Crucial role of FcgammaRIIa (CD32) in assessment of functional anti-Streptococcus pneumoniae antibody activity in human sera

Polyfunctional antibodies: a path towards precision vaccines for vulnerable populations

Vaccine efficacy determined within the controlled environment of a clinical trial is usually substantially greater than real-world vaccine effectiveness. Typically, this results from reduced protection of immunologically vulnerable populations, such as children, elderly individuals and people with chronic comorbidities. Consequently, these high-risk groups are frequently recommended tailored immunisation schedules to boost responses. In addition, diverse groups of healthy adults may also be variably protected by the same vaccine regimen. Current population-based vaccination strategies that consider basic clinical parameters offer a glimpse into what may be achievable if more nuanced aspects of the immune response are considered in vaccine design. To date, vaccine development has been largely empirical. However, next-generation approaches require more rational strategies. We foresee a generation of precision vaccines that consider the mechanistic basis of vaccine response variations associated with both immunogenetic and baseline health differences. Recent efforts have highlighted the importance of balanced and diverse extra-neutralising antibody functions for vaccine-induced protection. However, in immunologically vulnerable populations, significant modulation of polyfunctional antibody responses that mediate both neutralisation and effector functions has been observed. Here, we review the current understanding of key genetic and inflammatory modulators of antibody polyfunctionality that affect vaccination outcomes and consider how this knowledge may be harnessed to tailor vaccine design for improved public health.

The Human FcγRII (CD32) Family of Leukocyte FcR in Health and Disease

FcγRs have been the focus of extensive research due to their key role linking innate and humoral immunity and their implication in both inflammatory and infectious disease. Within the human FcγR family FcγRII (activatory FcγRIIa and FcγRIIc, and inhibitory FcγRIIb) are unique in their ability to signal independent of the common γ chain. Through improved understanding of the structure of these receptors and how this affects their function we may be able to better understand how to target FcγR specific immune activation or inhibition, which will facilitate in the development of therapeutic monoclonal antibodies in patients where FcγRII activity may be desirable for efficacy. This review is focused on roles of the human FcγRII family members and their link to immunoregulation in healthy individuals and infection, autoimmunity and cancer.

Protection Against Invasive Infections in Children Caused by Encapsulated Bacteria

The encapsulated bacteria Streptococcus pneumoniae, Neisseria meningitis, Haemophilus influenzae, and Streptococcus agalactiae (Group B Streptococcus) have been responsible for the majority of severe infections in children for decades, specifically bacteremia and meningitis. Isolates which cause invasive disease are usually surrounded by a polysaccharide capsule, which is a major virulence factor and the key antigen in protective protein-polysaccharide conjugate vaccines. Protection against these bacteria is largely mediated via polysaccharide-specific antibody and complement, although the contribution of these and other components, and the precise mechanisms, vary between species and include opsonophagocytosis and complement-dependent bacteriolysis. Further studies are required to more precisely elucidate mechanisms of protection against non-type b H. influenzae and Group B Streptococcus.

FcγR requirements leading to successful immunotherapy

Monoclonal antibody (mAb) immunotherapy is currently experiencing an unprecedented amount of success, delivering blockbuster sales for the pharmaceutical industry. Having experienced several false dawns and overcoming technical issues which limited progress, we are now entering a golden period where mAbs are becoming a mainstay of treatment regimes for diseases ranging from cancer to autoimmunity. In this review, we discuss how these mAbs are most likely working and focus in particular on the key receptors that they interact with to precipitate their therapeutic effects. Although their targets may vary, their engagement with Fcγ receptors (FcγRs) on numerous immune effector cells is almost universal, and here we review their roles in delivering successful immunotherapy.

Functions of Antibodies

Antibodies can impact pathogens in the presence or in the absence of effector cells or effector molecules such as complement, and experiments can often sort out with precision the mechanisms by which an antibody inhibits a pathogen in vitro . In addition, in vivo models, particularly those engineered to knock in or knock out effector cells or effector molecules, are excellent tools for understanding antibody functions. However, it is highly likely that multiple antibody functions occur simultaneously or sequentially in the presence of an infecting organism in vivo . The most critical incentive for measuring antibody functions is to provide a basis for vaccine development and for the development of therapeutic antibodies. In this respect, some functions, such as virus neutralization, serve to inhibit the acquisition of a pathogen or limit its pathogenesis. However, antibodies can also enhance replication or contribute to pathogenesis. This review emphasizes those antibody functions that are potentially beneficial to the host. In addition, this review will focus on the effects of antibodies on organisms themselves, rather than on the toxins the organisms may produce.

FcγRIIa cross-talk with TLRs, IL-1R, and IFNγR selectively modulates cytokine production in human myeloid cells

Myeloid antigen-presenting cells (APCs) tailor immune responses to the pathogen involved through the production of specific pro- and anti-inflammatory cytokines. It is becoming increasingly clear that the ultimate cytokine profile produced by myeloid APCs crucially depends on interaction between multiple pathogen recognizing receptors. In this respect, we recently identified an important role for cross-talk between Fc gamma receptor IIa (FcγRIIa) and Toll-like receptors (TLRs) in human dendritic cells (DCs), which induces anti-bacterial immunity through the selective induction of TNFα and Th17-promoting cytokines. Here, we show that FcγRIIa-TLR cross-talk is not restricted to DCs, but is a common feature of various human myeloid APC subsets including monocytes and macrophages. Interestingly, FcγRIIa-TLR cross-talk in monocytes resulted in the induction of a cytokine profile distinct from that in DCs and macrophages, indicating that FcγRIIa stimulation induces cell-type and tissue specific responses. Surprisingly, we show that the FCGR2A H131R single nucleotide polymorphism (SNP), which is known to greatly affect FcγRIIa-mediated uptake of IgG2-opsonized bacteria, did not affect FcγRIIa-dependent cytokine production, indicating that these processes are differently regulated. In addition, we demonstrate that FcγRIIa selectively synergized with TLRs, IL-1R, and IFNγR, but did not affect cytokine production induced by other receptors such as C-type lectin receptor Dectin-1. Taken together, these data demonstrate that FcγRIIa-dependent modulation of cytokine production is more widespread than previously considered, and indicate that cross-talk of FcγRIIa with various receptors and in multiple cell types contributes to the induction of pathogen and tissue-specific immunity.

Antibody Engineering

Advanced molecular biology techniques developed during the past few decades have allowed the industry to exploit and commercialize the natural defense mechanisms that antibodies provide. This review discusses the latest advances in antibody-engineering technologies to enhance clinical efficacy and outcomes. For the constant regions, the choice of the antibody class and isotype has to be made carefully to suit the therapeutic applications. Engineering of the Fc region, either by direct targeted mutagenesis or by modifying the nature of its N -glycan, has played an important role in recent years in increasing half-life or controlling effector functions. The variable regions of the antibody are responsible for binding affinity and exquisite specificity to the target molecule, which together with the Fc determine the drug's efficacy and influence the drug dose required to obtain the desired effectiveness. A key requirement during antibody development is therefore to affinity mature the variable regions when necessary, so that they bind the therapeutic target with sufficiently high affinity to guarantee effective occupancy over prolonged periods. If the antibody was obtained from a non-human source, such as rodents, a humanization process has to be applied to minimize immunogenicity while maintaining the desired binding affinity and selectivity. Finally, we discuss the next next-generation antibodies, such as antibody-drug conjugates, bispecific antibodies, and immunocytokines, which are being developed to meet future challenges.

Isotype Diversification of IgG Antibodies to HIV Gag Proteins as a Therapeutic Vaccination Strategy for HIV Infection

The development of vaccines to treat and prevent human immunodeficiency virus (HIV) infection has been hampered by an incomplete understanding of "protective" immune responses against HIV. Natural control of HIV-1 infection is associated with T-cell responses against HIV-1 Gag proteins, particularly CD8⁺ T-cell responses restricted by "protective" HLA-B alleles, but other immune responses also contribute to immune control. These immune responses appear to include IgG antibodies to HIV-1 Gag proteins, interferon-a-dependant natural killer (NK) cell responses and plasmacytoid dendritic cell (pDC) responses. Here, it is proposed that isotype diversification of IgG antibodies against HIV-1 Gag proteins, to include IgG2, as well as IgG3 and IgG1 antibodies, will broaden the function of the antibody response and facilitate accessory cell responses against HIV-1 by NK cells and pDCs. We suggest that this should be investigated as a vaccination strategy for HIV-1 infection.

Three-dimensional structure of the human myeloma IgG2

Human immunoglobulin G, subclass 2 (hIgG2), plays an important role in immunity to bacterial pathogens and in numerous pathological conditions. However, there is a lack of information regarding the three-dimensional (3D) structure of the hIgG2 molecule. We used electron microscopy (EM), differential scanning microcalorimetry (DSC) and fluorescence for structural analysis of the hIgG2. DSC and fluorescence indicated two types of interaction between C_H1 domain of Fab (antigen-binding fragment/subunit) and C_H2 domain of Fc (complement fixation fragment/subunit) simultaneously present in the sample: close interaction, which increases the thermostability of both, C_H1 and C_H2 domains, and weak (or no) interaction, which is typical for most IgGs but not hIgG2. Thermodynamics could not determine if both types of interactions are present within a single molecule. To address this question, EM was used. We employed a single-particle reconstruction and negative staining approach to reveal the three-dimensional structure of the hIgG2. A three-dimensional model of hIgG2 was created at 1.78 nm resolution. The hIgG2 is asymmetrical: one Fab subunit is in close proximity to the upper portion of the Fc subunit (C_H2 domain) and the other Fab is distant from Fc. The plane of Fab subunits is nearly perpendicular to Fc. EM structure of the hIgG2 is in good agreement with thermodynamic data: a Fab distant from Fc should exhibit a lower melting temperature while a Fab interacting with Fc should exhibit a higher melting temperature. Both types of Fab subunits exist within one molecule resembling an A/B hIgG2 isoform introduced earlier on physicochemical level by Dillon et al. (2008). In such an arrangement, the access to the upper portion of Fc subunit is partially blocked by a Fab subunit. That might explain for instance why hIgG2 mildly activates complement and binds poorly to Fc receptors. Understanding of the three-dimensional structure of the hIgG2 should lead to better design of antibody-based therapeutics.

Protective effects of FCGR2A polymorphism in invasive pneumococcal diseases

BACKGROUND

Streptococcus pneumoniae is a major cause of pneumonia and meningitis. Several genetic polymorphisms have been described to explain differences in susceptibility and severity of encapsulated pathogen-related diseases. Among them, a functional FCGR2A polymorphism leading to amino acid change of histidine (H) to arginine (R) at position 131 appears to be a major candidate in adult invasive pneumococcal diseases (IPDs). However, previous reports need confirmation in a large, well-defined population.

METHODS

This prospective genetic association study was carried out in a 24-bed medical ICU of a tertiary teaching hospital over 7 years. DNA from all white patients with IPD (pneumonia or meningitis) was genotyped for the FcγRIIa-R/H131 polymorphism.

RESULTS

A total of 243 patients with IPD were enrolled; 202 (82%) had pneumonia, and 55 (22%) had meningitis. Mean age was 61 years, and mean Simplified Acute Physiology Score II was 50.4. One-half of the patients had bacteremia, and 84% of the cohort received mechanical ventilation. The hospital mortality rate was 31%. In the IPD group, the distribution of the FcγRIIa-R/H131 genotypes (H/H, 25%; H/R, 53%; R/R, 22%) was comparable with that in the white control group. Comparison of the FcγRIIa-R/R131 and the FcγRIIa-R/H131 + FcγRIIa-H/H131 groups did not demonstrate any difference for age, Simplified Acute Physiology Score II, origin of sepsis, and other comorbid conditions. However, the variant FcγRIIa-R/R131 genotype was independently associated with decreased hospital mortality (OR, 0.251; 95% CI, 0.098-0.645; P = .004).

CONCLUSIONS

In a well-defined population of patients with IPD, the frequency of the variant FcγRIIa-R131 does not differ from that of other critically ill patients. However, the FcγRIIa-R/R131 genotype was independently associated with increased survival, regardless of site of infection.

Isotype-switched immunoglobulin G antibodies to HIV Gag proteins may provide alternative or additional immune responses to 'protective' human leukocyte antigen-B alleles in HIV controllers

BACKGROUND

Natural control of HIV infection is associated with CD8 T-cell responses to Gag-encoded antigens of the HIV core and carriage of 'protective' human leukocyte antigen (HLA)-B alleles, but some HIV controllers do not possess these attributes. As slower HIV disease progression is associated with high levels of antibodies to HIV Gag proteins, we have examined antibodies to HIV proteins in controllers with and without 'protective' HLA-B alleles.

METHODS

Plasma from 32 HIV controllers and 21 noncontrollers was examined for immunoglobulin G1 (IgG1) and IgG2 antibodies to HIV proteins in virus lysates by western blot assay and to recombinant (r) p55 and gp140 by ELISA. Natural killer (NK) cell-activating antibodies and FcγRIIa-binding immune complexes were also assessed.

RESULTS

Plasma levels of IgG1 antibodies to HIV Gag (p18, p24, rp55) and Pol-encoded (p32, p51, p66) proteins were higher in HIV controllers. In contrast, IgG1 antibodies to Env proteins were less discriminatory, with only antigp120 levels being higher in controllers. High-level IgG2 antibodies to any Gag protein were most common in HIV controllers not carrying a 'protective' HLA-B allele, particularly HLA-B*57 (P = 0.016). HIV controllers without 'protective' HLA-B alleles also had higher plasma levels of IgG1 antip32 (P = 0.04). NK cell-activating antibodies to gp140 Env protein were higher in elite controllers but did not differentiate HIV controllers with or without 'protective' HLA-B alleles. IgG1 was increased in FcγRIIa-binding immune complexes from noncontrollers.

CONCLUSION

We hypothesize that isotype-switched (IgG2+) antibodies to HIV Gag proteins and possibly IgG1 antip32 may provide alternative or additional immune control mechanisms to HLA-restricted CD8 T-cell responses in HIV controllers.

Bordetella parapertussis survives the innate interaction with human neutrophils by impairing bactericidal trafficking inside the cell through a lipid raft-dependent mechanism mediated by the lipopolysaccharide O antigen

Whooping cough is a reemerging disease caused by two closely related pathogens, Bordetella pertussis and Bordetella parapertussis. The incidence of B. parapertussis in whooping cough cases has been increasing since the introduction of acellular pertussis vaccines containing purified antigens that are common to both strains. Recently published results demonstrated that these vaccines do not protect against B. parapertussis due to the presence of the O antigen on the bacterial surface that impairs antibody access to shared antigens. We have investigated the effect of the lack of opsonization of B. parapertussis on the outcome of its interaction with human neutrophils (polymorphonuclear leukocytes [PMNs]). In the absence of opsonic antibodies, PMN interaction with B. parapertussis resulted in nonbactericidal trafficking upon phagocytosis. A high percentage of nonopsonized B. parapertussis was found in nonacidic lysosome marker (lysosome-associated membrane protein [LAMP])-negative phagosomes with access to the host cell-recycling pathway of external nutrients, allowing bacterial survival as determined by intracellular CFU counts. The lipopolysaccharide (LPS) O antigen was found to be involved in directing B. parapertussis to PMN lipid rafts, eventually determining the nonbactericidal fate inside the PMN. IgG opsonization of B. parapertussis drastically changed this interaction by not only inducing efficient PMN phagocytosis but also promoting PMN bacterial killing. These data provide new insights into the immune mechanisms of hosts against B. parapertussis and document the crucial importance of opsonic antibodies in immunity to this pathogen.

Isotype and glycoform selection for antibody therapeutics

We live in a hostile environment but are protected by the innate and adaptive immune system. A major component of the latter is mediated by antibody molecules that bind to pathogens, with exquisite specificity, and the immune complex formed activates cellular mechanisms leading to the removal and destruction of the complex. Five classes of antibody are identified; however, the IgG class predominates in serum and a majority of monoclonal antibody (mAb) therapeutics are based on the IgG format. Selection within the antibody repertoire allows the generation of (mAb) having specificity for any selected target, including human antigens. This review focuses on the structure and function of the Fc region of IgG molecules that mediates biologic functions, within immune complexes, by interactions with cellular Fc receptors (FcγR) and/or the C1q component of complement. A property of IgG that is suited to its use as a therapeutic is the long catabolic half life of ~21 days, mediated through the structurally distinct neonatal Fc receptor (FcRn). Our understanding of structure/function relationships is such that we can contemplate engineering the IgG-Fc to enhance or eliminate biologic activities to generate therapeutics considered optimal for a given disease indication. There are four subclasses of human IgG that exhibit high sequence homology but a unique profile of biologic activities. The FcγR and the C1q binding functions are dependent on glycosylation of the IgG-Fc. Normal human serum IgG is comprised of multiple glycoforms and biologic activities, other than catabolism, varies between glycoforms.

The antibody paradigm: present and future development as a scaffold for biopharmaceutical drugs

Early studies of the humoral immune response revealed an apparent paradox: an infinite diversity of antibody specificities encoded within a finite genome. In consequence antibodies became a focus of interest for biochemists and geneticists. It resulted in the elucidation of the basic structural unit, the immunoglobulin (Ig) domain, comprised of ~ 100 amino acid residues that generate the characteristic "immunoglobulin (Ig) fold". The Ig fold has an anti-parallel ß-pleated sheet (barrel) structure that affords structural stability whilst the ß-bends allow for essentially infinite structural variation and functional diversity. This versatility is reflected in the Ig domain being the most widely utilised structural unit within the proteome. Human antibodies are comprised of multiple Ig domains and their structural diversity may be enhanced through the attachment of oligosaccharides. This review summarizes our current understanding of the immunoglobulin structure/function relationships and the application of protein and oligosaccharide engineering to further develop the Ig domain as a scaffold for the generation of new and novel antibody based therapeutics.

Vaccine-induced IgG2 anti-HIV p24 is associated with control of HIV in patients with a 'high-affinity' FcgammaRIIa genotype

OBJECTIVES

We have previously shown that vaccination with a recombinant fowlpox virus carrying the genes for HIV Gag-Pol and interferon-gamma (IFN-gamma) was associated with partial control of HIV replication after antiretroviral therapy (ART) was ceased but not with increased anti-HIV T-cell responses. Because IFN-gamma enhances IgG2 production, and IgG2 antibodies to HIV antigens and the 'high-affinity' polymorphism of FcgammaRIIa (the major Fc receptor for IgG2) have been associated with a favourable outcome of HIV infection, we examined the association of IgG2 antibodies to HIV p24 and 'high-affinity' polymorphisms of FcgammaRIIa with control of HIV replication in these patients.

METHODS

Plasma from weeks 0 (cessation of ART 1 week after the last vaccination), 9 and 20 was available from patients who had received the full construct vaccine, a partial construct (without IFN-gamma) or placebo. IgG2 and IgG1 anti-p24 and anti-gp41 were assayed and all patients were genotyped for the FcgammaRIIa 131 R/H polymorphism that affects IgG2 binding.

RESULTS

At week 0, IgG2 anti-p24 was present in five of nine full construct patients but none of 14 partial construct or placebo patients and was associated with a smaller increase in plasma HIV RNA over 20 weeks. Patients with IgG2 anti-p24 and the 'high-affinity' polymorphism of FcgammaRIIa exhibited lower HIV replication than other patients at week 20.

CONCLUSION

The role of IgG2 anti-HIV antibodies and FcgammaRIIa in the control of HIV replication should be investigated further. Inclusion of an IFN-gamma gene in DNA vaccine constructs might be a means of enhancing IgG2 antibody production.

FcγRs in health and disease

Genetic defects affecting the humoral immune response and especially the production of antibodies of the immunoglobulin G (IgG) isotype result in a heightened susceptibility to infections. Studies over the last years have demonstrated the crucial role of Fc-receptors for IgG (FcγRs) widely expressed on innate immune effector cells in mediating the protective function of IgG. During the last years, additional ligands interacting with FcγRs as well as additional receptors binding to IgG glycosylation variants have been identified. In this review, we discuss how the interaction of these different ligands with classical and novel Fcγ-receptors influences the immune response and which strategies microorganisms have developed to prevent them.

Allogeneic hematopoietic stem cell transplantation recipients have defects of both switched and igm memory B cells

Allogeneic hematopoietic stem cell transplant (HSCT) recipients were assessed to elucidate memory B cell defects underlying their increased susceptibility to infections, particularly by encapsulated bacteria. Circulating IgM memory B cells (CD19+, CD27+, IgM+) and switched memory B cells (CD19+, CD27+, IgM(-)) were enumerated in allogeneic HSCT recipients (n = 37) and healthy controls (n = 35). T lymphocyte subpopulations and serum levels of immunoglobulins, including IgG subclasses, and antibodies to pneumococcal polysaccharides were also assayed. Allogeneic HSCT recipients were deficient in both switched memory and IgM memory B cells compared to healthy controls (both P < .0001), irrespective of time post-HSCT. Switched memory B cell deficiency correlated with CD4+ T cell deficiency, and both correlated with serum levels of IgG1 (P < .0001), possibly reflecting impaired B cell isotype switching in germinal centres. "Steady-state" serum levels of antibodies to pneumococcal polysaccharides did not correlate with circulating memory B cells. Graft-versus-host disease (GVHD) was associated with lower IgM memory B cell counts and lower serum levels of IgG2, IgG4, IgA, and pneumococcal antibodies. The increased susceptibility of allogeneic HSCT patients to infection may reflect a combination of memory B cell defects, which are most common in patients with a history of GVHD.

Low-affinity Fcgamma receptors, autoimmunity and infection

Low-affinity Fcgamma receptors (FcgammaRs) mediate the effects of immunoglobulin G (IgG) antibodies on leukocytes, including recruitment to inflammatory lesions, phagocytosis, antibody-dependent cellular cytotoxicity, release of inflammatory mediators and regulation of B cell activation. These functions are an important part of the mammalian response to infection, but if deployed inappropriately can cause autoimmune disease. Although most FcgammaRs are activatory, there is also an inhibitory FcgammaR that, when bound to IgG immune complexes, is able to downregulate the effects of both the activatory FcgammaRs and the B cell receptor. This review discusses the role of the low-affinity FcgammaRs in a balanced immune response and how perturbations in FcgammaR function result in susceptibility to infection or autoimmunity.

Human immunoglobulin allotypes: possible implications for immunogenicity

More than twenty recombinant monoclonal antibodies are approved as therapeutics. Almost all of these are based on the whole IgG isotype format, but vary in the origin of the variable regions between mouse (chimeric), humanized mouse and fully human sequences; all of those with whole IgG format employ human constant region sequences. Currently, the opposing merits of the four IgG subclasses are considered with respect to the in vivo biological activities considered to be appropriate to the disease indication being treated. Human heavy chain genes also exhibit extensive structural polymorphism(s) and, being closely linked, are inherited as a haplotype. Polymorphisms (allotypes) within the IgG isotype were originally discovered and described using serological reagents derived from humans; demonstrating that allotypic variants can be immunogenic and provoke antibody responses as a result of allo-immunization. The serologically defined allotypes differ widely within and between population groups; therefore, a mAb of a given allotype will, inevitably, be delivered to a cohort of patients homozygous for the alternative allotype. This publication reviews the serologically defined human IgG allotypes and considers the potential for allotype differences to contribute to or potentiate immunogenicity.

Early atherosclerosis and IgG2 to bacteria are associated with FcgammaRIIa genotype in non-smokers

BACKGROUND

Involvement of low density lipoprotein (LDL) immune complexes (ICs) in atherogenesis has been proposed. Human FcgammaRIIa receptor (CD32) plays a crucial role in the phagocytosis of IgG(2) ICs and a functional point mutation 131His/Arg diminishes IgG(2) binding to the receptor.

STUDY DESIGN

We examined FcgammaRIIa-131His/Arg polymorphism, IgG(2) antibody titres to oxidized low-density lipoprotein (OxLDL) and Streptococcus pneumoniae cell wall polysaccharide (CWPS) and subclinical atherosclerosis in a large cohort of Finnish subjects (n = 1041).

RESULTS

Non-smoking subjects with homozygous 131His/His genotype had more premature atherosclerosis (P = 0.004) and higher IgG(2) to bacterial CWPS (P = 0.002) compared with other genotypes. Smoking subjects had significantly higher intima-media thickness (IMT) than that of non-smokers (P < 0.001) and genotype-dependent associations were indistinct. There was no association between FcgammaRIIa genotype and antibody titres to OxLDL.

CONCLUSIONS

Our data demonstrate that FcgammaRIIa 131His/Arg polymorphism is associated with subclinical atherosclerosis in non-smoking subjects. Furthermore, FcgammaRIIa genotype is associated with IgG(2) titres to bacterial CWPS, but not to OxLDL. These data propose possible involvement of FcgammaRIIa receptor in atherogenesis.

Antibody therapeutics:

Recombinant monoclonal antibody (rMAb) therapy may be instituted to achieve one of two broad outcomes: i) killing of cells or organisms (e.g., cancer cells, bacteria); and ii) neutralisation of soluble molecules (e.g., cytokines in chronic disease or toxins in infection). The choice of rMAb isotype is a critical decision in the development of a therapeutic antibody as it will determine the biological activities triggered in vivo. It is not possible, however, to accurately predict the in vivo activity because multiple parameters impact on the functional outcome, for example, IgG subclass, IgG-Fc glycoform, epitope density, cellular Fc receptors polymorphisms and so on. The present understanding of the molecular interactions between IgG-Fc and effector ligands in vitro has allowed the generation of new antibody structures with altered/improved effector function profiles that may prove optimal for given disease indications. Thus, when maximal antibody-dependent cell-mediated cytotoxicity activity is indicated a non-fucosylated IgG1 format may be optimal; when minimal activity is indicated an aglycosylated IgG2 may be the form of choice.

Ganglioside-specific IgG and IgA recruit leukocyte effector functions in Guillain-Barré syndrome

The capacity of ganglioside-specific autoantibodies to recruit leukocyte effector functions was studied. Serum samples from 87 patients with Guillain-Barré (GBS) or Miller Fisher syndrome (MFS), containing GM1-, GQ1b-, or GD1b-specific IgG or IgA, were tested for leukocyte activating capacity. Ganglioside-specific IgG antibodies generally induced leukocyte activation, irrespective of specificity. The magnitude of leukocyte degranulation correlated with GM1- and GQ1b-specific IgG titers, but not with disease severity. Finally, GM1-specific IgA activated leukocytes through the IgA receptor, FcalphaRI (CD89). Therefore, both ganglioside-specific IgG and IgA can recruit leukocyte effector functions, which may be relevant in the pathogenesis of GBS and MFS.

FcRn: an IgG receptor on phagocytes with a novel role in phagocytosis

Here, we report that the MHC class I-related neonatal Fc receptor (FcRn) is expressed within azurophilic and specific granules of neutrophils and relocates to phagolysosomes on phagocytosis of IgG-opsonized bacteria. We found FcRn to enhance phagocytosis in a pH-dependent manner which was independent of IgG recycling. IgG-opsonized bacteria were inefficiently phagocytosed by neutrophils from beta2M knock-out or FcRn alpha-chain knock-out mice, which both lack expression of FcRn. Similarly, low phagocytic activity was also observed with mutated IgG (H435A), which is incapable of binding to FcRn, while retaining normal binding to classical leukocyte Fcgamma receptors. Finally, a TAT peptide representing intracellular endocytosis and transport motifs within FcRn strongly inhibited IgG-mediated phagocytosis. These findings support a novel concept in which FcRn fulfills a major role in IgG-mediated phagocytosis.

Pneumococcal vaccine efficacy for mucosal pneumococcal infections depends on Fcγ receptor IIa polymorphism

IgG2 antibodies are the main antibody subclass produced after pneumococcal polysaccharide vaccination. For these antibodies to be effective, interaction with FcgammaIIa receptors on phagocytic cells is necessary. FcgammaRIIa displays a functional polymorphism with either a histidine (H) or arginine (R) at position 131. Interaction of IgG2 antibodies depends on the H131 allele, whereas this interaction is low to absent with the R131 allele. We tested the clinical efficacy of combined pneumococcal conjugate and pneumococcal polysaccharide vaccination according to FcgammaIIa-H/R131 genotype in a randomized double blind placebo controlled vaccination trial in children with a history of acute otitis media. We found a decisive role for the FcgammaIIa-H/R131 polymorphism on the clinical vaccine efficacy of combined pneumococcal conjugate and polysaccharide vaccinations. RR homozygotes showed a significant increase in recurrence of acute otitis media after pneumococcal vaccinations. This cannot be explained by differences in the pneumococcal specific antibody response or differences in nasopharyngeal pneumococcal carriage, but may be explained by less efficient interaction of FcgammaRIIa with polysaccharide-induced IgG2 anti-pneumococcal antibodies in RR homozygotes. Our data show that the genetic make-up of individuals or populations under study should be considered while evaluating vaccine efficacy trials.

Immunologic screening of children with recurrent otitis media

Some 5% to 10% of all infants and toddlers suffer from four or more episodes of otitis per year. Usually, this is a temporary problem that resolves with increasing age. In a minority of cases, otitis episodes are frequent or have an abnormal course, with complications such as mastoiditis. In these cases, immunologic screening is indicated, to exclude an immunodeficiency. Agammaglobulinemia or hypogammaglobulinemia is rare among these patients. Other immune defects that occur more often are deficient or lowered immunoglobulin (Ig)A or decreased levels of one or more IgG subclass, in particular IgG2. The specific antibody response to bacterial capsular polysaccharides often is disturbed. These findings can give direction to the treatment of children with frequent, recurrent otitis.

Polymorphism of the Fcgamma receptor IIA and malaria morbidity

Fc receptors (FcRs) are expressed on the surface of all types of cells of the immune system. They bind the Fc portion of immunoglobulin (Ig), thereby bridging specific antigen recognition by antibodies with cellular effector mechanisms. FcγRIIA, one of the three receptors for human IgG, is a low-affinity receptor for monomeric IgG, but binds IgG immune complexes efficiently. FcγRIIA is believed to play a major role in eliciting monocyte- and macrophage-mediated effector responses against blood-stage malaria parasites. A G → A single nucleotide polymorphism, which causes an arginine (R) to be replaced with histidine (H) at position 131, defines two allotypes which difer in their avidity for complexed human IgG₂ and IgG₃. Because FcγRIIA-H131 is the only FcγR allotype which interacts efficiently with human IgG_2, this polymorphism may determine whether parasite-specific IgG₂ may or may not elicit cooperation with cellular imune responses during blood-stage malaria infection. Here, we review data from four published case-control studies describing associations between FcγRIIA R/H131 polymorphism and malaria-related outcomes and discuss possible reasons for some incongruities found in these available results.

Approach to validating an opsonophagocytic assay for Streptococcus pneumoniae

Streptococcus pneumoniae (pneumococcus) polysaccharide serotype-specific antibodies that have opsonophagocytic activity are considered a primary mechanism of host defense against pneumococcal disease. In vitro opsonophagocytic assays (OPAs) with antibody and complement to mediate opsonophagocytic killing of bacteria have been designed and developed as an adjunct to the standardized serum immunoglobulin G antipneumococcal capsular polysaccharide enzyme immunoassay to assess the effectiveness of pneumococcal vaccines. OPA presents challenges for assay standardization and assay precision due to the multiple biologically active and labile components involved in the assay, including human polymorphonuclear leukocytes or cultured effector cells, bacteria, and complement. Control of these biologically labile components is critical for consistent assay performance. An approach to validating the performance of the assay in accordance with International Conference for Harmonization guidelines, including its specificity, intermediate precision, accuracy, linearity, and robustness, is presented. Furthermore, we established parameters for universal reagents and standardization of the use of these reagents to ensure the interlaboratory reproducibility and validation of new methodologies.

Neutrophil responses to CRP are not dependent on polymorphism of human FcgammaRIIA (R131H)

IgG2a mediated in vitro phagocytosis is less effective for individuals homozygous for Fcgamma RIIaR131 allele and such individuals are also more susceptible to certain infections. It has been reported that CRP binds to Fcgamma RIIaR131 but not Fcgamma RIIaH131 and since Fcgamma RIIa is also a major Fc receptor on neutrophils it would be expected that normal healthy donors who did not have at least one copy of Fcgamma RIIaR131 would not respond to CRP. We examined responses reported to be dependent on FcgammaRIIa but no difference between groups was observed in CRP mediated phagocytosis of S. pneumoniae, reactive oxygen production, or IL-8 synthesis. This suggests that either neutrophil receptors other than Fcgamma RIIa are responsible for CRP mediated responses or differences in CRP binding to the forms of Fcgamma RIIa are comparatively minor.

Immunoglobulin G Fc(gamma) receptor expression on polymorphonuclear cells in bronchoalveolar lavage fluid of HIV-infected and HIV-seronegative patients with bacterial pneumonia

This study was designed to test the hypothesis that impaired neutrophil function might contribute to the development of bacterial pneumonia in patients with HIV-infection. Numbers of inflammatory cells and immunoglobulin G Fcgamma receptor (IgG FcgammaR) I, II, III levels were investigated in bronchoalveolar lavage (BAL) fluid of HIV-seronegative and HIV-infected patients with bacterial pneumonia. The 99 patients were classified into three groups: I: HIV-seronegative and pneumonia (n = 40); II: HIV-infected and pneumonia (n = 19); III: HIV-seronegative with other pulmonary diseases than pneumonia (n = 40). The results of groups I and II, II and III, and I and III were compared. The percentage of alveolar macrophages was significantly lower (group II vs. III: p = 0.005, group I vs. III: p = 0.001), that of neutrophils increased significantly in patients with pneumonia (group II vs. III: p = 0.02, group I vs. III: p = 0.01). Lymphocytes differed only between groups I and III (p = 0.04). Although only the expression of FcgammaRI was significantly higher in HIV-seronegative pneumonia patients compared to those without pneumonia (p = 0.01), the mean expression of all three receptors was lower in the HIV-infected group, with that of FcgammaRI approaching statistical significance. This report provides first evidence that altered FcgammaR expression on BAL neutrophils might contribute to the increased susceptibility of HIV-infected patients to bacterial pneumonia.

Association of the Fc gamma receptor IIA-R/R131 genotype with myasthenia gravis in Dutch patients

Myasthenia gravis (MG) susceptibility is partially determined by allelic heterogeneity of immune-modulatory genes. IgG receptors (FcgammaR) link the humoral and cellular branches of the immune system, and regulate immune responses and inflammation. Three FcgammaR subclasses (FcgammaRIIa, FcgammaRIIIa, and FcgammaRIIIb) exhibit functional polymorphisms, which affect efficiency of FcgammaR-mediated functions. FcgammaRIIa genotypes, but not FcgammaRIIIa and FcgammaRIIIb genotypes, were differentially distributed among 107 MG patients as compared to 239 healthy controls (Pz.Lt;0.01), with a relative increase of the FcgammaRIIa-R/R131 genotype (Odds ratio 2.4, 95% confidence interval 1.4-3.9). These data suggest that the FcgammaRIIa-R/R131 genotype is a marker for susceptibility to MG.

Modulation of polymorphonuclear cell interleukin-8 secretion by human monoclonal antibodies to type 8 pneumococcal capsular polysaccharide

Pneumococcal capsular polysaccharide (PS) vaccines induce type-specific immunoglobulin M (IgM), IgG, and IgA. Type-specific IgG to the PS is sufficient to confer protection against the homologous serotype of the pneumococcus, but the efficacies of type-specific IgM and IgA are less well understood. We examined the in vitro activities and efficacies in mice of two human monoclonal antibodies (MAbs) to type 8 PS, NAD (IgA) and D11 (IgM). MAb-mediated opsonophagocytic killing was evaluated after coculture of type 8 pneumococci with human polymorphonuclear cells (PMNs), type-specific or control MAbs, and human complement sources. The effects of the MAbs on PMN interleukin-8 (IL-8) and IL-6 secretion were determined in supernatants from cocultures containing pneumococci and PMNs by enzyme-linked immunosorbent assay. MAb efficacy was determined in an intratracheal model of type 8 infection in mice with classical complement pathway deficiency. Both MAbs were protective in 100% of infected mice. Neither MAb promoted a significant amount of killing of type 8 pneumococci compared to its isotype control MAb. Both type-specific MAbs mediated complement-dependent modulation of PMN IL-8 secretion, with increased secretion at effector/target (E:T) ratios of 500:1 and 50:1 and reduced secretion at 1:5. Trypan blue staining revealed that PMNs cocultured with D11 were less viable at an E:T ratio of 1:5 than PMNs cocultured with the control MAb. PMN IL-6 secretion was increased by both type-specific and control MAbs. These results suggest that certain type-specific IgM and IgAs might contribute to host defense by modulation of the inflammatory response to pneumococci.

Immunoglobulins in otitis-prone children

Defective or immature antibody responses to pathogens in children may explain the increased susceptibility to acute otitis media (AOM) in otitis-prone children. In literature, data on immunology have been based on studies of small groups of severely otitis-prone children and have not been consistent. Humoral immune status was assessed in 365 children, 1-7 years old, with two or more documented episodes of AOM in the previous year. Children with 4 or more episodes in the preceding year were defined as otitis-prone. Serum immunoglobulin levels were determined by radial immunodiffusion. Immunoglobulin levels of otitis-prone children were compared with those of children who had experienced 2-3 AOM episodes per year. Children with recurrent episodes of AOM were found to have normal or increased serum IgA, IgM, IgG, and IgG1 levels compared with normal values for age, whereas the serum IgG2 levels were mostly in the lower normal range. Twenty-two percent of all children showed IgG2 levels lower than 2 SD below the age-specific mean. Interestingly, the otitis-prone group of children showed significantly lower median and mean levels for all immunoglobulins compared with those children with only 2-3 previous AOM episodes. Lower immunoglobulin levels in otitis-prone children suggest a generalized decreased antibody response in otitis-prone children.

Central role of complement in passive protection by human IgG1 and IgG2 anti-pneumococcal antibodies in mice

Streptococcus pneumoniae is an important cause of morbitity and mortality worldwide. Capsule-specific IgG1 and IgG2 Abs are induced upon vaccination with polysaccharide-based vaccines that mediate host protection. We compared the protective capacity of human recombinant serogroup 6-specific IgG1 and IgG2 Abs in mice deficient for either leukocyte FcR or complement factors. Human IgG1 was found to interact with mouse leukocyte FcR in vitro, whereas human IgG2 did not. Both subclasses induced complement activation, resulting in C3c deposition on pneumococcal surfaces. Passive immunization of C57BL/6 mice with either subclass before intranasal challenge with serotype 6A induced similar degrees of protection. FcgammaRI- and III-deficient mice, as well as the combined FcgammaRI, II, and III knockout mice, were protected by passive immunization, indicating FcR not to be essential for protection. C1q or C2/factor B knockout mice, however, were not protected by passive immunization. Passively immunized C2/factor B(-/-) mice displayed higher bacteremic load than C1q(-/-) mice, supporting an important protective role of the alternative complement pathway. Spleens from wild-type and C1q(-/-) mice showed hyperemia and thrombotic vessel occlusion, as a result of septicemic shock. Notably, thrombus formation was absent in spleens of C2/factor B(-/-) mice, suggesting that the alternative complement pathway contributes to shock-induced intravascular coagulation. These studies demonstrate complement to play a central role in Ab-mediated protection against pneumococcal infection in vivo, as well as in bacteremia-associated thrombotic complications.

FcgammaRIIA polymorphisms in Streptococcus pneumoniae infection

Invasive pneumococcal disease continues to be a major cause of morbidity and mortality among children and adults worldwide. Effective host defence against Streptococcus pneumoniae depends on immunoglobulin G-mediated phagocytosis of the bacteria and it has been shown in vitro that the FcgammaRIIA polymorphism (FcgammaRIIA-R131 vs FcgammaRIIA-H131) determines the capacity of immunoglobulin G2-mediated phagocytosis via this receptor. In this study, we evaluated FcgammaRIIA polymorphisms in children with pneumococcal sepsis and a number of control groups in order to investigate a possible association of FcgammaRIIA genotypes with Streptococcus pneumoniae infection. The distribution of the genotypes differed in these populations. The frequency of homozygosity for FcgammaRIIA-R/R131 in the patients was significantly higher than that in the healthy random donor population (43%vs 21%, P < 0.05). The frequencies of FcgammaRIIA-H/H131 were similar among all groups of individuals, while the incidence of the heterozygous FcgammaRIIA-R/H131 was lower (35%vs 52%, P < 0.05). Thus, it appears that the FcgammaRIIA-H131 polymorphic form, even in the heterozygous form, may be protective for pneumococcal sepsis and children with FcgammaRIIA-R/R131 genotype could be more at risk of infection with invasive Streptococcus pneu-moniae.

Anti-GM1 IgG antibodies induce leukocyte effector functions via Fcgamma receptors

Guillain-Barré syndrome (GBS) is an immune-mediated neuropathy, in which leukocytes and humoral components of the immune system proposedly initiate localized inflammation. An important pathogenic role for anti-GM1 ganglioside antibodies has been suggested. Therefore, we evaluated anti-GM1 IgG antibody-induced leukocyte effector functions such as degranulation and phagocytosis using serum of 24 GBS patients. Serum without anti-GM1 antibodies of 9 GBS patients as well as pooled serum from healthy individuals served as controls. Ten out of 15 (67%) of anti-GM1 IgG positive sera were capable of inducing leukocyte degranulation, and 8 out of 15 (53%) of anti-GM1 IgG positive sera were capable of inducing phagocytosis of GM1-coated beads. In all of these sera anti-GM1 antibody titers were >or=1:800. No leukocyte degranulation or phagocytosis was observed in control sera. Leukocyte activation was completely abrogated in the presence of IgG receptor (FcgammaR) blocking antibodies, suggesting a crucial role for leukocyte FcgammaR in GBS pathogenesis. No correlation of antibody titers with the extent of leukocyte activation, or severity of disease was observed. These data document the capacity of anti-GM1 IgG antibodies to activate leukocyte inflammatory functions, and suggest an important role for anti-ganglioside IgG antibodies in the pathogenesis of GBS.

FcgammaR polymorphisms: Implications for function, disease susceptibility and immunotherapy

Leukocyte Fcgamma receptors (FcgammaR) confer potent cellular effector functions to the specificity of IgG. FcgammaR-induced leukocyte functions, including antibody-dependent cellular cytotoxicity, phagocytosis, superoxide generation, degranulation, cytokine production and regulation of antibody production, are essential for host defense and immune regulation. The efficacy of IgG-induced FcgammaR function displays inter-individual heterogeneity due to genetic polymorphisms of three FcgammaR subclasses, FcgammaRIIa (CD32a), FcgammaRIIIa (CD16a), and FcgammaRIIIb (CD16b). FcgammaR polymorphisms have been associated with infectious and autoimmune disease, or with disease severity. FcgammaR polymorphisms may furthermore serve as markers for therapeutic efficacy and side-effects of treatment with monoclonal antibodies. In this review, FcgammaR function and the relevance of FcgammaR polymorphisms as prognostic markers for inflammatory disease and antibody-based immunotherapy are discussed.

Risk factors in HIV-1-infected patients developing repetitive bacterial infections: toxicological, clinical, specific antibody class responses, opsonophagocytosis and Fc(gamma) RIIa polymorphism characteristics

The aim of the study was to determine possible factors related to the risk of developing recurrent bacterial respiratory tract infections in HIV-1-infected patients, regardless of the degree of immune cellular impairment. Thirty-three HIV-1 seropositive patients with previous repetitive bacterial respiratory tract infections (case group), 33 HIV-1 seropositive controls (matched by CD4-cell counts) without these antecedents and 27 healthy controls were studied before and after administration of pneumococcal and Haemophilus influenzae type b vaccines. Clinical or toxicological variables, cutaneous tests, complement factors, beta2-microglobulin, serum IgM, IgA, IgG and subclasses, specific antibodies (IgG, IgG2, IgA) against pneumococcal vaccine and polyribosylribitol phosphate (PRP), their avidity, opsonophagocytosis and IgG(2)m and Fc(gamma)RIIa allotypes were determined. A history of drug abuse (P = 0.001), less likelihood of receiving high activity antiretroviral treatment high activity antiretroviral treatment (HAART) (P = 0.01), higher levels of HIV-1 viral load (P < 0.05), serum IgG (P < 0.01) and beta2-microglobulin (P < 0.01) were observed in the case group. Also, a lower increase in specific antibodies to pneumococcal vaccine and PRP was demonstrated in the cases in comparison with the two control groups. No differences were observed in the avidity of antibodies, opsonophagocytic capacity or IgG(2)m and Fc(gamma)RIIa allotypes between the three groups. These data indicate that vaccination strategies against encapsulated bacteria can be unsuccessful in the HIV-1-infected patients presenting repetitive bacterial respiratory tract infections if behavioural aspects or measures to improve adherence to HAART therapies are not considered.

Fcgamma receptor-mediated phagocytosis of Plasmodium falciparum-infected erythrocytes in vitro

Although convincing evidence exists for the role of immunoglobulin G (IgG) antibodies in immunity to malaria, antibody titres do not usually predict protection. In this study we have assessed the interaction between Plasmodium falciparum-infected erythrocytes (PE), opsonized with immune serum containing different amounts of IgG antibody isotypes, with either THP-1 cells, ex-vivo human monocytes or IIAI.6 transfectant cells expressing Fc(gamma)RIIa-Arg/Arg131 or -His/His131 allotypes. Our results show that PMA-treated THP-1 cells were capable of phagocytosing serum-opsonized PE by Fc(gamma)RI (CD64) and Fc(gamma)RIIa (CD32), acting synergistically. The known Fc(gamma)RIIa polymorphism motivated us to examine its influence on IgG isotype-mediated phagocytosis of opsonized PE with human monocytes and the IIAI.6 transfectant cells expressing either allelic forms. Regardless of the cell type, PE phagocytosis with Fc(gamma)RIIa-His/His131 was highest following opsonization with a predominantly IgG3-containing immune serum pool. In contrast, PE phagocytosis with Fc(gamma)RIIa-Arg/Arg131 tended to be higher with an IgG1-containing pool. These results suggest a genetically determined influence of effector cell phenotype on IgG antibody-pathogen interaction in P. falciparum malaria.

Current state of pneumococcal vaccines

Streptococcus pneumoniae is a leading cause of bacterial pneumonia, meningitis, and acute otitis media in children and adults worldwide. According to World Health Organization estimates, at least 1 million children under 5 years of age die each year from pneumococcal pneumonia. The emergence of resistant strains necessitates the development of an effective vaccine with a large serotype coverage. The 11 most common serotypes cause 72-83% of all serious pneumococcal diseases worldwide. Currently marketed 23-valent pneumococcal polysaccharide vaccine provides large serotype coverage and offers a less expensive option. However, it is efficacious only in adults but not in infants. Conjugate vaccines offer a solution by generating immunological memory already at early age. A recently licensed 7-valent conjugate vaccine is immunogenic and efficacious in infants. Its serotype coverage might be sufficient in Europe and North America, but not in Africa, Asia and Oceania. A need exists to develop pneumococcal vaccines with lower cost and larger serotype coverage. Several 11-valent pneumococcal conjugate vaccines are being evaluated in phase I-III trials. This study reviews the current state of pneumococcal problem and pneumococcal vaccines in clinical use.

Fcgamma receptors in autoimmune diseases

Fcgamma-receptors (Fcgamma-R) recognise the Fc portion of IgG and thus form a link between humoral and cellular immunity. These receptors are expressed by a variety of immune cells, and they function in the binding of immune complexes or IgG-opsonised particles, such as microbial pathogens. The are three major types of Fcgamma-R, namely Fcgamma-RI (CD64), Fcgamma-RII (CD32) and Fcgamma-RIII (CD16), and these differ in their ability to bind IgG and complexes. There are many isoforms of these receptors and a number of recently identified polymorphisms in their structure. This review describes the structure and function of these Fcgamma-Rs, and highlights how gene deficiencies and polymorphisms may contribute to the pathology of human diseases.

Immunoglobulin A-mediated protection against Bordetella pertussis infection

Infection with Bordetella pertussis, the causative agent of pertussis (whooping cough) in humans, is followed by the production of antibodies of several isotypes, including immunoglobulin A (IgA). Little is known, however, about the role of IgA in immunity against pertussis. Therefore, we studied targeting of B. pertussis to the myeloid receptor for IgA, FcalphaRI (CD89), using either IgA purified from immune sera of pertussis patients or bispecific antibodies directed against B. pertussis and FcalphaRI (CD89 BsAb). Both IgA and CD89 BsAb facilitated FcalphaRI-mediated binding, phagocytosis, and bacterial killing by human polymorphonuclear leukocytes (PMNL) and PMNL originating from human FcalphaRI-transgenic mice. Importantly, FcalphaRI targeting resulted in enhanced bacterial clearance in lungs of transgenic mice. These data support the capacity of IgA to induce anti-B. pertussis effector functions via the myeloid IgA receptor, FcalphaRI. Increasing the amount of IgA antibodies induced by pertussis vaccines may result in higher vaccine efficacy.

Mononuclear cells ingest E. coli opsonized by investigational intravenous immunoglobulin preparations in the absence of complement more efficiently than polymorphonuclear phagocytes

OBJECTIVE

The increasing use of intravenous immunoglobulins (IVIg) to treat systemic inflammatory, mostly autoimmune, diseases raises the concern that their competition for FcgammaReceptor (FcgammaR) binding with immune complexes might interfere with the removal of opsonized microbial agents more than it favoured their opsonophagocytosis.

METHODS

Using a sensitive FACS-based phagocytosis assay allowing for the identification of the phagocytosing cell-type, we have tested the relative capacity of Pentaglobin and two investigational IVIg preparations, IVIgG and IVIgM, to promote E. coli phagocytosis. The plasma IL-8 levels 4 h after phagocytosis were measured by ELISA.

RESULTS

Heat inactivated, complement (C) depleted serum allowed about 25% polymorphonuclear leukocytes (PMNLs) and 15% monocytes (MNs) of 16 donors to phagocyte E. coli. The addition of IVIg favoured mean phagocytosis by MNs to 55%, but by PMNLs only to 25%. The addition of fresh autologous plasma as a C source resulted in a boost of phagocytosing capacity to reach >90% of the PMNLs and >72% of the MNs. Blockade of FcgammaRII and FcgammaRI using monoclonal antibodies (mAb) reduced phagocytosis and IL-8 release.

CONCLUSION

We conclude that excess IVIgG/M preparations added to a phagocytic system of PMNLs and MNs, without preopsonization of E. coli, promotes phagocytosis as expected, but MNs rather than PMNLs do this more thoroughly on their own.

Flow cytometry-based phagocytosis assay for sensitive detection of opsonic activity of pneumococcal capsular polysaccharide antibodies in human sera

The development of efficient vaccines against Streptococcus pneumoniae is of major importance for public health. The efficacy of pneumococcal vaccination and induced protection are thought to be reflected by the opsonic antibody titers in sera from vaccines. We describe a novel two-color flow cytometry technique for quantification of antibody-mediated pneumococcal phagocytosis. Serum-opsonised fluorescein-isothiocyanate (FITC)-labelled S. pneumoniae were allowed to attach to neutrophils, split into two aliquots and further incubated either at 4 degrees C (to avoid phagocytosis) or 37 degrees C (to allow phagocytosis). Cell-surface residual opsonic IgG was detected by phycoerythrin (PE)-conjugated anti-human IgG in both samples. The fraction of FITC-labelled bacteria phagocytosed via antibody (F(i)) could be estimated from FITC and PE labels, and reflected the opsonic activity of sera. The technique displayed high sensitivity for the detection of opsonic antibodies, as shown by experiments using pre- and post-immune sera, which documented significantly increased phagocytosis after vaccination, and the observed increase in phagocytosis rates at higher antibody levels. The intrinsic variation of the assay was low, and could be further reduced by the use of effector cells from donors with similar IgG receptor (FcgammaR) allotypes. The method described in this study should be generally applicable to test vaccine efficacy, to evaluate the interaction of bacteria and phagocytes, and to discriminate between antibody-mediated and antibody-independent interactions between bacteria and phagocytes.

Activity of human IgG and IgA subclasses in immune defense against Neisseria meningitidis serogroup B

Both IgG and IgA Abs have been implicated in host defense against bacterial infections, although their relative contributions remain unclear. We generated a unique panel of human chimeric Abs of all human IgG and IgA subclasses with identical V genes against porin A, a major subcapsular protein Ag of Neisseria meningitidis and a vaccine candidate. Chimeric Abs were produced in baby hamster kidney cells, and IgA-producing clones were cotransfected with human J chain and/or human secretory component. Although IgG (isotypes IgG1-3) mediated efficient complement-dependent lysis, IgA was unable to. However, IgA proved equally active to IgG in stimulating polymorphonuclear leukocyte respiratory burst. Remarkably, although porin-specific monomeric, dimeric, and polymeric IgA triggered efficient phagocytosis, secretory IgA did not. These studies reveal unique and nonoverlapping roles for IgG and IgA Abs in defense against meningococcal infections.

Comparison of a classical phagocytosis assay and a flow cytometry assay for assessment of the phagocytic capacity of sera from adults vaccinated with a pneumococcal conjugate vaccine

Antibody- and complement-mediated phagocytosis is the main defense mechanism against Streptococcus pneumoniae. A standardized, easy to perform phagocytosis assay for pneumococci would be a great asset for the evaluation of the potential efficacy of (experimental) pneumococcal vaccines. Such an assay could replace the laborious phagocytosis assay of viable pneumococci (classical killing assay). Therefore, a newly developed phagocytosis assay based on flow cytometry (flow assay) was compared with the conventional killing assay and enzyme-linked immunosorbent assay (ELISA), using sera obtained from adults pre- and postvaccination with either a bivalent conjugate, a tetravalent conjugate, or the 23-valent polysaccharide vaccine. Highly significant correlations were observed between flow assay phagocytosis titers, killing assay phagocytosis titers, and ELISA antibody titers for serotype 6B and 23F as well. For serotype 19F, strong correlations were only observed between killing assay and ELISA titers. A potential drawback of the flow assay might be the low sensitivity compared with that of the killing assay. The choice of what assay to use, however, will depend on the objectives of the assay. When speed, easy performance, sample throughput, improved worker safety, absence of influence of antibiotics, and absence of false positives are the major criteria, the flow assay is the method of choice. When higher sensitivity is the major requirement, the classical killing assay should be used.

Evaluation of human FcgammaRIIA (CD32) and FcgammaRIIIB (CD16) polymorphisms in Caucasians and African-Americans using salivary DNA

Two classes of low-affinity receptors for the Fc region of immunoglobulin G (IgG) (FcgammaR) are constitutively expressed on resting human neutrophils. These receptors, termed FcgammaRIIa (CD32) and FcgammaRIIIb (CD16), display biallelic polymorphisms which have functional consequences with respect to binding and/or ingestion of targets opsonized by human IgG subclass antibodies. The H131-R131 polymorphism of CD32 influences binding of human IgG2 and, to a lesser extent, human IgG3 to neutrophils. The neutrophil antigen (NA1-NA2) polymorphism of CD16 influences the efficiency of phagocytosis of bacteria opsonized by human IgG1 and IgG3. These polymorphisms may influence host susceptibility to certain infectious and/or autoimmune diseases, prompting interest in the development of facile methods for determination of CD32 and CD16 genotype in various clinical settings. We previously reported that genomic DNA from saliva is a suitable alternative to DNA from blood in PCR-based analyses of CD32 and CD16 polymorphisms. In the present study, we utilized for the first time this salivary DNA-based methodology to define CD32 and CD16 genotypes in 271 Caucasian and 118 African-American subjects and to investigate possible linkage disequilibrium between certain CD32 and CD16 genotypes in these two ethnic groups. H131 and R131 gene frequencies were 0.45 and 0.55, respectively, among Caucasians and 0.59 among African-Americans. NA1 and NA2 gene frequencies were 0.38 and 0.62 among Caucasians and 0. 39 and 0.61 among African-Americans. Since FcgammaRIIa and FcgammaRIIIb synergize in triggering neutrophils, we also assessed the frequency of different CD32 and CD16 genotype combinations in these two groups. In both groups, the R/R131-NA2/NA2 genotype combination was more common than the H/H131-NA1/NA1 combination (threefold for Caucasians versus sevenfold for African-Americans). Whether individuals with the combined R/R131-NA2/NA2 genotype are at greater risk for development of infectious and/or autoimmune diseases requires further investigation, which can be conveniently performed using DNA from saliva rather than blood.

Association between FcgammaRIIa-R131 allotype and bacteremic pneumococcal pneumonia

Human FcgammaRIIa has 2 codominantly expressed allotypes, which differ greatly in their ability to ligate immunoglobulin G2 (IgG2). Whereas FcgammaRIIa-R131 binds only weakly to IgG2, FcgammaRIIa-H131 binds to it efficiently and might be primarily responsible for the phagocytosis of IgG2-opsonized bacteria. IgG2 plays a pivotal role in defense against pneumococcal infection. This prospective study showed that 50% of patients with bacteremic pneumococcal pneumonia were homozygous for FcgammaRIIa-R131, compared with 28% with nonbacteremic pneumococcal pneumonia and 29% of uninfected controls (P<.05). The gene frequency of FcgammaRIIa-R131 was 0.67 in bacteremic patients, significantly higher than in the other groups (P<.05). All bacteremic patients who died within 1 week of hospitalization were homozygous for FcgammaRIIa-R131. Therefore, the severity of pneumococcal infection may, in part, be genetically mediated. Taken together with similar findings in cases of meningococcal disease, these results suggest that such genetic factors may be generalizable to infections caused by encapsulated bacteria.