Antibodies to O-antigen of lipopolysaccharide are protective against neonatal infection with Escherichia coli K1

Intrinsic B cell TLR-BCR linked coengagement induces class-switched, hypermutated, neutralizing antibody responses in absence of T cells

Maturation of antibody responses entails somatic hypermutation (SHM), class-switch DNA recombination (CSR), plasma cell differentiation, and generation of memory B cells, and it is thought to require T cell help. We showed that B cell Toll-like receptor 4 (TLR4)-B cell receptor (BCR) (receptor for antigen) coengagement by 4-hydroxy-3-nitrophenyl acetyl (NP)-lipopolysaccharide (LPS) (Escherichia coli lipid A polysaccharide O-antigen) or TLR5-BCR coengagement by Salmonella flagellin induces mature antibody responses to NP and flagellin in Tcrβ-/-Tcrδ-/- and NSG/B mice. TLR-BCR coengagement required linkage of TLR and BCR ligands, "linked coengagement." This induced B cell CSR/SHM, germinal center-like differentiation, clonal expansion, intraconal diversification, plasma cell differentiation, and an anamnestic antibody response. In Tcrβ-/-Tcrδ-/- mice, linked coengagement of TLR4-BCR by LPS or TLR5-BCR by flagellin induced protective antibodies against E. coli or Salmonella Typhimurium. Our findings unveiled a critical role of B cell TLRs in inducing neutralizing antibody responses, including those to microbial pathogens, without T cell help.

Immunogenicity and safety of a tetravalent E. coli O-antigen bioconjugate vaccine in animal models

BACKGROUND

Extra-intestinal pathogenic Escherichia coli (ExPEC) are major human pathogens; however, no protective vaccine is currently available. We assessed in animal models the immunogenicity and safety of a 4-valent E. coli conjugate vaccine (ExPEC-4V, serotypes O1, O2, O6 and O25 conjugated to Exotoxin A from Pseudomonas aeruginosa (EPA)) produced using a novel in vivo bioconjugation method.

METHODS

Three doses of ExPEC-4V (with or without aluminum hydroxide) were administered to rabbits (2μg or 20μg per O-antigen, subcutaneously), mice (0.2μg or 2μg per O-antigen, subcutaneously) and rats (0.4μg or 4μg per O-antigen, intramuscularly). Antibody persistence and boostability were evaluated in rats using O6-EPA monovalent conjugate (0.4μg O-antigen/dose, intramuscularly). Toxicity was assessed in rats (16μg total polysaccharide, intramuscularly). Serum IgG and IgM antibodies were measured by ELISA.

RESULTS

Robust antigen-specific IgG responses were observed in all animal models, with increased responses in rabbits when administered with adjuvant. O antigen-specific antibody responses persisted up to 168days post-priming. Booster immunization induced a rapid recall response. Toxicity of ExPEC-4V when administered to rats was considered to be at the no observed adverse effect level.

CONCLUSIONS

ExPEC-4V conjugate vaccine showed good immunogenicity and tolerability in animal models supporting progression to clinical evaluation.

Sepsis: multiple abnormalities, heterogeneous responses, and evolving understanding

Sepsis represents the host's systemic inflammatory response to a severe infection. It causes substantial human morbidity resulting in hundreds of thousands of deaths each year. Despite decades of intense research, the basic mechanisms still remain elusive. In either experimental animal models of sepsis or human patients, there are substantial physiological changes, many of which may result in subsequent organ injury. Variations in age, gender, and medical comorbidities including diabetes and renal failure create additional complexity that influence the outcomes in septic patients. Specific system-based alterations, such as the coagulopathy observed in sepsis, offer both potential insight and possible therapeutic targets. Intracellular stress induces changes in the endoplasmic reticulum yielding misfolded proteins that contribute to the underlying pathophysiological changes. With these multiple changes it is difficult to precisely classify an individual's response in sepsis as proinflammatory or immunosuppressed. This heterogeneity also may explain why most therapeutic interventions have not improved survival. Given the complexity of sepsis, biomarkers and mathematical models offer potential guidance once they have been carefully validated. This review discusses each of these important factors to provide a framework for understanding the complex and current challenges of managing the septic patient. Clinical trial failures and the therapeutic interventions that have proven successful are also discussed.

Identification of protective and broadly conserved vaccine antigens from the genome of extraintestinal pathogenic Escherichia coli

Extraintestinal pathogenic Escherichia coli (ExPEC) are a common cause of disease in both mammals and birds. A vaccine to prevent such infections would be desirable given the increasing antibiotic resistance of these bacteria. We have determined the genome sequence of ExPEC IHE3034 (ST95) isolated from a case of neonatal meningitis and compared this to available genome sequences of other ExPEC strains and a few nonpathogenic E. coli. We found 19 genomic islands present in the genome of IHE3034, which are absent in the nonpathogenic E. coli isolates. By using subtractive reverse vaccinology we identified 230 antigens present in ExPEC but absent (or present with low similarity) in nonpathogenic strains. Nine antigens were protective in a mouse challenge model. Some of them were also present in other pathogenic non-ExPEC strains, suggesting that a broadly protective E. coli vaccine may be possible. The gene encoding the most protective antigen was detected in most of the E. coli isolates, highly conserved in sequence and found to be exported by a type II secretion system which seems to be nonfunctional in nonpathogenic strains.

Enhanced protection by use of a combination of anticapsule and antilipopolysaccharide monoclonal antibodies against lethal Escherichia coli O18K5 infection of mice

To study antibody-mediated protection against Escherichia coli peritonitis in BALB/c mice, monoclonal antibodies (MAbs) were generated against the capsule (K5) and the lipopolysaccharide (O18) of E. coli. Flow cytometric analysis with two selected immunoglobulin M MAbs revealed that bacteria were antigenically heterogeneous. Arbitrarily, three subpopulations in E. coli O18K5 cultures could be distinguished by double immunofluorescence. A subpopulation bound only the anti-K5 MAb, and another subpopulation bound only the anti-O18 MAb. An intermediate subpopulation, however, bound both MAbs. In agreement with this result, combinations of both MAbs enhanced phagocytosis of fluorescein isothiocyanate-labeled bacteria by human polymorphonuclear leukocytes and mouse macrophage J774 cells as well. In protection experiments, combinations of both MAbs, preincubated with 3 50% lethal doses of E. coli O18K5, protected all mice upon intraperitoneal challenge. Relatively high doses of either MAb alone proved to be not fully protective in this infection model. Protection of mice by the combination of MAbs was associated with significantly lower (P < 0.02) tumor necrosis factor levels in serum 90 min after challenge compared with any other treatment group. Similarly, prophylactic administration of MAbs yielded significantly lower (P < 0.01) tumor necrosis factor levels in mice that received the combination of MAbs than in any other treatment group.

Comparative immunogenicity of conjugates composed of Escherichia coli O111 O-specific polysaccharide, prepared by treatment with acetic acid or hydrazine, bound to tetanus toxoid by two synthetic schemes

Escherichia coli O111, of various H types and virulence factors, causes enteritis throughout the world, especially in young children. This O type is found rarely in healthy individuals. Serum antibodies to the O-specific polysaccharide of O111 lipopolysaccharide (LPS) protect mice and dogs against infection with this E. coli serotype. The O111 O-specific polysaccharide is composed of a pentasaccharide repeat unit with two colitoses bound to the C-3 and C-6 of glucose in a trisaccharide backbone; this structure is identical to that of Salmonella adelaide (O35), another enteric pathogen. Nonpyrogenic O111 O-specific polysaccharide was prepared by treatment of its LPS with acetic acid (O-SP) or the organic base hydrazine (DeA-LPS). The O-SP had a reduced concentration of colitose. These products were derivatized with adipic acid dihydrazide (ADH) or thiolated with N-succinimidyl-3(2-pyridyldithio) propionate (SPDP). The four derivatives were covalently bound to tetanus toxoid (TT) by carbodiimide-mediated condensation or with SPDP to form conjugates. Immunization of BALB/c and general-purpose mice by a clinically acceptable route showed that DeA-LPS-TTADH, of the four conjugates, elicited the highest level of LPS antibodies. Possible reasons to explain this differential immunogenicity between the four conjugates are discussed.

Influence of some bacterial and host factors on colonization and invasiveness of Escherichia coli K1 in neonatal rats

Of 209 healthy infants examined, 44 (21.1%) carried Escherichia coli K1 in their feces. Of these 44 isolates, 36 (81.8%) were attributed to 10 different known clonal groups of E. coli K1 and 4 isolates represented unknown types. The influence of mannose-resistant (MR) adhesins, aerobactin production, and resistance to serum on colonization and invasiveness of E. coli K1 in orally infected inbred LEW baby rats was investigated. Strains expressing MR adhesins had significantly higher colonization and invasion rates than non-MR strains did. Mixed-infection experiments of LEW rats revealed interactions between different types of E. coli K1 strains affecting colonization and invasion rats. P-fimbriated strains appeared to have a selective advantage for colonization. The bacteremic potentials of different E. coli K1 strains could not be associated with their resistance to sera from LEW rats free of members of the family Enterobacteriaceae. No differences in virulence between fecal E. coli K1 isolates and clinical isolates from diseased humans were found. An influence of the major histocompatibility complex on host susceptibility to invasive E. coli K1 was indicated by comparing the parental LEW rat strain with different congenic LEW strains (RT1).

Estimation of protective levels of anti-O-specific lipopolysaccharide immunoglobulin G antibody against experimental Escherichia coli infection

Serum obtained after immunization with an O18 polysaccharide-toxin A conjugate vaccine was evaluated for the estimation of protective levels of anti-O-specific lipopolysaccharide (LPS) immunoglobulin G (IgG) antibody against bacteremia and death caused by a homologous serotype of Escherichia coli K1 strains. Passive transfer of rabbit serum conferred significant protection from a lethal E. coli infection in a neonatal rat model. The overall incidence of bacteremia and mortality was 4% in rat pups receiving undiluted postvaccination serum, while that in control animals was 100% (P < 0.001). The overall incidences of bacteremia were 5 and 72% for animals with serum anti-O18 LPS IgG concentrations of > 1.0 and < 1.0 microgram/ml, respectively, while the overall incidences of mortality for animals with serum anti-O18 LPS IgG levels of > 1.0 and < 1.0 microgram/ml were 0 and 72%, respectively (P < 0.001). Protection against E. coli infection was also demonstrated with human anti-O18 polysaccharide IgG. None of the animals with human anti-O18 LPS IgG levels of > 1 microgram/ml had bacteremia after bacterial challenge, whereas all animals with bacteremia at 18 h had levels of < 1 microgram/ml. These findings suggest that serum anti-O18 LPS IgG concentrations of > 1.0 microgram/ml may provide protection against bacteremia and death caused by a homologous E. coli K1 infection.

Historic aspects of intravenous immunoglobulin therapy

The earliest preparations of immunoglobulins (Ig) decreased the susceptibility of agammaglobulinemic patients to infections caused by pneumococci, Haemophilus influenzae, meningococci, streptococci, and Pseudomonas aeruginosa. Intramuscular administration of such preparations was painful and traumatic, especially for children. Ethanol-fractionated Ig could not be administered intravenously (IV) because the IgG molecules tended to aggregate and thus were more likely to produce anaphylactoid reactions. New Ig preparations, isolated at low pH (e.g., pH 4) in the presence of traces of pepsin to inhibit reaggregation, were well tolerated when administered IV. Thus a new era of treatment and prophylaxis of disease using IV Ig (IVIG) was launched. The IVIG preparations revolutionized the management of virtually all immunodeficiency syndromes characterized by failure of antibody responses. Amelioration of antibody deficiency secondary to certain chronic diseases or surgical trauma can be achieved with these preparations. Newer uses of IVIG include treatment of some autoimmune diseases; in some conditions, the beneficial influences may be attributable to antiidiotype antibodies present in the IVIG. Another likely explanation is that IVIG inhibits damage to cells and tissues by antibody-mediated cellular cytotoxicity or blocks phagocytosis that is facilitated by Fc receptor mechanisms. The value of IVIG in preventing infection in patients undergoing bone marrow or organ transplantation and in the treatment and prophylaxis of life-threatening infections in neonates and premature infants also is reviewed.

Use of hybridoma immunoglobulin switch variants in the analysis of the protective properties of anti-lipopolysaccharide antibodies in Escherichia coli K1 infection

Functional properties of rat immunoglobulins obtained from hybridoma isotype switch variants were studied in vivo in a rat model for neonatal bacterial sepsis. Escherichia coli 018:K1, a common cause of human neonatal sepsis and meningitis, was injected intravenously into 6-day-old rats after incubation with 018-specific antibodies IgM, IgG1, IgG2a, IgG2b, IgG2c, IgE and IgA. The clearance of bacteria treated with saline or IgE was low, whereas monoclonal antibodies of other isotypes triggered hepatic sequestration and killing of the K1 E. coli cells. All four IgG subclasses were more efficient than IgM and IgA. Comparable results were obtained upon injecting antibodies into rats with an established fulminating bacteraemia. IgM was inactive in animals depleted of complement with cobra-venom factor (CVF), whereas IgG2b was able to trigger hepatic clearance independently of complement.

Isolation of rat IgM to IgG hybridoma isotype switch variants and analysis of the efficiency of rat Ig in complement activation

Sequential sublining was used in combination with enzyme-linked immunosorbent assays to isolate mu----gamma isotype switch variants of the rat IgM secreting mouse-rat B cell hybridoma line BA1.8. Switch variants to all four subclasses of IgG were obtained. The variant antibodies retained the antigen specificity of the parental IgM for the O18 (lipopolysaccharide) antigen of Escherichia coli. In sodium dodecyl sulfate-polyacrylamide gels the apparent molecular mass of the gamma heavy chains decreased in the order gamma 2b greater than gamma 1 greater than gamma 2c greater than gamma 2a. IgM, IgG1, IgG2a, IgG2b and IgG2c of the BA1.8 variant family and IgG2b, IgE and IgA of the previously described BA1.2 family were used for a comparative analysis of the capacity of rat Ig to activate complement. Efficient lysis of sheep erythrocytes coated with the O18 antigen was observed with IgM and all IgG subclasses, but no lysis was triggered by IgE or IgA. One hundred to 1000 IgG molecules were required to mediate the same hemolytic activity as one IgM molecule. The four IgG subclasses were equally efficient at mediating lysis by rat or human complement, while IgG2a was less efficient with guinea pig complement than the other three IgG subclasses. Antibody-triggered binding of C3 to pathogenic O18:K1 E. coli bacteria was measured using serum containing 125I-labeled C3. K1-encapsulated strains did not fix C3 efficiently in the absence of specific antibodies while acapsular mutants fixed C3 via the alternative pathway. IgM and all IgG subclasses triggered C3 binding to the K1 encapsulated bacteria. The capacity of IgM to mediate C3 fixation was not greater than that observed with IgG.

Host factors in the resistance of newborn mice to K1 Escherichia coli infection

It is not clear which factors are responsible for the deficient resistance of human neonates to K1 E. coli sepsis and meningitis. To evaluate the relative importance of different defense mechanisms against bacterial invasion, we have analyzed the sensitivity of newborn mice with known immune deficiencies to infection after oral challenge with virulent K1 E. coli. T and B lymphocyte and complement (C5) defects had no significant effect on natural resistance. In contrast, both endotoxin-hyporesponsive mouse strains tested were highly sensitive. This susceptibility to infection was strongly age dependent. Infant endotoxin-hyporesponsive mice were killed by i.p. injection of less than ten virulent K1 E. coli cells. In contrast, endotoxin-responsive animals and F1 hybrids derived from crosses between endotoxin-responsive and hyporesponsive mice survived an injection with up to 10(4) bacteria. Mutants of a virulent 018:K1 E. coli strain defective in the synthesis of the capsular polysaccharide or the O-antigen of lipopolysaccharide were avirulent as were 01:K1 bacteria, which are under-represented among E. coli isolates from neonatal meningitis. Endotoxin-hyporesponsive mice were protected from lethal bacterial challenge by monoclonal IgG specific for the O-antigen of the challenge strain or by human recombinant interleukin 1. A fulminant bacterial multiplication in the bloodstream of endotoxin-hyporesponsive mice was observed after i.v. injection of 100 virulent K1 E. coli cells. Persistent bacteremia with 10(5) to 10(6) bacteria per ml of blood resulted in death of the animals one to two days after challenge. In the bloodstream of endotoxin-responsive mice the bacteria proliferated to a comparable extent within the first 6 h after challenge. Thereafter they were rapidly cleared from the circulation and the animals recovered from the infection.

Unexpected effects of absorbed normal rabbit serum and bovine serum albumin on survival of Haemophilus influenzae type b in the infant rat

In the course of using the infant rat model to determine the ability of various rabbit antisera to protect against challenge by Haemophilus influenzae type b we made two unexpected observations. In these experiments 4-day-old rats were inoculated s.c. on the dorsum with either rabbit serum or physiological buffers (sham serum) and then were challenged the next day with H. influenzae type b injected i.p. Bacteremia, as a marker for disease, was measured 24 h later on day 6. We observed the following. (i) Pre-immune, i.e., normal rabbit serum, containing minimal levels of antibodies to outer membrane proteins and depleted of antibodies to capsule and lipopolysaccharide, nevertheless significantly (P less than 0.01) protected the rats from challenge with H. influenzae type b when compared to a sham inoculation of buffer; (ii) In the absence of a serum inoculation on day 4 (a buffer was used as a sham serum inoculation), the levels of bacteremia obtained after inoculation with bacteria on day 5 depended upon the composition of the buffer in which the H. influenzae inoculum was suspended. Use of phosphate buffered saline (PBS) resulted in higher levels of bacteremia than PBS containing 0.5% bovine serum albumin (PBS-BSA) (P less than 0.001), i.e. the BSA apparently acted to protect the rats from H. influenzae infection. In fact the use of PBS-BSA as an inoculum buffer masked the protective effect noted above of the absorbed normal rabbit serum.

Mechanisms of specific immunological unresponsiveness to bacterial lipopolysaccharides

Low-dose priming of mice with Escherichia coli O113 lipopolysaccharide (LPS) results in the development of immunological memory, whereas low-dose priming with E. coli O55 LPS or Serratia marcescens LPS induces significant antigen-specific unresponsiveness. All three preparations of LPS induced proliferation of mouse splenocytes with similar time course and [3H]thymidine uptake. There was no correlation between the small amounts of serum antibody detected by enzyme-linked immunosorbent assay after low-dose priming and the subsequent generation of either memory or unresponsiveness. Further, the passive transfer of small amounts of LPS-specific antibody had no significant effect on the magnitude of the plaque-forming cell (PFC) response elicited after subsequent immunization. Reduction of the PFC response to E. coli O55 LPS occurred after low-dose priming of nu/nu (as well as nu/+) mice; however, unresponsiveness could not be generated in nu/nu mice by low-dose priming with S. marcescens LPS. Thus, although the development of low-dose unresponsiveness to S. marcescens LPS appears to involve T cells, the response of E. coli O55 LPS does not. Enhancement of the primary PFC response to S. marcescens LPS could be transferred with low-dose primed spleen cells depleted of Lyt-2+ T cells; this suggests that the magnitude of the PFC response to this preparation of LPS is negatively influenced by Lyt-2+ T cells and positively influenced by Lyt-2- spleen cells (i.e., L3T4+ T cells). These findings indicate that T cells appear to be involved in regulating the magnitude of the antibody response to some types of bacterial LPS.

Protective ability and specificity of convalescent serum from calves with Haemophilus somnus pneumonia

The ability of convalescent serum to passively protect calves against Haemophilus somnus-induced pneumonia was studied. Preimmune and convalescent serum were obtained from calves before or after recovery from experimental chronic H. somnus pneumonia. Passive protection was assessed in another group of calves by intrabronchial inoculation of H. somnus that had been incubated with preimmune or convalescent serum. Each calf was inoculated with each treatment in alternating caudal lung lobes. Twenty-four hours after inoculation almost no pneumonia was present in lungs inoculated with bacteria incubated with convalescent serum, whereas severe pneumonia was present in lungs inoculated with bacteria incubated with preimmune serum. Quantitation of calf pneumonia in both treatment groups indicated a significantly different protective capacity between convalescent serum and preimmune serum (P less than 0.0005). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by Western blotting of purified H. somnus lipopolysaccharide resulted in intense reactivity with convalescent serum, but no reactivity was detected with preimmune serum. After sodium dodecyl sulfate-polyacrylamide gel electrophoresis of H. somnus outer membrane-enriched fractions, Western blots with convalescent serum gave intense reactions against H. somnus outer membrane antigens with apparent molecular masses of 78 and 40 kilodaltons and weaker reactions with 60-, 34-, 31-, 29-, 18-, and 15-kilodalton outer membrane antigens. No reactivity was detected with preimmune serum. Antibodies eluted from H. somnus after adsorption of convalescent serum reacted almost identically to unadsorbed convalescent serum in Western blots against bacterial outer membrane-enriched fractions. Thus, most of the antigens recognized by convalescent serum are likely to be on the bacterial surface and accessible to antibody. Surface antigens recognized by protective convalescent serum are candidate antigens for a subunit vaccine against H. somnus pneumonia.

Isolation of rat immunoglobulin class switch variants of rat-mouse hybridomas by enzyme-linked immunosorbent assay and sequential sublining

The use of sequential sublining in combination with highly specific and sensitive enzyme-linked immunosorbent assays for the isolation of spontaneous rat Ig heavy chain class switch variants is described. These methods allowed us to isolate switch variants from mouse-rat hybridoma lines secreting monoclonal rat antibodies. Switch variants from IgM to IgG2a, from IgG2a or IgG2b to IgE and from IgE to IgA were obtained. Members of the BA1.2 family, which consists of IgG2b, IgE and IgA antibodies are shown to exhibit identical rhamnose-inhibitable binding to the O18A antigen of Escherichia coli and to the paratope-associated anti-idiotypic antibody BA114.

Monoclonal antibodies reactive with K1-encapsulated Escherichia coli lipopolysaccharide are opsonic and protect mice against lethal challenge

Seven murine monoclonal antibodies (MAbs) directed against O-side-chain determinants of the K1-encapsulated Bortolussi strain of Escherichia coli (O18:K1:H7) were evaluated for their in vitro and in vivo activities. All the MAbs reacted well in Western blots against E. coli O18 lipopolysaccharide antigens. Two MAbs of the immunoglobulin G (IgG) class promoted in vitro opsonophagocytosis and protected mice lethally challenged with bacteria. Two IgM MAbs showed partial protection, although they had no in vitro opsonic activity, and the remaining three IgM MAbs showed no apparent functional activities. Monoclonal IgG antibodies against bacterial lipopolysaccharide can be opsonic and protective in spite of the presence of the K1 capsule on the bacterium.

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and monoclonal antibodies as tools for the subgrouping of Escherichia coli lipopolysaccharide O18 and O23 antigens

The lipopolysaccharide (LPS) of Escherichia coli O18 isolated from a wide variety of sources was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Four different LPS types, designated O18A, O18A1, O18B, and O18B1, were identified. Most O18 strains possess O18A, O18A1, or O18B LPS types, and these types are clonally associated. A reference test strain with the classical O18ab designation possessed O18B LPS, while two reference O18ac strains possessed O18A and O18A1 LPS, respectively. A panel of 15 anti-O18A B-cell hybridomas was isolated. Enzyme-linked immunosorbent assays revealed that some of the monoclonal antibodies produced by these cells recognize different epitopes. Four of these antibodies suffice to distinguish the four O18 types. Numerous strains whose LPS had been typed by SDS-PAGE were tested by agglutination with seven monoclonal antibodies whose specificities had been determined by enzyme-linked immunosorbent assays. The results indicated a perfect correlation between the two methods. Rabbit antisera raised against O18A bacteria agglutinated boiled bacteria of each of the O18 LPS types efficiently. The antisera were adsorbed with bacteria possessing each of the LPS types. The adsorbed sera only distinguished between two groups: O18A and O18A1 versus O18B and O18B1, as shown by agglutination assays and Western blotting. E. coli O4 and O23 and Serratia marcescens O8 antigens, which are reputed to cross-react with O18, were also analyzed. One O4, one O8, and four O23 strains were tested. All made an LPS which was distinguishable from O18 LPS types by SDS-PAGE. Each O23 strain synthesized a different LPS, and three of them synthesized only few short chains. Some of the monoclonal antibodies reacted with O4, O8, and O23A LPSs. The results are interpreted as indicating that numerous E. coli O serogroups will prove to be chemically heterogeneous and that future analyses of subgroup heterogeneity should be guided by results from SDS-PAGE and rely preferentially on monoclonal antibodies as opposed to rabbit hyperimmune sera.