-12, and T cell-produced IFN-gamma. Bone marrow chimera experiments verified that bone marrow cell-expressed C5aR is required for optimal differentiation into IFN-gamma-producing effector T cells. Overall, our results indicate that APC-expressed DAF regulates local production/activation of C5a following cognate T cell/APC interactions. Through binding to its receptor on APCs the C5a up-regulates IL-12 production, this in turn, contributes to directing T cell differentiation toward an IFN-gamma-producing phenotype. The findings have implications for design of therapies aimed at altering pathologic T cell immunity.

B cells in glomerulonephritis: focus on lupus nephritis

The production of pathogenic antibody has been traditionally viewed as the principle contribution of B cells to the pathogenesis of immune-mediated glomerulonephritis. However, it is increasingly appreciated that B cells play a much broader role in such diseases, functioning as antigen-presenting cells, regulators of T cells, dendritic cells, and macrophages and orchestrators of local lymphatic expansion. In this review, we provide an overview of basic B cell biology and consider the evidence implicating B cells in one of the archetypal immune-mediated glomerulonephritides, lupus nephritis.

Inhibition of Terminal Complement Components in Presensitized Transplant Recipients Prevents Antibody-Mediated Rejection Leading to Long-Term Graft Survival and Accommodation

Ab-mediated rejection (AMR) remains the primary obstacle in presensitized patients following organ transplantation, as it is refractory to anti-T cell therapy and can lead to early graft loss. Complement plays an important role in the process of AMR. In the present study, a murine model was designed to mimic AMR in presensitized patients. This model was used to evaluate the effect of blocking the fifth complement component (C5) with an anti-C5 mAb on prevention of graft rejection. BALB/c recipients were presensitized with C3H donor skin grafts 7 days before heart transplantation from the same donor strain. Heart grafts, transplanted when circulating anti-donor IgG Abs were at peak levels, were rejected in 3 days. Graft rejection was characterized by microvascular thrombosis and extensive deposition of Ab and complement in the grafts, consistent with AMR. Anti-C5 administration completely blocked terminal complement activity and local C5 deposition, and in combination with cyclosporine and short-term cyclophosphamide treatment, it effectively prevented heart graft rejection. These recipients achieved permanent graft survival for >100 days with normal histology despite the presence of systemic and intragraft anti-donor Abs and complement, suggesting ongoing accommodation. Furthermore, double-transplant experiments demonstrated that immunological alterations in both the graft and the recipient were required for successful graft accommodation to occur. These data suggest that terminal complement blockade with a functionally blocking Ab represents a promising therapeutic approach to prevent AMR in presensitized recipients.

Natural protection from zoonosis by alpha-gal epitopes on virus particles in xenotransmission

Clinical transplantation has become one of the preferred treatments for end-stage organ failure, and one of the novel approaches being pursued to overcome the limited supply of human organs involves the use of organs from other species. The pig appears to be a near ideal animal due to proximity to humans, domestication, and ability to procreate. The presence of Gal-alpha1,3-Gal residues on the surfaces of pig cells is a major immunological obstacle to xenotransplantation. Alpha1,3galactosyltransferase (alpha1,3GT) catalyzes the synthesis of Gal alpha 1-3Gal beta 1-4GlcNAc-R (alpha-gal epitope) on the glycoproteins and glycolipids of non-primate mammals, but this does not occur in humans. Moreover, the alpha-gal epitope causes hyperacute rejection of pig organs in humans, and thus, the elimination of this antigen from pig tissues is highly desirable. Recently, concerns have been raised that the risk of virus transmission from such pigs may be increased due to the absence of alpha-gal on their viral particles. In this study, transgenic cells expressing alpha1,3GT were selected using 1.25 mg/ml neomycin. The development of HeLa cells expressing alpha1,3GT now allows accurate studies to be conducted on the function of the alpha-gal epitope in xenotransmission. The expressions of alpha-gal epitopes on HeLa/alpha-gal cells were demonstrated by flow cytometry and confocal microscopy using cells stained with IB4-fluorescein isothiocyanate lectin. Vaccinia viruses propagated in HeLa/alpha-gal cells also expressed alpha-gal on their viral envelopes and were more sensitive to inactivation by human sera than vaccinia virus propagated in HeLa cells. Moreover, neutralization of vaccinia virus was inhibited in human serum by 10 mm ethylene glycol bis(beta-aminoethylether)tetraacetic acid (EDTA) treatment. Our data indicated that alpha-gal epitopes are one of the major barriers to zoonosis via xenotransmission.

Molecular cloning and characterisation of a proteinase inhibitor, alpha 2-macroglobulin (α2-M) from the haemocytes of tiger shrimp Penaeus monodon

An alpha 2-macroglobulin (alpha2-M) gene was cloned from the haemocytes of tiger shrimp Penaeus monodon by RT-PCR, cloning and sequencing of overlapping PCR and rapid amplification of cDNA ends (RACE) method. Analysis of the nucleotide sequence revealed that the alpha2-M cDNA consists of 4876 bp with an open reading frame (ORF) of 4494 bp, a 52 bp 5'-untranslated region, and a 327 bp 3'-untranslated region containing a poly A signal. The open reading frame encodes a protein of 1498 amino acids with 18 residues signal sequence. The predicted molecular mass of the mature protein (1480 amino acids) is 167.7 kDa with an estimated pI of 5.30. The P. monodon alpha2-M sequence contains putative functional domains including a GCGEQNM thioester region, a bait region, and a receptor-binding domain which are present in other invertebrate and vertebrate alpha2-Ms. Sequence comparison showed that alpha2-M deduced amino acid sequence of P. monodon has an overall similarity of 85, 52 and 49% to that of kuruma shrimp Marsupenaeus japonicus, American horseshoe crab Limulus polyphemus and mud crab Scylla serrata, respectively. Alignment of the deduced amino acid sequence to other species alpha2-M showed that the overall structure is evolutionarily conserved and phylogenetic analysis revealed that P. monodon alpha2-M is closely related to other arthropod alpha2-M, and displays the highest similarity to M. japonicus alpha2-M. The alpha2-M was mainly expressed in haemocytes, but not in eyestalk, gill, muscle, hepatopancreas, and intestine. Quantitative real-time RT-PCR analysis showed that alpha2-M mRNA transcript in haemocytes of P. monodon increased significantly in 12, 24 and 48 h post-peptidoglycan (PG) injection, but returned to the original values in 72 h post-PG injection.

Salicylic acid and a chitin elicitor both control expression of the CAD1 gene involved in the plant immunity of Arabidopsis

The Arabidopsis mutant cad1 (constitutively activated cell death 1) shows a phenotype that mimics hypersensitive response (HR)-like cell death. The CAD1 gene, which encodes a protein containing a domain with significant homology to the MACPF (membrane attach complex and perforin) domain of complement components and perforin, is likely to control plant immunity negatively and has a W-box cis-element in its promoter region. We found that expression of the CAD1 gene and other W-box containing genes, such as NPR1 and PR2, was promoted by salicylic acid (SA) and benzothiadiazole (BTH) as a SA agonist. The CAD1 gene was also stimulated by a purified chitin oligosaccharide elicitor (degree of polymerization = 8). This latter control was not under SA, because CAD1 expression was not suppressed in 35SnahG transgenic plants, which are unable to accumulate SA. These expression profiles were confirmed by promoter analysis using pCAD1::GUS transgenic plants. The CAD1 expression promoted by BTH and the chitin elicitor was not suppressed in the npr1 mutant, which is insensitive to SA signaling. These results indicate that the CAD1 gene is regulated by two distinct pathways involving SA and a chitin elicitor: viz., SA signaling mediated through an NPR1-independent pathway, and chitin elicitor signaling, through an SA-independent pathway. Three CAD1 homologs that have multiple W-box elements in their promoters were also found to be under the control of SA.

Complement Activation via Alternative Pathway Is Critical in the Development of Laser-Induced Choroidal Neovascularization: Role of Factor B and Factor H

The objective of this study was to explore the role of classical, lectin, and alternative pathways of complement activation in laser-induced choroidal neovascularization (CNV). The classical and alternative pathways were blocked in C57BL/6 mice by small interfering RNAs (siRNA) directed against C1q and factor B, respectively. C4(-/-) mice developed CNV similar to their wild-type controls and inhibition of C1q by siRNA had no effect on the development of CNV. In contrast, CNV was significantly inhibited (p < 0.001) in C5(-/-) mice and C57BL/6 mice treated with factor B siRNA. Inhibition of the alternative pathway by factor B siRNA resulted in decreased levels of membrane attack complex and angiogenic factors-vascular endothelial growth factor and TGF-beta2. Furthermore, factor B was up-regulated in complement sufficient C57BL/6 mice at day 1 postlaser and remained elevated at day 7. Significantly reduced levels of factor H were observed at day 3 in these animals. In conclusion, our results demonstrate that activation of the factor B-dependent alternative pathway, but not the classical or lectin pathways, was essential for the development of CNV in mouse model of laser-induced CNV. Thus, specific blockade of the alternative pathway may represent a therapeutically relevant strategy for the inhibition of CNV.

The complement system plays a critical role in the development of experimental autoimmune anterior uveitis

PURPOSE

The role of complement in ocular autoimmunity was explored in a experimental autoimmune anterior uveitis (EAAU) animal model.

METHODS

EAAU was induced in Lewis rats by immunization with bovine melanin-associated antigen. Complement activation in the eye was monitored by Western blot for iC3b. The importance of complement to the development of EAAU was studied by comparing the course of intraocular inflammation in normal Lewis rats (complement-sufficient) with cobra venom factor-treated rats (complement-depleted). Eyes were harvested from both complement-sufficient and complement-depleted rats for mRNA and protein analysis for IFN-gamma, IL-10, and interferon-inducible protein (IP)-10. Intracellular adhesion molecule (ICAM)-1 and leukocyte-endothelial cell adhesion molecule (LECAM)-1 were detected by immunofluorescent staining. OX-42 was used to investigate the importance of iC3b and CR3 interaction in EAAU.

RESULTS

There was a correlation between ocular complement activation and disease progression in EAAU. The incidence, duration, and severity of disease were dramatically reduced after active immunization in complement-depleted rats. Complement depletion also completely suppressed adoptive transfer EAAU. The presence of complement was critical for local production of cytokines (IFN-gamma and IL-10), chemokines (IP-10), and adhesion molecules (ICAM-1 and LECAM-1) during EAAU. Furthermore, intraocular complement activation, specifically iC3b production and engagement of complement receptor 3 (CR3), had a significant impact on disease activity in EAAU.

CONCLUSIONS

The study provided the novel finding that complement activation plays a central role in the pathogenesis of ocular autoimmunity and may serve as a potential target for therapeutic intervention.

Targeted complement inhibition by C3d recognition ameliorates tissue injury without apparent increase in susceptibility to infection

Previous studies indicate a pivotal role for complement in mediating both local and remote injury following ischemia and reperfusion of the intestine. Here, we report on the use of a mouse model of intestinal ischemia/reperfusion injury to investigate the strategy of targeting complement inhibition to sites of complement activation by linking an iC3b/C3dg-binding fragment of mouse complement receptor 2 (CR2) to a mouse complement-inhibitory protein, Crry. We show that the novel CR2-Crry fusion protein targets sites of local and remote (lung) complement activation following intestinal ischemia and reperfusion injury and that CR2-Crry requires a 10-fold lower dose than its systemic counterpart, Crry-Ig, to provide equivalent protection from both local and remote injury. CR2-Crry has a significantly shorter serum half-life than Crry-Ig and, unlike Crry-Ig, had no significant effect on serum complement activity at minimum effective therapeutic doses. Furthermore, the minimum effective dose of Crry-Ig significantly enhanced susceptibility to infection in a mouse model of acute septic peritonitis, whereas the effect of CR2-Crry on susceptibility to infection was indistinguishable from that of PBS control. Thus, compared with systemic inhibition, CR2-mediated targeting of a complement inhibitor of activation improved bioavailability, significantly enhanced efficacy, and maintained host resistance to infection.

Brain microglia and blood-derived macrophages: molecular profiles and functional roles in multiple sclerosis and animal models of autoimmune demyelinating disease

Microglia and macrophages, one a brain-resident, the other a mostly hematogenous cell type, represent two related cell types involved in the brain pathology in multiple sclerosis and its autoimmune animal model, the experimental allergic encephalomyelitis. Together, they perform a variety of different functions: they are the primary sensors of brain pathology, they are rapidly recruited to sites of infection, trauma or autoimmune inflammation in experimental allergic encephalomyelitis and multiple sclerosis and they are competent presenters of antigen and interact with T cells recruited to the inflamed CNS. They also synthesise a variety of molecules, such as cytokines (TNF, interleukins), chemokines, accessory molecules (B7, CD40), complement, cell adhesion glycoproteins (integrins, selectins), reactive oxygen radicals and neurotrophins, that could exert a damaging or a protective effect on adjacent axons, myelin and oligodendrocytes. The current review will give a detailed summary on their cellular response, describe the different classes of molecules expressed and their attribution to the blood derived or brain-resident macrophages and then discuss how these molecules contribute to the neuropathology. Recent advances using chimaeric and genetically modified mice have been particularly telling about the specific, overlapping and nonoverlapping roles of macrophages and microglia in the demyelinating disease. Interestingly, they point to a crucial role of hematogenous macrophages in initiating inflammation and myelin removal, and that of microglia in checking excessive response and in the induction and maintenance of remission.

Role of complement and complement membrane attack complex in laser-induced choroidal neovascularization

Choroidal neovascularization (CNV), or choroidal angiogenesis, is the hallmark of age-related macular degeneration and a leading cause of visual loss after age 55. The pathogenesis of new choroidal vessel formation is poorly understood. Although inflammation has been implicated in the development of CNV, the role of complement in CNV has not been explored experimentally. A reliable way to produce CNV in animals is to rupture Bruch's membrane with laser photocoagulation. A murine model of laser-induced CNV in C57BL/6 mice revealed the deposition of C3 and membrane attack complex (MAC) in the neovascular complex. CNV was inhibited by complement depletion using cobra venom factor and did not develop in C3(-/-) mice. Anti-murine C6 Abs in C57BL/6 mice inhibited MAC formation and also resulted in the inhibition of CNV. Vascular endothelial growth factor, TGF-beta2, and beta-fibroblast growth factor were elevated in C57BL/6 mice after laser-induced CNV; complement depletion resulted in a marked reduction in the level of these angiogenic factors. Thus, activation of complement, specifically the formation of MAC, is essential for the development of laser- induced choroidal angiogenesis in mice. It is possible that a similar mechanism may be involved in the pathophysiology of other angiogenesis essential diseases.

Effect of complement component C3 deficiency on experimental Lyme borreliosis in mice

Mice deficient in complement component C3 (C3(-/-)) and syngeneic C57BL/6 control mice were challenged with Borrelia burgdorferi to determine the role of complement in immune clearance and joint histopathology during experimental Lyme borreliosis. Tibiotarsal joint, ear, and heart tissues were monitored for spirochete numbers at 2, 4, 8, and 12 weeks postinoculation with 10(5) B. burgdorferi B31 clone 5A4 by using quantitative real-time PCR. The spirochete load in joint and ear tissue remained higher in the C3(-/-) mice than in the wild-type counterparts throughout the 12-week study, whereas the numbers in heart tissue of both groups of mice decreased substantially at 8 to 12 weeks postinfection. Histopathology scores for joint tissue were generally higher in the C3(-/-) mice compared to C57BL/6 controls at 2 and 4 weeks postinfection, which may reflect the presence of higher numbers of bacteria in the joints at these early time points. Levels of anti-B. burgdorferi immunoglobulin G tended to be reduced in the C3(-/-) mice compared to control mice. Furthermore, a 5.5-fold-lower number of the complement-sensitive Borrelia garinii was needed to infect C3(-/-) mice compared to C57BL/6 mice, indicating that its sensitivity to complement is one barrier to infection of the mouse model by B. garinii. These results indicate that the complement system may be important in controlling the early dissemination and progression of B. burgdorferi infection.

Tolerance is dependent on complement C3 fragment iC3b binding to antigen-presenting cells

Systemic tolerance can be induced by the introduction of antigen into an immune-privileged site. Here we investigated the role of complement in the induction of tolerance after intraocular injection. We found that the development of antigen-specific tolerance is dependent on a complement activation product. The ligation of the complement C3 activation product iC3b to complement receptor type 3 (the iC3b receptor) on antigen-presenting cells resulted in the sequential production of transforming growth factor-beta2 and interleukin-10, which is essential for the induction of tolerance. These observations may extend to the development of both neonatal tolerance and other forms of acquired tolerance.

Molecular cloning and structural characterization of the hagfish proteinase inhibitor of the alpha-2-macroglobulin family

The "most primitive" living vertebrate the hagfish has a dimeric proteinase inhibitor, a protein homologous to human alpha2-macroglobulin, in its plasma at high concentration. Although the hagfish proteinase inhibitor has been isolated and its function and quaternary structure studied, its primary structure, subunit composition and fragmentation process remain unclear. In this study, hagfish proteinase inhibitor cDNA was cloned, sequenced and cDNA-deduced amino acid sequence was analyzed. A large fraction of homosubunits in the dimeric structure of the protein has undergone a cleavage at a specific arginyl residue (Arg833) while the rest retained their chain integrity without being processed. Thus random combinations of processed and nonprocessed subunits in the dimeric structure of this protein result in different molecular conformers and generate a complicated multiband pattern in SDS-PAGE. It was further demonstrated by proteolytic analysis that the hagfish inhibitor has no susceptible arginyl residues within its bait region and thus incapable of trapping arginine specific proteinases. This implies that the specific subunit cleavage at Arg833 was caused by an unknown arginine specific proteinase which escaped from the entrapment by the hagfish inhibitor.

Thioester function is conserved in SpC3, the sea urchin homologue of the complement component C3

The amino acid sequence of the thioester site in the alpha chain of SpC3, the sea urchin homologue of C3, is conserved. This implies a conserved function of covalent bond formation with amine or hydroxyl groups on target molecules. When coelomic fluid (CF) was incubated with 14C-methylamine, a classic assay for thioester binding function, the alpha chain became labeled. When CF was treated to induce autolysis, peptide bond cleavage occurred at the thioester site. Autolysis could be blocked or reduced by pre-treating CF with either methylamine or yeast, both of which are known to bind to thioester sites C3 proteins from other organisms. The data suggest that SpC3 can bind to target cell surfaces, constituting indirect evidence that it can covalently bind to pathogen surfaces and function as an opsonin in vivo. This activity may be an important aspect of host defense in the sea urchin.

Flavoxobin, a serine protease from Trimeresurus flavoviridis (habu snake) venom, independently cleaves Arg726-Ser727 of human C3 and acts as a novel, heterologous C3 convertase

We have recently shown that crude Trimeresurus flavoviridis (habu snake) venom has a strong capability for activating the human alternative complement system. To identify the active component, the crude venom was fractionated and purified by serial chromatography using Sephadex G-100, CM-cellulose C-52, diethylaminoethyl-Toyopearl 650M, and Butyl-Toyopearl, and the active fractions were evaluated by the C3a-releasing and soluble membrane attack complex-forming activities. Two peak fractions with the highest activities were detected after gel filtration and ion exchange chromatography, and the first fraction was purified to homogeneity. The homogeneous protein was examined for its N-terminal amino acid sequence by Edman degradation. The determined sequence of 25 amino acids completely coincided with that of a previously reported serine protease with coagulant activity, flavoxobin, purified from the same snake venom. To elucidate the molecular mechanism of the complement activation, the reactive products of the mixture of the purified human C3 and flavoxobin were examined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The digesting pattern revealed that flavoxobin cleaves the alpha chain of the C3 molecule into two fragments. The N-terminal amino acid sequences for the remnant fragments of C3 disclosed that flavoxobin severs the human C3 at the Arg726-Ser727 site to form C3b and C3a the way C3bBb, the human alternative C3 convertase, does. In conclusion, flavoxobin acts as a novel, heterologous C3 convertase that independently cleaves human C3 and kick-starts the complement cascade.

Deletion of wboA enhances activation of the lectin pathway of complement in Brucella abortus and Brucella melitensis

Brucella spp. are gram-negative intracellular pathogens that survive and multiply within phagocytic cells of their hosts. Smooth organisms present O polysaccharides (OPS) on their surface. These OPS help the bacteria avoid the bactericidal action of serum. The wboA gene, coding for the enzyme glycosyltransferase, is essential for the synthesis of O chain in Brucella. In this study, the sensitivity to serum of smooth, virulent Brucella melitensis 16M and B. abortus 2308, rough wboA mutants VTRM1, RA1, and WRR51 derived from these two Brucella species, and the B. abortus vaccine strain RB51 was assayed using normal nonimmune human serum (NHS). The deposition of complement components and mannose-binding lectin (MBL) on the bacterial surface was detected by flow cytometry. Rough B. abortus mutants were more sensitive to the bactericidal action of NHS than were rough B. melitensis mutants. Complement components were deposited on smooth strains at a slower rate compared to rough strains. Deposition of iC3b and C5b-9 and bacterial killing occurred when bacteria were treated with C1q-depleted, but not with C2-depleted serum or NHS in the presence of Mg-EGTA. These results indicate that (i) OPS-deficient strains derived from B. melitensis 16M are more resistant to the bactericidal action of NHS than OPS-deficient strains derived from B. abortus 2308, (ii) both the classical and the MBL-mediated pathways are involved in complement deposition and complement-mediated killing of Brucella, and (iii) the alternative pathway is not activated by smooth or rough brucellae.

Incorporation of decay-accelerating factor into the baculovirus envelope generates complement-resistant gene transfer vectors

Baculovirus vectors are an efficient means to deliver genes into hepatocytes in vitro. In experiments that exclude components of the complement system, gene transfer is facilitated. Therefore, the complement system has been defined to represent a potent primary barrier to direct application of baculoviruses in vivo. Here we have genetically manipulated baculoviruses so that the complement-regulatory protein human decay- accelerating factor (DAF) is incorporated into the viral envelope. We found that this modification protected baculovirus vectors against complement-mediated inactivation. Complement-resistant baculovirus vectors were additionally analyzed by immunoblotting and electron microscopy, showing the extent of envelope-incorporated DAF and shape of complement-resistant baculoviruses after exposure to complement. This modified baculovirus vector allowed for an enhanced gene transfer into complement-sufficient neonatal rats in vivo, and thus represents a step in the development of improved alternative viral vectors for gene therapy.

Pursuit of optimal carbohydrate-based anticancer vaccines: preparation of a multiantigenic unimolecular glycopeptide containing the Tn, MBr1, and Lewis(y) antigens

A novel preparation of nonnatural glycoamino acids starting from n-pentenyl glycosides is described. The approach involves a Horner-Emmons olefination with a suitably protected glycine-derived phosphonate, followed by catalytic asymmetric hydrogenation, which proceeds with excellent diastereomeric selectivity. The synthetic methodology was useful for the preparation of glycoamino acids containing the Tn antigen, the MBr1 antigen (Globo-H), the Le(y) antigen, and lactose. These glycoamino acids can also serve as units for peptide synthesis. The synthesis of polyvalent glycopeptides containing three different antitumor antigens is described (28 and 29), and these have been prepared for conjugation to carrier protein in order to access the immunogenicity for tumor immunotherapy applications.

Immunology 101 at poxvirus U: immune evasion genes

Poxviruses, unlike some other large DNA viruses, do not undergo a latent stage but rely on the expression of viral proteins to evade host immune responses. Of the many poxviral evasion genes identified, most target cytokines or other innate immune defenses. Resistance to interferons appears to be a priority as there are viral proteins that prevent their induction, receptor binding, and action. Additional poxviral proteins inhibit complement activation, chemokines, IL-1 beta and tumor necrosis factor. The identification of viral immune evasion genes and the determination of their roles in virus survival and spread contribute to our understanding of immunology and microbiology.

Complement C4 inhibits systemic autoimmunity through a mechanism independent of complement receptors CR1 and CR2

The complement system enhances antibody responses to T-dependent antigens, but paradoxically, deficiencies in C1 and C4 are strongly linked to autoantibody production in humans. In mice, disruption of the C1qa gene also results in spontaneous autoimmunity. Moreover, deficiencies in C4 or complement receptors 1 and 2 (CR1/CR2) lead to reduced selection against autoreactive B cells and impaired humoral responses. These observations suggest that C1 and C4 act through CR1/CR2 to enhance humoral immunity and somehow suppress autoimmunity. Here we report high titers of spontaneous antinuclear antibody (ANA) in C4(-/)- mice. This systemic lupus erythematosus-like autoimmunity is highly penetrant; by 10 mo of age, all C4(-)(/)- females and most males produced ANA. In contrast, titers and frequencies of ANA in Cr2(-)(/)- mice, which are deficient in CR1 and CR2, never rose significantly above those in normal controls. Glomerular deposition of immune complexes (ICs), glomerulonephritis, and splenomegaly were observed in C4(-)(/)- but not Cr2(-)(/)- mice. C4(-)(/)-, but not Cr2(-)(/)-, mice accumulate activated T and B cells. Clearance of circulating ICs is impaired in preautoimmune C4(-)(/)-, but not Cr2(-)(/)-, mice. C4 deficiency causes spontaneous, lupus-like autoimmunity through a mechanism that is independent of CR1/CR2.

Biodistribution of liposomes and C3 fragments associated with liposomes: evaluation of their relationship

The biodistribution of liposomes with two different kind phospholipids (hydrogenated egg phosphatidylcholine and egg phosphatidylcholine) plus cholesterol (CHOL) were investigated after intravenous administration to rats. Elimination of liposomes from blood circulation was affected by the lipid composition. It appeared that the inclusion of CHOL in liposomes accelerates the rate of liposome uptake by liver, resulting in rapid elimination of liposomes. The amount of C3 fragments bound to liposomes was quantitatively determined to assess the contribution of the complement system to liposome accumulation into organs and liposome destabilization in vivo and in vitro. The amount of bound C3 fragments was directly proportional to CHOL content, and the amount was also proportional to the CLh, CLs as well as CLrel. This relationship suggests that the complement system is responsible for the elimination of liposomes from blood circulation, presumably as a consequence of opsonization by C3 fragments and assembly of membrane attack complex (MAC) onto liposomes. In addition, substitution of cholesteryl methyl ether into the liposome formulation for CHOL significantly diminished not only the binding of C3 fragments but also the CLh, CLs and CLrel, resulting in increased mean resident time (MRT) of the liposomes. This result suggests that the hydroxyl-group on CHOL is a binding site for C3 fragments on the liposomes and that CHOL in a liposome formulation promotes the accumulation of liposomes into the liver and spleen, probably due to their uptake by phagocytic cells, and impairs the stability of the liposomes in blood circulation, via a mechanism involving the complement system.

Expression of complement messenger RNAs by human endothelial cells

This study evaluated complement mRNA expression in human brain microvessel endothelial cells (HBMEC), human umbilical vein endothelial cells (HUVEC), and cells of the human derived ECV304 line. Cerebral endothelial cells and HUVEC expressed detectable levels of complement gene mRNAs for the C1q B-chain, C1r, C1s, C2, C3, C4, C5, C7, C8 gamma-subunit and C9. In addition to C6 mRNA, C1q and C9 were not detected in ECV304 cells. These results indicate that endothelial cells may be a source of complement proteins in brain and other organs of the body.

Generation of complement fragment C5a in milk is variable among cows

The appearance of chemotactic fragments of complement at sites of infection is an important component of innate immunity. The contribution of C5a, the most biologically active complement fragment, to the recruitment of phagocytes in milk is not well defined, in particular the amount of C5a that is released in normal milk before inflammation. The generation of C5a in normal milk upon activation of complement by invading bacteria depends on the amount of available C5 and on the activity of the C3/C5-convertase of the alternative pathway. Concentrations of C5 were measured in one fore and one rear uninfected quarter of 19 Holstein cows. Values were consistent within cows, but widely dispersed among cows (0.19 to 1.94% blood concentration). C5 concentrations in milk were loosely related to concentrations in blood. By comparison, the range of milk concentrations of C3 (1.4 to 4.4%, mean 2.46 +/- 0.63% of blood concentration) was narrower. Two groups of six cows with high milk concentrations of C5 (cows H5: mean = 1.31%) and six cows with low milk concentrations of C5 (cow L5: mean = 0.21%) were constituted for further analysis of complement activation. There was a positive correlation between concentrations in milk of BSA and C5, but not between concentrations of BSA and C3. The activities of the C3- and C5-convertases were assessed through the deposition on complement-activating bacteria (Streptococcus agalactiae) of C3 and C5 fragments, respectively. The deposition of C3 was 1.7-fold higher, and the deposition of C5 was 2.75-fold higher in milk from H5 cows than in milk of L5 cows. Higher concentrations of C5 and better functioning of C5-convertase were mirrored by a much higher concentration of C5a in milk from H5 cows (12.30 ng/ml) than in milk of L5 cows (0.76 ng/ml) after activation of complement with zymosan. These results indicate that cows differed widely in their capacity to generate C5a in milk before inflammation, and that milk C5 concentrations were a primary limiting factor for C5a generation. Cows with the lowest milk concentrations of C5 are likely unable to use the complement system for the initial recruitment of leukocytes.

Immunologic response to infection and its role in septic shock

In summary, the invasion of bacteria across mucosal surfaces is met with a vigorous host response that includes complement, antibody formation (thymus-independent and eventually thymus-dependent), phagocytosis, production of antibacterial peptides and proteins, the production of cytokines that result in activation of phagocytes and endothelial cells to attract more phagocytes, and the formation of fibrin to limit the spread of infection. The best summary of immune response to infection was written by Lewis Thomas in 1974.

Molecular aspects of complement-mediated bacterial killing. Periplasmic conversion of C9 from a protoxin to a toxin

As part of the membrane attack complex complement protein C9 is responsible for direct killing of bacteria. Here we show that in the periplasmic space of an Escherichia coli cell C9 is converted from a protoxin to a toxin by periplasmic conditions missing in spheroplasts. This conversion is independent of the pathway by which C9 enters the periplasm. Both, C9 shocked into the periplasm and plasmid-expressed C9 targeted to the periplasm via a signal sequence are toxic. Toxicity requires disulfide-linked C9 because export into the periplasm of cells defective in disulfide bond synthesis (dsbA and dsbB mutants) is not toxic unless N-acetylcysteine is added externally to promote cystines. A N-terminal fragment, C9[1-144], is not toxic nor is cytoplasmically expressed C9, even in trxB mutants that are able to form disulfide bonds in the cytoplasm. Importantly, expression of full-length C9 in complement-resistant cells has no effect on their viability. Expression and translocation into the periplasm may provide a novel model to identify molecular mechanisms of other bactericidal disulfide-linked proteins and to investigate the nature of bacterial complement resistance.

Complement systems in invertebrates. The ancient alternative and lectin pathways

The complement system in higher vertebrates is composed of about thirty proteins that function in three activation cascades and converge in a single terminal pathway. It is believed that these cascades, as they function in the higher vertebrates, evolved from a few ancestral genes through a combination of gene duplications and divergences plus pathway duplication (perhaps as a result of genome duplication). Evidence of this evolutionary history is based on sequence analysis of complement components from animals in the vertebrate lineage. There are fewer components and reduced or absent pathways in lower vertebrates compared to mammals. Modern examples of the putatively ancestral complement system have been identified in sea urchins and tunicates, members of the echinoderm phylum and the protochordate subphylum, which are sister groups to the vertebrates. Thus far, this simpler system is composed of homologues of C3, factor B, and mannose binding protein associated serine protease suggesting the presence of simpler alternative and lectin pathways. Additional components are predicted to be present. A complete analysis of this invertebrate defense system, which evolved before the invention of rearranging genes, will provide keys to the primitive beginnings of innate immunity in the deuterostome lineage of animals.

Benzenesulfonamide derivatives of 2-substituted 4H-3,1-benzoxazin-4-ones and benzthiazin-4-ones as inhibitors of complement C1r protease

A series of 2-sulfonyl-4H-3,1-benzoxazinones was prepared that inhibit C1r protease in vitro. Several compounds were found to be selective for C1r verses the related serine protease trypsin. Selected compounds demonstrated functional activity in a hemolysis assay.