Complement activation by polyclonal immunoglobulin G1 and G2 antibodies against Staphylococcus aureus, Haemophilus influenzae type b, and tetanus toxoid

Vaccination with LytA, LytC, or Pce of Streptococcus pneumoniae Protects against Sepsis by Inducing IgGs That Activate the Complement System

The emergence of non-vaccine serotypes of Streptococcus pneumoniae after the use of vaccines based in capsular polysaccharides demonstrates the need of a broader protection vaccine based in protein antigens and widely conserved. In this study, we characterized three important virulence factors of S. pneumoniae namely LytA, LytC, and Pce as vaccine candidates. These proteins are choline-binding proteins that belong to the cell wall hydrolases’ family. Immunization of mice with LytA, LytC, or Pce induced high titers of immunoglobulins G (IgGs) of different subclasses, with IgG1, IgG2a, and IgG2b as the predominant immunoglobulins raised. These antibodies activated the classical pathway of the complement system by increasing the recognition of C1q on the surface of pneumococcal strains of different serotypes. Consequently, the key complement component C3 recognized more efficiently these strains in the presence of specific antibodies elicited by these proteins, activating, therefore, the phagocytosis. Finally, a mouse sepsis model of infection was established, confirming that vaccination with these proteins controlled bacterial replication in the bloodstream, increasing the survival rate. Overall, these results demonstrate that LytA, LytC, and Pce can be protein antigens to be contained in a future universal vaccine against S. pneumoniae.

Igg Subclasses in Capd Patients

Objective

To make a comparison of serum levels of immunoglobulin G (lgG) subclasses in adult continuous ambulatory peritoneal dialysis (CAPD) patients with those in age-and sex-matched hemodialysis patients and healthy volunteers, and to analyze the contribution of removal of these proteins in peritoneal effluent to their plasma values.

Design

A cross-sectional study.

Setting

A renal unit of a university hospital.

Patients

Twenty-three CAPD patients, 21 hemodialysis patients, and 21 healthy volunteers. Peritoneal transport studies were done in 8 of the 23 CAPD patients.

Methods

IgG subclasses were measured in serum by nephelometry. For the peritoneal transport studies an ELISA method on ethylenediamine tetracetic acid plasma was used. The same method was used in seven-to ten-fold concentrated peritoneal dialysate.

Results

CAPD patients had lower IgG2 and IgG4 1evels than hemodialysis patients and healthy volunteers (p < 0.01). lgG2 values below 1.5 glL were present in 43% of the CAPD patients (p < 0.001 compared to healthy volunteers). Peritonitis incidence was not different between CAPD patients with low or normal IgG2 plasma levels. Peritoneal clearance of IgG3 was lower than that of the other subclasses. Evidence was obtained for a depressed synthesis of IgG2 and IgG4 in CAPD patients. The hypothesis that interleukin-2 may be involved in the low synthesis rate of IgG2 is discussed.

Conclusion

Low serum IgG2 and IgG4 1evels are present in stable, adult CAPD patients. These were not caused by increased peritonealloss, but by decreased synthesis.

Novel Intervention in the Aging Population: A Primary Meningococcal Vaccine Inducing Protective IgM Responses in Middle-Aged Adults

Introduction

Vaccine responses are often reduced in the elderly, leaving part of the elderly population vulnerable to infectious diseases. Timely vaccination may offer a solution for strengthening memory immunity before reaching old age, which classifies middle-aged persons as a target age group for vaccine interventions. However, knowledge regarding the immunogenicity of primary immunizations in middle-aged adults is lacking. We determined the immunogenicity of a primary meningococcal vaccine towards which no or (very) low pre-vaccination immunity exists in middle-aged adults (NTR4636).

Methods

A vaccine containing multiple meningococcal groups (tetravalent) conjugated to tetanus toxoid (MenACWY-TT) was administered to middle-aged adults (50–65 years of age, N = 204) in a phase IV single-center and open-label study. Blood samples were taken pre-, 7 days, 28 days, and 1 year post-vaccination. Functional antibody titers were measured with the serum bactericidal assay (SBA). Meningococcal- and tetanus-specific antibody responses were determined with a fluorescent bead-based multiplex immunoassay. A bi-exponential decay model was used to estimate long-term protection.

Results

In the majority of the participants, the meningococcal vaccine clearly induced naïve responses to meningococci W (MenW) and meningococci Y (MenY) as compared to a booster response to meningococci C (MenC). After 28 days, 94, 99, and 97% of the participants possessed a protective SBA titer for MenC, MenW, and MenY, respectively, which was maintained in 76, 94, and 86% 1 year post-vaccination. At this 1-year time point, significantly lower SBA titers were found in participants without a pre-vaccination SBA titer. Overall, protective antibody titers were predicted to persist after 10 years in 40–60% of the participants. The SBA titers correlated well with the meningococcal-specific IgM responses, especially for MenW and MenY. Interestingly, these IgM responses were negatively correlated with age.

Conclusion

Primary immunization with a tetravalent meningococcal vaccine was highly immunogenic in middle-aged adults, inducing protective antibody titers in the vast majority of the participants lasting for at least 1 year. The age-related decrease in highly functional IgM responses argues in favor of vaccination against de novo antigens before reaching old age and, hence, middle-aged persons are an age group of interest for future vaccine interventions to protect the aging population.

Immunization with LytB protein of Streptococcus pneumoniae activates complement-mediated phagocytosis and induces protection against pneumonia and sepsis

The cell wall glucosaminidase LytB of Streptococcus pneumoniae is a surface exposed protein involved in daughter cell separation, biofilm formation and contributes to different aspects of the pathogenesis process. In this study we have characterized the antibody responses after immunization of mice with LytB in the presence of alhydrogel as an adjuvant. Enzyme-linked immunosorbent assays measuring different subclasses of immunoglobulin G, demonstrated that the antibody responses to LytB were predominantly IgG1 and IgG2b, followed by IgG3 and IgG2a subclasses. Complement-mediated immunity against two different pneumococcal serotypes was investigated using sera from immunized mice. Immunization with LytB increased the recognition of S. pneumoniae by complement components C1q and C3b demonstrating that anti-LytB antibodies trigger activation of the classical pathway. Phagocytosis assays showed that serum containing antibodies to LytB stimulates neutrophil-mediated phagocytosis against S. pneumoniae. Animal models of infection including invasive pneumonia and sepsis were performed with two different clinical isolates. Vaccination with LytB increased bacterial clearance and induced protection demonstrating that LytB might be a good candidate to be considered in a future protein-based vaccine against S. pneumoniae.

Neisseria meningitidis Group A IgG1 and IgG2 Subclass Immune Response in African Children Aged 12-23 Months Following Meningococcal Vaccination

Background. A group A meningococcal conjugate vaccine, PsA-TT, was licensed in 2010 and was previously studied in a phase 2 clinical trial to evaluate its safety and immunogenicity in African children 12–23 months of age.

Methods. Subjects received either PsA-TT; meningococcal group A, C, W, Y polysaccharide vaccine (PsACWY); or Haemophilus influenzae type b conjugate vaccine (Hib-TT). Forty weeks following primary vaccination, the 3 groups were further randomized to receive either PsA-TT, one-fifth dose of PsACWY, or Hib-TT. Group A–specific immunoglobulin G (IgG) subclass response was characterized using an enzyme-linked immunosorbent assay.

Results. The predominant IgG subclass response, regardless of vaccine, was IgG1. One month following primary vaccination, the geometric mean concentrations (GMCs) of IgG1 and IgG2 in the PsA-TT group were 21.73 µg/mL and 6.27 µg/mL, whereas in the PsACWY group the mean GMCs were 2.01 µg/mL and 0.97 µg/mL, respectively (P < .0001). Group A–specific IgG1 and IgG2 GMCs remained greater in the PsA-TT group than in the PsACWY group 40 weeks following primary vaccination (P < .0001). One week following revaccination, those given 2 doses of PsA-TT had the greatest IgG1 and IgG2 GMCs of 125.23 µg/mL and 36.12 µg/mL, respectively (P = .0008), and demonstrated a significant increase in IgG1:IgG2 mean ratio, indicative of the T-cell–dependent response associated with conjugate vaccines.

Conclusions. Vaccination of African children aged 12–24 months with either PsA-TT or PsACWY elicited a predominantly IgG1 response. The IgG1:IgG2 mean ratio decreased following successive vaccination with PsACWY, indicating a shift toward IgG2, suggestive of the T-cell–independent immune response commonly associated with polysaccharide antigens.

Clinical Trials Registration. SRCTN78147026.

Influence of IgG Subclass on Human Antimannan Antibody-Mediated Resistance to Hematogenously Disseminated Candidiasis in Mice

Candida albicans is a yeast-like pathogen and can cause life-threatening systemic candidiasis. Its cell surface is enriched with mannan that is resistant to complement activation. Previously, we developed the recombinant human IgG1 antimannan antibody M1g1. M1g1 was found to promote complement activation and phagocytosis and protect mice from systemic candidiasis. Here, we evaluate the influence of IgG subclass on antimannan antibody-mediated protection. Three IgG subclass variants of M1g1 were constructed: M1g2, M1g3, and M1g4. The IgG subclass identity for each variant was confirmed with DNA sequence and subclass-specific antibodies. These variants contain identical M1 Fabs and exhibited similar binding affinities for C. albicans yeast and purified mannan. Yeast cells and hyphae recovered from the kidney of antibody-treated mice with systemic candidiasis showed uniform binding of each variant, indicating constitutive expression of the M1 epitope and antibody opsonization in the kidney. All variants promoted deposition of both murine and human C3 onto the yeast cell surface, with M1g4 showing delayed activation, as determined by flow cytometry and immunofluorescence microscopy. M1g4-mediated complement activation was found to be associated with its M1 Fab that activates the alternative pathway in an Fc-independent manner. Treatment with each subclass variant extended the survival of mice with systemic candidiasis (P < 0.001). However, treatment with M1g1, M1g3, or M1g4, but not with M1g2, also reduced the kidney fungal burden (P < 0.001). Thus, the role of human antimannan antibody in host resistance to systemic candidiasis is influenced by its IgG subclass.

Neonatal infections in Saudi Arabia: association with C-reactive protein, CRP -286 (C>T>A) gene polymorphism and IgG antibodies

Background

C-reactive protein (CRP) is a nonspecific, acute-phase protein that rises in response to infectious and non-infectious inflammatory processes. Infections are the single largest cause of neonatal deaths globally.

The primary aim of this study is to examine the association between CRP gene polymorphism and serum levels of CRP in correlation with early onset sepsis (EOS) infection in newborns living in Taif city, Saudi Arabia. The second aim is to examine the relationship between specific IgG/IgG subclasses and early onset sepsis (EOS) infection among these newborns.

Methods

Staphylococcus aureus (S. aureus) is one of the most common organisms related to sepsis infection in the newborn at King Abdel Aziz Specialist Hospital (KAASH). This study was conducted in Taif city, at KAASH’s neonatal intensive care unit between March and August 2012. Neonates were consecutively enrolled onto the study having met our inclusion criteria (as per our research protocol).

The CRP concentration level was analysed using NycoCard® CRP Single Test. CRP -286 (C>T>A) A polymorphisms were analyzed using Pyrosequencing technology for CRP genotyping. IgG subclasses were analysed in the study population using ELISA.

Result

Logistic regression analyses showed that the AA and AC genotypes were negatively associated amongst EOS neonates compared to suspected neonates. The frequency of CC and CT were significantly associated with the EOS neonates compared to the suspected group. The levels of specific IgG1, IgG2 and IgG3 antibodies were significantly lower amongst EOS compared to the suspected group.

Conclusions

Taken together, the CRP-286 (C>T>A) A genotype polymorphism and specific IgG antibodies isotype levels can contribute to a reduced risk of EOS. Furthermore, CRP has a potential use in detecting EOS in neonates, which may mean earlier detection and management of EOS and subsequently better clinical outcome.

An engineered Fc variant of an IgG eliminates all immune effector functions via structural perturbations

The Fc variant of IgG2, designated as IgG2σ, was engineered with V234A/G237A /P238S/H268A/V309L/A330S/P331S substitutions to eliminate affinity for Fcγ receptors and C1q complement protein and consequently, immune effector functions. IgG2σ was compared to other previously well-characterized Fc 'muted' variants, including aglycosylated IgG1, IgG2m4 (H268Q/V309L/A330S/P331S, changes to IgG4), and IgG4 ProAlaAla (S228P/L234A/L235A) in its capacity to bind FcγRs and activate various immune-stimulatory responses. In contrast to the previously characterized muted Fc variants, which retain selective FcγR binding and effector functions, IgG2σ shows no detectable binding to the Fcγ receptors in affinity and avidity measurements, nor any detectable antibody-dependent cytotoxicity, phagocytosis, complement activity, or Fc-mediated cytokine release. Moreover, IgG2σ shows minimal immunogenic potential by T-cell epitope analysis. The circulating half-life of IgG2σ in monkeys is extended relative to IgG1 and IgG2, in spite of similar in vitro binding to recombinant FcRn. The three-dimensional structure of the Fc, needed for assessing the basis for the absence of effector function, was compared with that of IgG2 revealing a number of conformational differences near the hinge region of the CH2 domain that result from the amino acid substitutions. Modeling reveals that at least one of the key interactions with FcγRs is disrupted by a conformational change that reorients P329 to a position that prevents it from interacting with conserved W90 and W113 residues of the FcγRs. Inspection of the structure also indicated significant changes to the conformations of D270 and P329 in the CH2 domain that could negatively impact C1q binding. Thus, structural perturbations of the Fc provide a rationale for the loss of function. In toto, these properties of IgG2σ suggest that it is a superior alternative to previously described IgG variants of minimal effector function, for future therapeutic applications of non-immunostimulatory mAb and Fc-fusion platforms.

Leveraging SBDD in protein therapeutic development: antibody engineering

Antibodies make up the largest, growing segment of protein therapeutics in the pharmaceutical and biotechnology industries. The development or engineering of therapeutic antibodies is based to a large extent on our knowledge of antibody structure and requires sophisticated methods that continue to evolve. In this chapter, after a review of what is known about the structure and functional properties of antibodies, the current, state-of-the-art antibody engineering methods are described. These methods include antibody humanization, antigen-affinity optimization, Fc engineering for modulated effector function and extended half-life, and engineering for improved stability and biophysical properties. X-ray crystallographic structures of antibody fragments and their complexes can play a critical role in guiding and, in some cases, accelerating these processes. These approaches represent guidelines for developing antibody therapeutics with the desired affinity, effector function, and biophysical properties.

Immunity against Neisseria meningitidis serogroup C in the Dutch population before and after introduction of the meningococcal c conjugate vaccine

Background

In 2002 a Meningococcal serogroup C (MenC) conjugate vaccine, with tetanus toxoid as carrier protein, was introduced in the Netherlands as a single-dose at 14 months of age. A catch-up campaign was performed targeting all individuals aged 14 months to 18 years. We determined the MenC-specific immunity before and after introduction of the MenC conjugate (MenCC) vaccine.

Methods and Findings

Two cross-sectional population-based serum banks, collected in 1995/1996 (n = 8539) and in 2006/2007 (n = 6386), were used for this study. The main outcome measurements were the levels of MenC polysaccharide(PS)-specific IgG and serum bactericidal antibodies (SBA) after routine immunization, 4–5 years after catch-up immunization or by natural immunity. There was an increasing persistence of PS-specific IgG and SBA with age in the catch-up immunized cohorts 4–5 years after their MenCC immunization (MenC PS-specific IgG, 0.25 µg/ml (95%CI: 0.19–0.31 µg/ml) at age 6 years, gradually increasing to 2.34 µg/ml,(95%CI: 1.70–3.32 µg/ml) at age 21–22 years). A comparable pattern was found for antibodies against the carrier protein in children immunized above 9 years of age. In case of vaccination before the age of 5 years, PS-specific IgG was rapidly lost. For all age-cohorts together, SBA seroprevalence (≥8) increased from 19.7% to 43.0% in the pre- and post-MenC introduction eras, respectively. In non-immunized adults the SBA seroprevalence was not significantly different between the pre- and post-MenC introduction periods, whereas PS-specific IgG was significantly lower in the post-MenC vaccination (GMT, age ≥25 years, 0.10 µg/ml) era compared to the pre-vaccination (GMT, age ≥25 years, 0.43 µg/ml) era.

Conclusion

MenCC vaccination administered above 5 years of age induced high IgG levels compared to natural exposure, increasing with age. In children below 14 months of age and non-immunized cohorts lower IgG levels were observed compared to the pre-vaccination era, whereas functional levels remained similar in adults. Whether the lower IgG poses individuals at increased risk for MenC disease should be carefully monitored. Large-scale introduction of a MenCC vaccine has led to improved protection in adolescents, but in infants a single-dose schedule may not provide sufficient protection on the long-term and therefore a booster-dose early in adolescence should be considered.

Solution insights into the structure of the Efb/C3 complement inhibitory complex as revealed by lysine acetylation and mass spectrometry

The extracellular fibrinogen-binding protein (Efb), an immunosuppressive and anti-inflammatory protein secreted by Staphylococcus aureus, has been identified as a potent inhibitor of complement-mediated innate immunity. Efb functions by binding to and disrupting the function of complement component 3 (C3). In a recent study, we presented a high-resolution co-crystal structure of the complement inhibitory domain of Efb (Efb-C) bound to its cognate domain (C3d) from human C3 and employed a series of structure/function analyses that provided evidence for an entirely new, conformational change-based mechanism of complement inhibition. To better understand the Efb/C3 complex and its downstream effects on C3 inhibition, we investigated the solvent-accessibility and protein interface of Efb(-C)/C3d using a method of lysine acetylation, proteolytic digestion, and mass spectrometric analysis. Lysine modification in Efb was monitored by the mass increment of lysine-containing fragments. Besides confirming the binding sites observed in co-crystal structure study, the in-solution data presented here suggest additional contacting point(s) between the proteins that were not revealed by crystallography. The results of this study demonstrate that solution-based analysis of protein-protein interactions can provide important complementary information on the nature of protein-protein interactions.

Characterization of Ehp, a secreted complement inhibitory protein from Staphylococcus aureus

We report here the discovery and characterization of Ehp, a new secreted Staphylococcus aureus protein that potently inhibits the alternative complement activation pathway. Ehp was identified through a genomic scan as an uncharacterized secreted protein from S. aureus, and immunoblotting of conditioned S. aureus culture medium revealed that the Ehp protein was secreted at the highest levels during log-phase bacterial growth. The mature Ehp polypeptide is composed of 80 residues and is 44% identical to the complement inhibitory domain of S. aureus Efb (extracellular fibrinogen-binding protein). We observed preferential binding by Ehp to native and hydrolyzed C3 relative to fully active C3b and found that Ehp formed a subnanomolar affinity complex with these various forms of C3 by binding to its thioester-containing C3d domain. Site-directed mutagenesis demonstrated that Arg(75) and Asn(82) are important in forming the Ehp.C3d complex, but loss of these side chains did not completely disrupt Ehp/C3d binding. This suggested the presence of a second C3d-binding site in Ehp, which was mapped to the proximity of Ehp Asn(63). Further molecular level details of the Ehp/C3d interaction were revealed by solving the 2.7-A crystal structure of an Ehp.C3d complex in which the low affinity site had been mutationally inactivated. Ehp potently inhibited C3b deposition onto sensitized surfaces by the alternative complement activation pathway. This inhibition was directly related to Ehp/C3d binding and was more potent than that seen for Efb-C. An altered conformation in Ehp-bound C3 was detected by monoclonal antibody C3-9, which is specific for a neoantigen exposed in activated forms of C3. Our results suggest that increased inhibitory potency of Ehp relative to Efb-C is derived from the second C3-binding site in this new protein.

A structural basis for complement inhibition by Staphylococcus aureus

To provide insight into bacterial suppression of complement-mediated immunity, we present here structures of a bacterial complement inhibitory protein, both free and bound to its complement target. The 1.25-A structure of the complement component C3-inhibitory domain of Staphylococcus aureus extracellular fibrinogen-binding protein (Efb-C) demonstrated a helical motif involved in complement regulation, whereas the 2.2-A structure of Efb-C bound to the C3d domain of human C3 allowed insight into the recognition of complement proteins by invading pathogens. Our structure-function studies provided evidence for a previously unrecognized mode of complement inhibition whereby Efb-C binds to native C3 and alters the solution conformation of C3 in a way that renders it unable to participate in successful 'downstream' activation of the complement response.

Assignment of weight-based immunoglobulin G1 (IgG1) and IgG2 units in antipneumococcal reference serum lot 89-S(F) for pneumococcal polysaccharide serotypes 1, 4, 5, 7F, 9V, and 18C

Weight-based assignments for immunoglobulin G1 (IgG1) and IgG2 subclass antibodies to Streptococcus pneumoniae capsular polysaccharides (PnPs) in antipneumococcal standard reference serum lot 89-S (lot 89-S), also known as lot 89-SF, have been determined for serotypes 1, 4, 5, 7F, 9V, and 18C. This extends the usefulness of lot 89-S beyond the IgG1 and IgG2 subclass assignments for serotypes 3, 6B, 14, 19F, and 23F made previously (A. Soininen, H. Kayhty, I. Seppala, and T. Wuorimaa, Clin. Diagn. Lab. Immunol. 5:561-566, 1998) to cover 11 major serotypes associated with the highest percentage of pneumococcal disease worldwide. A method of equivalence of absorbances in enzyme immunosorbent assays was used to determine the IgG1 and IgG2 antibody concentrations for the additional serotypes in lot 89-S, based on the subclass values previously assigned for PnPs serotypes 6B, 14, and 23F. This cross-standardization method assures consistency with previous antibody assignments in that reference serum. The newly assigned subclass values for serotype 9V, and previously assigned values for serotype 14, were used to quantitate PnPs antibodies in sera from adult and pediatric subjects immunized with a pneumococcal conjugate vaccine. There was a predominance of IgG1 anti-PnPs antibodies in pediatric sera and IgG2 anti-PnPs antibodies in the adult sera. The IgG1 and IgG2 subclass assignments for the 11 PnPs serotypes in antipneumococcal standard reference serum lot 89-S are useful for quantitating and characterizing immune responses to pneumococcal infection and vaccination regimens.

Concentration and avidity of anti-Haemophilus influenzae type b (Hib) antibodies in serum samples obtained from patients for whom Hib vaccination failed

Haemophilus influenzae type b (Hib) conjugate vaccines are extremely effective in protecting infants and children from invasive Hib infections; however, vaccine failures do occur. The anti-Hib antibody production was studied both quantitatively and qualitatively in 12 patients who experienced Hib failure, all of whom had normal serum immunoglobulin concentrations and all of whom were without clinical risk factors for invasive Hib disease. Both anti-Hib antibody concentration and immunoglobulin-G2 anti-Hib antibody avidity were significantly lower in patients who experienced Hib failure, at onset of disease and after reconvalescence, when compared with controls. This finding suggests that the patients who developed invasive Hib disease--despite having received 3-4 Hib conjugate vaccinations--were inadequately primed by these vaccinations.

Clinical relevance of lower Hib response in DTPa-based combination vaccines

Combination vaccines are essential to enable administration of all the required antigens in routine infant immunisation schedules at any single visit. Some combinations of diphtheria-tetanus-acellular pertussis (DTPa) with Haemophilus influenzae type b (Hib) conjugate vaccines have been shown to result in lower Hib titres than when Hib is administered separately. While confirming that a primary series with a DTPa-HBV-IPV/Hib combination gives lower antibody levels than separate Hib conjugates, we show that the nature (isotype and IgG subclasses) and function (avidity and opsonic activity) of the antibodies are the same, and immunologic memory is induced. It is likely therefore that the DTPa-HBV-IPV/Hib combination will be efficacious against Hib disease.

Functional human monoclonal antibodies of all isotypes constructed from phage display library-derived single-chain Fv antibody fragments

We have constructed a series of eukaryotic expression vectors that permit the rapid conversion of single chain (sc) Fv antibody fragments, derived from semi-synthetic phage display libraries, into intact fully human monoclonal antibodies (mAb) of each isotype. As a model, a scFv fragment specific for sheep red blood cells (SRBC) was isolated from a semi-synthetic phage antibody (Ab) display library, and used to produce human mAbs of IgM, IgG1-IgG4, IgA1, IgA2m(1) and IgE isotype in vitro in stably transfected cells. N-terminal protein sequence analysis of purified immunoglobulin heavy (H) and light (L) chains revealed precise proteolytic removal of the leader peptide. Biochemical analysis of purified recombinant human mAbs demonstrated that properly glycosylated molecules of the correct molecular size were produced. The IgG and IgA mAbs retained SRBC-binding activity, interacted with different Fc receptor-transfectants, and induced complement-mediated hemolysis and Ab-dependent phagocytosis of SRBC by neutrophils in a pattern consistent with the immunoglobulin (Ig) H chain isotype. We conclude that in vitro produced recombinant human mAbs constructed from phage display library-derived scFv fragments mirror their natural counterparts and may represent a source of mAbs for use in human therapy.

The role of Fcgamma receptor polymorphisms and C3 in the immune defence against Neisseria meningitidis in complement-deficient individuals

Individuals with either a late (C5-9) complement component deficiency (LCCD) or properdin deficiency are at increased risk to develop meningococcal disease, often due to serogroups W135 and Y. Anti-meningococcal defence in both LCCD persons and properdin-deficient individuals without bactericidal antibodies depends mainly on phagocytosis. Three types of opsonin receptors are involved in phagocytosis by polymorphonuclear cells (PMN). These represent the polymorphic FcgammaRIIa (CD32) and FcgammaRIIIb (CD16b) receptors, and the C3 receptor CR3 (CD11b/CD18). When the distribution of FcgammaRIIa and FcgammaRIIIb allotypes was assessed in 15 LCCD and in 15 properdin-deficient patients with/without previous meningococcal disease, we found the combination of FcgammaRIIa-R/R131 with FcgammaRIIIb-NA2/NA2 allotypes to be associated with previous meningococcal disease (odds ratio 13.9, Fisher's test P = 0.036). No such relation was observed in the properdin-deficient patients. The importance of FcgammaRIIa allotypes was also demonstrated using in vitro phagocytosis assays. PMN from FcgammaRIIa-R/R131 homozygous donors internalized IgG2 opsonized meningococci W135 significantly (P < 0.05) less than PMN from FcgammaRIIa-H/H131 donors. When properdin-deficient serum was tested, it was observed that reconstitution with properdin resulted in enhanced PMN phagocytosis of the W135 meningococci (P = 0.001). This enhanced phagocytosis was parallelled by an increase in C3 deposition onto the opsonized meningococci W135 (r = 0.6568, P = 0. 01). We conclude that the occurrence of meningococcal disease in LCCD patients is associated with certain FcgammaR allotypes. Properdin-deficient individuals are susceptible to meningococcal disease because of an insufficient C3 deposition on the surface of meningococci, resulting in insufficient phagocytosis.

Assignment of immunoglobulin G1 and G2 concentrations to pneumococcal capsular polysaccharides 3, 6B, 14, 19F, and 23F in pneumococcal reference serum 89-SF

We describe standardization of an enzyme-linked immunosorbent assay (ELISA) for measuring immunoglobulin G1 (IgG1) and IgG2 concentrations of antibodies to pneumococcal capsular polysaccharide (Pnc PS). The ELISA uses a human pneumococcal reference serum pool, lot 89-SF, as a reference. IgG1 and IgG2 concentrations were assigned to antibodies to Pnc PS serotypes 3, 6B, 14, 19F, and 23F in 89-SF by ELISA using affinity-purified monoisotypic IgG1 and IgG2 preparations. The sum of IgG1 and IgG2 concentrations in 89-SF agrees well with the previously assigned IgG concentrations. The IgG1 and IgG2 values in 89-SF were used to measure antibodies to Pnc PS 6B, 14, and 23F in adult pre- and postimmunization sera and the sum of IgG1 and IgG2 concentrations correlated well with the IgG values. Furthermore, the IgG2/IgG1 ratio did not affect the detection of IgG1, the isotype usually represented by a lower concentration.

Opsonophagocytic and bactericidal activity mediated by purified IgG subclass antibodies after vaccination with the Norwegian group B meningococcal vaccine

To study how the different immunoglobulin (Ig)G subclass antibodies may confer protection against systemic meningococcal disease, we isolated IgG1, IgG2 and IgG3 antibodies from plasma from vaccinees immunized with the Norwegian meningococcal outer membrane vesicle vaccine. Four IgG1, one IgG2 and four IgG3 preparations were purified. The IgG2 and IgG3 subclass preparations were free from contaminating subclasses, whereas the IgG1 preparations contained from 0 to 14% of IgG2 and/or IgG3. Immunoblotting against whole-cell meningococcal antigens showed broad specificities of the various preparations, both within and between subclasses. These subclass preparations were tested for opsonophagocytic and bactericidal activity. As targets we used two different variants of the meningococcal vaccine strain, with low (44/76-SL) and high (44/76-1) expression of the outer membrane protein Opc. Using polymorphonuclear leucocytes as effector cells in the presence of human complement, all three IgG subclass preparations revealed high, and similar, opsonophagocytic activities against 44/76-SL, whereas against 44/76-1 the IgG2 preparation showed a reduced activity and most IgG3 preparations were slightly more active than the IgG1 preparations. Regarding bactericidal activity, all the three subclasses were highly active against 44/76-SL. Against 44/76-1 the bactericidal activities were somewhat more varied: all IgG1 and three IgG3 preparations exhibited higher activities than against 44/76-SL. Due to the low concentration in the IgG2 preparations, only a weak activity was seen against 44/76-1. One IgG3 preparation that was highly opsonophagocytic revealed no bactericidal activity against either of the two bacterial variants examined. In conclusion, we have shown that the IgG subclass effector functions differ from person to person, but that antibodies of IgG1, IgG2 and IgG3 subclasses, judged by their behaviour in the functional tests, may all contribute to protection against meningococcal disease.

The immune response to staphylococcal antigens in mice depleted of macrophages by Cl2MDP-liposomes

To investigate the role of macrophages in the induction of the production of antibody to staphylococcal antigens, we used Cl2MDP (clodronate) liposomes as a tool for local macrophage depletion. Macrophage depletion caused in mice by intraperitoneal (i.p.) injection of Cl2MDP liposomes was associated with a reduction in the clearance of Staphylococcus aureus Cowan 1 bacteria from the tissues of infected animals and with a marked decrease in the bactericidal activity of macrophages escaping from the lethal effect of clodronate. Despite the functional defect of macrophages, the mice treated with Cl2MDP liposomes two days before the injection of alpha-toxin (toxoid) or whole heat-killed S. aureus Cowan 1 bacteria, demonstrated an enhancement in the production of anti-staphylococcal alpha-toxin IgM and anti-collagen-binding protein IgG. A similar enhancement of antistaphylococcal antibody synthesis was observed in mice after receiving phosphate buffered saline (PBS) encapsulated in liposomes.

Specific immune response to staphylococcal antigens during long-lasting biomaterial implantation

Biomaterial-associated infections caused by staphylococci are one of the main therapeutic problems in modern medicine. There is no doubt that local disfunction of polymorphonuclear leukocytes and macrophages predisposes to such infections. However, it is not clear how implantation of a foreign body influences the antibacterial immune response. We analyzed some parameters of the specific immune response to staphylococcal antigens, in mice implanted for 3 months with heparinized polyethylene. Three weeks before the evaluation of the immune response, mice (implanted and non-implanted) were infected i.p. with 2 x 10(7) cells of Staphylococcus aureus Cowan 1. The proliferation of splenocytes was determined on the basis of [3H]thymidine incorporation in cultures stimulated with staphylococcal lipoteichoic acid, protein A, alpha-toxin, or phytohemagglutinin. Moreover, the level of specific antibodies to staphylococcal antigens was determined in serum samples (ELISA with the antigens lipoteichoic acid, protein A, and alpha-toxin). The data obtained indicate that long-lasting implantation caused evident changes in proliferative activity of lymphocytes and in humoral response to staphylococcal antigens. It enhanced spontaneous and lipoteichoic acid- or alpha-toxin-stimulated proliferation of splenocytes, in vitro. In contrast, heparinized polyethylene-implanted animals showed a significant decrease in the production of anti-protein A IgG2b and anti-alpha-toxin IgG2a and IgG2b.

Complement-mediated serum sensitivity among spirochetes that cause Lyme disease

Borrelia burgdorferi-related isolates were tested for their sensitivity to normal human serum (NHS) and their ability to activate complement. By dark-field microscopy, electron microscopy, and subsurface plating, it was shown that exposure of a Borrelia garinii isolate to 10% or more NHS resulted in immobilization, blebbing, and killing of the spirochetes. These effects were mediated by complement, since they were not seen after heat treatment of NHS, in the presence of EDTA, or in an agammaglobulinemic serum. All seven B. garinii type 5 or 6 and all four VS116/M19 strains were serum sensitive, whereas all eight Borrelia afzelii, five of eight B. garinii type 4, and three of seven B. burgdorferi sensu stricto isolates were serum resistant. The other isolates were partially serum sensitive. Four serum-sensitive B. garinii isolates had been isolated from human cerebrospinal fluid. Most likely, activation of both the alternative pathway and the classical pathway of complement was involved, since bactericidal activity was diminished in properdin-deficient sera as well as in a C1q-depleted serum and in a C4-deficient serum. Bactericidal activity could be restored when a serum lacking C1q or C4 was mixed with a properdin-deficient serum. Isolates with various genetic backgrounds were equally able to activate C3 as measured by enzyme-linked immunosorbent assay. In the presence of Mg-EGTA, C3 was activated by all isolates after exposure to > or = 10% NHS. This study shows that B. burgdorferi-related spirochetes can be either serum sensitive or serum resistant in vitro and that this characteristic is associated with their genetic background.

Immunoglobulin G2 antibodies promote neutrophil killing of Actinobacillus actinomycetemcomitans

Sera from patients with localized juvenile periodontitis (LJP) often contain markedly elevated levels of immunoglobulin G2 (IgG2) antibodies reactive to cell envelope constituents of Actinobacillus actinomycetemcomitans. The objective of this study was to determine if these IgG2 antibodies are capable of supporting phagocytosis and killing of A. actinomycetemcomitans by human neutrophils. Polyclonal IgG2 antibodies were prepared from high-titer LJP serum by affinity chromatography, yielding a preparation which was > 99% subclass restricted and retained immunoreactivity to A. actinomycetemcomitans antigens. Affinity-purified IgG2 antibodies were evaluated by an in vitro opsonophagocytic assay that employed neutrophils obtained from donors who were homozygous for the H131 allotype of Fc gamma receptor type IIa (CD32), which efficiently binds human IgG2 antibodies. Affinity-purified IgG2 antibodies from LJP serum but not from sera of periodontally healthy individuals promoted phagocytosis and killing of A. actinomycetemcomitans. The expression of IgG2-dependent opsonic activity required the presence of complement. Incubation of A. actinomycetemcomitans with neutrophils in the presence of an optimal concentration of LJP IgG2 (50 micrograms/ml) and 5% hypogammaglobulinemic serum (as a complement source) resulted in a > 1 log10 reduction in bacterial viability within 30 min. The opsonic activity of IgG2 antibodies was found to be comparable to that observed with affinity-purified IgG1 antibodies. Moreover, IgG1 antibodies interacted synergistically with IgG2 antibodies in promoting opsonophagocytosis of A. actinomycetemcomitans. The results of this study indicate that LJP serum contains IgG2 antibodies which, when employed in conjunction with neutrophils that express Fc gamma receptors capable of recognizing this subclass, are opsonic for A. actinomycetemcomitans.

Role of neutrophil Fc gamma RIIa (CD32) and Fc gamma RIIIb (CD16) polymorphic forms in phagocytosis of human IgG1- and IgG3-opsonized bacteria and erythrocytes

The four subclasses of IgG have different biological activities associated with their Fc regions. Fc gamma receptors on leucocytes (Fc gamma R) mediate binding and phagocytosis of opsonized particles. Two structurally and functionally distinct allelic polymorphisms of the Fc gamma R have been defined: the H/R131 forms of Fc gamma RIIa (CD32), and the neutrophil antigen 1 (NA1)/NA2 forms of Fc gamma RIIIb (CD16). In this study the activities of allotypes of CD16 are analysed with antibacterial IgG subclass antibodies and with IgG1 and IgG3 anti-Rhesus D, and the activities of CD32 with IgG1 and IgG3 anti-Rhesus D. With respect to the allotypes of CD16, polymorphonuclear leucocytes (PMN) homozygous for Fc gamma RIIb-NA2 exhibited a lower (21-25%) IgG1-mediated phagocytosis of Staphylococcus aureus strain Wood (STAW), Haemophilus influenzae type b (Hib), and Neisseria meningitidis group B (NMen) than IIIb-NA1 PMN. The difference was apparent only when the micro-organisms were opsonized in the absence of complement, and was furthermore enhanced (34-52%) upon blockade of Fc gamma RIIa. In addition, monoclonal IgG3 anti-D-mediated rosette formation and phagocytosis was consistently found to be lower (16%) with Fc gamma RIIIb-NA2 than with IIIb-NA1 PMN. For the allotypes of CD32 we now show that IgG3 anti-D sensitized erythrocytes formed more (50%) rosettes and were phagocytosed at a higher rate with PMN carrying Fc gamma RIIa-H131 than with PMN carrying IIa-R131. Heterozygous Fc gamma RIIa-H/R131 PMN exhibited intermediate phagocytic activity in this respect. This study illustrates a critical role of Fc gamma R allotypes in functional interactions with biologically relevant IgG subclass antibodies.