Activation of the alternative complement pathway by pneumococcal lipoteichoic acid

Kidney diseases

Dysregulation and accelerated activation of the alternative pathway (AP) of complement is known to cause or accentuate several pathologic conditions in which kidney injury leads to the appearance of hematuria and proteinuria and ultimately to the development of chronic renal failure. Multiple genetic and acquired defects involving plasma- and membrane-associated proteins are probably necessary to impair the protection of host tissues and to confer a significant predisposition to AP-mediated kidney diseases. This review aims to explore how our current understanding will make it possible to identify the mechanisms that underlie AP-mediated kidney diseases and to discuss the available clinical evidence that supports complement-directed therapies. Although the value of limiting uncontrolled complement activation has long been recognized, incorporating complement-targeted treatments into clinical use has proved challenging. Availability of anti-complement therapy has dramatically transformed the outcome of atypical hemolytic uremic syndrome, one of the most severe kidney diseases. Innovative drugs that directly counteract AP dysregulation have also opened new perspectives for the management of other kidney diseases in which complement activation is involved. However, gained experience indicates that the choice of drug should be tailored to each patient's characteristics, including clinical, histologic, genetic, and biochemical parameters. Successfully treating patients requires further research in the field and close collaboration between clinicians and researchers who have special expertise in the complement system.

Streptococcus pneumoniae interactions with the complement system

Host innate and adaptive immunity to infection with Streptococcus pneumoniae is critically dependent on the complement system, demonstrated by the high incidence of invasive S. pneumoniae infection in people with inherited deficiency of complement components. The complement system is activated by S. pneumoniae through multiple mechanisms. The classical complement pathway is activated by recognition of S. pneumoniae by C-reactive protein, serum amyloid P, C1q, SIGN-R1, or natural or acquired antibody. Some S. pneumoniae strains are also recognised by ficolins to activate the mannose binding lectin (MBL) activation pathway. Complement activation is then amplified by the alternative complement pathway, which can also be activated by S. pneumoniae directly. Complement activation results in covalent linkage of the opsonic complement factors C3b and iC3b to the S. pneumoniae surface which promote phagocytic clearance, along with complement-mediated immune adherence to erythrocytes, thereby protecting against septicaemia. The role of complement for mucosal immunity to S. pneumoniae is less clear. Given the major role of complement in controlling infection with S. pneumoniae, it is perhaps unsurprising that S. pneumoniae has evolved multiple mechanisms of complement evasion, including the capsule, multiple surface proteins, and the toxin pneumolysin. There is considerable variation between S. pneumoniae capsular serotypes and genotypes with regards to sensitivity to complement which correlates with ability to cause invasive infections. However, at present we only have a limited understanding of the main mechanisms causing variations in complement sensitivity between S. pneumoniae strains and to non-pathogenic streptococci.

Acute post-streptococcal glomerulonephritis: analysis of the pathogenesis

Increasing evidence supports a central role of the immune system in acute post streptococcal glomerulonephritis (APSGN), but the current view of how streptococcal biology affects immunity, and vice versa, remains to be clarified. Renal glomerular immune complex deposition is critical in the initiation of APSGN; however, mechanisms previous to immune complex formation could modulate the initiation and the progression of the disease. Initial and late renal events involved in the nephritis can also be related to host factors and streptococcal factors. In this review we describe the mechanisms reported for the APSGN pathogenesis, the interactions of streptococcal products with renal cells and leukocytes, the possible effects of different nephritogenic antigens in the renal environment and the possibility that APSGN is not just due to a single streptococcal antigen and its antibody; instead, kidney damage may be the result of different factors acting at the same time related to both streptococcus and host factors. Addressing these points should help us to better understand APSGN physiopathology.

Inhibition of Post-Meningitic Cochlear Injury with Cerebrospinal Fluid Irrigation

OBJECTIVE: Labyrinthitis ossificans, the pathologic ossification of the otic capsule associated with profound deafness and loss of vestibular function occurs frequently as a sequella of bacterial meningitis and subsequent purulent labyrinthitis. Experimentally, in Streptococcus pneumoniae meningitis, it has been shown that a vigorous inflammatory response to teichoic acids in the bacterial cell wall contributes to cochlear damage and subsequent fibrosis and ossification. The hypothesis of this study is that a dilution of concentration of inflammatory mediators through cerebrospinal fluid (CSF) irrigation will lead to a reduction in both inner ear pathology and permanent hearing loss.

STUDY DESIGN AND SETTING: Auditory brainstem response testing was used to determine baseline hearing thresholds in 20 Mongolian gerbils (12 irrigated, 8 sham irrigated animals) at 32 kHz, 16 kHz, 8 kHz, and 4 kHz frequencies. Their thresholds at 14 days and 120 days post-procedure were also obtained. Streptococcus pneumoniae meningitis was induced in both groups of animals by intrathecal (i.t.) injection of bacteria. Both groups received penicillin treatment. Forty-eight hours after inoculation, both groups were implanted with i.t. inflow and outflow catheters. The irrigated group was infused continuously with artificial CSF over 36 hr at a rate of 70 μL/hr and the outflow sampled. The tubing in the sham irrigated group was clamped (without sampling). They were sacrificed at 120 days post-procedure and histomorphometric analysis carried out. The concentration of interleukin 1β (IL-1β) for the CSF samples from the irrigated group were compared to samples collected from an additional control group of 8 non-irrigated meningitic gerbils. IL-1β was chosen to study because it is a potent pro-inflammatory cytokines in bacterial meningitis that is unaffected by the neurosurgical trauma of the experimental protocol.

RESULTS: Twenty animals survived the meningitis (6 irrigation, 6 sham irrigation, 8 non-irrigation meningitic controls). At Days 14 and 120 post-infection, the irrigated animals manifested significantly less hearing loss with a mean loss of 28.82 dB compared to the sham irrigation group mean loss of 40.76 dB ( P < 0.03). The degree of hearing loss in both groups was frequency-dependent with greater loss at higher frequencies (mean loss = 22.4 dB at 32 kHz, 23.0 dB at 16 kHz, 18.6 dB at 8 kHz, and 12.5 dB at 4 kHz). Histomorphometric analysis demonstrated a marked reduction in degeneration of the spiral ligament, spiral ganglion cells, and stria vascularis in experimental animals as compared to controls. Immunohistochemistry showed a significant reduction in IL-β1 concentrations in the irrigated animals compared to the non-irrigated, infected controls ( P < 0.03).

CONCLUSIONS: Irrigation of CSF resulted in a significant reduction in post-meningitic cochlear injury when compared to controls. This model for continuous cerebrospinal fluid irrigation provides a means to evaluate the effects of a dilution of inflammatory mediators on hearing loss and labyrinthitis ossificans after bacterial meningitis.

SIGNIFICANCE: Despite advances in the prevention of meningitis and improved antibiotic treatment, bacterial meningitis continues to have significant associated morbidity. This study provides insight into some of the mechanisms responsible for post-meningitic hearing loss and labyrinthitis ossificans and presents a novel approach to reduce these complications.

EBM rating: C-4

The Effects of Superoxide Dismutase in Gerbils with Bacterial Meningitis

BACKGROUND: Inflammatory products, such as oxygen radicals generated during the course of bacterial meningitis, can damage nerve endings, hair cells, and/or supporting cells in the cochlea. Superoxide dismutase (SOD), an O2-scavenger, has been shown to play an important role in the protection against radical toxicity in various animal experiments.

OBJECTIVE: To study the antioxidant effects of SOD on the inflammatory response of gerbils with bacterial meningitis.

STUDY DESIGN: Meningitis was induced in three groups of 10 gerbils by intrathecal (IT) injection of Streptococcus pneumoniae into the cisterna magna. Group 1 received IT SOD, group 2 received intramuscular (IM) SOD, and group 3, the control group, received IM normal saline. Histologic data and auditory brainstem responses (ABR) were obtained from each gerbil.

RESULTS: Fibrosis and/or neo-ossification were near absent in the IT SOD group and significantly less fibrosis occurred in the IM group (IT vs. IM: P = 0.010; IT vs. control group: P = 0.001). The amount of surviving spiral ganglion cells correlated inversely with the extent of fibrosis (r = −0.753, P < 0.00001).

CONCLUSIONS: IT injection of SOD significantly reduced cochlear fibrosis and neo-ossification, reduced the spiral ganglion cell loss, and decreased damage of the cochlear components following bacterial meningitis.

Pneumococcal wall teichoic acid is required for the pathogenesis of Streptococcus pneumoniae in murine models

Pneumococcal asymptomatic colonization of the respiratory tracts is a major risk for invasive pneumococcal disease. We have previously shown that pneumococcal wall teichoic acid (WTA) was involved in pneumococcal infection of sepsis and adherence to epithelial and endothelial cells. In this study, we investigated the contribution of pneumococcal WTA to bacterial colonization and dissemination in murine models. The result showed that nasopharynx colonizing D39 bacterial cells have a distinct phenotype showing an increased exposure of teichoic acids relative to medium-grown bacteria. The WTA-deficient mutants were impaired in their colonization to the nasopharynx and lungs, and led to a mild inflammation in the lungs at 36 h post-inoculation. Pretreatment of the murine nares with WTA reduced the ability of wild type D39 bacteria to colonize the nasopharynx. In addition, the WTA-deficient strain was impaired in its ability to invade the blood and brain following intranasal administration. WTA-deficient D39 strain was reduced in C3 deposition but was more susceptible to the killing by the neutrophils as compared with its parent strain. Our results also demonstrated that the WTA enhanced pneumococcal colonization and dissemination independently of the host strains. These results indicate that WTA plays an important role in pneumococcal pathogenesis, both in colonization and dissemination processes.

Structural reevaluation of Streptococcus pneumoniae Lipoteichoic acid and new insights into its immunostimulatory potency

Streptococcus pneumoniae is a Gram-positive human pathogen with a complex lipoteichoic acid (pnLTA) structure. Because the current structural model for pnLTA shows substantial inconsistencies, we reinvestigated purified and, more importantly, O-deacylated pnLTA, which is most suitable for NMR spectroscopy and electrospray ionization-MS spectrometry. We analyzed pnLTA of nonencapsulated pneumococcal strains D39Δcps and TIGR4Δcps, respectively. The data obtained allowed us to (re)define (i) the position and linkage of the repeating unit, (ii) the putative α-GalpNAc substitution at the ribitiol 5-phosphate (Rib-ol-5-P), and (iii) the length of (i.e. the number of repeating units in) the pnLTA chain. We here also describe for the first time that the terminal sugar residues in the pnLTA (Forssman disaccharide; α-D-GalpNAc-(1→3)-β-D-GalpNAc-(1→)), responsible for the cross-reactivity with anti-Forssman antigen antibodies, can be heterogeneous with respect to its degree of phosphorylcholine substitution in both O-6-positions. To assess the proinflammatory potency of pnLTA, we generated a (lipopeptide-free) Δlgt mutant of strain D39Δcps, isolated its pnLTA, and showed that it is capable of inducing IL-6 release in human mononuclear cells, independent of TLR2 activation. This finding was quite in contrast to LTA of the Staphylococcus aureus SA113Δlgt mutant, which did not activate human mononuclear cells in our experiments. Remarkably, this is also contrary to various other reports showing a proinflammatory potency of S. aureus LTA. Taken together, our study refines the structure of pnLTA and indicates that pneumococcal and S. aureus LTAs differ not only in their structure but also in their bioactivity.

Systematic review of membrane components of gram-positive bacteria responsible as pyrogens for inducing human monocyte/macrophage cytokine release

Fifty years after the elucidation of lipopolysaccharides (LPS, endotoxin) as the principal structure of Gram-negative bacteria activating the human immune system, its Gram-positive counterpart is still under debate. Pyrogen tests based on the human monocyte activation have been validated for LPS detection as an alternative to the rabbit test and, increasingly, the limulus amebocyte lysate test. For full replacement, international validations with non-endotoxin pyrogens are in preparation. Following evidence-based medicine approaches, a systematic review of existing evidence as to the structural nature of the Gram-positive pyrogen was undertaken. For the three major constituents suggested, i.e., peptidoglycan, lipoteichoic acids (LTA), and bacterial lipoproteins (LP), the questions to be answered and a search strategy for relevant literature was developed, starting in MedLine. The evaluation was based on the Koch–Dale criteria for a mediator of an effect. A total of 380 articles for peptidoglycan, 391 for LP, and 285 for LTA were retrieved of which 12, 8, and 24, respectively, fulfilled inclusion criteria. The compiled data suggest that for peptidoglycan two Koch–Dale criteria are fulfilled, four for LTA, and two for bacterial LP. In conclusion, based on the best currently available evidence, LTA is the only substance that fulfills all criteria. LTA has been isolated from a large number of bacteria, results in cytokine release patterns inducible also with synthetic LTA. Reduction in bacterial cytokine induction with an inhibitor for LTA was shown. However, this systematic review cannot exclude the possibility that other stimulatory compounds complement or substitute for LTA in being the counterpart to LPS in some Gram-positive bacteria.

Acute presentation and persistent glomerulonephritis following streptococcal infection in a patient with heterozygous complement factor H-related protein 5 deficiency

Acute poststreptococcal glomerulonephritis is a common cause of acute nephritis in children. Transient hypocomplementemia and complete recovery are typical, with only a minority developing chronic disease. We describe a young girl who developed persistent kidney disease and hypocomplementemia after a streptococcal throat infection. Kidney biopsy 1 year after presentation showed isolated glomerular complement C3 deposition, membranoproliferative changes, and subendothelial, intramembranous and occasional subepithelial electron-dense deposits consistent with C3 glomerulopathy. Complement gene screening revealed a heterozygous single nucleotide insertion in exon 4 of the complement factor H-related protein 5 gene (CFHR5), resulting in a premature stop codon. This variant was not detected in 198 controls. Serum CFHR5 levels were reduced. The mother and sister of the index patient were heterozygous for the sequence variant, with no overt evidence of kidney disease. We speculate that this heterozygous CFHR5 sequence variant is a risk factor for the development of chronic kidney disease after streptococcal infection.

Chemical synthesis of bacterial lipoteichoic acids: an insight on its biological significance

During infections caused by Gram-negative bacteria, lipopolysaccharide (LPS, endotoxin) has a dominant role leading to fulminant pro-inflammatory reactions in the host. As there is no LPS in Gram-positive bacteria, other microbial cell wall components have been identified to be the causative agent for the pro-inflammatory activity since also Gram-positive bacterial infections lead to comparable clinical symptoms and reactions. On search for the "Gram-positive endotoxin" a widely accepted hypothesis has been raised in that the lipoteichoic acids (LTAs) serve as pathogen-associated molecular patterns (PAMPs) during Gram-positive sepsis, although the amount necessary for a pro-inflammatory in vitro response is several orders of magnitude higher than that for LPS. Therefore, LTA cannot be considered to be "the (endo)toxin of Gram-positive bacteria". Although LPS and LTA show structural relatedness (amphiphilic, negatively charged glycophospholipids), they are structurally quite different from each other and one might expect that they are also recognized by different receptors of the innate immune system, the so called toll-like receptors 4 and 2 (TLR4 and TLR2), respectively. Based on their chemical structure, the LTAs were classified into four types (type I-IV) of which we have carefully investigated the LTA of Staphylococcus aureus (type I), Lactococcus garvieae (type II) and Streptococcus pneumoniae (type IV). Hence, these LTAs have been synthesized in our group and biologically evaluated with respect to their potency to activate cytokines in transiently TLR2/CD14-transfected human endothelial kidney cells (HEK 293) or human macrophages and whole blood cells. Although LTA of type I and IV are structurally quite different, especially in their hydrophilic moiety, they originally were believed to interact with the same receptor (TLR2). Hence, the chemical syntheses leading to structurally defined, non-contaminated stimuli have a major impact on the outcome and interpretation of these biological studies of the innate immune system. With this material, it became evident that synthetic LTA from S. aureus and S. pneumoniae are not recognized by TLR2. Instead, another receptor of the innate immune system, the lectin pathway of the complement, known since many years to interact with LTA in quite a specific way, has gained increasing attractivity. With the help of synthetic LTA we obtained first evidences that this receptor is indeed the pathogen recognition receptor (PRR) for LTA.

Versatility of choline metabolism and choline-binding proteins in Streptococcus pneumoniae and commensal streptococci

The pneumococcal choline-containing teichoic acids are targeted by cholinebinding proteins (CBPs), major surface components implicated in the interaction with host cells and bacterial cell physiology. CBPs also occur in closely related commensal species, Streptococcus oralis and Streptococcus mitis, and many strains of these species contain choline in their cell wall. Physiologically relevant CBPs including cell wall lytic enzymes are highly conserved between Streptococcus pneumoniae and S. mitis. In contrast, the virulence-associated CBPs, CbpA, PspA and PcpA, are S. pneumoniae specific and are thus relevant for the characteristic properties of this species.

Acute post-streptococcal glomerulonephritis associated with prolonged hypocomplementaemia

The case of a 6-year-old boy who presented with acute post-streptococcal glomerulonephritis is reported. C3 levels and complement alternative pathway activity remained low for at least 10 months after presentation, before returning to normal. There was no evidence of other renal disease. This case highlights that hypocomplementaemia in acute post-streptococcal glomerulonephritis may persist for several months, and that prolonged hypocomplementaemia does not exclude this diagnosis.

A new model of pneumococcal lipoteichoic acid structure resolves biochemical, biosynthetic, and serologic inconsistencies of the current model

Lipoteichoic acid (LTA) is an essential bacterial membrane polysaccharide (cell wall component) that is attached to the membrane via a lipid anchor. According to the currently accepted structure of pneumococcal LTA, the polysaccharide is comprised of several repeating units, each of which starts with glucose and ends with ribitol, with the lipid anchor predicted to be Glc(beta1-->3)AATGal(beta1-->3)Glc(alpha1-->3)-acyl(2)Gro, where AATGal is 2-acetamido-4-amino-2,4,6-trideoxy-D-galactose. However, this lipid anchor has not been detected in pneumococcal membranes. Furthermore, the currently accepted structure does not explain the Forssman antigen properties of LTA and predicts a molecular weight for LTA that is larger than its actual observed molecular weight. To resolve these problems, we used mass spectrometry to analyze the structure of LTA isolated from several pneumococcal strains. Our study found that the R36A pneumococcal strain produces LTA that is more representative of pneumococci than that previously characterized from the R6 strain. Analysis of LTA fragments obtained after hydrofluoric acid and nitrous treatments showed that the fragments were consistent with an LTA nonreducing terminus consisting of GalNAc(alpha1-->3)GalNAc(beta1-->, which is the minimal structure for the Forssman antigen. Based on these data, we propose a revised model of LTA structure: its polysaccharide repeating unit begins with GalNAc and ends with AATGal, and its lipid anchor is Glc(alpha1-->3)-acyl(2)Gro, a common lipid anchor found in pneumococcal membranes. This new model accurately predicts the observed molecular weights. The revised model should facilitate investigation of the relationship between LTA's structure and its function.

Monoclonal antibodies can affect complement deposition on the capsule of the pathogenic fungus Cryptococcus neoformans by both classical pathway activation and steric hindrance

The capsule of the human pathogenic fungus Cryptococcus neoformans presents the immune system with a formidable problem for phagocytosis. Capsule-mediated activation of the alternative complement (C) pathway results in component 3 (particularly, C3) binding to the capsule near the cell wall surface. Hence, for cells with large capsule, C3 cannot interact with the complement receptor (CR) and is not opsonic. However, C activation in either immune serum or in the presence of monoclonal antibody (mAb) to capsular polysaccharide localizes C3 to the capsular edge. When C. neoformans cells were coated with both C and antibody (Ab) opsonins, Ab bound first and promoted C3 deposition at the edge of the capsule. The mechanism for the Ab-mediated change in C3 localization to the capsule edge involved both classical C pathway activation and steric hindrance preventing C3 penetration into the capsule. The change in C3 localization changed the mode of phagocytosis in macrophages, such that localizing C3 at the edge of the capsule allowed phagocytosis through C3-CR3 and C3-CR4 interactions, which did not occur in serum without Ab. These findings reveal a new mechanism of Ab action whereby Abs affect the location of C3 and its interaction with its receptor in macrophages depending on the immunoglobulin concentration.

Role of Tumor Necrosis Factor-α in Sensorineural Hearing Loss after Bacterial Meningitis

HYPOTHESIS

Blockade of tumor necrosis factor-alpha with tumor necrosis factor-alpha antibody will reduce the extent of cochlear injury and hearing loss associated with Streptococcus pneumoniae meningitis.

BACKGROUND

Inflammatory mediators play a significant role in the morbidity associated with bacterial meningitis, including hearing loss and labyrinthitis ossificans. Previous studies have shown the attenuation of hearing loss by the nonspecific blockade of such pathways.

METHODS

Fifty Mongolian gerbils were divided into four groups. Auditory brainstem response testing was conducted to measure hearing thresholds. Streptococcus pneumoniae meningitis was induced in Groups 1 and 2. Group 2 was then given a single intraperitoneal injection of tumor necrosis factor-alpha antibody, whereas Group 1 received phosphate-buffered saline. Uninfected animals in Groups 3 and 4 were implanted with osmotic pumps that delivered a continuous 8-day intrathecal flow of either tumor necrosis factor-alpha (Group 4) or phosphate-buffered saline (Group 3). After 6 weeks, auditory brainstem response testing was repeated. The cochleas were harvested and analyzed histomorphometrically.

RESULTS

Group 2 animals with Streptococcus pneumoniae meningitis that also received tumor necrosis factor-alpha antibody developed significantly less hearing loss than Group 1 animals with meningitis alone. The decrease in the average threshold at 4, 8, 16, and 32 kHz was 31, 30, 25, and 28 dB sound pressure level, respectively (p < 0.0092 for each). Furthermore, histomorphometric analysis showed significantly less damage to the organ of Corti, spiral ganglion, spiral ligament, and stria vascularis in Group 2. Conversely, tumor necrosis factor-alpha induced meningitis animals (Group 3) showed increased hearing loss compared with phosphate-buffered saline controls (Group 4), with p < 0.0001 at all frequencies.

CONCLUSION

Tumor necrosis factor-alpha plays an important role in cochlear injury after bacterial meningitis. Blockade of tumor necrosis factor-alpha reduces postmeningitic hearing loss and cochlear injury. Induction of meningitis with intrathecal tumor necrosis factor-alpha also resulted in hearing loss and cochlear injury similar to bacterial meningitis.

Immunoglobulin M-enriched intravenous immunoglobulin inhibits classical pathway complement activation, but not bactericidal activity of human serum

Acute or even hyperacute humoral graft rejection, mediated by classical pathway complement activation, occurs in allo- and xenotransplantation due to preformed anti-graft antibodies. Intravenous immunoglobulin (IVIg) preparations can prevent complement-mediated tissue injury and delay hyperacute xenograft rejection. It is known that IgM-enriched IVIg (IVIgM) has a higher capacity to block complement than IVIgG. Different IVIgs were therefore tested for specificity of complement inhibition and effect on anti-bacterial activity of human serum. IVIgM-I (Pentaglobin), 12% IgM), IVIgM-II (IgM-fraction of IVIgM-I, 60% IgM), and three different IVIgG (all >95% IgG) were used. The known complement inhibitor dextran sulfate was used as control. Hemolytic assays were performed to analyze pathway-specificity of complement inhibition. Effects of IVIg on complement deposition on pig cells and Escherichia coli were assessed by flow cytometry and cytotoxicity as well as bactericidal assays. Complement inhibition by IVIgM was specific for the classical pathway, with IC50 values of 0.8 mg/ml for IVIgM-II and 1.7 mg/ml for IVIgM-I in the CH50 assay. Only minimal inhibition of the lectin pathway was seen with IVIgM-II (IC50 15.5 mg/ml); no alternative pathway inhibition was observed. IVIgG did not inhibit complement in any hemolytic assay. Classical pathway complement inhibition by IVIgM was confirmed in an in vitro xenotransplantation model with PK15 cells. In contrast, IVIgM did not inhibit (mainly alternative pathway mediated) killing of E. coli by human serum. In conclusion, IgM-enriched IVIg is a specific inhibitor of the classical complement pathway, leaving the alternative pathway intact, which is an important natural anti-bacterial defense, especially for immunosuppressed patients.

Tissue reactions to a component of root canal system bacteria: lipoteichoic acid

Lipoteichoic acid (LTA), present in Gram-positive microorganisms, has physiochemical characteristics that allow it to act as an immunogen. Due to polymicrobial characteristics of root canal infections, LTA can participate in the development of periapical disease. The reaction of the rat subcutaneous tissue to Teflon tube implants, filled with Fibrinol soaked in lipoteichoic acid (concentration of 150 microg/ml), was observed. Lipoteichoic acid provoked an inflammatory tissue reaction.

Receptors, mediators, and mechanisms involved in bacterial sepsis and septic shock

Bacterial sepsis and septic shock result from the overproduction of inflammatory mediators as a consequence of the interaction of the immune system with bacteria and bacterial wall constituents in the body. Bacterial cell wall constituents such as lipopolysaccharide, peptidoglycans, and lipoteichoic acid are particularly responsible for the deleterious effects of bacteria. These constituents interact in the body with a large number of proteins and receptors, and this interaction determines the eventual inflammatory effect of the compounds. Within the circulation bacterial constituents interact with proteins such as plasma lipoproteins and lipopolysaccharide binding protein. The interaction of the bacterial constituents with receptors on the surface of mononuclear cells is mainly responsible for the induction of proinflammatory mediators by the bacterial constituents. The role of individual receptors such as the toll-like receptors and CD14 in the induction of proinflammatory cytokines and adhesion molecules is discussed in detail. In addition, the roles of a number of other receptors that bind bacterial compounds such as scavenger receptors and their modulating role in inflammation are described. Finally, the therapies for the treatment of bacterial sepsis and septic shock are discussed in relation to the action of the aforementioned receptors and proteins.

Activation of complement pathways in xenotransplantation: an in vitro study

Pig-to-human xenotransplantation faces the problem of hyperacute graft rejection due to the presence of human naturally occurring antibodies against the disaccharide Galalpha1-3Gal (anti-Gal antibodies) expressed on pig endothelium. Antibody-mediated complement activation is usually referred to as classical pathway activation. In this study we examined if the alternative complement pathway is also directly activated through anti-Gal antibodies or if the classical pathway is indispensable. We therefore developed a hemolysis test with rabbit erythrocytes (E), which have an activating surface for the alternative complement pathway and express abundant amounts of Galalpha1-3Gal, and used this assay in addition to the standard complement tests CH50 and AP50. In this rabbit E CH50 (RECH50) assay we were able to study activation of both major complement pathways simultaneously. FACS analysis was used to trace complement and antibody deposition on rabbit E. Anti-Gal depletion of human serum by immunoabsorption revealed a 65% reduction of rabbit E hemolysis in the RECH50 test (value before absorption: 28 +/- 5.8, after absorption: 9.9 +/- 2.8, P<0.001), but only a 35% reduction of lysis in the AP50 test (AP50 before 11.3 +/- 2.1, after 7.4 +/- 2.0, P<0.002). Repletion with purified anti-Gal fully restored hemolysis in both assays. Serum depleted of Clq showed a reduced lysis of rabbit E as compared to normal human serum; this effect increased with higher serum dilutions. The reciprocal picture, i.e. less effect on hemolysis with increasing dilution, was seen with factor D depleted serum. Comparison of the RECH50 values with the AP50 values revealed an 8.4-fold increase of lysis in the RECH50 test, in which both complement pathways are running. By FACS analysis, complement deposition on rabbit E was determined and components of the classical pathway were found, especially in sera where the alternative pathway was disrupted. We conclude that in our model anti-Gal induce lysis via both classical and alternative complement pathways, but that the alternative pathway activation is of minor importance. In addition, we saw that with higher serum dilutions, the classical pathway (i.e. anti-Gal-mediated lysis) takes a predominant role in lysing the rabbit E. As anti-Gal-mediated activation of the alternative complement cascade seems of minor importance based on our results, and as there are only few surfaces in transplanted organs that would favor the alternative pathway to be executed, the specific inhibition of early steps of the classical pathway appears as a realistic strategy in pig-to-primate xenotransplantation that-to the benefit of the patient-leaves the mainly anti-bacterial defense by the alternative pathway intact.

Induction of labyrinthitis ossificans after pneumococcal meningitis: an animal model

Newly formed disorganized bone fills the open spaces within the otic capsule in various pathologic conditions, resulting in labyrinthitis ossificans. The pathologic mechanisms of this disease remain poorly understood. To better study the sequence of events and contributing mechanisms involved in labyrinthitis ossificans, an animal model was developed. Three groups of Mongolian gerbils received either an intralabyrinthine injection of normal saline solution (group 1) or Streptococcus pneumoniae polysaccharide capsule antigens (groups 2 and 3). The temporal bones were harvested after 3 months and serially sectioned. None of the eight control animals (group 1), which received intralabyrinthine injections of normal saline solution had any histologic changes in their temporal bones. Nine of the surviving 19 animals in groups 2 and 3 had fibrosis or evidence of early ossification. A fourth group of Mongolian gerbils received two intrathecal injections of live S. pneumoniae organisms. The temporal bones were harvested after 3 months and serially sectioned. Fourteen of the surviving 15 animals had fibrosis or ossification or both. This animal model will provide a method for study of the mechanisms of labyrinthitis ossificans.

Small-angle X-ray scattering analysis of pneumococcal lipoteichoic acid phase structure

X-Ray-scattering analysis was performed on micelles of lipoteichoic acid from Streptococcus pneumoniae. This lipoteichoic acid differs from poly(glycerophosphate) and poly(glycosylglycerophosphate) lipoteichoic acids by a unique positively charged diacylglyceroglycolipid anchor and a complex structure of the hydrophilic chain which is composed of zwitterionic tetrasaccharide ribitol phosphate repeats each carrying two zwitterionic phosphocholine substituents. The size distribution of the lipoteichoic acid micelles was sufficiently homogenous to determine their size and some related molecular parameters by small-angle scattering analysis. Nearly independent on the ionic strength of the aqueous dispersion, an average micelle contained about 150 lipoteichoic acid molecules arranged in a spherical assembly with a diameter of about 23 nm, whereby the hydrophilic region occupied an outer shell of about 8.5 nm thickness. Taken this as the average chain length of LTA in the micelle and 7.2 repeats per chain, each repeat contributed about 1.2 nm to the thickness of the hydrophilic shell as compared to 2.4 nm for a fully extended chain conformation and 1.1 nm estimated for a more helical arrangement [Klein, R. A., Hartmann, R., Egge, H., Behr, T. & Fischer, W. (1994) Carbohydr. Res. 256, 189-222]. A comparison with the micelle of poly(glycerophosphate) lipoteichoic acid of Staphylococcus aureus suggests that the supramolecular structure is largely independent of the structure of the hydrophilic chain and solely dictated by the small cross-sectional area of the diacylglycerol moiety common to all lipoteichoic acids and lipoglycans of gram-positive bacteria.

Detection of C-polysaccharide in serum of patients with Streptococcus pneumoniae bacteraemia

AIM

To investigate the fate of Streptococcus pneumoniae C-polysaccharide antigen in serum in patients with S pneumoniae bacteraemia.

METHOD

In vitro dissociation experiments were performed to demonstrate that C-polysaccharide was masked by ligands in normal and acute phase serum. Serum samples from 22 patients with S pneumoniae bacteraemia were treated to dissociate immune complexes and then tested for C-polysaccharide by enzyme linked immunosorbent assay (ELISA).

RESULTS

C-polysaccharide antigen was masked in normal and acute phase serum but could be released by EDTA treatment and detected by ELISA. Antigen was found in six patients ranging in concentration from 2.5 to 200 ng/ml. Patients with detectable antigen were more likely to die than those in whom antigen was not detected.

CONCLUSION

This study demonstrates that C-polysaccharide antigen commonly circulates in patients with S pneumoniae bacteraemia but its presence is masked by ligands present in serum.

The aqueous solution structure of the tetrasaccharide-ribitol repeat-unit from the lipoteichoic acid of Streptococcus pneumoniae strain R6 determined using a combination of NMR spectroscopy and computer calculations

High-resolution 1D- and 2D-correlation 1H NMR and 13C NMR, at 500 and 125 MHz, respectively, permitted assignment of the majority of the resonances in the per-N-acetylated, phosphorylated tetrasaccharide-ribitol repeat-unit, and in the complete polymer (n = 5 - 7) containing between five and seven repeating units attached to the deacylated lipid anchor, for the lipoteichoic acid from Streptococcus pneumoniae strain R6; the 31P resonances were also assigned. Comparison of the 31P spectra obtained for the per-N-acetylated oligosaccharide and for the oligosaccharide having the AATG 4-NH2 group still free, indicate a conformational difference brought about by interaction between the amino group and the neighboring phosphate group.

In vitro activation of the classical pathway of complement by a streptococcal lipoteichoic acid

The purpose of this study was to find whether a glycerolphosphate-containing lipoteichoic acid prepared from Streptococcus sobrinus OMZ 176 cells would activate the classical pathway of complement while in solution. Reference activators were lipopolysaccharide from Escherichia coli 0111:B4 and heat-aggregated immunoglobulin G. Serum samples were taken from healthy students. Analysis through crossed immunoelectrophoresis showed that lipoteichoic acid caused an almost complete dissociation of the C1qrs macromolecule. All activators decreased the area of and slowed the electrophoretic mobility of the C4 protein peaks, with lipoteichoic acid causing the most pronounced alterations. Electroimmunoassays showed that lipoteichoic acid separately, yielded detectable amounts of free C1r2s2 subunits; it also generated significantly more trimer complexes between C1r, C1s and C1 inhibitor (C1INH) than did the other two activators. Lipoteichoic acid was, however, a comparatively weak inducer of tetramer C1INH-C1r-C1s-C1INH complexes. Analysis through Western blotting showed that all activators accelerated consumption of C1r, induced complex formations between C1INH and C1s and produced cleavage products of C2. Altogether, the immunochemical analysis gave clear evidence of classical pathway activation by lipoteichoic acid, but its activation profile differed from those seen with lipopolysaccharide and aggregated immunoglobulin G.

Teichoic acid and lipoteichoic acid of Streptococcus pneumoniae possess identical chain structures. A reinvestigation of teichoid acid (C polysaccharide)

Teichoic acid (C polysaccharide) was extracted and purified from Streptococcus pneumoniae R6 with standard procedures except that lipoteichoic acid was extracted first. The dephosphorylated repeating unit was isolated after hydrolysis with 48% (by mass) HF, the bis(phosphocholine)-containing repeating unit was isolated by alkali hydrolysis, anion-exchange chromatography and phosphomonoester cleavage. On the basis of compositional analysis, fast-atom-bombardment mass spectrometry and NMR spectroscopy the following structure is proposed: [formula: see text] where AATGal is 2-acetamido-4-amino-2,4,6-trideoxy-D-galactose. The repeating units are linked to each other by phosphodiester bonds between O5 of the ribitol and O6 of the glucopyranosyl residue of adjacent units. This chain structure is identical with that previously established for pneumococcal lipoteichoic acid [Behr, T., Fischer, W., Peter-Katalinić, J. & Egge, H. (1992) Eur. J. Biochem. 207, 1063-1075]. This represents a unique situation because in other Gram-positive bacteria teichoic and lipoteichoic acids are structurally unrelated.

Effects of a streptococcal lipoteichoic acid on complement activation in vitro

This study describes activation of serum complement by lipoteichoic acid (LTA) from Streptococcus mutans OMZ 176, while in solution. Serum from 16 healthy students was taken. Test samples were incubated with increasing doses (1-5,000 micrograms/ml) of LTA or lipopolysaccharide (LPS) from Escherichia coli 0111:B4 for 1 h at 37 degrees C; then assayed for degradation of C3, C4 or factor B by crossed immunoelectrophoresis. Each preparation caused a significant (p < 0.05) dose-dependent conversion of C3. The response curves obtained were not statistically different. LPS was a stronger activator of the alternative pathway than LTA, as judged from analysis of C3 degradation in the presence of Mg2+/EGTA, and from their effects on factor B cleavage. LTA caused, however, pronounced alterations in the shape of C4 precipitation in the gels. Functional (hemolytic) assays showed that, when tested at 200 micrograms/ml, LTA and LPS triggered significant (p < 0.05) consumptions of both classical and alternative pathway proteins. LPS was a significantly (p < 0.05) stronger activator than LTA. Apparently, the C3 degradation found for this LTA involved the alternative pathway to a small extent; thus some other mechanism of fluid-phase C3 cleavage seemed also to be operative.

Modification of resistance to Streptococcus pneumoniae by dietary ethanol, immunization, and murine retroviral infection

Hallmarks of the acquired immune deficiency syndrome (AIDS) are immunologic alterations, frequently associated with opportunistic infections. To study such associations, LP-BM5 murine retrovirus infection was used as a murine model of AIDS. Retrovirally infected and uninfected mice were fed a 5% (v/v) ethanol diet for 55 days and then fed a 7% v/v ethanol diet for the final 7 days to assert the role of ethanol as a cofactor in development of murine AIDS. There was a reduction in polymorphonuclear neutrophils count in ethanol-fed groups. Neutrophils increased in retrovirus-infected groups, except those vaccinated 10 days before challenge with live bacteria. The percentage of splenic lymphocytes in the retrovirus-infected group was reduced in comparison with controls. Survival of the mice challenged intraperitoneally with Streptococcus pneumoniae was increased by vaccination and suppressed by dietary alcohol. Retrovirus infection caused a much faster death rate after bacterial challenge than nonretrovirus infected controls. Vaccination played an important role in delaying the death rate in all treated groups. Transferring spleen cells from healthy, unimmunized mice also enabled the retrovirally infected mice to survive the bacterial infection longer. Enhancement of resistance to S. pneumoniae by vaccination and transfer of immunocompetent cells to mice immunosuppressed by retroviral infection show the potential to use immunomodulation to affect disease resistance in AIDS.

Macrophage response to bacteria: induction of marked secretory and cellular activities by lipoteichoic acids

Lipoteichoic acids (LTAs) from various bacterial species, including Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae, Enterococcus faecalis, and Listeria monocytogenes, were examined for the ability to induce secretory and cellular responses in a pure population of bone marrow-derived mononuclear phagocytes. Some of the highly purified LTAs, in particular LTAs from Bacillus subtilis, S. pyogenes, E. faecalis, and Enterococcus hirae, were able to affect each of the macrophage parameters measured, i.e., reductive capacity, secretion of tumor necrosis factor and nitrite, and tumoricidal activity. As after stimulation with whole organisms or other bacterial products, secretion of tumor necrosis factor induced by these LTAs reached its maximum within the first few hours of the interaction, while secretion of nitrite and tumoricidal activity required 24 to 36 h for full expression. Other purified LTAs, i.e., LTAs from Streptococcus sanguis, S. pneumoniae, and L. monocytogenes, as well as lipomannan from Micrococcus luteus affected only some of these parameters, while native LTA from S. aureus was inactive. There was no obvious correlation between biological activity and chain length, kind of glycosyl substituents, glycolipid structures, or fatty acid composition of LTAs. Deacylation of LTAs resulted in a complete loss of activity, and deacylated LTAs did not impair the activity of their acylated counterparts, suggesting that acyl chains may be essential for binding of LTA to the cell surface. The results demonstrate that some LTA species are potent inducers of macrophage secretory and cellular activities.

The structure of pneumococcal lipoteichoic acid. Improved preparation, chemical and mass spectrometric studies

Pneumococcal lipoteichoic acid was extracted and purified by a novel, quick and effective procedure. Structural analysis included methylation, periodate oxidation, Smith degradation, oxidation with CrO3, and fast-atom-bombardment mass spectrometry. Hydrolysis with 48% (by mass) HF and subsequent phase partition yielded the lipid anchor (I), the dephosphorylated repeating unit of the chain (II) and a cleavage product of the latter (III). The proposed structures are: (I) Glc(beta 1----3)AATGal(beta 1----3)Glc(alpha 1----3)acyl2Gro, (II) Glc(beta 1----3)AATGal(alpha 1----4)GalNAc(alpha 1----3)GalNAc(beta 1----1)ribitol and (III) Glc(beta 1----3)AATGal(alpha 1----4)GalNAc(alpha 1----3)GalNAc, where AATGal is 2-acetamido-4-amino-2,4,6-trideoxygalactose, and all sugars are in the pyranose form and belong to the D-series. Alkaline phosphodiester cleavage of lipoteichoic acid, followed by treatment with phosphomonoesterase, resulted in the formation of II and IV, with IV as the prevailing species: [sequence: see text] The linkage between the repeating units was established as phosphodiester bond between ribitol 5-phosphate and position 6 of the glucosyl residue of adjacent units. The chain was shown to be linked to the lipid anchor by a phosphodiester between its ribitol 5-phosphate terminus and position 6 of the non-reducing glucosyl terminus of I. The lipoteichoic acid is polydisperse: the chain length may vary between 2 and 8 repeating units and variations were also observed for the fatty acid composition of the diacylglycerol moiety. Preliminary results suggest that repeating units II and IV are enriched in separate molecular species. All species were associated with Forssman antigenicity, albeit to a various extent when related to the non-phosphocholine phosphorus. Owing to its unique structure, the described macroamphiphile may be classified as atypical lipoteichoic acid.

Heterogeneity of lipoteichoic acid detected by anion exchange chromatography

A complex polydispersity became apparent when the poly(glycerophosphate) lipoteichoic acid of Enterococcus faecalis was chromatographed on DEAE-Sephadex. The chain length varied between 13 and 33 glycerophosphate residues per lipid anchor. In parallel, the extent of chain glycosylation increased from 0.2 to 0.4 diglucosyl residues per glycerophosphate unit. Substitution with D-alanine ester showed a reverse distribution dropping with increasing chain length from 0.53 to 0.23 mol D-alanine per mol phosphorus. Variations in the fatty acid composition were also observed. The results extent and modify the current picture of lipoteichoic acid biosynthesis. They further suggest that during infection the mammalian organism may be confronted particularly with long-chain less hydrophobic molecular species.

Stimulation of monokine production by lipoteichoic acids

Lipoteichoic acids (LTAs) isolated from bacterial species, including Staphylococcus aureus, Streptococcus pyogenes A, Enterococcus faecalis, Streptococcus pneumoniae, and Listeria monocytogenes, were tested for their ability to stimulate the production of interleukin-1 beta (IL-1 beta), IL-6, and tumor necrosis factor alpha in cultured human monocytes. LTAs from S. aureus and S. pneumoniae failed to induce monokine production when applied in the concentration range of 0.05 to 5.0 micrograms/ml. However, LTAs from several enterococcal species (0.5 to 5 micrograms/ml) induced the release of all three monokines at levels similar to those observed after lipopolysaccharide stimulation. The kinetics of IL-1 beta and tumor necrosis factor alpha release elicited by LTAs closely resembled those observed following lipopolysaccharide application. Cytokine production occurred in the presence of both fetal calf serum and autologous human serum. Hence, it was not dependent on complement activation and could not be suppressed by naturally occurring human antibodies. Deacylation caused the total loss of monocyte stimulatory capacity. Deacylated LTAs were unable to prevent monocyte activation by intact LTAs, so primary binding of these molecules probably does not involve a simple interaction of a membrane receptor with the hydrophilic portion of the molecule. The results identify some species of LTAs as inducers of monokine production in human monocytes.

The ability to sensitize host cells for destruction by autologous complement is a general property of lipoteichoic acid

Previous studies have demonstrated that lipoteichoic acid (LTA) from Streptococcus pneumoniae binds to erythrocytes and renders them susceptible to lysis by autologous complement. The present study was performed to determine whether LTA from two other gram-positive bacterial species had the ability to render mammalian cells susceptible to lysis by autologous complement. Human erythrocytes were sensitized with LTA from S. pneumoniae, Streptococcus pyogenes, or Lactobacillus fermentum. Under incubation in normal autologous serum, lysis was observed with each of the LTA-sensitized erythrocyte preparations. When erythrocytes from a C2-deficient patient were sensitized with the LTA preparations and then incubated in autologous, C2-deficient serum, the erythrocytes sensitized with S. pyogenes or L. fermentum LTA demonstrated relatively little lysis, whereas the erythrocytes sensitized with S. pneumoniae LTA yielded near-total lysis. After reconstitution of the C2-deficient serum with purified human C2, lysis was observed with all three LTA preparations. When erythrocytes from an agammaglobulinemic patient were sensitized with either the S. pyogenes or the L. fermentum LTA, they were not lysed in the presence of autologous agammaglobulinemic serum, whereas the erythrocytes sensitized with S. pneumoniae LTA were completely lysed. Serum obtained from the agammaglobulinemic patient after reconstitution with intravenous pooled gamma globulin was able to lyse autologous erythrocytes sensitized with each of the three LTA preparations. These results demonstrate that the ability to render host cells susceptible to lysis by autologous complement is a general property of LTA. Whether activation of the autologous complement occurs by the classical or alternative pathways and whether it is antibody dependent depends on the nature of the bacterial LTA.

Interaction of purified lipoteichoic acid with the classical complement pathway

Glycerophosphate-containing lipoteichoic acids (LTAs) interact with the first component of the classical complement pathway (C1). This resulted in the activation of the classical complement pathway in serum, shown by the consumption of C1, C2, and C4. The dose-dependent interaction of LTAs with purified C1 and C1q was dependent on the negative charges of the phosphate groups of LTA. It was reduced by charge compensation through D-alanine ester substituents and by sterical hindrance through di- and trihexosyl residues linked to position 2 of the glycerol moieties. The charge density of LTA may also play a role: poly(digalactosylglycerophosphate) LTAs, in which the phosphate groups are in a greater distance from each other, were less effective, and the loss of micellar organization by deacylation of LTA drastically reduced the complement activation capacity.

Bacterial lipoteichoic acid sensitizes host cells for destruction by autologous complement

Lipoteichoic acids (LTA) released by gram-positive bacteria can spontaneously bind to mammalian cell surfaces. In the present study, erythrocytes (E) sensitized with pneumococcal LTA (LTA-E) were used as a model system to determine if LTA could render host cells susceptible to damage by autologous complement. Complement (C)-mediated lysis of LTA-E from normal rats and normal humans occurred when these cells were incubated in their respective autologous sera in vitro. In addition, when LTA-E from a C2-deficient human and from C4-deficient guinea pigs were incubated in their autologous sera, there was significant lysis in vitro, demonstrating a role for the alternative pathway. The in vivo survival of 51Cr-labeled autologous LTA-E was also studied. Only 2.9% of autologous LTA-E remained in the circulation of normal rats after 90 min. In contrast, 31.2% of autologous LTA-E remained in the circulation of rats depleted of C3. Intravascular hemolysis accounted for the clearance of LTA-E in the normal rats, whereas liver sequestration was responsible for clearance in the C3-depleted rats. These results demonstrate that LTA can render the host's cells susceptible to damage by its own complement system, establishing this as a possible mechanism of tissue damage in natural bacterial infections.