Structure and Biological Activity of Immunoglobulins

Antibody constant region peptides can display immunomodulatory activity through activation of the Dectin-1 signalling pathway

We previously reported that a synthetic peptide with sequence identical to a CDR of a mouse monoclonal antibody specific for difucosyl human blood group A exerted an immunomodulatory activity on murine macrophages. It was therapeutic against systemic candidiasis without possessing direct candidacidal properties. Here we demonstrate that a selected peptide, N10K, putatively deriving from the enzymatic cleavage of the constant region (Fc) of human IgG₁, is able to induce IL-6 secretion and pIkB-α activation. More importantly, it causes an up-regulation of Dectin-1 expression. This leads to an increased activation of β-glucan-induced pSyk, CARD9 and pIkB-α, and an increase in the production of pro-inflammatory cytokines such as IL-6, IL-12, IL-1β and TNF-α. The increased activation of this pathway coincides with an augmented phagocytosis of non opsonized Candida albicans cells by monocytes. The findings suggest that some Fc-peptides, potentially deriving from the proteolysis of immunoglobulins, may cause an unexpected immunoregulation in a way reminiscent of innate immunity molecules.

Peptides of the constant region of antibodies display fungicidal activity

Synthetic peptides with sequences identical to fragments of the constant region of different classes (IgG, IgM, IgA) of antibodies (Fc-peptides) exerted a fungicidal activity in vitro against pathogenic yeasts, such as Candida albicans, Candida glabrata, Cryptococcus neoformans, and Malassezia furfur, including caspofungin and triazole resistant strains. Alanine-substituted derivatives of fungicidal Fc-peptides, tested to evaluate the critical role of each residue, displayed unaltered, increased or decreased candidacidal activity in vitro. An Fc-peptide, included in all human IgGs, displayed a therapeutic effect against experimental mucosal and systemic candidiasis in mouse models. It is intriguing to hypothesize that some Fc-peptides may influence the antifungal immune response and constitute the basis for devising new antifungal agents.

LOCALIZATION OF ANTIBODIES BY ELECTRON MICROSCOPY IN DEVELOPING ANTIBODY-PRODUCING CELLS

The development of antibody-producing cells in the early stages of the secondary or hyperimmune response has been studied with the electron microscope in lymph nodes of adult chinchilla rabbits immunized with ferritin or apoferritin. The intracellular distribution of antiferritin antibodies was determined in the lymph node cells at 1 to 5 days after a booster injection, employing the labelling technique previously used by the authors (12) to demonstrate the localization of antibodies in mature plasma cells. Antibodies were first detected at 48 hours in blasts; i.e., cells which have a poorly developed endoplasmic reticulum and a cytoplasm filled with many ribosomes grouped in clusters. The label was subsequently found in forms intermediate between blasts and plasma cells (plasmoblasts, immature plasma cells), in which the endoplasmic reticulum appeared progressively more developed. Antiferritin antibodies were also found in cells in mitosis. In all the above cell types, antigen-antibody precipitates were consistently found in the perinuclear space and in the cisternae of the granular endoplasmic reticulum, from an early stage in the development of the latter. Evidence was also obtained for the presence of antibody in the Golgi area. The results are discussed in relation to the possible cellular sites of antibody synthesis.

Structural mapping of immunoglobulin subclasses using multiplexed bead flow cytometry

Monoclonal hybridomas secrete immunoglobulin molecules with a single specificity and distinct class/subclass structure. The determination of immunoglobulin structure can be used to facilitate hybridoma colony management and predict monoclonality. In this report, we used multiplexed bead flow cytometry to define hybridoma class/subclass. The assay was sufficiently sensitive to detect 50 ng/mL of antibody. The multiplexed bead assay efficiently defined traditional class/subclass determinants as well as more subtle patterns of crossreactivity. Further, the assay was combined with Poisson statistics to provide a numerical estimate of hybridoma monoclonality. The sensitivity and flexibility of this approach suggests the utility of multiplexed bead flow cytometry in the early management of immunoglobulin-secreting hybridomas.

Effect of polyclonal immunoglobulins on neutrophil phagocytic capacity and reactive oxygen production in patients with gram-negative septicemia

The effect of immunoglobulin (Ig) preparations on neutrophil phagocytic ability and oxidative burst in response to Escherichia coli stimulation was analyzed in 14 patients with gram-negative septicemia by an ex vivo whole blood assay using flow cytometry. In patients, neutrophils exhibited a decreased capacity to phagocytize E. coli and generate reactive oxygen products compared to healthy controls (median -68%, P < 0.01). The addition of both 7S-Ig and 19S-Ig enriched preparations in vitro resulted in a dose-dependent increase in neutrophil reactive oxygen production at concentrations of 10 g/l (median +153% and +211%, P < 0.01, respectively) and 20 g/l (median +205% and +282%, P < 0.01, respectively). A decreased neutrophil phagocytic ability was seen in patients with septicemia (median -58%) compared to healthy controls (P < 0.01). Again, the addition of 7S and 19S-Igs enhanced the phagocytic ability in a dose-dependent manner (10 g/l: median +56 and +126%; 20 g/l: median +126% and +165%, P < 0.01 for all). It can be concluded that both polyclonal Igs can increase depressed neutrophil reactive oxygen production and neutrophil phagocytosis in patients with gram-negative septicemia.

STUDIES ON HUMAN ANTIBODIES. 3. AMINO ACID COMPOSITION OF FOUR ANTIBODIES FROM ONE INDIVIDUAL

Antilevan, antidextran, antiteichoic acid, anti-"A" and normal γG, all isolated from a single human, as well as pooled γG and γM, were analyzed for amino acid content. Differences ranging from 7 to 32 residues were found among these antibodies in glycine, valine, leucine, tyrosine, arginine, lysine, and threonine content. These conspicuous differences in amino acid composition were not correlated with the Gm type of these antibodies. Hydroxylysine was found in two of the four antibodies, antidextran and antiteichoic acid, and in γM- immunoglobulin. No hydroxylysine was found in γG- globulins. These findings probably account for discrepancies in the literature concerning the occurrence of this residue in antibody protein.

Comparison of two types of intravenous immunoglobulins in the treatment of neonatal sepsis

In a prospective double-blind study, standard intravenous immunoglobulin (IVIG) was compared with an IgM-enriched IVIG in the treatment of neonatal sepsis. The two treatment groups were also compared with matched controls. One hundred and thirty babies (65 in each group) ranging from 0 to 24 days old, 480 to 4200 g in weight and born between 24 and 42 weeks of gestation who had, or were suspected of having, sepsis were given either standard IVIG or IgM-enriched IVIG (250 mg/kg per day) for 4 days in addition to supportive and antibiotic therapy. A further 65 babies who received similar supportive, antibiotic and fluids but not IVIG were used as matched controls. Mortality from infection in 'culture proven sepsis' was 3/44 (6.8%) in the IgM-enriched IVIG group, 6/42 (14.2%) in the standard IVIG group, and 11/43 (25.5%) in the control group (P = 0.017, IgM versus control, P = 0.19 standard IVIG versus control). There was no statistical difference in the outcome between the two immunoglobulin therapy groups (P = 0.25). The study indicates that IVIG improves outcome in neonatal sepsis when used as an adjunct to supportive and antibiotic therapy, but larger studies are required to confirm this.

Characterization of the cooperative cross-linking of doxorubicin N-hydroxysuccinimide ester derivatives to water soluble proteins

Protein-anthracycline interactions have been examined by using reactive N-hydroxysuccinimide ester derivatives of doxorubicin. These compounds cross-link to lysine epsilon-amino groups with high efficiency and offer the possibility for structural studies of protein-anthracycline complex formation by using gel filtration, ultracentrifugation and spectrophotometric methods. The results are in accordance with association of anthracycline to the hydrophobic ligand binding cavities of serum albumin. The results for proteins not having hydrophobic domains (IgG, serum transferrin, lactotransferrin, ovotransferrin) suggest that complex formation is cooperative and involves two steps: initial self-association of anthracycline into aggregated structures and subsequent binding of protein at the aggregate surface. With serum transferrin, anthracycline self-association makes possible the assembly of stable nanometer-sized protein-anthracycline particles held together by non-covalent bonds. This reaction, which is highly reproducible and efficient, may have applications in the field of development of anthracycline carrier systems.

A microplate method for the determination of amino groups in monoclonal antibodies

The total number of amino groups in monoclonal antibodies can be estimated via a colorimetric assay using trinitrobenzene sulfonic acid (A.F.S.A. Habeeb (1966) Anal. Biochem. 14, 328-336). This work describes the adaptation of this method to a microtiter plate in order to rapidly and reliably assay large numbers of samples.

Oxidative cross-linking of immune complexes by human polymorphonuclear leukocytes

Incubation of human serum albumin-anti-human serum albumin immune complexes bound to a plastic surface, with human polymorphonuclear leukocytes for 1 h at 37 degrees C resulted in covalent cross-linking of 8.5% +/- 0.5 of the complexes, corresponding to a minimum rate of 700 antibody molecules per cell per minute. Similar results were obtained with IgG-anti-IgG and type II collagen-anticollagen II human antibodies. Cross-linking was defined as the antibody remaining attached to plastic-bound antigen after extraction with 3 M MgCl2 and 0.1 N HCl solutions. The effects of addition of oxygen radical scavengers, heme-enzyme inhibitors, and omission of Cl- indicated that the cross-linking process was mediated by the myeloperoxidase-H2O2-Cl- system. Cross-linking was also obtained with cell lysates, polymorphonuclear granules, and purified human myeloperoxidase in the presence of a steady flux of H2O2 provided by glucose oxidase-glucose. Cross-linking by the cell-free systems was also abolished by sodium azide or omission of chloride ions. Cross-linked immune complexes were also generated by incubation with 20 to 50 microM solutions of freshly distilled hypochlorous acid. Addition of 10 mM hypochlorous acid to soluble IgG resulted in the formation of protein precipitates insoluble in 5 M guanidine, 0.1 N HCl, or boiling 2.3 M sodium dodecyl sulfate-1.4 M 2-mercaptoethanol. The remaining soluble IgG contained fluorescent high molecular aggregates (ex: 360 nm; em: 454 nm). Oxidative cross-linking of antigen-antibody molecules, and of immune complexes to connective tissue macromolecules may play a pathogenic role in acute and chronic inflammatory processes.

Gene conversion in human immunoglobulin gamma locus shown by unusual location of IgG allotypes

The constant region of the gamma 1, gamma 2 and gamma 3 heavy chains of the human IgG1, IgG2 and IgG3 immunoglobulins carries antigenic determinants or G1m, G2m and G3m allotypes, which are genetic markers of these subclasses. The exceptional presence on gamma 1 and gamma 2 chains of Gm allotypes usually located on the CH3 domain of gamma 3 shows an unexpected clustering of base changes and subsequent identity of short DNA sequences in the CH3 exon of the non-allelic gamma 1, gamma 2 and gamma 3 genes. Such clusters of substitutions are not easily explained on the classical basis of point mutations. A gene conversion, which substituted a segment of the gamma 1 or gamma 2 gene with the homologous region of the non-allelic gamma 3 gene, is more likely. Other examples of possible conversion involving the gamma genes are described. The conservation or the restoration of short sequences produced by the conversion events might be related to the biological properties of the constant region of the heavy chains.

The antibody paradox: trying on a pair of genes

Rodney Porter's separation of antibody molecules into Fab and Fc fragments engendered the notion that a single antibody polypeptide chain might be coded by two or more genes. This concept profoundly influenced the development of molecular immunology over the past 25 years. Our current knowledge of antibody gene organization has enabled investigators to recombine antibody genes to create 'chimeric' antibodies with a number of potentially useful applications.

Fragmentation and reduction of bovine secretory component. Preparation of a biologically active fragment and some evidence for a multiple-domain structure

A tryptic fragment (A) of Mr 25000 was prepared from bovine secretory component. The fragment binds polymeric immunoglobulin, although 9 times less effectively than secretory component on a molar basis. The fragment has four buried half-cystine residues and two exposed half-cystine residues. It gives rise to two fragments of Mr 11000-13000 on prolonged digestion with trypsin, and these do not bind polymeric immunoglobulin. It is proposed that fragment A consists of two immunoglobulin-like domains. Bovine secretory component was found to have 9-11 buried half-cystine residues and four exposed half-cystine residues. Reduction and alkylation of the exposed residues decreases the binding of polymeric immunoglobulin by 3-fold. Initial tryptic cleavage of bovine secretory component gives a fragment (Q) disulphide-bridged to a further fragment (T). Fragment Q is similar in size to a three-domain immunoglobulin fragment, and fragment T is similar in size to a two-domain immunoglobulin fragment. The two-domain fragment A is derived from fragment Q by further tryptic cleavage. The results are compatible with the proposal by Mostov, Friedlander & Blobel [(1984) Nature (London) 308, 37-43] that secretory component consists of multiple immunoglobulin-like domains. The results also indicate that optimal binding of polymeric immunoglobulin involves several domains stabilized by an exposed disulphide bridge.

Structural studies on dissociated porcine gastric mucus gel

Reduction of the insoluble porcine mucus gel with dithiothreitol, followed by molecular sieve chromatography, resulted in the recovery of the two fragments of molecular size 2.54 X 10(6) and 1.75 X 10(6). Both gastric mucus glycoproteins have been cleaved by pronase, papain or trypsin into fragments whose molecular sizes range from 5.60 X 10(5) to 1.87 X 10(6) daltons. On reduction, the enzyme-treated fragments were further dissociated into smaller fragments which have been characterized by SDS-PAGE. The evidence indicates that the pol: neric structure of gastric mucus differs from that of mucus of varying origins, in particular with respect to the size and number of component subunits per native molecule.

Immune response to lactate dehydrogenase-elevating virus: isolation of infectious virus-immunoglobulin G complexes and quantitation of specific antiviral immunoglobulin G response in wild-type and nude mice

Lactate dehydrogenase-elevating virus (LDV) causes a normally benign persistent infection of mice, resulting in a life-long viremia characterized by the presence of circulating infectious immune complexes, impaired clearance of certain enzymes from the blood, and modification of the host immune response to various heterologous antigens. In this study, we isolated infectious immunoglobulin G (IgG)-LDV complexes in the plasma of persistently infected mice by adsorption to and elution from protein A-Sepharose CL-4B. We found that practically all infectious LDV in the plasma of persistently infected mice is complexed to IgG. LDV infectivity in these complexes was partially neutralized, but could be reactivated by treatment with 2-mercaptoethanol. We also quantitated total plasma IgG and anti-LDV IgG in wild-type and nude Swiss and BALB/c mice as a function of the time after infection with LDV by radial immunodiffusion and an enzyme-linked immunosorbent assay, respectively. Total plasma IgG levels nearly doubled in BALB/c mice during 150 days of infection. IgG levels in uninfected nude mice were only 20% of those in uninfected BALB/c mice, but during infection with LDV increased to approximately those found in uninfected BALB/c mice. Anti-LDV IgG levels were almost as high in nude mice as in normal BALB/c mice. Isoelectric focusing of purified IgG from BALB/c mice showed that LDV infection resulted in the enhanced synthesis of all 16 normal IgG fractions that we could separate by this method, which suggests that LDV infection results in polyclonal activation of IgG-producing lymphocytes.

Immune response to lactate dehydrogenase-elevating virus: serologically specific rabbit neutralizing antibody to the virus

A rabbit was immunized with large amounts of the lactate dehydrogenase-elevating virus (LDV) over a 9-month period. The plasma from this rabbit possessed an anti-LDV IgG titer of 1:80,000 as measured by the enzyme-linked immunosorbent assay (ELISA) method and a neutralizing titer of 1:1,000 for the homologous strain of LDV. LDV neutralization at 4 degrees C followed single-hit kinetics. In contrast, mouse anti-LDV IgG in plasma of chronically LDV-infected mice failed to neutralize LDV at 4 degrees C and neutralization at 37 degrees C was slow, biphasic, and inefficient compared with the neutralization caused by rabbit anti-LDV IgG, even though high levels of anti-LDV IgG were detectable in mouse plasma by the ELISA method. Rabbit anti-LDV IgG neutralized one heterologous strain of LDV as rapidly as it did the homologous strain, but failed to significantly neutralize five other strains of LDV, all of which were originally isolated from different mouse strains bearing transplantable tumors. The results indicate clear serological differences between LDV strains. Cross-reactions between the strains, however, were observed by ELISA, using the antibody induced during persistent infection of mice with each LDV strain. Immunoglobulin M (IgM) from mice infected for 15 days with the various strains bound equally to our LDV strain. IgG obtained from 2-month-infected mice also cross-reacted, but to a varying extent which partly correlated with the specificity detected by neutralization. Both rabbit and mouse anti-LDV IgG enhanced the infectivity of LDV at a low multiplicity of infection for primary cultures of peritoneal mouse macrophages.

The role of bacterial surface structures in pathogenesis

Modern research has revealed that the true surfaces of animal cells consist of polysaccharide chains that are linked to proteins hydrophobically anchored in the membrane and protrude to form a dense glycocalyx. It has become increasingly clear that most pathogenic bacteria must position themselves at the surface of their "target" cell in order to exert their toxic or otherwise deleterious effects. The true surface of most pathogenic bacteria has also been recently shown to consist of a protruding mass of polysaccharide chains--the bacterial glycocalyx--that is composed of teichoic acids in many gram-positive species and of acid polysaccharides in many gram-negative organisms. Through this bacterial glycocalyx certain cell surface proteins and organized protein structures (e.g., pili) are known to project, so that the bacterial surface is a mosaic of polysaccharides and proteins; both of these types of molecules have been implicated in instances of specific pathogenic adhesion. Besides their role in specific adhesion to target cells, these surface components interpose a highly charged, and often very extensive, barrier that can prevent the penetration of antibodies and antibiotics to their target sites in the bacterial cell. They may also frustrate mucociliary clearance, phagocytosis, and other clearance mechanisms of the host. We will discuss the chemical and physical nature of these bacterial surface components that mediate pathogenic adhesion and counteract host defense mechanisms sufficiently to allow infections to become established.

Local immune system of nasal nucosa in inflammation. IgA distribution and secretory activity

The local immune system of human mucosa (inferior turbinates and nasal polyps) from 53 humans with chronic inflammation was studied by examining the distribution and secretory activity of IgA using an immunofluorescent technique. Diffuse fluorescence for IgA was observed throughout the connective tissue ground substance in every nasal turbinate and nasal polyp. In mucosa with mild inflammatory changes, most IgA seemed to be produced locally by IgA-producing plasma cells. However, in severely inflamed nasal mucosa, including nasal polyps, plasma cells were relatively fewer in number and the distribution of IgA generally corresponded to the pattern of fibrin deposition. Secretory activity of IgA in the glandular cells of nasal turbinates was well retained. Only in dilated glandular ductules in nasal polyps was the secretory activity impaired.

Protein thiolation and reversible protein-protein conjugation. N-Succinimidyl 3-(2-pyridyldithio)propionate, a new heterobifunctional reagent

A heterobifunctional reagent, N-succinimidyl 3-(2-pyridyldithio)propionate, was synthesized. Its N-hydroxysuccinimide ester group reacts with amino groups and the 2-pyridyl disulphide structure reacts with aliphatic thiols. A new thiolation procedure for proteins is based on this reagent. The procedure involves two steps. First, 2-pyridyl disulphide structures are introduced into the protein by the reaction of some of its amino groups with the N-hydroxysuccinimide ester sie of the reagent. The protein-bound 2-pyridyl disulphide structures are then reduced with dithiothreitol. This reaction can be carried out without concomitant reduction of native disulphide bonds. The technique has been used for the introduction of thiol groups de novo into ribonuclease, gamma-globulin, alpha-amylase and horseradish peroxidase. N-Succinimidyl 3-(2-pyridyldithio)propionate can also be used for the preparation of protein-protein conjugates. This application is based on the fact that protein-2-pyridyl disulphide derivatives (formed from the reaction of non-thiol proteins with the reagent) react with thiol-containing proteins (with native thiols or thiolated by, for example, the method described above) via thiol-disulphide exchange to form disulphide-linked protein-protein conjugates. This conjugation technique has been used for the preparation of an alpha-amylase-urease, a ribonuclease-albumin and a peroxidase-rabbit anti-(human transferrin) antibody conjugate. The disulphide bridges between the protein molecules can easily be split by reduction or by thiol-disulphide exchange. Thus conjugation is reversible. This has been demonstrated by scission of the ribonuclease-albumin and the alpha-amylase-urease conjugate into their components with dithiothreitol. N-Succinimidyl 3-(2-pyridyldithio)propionate has been prepared in crystalline form, in which state (if protected against humidity) it is stable on storage at room temperature (23 degrees C).

Myasthenic antibodies cross-link acetylcholine receptors to accelerate degradation

The decrease of acetylcholine receptors at neuromuscular junctions of myasthenic patients has been attributed to an antibody-mediated autoimmune process that accelerates receptor degradation. We studied the mechanism of this process in skeletal-muscle cultures, using intact antibodies and antibody fragments. Addition of myasthenic IgG or its divalent fragment, F(ab')2, to cultures accelerated the rate of acetylcholine-receptor degradation threefold. By contrast, the monovalent fragment, Fab, from myasthenic serum had no effect on degradation, although it bound to acetylcholine receptors. Addition of a second, "piggyback" antibody to cross-link the Fab:receptor complexes resulted in a threefold increase of the degradation rate. Similarly, when acetylcholine receptors with bound alpha-bungarotoxin were cross-linked by the addition of specific antibody against alpha-bungarotoxin, the degradation rate increased approximately threefold. The effect of myasthenic patients' antibodies in accelerating degradation of acetylcholine receptors is attributed to their ability to cross-link the receptors.

Immunoglobulin gamma chains of a monotreme mammal, the echidna (Tachyglossus aculeatus): amino acid composition and partial amino acid sequence

The echidna represents the lowest stage in phylogeny at which molecules clearly homologous to IgG antibodies appear to occur. We provide evidence that a fraction of gamma chains possess an unblocked N terminal sequence comparable to the VHIII sub-group of human gamma chains and that glycine is the C-terminal residue. Statistical comparison of amino acid composition of the component chains with other immunoglobulin heavy chains suggests that echidna gamma chains are more closely related to eutherian gamma chains than to the 7S Ig heavy chains from amphibia or aves. The results are consistent with our view that true gamma-type heavy chains did not appear in evolution until after the mammalian line diverged from the stem reptiles.

Modification of antityphus antibodies on passage through the gut of the human body louse with discussion of some epidemiologic and evolutionary implications

Evidence is presented to indicate that proteolytic and perhaps other enzymes of the louse midgut, essential to the nutrition of the louse, perform molecular dissection on the antirickettsial antibodies present in the blood of a typhus-immune host that selectively destroys, along with other functions, the portion of the antibody that determines the only known function by which antirickettsial antibodies may operate in host defense mechanisms, namely, opsonization of rickettsiae for enhanced ingestion by professional phagocytes and subsequent destruction. The epidemiologic significance of these findings is discussed in relation to the progressive destruction of cells that produce digestive enzymes of the louse midgut that occurs with progressive rickettsial infection, and the possibility of a negative feedback mechanism in transmission is introduced. Speculations that involve evolutionary concepts of both convergent and divergent varieties with respect to rickettsiae, potentially operational in a system that consists of an obligate blood-sucking arthropod vector and a vertebrate host capable of adaptive responses to both vector and rickettsial agent, are presented.